High Resolution AMR Compass

Honeywell

Advisor Dr. Andy PeczalskiAdvisor Professor Beth StadlerPat AlbersmanJeff AymondDan BeckvallMarcus EllsonPatrick Hermans

Abstract

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This project’s purpose is to improve the accuracy of a digital compass by using multiple compass IC’s.

These will work together to collectively improve the accuracy of the overall

system.

Project Motivation

• Magnetic ICs in High Demand• Navigation• HDD• Proximity sensing• Position sensing

• Increasing Accuracy is Required• Decreasing Size is also Beneficial

HoneywellImages from http://phermans.com/w/images/e/e2/HMC105X.pdf

Current Technology

• Anisotropic Magnetoresistance

• Wheatstone bridge

HoneywellImages from http://phermans.com/w/images/9/9f/Appl_note_for_position_sensing.pdf

Current Technology

• Analog– 1, 2 or 3 axes sensing– Direct access to bridge– Navigational accuracy depends on ability to read voltages

• Digital– 2 or 3 axes – Internal heading calculation– Accurate to 1 degree

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Future Technology

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•What is the next step?

•Nanowires•AMR sensing abilities•Decreased size•Decreased sensitivity

Images from Prof. Beth Stadler

Project Description

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•Feasibility study for the use of nanowires•Not actually working with nanowires •Trying to increase accuracy by using multiple bridges as would be required with nanowires•Providing Honeywell with a new use for nanowires

Project Description

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One benchmark is to try to increase the accuracy of the system by the

number of sensors used.

Increased precision and repeatability is also desired.

Project Description

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Customized hardware is necessary to implement the multiple sensor system.

Customized software will be required to manage the implementation.

Chosen IC: HMC 6352

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•Digital 2-axis compass•On board ADC•Modifiable sensing range•Speaks I2C•Small package•Improvable accuracy•Barber pole bridges

Image from http://phermans.com/w/images/9/9d/HMC6352.pdf

Software & Algorithms• Modeling & Simulations

• Matlab

• Firmware•MPLab & CCS Compiler

•User Interface•Visual Basic (VB)

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Sensor Modeling

• Goal: Parameters-> M-file -> Sensor Data

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• Consists of Many Sub-functions• Noise, Bridge, OpAmp, A2D

• Needs to model real world situations

MATLAB

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• Successfully used to simulate single and multiple sensors before our hardware could be designed

• Provided a vehicle to test the performance of our heading calculation algorithms

• Totaled 1702 lines of MATLAB code

Sensor Placement

• The placement of the sensors must create a system accurate across 360 degrees

• Each individual bridge of each sensor can be simulated independently in MATLAB

• Multiple arrangements can be simulated to determine the best implementation

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Orientation Simulations

• Single IC Senor Output Wave Form:

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• Data Appears Evenly Spaced• ICs at: 0, 36, 72, 108, 144, 180, 216, 252, 288, 324 Degrees

0 50 100 150 200 250 300 350 400-600

-400

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0

200

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600ICs Binary Outputs

B Field Angle

ICs B

inary

Outp

uts

0 50 100 150 200 250 300 350 400-600

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0

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600ICs Binary Outputs

B Field Angle

ICs B

inary

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Orientation Simulations

• Single IC Senor Output Wave Form:

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• Data Evenly Spaced• ICs at: 0, 9, 18, 27, 36, 45, 54, 63, 72, 81 Degrees

0 50 100 150 200 250 300 350 400-600

-400

-200

0

200

400

600ICs Binary Outputs

B Field Angle

ICs B

inary

Outp

uts

0 50 100 150 200 250 300 350 400-600

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0

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600ICs Binary Outputs

B Field Angle

ICs B

inary

Outp

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MicroController C Code

• Written in MPLab – Version 8.0

• CCS complier– Version 4

• Run on PIC 18f4550• 1326 Lines of C

– 2532 Lines of Assembly

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Sensor Communication

• Sensor Commands– Heading

• Adjusted voltages• Raw voltages

– Calibrate– Re-address– Number of Summed measurements

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Serial Communication

• Allows Compass to display results • Very helpful in debugging• Allows for VB to control sensor• Easy to implement in CCS• 115200 Baud allowable from the 20Mhz

crystal

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Weighted Averaging

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Visual Basic (VB) Interface

• Provides an end-user interface• Synchronizes the compass and the rotation

table used to accurately measure moves• Allows for automated data acquisition• Provides a repeatable test benching system• Requires a third board to handle adjusted

ground on PMC• Total of 4733 Lines of Code

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Visual Basic (VB) Interface

Commands to perform repeatable data acquisition and benchmark tests.

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Personal Computer(VB)

PIC18F4520(C)

PMC Controller

Rot. Table

Sensors

Serial Serial

I2C

Parallel

Hardware: Abstract

• One compass, two boards– Main Board

• Microcontroller

– Daughter Board• Sensors

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Hardware: Main Board

• Essentially a controller board– Microcontroller– RS-232 Communication– I2C Communication– Interfacing

• Daughter Board• Front Panel

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Initial Design: Daughter Board

• Three functional systems– Sensor array– Power MUX– Laser

• Constraint: One of the dimensions must be less than 3.5” – Opening of zero-gauss

chamber is 3.5” in diameter

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3.492”

3.132”

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Daughter BoardI2C Bus

Data

Clock

• Design challenge:– Need to assign unique address to each sensor– Each sensor is factory installed with address 0x42– In order to change addresses, a command must be

sent to a sensor on the bus– This command message contains:

– How to change address of individual sensor if every sensor is receiving the command?

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Daughter BoardPower MUX

Start Address [Ack] Command [Ack] Stop

• Solution: Need to isolate communication to individual sensor

• How?– Burn-in Socket

– Use a network of jumpers– Multiplex I2C to each sensor– Multiplex power to each sensor

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Daughter BoardPower MUX

Photo taken from http://www.locknest.com/newsite/products/qfn/index.htm

• We chose to multiplex power– Advantages

• Saves power• Simplifies troubleshooting

– Disadvantages• Signal loss through MUX• Other unknowns…

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Daughter BoardPower MUX

Problems with Initial Design• Problems

– Main Board• None

– Daughter Board• I2C bus

– When powered off, the sensors interfere with I2C bus– 5V data signal is pulled down to 2.5V– Therefore communication will not work

– Problems not related to design• Sensor 3 will not communicate• Will not hinder project; algorithm will still work• Slight loss of sensitivity at sensor 3’s axes of sensitivity (27°

and 117 °)

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Changes to Initial Design

• I2C bus fix– Remove MUX and feed power to all sensors

– Cut I2C traces– Add jumpers to I2C vias and address them one by one– Connect all jumpers to I2C bus

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Changes to Initial Design

• Other changes– No laser mount

• Laser mounted directly to plexi-glass case• Saves cost ($25)

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Proposed Final Design

• Due to I2C bus issues, our current design does not work

• Two options1. Power all sensors and use burn-in or jumpers

socket to isolate sensors2. Multiplex I2C bus3. Add Physical Jumpers to the I2C bus to individual

connect one sensor at a time

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Testing

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Prototype Final

Test Setup

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Accuracy

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Precision

RepeatabilityCompare

Compare

ß field

Compare

Prototype Testing

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•Given one sensor

•CCS compiler

Final Testing

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Elements of Final testing

•Pretesting to determine zero gauss values

•Pretesting to determine IC positional offsets

•Testing to obtain compass specs•Accuracy, Precision, Repeatability

Pre-testing (zero gauss)

1. Place sensors in the zero gauss chamber2. Rotate 360 deg. while taking readings3. Analyze data and get zero gauss values

This determines what value we should see when the IC is experiencing zero gauss, aka: parallel to the field direction.

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Pre-testing (offsets)

1. Place sensors in artificial magnetic field2. Run VB script that finds sensor locations

• Uses the zero gauss value of each chip• Works using relativity, sensor 1 = 0, sensor2 = ? From 1• Bang bang control

3. Analyze data and find chip placements 4. Hardcode this to software

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Accuracy

Test Procedure1. Determine the B field

• Find the zero crossing on each axis• B field should be 90 degrees from zero crossing• Average the 20 axes results

2. Take measurement 3. Compare result to actual4. Rotate to different position5. Repeat steps 2-5

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23 deg

113 deg

Results

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Results Comprise of:

•Determining Specs

•Comparison of Specs to Controls

•Ways to improve

•Future for Nanowires?

Results: Control Comparisons

• First Control is the Sensor Heading output– We Don’t know how they compute this

• Second Control is performing arctan(x/y) on a single designated sensor

• These will be compared with our computation of arctan(x/y) of multiple sensors averaged

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Results: Specs - Repeatability

• Comprised of 5 readings taken at 0, 90, 180,270

• Our Product: Min = +- 0.015 Max = +-0.089

• Control: Min = +- 0.033 Max = +-0.051

• Honeywell: Min = +- 0.030 Max = +- 0.120

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Results: Specs - Precision

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Results: Specs - Accuracy

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How Can We Improve

• Currently using arcTan(x/y) to compute heading– This assumes we have X and Y which need to be

90 degrees apart– In practice this is not true, we found this is

actually only within +-8 degrees

• Use different algorithms, better weighting• More Sensors

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Future For Nanowires?

• Nanowires are inherently less accurate• Means greater room for improvement• Small enough to use more than 10 bridges• Weighting should have more of an effect• Will have completely different obstacles • All in all, from the results of this feasibility test

they look very promising

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Conclusion

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•Questions/ Comments?

•Thanks for your Attention and Time!