Customer Needs Assessment:



Staff | |

|Team Member |Discipline |Role |

|Wayne Walter |ME |Faculty Guide |

|Erin Gillespie |ME |Consultant |

|Anthony Squaire |ISE |Project Manager |

|Alan Mattice |ME |Lead Engineer |

|Brian Bullen |ME |ME Support |

|Cody Ture |ME |ME Support |

|Charles Trumble |ME |ME Support |

|Jeff Cowan |EE |EE Support |

|Aron Khan |EE |EE Support |

|Andre McRucker |CE |CE Support |

Open Architecture Thruster Design for Underwater ROV

Detailed Design Review

Project 08454

Introduction

The primary objective of the Underwater Equipment Technology project is to take the underwater ROV that was constructed by the Underwater ROV Project 06606 during the 2005-2006 Senior Design sequence and expand its capabilities. Currently, the ROV is submersible to a depth of 400 feet of water and is equipped with a digital video system, which allows the user to search for or observe underwater objects. The current projects will focus on improving lighting, thrusters, and equipment housing.

The mission of this team will focus on improving propeller shape and selecting other components to create an optimized thruster. The team will use hardware and software that is very similar to that used with the scaleable land-based robot and the lighting system. The team will also use the same housing to enclose the thruster system that is used to enclose the lighting system.

The figure below is a sub function flow diagram depicting how the team plans to engage the thruster project.

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Customer Needs

The establishment and clarification of customer needs is a very important part of the concept development process. The Thruster Team developed these customer needs after meeting with their customers Dr. Hensel, Dresser Rand, and Hydroacoustics. The customer needs were then translated from the voice of the customer to engineering metrics and specifications stating what the team will attempt to achieve in order to satisfy all customer needs.

Customer Needs Assessment:

1. The thruster must have a powerful motor

2. The motor must have a high torque rating

3. The thruster must have a long operation life per battery charge

4. The motor must have a lifetime of at least 168 hours

5. The thruster must fit on the ROV frame

6. The thruster must be easy to mount on the ROV frame

7. The thruster must use standard (off-the-shelf) fittings and connections

8. The seals must be able to withstand at least 400 ft. (180 psi) of water pressure

9. The thruster must be able to travel in the reverse direction

10. The thruster must give off very little vibration when being operated

11. The thruster must be operational in at least 400 ft. (180 psi) of water

12. The thruster must be less than or of the same weight as the Seabotix thruster that is currently being used

13. The thruster must have a volume comparable to the Seabotix thruster that is currently being used

14. The thruster must be operational in temperatures that range from 32º-80º Fahrenheit

15. The thruster must be modular

16. The thruster must be open source

17. The thruster must be open architecture

18. The thruster must comply with federal, state, and local laws

19. The thruster must comply with RIT’s policies and procedures

20. The thruster must be durable

21. The thruster must have a lifetime of at least 168 hours

Specifications

After defining the customer needs, the needs were then translated into detailed statements and specific values that the Thruster Team collectively agreed on. The specifications developed for the thrusters are listed below. All specifications will attempt to be satisfied, however some tradeoffs might occur due to time, space, and money constraints.

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Design Space/Metrics

The key areas of the thruster design that the team has decided to focus on are Power Consumption, Torque, and Cost. The team’s goal is to create a thruster that consumes an amount of power that is less than or equal to that of the currently used Seabotix thruster, that costs less than or equal to that of the thruster and has more torque output (thrust) than the currently used thruster.

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The Seabotix thruster curves that were tested are provided.

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Motor

The currently used thruster on the ROV contains a brushed DC motor. The team’s new design will use a Permanent Magnet Synchronous (Brushless DC) Motor. Brushless motors have an average efficiency of 85% to 95%, do not have a commutator which decreases the friction caused by the brushes rubbing against the rotor assembly, and have identical torque speed characteristics to brushed motors.

Brushless motors also do not spark. This is desirable since the drive electronics for the motor will be in close proximity. Hall Effect sensors will be used to provide information about the position of the motor to the drive electronics, which then activate the phase winding to achieve the desired direction and speed of rotation. The Hall sensors will help the implementation of a closed loop control system.

Four brushless motors that were suitable to our needs were found and compared to the Seabotix motor in the two tables shown:

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After careful comparison the BLWRPG-17 motor was chosen because its rated voltage is at 24 and because it satisfied the greatest number of criteria outlined in our design space above. The Anaheim motor uses less power, outputs more thrust, and cost less than any of the other considered motors.

Anaheim Automation BLWRPG170S-24V-4200 Brushless DC Motor

Anaheim offers this motor with several gearing options over a various range. The team chose a 4.9:1 gear ratio, which will drive the motor at about 850 RPM. Since this company offers a 30-day evaluation period with their products, the motor will be ordered and tested. The team will test each of their proposed propeller concepts and record how they perform. The team will chose a propeller based on which has the best output performance.

Motor Driver

This is a schematic of the Fully Integrated DMOS Driver for Three-Phase Brushless DC Motors that will be used to operate the BLWRPG Motor.

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Implementation of the Motor Driver for testing

Housing Drawings/Schematics

These are the drawing for the coupling membrane, impeller shaft, impeller spacer, and ducting brace (in that order). The shaft, brace, and spacer will be composed of aluminum. The membrane however will be composed of stainless steel.

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Heat Transfer Analysis

This analysis is designed to evaluate the team’s proposed thruster design that was in respect to its thermal dynamics. From this analysis, an understanding will be gained of the heat flow through the design and how much power can be put into the thruster while still keeping the designed components under maximum working parameters. Four test cases will be looked at, each in four different outside flow environments. The test cases will include, the bare thruster with no additional thermal dissipation, adding a heat sink to the motor to increase conduction to the housing, changing the inner fluid of the thruster to increase convection (air to oil), and combining the heat sink and change of fluid. The outer flow conditions will change since the ROV will not always operate in standard conditions. The extreme temperatures ranging from surface conditions (75oF) to bottom conditions (38oF) are important, along with the flow of the water over the housing body, which will range from 0.5 to 3.0 knots.

Assumptions (for all cases)

• The temperature of all components (TM, TMC, THB) is not to exceed 90oC (194oF)

• Neglect effects of radiation; the heat transfer then depends on the modes that are being looked at in the analysis (conduction and convection)

• Heat Generation throughout each component is homogeneous and steady state

• Look at steady state conditions so that the convective coefficients for both fluids (inner and outer) can be considered to be roughly constant

• The partition of the housing that separates the motor from the electrical components is adiabatic (based on the fact that the gradient between the two compartments is minimal when compared to the temperature gradient of the compartments versus the outside)

• Look at the extreme outside temperatures: 38oF (3.3oC) < To < 75oF (23.9oC)

• Aluminum housing [k ~ 136 Btu/hr-ft-F (240 W/m-K)]

• Motor is made mostly of steel [k ~ 26.58 Btu/hr-ft-F (46 W/m-K)]

• Inner fluid of thruster has little to no movement so looking at the worst case convection [Air: h ~ 1.057 Btu/hr-ft2-F (6 W/m2-K), Soybean Oil: h ~ 34.74 Btu/hr-ft2-F (197.25 W/m2-K)]

• Assume Lumped thermal capacitance for the housing as:

o The characteristic length needed to calculate the Biot number:

o [pic]

o At the inner surface: [pic]

o At the outer surface when flow conditions are optimum for heat transfer: [pic]

• Outside fluid is homogenous water that is not fully developed, but the boundary layer is small enough to neglect for thermal analysis and moving at constant speeds of 3 knots [5.06 ft/s] and 0.5 knots [0.844 ft/s], as these speeds correspond to a horizontal thruster at full power (3 knots) and the vertical thruster at full power (0.5 knots). If the flow is looked at as flowing over a flat plate with a width of infinitely small size, [pic], then the following Reynolds numbers are found:

o At 38oF and 3 knots: [pic]

o At 38oF and 0.5 knots: [pic]

o At 75oF and 3 knots: [pic]

o At 75oF and 0.5 knots: [pic]

• The temperature of the outer surface of the housing can be modeled as an isothermal flat plate as the aluminum is highly conductive and will spread heat through itself easily and the assumption of Lumped Thermal Capacitance has already been shown. Using this assumption, an equation to find the Nusselt number for this scenario exists, which will lead to finding the convective coefficients:

o [pic], [pic]

o Prandtl number at 38oF = 10.916

o Prandtl number at 75oF = 5.9429

o [pic] (Values tabulated in Data Worksheet

Test Cases:

Case 1: No Heat Sink and No Dissipative Fluid

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Solution:

Find equivalent resistance for the entire circuit:

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Case 2: Heat Sink on the Motor and No Dissipative Fluid

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Solution:

Find equivalent resistance for the entire circuit:

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Case 3: No Heat Sink and a Dissipative Fluid

Same Figures as Case 1 and same solution only using [pic] instead of [pic] for the motor circuit

Case 4: Heat Sink on the Motor and a Dissipative Fluid

Same Figures as Case 2 and same solution only using [pic] instead of [pic] for the motor circuit

Results/Conclusions:

The heat dissipation of this design is very pivotal and can influence whether the final design is successful. In this early look, the design as is appears to be acceptable in allowing adequate heat loss to the surroundings to keep all components under their maximum operating temperatures. Table 1 gives a brief look at the results for all test cases at the four outside flow conditions:

| |Maximum Heat Transfer |

|  |To = 38F, 5.06 ft/s |To = 38F, 0.844 ft/s |To = 75F, 5.06 ft/s |To = 75F, 0.844 ft/s |

|  |[Btu/hr] |[Btu/hr] |[Btu/hr] |[Btu/hr] |

|Test Case 1 |2555.42 |2014.89 |1970.90 |1566.88 |

|Test Case 2 |2836.74 |2160.77 |2192.08 |1684.28 |

|Test Case 3 |2794.25 |2139.46 |2158.62 |1667.09 |

|Test Case 4 |1019.80 |978.33 |666.14 |636.02 |

In the first two outside flow conditions, the heat flows are at their highest because of the larger temperature gradient. The higher speed shows the highest heat flow because this flow condition allows for a higher heat transfer coefficient of the fluid.

The heat transfer for all cases seems to be more than adequate for the heat load that will be introduced into the housing of the thruster, a mere 58.01 Btu/hr. Curiously, the heat transfer for when both a heat sink and dissipative fluid are used is very low. This could be a result of extra resistance to heat flow added by adding the heat sink in the way of the more efficient convection of the oil. But it seems that best, safest and cheapest solution to adding more heat transfer from the housing is to add a simple disc of aluminum to the end of the motor as a heat sink to help conduct heat to the housing surface.

Sources:

Note: Any table or page reference in this document refers to the book:

• Fundamentals of Heat and Mass Transfer, Sixth Edition . Incropera, Frank P., DeWitt, David P., Bergman, Theodore L., Lavine, Adrienne S. . 2007 . Jon Wiley and Sons Inc.

• “Fluid Properties Calculator” . 1997 . Microelectronics Heat Transfer Laboratory .

• “Basic Mechanical and Thermal Properties of Silicon” . Virginia Semiconductor Inc. .

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Test Case 1

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Test Case 2

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Test Case 3

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Test Case 4

Sealing

To seal the cap on the enclosure:

One end of the thruster enclosure will be separable from the enclosure to allow servicing of the motor inside. The cap of the enclosure will have a step machined around the edge that will line up with the machined flat surface of the enclosure. There will be a rubber o-ring lined around the step of the cap to ensure a watertight seal. This method was chosen because Dan Scoville at Hydroacoustics currently uses it and recommended it as being cost efficient and reliable. The material of the o-ring will likely be nitrile because the material is durable and is considered inexpensive.

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Picture of step around edge for the cap

The seal between the motor shaft and enclosure:

It was decided to use a magnetic coupling to transfer the rotational energy of the motor shaft to the propeller because our competitor Tecnadyne uses the same method for their thruster. This coupling will allow the motor to continue to rotate inside the enclosure when the propeller is caught with debris which will save the motor from failure from heat build-up if it were physically connected as one piece.

It was decided to use co-axial magnetic couplings because the inner hub will be sealed in the enclosure with a stainless steel barrier, and the outer hub is already sealed. Magnetic Technologies manufactures co-axial couplings that will be reliable up to 450psi.

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Picture of co-axial magnetic coupling

The team will use the MTC-0.3 coupling which has a barrier made out of stainless steel and has a synchronous design, which will not allow any slip at any speed unless the propeller is caught.

Condensation

Controlling condensation inside the enclosure:

Some of the moist air inside the enclosure when the thruster is built may condense when the enclosure is submerged to temperatures around 40°F. This condensate may affect the performance of the H-bridge, microcontroller, magnetic coupling, or the motor inside the enclosure. One way of minimizing the condensation is to add calcium sulfate to the enclosure. This powdered substance will absorb moisture as the air condenses. Another substance that the team is considering and leaning towards using is silica gel. This gel will readily absorb moisture, and can be regenerated by heating it up to 300°F. 450 grams of silica gel can be purchased for around $10.

Propeller

Given the scope of the project and the time given to complete the project, it was decided that designing a propeller from the ground up would be impractical. Consultation with the customer and members of the original ROV team revealed that thruster manufacturers often choose propeller geometries empirically. In looking at the propeller designs of the Seabotix and Tecnadyne thrusters, both resembled geometries typically used to move air rather than water. Since the small motors will need to be geared down to produce ample torque, it was decided that an aggressive propeller design was best. Propellers designed to move air are the only type found in the team’s needed size range of 3 to 5 inch diameter. These propellers come in a variety of different geometries and materials and are very inexpensive and easy to modify.

The initial design schedule called for propeller testing during the first half of project. Unfortunately, the test rig we constructed to benchmark the Seabotix thruster did not have enough power to develop propeller efficiency curves over a useful range. It also lacked the ability to measure the rotational speed of the motor shaft, a crucial component of quantifying propeller efficiency. As a result, propeller testing has been postponed until we have a sealed functional motor and enclosure. The motor’s Hall sensor will allow the team to accurately measure revolutions per minute while using the final housing as the test platform. This will ensure that the team’s flow characteristics are applicable.

Two types of fan geometries have been chosen for consideration. The single vertical axis thruster will require relatively equal forward/reverse thrust. For this application, a modified two or three bladed airplane propeller, similar to the Seabotix thruster design, will be used. For the remaining thrusters, since forward biased thrust characteristics are more acceptable, wide blade, high pitch designs with significant curvature will be used. Since quiet air moving fans have hydrodynamic characteristics, “low noise” fans will be primarily tested. Fan diameters from 80mm to 120 mm will be tested in combination with multiple gear ratios and nozzle configurations.

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Research into ducted propellers has revealed that the use of a nozzle will potentially increase thrust by 40% with the right configurations. The team’s application will require an accelerating nozzle machined from either PVC or billet nylon. Naval research texts recommend that when using accelerating nozzles, wide blades with little clearance to the inner diameter of the nozzle reduces cavitations at the tip (loss of power).

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Control

The motor control is required in order to operate and optimize the performance of the thruster. The control unit needs to be able to do the following:

• Fit in the thruster housing unit

• Have enough PWM channels to operate a BLWRPG motor

• Use power efficiently

• Operate the motor in both the forward and reverse direction

The control unit will be a slave to the current control unit on the Hydroacoustic ROV (ATmega128). The control unit will communicate with the ATmega128 through a RS-485 cable through which it will listen for messages that pertain to it. All other messages will be disregarded. In order to operate the motor in both the forward and reverse mode, the control unit will control an H-Bridge chip. As an improvement from the previously designed project, the motor of the thruster will give feedback to the control unit. The BLWRPG motor contains a Hall Effect sensor, which will send a signal to the control unit containing the speed of the motor. This feedback can be used to increase or decrease the voltage to a motor if the motor is not running at the same speed as the other motors.

Since a microcontroller will be used, the source code will not need to be loaded before each use of a thruster and the microcontroller’s design can be changed easily allowing the thruster design to be modular and scalable for future design team use.

Since the ATmega168 is designed for low power consumption it meets the team’s power consumption design criteria. The ATmega168 draws 250 μA at 1 MHz when it is in active mode and has a built-in Hall Effect encoder which makes it suitable for controlling the BLWRPG motor.

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Figure 1: Motor Thermal Circuit for Case 1

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Figure 2: Microcontroller and H-Bridge Thermal Circuit for Case 1

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Since contact resistances are negligible, TM~THS

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Figure 3: Motor Thermal Circuit for Case 2

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Figure 4: Microcontroller and H-Bridge Thermal Circuit for Case 2

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Symbol Legend

RHS,conv = Convective Heat Sink Resistance ([pic])

RMC,conv = Convective Microcontroller Resistance ([pic])

RMC,cond = Conductive Microcontroller Resistance ([pic])

RHB,conv = Convective H-Bridge Resistance ([pic])

RHB,cond = Conductive H-Bridge Resistance ([pic])

RIF,conv = Inner Fluid Convective Resistance ([pic])

RH,cond = Conductive Housing Resistance ([pic])

ROF,conv = Outer Fluid Convective Resistance ([pic])

TM = Motor Temperature (194oF)

TMC = Microcontroller Temperature (194oF)

THB = H-Bridge Temperature (194oF)

Ti = Inner Fluid Temperature

TH,i = Inner Surface Temperature of the Housing

TH,o = Outer Surface Temperature of the Housing

To = Outer Fluid Temperature (38oF and 75oF)

RM,conv = Convective Motor Resistance (±-@É[pic])

RM,cond = Conductive Motor Resistance ([pic])

RM,cond,HS = Conductive Motor/ Heat Sink Resistance ([pic])

Table 1: Results for the maximum allowed heat dissipation for each test case

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