University of Minnesota
Magnetic Compass Design
Increasing Accuracy with Multiple Sensors
5-13-08
Prof. Beth Stadler
Dr. Andrzej Peczalski
Beckvall, Daniel P
Ellson, Marcus P
Hermans, Patrick M
Aymond, Jeffrey A
Albersman, Patrick B
Table of Contents
Abstract ......................................................................................................................................................... 4
Motivation & Background ............................................................................................................................. 4
Prior Work & Solutions ................................................................................................................................. 4
Requirements & Specifications ..................................................................................................................... 6
Concept Design & Description ...................................................................................................................... 7
Sensor Selection ........................................................................................................................................ 7
Hardware Overview .................................................................................................................................. 8
Initial Design.............................................................................................................................................. 8
Final Design ............................................................................................................................................... 9
Main Board................................................................................................................................................ 9
Daughter Board ....................................................................................................................................... 10
Software Overview.................................................................................................................................. 11
Compass Firmware Concept Design ................................................................................................... 11
Compass Firmware Design Description .............................................................................................. 12
Control Computer Concept Design ..................................................................................................... 15
Control Computer Design Description ................................................................................................ 16
Design Evaluation........................................................................................................................................ 17
Prototype ................................................................................................................................................ 17
Test Methods .......................................................................................................................................... 17
Accuracy .............................................................................................................................................. 17
Precision .............................................................................................................................................. 18
Repeatability ....................................................................................................................................... 18
Results ..................................................................................................................................................... 19
Conclusion & Recommendations ................................................................................................................ 20
Project Review ........................................................................................................................................ 20
Future Work ............................................................................................................................................ 22
Reflection ................................................................................................................................................ 22
Research References ................................................................................................................................... 24
Related documents ................................................................................................................................. 24
Appendices.................................................................................................................................................. 24
Data sheets ............................................................................................................................................. 24
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Abstract
The purpose of the team’s project is to increase the accuracy of a single AMR IC compass by
incorporating multiple ICs. The goal is to increase the accuracy of a magnetic sensor by the number of
multiple ICs used. This is done by obtaining data from within the most accurate regions of operation for
each sensor and applying a weighted averaging algorithm. Sensors and test equipment were provided
by Honeywell and the University of Minnesota’s Electrical Engineering Department.
Motivation & Background
Magnetic sensors are used in navigations systems, magnetic hard drives, proximity sensors, position
sensors just to name a few areas of applications. With this said, it is becoming more and more
important to minimize the size of such sensors while maintaining or increasing accuracy and resolution.
A proposed method of decreasing sensor size is to use nanowire technology, which has the potential to
create much smaller IC’s than in use today. However, these individual nanowires are not very accurate.
The question then becomes whether or not combining multiple nanowire bridges together would boost
accuracy to match or exceed current IC sensors while still being much smaller in overall size. With this
being the motivation, the group’s task is to determine whether multiple sensors can be combined to
give a significant increase in accuracy and resolution.
Due to the fact that nanowire technology is still in its early stages and requires highly sophisticated
equipment and procedures, the project can be simplified to a proof of concept using alternate sensors.
With personal navigation becoming more and more in demand amongst today’s consumers it has
sparked the interest of many magnetic IC sensor companies to create chips for use in various types of
compasses. For example, Honeywell manufactures several ICs with the sensitivity required to measure
the earth’s magnetic field and provide electronic developers with the ability to sense direction
accurately. These magnetic and compass ICs are very small and fairy low cost which will work well for
the group’s proof of concept.
The goal of the team project is to combine many independent magnetic or compass ICs together to
significantly increase the resulting accuracy and resolution. Since this is a proof of concept size, cost,
power consumption, and the like are not important in the final product. It is assumed that the outcome
of combining multiple ICs to increase accuracy will be the same for combining multiple nanowire bridges
in the future. If all goes well, this will have produced reasonable evidence and algorithms that combing
multiple nanowire bridges with low accuracy can be combined to create a highly accurate and small
magnetic sensor solution.
Prior Work & Solutions
Since the project was proposed to us by Dr. Peczalski, there was no previous work for this specific
project. To the best of the group’s knowledge and research, no one has done any research on using
nanowires for the construction of wheatstone bridge. Furthermore, to the best of the group’s
knowledge no other devices current use multiple IC chips in various orientations to increase
performance. However, within some IC sensors multiple magnetoresistive wheatstone bridges are used
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to broaden overall range and in some case increase accuracy. The extent of this while remaining
coplanar has been limited to two bridges and therefore has not yet been fully explored as this project
does.
It is important, however, to have an understanding of how other digital compasses and similar
navigation devices have been designed. Due to the fact that anisotropic magnetoresistive (AMR)
sensors will be used for this test of concept, the scope of this description will therefore be limited to
AMR sensors and the IC’s which use them.
AMR sensors measure the orientation of a magnetic field by means of a differential voltage that is
produced by a wheatstone bridge configuration of four magnetoresistive elements. The wheatstone
bridge is shown below in Figure 1.0, where the pink strips are the magnetoresistive elements, and the
voltage measured across the center is the differential voltage produced.
Figure 1.0: AMR Wheatstone Bridge
This configuration works to create a differential voltage that varies with respect to the direction of the
applied magnetic field due to that fact that the resistive properties of the individual magnetoresistive
elements change resulting in different voltage drops around the bridge. The output differential voltage
as a function of the applied magnetic field direction with respect to the bridge orientation is shown
below in Figure 2.0.
Figure 2.0: AMR Wheatstone Bridge Voltage Output
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From Figure 2.0 above there are a few points of interest. First, it is important to note that there is a
window from -45 degrees to +45 degrees before all output values are repeated. This means there is
only a 90 degree window for which a magnetic field direction can be determined using only one bridge.
It is also important to note that this function is not linear. This means that at some positions a small
change in magnetic field direction will cause a large voltage change than at others. Obviously, where
the voltage change is greatest for the smallest direction change the more accurate the sensor will be.
This happens during the linear region shown from roughly -35 degrees to +35 degrees. These items of
interest need to be taken into account when formulating a working compass and compensated for.
Other designs have compensated for the limited 90 degree range by introducing multiple bridges and
Hall Effect sensors to specify which quadrant or hemisphere the measured magnetic field is within. As a
bonus, this also easily increases accuracy by creating more linear regions which can be staggered to
create effectively one continual linear range. Figure 3.0 below shows the plot of a device using two
bridges positioned 90 degrees apart with an addition of a Hall Effect sensor.
Figure 3.0: Dual Bridge with Hall Effect Sensor
Using Figure 3.0 above it can be seen that whenever sensor A or B is outside its linear region the other
sensor has entered it linear region, always supplying a very accurate voltage reading. Furthermore,
using the different signs from each of the bridge output and the Hall Effect sensor the resulting direction
can be placed within the correct quadrant extending the range to a full 360 degrees.
To further increase the complexity of using these devices is the fact that the output voltages produced
are fairly low, less than 1 volt. Therefore, amplification is generally required to allow for sufficient
accuracy. This often posses an additional challenge because the amplifiers need to be tuned with great
precision. Fortunately, this has been perfected and there are many devices available that will perform
the amplification needed and many that also provide a digital output that can easily be interfaced to a
microcontroller for processing.
Requirements & Specifications
The requirements for the final product are straight forward and simple. It should consist of two
compasses or the equivalent, from which one will be composed of a single AMR sensor and the other of
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multiple identical AMR sensors. The performance specifications for each compass should be
determined accurately and identically for each device to be used for comparison purposes. Specifically,
each compass should be tested for repeatability, precision, and accuracy with the goal to increase each
by X-fold, where X represents the number of chips used in the multiple IC compass design.
Because this project is a proof of concept there are no strict restrictions on power consumption, overall
size, or budget. The only requirement is that the direction should be computed and displayed within a
reasonable time frame, such as within approximately 1 second.
When the product is completed and testing is done to determine the specs for each compass a final
conclusion should be made as to whether multiple sensors can significantly increase accuracy over an
individual sensor. Furthermore, if the proof of concepts passes some thought should be done to
determine if this concept will/should hold true for nanowire sensors in the future.
Concept Design & Description
Sensor Selection
The bulk of this design revolves around the AMR IC sensor selected. Andy has given us the choice to use
any of three sensors made by Honeywell; the HMC1042I, the HMC105x series or the HMC6352. After
many in-depth data sheet reviews and debates were placed over the conclusion was made to go with
the HMC6352.
One of the main reasons the HMC6352 was selected by the team was its I2C capabilities. I2C is a
communication protocol that allows you to communicate to multiple devices using their addresses as a
way to distinguish one from another. In the case of the HMC6352, each chip can calculate its own
heading, voltage readings, or (X Y) coordinate. This will allow us to greatly simplify the circuit. Without
these capabilities the team would be forced to have many analog to digital conversions along with
several operational amplifiers. The processing capabilities far outweigh its only foreseeable down side,
its field range of measurement. It field range can measure magnetic fields anywhere from 0.10 to 0.75
gauss. This is fine for applications such as navigation systems because the earth’s magnetic field around
the upper Midwest is around 0.6 gauss. The only foreseeable problem is getting a constant magnetic
field to test in. If the team were using another sensor that can measure fields at ± 6 gauss the team
could cancel out any of the earth’s magnetic field with a stronger artificial magnetic field created by
winding coils.
The last reason that the HMC6352 was chosen was that its accuracy is slightly less than the other
available sensors. The HMC6352’s specs say that it is accurate to within 2.5 degrees. This is less
accurate then the other sensors that are accurate within 1 degree. This is good because it will allow the
team to see more definitive results. Although the team has a highly accurate rotational table that will
allow for us to measure precision, other resources like a highly accurate directional gauss meter might
be needed to see significant improvement in accuracy. With less accuracy the team will be able to
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answer the general question about using multiple sensors more definitively. With everything taken into
account the team settled on the HMC6352 as the sensor that would be used in the final circuit.
Hardware Overview
The basic concept of the hardware design is to use two separate boards, one containing a
microcontroller as well as power and the proper communication circuitry needed to interface with a PC,
and the other containing the array of sensors and a laser for demonstration purposes. This idea of using
two separate boards serves multiple purposes. First, having the components on separate boards makes
each board smaller and less bulky. This will be important when it comes time to mount the product to
the rotation table for testing. Having the microcontroller on one board and the AMR IC’s on the other
will enable the daughter board to be remained mounted while the main board can be removed and
have its flash updated or modified. Separate boards also serve as a safety net; if one boards fails, or has
been constructed incorrectly, the entire project is not lost. As an added bonus if the desired orientation
or layout of the AMR IC’s is to be changed a new daughter board can simply be constructed. The design
will also allow for variations in number of IC’s as well as orientations without the reconstruction of the
main board. Using two boards should also make the group’s testing and configuration processes easier.
Having the sensors on a separate smaller board will allow us to more easily use reflow soldering on the
sensor array. This will help the sensors align more accurately to their PCB pads. Once the sensors are
mounted to their board, the daughter board as a whole can be treated like an individual sensor which
will allow for easier breadboard level testing and calibration.
Initial Design
Our initially proposed design is shown below as a block diagram layout in Figure 4.0. As can be seen, it
consists of two separate boards connected by an 8 wire cable. The main board on the right contains the
microcontroller that will be used to control the various devices and compute a direction to be displayed.
Also on this board are all the peripherals the team feels are required to accurately gather data and
troubleshoot any problems that may arise.
The daughter board contains the array of AMR IC chips used to gather data about the direction of the
applied magnetic field. It also contains other minimal hardware required to operate the IC’s and a laser
to precisely depict and set the orientation of the daughter board. A multiplexer is needed to provide
power to each sensor individually. This MUX creates several advantages. First, this configuration saves
power. Since only one sensor is needed at a time, it is wasteful to have any other sensors powered on.
Second, this achieves isolation of the communication bus. This will allow us to change the address of
any sensor if needed, and simplifies troubleshooting, since we do not have to worry about any problems
arising from the other sensors, since they are not powered. Finally, a voltage regulator and 9V battery
are needed to provide power for the board.
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Figure 4.0: Proposed Block Diagram Layout
Final Design
Several changes were made to the initial design of the daughter board. First of all, the 9V battery and
voltage regulator have been removed, as we have decided to feed power and ground through the
parallel connector from the main board. Second, we have removed the power MUX and applied power
to all the sensors at the same time. This was done because we found that we cannot leave the
unpowered sensors connected to the I2C communication bus, because they interfere with
communications.
In the future, we would have to face the problem of how to isolate communication between one sensor
and the main board on the I2C bus, because this is the only way to assign a unique address to each
sensor. This can be done in two ways. One way would be to use a burn-in socket to address the sensors
one by one prior to assembly onto the daughter board. Another way would be to add a jumper next to
each sensor to open/short the I2C connection between the sensor and the bus.
Main Board
The main board shown in Figure 5.0 contains the all the equipment to control and collect data from the
AMR IC’s located on the daughter board. Great lengths have been taken to ensure that this board can
handle everything required while at the same time be very versatile and adaptable in case something
has been overlooked or requires a change.
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Figure 5.0: Main Board Layout
The microcontroller is the key component to this board and the entire project. It will be used to
communicate over I2C to the AMR ICs as well as to a computer running hyper terminal or LabVIEW via a
serial connection. At the same time it will also be required to display pertinent data to the onboard LCD
and status LEDs. To increase the usability of the board a built in programming port has been added
along with an area to probe various control lines and pins for troubleshooting purposes.
Other hardware that can be found on the main board includes an optional EEPROM to be used if extra
memory is needed, 3 status/debugging LEDs, 3 diagnostic pushbuttons, and an RS-232 converter chip.
The parallel connector will require a minimum of 5 signals to be sent to the Daughter Board. They are as
follows: 1 for laser control, 2 for I2C, 1 for power, and finally 1 for ground.
Daughter Board
The daughter board, shown in Figure 6.0, contains the AMR ICs that will be used to determine the
magnetic field direction. An onboard laser can be used to pinpoint the board direction from which it can
be calibrated.
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Figure 6.0: Daughter Board Layout
The layout of the board is pretty easy to follow as the design is very basic. Surrounding each sensor are
the following: one power LED with current limiting resistor, one decoupling capacitor, and two optional
R-C feedback networks (one for each axis) that can be used to tune the sensitivity of the sensor. These
R-C networks are not populated on our final design, because they are not needed. If we wanted to
experiment, however, they could be added in the future. It is important to note that there are two
potentiometers in the top right corner that are in series with the two resistors on the R-C feedback
network of sensor 11, which is located on the top right of the board. Since this is our extra or “control”
sensor, we can investigate the use of sensitivity by changing the resistance provided by the
potentiometers.
Software Overview
Compass Firmware Concept Design
Based on our collective professional and academic experience we decided to wire our compass firmware
in C. All of us have had at least one class use C as a native programming language and many of us have
actually worked with it in the field. Once the platform was decided we needed to determine how we
were going to translate our high level C into a .hex programming file for the microcontroller to use. We
had access to different compilers that would translate our C for Microchip’s microcontrollers. We
decided that we would use the default C18 compiler unless we ran into trouble in the development
stage in which case we could try our CCS compiler.
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We developed different methods to process the sensor headings using an iterative process that
dissociates X and Y bridges from one another and then iterates them against one another using
arctangent amongst other trig functions. The function is shown below with the 2 bridge voltages
represented by X1 and X2 and their respective displacement from the azimuth, or their offset,
represented by θ1 and θ2.
We also came up with a method for calculating headings that simply used the associated X and Y bridge
voltages and a weighted sum that gave sensors which were closest to their linear region the highest
weight. Below is a graph of the sensor weights
Sensor Weight
X Voltage
-180
-135
-90
Y Voltage
-45
0
Sensor Weight
45
90
135
180
Compass Firmware Design Description
Due to problems we were having with I2C communication we elected to use the CCS compiler. After
several stages of data gathering with our dissociated bridge algorithm we decided to implement the
weighted arctangent. This decision was reached based on our time frame and the quality of the data we
were gathering from the actual sensors.
For an operating system structure we decided to do a do a non-real-time system that waits for a request
before doing any data gathering or heading calculations. A description is below:
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Device
is
turned
on
Initialize
device
Hyper-terminal or
VB interface?
VB
Indicate to VB
that Compass is
ready to receive
commands
Begin Running
Loop
Hyper-terminal
Print Menu for
User
Receive
“Get Heading”
Command
Communicate with
sensors to get their
seen voltages
Apply our tuning
offsets and
normalize voltages
Calculate headings
Using ArcTan
Apply Weighted
average priority
based on voltages
and determine
weighted average
VB
Send heading to VB
to be displayed
Hyper-terminal
End Running Loop
Display Heading to
Hyper-terminal
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The most critical portions of our code were the data acquisition from the sensors and the bridge voltage
processing. The sensor data acquisition portion is below.
This code segment first gets
the correct sensor address
from the sensor address array
to use as the address to send
to for the rest of the function.
It then modifies a variable in
the sensor RAM to indicate to
the sensor that it should return
headings. We then tell the
sensor that we want to take a
reading using the hex 0x41
command. We then wait for
7ms as indicated in the data
sheet to allow for the sensor to
get a heading. After the delay
the read command is sent to the sensor and we then send clocks over I2C recording two consecutive
bytes into two variables. These storage variables are then concatenated together and converted to
floating point values for eventual heading processing. This same process is done when collecting the
bridge voltage values, the only difference is the command sent to the sensor RAM; 0x03 for X voltage
and 0x04 for Y voltage.
The other important portion of our code is the actual heading calculations.
Our calculation
code first runs
through all the
sensors applying
arctangent to
their voltages
and then
converts from
radians back to
degrees.
Because
arctangent gives
a value from -90
to 90 we then
need to adjust
certain heading
readings based
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on the sign of their respective X and Y voltages. After the heading is correctly adjusted we remove the
rotational offset from each sensors heading so that it is referencing the same azimuth. We then check to
see if the individual sensors heading is the max or the min for error checking purposes.
The sensors weights are then computed using the function we derived during conceptual design. The
weight is determined first as a value between zero and one, then scaled and offset on the next line
allowing us to tune the values during testing. We ultimately ended up using sensor weights from 1 to
50. Each sensors weight is then added to a sum for eventual averaging.
After our loop has run through all sensors we
check to see if the difference between the
maximum and minimum heading is greater
than 340. If this is the case then we know that
sensor array is crossing the 0-360° border and
in order to average properly we need to add
360 to the headings around 0.
We then reset the master heading and
calculate a new one by summing up products
of each sensors weight with its heading. After
summing we then divide off the total of all the
weights in accordance with standard averaging
mathematics.
We then check to see if each heading is bogus
(greater than 360 or less than 0) for both the
individual sensor headings and the overall
processed heading.
As is true with all software development our compass firmware went through many revisions.
Control Computer Concept Design
We determined that there were multiple different development environments that could use to
interface our compass and rotational table to a host PC. The first platform we considered was LabVIEW
due to its flexibility, fancy graphics and possibly pre-written code. We also realized that Visual Basic (VB)
could be easily set up to communicate over serial and parse off incoming ASCII. Other than controlling
the rotational table, the biggest job of our host PC software was to gather data for display we also
considered recording the compass’ output while controlling the rotational table to allow for automated
data acquisition.
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Control Computer Design Description
Again due to our collective experience, we decided to use VB for our controlling computer. VB allowed
us to easily communicate to the compass and the rotational table allowing for implementation of
automated testing routines and general data acquisition.
Above is a screenshot of our controller for the rotational table. Our team was able to get a hold of a
manual that allowed us to control many aspects of the table that can’t be controlled using the standard
on-board button interface.
To the right is a screenshot of our compass
control interface. Most of the data is passed
directly into the background code using
ASCII so there are not as many controls on
this screen when compared to the
rotational table interface.
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Design Evaluation
Prototype
As described previously our protoype consists of a separate main board and daughter board
components. As shown in the photo below the daughter board is mounted on top of the high accuracy
rotational table by means of a PVC pedestal. This places the daughter board containing the magnetic
sensors directly in the middle of the magnetic field generated by the two large Helmoltz Coils, ideal for
simulating a consistent magnetic field. It is important to note that the daughter board is also firmly
attached to the rotational table which will be used to accuratly rotate the device to known positions for
data aquistion and testing.
Test Methods
In order to meet the remainder of the requirements stated above, the following testing procedures have
been determined and implemented into the VB software to compute the specs for both the individual IC
and multiple IC compass readings. The following test plan describes what has been implemented and
the exact code can be found in the Appendix.
Accuracy
Definition: Accuracy is most easily understood and is usually reported as maximum possible difference
from the true value. In the team’s case with a compass it can be expressed as how near to the true
reading it is, for example ±5 degrees.
Method: First determine true north and set the PMC table to 0 when pointing in this direction. This is
done by rotating each bridge until its output is zero, and recording the position for each of the 20
bridges. Use each known bridge placement offset to determine north with respect to sensor 1 bridge x.
Once the VB program has found this position it moves there and sets the PMC heading to 0. Now the
PMC represents the absolute correct position and can be compared to computed headings from the
compass. For our purposes we then rotated the compass a full 360 degrees stopping every 1 degree to
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take compass readings. The data was then analyzed to determine the minimum, maximum, average,
and standard deviation of the readings from the PMC.
Precision
Definition: Precision can be defined as the smallest unit increment a product can measure. For
example, if the team’s compass is rotate a known 10.0 degrees and the compass readout changes by
10.1 degrees it could be said to have a precision of 0.1 degrees. This is different from accuracy in the
sense that it may have a very precise reading, let’s say within tenths of a degree, but may have a large
offset representing bad accuracy.
Method:
1. VB will request the compass headings and the PMC position and write them to a text file.
2. The table is then rotated 10 degrees and VB will again request the headings and the PMC position
and save them to the text file.
3. Repeat steps 1 and 2 for an entire 360 degrees.
4. The data file can then be imported into Excel and analyzed. The amount moved is the difference in
PMC positions and should be identical to the change in headings. Any discrepancy here is classified as
‘precision’, here again the minimum, maximum, average, and standard deviation will be computed.
Repeatability
Definition: Repeatability may seem as if it is a function of accuracy and precision but it is not. The best
way to describe this is with an example. Assume the compass at hand has an accuracy of ±5 degrees
and a precision of 0.1 degrees. Now assume that when positioned at 0 degrees the compass reads 4.1
degrees, very inaccurate but with moderate precision. Now rotate the compass around and then place it
again facing 0 degrees. If the compass were to now read 4.2 degrees it would have good repeatability of
0.1 degrees, if it now read 2.1 degrees it would have bad repeatability, of 2 degrees.
Method:
1. The rotational table is stopped and held in one place and VB request compass headings and saves
them to a text file.
2. VB pauses for 1 second and then requests compass headings which are appended to the text file.
3. Repeat step 2 ten times.
4. Move the rotational table 10 degrees and repeat steps 2 and 3.
5. Repeat step 4 until a full 360 degree range has been covered.
6. The data file can then be imported into Xcel and analyzed. The amount of variation between
readings at any given location is then considered the range of repeatability. Example: 4 readings taken
are 10.0, 10.2, 10.2, 9.9, the max variation is 10.2-9.9=0.3, and therefore the repeatability here is +-0.15
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degrees. The repeatability is computed at each stop and then minimum, maximum, average, and
standard deviations are computed for the full 360 degrees.
Results
It was determined that multiple sensors working together can indeed increase the precision, accuracy,
and repeatability of the system. The table below shows the overall improvements in each of these
areas. This shows that this type of implementation is very feasible and could potentially lead to more
accurate digital compasses in the near future.
Results
Single Sensor Sensor Array Improvement
Accuracy
2.169
1.135
47.65%
Precision
0.162
0.078
51.69%
Repeatability
0.174
0.096
44.70%
The graphs below depict the accuracy, precision, and repeatability over a full 360 degree range to get a
further understanding of where the above numbers come from.
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From the graphs it is obvious that our results have some sort of sinusoidal error to them. This may be
due to manufacturing inaccuracies. The x and y bridges are assumed to be 90 degrees apart making
them sine and cosine waves with respect to each other. In practice we found that they are actually 93
to 98 degrees apart and therefore are not actually outputting sine and cosine data points. Plugging this
into arctangent as our heading computation does produce the oscillation around the true value as we
see in our accuracy plot. Now that this is known, a better algorithm can be computed to account for the
true bridge positions.
Unfortunately, we were unable to increase accuracy by tenfold as desired. It was however, proven that
multiple sensors can improve accuracy substantially. Throughout the process problems and data plots
have given rise to ideas for better algorithms and a better understanding of what is really happening.
This provides room for future improvement in algorithms and methods used. It is also important to note
that it should be much easier to increase accuracy with a device much less accurate than trying to
improve an already high accuracy device as we have done. All in all, the future for nanowire
implementation appears to be promising if the results of this feasibility test have any validity.
Conclusion & Recommendations
The team did not meet the tenfold accuracy improvement goal that was set at the beginning of the
term. The team was able to achieve about 2 fold improvement on all of the areas of interest outlined in
the test bench; accuracy, precision and repeatability. This inability to achieve tenfold improvement will
be discussed in detail.
Project Review
Besides the obvious cost of man hours, the main cost of the project was in the three unique boards that
the team designed and populated. These boards were the main board, the daughter board and the
translation board. Below are the costs of the parts to populate the boards as well as the cost of the
boards themselves.
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Part
Supplier
Part Location
Cost to TEAM
HMC6352
Honeywell (10)
Daughter Board
0.00
PIC18F4520
Microchip (sample)
Main Board
7.90
GDM 1602K
Sparkfun
Main Board
15.95
74HCT4067DB
DigiKey
Daughter Board
2.37
Programming Jack
DigiKey
Main Board
1.63
Serial Jack
DigiKey
Main Board
1.90
Connection
DigiKey
Main\Daughter Board
8.48
Laser D6505I
DigiKey
Daughter Board
9.00
12.0V Power Supply
DigiKey
Main Board
13.38
Power Switch
DigiKey
Main Board
5.30
PCB manufactured
Ultimate Electronics
N/A
66.66
Serial Jack (2)
DigiKey
Main Board
2.80
Voltage Regulator
DigiKey
Main Board
1.66
Power Connector
DigiKey
Main Board
0.42
Toggle Switch
DigiKey
Main Board
1.03
BJT (laser)
DigiKey
Daughter Board
0.59
Other Circuitry
DigiKey
All Boards
8.21
Total 147.28
Though the cost of the entire project well exceeded 147.28 dollars, if the team wanted to replicate the
boards, it would only cost 147.28 dollars to do so. It should be noted that each sensor costs 35 dollars
but the team was given a total of 12 HMC6532 sensors (one breakout board) to use for free. These
sensors were provided by Honeywell.
The design strengths of the daughter board and mother board were by far their flexibility. This flexibility
allowed the team to complete the tasks in multiple ways. For example, the team was not able to display
the results on the LCD screen due to timing issues. However, the group had planned multiple ways to
display. The final solution the team used to display was on a PC that ran a VB script. Another example of
flexible engineering was in the daughter board design. When designing the daughter board, traces were
made so that if a RC network could be placed on each axes so that the gain could be brought down. This
allowed the team to order the PCBs without finalizing the testing environment (the magnetic range).
These two examples showed the team’s greatest strength in design.
The biggest weakness was the design of the sensors on the daughter board. After testing the team
realized that a 0-180 orientation would have suited the groups daughter board the best. This was due to
the unknown double sinusoidal error seen in the results. Though it was not possible to realize without
full testing, the chip placement is permanent and could be improved upon.
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Future Work
If one was to redo the project two main recommendations would be made; place the sensors in a 0-180
degree fashion and add jumpers to the I2C bus.
First, placing the sensors in a 0-180 degree fashion would significantly reduce the error because it is now
known to be double sinusoidal. This is because if the sensors were placed from 0-180 degrees, for every
sensor that has a positive error there will be another sensor that has about the same negative error.
This offsetting of two equal errors along with the weighting system will significantly minimize the false
displacement from the true direction of the magnetic field. The figure below shows the placement of
sensor 0-180 degrees.
Secondly, the team realized that the internal workings of the HMC6532 included a PIC16F819 core. This
knowledge is useful in designing the I2C bus. The problems described earlier could be avoided by
powering all the chips and having jumpers on the I2C bus. This would allow the team to address each
sensor individually. A sensor would be connected to the I2C bus via a jumper and then readdressed. This
process would be repeated until every sensor was readdressed properly.
Reflection
Looking back at the design process a lot of things were learned. The biggest thing that was learned was
“anything that can go wrong will go wrong” (Murphy’s Law). This was realized in many aspects of the
project. The team had problems with the PMC for the rotational table, the I2C bus and the Honeywell
sensors just to name a few. The team learned that there will be problems that you can see coming and
ones that you may not. With a little perseverance everything will turn out fine.
Managing our time was another lesson learned. The timeline was useful because it helped map out the
progress and realize the critical path. A milestone in the project was the manufactured PCBs. This was
on the critical path and special attention was given to it when it was delayed. This helped the team
successfully complete the project on time.
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Figure 7.0: Project Gantt Chart
In conclusion, the team was successful in completing the project. Though the team did not meet the
goal of tenfold improvement, the team did successfully build and tested the magnetic compass. The
team was able to achieve twofold improvement and has great insight on how to improve in the future.
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Research References
Related documents
1. A New Perspective on Magnetic Field Sensing. Michael J. Caruso, Dr. Carl H. Smith,
Tamara Bratland, Robert Schneider. May 1998. Honeywell International Inc. Feb
2008. http://phermans.com/w/images/4/4d/Magnetic_sensing.pdf
2. Linear / Angular / Rotary Displacement Sensors. Aug 2000. Honeywell International
Inc. Feb 2008. http://phermans.com/w/images/b/b6/Hmc15011512_displacment_AMR_sensor.pdf
3. APPLICATIONS OF MAGNETIC POSITION SENSORS. Jan 2002. Honeywell
International Inc. Feb 2008.
http://phermans.com/w/images/9/9f/Appl_note_for_position_sensing.pdf
4. High Resolution Compass with Multiple Anisotropic Magnetic Sensors. Daniel
Beckvall, Marcus Ellson, Patrick Hermans, Jeffery Aymond, Patrick Albersman. Feb
2008. University of Minnesota EE 4951 Group 4. Feb 2008.
http://phermans.com/w/index.php?title=High_Resolution_Compass_with_Multiple_
Anisotropic_Magnetic_Sensors
Appendices
Data sheets
1. 2-Axis Magnetic Sensor HMC1042L. Nov 2006. Honeywell International Inc. Feb
2008. http://www.ssec.honeywell.com/magnetic/datasheets/hmc1042L.pdf
2. 1, 2 and 3 Axis Magnetic Sensors HMC1051/HMC1052/HMC1053. Mar 2006.
Honeywell International Inc. Feb 2008.
http://www.ssec.honeywell.com/magnetic/datasheets/HMC105X.pdf
3. 2-Axis Compass with Algorithms HMC6352. Jan 2006. Honeywell International Inc.
Feb 2008. < http://www.ssec.honeywell.com/magnetic/datasheets/HMC6352.pdf>
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4. PIC18F2420/2520/4420/4520 Data Sheet. 2007. Microchip Tech. Inc. Feb 2008 <
http://ww1.microchip.com/downloads/en/DeviceDoc/39631D.pdf>
5.
HD44780U (LCD-II). Dec 2001. Hitachi. Feb 2008.
http://www.sparkfun.com/datasheets/LCD/HD44780.pdf
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