Introduction:
1 Introduction: 2
2 Recognize and Quantify Need 2
2.1 Project Mission Statement: 2
2.2 Company Background 2
2.3 Calibration: An element of Metrology 2
2.4 Product Description 2
2.5 Scope Limitations 2
2.6 Stakeholders: 2
2.7 Key Business Goals: 2
2.8 Top Level Critical Financial Parameters 2
2.9 Financial Analysis 2
2.10 Primary Market 2
2.11 Secondary Market 2
2.12 Order Qualifiers 2
2.13 Order Winners 2
2.14 Formal Statement of Work 2
3 Concept Development 2
3.1 Concept Development Process 2
3.2 Brainstorming Session 2
3.3 Conceptual Level Drawing 2
3.4 Mechanical Concepts 2
3.4.1 Plunger Depression 2
3.4.1.1 Stepper Motor 2
3.4.1.2 Pneumatic Systems 2
3.4.2 Stand 2
3.4.2.1 Stand with vertical movement 2
3.4.2.2 Stand with radial movement 2
3.4.2.3 The Basic Stand 2
3.4.3 Vial Holder 2
3.4.3.1 The Simple Block 2
3.4.3.2 The CMM Set up 2
3.5 Electrical Concept 2
3.5.1 Micro-controller 2
3.5.1.1 8051 Micro-controller 2
3.5.1.2 DIOS Micro-controller 2
3.5.1.3 PIC Micro-controller 2
3.5.1.4 BASIC Stamp Micro-controller 2
3.5.2 Vertical Movement Sensors 2
3.5.2.1 Hall Effect Sensor 2
3.5.2.2 Reed Switch 2
3.5.2.3 Magneto Resistive Devices 2
3.5.3 Radial Movement Sensors 2
3.5.3.1 Infra Red Proximity Detector (IRPD) 2
3.5.3.2 Potentiometer 2
3.5.3.3 Optical Interrupter 2
4 Feasibility Assessment 2
4.1 The Stand 2
4.1.1 Stand Concepts: 2
4.1.2 Attributes: 2
4.1.3 Level of Attainment Analysis 2
4.1.4 Critical/Important Attribute List 2
4.1.5 Second Level of Attainment Analysis 2
4.1.6 Performance Feasibility 2
4.1.7 Economic Feasibility 2
4.1.8 Technical Feasibility 2
4.1.9 Schedule Feasibility 2
4.1.10 Final Decision 2
4.2 Plunger Depression 2
4.2.1 Plunger Depression Attributes: 2
4.2.2 Concepts: 2
4.2.3 Level of Attainment Analysis 2
4.2.4 Performance Feasibility 2
4.2.5 Economic Feasibility 2
4.2.6 Technical Feasibility 2
4.2.7 Schedule Feasibility 2
4.2.8 Final Decision 2
4.3 The Micro-controller 2
4.3.1 Micro-controller Concepts: 2
4.3.2 Attributes: 2
4.3.3 Level of Attainment Analysis 2
4.3.4 Performance Feasibility 2
4.3.5 Economic Feasibility 2
4.3.6 Technical Feasibility 2
4.3.7 Schedule Feasibility 2
4.3.8 Final Decision 2
4.4 Sensors for Vertical Tracking 2
4.4.1 Concepts for Vertical Tracking: 2
4.4.2 Vertical Tracking Attributes: 2
4.4.3 Level of Attainment Analysis 2
4.4.4 Performance Feasibility 2
4.4.5 Economic Feasibility 2
4.4.6 Technical and Schedule Feasibility 2
4.4.7 Final Decision 2
4.5 Radial Movement Sensors 2
4.5.1 Concepts for radial tracking: 2
4.5.2 Attributes: 2
4.5.3 Level of Attainment Analysis 2
4.5.4 Performance Feasibility 2
5 Specifications, Analysis and Synthesis 26
5.1 Mechanical Analysis & Synthesis 2
5.1.1 Problem Statement 2
5.1.2 Known Information 2
5.1.3 Desired Information 2
5.1.4 Assumptions 2
5.1.5 Data and Analysis 2
5.1.6 Quality Review 2
5.2 Electrical Analysis 2
5.2.1 Design Specifications: 2
5.2.2 Micro-controller Specifications: 2
5.2.3 Sensor Specifications: 2
5.2.4 Power Supply Specifications: 2
5.2.5 Micro-controller 2
5.2.5.1 Selection of Micro-controller 2
5.2.5.2 Integration with Electrical System 2
5.2.5.3 Inputs: 2
5.2.5.4 Outputs: 2
5.2.5.5 Serial: 2
6 Bill of Materials 2
Appendix A: System Schematic 2
Appendix B: Electrical System 2
Appendix C: Flow Chart of System 2
Appendix D: Beer-Lambert’s Law 61
Appendix E: Hall Effect 61
Introduction:
Pipettes are widely used for delivering liquids in chemistry and life sciences laboratories, where their accuracy and precision are critical to achieving good results. In recent years, much attention has been focused on how best to calibrate these devices in centralized metrology laboratories, but little attention has been given to assuring good quality of liquid delivery in the place of use. Transcat has noted this and is currently looking into ways to calibrate these devices so as to expand the range of services.
Current calibration procedures require a technician with many hours of training to manually operate a pipette and measure how much fluid is transported compared to how much is metered on the pipette. Tools that are used for this form of calibration are the Artel PCS ® colorimetric calibrator and the Artel Pipette Tracker® software. These tools take the guesswork out of measuring the fluid delivered and also perform the statistical equations needed to find the accuracy and precision of the micropipette. To be specific the technician inserts the fluid from the pipette into a vial that is inside of the PCS®, which then uses Beer-Lambert’s Law to find the volume that is inside the vial. We will explain this law later in the paper. The PCS® then uses serial communication to send the results to the Pipette Tracker® software which then perform statistical equations.
Unfortunately the human interface tends to create quite a bit of variability in the calibration process. That is why Transcat desired to create a system which will automate the system as much as possible so as to reduce the variability and increase the reliability of calibration process. Our current design will accomplish this primarily by use of pneumatics.
The semi automatic pipette calibrator will require the technician to start by inputting the unique code for the pipette in computer interface. The technician will then be prompted to move the device over the vial containing the fluid, which is the starting position. The device will then submerge the pipette tip into the fluid and, once told to by the technician; will aspirate the liquid into the pipette tip. The pipette will then be raised and the technician will be prompted to move the device over the calibrator. Again, the device will insert the pipette tip into a vial. Then the technician will prompt the device to first dispense the fluid and then clear out the tip. The pipette will be raised again and the process will be repeated. This cycle will go on for 15 times, with three runs each at five equidistant volumes within the range of the pipette.
Recognize and Quantify Need
1 Project Mission Statement:
The goal of student engineering design team was to investigate viable processes and approaches to semi automate the pipette calibration process for as many pipette as possible. The final design consist of several pneumatic cylinders, a manually turning stand, several sensors to find the precise positioning of pipette, a computer interface for the technician, as well as the Artel PCS® and the Pipette Tracker® software.
2 Company Background
Transcat is one of North America's leading providers of calibration services and instrumentations. Since its' incorporation in 1964, Transmation (now Transcat) concentrated on developing, servicing and distributing electronic instrumentation used in the monitoring, calibration, and supervision. The primary markets for Transcat's services are process, life sciences, manufacturing, communications, automotive, and aerospace industries.
Transcat, Inc.'s former manufacturing organization located in Rochester, New York, was comprised of the Transmation Instrument Division, established in 1964, and Altek Industries, which was purchased by Transmation, Inc. in 1996. The two groups combined in 1999 to form the Products Group in 1980, the Transcat (short for "Transmation Catalog") Division was established as a catalog sales operation to offer customers a single source for calibration and test instrumentation. This operation has grown into a full-fledged industrial distribution network for not only Transmation and Altek products, but also those of more than 200 other manufacturers.
As catalog sales increased at a dramatic pace, it became evident that this type of equipment would require periodic recalibration and general maintenance in order to perform at peak level. Thus, in 1988, Transcat opened the first of many calibration laboratories to service customer equipment. The purchase of E.I.L. in 1997 and MeterMaster in 1999 (both distributors & calibration service organizations with laboratories located throughout the United States and Canada) established Transcat as the leading calibration service provider in the U.S. Finally, the newest business unit of Transcat established in 1999, , provides customers with an "Internet" channel for the purchase of calibration equipment and tools. In late 2001, as part of their strategy to divest non-core businesses, Transcat sold both the Products Group Division and the MAC (Measurement and Control) Division. This allowed them to focus on providing innovative, quality products and calibration services to their customers. Upon completion of the sale, the company name was changed from Transmation, Inc. to Transcat, Inc. Today, Transcat, Inc. employs 230 talented individuals throughout the U.S., Canada, and China.
3 Calibration: An element of Metrology
Metrology is a branch of science that was created in France and consists of the quantification of weights and measurement. Metrology is used in everyday life to make sure that the instruments and the systems that we use can accurately and reliably perform its designated task. Due to many cases of fraud in market place there were laws that were created to regulate these measurements. Today scientists instead of lawyers handle the regulation of measurement, and they are formed at an international basis so as to create a common base for all researchers to work on. With this cooperation, the level of precision of measurements has risen considerably, improving not only the methods of research but also improving the quality of the products created by this research.
Of particular interest to quality control (QC) is the element of metrology called calibration. Calibration is the process of comparing measurements, made by an instrument with a standard. The instrument, which is of unconfirmed accuracy, is referred as the unit under test (UUT) and the instrument of known accuracy is known as a measurement standard. Calibration is performed to establish the accuracy of the UUT for measurements. Instruments that do not meet the standards are adjusted and then tested again until they meet the standards.
4 Product Description
The primary goal of students on the Transcat design team was to design, build, test, and debug a working prototype device that incorporates a majority of the design and will work under the specifications listed below:
• Test Stand: A mechanical setup that will hold the pipette in place as the calibration is performed. The test stand will move in two directions, vertically and either rotationally or horizontally.
• Plunger Depression Device: A mechanical device that will insure the proper amount of force will be applied for the depression of the plunger for both the dispensing and tip blow out phases of pipette evacuation.
• Automated Digital Controls for some of the above features and for the data interface to the PCS® and Pipette Tracker® software.
• User Interface for the technician. This interface will allow the technician to properly use the semi-automated system. This could be either a physical or graphical interface.
5 Scope Limitations
The prototype was fully designed by the end of the fall quarter and a working device will be completed by the end of the spring quarter. At the end of the fall quarter the senior design team presented the preliminary design, detail sketches, and cost for the components needed to build the prototype. During the spring quarter, the design team will participate in on-site testing, data collection, and evaluation of prototype device.
As with any design, though, there are limitations to the scope. This simply means the design will only be able to perform a specific set of tasks. The tasks that we are designing for will meet the customer requirements as discussed between the team and the customer.
The design team will be responsible for a device that:
• Automates the human element of the calibration process
o Depressing the plunger
o Raising and lowering the pipette
• Maintains a .25% accuracy in liquid dispensed during the calibration process
• Performs three measurements at each of five equidistant points within the volumetric range of the pipette.
• Keeps the pipette vertical within +/- 1 degree
• Prevent light from contaminating the photosensitive fluids
• Reduce human vibrations by use of semi-automation.
• Allows human control over the processes being performed by use of the Graphical User Interface.
Has a complete user’s manual for future reference.
The design team will not be responsible for:
• Taking apart the micropipettes to perform manual replacement of parts
• Calibrating the Artel PCS® colorimetric pipette calibrator.
• Initiating changes to the Artel Pipette Tracker® software.
• Training individual users on the setup after the product has been integrated into the everyday process of calibration.
6 Stakeholders:
The primary stakeholder for the research and development of the semi automated system is Transcat, since the device will be integrated with the PCS® and Pipette Tracker® software. The students are the secondary stakeholders in this project because it will satisfy the engineering curriculum requirements for the students in the design team and it will serve as a learning experience for them. Eventually, the entire calibration industry could be a stakeholder, especially seeing as not much work has been done in the field of automating colorimetric calibration.
7 Key Business Goals:
In the field of calibration slight differences in precision can make the difference between success and failure. After all, everyday more and more companies are providing calibration services making the competition very tight. Because of this is necessary for the variation to be as small as possible. Seeing as the laboratories are at controlled environments, one can rule out variation due to surrounding and concentrate instead on the variations caused by the technicians. The main way to get rid of this is, of course, by removing as much interaction as possible by use of automation. This prototype aspires to do as much of this as possible, giving Transcat the competitive edge needed to succeed in this field of calibration.
8 Top Level Critical Financial Parameters
The top level critical financial parameters related to the project are associated with the following components that are needed to build the prototype.
• The pneumatic cylinders
• The controls for the pneumatic cylinders
• The micro-controller for the system
9 Financial Analysis
The team has been given a definite budget of $500 by Transcat. Therefore, it is important that the team create a low cost system to fit this budget. The most costly component will likely be the pneumatic cylinders that will actuate the system. The total cost of the system will depend on the following components:
• The cylinder
• Microprocessor
• The sensors
• The pneumatic controls
• Bearings for rotation
• The pipette holding fixture
• Raw materials needed to create the base
• Magnets to be put on the cylinder piston head
• Miscellaneous electrical and mechanical components
10 Primary Market
The primary market will be Transcat because the system is customized to be used in their lab.
11 Secondary Market
The secondary market will be any other company that is interested in providing colorimetric calibration services for micropipettes. In order for this system to be part of the secondary market, modification will have to be made to the design to enable it to be mass-produced.
12 Order Qualifiers
The primary market requires is that this prototype performs calibration runs with a variation at or under 0.25% of volume dispensed. The setup will have to be able to fit in the laboratory.
13 Order Winners
• Universal acceptance of pipettes
• Reduce test time
• Simpler calibration procedure
• Simpler setup
• Meeting all the project requirements
14 Formal Statement of Work
The RIT engineering designing team shall work closely with Transcat technical staff to understand customer’s needs and incorporate these requirements into the final product. The team will research through the appropriate means to identify the various methods to which they will be able to automate the colorimetric calibration process. The design team will take part constructive and on going discussion to understand the need for the balance between the desires of the engineers, the needs of the customer, and product feasibility. The team will also create appropriate design and specification options that will be reviewed by Transcat. The design team is primarily concerned with building a working prototype that incorporates all the project requirements that were agreed upon with Transcat. In addition, the team will perform testing, data collection, and evaluation of the prototype period. The design team will prepare a professional written report including complete technical specification, construction information, and experimental results, along with other information that will be presented to the corporate managers.
Transcat will provide the engineering design team with a technical point of contact that will provide guidance, along with feedback and field engineering information pertaining to the operation requirements and use of the system. In addition, they will provide access to relevant support information, including documentation and case studies, which will be needed to complete the project, along with access to their local calibration laboratory for insight operation and testing evaluation. Financial support for all components needed in the build will also be provided. All measurement instruments and components associated with project are subject to Transcat review and approval.
Agreed by:
Jeff Youngs ____________________ Date:_______(Design Team Leader)
Jon Schneider___________________Date:_______(ME Design Team Member)
Ashish Rathour__________________Date:_______( EE Design Team Member)
Glenn Carroll____________________Date:_______(EE Design Team Member)
Tai To_________________________Date:________(EE Design Team Member)
Mayank Rathour_________________Date:________(EE Design Team Member)
Howard Zion____________________Date:________(Customer Representative)
Rainer Stellrecht_________________Date:________(Customer Representative)
George Slack___________________Date:________(Faculty Coordinator)
Mark Hopkins___________________Date:________(Faculty Mentor)
Wayne Walter___________________Date:________(Faculty Mentor)
Concept Development
1 Concept Development Process
The concept development process has the main objective of developing several design objectives that will each meet the design specifications and to improve these ideas through interaction among team members. The design team use brainstorming technique to produce variety of concepts that would be used in further discussion. Team members also sketch conceptual drawings to provide a clear picture of some of the proposed concepts. Finally, the team use the design specifications to develop the ideas into a useful form.
2 Brainstorming Session
In the brainstorming session each of the team members pitched in to create a large quantity of concept that could be used in creation of prototype. The design team listed concepts on different design components of the project. A list of these concepts was made and each of the team members was given two votes for each component so that they could identify which concept that they thought best met the design of specifications. The voting also helped in consolidation of concepts. Table 1 shows the ideas generated during the mechanical portion of the brainstorming session and how popular each concept was.
|Design Component | |Concept |Votes |Rank |
| | | | | |
|Plunger depression |1 |Stepper motor |7 |1 |
| |2 |Pneumatic system |5 |2 |
| |3 |Hydraulic system |0 |3 |
| |4 |Spring Loading |0 |3 |
| |5 |Ratcheting mechanism |0 |3 |
| | | | | |
|Stand |1 |No stand |1 |3 |
| |2 |Basic stand |3 |2 |
| |3 |Stand with vertical movement |4 |1 |
| |4 |Stand with radial movement |4 |1 |
| |5 |Clamps |0 |4 |
| | | | | |
|Vial Holder |1 |Block |12 |1 |
| |2 |CMM setup (Grid) |0 |2 |
Table 1 Mechanical Brainstorming Session
Afterwards the team voted on the electrical concepts of the prototype. The concept and their popularity are listed in Table 2.
|Design Component | |Concept |Votes |Rank |
| | | | | |
|Microprocessor |1 |8051 |2 |2 |
| |2 |Pic Micro |7 |1 |
| |3 |Basic Stamp |1 |3 |
| |4 |DIOS |2 |2 |
| | | | | |
|Vertical Movement Sensors |1 |Hall Effect Sensor |11 |1 |
| |2 |Reed Switch |0 |3 |
| |3 |Magneto Resistive Device |1 |2 |
| | | | | |
|Radial Movement Sensors |1 |Infra Red ( IRPD) |1 |2 |
| |2 |Sonar |0 |3 |
| |3 |Potentiometer |1 |2 |
| |4 |Optical Interrupter |10 |1 |
| |5 |Hair Trigger Switch |0 |3 |
Table 2 Electrical Brainstorming Session
3 Conceptual Level Drawing
The team members that have the most popular designs then created conceptual level drawings of these designs. These drawings were used to help the team obtain an idea of how the concept would work. These conceptual drawings can be found in appendix A.
4 Mechanical Concepts
Each of the design components has the list of concepts related to them. The design team discussed top two or three concepts for each design component.
1 Plunger Depression
In order to transport the fluids, the operator needs to depress the plunger and release it in the fluid to draw it into the pipette tip, and then dispense the fluid and blow out the tip. As one can imagine this was one of the most important parts of the project. Therefore, much emphasis was put into in depth explanations of the top two concepts.
1 Stepper Motor
This concept had two possible implementations. Each of them used the precise movement of the stepper motor to its advantage. The first idea was to attach a gear to the shaft of the stepper motor and use the gear to drive a pinion gear. This pinion would actuate vertically in order to either depress or release the plunger of the pipette. The simplicity of this design was rather attractive to the team. The other idea was to attach a lead screw to the shaft of the stepper motor. A nut would be attached to the lead screw. In order to prevent the nut from merely turning along with the screw, the nut had to be properly restrained. There were several ideas as how to do this, but the winning idea had the nut constrained by two arms that would run on two tracks that are part of the setup. Several team members noted that this setup would be rather difficult to repair should any problems arise. This was noted and brought up later in the feasibility assessment.
2 Pneumatic Systems
This concept was originally unpopular due to the misconception that the system would be very expensive. This was due to the fact that most of the pneumatic systems that the team had viewed included an expensive system of machine made servos. After this misunderstanding was cleared, the team was more eager to work with this concept. At the time, the team felt that a simple pneumatic cylinder with a spring return system would suffice. As per the control of the pneumatic cylinder, the team felt that there were only two points of interest; that of the first phase of depression and the blowout phase. Each of these phases had their own unique range of pressures allowed and these pressures could easily be converted to forces by taking into account the surface area of the piston head. Therefore, one could control the system by using two distinct pressures, each near the separation point between the two phases. All one would need to go from one phase to the other would be a simple switching mechanism for the two distinct pressures.
2 Stand
Seeing as this part of the design would be holding the actual pipette in place, this is easily one of the most important parts of the design. At the time the team was divided on whether or not to actually have this stand move. The top three concepts are listed below:
1 Stand with vertical movement
This concept was one of the hardest to conceptualize. The reason for this was the fact that it would have to act against the forces of gravity and moving in accurate manner. One of the first ideas that came up was to use a stepper motor with a gear attach to its shaft and to have this gear interact with a pinion gear. This time, however, the stepper would be the moving part instead of the pinion. After consideration, we found that this would not be usable due to two reasons: the resolution of movement would be too inaccurate and even if the gear teeth were small enough to accommodate for accuracy, the teeth would then not be able to support the relatively heavy load. It was at this time that we considered the use of a pneumatic cylinder for the linear actuation. The piston would be double acting, in which there would be two air supplies, one in the inlet orifice and the other in the exhaust orifice. These two supplies would act against each other so as to let the piston inside the cylinder move in a gradual manner instead of slamming it into its fully opened and fully closed positions.
2 Stand with radial movement
Though the title of this concept refers only to radial movement, there was talk of horizontal movement along with radial movement as we discussed this concept.
When it comes to horizontal movement, we had quite a few ideas. One of the first ideas was to use a manually operated trolley system. In this idea, a simple “trolley”, which was actually a piece of metal with four wheels attached to it, would have its wheels inside of an extruded piece of plastic. One could then move the trolley back and forth with a fairly smooth movement. One of the main problems, however, was the fact that there would be no speed control, thereby allowing for the possibility of the trolley coming to an extremely abrupt stop. We found, after a few rounds of experimentation, that abrupt stops such as these tend to cause extreme errors in calibration, mainly due to the fluid becoming stuck in the tip. After we done with this idea, we advanced to the possibility of moving the calibration fluid and calibrator instead of moving the pipette. To be specific, we thought it would be a good idea to attach a table to the top of a pneumatic cylinder. This cylinder would also be double acting so as to reduce the possibility of jerky movements in the process. However, as will be seen in the feasibility assessment, this option was rather expensive, especially for a small budget such as ours.
When it came to radial movement we had a fair share of ideas as well. One of the first ideas was to automate this by use of a rotating turntable. In theory, this idea would work rather well. We would merely need to use a DC Motor to turn the whole set up and use a system of bearings to make the movements as smooth as possible. However, as with the other table idea, we found that this was a rather expensive method and that it would be fairly hard to control. The team’s other idea was that of manually moving the stand by use of a simple handle. Again, we would have to use a system of bearings in order to make the movements smooth, but with this setup, it would be rather easy to create a system of stops to properly restrain the movement. The simplicity of the set up made particularly attractive.
3 The Basic Stand
This setup merely called for the use of a rod that was fixed to the ground. We thought of this concept when we were putting together an experimental setup for preliminary data collection. In the experimental setup, we had a ring stand that we attached specially made clamps to. Seeing as the results that were obtained from this experimental setup weren’t very inaccurate, this made the possibility of using such a simple set up in our final design seem very plausible. However, we also had to think about how inconvenient such a set up would be for the technician that would have to use it.
3 Vial Holder
Though it may seem a very simple component to the system, the restraint of the vial plays a very critical role in the calibration process. After all, one needs to have a common place for the vial to be located at if one wants to efficiently use their time when calibrating. Also, a restraint would help to prevent the vial from spilling in case an accident should happen.
1 The Simple Block
As the title suggests, this setup is quite simple. One merely has to get a block of certain material, possibly wood, and drill a hole into it, so that the vial will fit into the block. This will be enough to satisfy the two criteria that the team set for this design component.
2 The CMM Set up
This set up is based on the system used by Coordinate Measurement Machinery (CMM). In this set up, there is a grid on the table and each of the coordinates has a screw hole in it. The purpose of these holes is to allow various attachments to be placed on the table in a secure fashion. This is very useful for securing parts that are unusual shapes, but we are merely working with a commonly shaped vial. Thus, the design would be too advanced for our needs. Still, the grid is rather useful for a coordinate system for the movement of the pipette. However, the two positions that the pipette would be moving to will be fixed, negating the need for a coordinate system.
5 Electrical Concept
1 Micro-controller
In order to maintain control of the system, we found it necessary to include a micro-controller. This component would keep track of all the actions and, depending on state of the system, would tell the system to perform certain actions. This is the heart of the electrical system.
1 8051 Micro-controller
The 8051 is an 8-bit microprocessor originally designed in the 1980's by Intel that has gained great popularity since its introduction. Its standard form includes several standard on-chip peripherals, including timers, counters, and UART's, plus 4k bytes of on-chip program memory and 128 bytes (note: bytes, not Kbytes) of data memory, making single-chip implementations possible. Its hundreds of derivatives, manufactured by several different companies (like Philips) include even more on-chip peripherals, such as analog-digital converters, pulse-width modulators, I2C bus interfaces, etc. Costing only a few dollars per IC, the 8051 is estimated to be used in a large percentage of all embedded system products.
2 DIOS Micro-controller
The DIOS Micro-controller made by Kronos Robotics is an 8 bit micro controller that comes in both 28 and 40 pin packages. The 40-pin package has 33 I/O pins, 8 A/D ports, and a 40 MHz internal clock. It includes 32 Kbytes of program space (16 Kbytes for user programs), 256 bytes of internal memory, and a programming port that, along with the RS232 interface included on the carrier board can be used to communicate with the PC. The DIOS uses a form of the BASIC language and can interface with a simple Visual Basic GUI on a PC. The compiler software is available for free from Kronos Robotics.
3 PIC Micro-controller
The PIC Micro-controller is yet another 8-bit micro-controller. PICs are available in a wide range of packages and amount of pins. They are also available in a wide range of internal clock speeds and a considerably large range of memory size. There is a wide range of compilers available for them, however the better ones are not free. An evaluation board is not necessary for its operation, however a programming board or socket is required to program the PIC. These range in price from the affordable $50, designed for on PIC in particular, to the extreme $1000 programmers that can program any PIC you throw at it
4 BASIC Stamp Micro-controller
BASIC Stamp is a micro-controller made by Parallax. They are available in 24 and 40 pin packages. Like the DIOS, they run on a form of the BASIC language, making them easy to program. They have a serial port for programming and interfacing with a PC.
2 Vertical Movement Sensors
This form of sensor would keep track of the vertical movement of the stand. After much research, we found that some of the easiest and most widely accepted sensors had to do with magnetism. This formed our concept development, as one can see from table 2.
1 Hall Effect Sensor
This proximity sensor is very cheap and very compact. These sensors are basically mounted to the side of the stand and keep track of any magnetic fields around them. The theory behind the Hall effect is talked about in more detail in Appendix E. These sensors are especially useful in pneumatic setups, because most suppliers readily supply air cylinder with magnets on piston heads. This way, one can see where the piston head is by keeping track of the magnetic field around it.
2 Reed Switch
This sensor is not a sensor at all. It is, as the name implies, merely a switch. Its resolution is rather poor, with it closing the circuit it is on when a magnetic field is up to 0.2 inches away and opening the circuit when the magnetic field is at least 0.3 inches away. While these measurements may seem small, they could easily ruin our system if they presented themselves.
3 Magneto Resistive Devices
These devices are quite similar to Hall Effect sensors. The main difference between these two sensors is that the MRD sensors measure the change in a magnetic field instead of the presence of one. This is accomplished by a variable resistor in the device that changes value as the strength of the magnetic field changes. This would be very useful if we were to keep track of how far away a piston head is, if that piston head has a magnet on it.
3 Radial Movement Sensors
At this point of our project, we had decided to use radial movement instead of horizontal movement. The fact that the path of motion is curved instead of straight may for a fairly difficult concept development. However, this difficulty inspired us to think of some fairly inventive sensor setups, as one can see below.
1 Infra Red Proximity Detector (IRPD)
The IRPD is a proximity sensor, that is to say that it merely detects if object is there or not. The IRPD acts both as a transmitter and receiver of an infra red light beam. The beam reflects back to the sensor and is only noticed by the receiver if the distance of reflection is small enough so that the infra red light does not diffract too much. This is a fairly cheap and easy to mount sensor, but it is very limited to its uses in a real world scenario.
2 Potentiometer
The potentiometer is a variable resistor that gains resistance as the knob is turned further away from its starting point. This is achieved by setting one end of a circuit at the starting point and the other end of the circuit on a piece of metal that is attached to the knob of the potentiometer. The piece of metal travels on a resistive film, and as it goes further away from the starting point, there is more and more resistive film in the circuit, increasing the resistance. One can keep track of just how far the knob has rotated by either keeping track of voltage in the circuit or finding the RC Time constant for a circuit containing the potentiometer and a common capacitor. Either way, one has to experimentally find some constant values for certain known angles of rotation. One can then linearly interpolate values based on these known constants. We can use this to our advantage by attaching a potentiometer to the stand and seeing how much the knob turns as the shaft it is attached to moves.
3 Optical Interrupter
This sensor is very similar to the IRPD in concept. The difference, though, is that the transmitter and receiver of the infrared light are two different parts. The Optical Interrupter keeps track of any objects in its limited vicinity by seeing if any infra red light reaches the receiver. If not, something is in the path of the infrared light. Usually people use incremental encoder discs in conjunction with the optical interrupter. To be specific, the incremental encoder discs have a series of translucent lines that are etched into an opaque surface. Whenever the opaque portion of the disc is inside of the proximity of the optical interrupter, there is no infra red light that gets to the receiver. When the translucent portion of the disc is between the two parts of the optical encoder, some infra red light gets to the receiver. One can easily use this to his or her advantage by making the translucent portions into thin lines that stand for certain discrete positions that the stand can be at. For our setup, we would permanently attach the incremental encoder disc to the stand. The optical interrupter would be mounted so as to allow the encoder disc to be equidistantly between the transmitter and receiver. As the shaft rotates, the disc rotates and lines in the disc rotate as well.
Feasibility Assessment
As a result of brainstorming and initial research, multiple concepts were created for each of the design components. All of these concepts were submitted for a feasibility assessment. The objective of the feasibility assessment was to understand what was involved with each concept and decide which concept had the highest probability of success. Each concept was evaluated on the feasibility of its’ technical, economic, scheduling and performance aspects. Each concept was assigned a rating between -, 0 and + for each of the attributes. A score of 0 was a representation of a concept grading the same as the baseline. A score of - represents a design that is slightly worse that the baseline design and + represents a design that is an improvement over the baseline design. A concept that received a - meant that it had very low feasibility. These parameters were used to judge the concepts. Afterwards, we described the feasibility of the designs in more detail. The discussion took on four aspects:
1. Performance Feasibility
2. Economic Feasibility
3. Technical Feasibility
4. Schedule Feasibility
The following sections present the feasibility for each of the design component concepts.
1 The Stand
The first design component that we performed feasibility assessment on was the most important component in the design. Because of this, we put the most effort into this feasibility assessment, as can be seen by the number of attributes used in the Pugh Analysis.
1 Stand Concepts:
Baseline Concept: Stationary Stand
A: Pneumatic Cylinder/Handle
B: Hanging Box /Trolley
C: Rotating Table
D: Pneumatic table movement/Pneumatic Stand
2 Attributes:
1. Horizontal Movement
2. Vertical Movement
3. Radial Movement
4. Smoothness of operation
5. Stability
6. Repeatability
7. User-friendly
8. Ability to support weight
9. Cost-efficient
10. Ease of setup
11. Ease of control
12. Ease of build
3 Level of Attainment Analysis
|Attribute |A |B |C |D |
|1 |0 |+ |0 |+ |
|2 |+ |0 |0 |+ |
|3 |+ |0 |+ |0 |
|4 |+ |0 |+ |+ |
|5 |- |- |- |- |
|6 |+ |+ |+ |+ |
|7 |+ |+ |+ |+ |
|8 |0 |- |0 |0 |
|9 |- |- |- |- |
|10 |0 |- |0 |0 |
|11 |+ |0 |+ |+ |
|12 |- |- |- |- |
Table 3 Pugh Analysis of The Test Stands: Baseline = Stationary Stand
As one can see from the Pugh analysis in Table 3, there was a tie in feasibility between concepts A & D: the two pneumatic stands. To take care of this, we decided to go through another round of Pugh Analysis. This time, we decided to reduce the amount of concepts to only A & D and reduce the attributes to the most important ones. This more specific Pugh analysis can be seen in Table 4. We agreed that the following attributes were most important:
4 Critical/Important Attribute List
1. Horizontal Movement
2. Vertical Movement
3. Radial Movement
4. Smoothness of operation
5. Stability
6. Cost-efficient
7. Ability to support weight
8. Cost-efficient
5 Second Level of Attainment Analysis
| |A |D |
|1 |0 |+ |
|2 |+ |+ |
|3 |+ |0 |
|4 |+ |+ |
|5 |- |- |
|6 |- |- |
|7 |0 |0 |
|8 |+ |+ |
Table 4 Second Pugh Analysis of The Test Stands: Baseline = Stationary Stand
Again, there was no clear winner, so we decided to go further into the analysis. We considered all the aspects of assessment as were discussed beforehand. Seeing as we had already ruled out two of the concepts, we decided to only analyze the two pneumatic stands.
6 Performance Feasibility
Both of these designs require the use of pneumatic cylinders. The first concept requires only two cylinders, while the second requires three. Seeing as we decided to use a valve bank, this was not a true problem. The main problem was that of the pressure required.
After talking with an industrial representative, we found that all of the pneumatic components would be able to run properly on 60-80 psi, a level that is usually found in laboratories. While we were there, though, the representative brought up a problem that we had not thought of yet.
The problem was that putting a table on a pneumatic cylinder introduces quite a bit of vibration when the cylinder gets to its two end points. This is due to the fact that the cylinder operates, on a base level, with two mechanical stop points. When these stop points are hit, the system comes to an abrupt stop and, as a result, has quite a bit of vibration. We would be able to combat this by making the cylinder double acting, in which both the exhaust and inlet ports would have air pressure sent through them. In this setup, as one slowly reduces the pressure on one side, one can either slowly speed up or slow down the system. Still, there would be some vibration. Still, the other setup is mainly hands-on and can also have quite a bit of vibration, depending on how hard the technician swings it. However, one can handle this kind of vibration, seeing as this only affects the performance of a single pipette. The vibration from the table, however, would affect the calibrator, which would effect all future calibrations. Thus, in this assessment, we decided that the concept of the manually moved stand would be the best.
7 Economic Feasibility
As was said before, both setups center on the pneumatic cylinders Therefore, each of the designs have the same base cost for the vertically and radial moving cylinder that holds the pipette and the other cylinder. The main difference will be in the cost of the pneumatically controlled table.
When we talked to the pneumatic representative, we found that a common table setup, including pressure control, tracks and sensors, would cost in a range from $200-$300. This would consume most of our budget, leaving little room for expenses that may come up later in the design. The manually controlled setup, however, would have a main cost of bearings so that the setup can rotate properly without wearing out the shaft of the pneumatic cylinder. The cost for this is much less than that of the table setup. Thus, the manually controlled setup has the definitive advantage in this feasibility, as well.
8 Technical Feasibility
In this form of feasibility, we assessed how well we could apply our knowledge to creating a working component. For the automated setup, we would have to know how to machine a serviceable track from extruded metal so as to reduce costs. This is a very rudimentary process, so we could easily perform this task. Also, we would have to create a form of locomotion for the table, most likely through wheels that would run on the track. We were fairly unclear as to how to do this, so our knowledge in this aspect was lacking. Finally, we would have to find a way to smoothly actuate both cylinders. We had done some research on this and, with the help of our Mechanical Engineering Advisor, Professor Wellin, and the pneumatic representative; we felt that we had a solid base of knowledge from which we could easily control the system. When it came to the manual setup, we had to know how to move the main cylinder manually. Seeing as this will be a simple setup in which the cylinder will move with the help of bearings in the table on which it stands, this would not be too hard for our level of knowledge. As per the actuation of the cylinder, it would be the same as the automated setup, but we would only have to control one piston, making it easier as a whole. Again, the manual setup has the advantage in the feasibility assessment.
10 Schedule Feasibility
Seeing as both designs are quite alike, the time schedules for the two setups will also be similar. However, as was talked about in the Technical Feasibility, we would have to do more machining for the automated setup, creating a longer time schedule for setup. Also, the automated setup would have to have extra programming and sensors to control the pneumatic cylinder on which the table stands. This would also increase the time schedule for setup. Seeing as we need to concentrate more on improving the setup than creating the initial setup, it would be best if we had as little initial setup time as is possible. Thus, the manual setup has the time advantage.
11 Final Decision
After going through all four forms of feasibility assessment, we can conclude that the manual setup is the best for what our project.
3 Plunger Depression
This design component was a very important component in the design. After all, if the pipette plunger cannot be depressed, one cannot transport fluid from the vial to the calibrator. However, this setup is fairly simple, so we didn’t have to account for too many attributes, as can be seen in the Pugh Analysis in Table 5.
1 Plunger Depression Attributes:
1. Repeatability
2. Two stokes
3. Accuracy I force/distance
4. Ease of replacement
5. Ease of setup
6. Ease of control
2 Concepts:
Baseline = Pneumatic Cylinder
A. Stepper motor with a lead screw
B. Spring loaded actuation
3 Level of Attainment Analysis
|Attribute |A |B |
|1 |0 |- |
|2 |0 |- |
|3 |+ |- |
|4 |- |+ |
|5 |- |+ |
|6 |- |- |
Table 5 Pugh Analysis of Plunger Depression: Baseline = Pneumatic Cylinder
As one can see from the Pugh Analysis, the baseline concept was the winning concept. In order to reinforce this finding, we went through the four steps of feasibility assessment.
4 Performance Feasibility
When we looked at the concepts, we decided right away that the spring-loaded actuation was not feasible. From a mechanical engineering viewpoint, whenever a spring is included in the system, there are many variances that come into play. Also, any performance from the system will be fairly rough due to the fact that the springs may not move unless a certain load is achieved, reducing the gradual nature of the actuation. Seeing as our experimentation has concluded that gradual actuation is preferred over rough actuation, we decided that this concept isn’t feasible from a performance standpoint.
Afterward, we compared the pneumatic cylinder to the stepper motor. When it comes to the precision of movement, the stepper motor has the advantage. After all, the stepper has a system of discrete steps that, depending on the controller, can be controlled to an accuracy of 1/64 of a step, or about 1/32 of a degree. The true resolution, however, depends on the number of threads per inch on the lead screw. Seeing as this parameter can be rather small, this is not a problem.
When it comes to the cylinder, our control of the actuation is through force instead of length measurement. For our means, we can control this parameter by finding the two pressures needed to get to each level of actuation. After experimentation, we found two values that would work for all models. This method of actuation is preferable, seeing as the pipettes may not be mounted the same way for each case. By using force stops, we can ignore differences in length. We can do this with the stepper as well, but a force sensor will need to be added to the design. Therefore, the slight advantage for performance goes to the stepper motor.
5 Economic Feasibility
For this case, we decided to create a miniature Bill of Materials so as to make a more concrete analysis of our design. The approximate costs are listed in Table 6 for the pneumatic system, and Table 7 for the stepper motor system.
|Part |Qty |Cost |Total Cost |
|Cylinder |1 |~$35 |~35 |
|Regulators |2 |~$10 |~20 |
|Switch Between Regulators |1 |~$5 |~5 |
|Total | | |~$60 |
Table 6 Estimated Cost of Pneumatic System
|Part |Qty |Cost |Total Cost |
|Stepper Motor |1 |~$20 |~$20 |
|Force Sensor |1 |~$25 |~$25 |
|Stepper Controller |1 |~$5 |~$5 |
|Lead Screw |1 |~$10 |~$10 |
|Total | | |~$60 |
Table 7 Estimated Cost of Stepper Motor System
As one can see, there is no clear advantage to either side. The only advantage that one could possibly think of would be the fact that the pneumatic system would be able to be bought all from the same supplier, making it possible to get bulk discounts.
6 Technical Feasibility
In this form of feasibility assessment, we thought not only about how much we knew about the subjects, but also about the simplicity of the design. When it comes to knowledge of the subjects, most of the team has had no real experience with either pneumatic cylinders or stepper motors. The team leader, though, has had extensive experience with use of stepper motors in robotic design. Also, some of our advisors are knowledgeable about the use of stepper motors. On the other hand, the team has advisors that are very knowledgeable about pneumatics and all of the principles of pneumatic use have been covered in the Mechanical Engineering syllabus.
When it comes to the simplicity of the design, however, the pneumatic system has the clear advantage. One merely needs to attach tubing to the regulators; the switch and the cylinder to do most of the setup. The stepper setup, however, tends to have a more complex setup. This is due to the fact that one has to constrain a nut that is on the lead screw, so that the nut moves vertically instead of having radial movement with the rotating screw. Thus, when we finished analyzing this feasibility, we found that the pneumatic setup had the advantage.
7 Schedule Feasibility
As was said in the technical assessment, the setup for the pneumatic system will be much simpler. This is important, seeing as we need to limit the initial setup time as much as possible to increase the time that can be spent on improving the system. Seeing as a simpler setup decreases setup time, we can say that the Pneumatic system has the clear advantage.
8 Final Decision
After going through all four forms of feasibility assessment, we can conclude that the pneumatic setup is the best for what our project.
4 The Micro-controller
This design component was a very important component in the design. This component would keep track of all the actions and, depending on state of the system, would tell the system to perform certain actions. We have taken an account of four commonly used micro-controllers for our Pugh Analysis.
1 Micro-controller Concepts:
Baseline Concept: PIC Micro
A. 8051
B. Basic stamp-2
C. DIOS
2 Attributes:
1. Input/outputs
2. Applicability of code
3. Compatibility with PC
4. Ease of programming
5. Clock speed
6. Memory
7. Voltage requirement (small?)
8. Documentation on controller
9. Knowledge of controller
10. Cost
11. Cost of accessories
3 Level of Attainment Analysis
|Attribute |A |B |C |
|1 |- |- |0 |
|2 |0 |- |0 |
|3 |0 |0 |+ |
|4 |0 |+ |+ |
|5 |+ |0 |+ |
|6 |0 |0 |- |
|7 |0 |0 |0 |
|8 |0 |0 |+ |
|9 |- |+ |0 |
|10 |0 |- |- |
|11 |0 |- |+ |
Table 8 Pugh Analysis of Micro controller: Baseline = PIC Micro
As one can see from the Pugh Analysis in Table 8, the DIOS micro-controller was the winning concept overall. Also, one can see that the 8051 and PicMicro micro-controllers were about equal. To further understand why each concept was feasible or not, we performed the four types of Feasibility Assessment.
4 Performance Feasibility
When it comes to interfacing with a PC, all of the micro-controllers start off with an equal footing. However, certain micro-controllers, such as the 8051 and the PicMicro, need to have a serial communications port added on to a custom-made breadboard in order to properly communicate with the computer. The Basic Stamp and DIOS, however, both have readily available and in-stock breadboards that have the capability for serial communication already built in. However, the DIOS controller board has the unique advantage of direct solenoid/servo control from the board.
When it comes to the amount of I/O pins, the more the better, seeing as more pins means more commands that can be sent out. The PicMicro and DIOS both have up to 40 pins, around 32 of which are programmable. The other concepts have less total pins and, therefore, less I/O pins. Thus, the team found that the DIOS had the largest advantage in this form of feasibility.
5 Economic Feasibility
When it comes to the price of the controller alone, the 8051 Micro controllers are much cheaper than Basic Stamp or DIOS micro-controllers. In fact, their prices are comparable with PIC micro controller. Still, this is not the only form of cost that the system can have.
There is also the cost of accessories for the system. In this case, the 8051 and the PicMicro again have equivalent prices. The Basic Stamp, being a Parralax product, has very expensive accessories when compared to the other micro-controllers. Finally, the cost of accessories for the DIOS micro-controller is relatively cheap when compared to the others. Seeing as we will have to add a fair number of accessories to the micro-controller, we can say that the DIOS, 8051 and PicMicro are on almost equivalent footing.
6 Technical Feasibility
One of the first attributes that come to mind when thinking of Technical Feasibility would definitely be the ease of programming. Seeing as the team leader has had extensive experience in programming systems with the Basic Stamp, the team decided that this would be one of the easiest micro-controllers to program. Still, the DIOS micro-controller also works with the Basic language. Therefore, the DIOS micro-controller could also easily be programmed. The other micro-controllers have a base language of Assembly, a language of which the Electrical Engineers had a good amount of experience, but not much on system-level programming.
Another attribute to consider would be the amount of documentation available for the micro-controller. All of the micro-controllers have some documentation, including books in the library. Still, the DIOS has an extensive web community behind it that is willing to give support to DIOS users free of charge. Therefore, one could say that, all in all, the DIOS micro-controller has the advantage.
7 Schedule Feasibility
As was said before, both the DIOS and the Basic Stamp had readily made carrier boards that could do a majority of the things we wanted to do. Seeing as, for the 8051 and the PicMicro, the team would have to modify a breadboard with all sorts of components, each which would have to be purchased with a lead time, one can easily see that the time for setting up the controllers is much longer for the 8051 and the PicMicro.
When it comes to programming time, the DIOS and Basic Stamp have very simple commands and the team leader knows most of the commands that would need to be used in any functionality. Seeing as not all the commands were known for the 8051 and the PicMicro, the team would have to lose valuable time learning how to program the controller. Thus, in this form of assessment, the DIOS and the Basic Stamp are tied.
8 Final Decision
As one can see in the four forms of feasibility assessment, the DIOS is the clear winner for the micro-controller.
6 Sensors for Vertical Tracking
In order to move the pipette from one place to another, one needs to clear any objects that lie in its path. In order to keep track of whether the pipette is high enough or not, one needs to install sensors. In the case of pneumatics, there are a few sensors that are accepted as the industry standards. We chose these sensors for the feasibility assessment.
1 Concepts for Vertical Tracking:
Baseline Concept: Hall Effect
A. Reed Switch
B. Magneto Resistive Device
2 Vertical Tracking Attributes:
1. Send signal to controller (feedback)
2. Easily mounted
3. Cost
4. Resolution/accuracy
5. Voltage/current needs
6. Simplicity of use
7. Knowledge of sensor
3 Level of Attainment Analysis
|Concept |A |B |
|1 |0 |- |
|2 |0 |- |
|3 |+ |- |
|4 |- |- |
|5 |0 |0 |
|6 |+ |- |
|7 |0 |- |
Table 9 Pugh Analysis of Vertical Tracking Sensors: Baseline = Hall Effect
By looking at Table 9, one would think that the reed switch would be the most feasible concept. This will change, however, once we go further in-depth in the feasibility assessment.
4 Performance Feasibility
When it comes to performance, the reed switch has a clear disadvantage. The switch activates when a strong enough magnetic field is about ¼ of an inch away and deactivates when the field goes about ½ of an inch away. This is not very good resolution at all, especially seeing as how the pipette requires a higher level of accuracy due to the need for the tip to go exactly into the fluid and not go too high or too low. Thus, the reed switch is not very viable.
The Hall effect sensor and the Magneto-resistive Device both have a better accuracy than the reed switch, but the two devices operate in different ways. The hall effect sensor senses if a magnetic field of a certain predetermined strength is present or not. If the field is any weaker than the predetermined strength, the sensor will not pick it up. This makes it so the sensor doesn’t pick up the magnet until it is right next to the sensor.
The Magneto-resistive Device, on the other hand, is not a proximity sensor at all. This device measures the change in a magnetic field. Thus, it can keep track of how far away a magnet is and how fast it is moving. The main problem is that of the speed of the signal. When it sends out a signal, it takes a while to go through its entire range and for the device to interpret this signal. Thus, the resolution of position is not that good for the Magneto-resistive Device. Thus, one can say that the Hall Effect sensor would be most feasible in this setup.
5 Economic Feasibility
All of the sensors are not very expensive. In addition, they can be mounted and attached to the micro-controller in almost the same way. Therefore, one could say that, unless the budget for the project was to be near its limit, one could choose either option at an almost equivalent economic feasibility. If one had to choose an option that was most feasible, however, the reed switch would be the obvious choice, as one can see from the Pugh Analysis.
6 Technical and Schedule Feasibility
Seeing as most of the team has worked with basic sensors and controls, none of these sensors should be outside of the expertise of the team members. However, the least easily set up option would be the Magneto-resistive Device. This is due to the fact that, instead of sending out a simple binary signal for the user to read, the user has to make some sort of setup to utilize the change in resistance in the MRD. One of the most common ways to do this is to create an RC circuit by using a common capacitor in conjunction with the MRD. Then, one can calculate the RC time constant, find several known position for certain time constants, and then linearly interpolate the data to find the positions for the RC time constants that one receives from the circuit. As one can see, this setup takes much more effort than the others, so this is the least feasible option, both in expertise and time taken to set it up.
7 Final Decision
After going through all four forms of feasibility assessment, we see that there is a tie between the Reed Switch and the Hall Effect Sensor as per how many forms of feasibility they were best in. However, for our project, the performance is the most important aspect of the design, at least as sensors go. Thus, one can say that the Hall Effect Sensor is the most feasible option for our setup.
7 Radial Movement Sensors
This component keeps track of the turning of the stand. Depending on the readout from the sensor, one can see if the pipette is over the vial or over the calibrator. From this, the micro-controller can tell the system to perform a certain set of actions that is suitable for this position. Therefore, we had to be careful when we performed the Pugh Analysis shown in Table 10.
1 Concepts for radial tracking:
Baseline Concept: Encoder disk and optical interrupter
A. IRPD (Infrared Proximity Device/Detector)
B. Sonar
C. Potentiometer
D. Hair trigger switch
2 Attributes:
1. Cost
2. Accuracy/resolution
3. Feedback
4. Ease of mounting
5. Simplicity of use
6. Knowledge of sensor
3 Level of Attainment Analysis
|Attribute |A |B |C |D |
|1 |+ |- |0 |+ |
|2 |- |- |0 |- |
|3 |0 |- |+ |0 |
|4 |+ |- |+ |+ |
|5 |0 |- |- |+ |
|6 |0 |0 |- |0 |
Table 10 Pugh Analysis of Sensors: Baseline = Optical Interrupter Setup
According to this analysis, the hair trigger switch is the winner. Still, as was shown in the last feasibility assessment, this does not always show what is the true winner. Therefore, we performed the four types of feasibility assessment again.
4 Performance Feasibility
The IRPD and hair trigger switches both share a common problem: that of resolution. For the IRPD, the range that it covers is about two inches, with much variability that can occur with the sensing. The hair trigger sensor also has a two-inch range, but the range is confined to the trigger on the switch. Seeing as the vials are less than an inch in diameter apiece, these ranges in sensing are not acceptable. Therefore, if the sensor is triggered at a distance of 1 ½ inches instead of the possible 2 inch range, the tip of the pipette would likely miss its mark.
When it comes to sonar, there is one key problem. This problem is the fact that once an object gets within the range of about one inch, the sonar sensor no longer notices it. This is not suitable for our system, seeing as our design would need to have the sensor close to the object to save space in the design.
The potentiometer and the optical interrupter both have very accurate sensing apparatus. The resistive film in the potentiometer lets one track the position very well with RC Time Constants and linear interpolation. The optical interrupter’s encoder disk lets one instantly know when one is at a reference point. Either way, they both do the job well.
5. Economic Feasibility
As one could see from the Pugh Analysis, the cheapest of the sensors are the IRPD and the hair trigger sensors. To be specific, the hair-trigger sensor is the least expensive of them all. When it comes to the IRPD, however, it is not much less expensive than the optical interrupter setup.
As per the sonar, not only is it the most expensive, but experience has shown that they are very temperamental and tend to need to be replaced regularly. Thus, this is far from a good choice for our system.
6. Technical Feasibility
When it comes to simplicity of use, one can’t get more simplistic than the switch. After all, it is purely a physical touch sensor with binary output. The IRPD is in the same vein; only it is a light wave sensor.
The sonar can be controlled with the proper conversion factor, either to inches or mm. Still, as was said before, the sensor is quite temperamental, so use of this sensor tends to give the user improper data that must be adjusted each time a new type of error is introduced.
When it comes to the potentiometer, one has to figure out the RC Time constants for certain positions and these Time constants change as the capacitors either wear out or are replaced. Thus, it is fairly hard to keep track of.
The optical interrupter has a physical encoder disk that has lines etched into it that are fixed in place. Thus, once one has mounted the encoder disk, one no longer has to worry about adjusting for position error.
7. Schedule Feasibility
All of the parts have to be mounted. The fastest mountings will be for the IRPD and the switch. The second fastest would likely be sonar, seeing as one also merely needs to mount it to be in place, but one also has to take time to find the RC Time Constants. Third would be the potentiometer, which one has to mount the shaft of directly to the cylinder. Lastly, the slowest to mount would be the optical encoder, seeing as one has to mount the encoder disk to the cylinder and then mount the mouth of the optical encoder around the disk.
When it comes to time spent correcting the errors in the system, the least time would be spent on the optical encoder, seeing as it has very little ability to have variance in it. The other sensors, however, would have plenty of time spent correcting the errors, perhaps even more than the time saved on mounting. Thus, the team decided that the optical encoder would be the most feasible.
8. Final Decision
After looking through all forms of feasibility assessment, we found that the optical encoder setup was the most feasible.
Specifications, Analysis and Synthesis
1 Mechanical Analysis & Synthesis
1 Problem Statement
During the calibration of a pipette there are many factors that affect the precise calibration of the pipette. Our goal is to minimize the factors that are causing the error in the calibration.
2 Known Information
The colorimetric or photometric method involves the analysis of volumes of diluted dye in a cell of known path length. According to the Beer- Lambert Relationship,if a beam of monochromatic light passes through homogeneous solutions of equal pathlength,the absorbance measured is proportional to the dye concentration.So,with this in mind, an unknown volume of dye can be pipetted into a known volume of diluent,the resulting dye concentration can be measured photometrically, and the volume can be calculated.
There are many techniques and tips available that will optimize pipetting performance and increase the reproducibility of results.
1. The equipment
1. Tips – It is advocated that only high quality tips, which optimize the pipette’s performance, be used. A high quality tip is one that has a smooth uniform interior with straight even sides that prevents the retention of liquids and minimizes surface wetting. Also, the tip should have a clean, hydrophobic surface and a perfectly centered opening in order to ensure the complete dispensing of the sample.
2. Liquid viscosity – since the pipette was originally factory calibrated using water, any liquid that has viscosity higher or lower than water will impact the volume dispensed.
2. The operator
1. Technique
a) Position – Pipette should be held vertical during the aspiration of liquids. Holding a pipette 30deg. off vertical can cause as much as 0.7% more liquid to be aspirated due to the impact of hydrostatic pressure.
b) Pre – Wetting /Pre – Rinsing Tips- Failing to pre-wet tips can cause inconsistency between samples since liquid in the initial samples adhere to the inside surfaces of the pipette tip, but liquid from later samples does not. Also, if a new volume is dialed in on pipette’s micrometer, better results will receive at the new volume by taking the old tip off and placing a new one on the shaft before commence pipetting.
c) Release of Plunger – Releasing the plunger abruptly can cause liquid to be bumped inside the pipette during a liquid transfer application. This can cause liquid to accumulate inside the instrument which in turn can be transferred to other samples causing variability in sample volume and the potential for cross contamination.
d) Immersion Depth- The pipette tip should only be inserted into the vessel containing the liquid to be transferred about 1-3mm. If the tip is immersed beyond this, the results could be erroneously high.
e) Thermal conductance- Thermal energy can be transferred from the operator’s hand to the air within the pipette (dead air) or even to the internal components themselves. This can have a dramatic impact on the amount of liquid dispensed due to effects of expansion and/ or contraction. To lessen this effect, it is recommended that some type of thermally insulated gloves like latex of cloth be worn.
2. Pipette micrometer setting – It is important to avoid significantly over dialing or under dialing the recommended range of pipette. Volume delivery performance may change radically and may become completely undefined.
3. The Environment
a) Temperature – The volume delivery performance specifications of pipettes have been referenced by most manufactures at room temperature which is defined as (20- 25) deg.Celsius.Any deviation from this specification can affect the amount of liquid dispensed due to the expansion or contraction of the internal components.
b) Barometric Pressure – Pressure is reduced by 1.06” Hg for every 1000’ of elevation, however, barometric pressure has only a small effect on the density formula, so the error encountered is not correcting for elevation is often ignored.
c) Relative humidity – This is the percentage of moisture in the air at a measured dry bulb temperature compared to the amount of moisture that the air can hold at that temperature if the air is 100%saturated. Under dry conditions, which are defined as less than 30% RH, it is extremely difficult to ensure an accurate measurement due to the rapid evaporation rate. Conversely, excessive humidity, which is defined as greater than 75%, can cause a measurement to be erroneously high due to condensation. Therefore, generally accepted guidelines for pipette volume delivery specify that relative humidity be maintained within the range of 45%-75%.
Concerning the operator, there are many factors in proper pipette techniques that play a major role in the calibration process. Factors that the operator have control of include the position of the pipette, pre-wetting the tips, release of the plunger, immersion depth, and the thermal conductance.
The environment is also a factor that affects the calibration of the pipette and such factors include vibration, evaporation, and temperature. All materials involved in the photometric must be at the same temperature as not to get any temperature gradients. Let the dye solutions, spectrophotometer, pipette, tips, etc. reach thermal equilibrium at room temperature overnight if possible to make results more accurate. Vibration creates air within the solution and can affect the calibration; therefore vibration is a concern of the system.
3 Desired Information
During the pipette calibration the goal is to figure out the largest error factor and then once that is taken care of work the way down as to minimize all errors and make the calibration more precise. The largest error factor right now is the “speed” at which the plunger is actuated. Some information needed is the amount of force to actuated the plunger at first stroke and clean out stroke.
4 Assumptions
There are a few assumptions that need to be listed within the system. The thermal equilibrium concern should be taken care of and by this one is to say there is no need for a controlled environment for the temperature of the system. The materials of the calibration should all be in the lab for at least a work day to make sure the equipment is all at thermal equilibrium. The system can also ignore any hydrostatic pressures caused when the tip of the pipette is placed into the test fluid. The pressure created during this process is so minimal that it can be neglected. Another important assumption is that there is no reason to concern the system with any evaporation concerns. The test vial will be very close to the calibration device and therefore with the laboratory environment there should be no issue of evaporation. The last assumption that is that the air supply in the lab is going to be no less then fifty pounds per square inch, as to design the air cylinders bore size around the supplied air pressure and the force required.
5 Data and Analysis
The problem with the pipette is the human interference and therefore if that can be diminished then the error may be able to also be diminished. In the tables below the experiment was held using different types of strategies to operate the pipette.
| |Pipette Calibration Result | | | |
| | | | | |
|Oxford Pipette | | | | |
|Range 10 - 100 uL | | | |
|SN# 014756 | | | | |
|8885-500945 | | | |Average for 80 uL |
|Accuracy +/-0.5 - +/-1uL | | |81.78571429 |
| | | |Average for 10uL |
| | | | |11.594 |
|Reading (uL) |Suck in/ Dispense/ Angle |Actual Reading (uL) |% Error |Percent Error with |
| | | | |respect to averages |
|80 |slow/slow/vertical |81.5 |1.875 |0.349344978 |
|80 |slow/fast/vertical |82 |2.5 |0.262008734 |
|80 |fast/fast/vertical |81.7 |2.125 |0.104803493 |
|80 |slow/slow/angle |81.3 |1.625 |0.593886463 |
|80 |slow/fast/angle |82.1 |2.625 |0.384279476 |
|80 |fast/slow/angle |81.9 |2.375 |0.139737991 |
|80 |fast/fast/angle |82 |2.5 |0.262008734 |
|10 |slow/slow/vertical |11.74 |17.4 |1.259272037 |
|10 |fast/slow/vertical |11.52 |15.2 |0.63826117 |
|10 |slow/fast/vertical |11.55 |15.5 |0.379506641 |
|10 |fast/fast/vertical |11.72 |17.2 |1.086769018 |
|10 |slow/slow/angle |11.44 |14.4 |1.328273245 |
| | | | | |
|Comment: All reading were off by 1-2 uL | | | |
|The pipette was out of tolerance. | | | |
|Varying the technique did not seem to matter at all or where dispense the fluid at an angle. | |
Table 11 Tests for the Oxford Brand Pipettes
|Fisherbrand pipette | | |Average for 100 uL |
|SN# N30775 | | | |99.725 |
|Range 20 - 200 uL | | |Average for 45 uL |
|Accuracy | | | |44.5075 |
| | | | |Average for 20 uL |
| | | | |19.7075 |
|Reading (uL) |Suck in/ Dispense/ Angle |Actual Reading (uL) |% Error |Percent Error based |
| | | | |on averages |
|100 |slow/slow/vertical |99.4 |0.6 |0.325896215 |
|100 |slow/slow/vertical |99.8 |0.2 |0.075206819 |
|100 |fast/fast/vertical |100.1 |0.1 |0.376034094 |
|100 |slow/fast/vertical |99.6 |0.4 |0.125344698 |
|45 |slow/slow/vertical |44.26 |1.64 |0.556086053 |
|45 |slow/slow/vertical |44.49 |1.13 |0.039319216 |
|45 |fast/fast/vertical |44.65 |0.78 |0.320170758 |
|45 |slow/fast/vertical |44.63 |0.82 |0.275234511 |
|20 |slow/slow/vertical |19.66 |1.7 |0.24102499 |
|20 |slow/slow/vertical |19.72 |1.4 |0.063427629 |
|20 |fast/fast/vertical |19.84 |0.8 |0.672332868 |
|20 |slow/fast/vertical |19.61 |1.95 |0.494735507 |
| | | | | |
|Comment: No need to put the tip of the pipette at an angle, it doesn’t make a difference.| | |
|Readings were very close to expected value. | | |
| | | |
Table 12 Tests for the Fisherbrand Pipette
As seen in Tables 11 and 12, there is much difference when the pipette is actuated and released in different manners. In Table 13, the different pipettes were tested to examine what amount of force was needed to actuate both levels of the plunger.
|Pipette |Volumetric |Level 1 |Level 2 |
| |Setting | | |
|Fisherbrand |20 uL |830g |2.4kg |
|(0-200) | | | |
| |50 uL |900g |2.4kg |
| |100 uL |900g |2.5kg |
| |150 uL |900g |2.4kg |
| |200 uL |900g |2.4kg |
|Ranin (0-20) |5 uL |900g |3.8kg |
| |10 uL |700g |4kg |
| |15 uL |700g |4kg |
|Oxford |10 uL |700g |2.8kg |
|(0-100) | | | |
| |50 uL |700g |2.5kg |
| |100 uL |700g |2.6kg |
Table 13 Force Requirements
The level one actuation is the amount of force used to inject the tip with the appropriate amount of fluid and then eject the fluid. The level two is the clean out stroke that is done at the time when all the substance is out of the tip and this is just to ensure that all of the fluid is out of the tip. When the force was converted to pounds of force it was calculated that approximately two pounds of force would be required to actuate level one and to make sure that the entire clean out stroke was actuated a force of eleven pounds will be used. The forces in Table 13 were found using calibrated weights that were set on top of the pipette plunger and with gravity constant after the suitable weight was on the pipette to actuate it; that weight was the required force. Since the pipette requires two different forces to run the calibration it would be a difficult process to accomplish with something like a stepper motor. The first problem with using a stepper motor is that not only would you need to have an output back to the motor to relay the force applied; but as the volume of the pipette changes so does the range of motion of the first stage actuation. This concludes that the best solution for the system is a pneumatic one because not only can the pneumatics take care of the two different forces but it can also handle the different ranges of travel. The next objective was to figure out what size pneumatic cylinder would be needed to operate the pipette. With the assumption that the air supply would be no less then 50 pounds per square inch, the equation below was used to find that a ¾” bore cylinder would be appropriate for this application.
[pic]
Clippard catalog series UDR-12 double acting cylinders will be appropriate for both pneumatic cylinders. For the cylinder that is going to actuate the plunger (cylinder #1) the range of actuation needs to be maximum distance of 1.14” therefore the designed cylinder will have an actuation length of 2”. The cylinder that will control the vertical movement of the system (cylinder #2) will have an actuation length of 6” this will account for clearance of the lid of the calibration device and capable of insertion of pipette tip into the test dye. According to Clippard, the ¾” bore cylinders have a ¼” rod, which, also according to Clippard, will handle a load of 190 pounds at a length of 5” until buckling occurs. The rod of cylinder 2 will actuate to a length of 6” the buckling load is still far greater then the applied load that it will experience. The maximum weight experienced by the rods is nowhere in the range of 190 pounds.
To control the pneumatic cylinders there will be pneumatic valves with regulators and speed control on each valve or inline with each valve. The speed control is important to minimize vibration with control of the vertical motion and most important to the actuation speed of the plunger. For this application the system will use adjustable flow control valves that will be inline meter out needle valves. The MFC series from Clippard is a good choice for the needle control valves. There will be three different air pressures required in the system make this possible there will have to be three air regulators that control each of the pressures to the valves. The three regulators that are needed are all at different ranges: 0 – 50 psi for the cylinder #2, 0 – 10 psi for cylinder #1 (first actuation), and 0 – 30 psi for cylinder #1 (second actuation). The MAR series from the Clippard catalog will work properly for all of these applications. The valves that will be electrically controlled are yet to be determined because the team may have the valves donated to them, so the rest of the design may be completed around these valves. For some specifications the valves will be controlled by 0 -24 VDC and will have a manual actuation so it can be operated without the electrical system.
6 Quality Review
After reading through this, I found that the report contains all of the facts that we had covered over the quarter in a concise manner. Still, this contains mostly information pertaining to the plunger actuation piston. If one were to think of the piston that acts as a stand, one would have to take into effect the weight of the setup, or:
[pic]
You could then apply the force equation again, this time on the larger piston in order to see if the pressure would be enough. I know from my experience with the representatives from Roessler that the piston should be able to support up to 6 pounds if the cylinder rises 4-8 inches, which is enough to clear the colorimetric calibrator. Speaking of which, we might want to add in the fact that the colorimetric calibrator is 5 inches high by itself and 8 inches high with an opened lid. Besides this, the analysis seems to be okay from a mechanical standpoint.
2 Electrical Analysis
1 Design Specifications:
The objective of this project is to fulfill all the requirements set forth by Transcat. The design is to reduce variability and error induced by human interaction with the pipettes and the calibration process.
2 Micro-controller Specifications:
• One RS232 serial port for PC communication (GUI)
• At least 8 I/O ports
• Low memory requirement (RAM size: 32 Bytes, EEPROM Size: 2K Bytes)
• Ease of programming (BASIC)
3 Sensor Specifications:
• 5V Logic
• Piston sensors must be able to accurately sense piston position
• Base sensors must be able to sense rotational position
• Sensor for cover must be able to determine if cover is open or closed
4 Power Supply Specifications:
• Must supply 5VDC and 24VDC
• Must use 120VAC 60Hz single phase power
Must be able to handle a maximum current draw of 1A as valves runs on 1W power.
5 Micro-controller
1 Selection of Micro-controller
The Basic Stamp was chosen for this project due to its ease of use and cost. The main constraint of the micro-controller is that we need RS232 serial communication to a PC as well as at least eight input/output lines.
Basic stamp 2 Module specs are:
• 24 pin DIP
• Microcontroller: PIC16C57
• Processor speed: 20 MHz
• Program Execution Speed: 4000 instructions/sec.
• RAM size: 32 Bytes (6I/O, 26 Variable)
• EEPROM (Program) Size: 2K Bytes, 500 instructions
• No. Of I/O pins: 16+ 2 Dedicated Serial
• Voltage Requirements: 5-15 V
• Current Draw @ 5V: 3mA Run /50uA Sleep
• Source/Sink Current per I/0:20mA/25mA
• Source/Sink Current per unit: 40mA/50mA per 8I/O pins
• PBASIC Commands: 42
• PC Programming Interface: Serial Port (9600 baud)
• Windows Text Editor: Stampw.exe (v1.04 and up)
2 Integration with Electrical System
The electrical system currently has five inputs and three outputs. The inputs are in the form of sensors. The three outputs are solenoid valves to control air flow. For the micro-controller to work in this situation, we will need to make a simple interface to make sure the sensors are on or off, with simply a 5V or 0V digital input to the micro-controller. A different interface is required for the output, as the solenoids valves require 24VDC and approximately 100mA, sometimes up to 200mA when they actively switch. Since the micro-controller can only provide 5VDC at around 20mA to an output pin, we will need to have a power FET or a driver to provide the necessary power to the valves. Also needed is a simple interface for RS232 communication with the PC. This serial communication will provide the initialization for the micro-controller as well as feedback and instructions to the user. The Stamp 2 Board has a built in RS232 interface, as well as room for an EEPROM, if additional memory is required as the project progresses.
3 Inputs:
• Piston Up
• Piston Down
• Dye Position Sensor
• Calibrator Position Sensor
• Calibrator Cover Sensor
4 Outputs:
• Piston Up/Down
• Stage 1 Pipette Activation
• Stage 2 Pipette Activation
5 Serial:
• Receive Command to Begin Run from user
• Send feedback messages to user
▪ (i.e. “Please rotate the pipette to the Dye Position”)
• Query user after each run if the wish to run again or end run
Bill of Materials
| Part |Qty |Unit Price |Total Price |
|DIOS Micro-controller 40 Pin |1 |$24.95 |$24.95 |
|DIOS Carrier Board #4 |1 |$39.95 |$39.95 |
|DIOS Compiler |1 |$0.00 |$0.00 |
|TI SN754410 Solenoid Driver |1 |$4.49 |$4.49 |
|Power Supply Radio Shack |1 |$17.99 |$17.99 |
|Misc. Electronics |1 |$10.00 |$10.00 |
|Magnetic Proximity Sensor |1 |$2.00 |$2.00 |
|Hall Effect Sensor |2 |$4.00 |$8.00 |
|Optical Interrupter Sensor |1 |$8.00 |$8.00 |
|Adjustable Air Regulator |3 |$5.00 |$15.00 |
|Adjustable Flow Control |4 |$32.30 |$129.20 |
|Pneumatic Cylinder 7/8" Bore |1 |$50.00 |$50.00 |
|Pneumatic Cylinder |1 |$30.00 |$30.00 |
|Solenoid Valve 2-way |2 |$18.00 |$36.00 |
|Solenoid Valve 3-way |1 |$25.00 |$25.00 |
|Raw Materials | | | |
|Shipping Costs |1 |$30.00 |$30.00 |
|Total | | |$420.58 |
Appendix A: System Schematic
[pic]
Appendix B: Electrical System
[pic]
Appendix C: Flow Chart of System
[pic]
[pic]
[pic]
Appendix D: Beer-Lambert’s Law
An unknown concentration can be found using a known absorbance of the liquid and applying Beer's law. The Beer-Lambert law (or Beer's law) is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:
c = A/(a(lambda) * b)
Where a(lambda) is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the liquid concentration. There is also a blank of known concentration and volume in the colorimeter. The system then uses the following equation to find the unknown volume:
Concentration (test) x Volume (test) = Concentration (blank) x Volume (blank)
Appendix E: Hall Effect
The sensors used to detect the vertical position of the cylinder are called Hall Effect sensors. The Hall effect is a phenomena of a voltage created transversely to the current applied in a conductor, when there is a magnetic field perpendicular to the current flow. The sensors respond to a presence or interruption of a magnetic field by producing either an analog or digital output proportional to the strength of the field.
Appendix F: STAMP 2 MODULE
[pic]
Appendix E: Fishbone Diagram
-----------------------
Pipette
The operator
Environment
Release of Plunger
Pre – Wetting
/ Pre – Rinsing Tips
Barometric Pressure
Position
Humidity
Pipette micrometer setting
Immersion Depth
Temperature
Error in Calibration Reading
Not smooth motion
Direct contact of sunlight
Calibration with different angles
Liquid viscosity
Tips
Calibration with variable speed
Other Factors
Calibration Method
Other Equipments
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