University of Idaho



University of Idaho

Department of Electrical and Computer Engineering

Department of Mechanical Engineering

Automated Touch Screen Linearity Tester

Sponsor: Advanced Input Devices, Coeur d’Alene, Idaho

November 18, 2003

By:

AXIS Inc., Student Design Team

Electrical Engineers:

Caroline Kawaguchi

kawa4852@uidaho.edu

Rick Engstrom

engs4998@uidaho.edu

Tim Matthews

matt6784@uidaho.edu

Mechanical Engineers:

Brock Schroeder

schr1523@uidaho.edu

Keith Ballenger

ball1367@uidaho.edu

Jason Stevens

stev5822@uidaho.edu

Sam Zimmerman,

graduate student mentor

zimm2280@uidaho.edu

Advisors: Dr. Joseph Feeley, Dr. Richard Wall and Dr. Steve Beyerlein

Table of Contents

1.0 Project Summary 3

1.1 Statement of objectives 3

1.2 Significance of project 3

1.3 Methods to be employed 3

2.0 Project Description 4

2.1 Objectives 4

2.2 Work Plan 4

2.3 Methods and Procedures 5

2.3.1 XY Table 5

2.3.2 Motors 5

2.3.3 Location Sensing 6

2.3.4 Z-axis Mechanism 7

2.3.5 Force Actuation Mechanism 7

2.3.6 Linearity/Skew Test 8

2.3.7 Resistance Test 9

2.3.8 Capacitance Test 10

2.3.9 Controller 10

2.3.9.1 Microcontroller Based Controller 10

2.3.9.2 Computer Based Controller 11

2.3.10 Graphical User Interface 12

2.4 Solution Impacts 12

2.5 Communication Plan 13

3.0 Schedule 15

4.0 Budget 15

5.0 Bibliography 16

Appendices 17

1. Project Summary

1. Statement of objectives

Team Axis Inc. proposes to design and construct an automated touch screen linearity tester (TSLT) for Advanced Input Devices (AID) of Coeur d’Alene, ID. The TSLT must test the actuation force, linearity, skew, and the resistance of touch screens. The TSLT will measure the force applied to the touch screen to actuate a signal, and collect the results of the linearity and resistance tests. The TSLT will immediately inform the test administrator of the test results on a pass/fail basis.

2. Significance of project

AID purchases their touch screens from external vendors. Each touch screen must pass the linearity and skew tests before they can calibrate the touch screen and integrate it into a product. AID would also like each screen tested for resistance and capacitance. Currently each screen is tested manually, which is slow and error-prone. Team Axis Inc. will automate this process to reduce error and expedite the process.

3. Methods to be employed

The TSLT will use an XY table to move a simulated finger or stylus in a set pattern on a touch screen. The TSLT will use a graphical user interface to set up and initiate the tests. The table will be controlled electronically with a micro controller or computer. The results of the tests will be plotted in a Microsoft Excel file for processing and saved for statistical analysis and future use.

2. Project Description

1. Objectives

Advanced Input Devices incorporates touch screens in some of their products. AID needs team Axis Inc. to develop a method and apparatus for testing four and five wire touch screens of different sizes and styles to verify each touch screen’s linearity. The primary tests must include a force actuation test, a linearity/skew test and a resistance test. The linearity test should produce an immediate pass/fail output. Secondary tests AID would also like, if possible, are a capacitance test and a life cycle test. The tests should be initiated by a graphical user interface (GUI) and the screens will be tested using a simulated finger and a stylus. The TSLT will run all tests automatically; the test administrator will only have to insert or remove screens and operate the GUI. The GUI will interface with Microsoft Excel for data analysis.

2. Work Plan

This project requires design work from Mechanical Engineering (ME) students and Electrical Engineering (EE) students. The ME students will be responsible for designing and constructing an apparatus that can accurately move a stylus or simulated finger around the touch screens. The EE students will be responsible for controlling the movement of this apparatus, recovering and processing data from each test, and designing a GUI to run the test.

3. Methods and Procedures

1. XY Table

The XY table is one of the major mechanical engineering project components. The XY table is raised over the touch screen and supports the testing apparatuses necessary to provide all required inputs to the touch screen. The table will trace a grid on the touch screen, which will be analyzed and plotted on a computer. The table will also position a force measuring apparatus in the center of the touch screen for a force actuation test.

The team has considered two options for the XY table: build and buy. The build option will be the lowest in cost, but will take more time to design and assemble. Two preliminary designs have been created and pricing is complete. Accurate machining and assembling, high quality bearings, and precision ball screws will enable the motion of the table to be precise with minimal friction and high speed operation. The ease of movement will ensure low error and low torque, enabling the use of lower cost stepper motors. The full purchase option is cost prohibitive and a partial purchase option will require a lot of modification. The purchase options have been ruled out.

2. Motors

The team will use stepper motors to produce the movement of the touch screen linearity tester. The advantages of using stepper motors are the precise step increments and lock torque characteristics. Stepper motors can be purchased with step increments as low as 0.9 degrees. The lock torque characteristic will prevent motor backlash and lower the error of the table. Servo motors were also considered. Servo motors must be incorporated with a feedback loop to prevent backlash. Unlike stepper motors, servo motors are susceptible to dust and dirt.

Stepper motors have high torque at low speed, and decreased torque at higher speeds. The torque requirements for the XY table movement are low due to the bearings and the ball screws. The low torque requirement will enable the stepper motor to be geared up, increasing the speed of the tester to allow for one hundred screens to be tested in a day as requested by the customer. A torque and speed analysis has been performed for the build option of the XY tables.

3. Location Sensing

Sensors provide feedback to the touch screen linearity tester (TSLT) control system. These sensors provide verification that the TSLT performs the desired motions. Sensors also prevent damage to components and the apparatus by stopping the machine before any extreme motions take place and eliminate the need for a physical stop. The sensors needed include strain gauges or a force transducer, encoders, and limit switches.

An encoder measures the position and speed of the stepper motor shaft. Encoders will provide active feedback for the position control system. Active feedback allows for not only control of the position of the stylus, but also control of the rate of motion in each axis. Controlling the rate of each axis enables the TSLT to traverse the stylus at angles other than forty five degrees through a differentiation between the X and Y axis stepper motor speeds.

Limit switches signal when an axis of the XY table, or the Z axis mechanism is about to reach the physical limit of travel. The TSLT will use this signal to stop the motion of the axis before it binds and causes damage to the table or a pinching hazard to the test administrator. Limit switches can also document the number of cycles in a cyclic test.

4. Z-axis Mechanism

The XY table suspends the Z axis mechanism over the touch screen by the XY table. The Z axis mechanism moves the force actuation and linearity tester devices into contact with the touch screen. A stepper motor will move the Z axis, and apply the force for the force actuation mechanism to measure. The force actuation mechanism will be held away from the touch screen while a pneumatic solenoid moves the linearity test mechanism into contact with the touch screen. The pneumatic solenoid is mounted to the side of the Z axis mechanism. Mounting the pneumatic solenoid to the side of the Z axis mechanism allows for greater clearance below the force actuation and linearity tester devices while raised. This also allows for the use of the simulated finger for the force actuation test and a stylus for the linearity test without the test administrator having to change the ends between tests.

5. Force Actuation Mechanism

The force applied to the screen during the force actuation test requires an accurate measurement to determine the performance of the touch screen. A couple of options to measure the force applied to the touch screen are available. Strain gauges can measure the force applied by the stylus to the touch screen through a cantilever beam[1]. A Force Transducer can measure the force directly[2]. A spring and linear motion potentiometer can measure the force by measuring the deflection of the spring.

6. Linearity/Skew Test

The TSLT will perform the linearity and skew tests simultaneously. To accomplish these tests, the TSLT will traverse a stylus across the touch screen in a grid pattern. The position the stylus touches is indicated by the touch screen with a voltage. The TSLT will relay the voltages from the touch screen to an Excel spreadsheet. The computer will tell the test administrator if the touch screen passes the linearity and skew tests after producing a plot. The grid pattern of the test consists of six parts (figure 1)[3].

1. Top edge of the screen (Y position 1)

2. Middle of the screen in the x-axis only (Y position 2)

3. Bottom of the screen (Y position 3)

4. Left edge of the (X position 1)

5. Middle of the screen in the y-axis only (X position 2)

6. Right edge of the screen (X position 3)

[pic]

Figure 1: Linearity Test Positions

The linearity test will map the raw-linearity of un-calibrated touch screens. The Excel spreadsheet will record the data as a plot of the XY coordinates touched.

Skew refers to the non-symmetry of a touch screen. The output lines from the traced grid pattern bow towards the center of the touch screen (Figure 2a). Bowing is normal, but a non-symmetric shape of the bow also known as skew can also cause the touch screen to fail. Figure 2 shows different amounts of skew. The gray lines are the actual lines traced. The black lines are the lines sensed by the touch screen. A symmetric touch screen (Figure 2a) will pass calibration. In Figure 2b skew is visible in the lower left hand corner of the screen. The severity of the skew determines weather or not the touch screen can be calibrated for use and thusly pass or fail the linearity test.

[pic]

Figure 2: Skew Examples

7. Resistance Test

The objective of the resistance test is to measure the resistance between the layers of the touch screen. To do this the TSLT will apply a force equal to that generated by a mass of 200 grams on the touch screen in a specific location, such as the center of the screen. Different touch screens may require this test at more than one location. Specifications for each touch screen will be used to determine if the resistance is within the bounds set by AID[2].

A Digital Multimeter can measure the resistance of the screen directly and get the readings between each layer. This is time consuming but reliable, and is currently the method used at AID. The TSLT will automate this test. A digital to analog converter will apply a known current to the touch screens then measure the voltage out. An analog to digital converter will then input this voltage to the computer and use relationship Resistance = Voltage/Current to find the resistance.

8. Capacitance Test

The capacitance test measures the capacitance between the layers of the touch screen. This test is not a primary test, but AID would like it included if possible. Team Axis Inc. is currently researching methods to accurately measure the capacitance between the layers of the touch screen, but has not found sufficient data to select a direction on this matter.

9. Controller

The XY table requires a control system to accurately test the touch screens. The Z-axis will also require precise control in order to apply the required force to the screen. Team Axis Inc. will design a control system to control the motion of the tester. There are a couple of possible options.

1. Microcontroller Based Controller

The first option considered is to use a microcontroller. The microcontroller would give precise directions to the motor devices and run the required tests. Programmable microcontrollers are not expensive and are versatile and easily adaptable. However; since a computer is already being used, the addition of a microcontroller may not be necessary. The computer will initiate the test by sending a signal to the micro controller. The micro controller will then perform all necessary tests and collect all test data. Once the micro controller has completed testing the touch screen the test data will be delivered to the computer for analysis. Figure 3 illustrates an example of a Micro controller based control system.

[pic]

Figure 3: Micro controller based control system schematic.

2. Computer Based Controller

The second alternative is to use a controller card, analog to digital converters, and C+ programming. A computer based controller card will direct the motors the same way that a microcontroller would, but it is specifically designed only to control movement. Each motor has a controller card that can be purchased. An analog to digital converter is needed to integrate these components. C+ programming will control the entire system. Figure 4 illustrates the computer based control system.

[pic]

Figure 4: Computer based control system schematic.

10. Graphical User Interface

The graphical user interface (GUI) will be programmed in Visual C or Visual Basic. To run a test, the test administrator will select the type of test (linearity, resistance, etc.). Then the test administrator will select the type of screen that is being tested. This is accomplished by selecting the pull down menu on the GUI and clicking on the touch screen profile that correlates to that touch screen. If the type of screen has not been tested before, the test administrator can input the parameters of the screen and save them as a future testing option by clicking on a new touch screen button. Once the screen is in place, the test administrator will click a “GO” button to begin the automated test.

The test will report data and results to a Microsoft Excel file and immediately display a “PASS” or “FAIL” upon completion of the test. This information will be saved to file and can be printed out to hard copy if necessary.

4. Solution Impacts

The economic impacts of this project will allow AID to test touch screens for proper functionality in a timely and organized manner. This project will also consolidate several tests performed on the touch screens saving AID time and in effect money. The automated touch screen linearity tester will test more screens in one day than is possible with the current process.

The reliability of the project is directly proportional to the quantity of time and quality of materials used to make the automated TSLT. We expect our finished product to last as long as AID needs to use it, without a change in performance. This will be accomplished through a comprehensive drawing package and parts list providing AID with the information necessary to perform any necessary maintenance to the machine.

The environmental impact and political considerations are nonexistent to this project due to the production scale. The only ethical or social concerns relate to displacement of employment positions. We do not expect that this product will significantly affect the number of jobs related to testing these screens as the persons currently tasked to perform the tests on the touch screens will operate the machine.

Many of the economic and environmental impacts of this project are reduced or negligible because this product will not be mass-produced. This product is a custom-designed tester for use by AID. Only one final model will be manufactured.

5. Communication Plan

Team Axis Inc. will meet at least once per week to ensure continuity between the Mechanical Engineers sub-team and the Electrical Engineers sub-team. The majority of the work will be done by dividing up and sub-teaming the mechanical and electrical engineering portions of the project. Both sub-teams will provide detailed short term schedules that look ahead to the coming weeks.

A weekly progress report will be sent to all members of the team, the mentor, all advising professors, and the engineers at AID. This progress report will cover the week’s activities and accomplishments, and the activities and goals for the coming week. The detailed short term schedules, progress reports, and the action item matrices will be displayed on the axis inc web page at: http//:seniordesign.engr.uidaho.edu/2003_2004/axisinc

The team will communicate with AID at least once per month by teleconference or by visiting AID. These meetings in conjunction with the information displayed on the team web sight will keep AID updated on the project and the team focused on the customer. Between these meeting times, questions from the team, and direction/suggestions from the engineers at AID will be sent via email through the designated customer contacts, Keith Ballenger (ME) and Rick Engstrom (EE).

3. Schedule

[pic]

Milestones/Deliverables

Written Proposal (1st draft) October 21st

Instructor Design Review October 23rd

Web site outlining design alternatives November 5th

Conceptual Design November 10th

Customer Design Review November 15th

Preliminary Design December 10th

Design Report December 10th

Detail Design January 30th

Working Prototype March 1st

Final Product Package April 30th

4. Budget

|Item [1], [2], [3], [4] |Cost |

|X/T Table |$2,000 |

|Shop Expenses |$500 |

|Z Axis Mechanism |$700 |

|Cart |$250 |

|Travel Costs |$350 |

|Controller and System Integration Components |$1,700 |

|Computer (Supplied by AID) |$0 |

|Total:  |$5,500 |

5. Bibliography

1) Futek Advanced Sensor Technology, Inc., , 14 November, 2003

2) OMEGA, http//:pptst/lcfa.html, 10 November, 2003

3) Advanced Input Devices internal document, functional spec Number: 9305-00772-001 revision 3, Advanced Input Devices Coeur d’Alene, Idaho 83815.

4) Advanced Micro Systems, Inc., , 5 November, 2003

5) MSC Industrial Supply Co., The Big book 2003/2004, Published by Quebecor World, August 2003.

6) Maurey Instruments, , 12 November, 2003

Appendices

A. Biographical Sketches

B. Design Alternatives

Appendix A

Biographical

Sketches

Appendix B

Design Alternatives

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