PNI TCM User Manual

User Manual

TCM

High-Performance Tilt-Compensated Compass Module

Table of Contents

1 COPYRIGHT & WARRANTY INFORMATION ................................................. 1

2 INTRODUCTION ......................................................................................... 2

3 SPECIFICATIONS ......................................................................................... 3

3.1 Characteristics & Requirements ........................................................... 3

3.2 Mechanical Drawings ............................................................................ 6

4 SET-UP ....................................................................................................... 8

4.1 Electrical Connections ........................................................................... 8

4.2 Installation Location .............................................................................. 9

4.2.1 Operate within the TCM’s dynamic range ................................... 9

4.2.2 Locate away from changing magnetic fields ............................... 9

4.2.3 Mount in a physically stable location .......................................... 9

4.2.4 Location-verification testing ........................................................ 9

4.3 Mechanical Mounting ......................................................................... 10

4.3.1 Pitch and Roll Conventions ........................................................ 10

4.3.2 Mounting Orientation ................................................................ 11

5 USER CALIBRATION .................................................................................. 12

5.1 Magnetic Calibration ........................................................................... 13

5.1.1 Full-Range Calibration ................................................................ 15

5.1.2 2D Calibration ............................................................................ 16

5.1.3 Limited Tilt Range Calibration .................................................... 17

5.1.4 Hard-Iron-Only Calibration ........................................................ 18

5.2 Accelerometer Calibration .................................................................. 18

5.2.1 Accelerometer-Only Calibration ................................................ 19

5.2.2 Mag-and-Accel Calibration ........................................................ 20

6 OPERATION WITH TCM STUDIO ............................................................... 21

6.1 Installation .......................................................................................... 21

6.2 Connection Tab ................................................................................... 22

6.2.1 Initial Connection ....................................................................... 22

6.2.2 Changing Baud Rate ................................................................... 22

6.2.3 Changing Modules ..................................................................... 23

6.3 Configuration Tab ............................................................................... 23

6.3.1 Mounting Options ...................................................................... 23

6.3.2 North Reference ......................................................................... 24

6.3.3 Endianess ................................................................................... 24

6.3.4 Output ........................................................................................ 25

6.3.5 Enable 3D Model ........................................................................ 25

6.3.6 Filter Setting (Taps) .................................................................... 25

6.3.7 Acquisition Settings .................................................................... 25

6.3.8 HPR During Calibration .............................................................. 26

6.3.9 Calibration Settings .................................................................... 26

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6.3.10 Default ........................................................................................ 27

6.3.11 Retrieve ...................................................................................... 27

6.4 Calibration Tab .................................................................................... 28

6.4.1 Samples ...................................................................................... 28

6.4.2 Calibration Results ..................................................................... 29

6.4.3 Current Configuration ................................................................ 30

6.4.4 Options ....................................................................................... 30

6.4.5 Clear ........................................................................................... 30

6.5 Test Tab ............................................................................................... 31

6.5.1 Current Reading ......................................................................... 31

6.5.2 3D Model .................................................................................... 31

6.5.3 Acquisition Settings .................................................................... 31

6.5.4 Sync Mode .................................................................................. 32

6.6 Log Data Tab ....................................................................................... 33

6.7 Graph Tab ............................................................................................ 34

6.8 System Log Tab ................................................................................... 35

7 OPERATION WITH PNI BINARY PROTOCOL ............................................... 36

7.1 Datagram Structure ............................................................................ 36

7.2 Parameter Formats ............................................................................. 37

7.3 Commands & Communication Frames ............................................... 39

7.3.1 kGetModInfo (frame ID 1d) ........................................................ 40

7.3.2 kGetModInfoResp (frame ID 2d) ................................................ 40

7.3.3 kSetDataComponents (frame ID 3d) .......................................... 41

7.3.4 kGetData (frame ID 4d) .............................................................. 42

7.3.5 kGetDataResp (frame ID 5d) ....................................................... 42

7.3.6 kSetConfig (frame ID 6d) ............................................................ 43

7.3.7 kGetConfig (frame ID 7d) ............................................................ 47

7.3.8 kGetConfigResp (frame ID 8d) .................................................... 47

7.3.9 kSave (frame ID 9d) .................................................................... 48

7.3.10 kStartCal (frame ID 10d) ............................................................. 48

7.3.11 kStopCal (frame ID 11d) .............................................................. 50

7.3.12 kSetFIRFilters (frame ID 12d) ...................................................... 50

7.3.13 kGetFIRFilters (frame ID 13d) ..................................................... 52

7.3.14 kGetFIRFiltersResp (frame ID 14d) ............................................. 52

7.3.15 kPowerDown (frame ID 15d) ...................................................... 52

7.3.16 kSaveDone (frame ID 16d) .......................................................... 53

7.3.17 kUserCalSampleCount (frame ID 17d) ........................................ 53

7.3.18 kCalScore (frame ID 18d) ............................................................ 53

7.3.19 kSetConfigDone (frame ID 19d) .................................................. 54

7.3.20 kSetFIRFiltersDone (frame ID 20d) ............................................. 54

7.3.21 kStartContinuousMode (frame ID 21d) ...................................... 54

7.3.22 kStopContinuousMode (frame ID 22d) ...................................... 54

7.3.23 kPowerUpDone (frame ID 23d) .................................................. 55

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7.3.24 kSetAcqParams (frame ID 24d) ................................................... 55

7.3.25 kGetAcqParams (frame ID 25d) .................................................. 56

7.3.26 kSetAcqParamsDone (frame ID 26d) .......................................... 56

7.3.27 kGetAcqParamsResp (frame ID 27d) .......................................... 56

7.3.28 kPowerDownDone (frame ID 28d) ............................................. 56

7.3.29 kFactoryMagCoeff (frame ID 29 d) ............................................. 56

7.3.30 kFactoryMagCoeffDone (frame ID 30 d) ..................................... 56

7.3.31 kTakeUserCalSample (frame ID 31d) .......................................... 57

7.3.32 kFactoryAccelCoeff (frame ID 36 d) ............................................ 57

7.3.33 kFactoryAccelCoeffDone (frame ID 37 d) ................................... 57

7.3.34 kSetSyncMode (frame ID 46 d) ................................................... 57

7.3.35 kSetSyncModeResp (frame ID 47 d) ........................................... 58

7.3.36 kSyncRead (frame ID 49 d) .......................................................... 58

7.4 Code Examples .................................................................................... 59

7.4.1 Header File & CRC-16 Function .................................................. 59

7.4.2 CommProtocol.h File ................................................................. 62

7.4.3 CommProtocol.cpp File .............................................................. 64

7.4.4 TCM.h File .................................................................................. 68

7.4.5 TCM.cpp File ............................................................................... 69

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List of Tables

Table 3-1: Performance Characteristics1 3
Table 3-2: Absolute Maximum Ratings 4
Table 3-3: Electrical Operating Requirements 4
Table 3-4: I/O Characteristics 5
Table 3-5: Environmental Requirements 5
Table 3-6: Mechanical Characteristics 5
Table 4-1: TCM Pin Descriptions 8
Table 5-1: Magnetic Calibration Mode Summary 14
Table 5-2: 12 Point Full-Range Calibration Pattern 16
Table 5-3: 12 Point 2D Calibration Pattern 17
Table 5-4: 12 Point Limited-Tilt Calibration Pattern 17
Table 5-5: 6 Point Hard-Iron-Only Calibration Pattern 18
Table 6-1: Mounting Orientations 24
Table 7-1: UART Configuration 36
Table 7-2: TCM Command Set 39
Table 7-3: Component Identifiers 41
Table 7-4: Configuration Identifiers 44
Table 7-5: Sample Points 45
Table 7-6: Recommended FIR Filter Tap Values 51

List of Figures

Figure 3-1: TCM XB Mechanical Drawing 6
Figure 3-2: TCM XB Pigtailed Cable Drawing 6
Figure 3-3: TCM MB Mechanical Drawing 7
Figure 4-1: Positive & Negative Roll and Pitch Definition 10
Figure 4-2: Mounting Orientations 11
Figure 5-1: 12 Point Full-Range Calibration 15
Figure 5-2: Accelerometer Calibration Starting Orientations 20
Figure 7-1: Datagram Structure 36

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1 Copyright & Warranty Information

© Copyright PNI Sensor Corporation 2009
All Rights Reserved. Reproduction, adaptation, or translation without prior written permission is prohibited, except

as allowed under copyright laws.
Revised July 2013. For most recent version visit our website at www.pnicorp.com
PNI Sensor Corporation

2331 Circadian Way
Santa Rosa, CA 95407, USA
Tel: [phone]
Fax: [phone]

Warranty and Limitation of Liability. PNI Sensor Corporation ("PNI") manufactures its TCM products (“Products”)

from parts and components that are new or equivalent to new in performance. PNI warrants that each Product to be
delivered hereunder, if properly used, will, for one year following the date of shipment unless a different warranty

time period for such Product is specified: (i) in PNI’s Price List in effect at time of order acceptance; or (ii) on PNI’s

web site (www.pnicorp.com) at time of order acceptance, be free from defects in material and workmanship and will
operate in accordance with PNI’s published specifications and documentation for the Product in effect at time of
order. PNI will make no changes to the specifications or manufacturing processes that affect form, fit, or function of
the Product without written notice to the OEM, however, PNI may at any time, without such notice, make minor
changes to specifications or manufacturing processes that do not affect the form, fit, or function of the Product. This

warranty will be void if the Products’ serial number, or other identification marks have been defaced, damaged, or

removed. This warranty does not cover wear and tear due to normal use, or damage to the Product as the result of
improper usage, neglect of care, alteration, accident, or unauthorized repair.

THE ABOVE WARRANTY IS IN LIEU OF ANY OTHER WARRANTY, WHETHER EXPRESS, IMPLIED, OR
STATUTORY, INCLUDING, BUT NOT LIMITED TO, ANY WARRANTY OF MERCHANTABILITY,
FITNESS FOR ANY PARTICULAR PURPOSE, OR ANY WARRANTY OTHERWISE ARISING OUT OF ANY
PROPOSAL, SPECIFICATION, OR SAMPLE. PNI NEITHER ASSUMES NOR AUTHORIZES ANY PERSON
TO ASSUME FOR IT ANY OTHER LIABILITY.

If any Product furnished hereunder fails to conform to the above warranty, OEM’s sole and exclusive remedy and
PNI’s sole and exclusive liability will be, at PNI’s option, to repair, replace, or credit OEM’s account with an

amount equal to the price paid for any such Product which fails during the applicable warranty period provided that
(i) OEM promptly notifies PNI in writing that such Product is defective and furnishes an explanation of the
deficiency; (ii) such Product is returned to PNI’s service facility at OEM’s risk and expense; and (iii) PNI is satisfied
that claimed deficiencies exist and were not caused by accident, misuse, neglect, alteration, repair, improper
installation, or improper testing. If a Product is defective, transportation charges for the return of the Product to
OEM within the United States and Canada will be paid by PNI. For all other locations, the warranty excludes all
costs of shipping, customs clearance, and other related charges. PNI will have a reasonable time to make repairs or

to replace the Product or to credit OEM’s account. PNI warrants any such repaired or replacement Product to be

free from defects in material and workmanship on the same terms as the Product originally purchased.
Except for the breach of warranty remedies set forth herein, or for personal injury, PNI shall have no liability for any

indirect or speculative damages (including, but not limited to, consequential, incidental, punitive and special
damages) relating to the use of or inability to use this Product, whether arising out of contract, negligence, tort, or

under any warranty theory, or for infringement of any other party’s intellectual property rights, irrespective of

whether PNI had advance notice of the possibility of any such damages, including, but not limited to, loss of use,

revenue or profit. In no event shall PNI’s total liability for all claims regarding a Product exceed the price paid for

the Product. PNI neither assumes nor authorizes any person to assume for it any other liabilities.
Some states and provinces do not allow limitations on how long an implied warranty lasts or the exclusion or

limitation of incidental or consequential damages, so the above limitations or exclusions may not apply to you. This
warranty gives you specific legal rights and you may have other rights that vary by state or province.

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2 Introduction

Thank you for purchasing PNI Sensor Corporation’s TCM XB (pn 12810) or TCM MB (pn

13095) tilt-compensated 3-axis digital compass. The TCM is a high-performance, low-power
consumption, tilt-compensated electronic compass module that incorporates PNI’s advanced
magnetic distortion compensation and calibration scoring algorithms to provide industry-leading
heading accuracy. The TCM combines PNI’s patented magneto-inductive sensors and
measurement circuit technology with a 3-axis MEMS accelerometer for unparalleled cost
effectiveness and performance.

PNI recognizes not all applications allow for significant tilt during calibration, so multiple
calibration methods are available to ensure optimized performance can be obtained in the real
world. These include Full-Range Calibration, when ≥45° of tilt is possible during calibration,
2D Calibration when constrained to calibration in a horizontal or near-horizontal plane, and
Limited-Tilt Calibration when tilt is constrained to <45° but >5° of tilt is possible.

PNI also recognizes conditions may change over time, and to maintain superior heading accuracy
it may be necessary to recalibrate the compass. So the TCM incorporates Hard-Iron-Only
Calibration to easily account for gradual changes in the local magnetic distorting components.
Plus, the accelerometer can be periodically recalibrated in the field to maintain maximum
accuracy.

These advantages make PNI’s TCM the choice for applications that require the highest accuracy
and performance anywhere in the world under a wide range of conditions. Applications for the
TCM include:

Unmanned vehicles – underwater (UUV), ground (UGV), & aerial (UAV)
Far target locaters and laser range finders
Dead reckoning systems
Systems in which the tilt angles used for calibration are physically constrained

With its many applications, the TCM incorporates a flexible and adaptable command set. Many
parameters are user-programmable, including reporting units, a wide range of sampling
configurations, output damping, and more.

We’re sure the TCM will help you to achieve the greatest performance from your system. Thank
you for selecting the TCM.

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Parameter

Value

Heading
Accuracy

≤65° of pitch after Full-Range
Calibration

<0.3° rms

≤80° of pitch after Full-Range
Calibration

<0.5° rms

≤5° of pitch after 2D calibration

<2.0° rms

≤2 times the calibration tilt angle when

using limited-tilt calibration2

<2.0° rms

Resolution

0.1°

Repeatability

0.05° rms

Attitude

Range
Pitch

± 90°

Roll

± 180°

Accuracy
Pitch

0.2° rms

Roll
≤65° of pitch

0.2° rms

≤80° of pitch

0.4° rms

≤86° of pitch

1.0° rms

Resolution

0.01°

Repeatability

0.05° rms

Maximum Operational Dip Angle3

85°

Magnetometers
Calibrated Field Range

± 125 µT

Resolution

0.05 µT

Repeatability

± 0.1 µT

3 Specifications

3.1 Characteristics & Requirements

Table 3-1: Performance Characteristics1

Footnotes:

1. Specifications are subject to change. Assumes the TCM is motionless and the local magnetic
field is clean relative to the user calibration.

2. For example, if the calibration was performed over ±10° of tilt, then the TCM would provide <2°
rms accuracy over ±20° of tilt.

3. Performance at maximum operational dip angle will be somewhat degraded.

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Parameter

Minimum

Maximum

Units

Supply Voltage

-0.3

+10

VDC

Storage Temperature

-40

+85

°C

Parameter

Value

Supply Voltage

TCM XB

3.8 to 9 VDC

TCM MB

3.3 to 9 VDC

Communication
Lines

TCM XB
High Level Input

2.4 V minimum

Low Level Input

0.6 V maximum

Output Voltage Swing

±5.2 V typ., ±5.0 V min.

Tx Output Resistance

300 Ω

TCM MB
High Level Input

2.0 V minimum

Low Level Input

0.8 V maximum

Output Voltage Swing

0 – 3.3 V typical

Tx Output Resistance

330 Ω

Average
Current Draw

TCM XB

@ max. sample rate

20 mA typical

@ 8 Hz sample rate

16 mA typical

TCM MB

@ max. sample rate

17 mA typical

@ 8 Hz sample rate

13 mA typical

Peak Current
Draw

During application of external
power

120 mA pk, 60 mA avg
over 2 ms

During logical power up/down or
Sync Trigger

135 mA pk, 60 mA avg
over 4 ms

Sleep Mode
Current Draw

TCM XB

0.3 mA typical

TCM MB

0.1 mA typical

CAUTION:

Table 3-2: Absolute Maximum Ratings

Stresses beyond those listed above may cause permanent damage to the device. These are
stress ratings only. Operation of the device at these or other conditions beyond those
indicated in the operational sections of the specifications is not implied.

Table 3-3: Electrical Operating Requirements

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Parameter

Value

Communication
Interface

TCM XB

RS232 UART

TCM MB

CMOS/TTL UART

Communication Protocol

PNI Binary

Communication Rate

300 to 115200 baud

Maximum Sample Rate1

~30 samples/sec

Time to Initial
Good Data2

Initial power up

<210 ms

Sleep Mode recovery

<80 ms

Parameter

Value

Operating Temperature1

-40C to +85C

Storage Temperature

-40C to +85C

Parameter

Value

Dimensions
(l x w x h)

TCM XB

35 x 43 x 13 mm

TCM MB

33 x 31 x 13 mm

Weight

TCM XB

6.8 gm

TCM MB

5.3 gm

Connector

TCM XB

9-pin Molex, pn 53780-0970

TCM MB

4-pin MIL-MAX, pn 850-10-004-10-001000

Table 3-4: I/O Characteristics

Footnotes:

1. The maximum sample rate is dependent on the strength of the magnetic
field, and typically will be from 25 to 32 samples/sec.

2. FIR taps set to “0”.

Table 3-5: Environmental Requirements

Footnote:

1. To meet performance specifications across this range,
recalibration will be necessary as the temperature varies.

Table 3-6: Mechanical Characteristics

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3.2 Mechanical Drawings

The default orientation is for the silk-screened arrow to point in the “forward” direction.

Figure 3-1: TCM XB Mechanical Drawing

Figure 3-2: TCM XB Pigtailed Cable Drawing

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The default orientation is for the silk-screened arrow to point in the “forward” direction.

Figure 3-3: TCM MB Mechanical Drawing

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Pin

Number1

TCM XB

TCM MB

9 Pin

Connector

Cable Wire

Color

4 Pin

Connector

1

GND

Black

GND

2

GND

Gray

Vin

3

GND

Green

UART Tx

4

NC

Orange

UART Rx

5

NC

Violet

6

NC

Brown

7

UART Tx

Yellow

8

UART Rx

Blue

9

Vin

Red

4 Set-Up

This section describes how to configure the TCM in your host system. To install the TCM into
your system, follow these steps:

Make electrical connections to the TCM.
Evaluate the TCM using TCM Studio or a binary terminal emulation program, such as

RealTerm or Tera Term, to ensure the compass generally works correctly.

Choose a mounting location.
Mechanically mount the TCM in the host system.
Perform a user calibration.

4.1 Electrical Connections

The TCM XB incorporates a 9 pin Molex connector, part number 53780-0970, which mates
with Molex part 51146-0900 or equivalent. The TCM MB incorporates a 4 pin Mil-Max
connector, part number 850-10-004-10-001000, which mates with Mill-Max part 851-XX­004-10-001000 or equivalent. The pin-out is given below in Table 4-1.

Table 4-1: TCM Pin Descriptions

Footnote:

1. For the TCM XB, pin #1 is indicated on Figure 3-1, while for the TCM MB, pin
#1 is the pin closest to the corner.

After making the electrical connections, it is a good idea to perform some simple tests to
ensure the TCM is working as expected. See Section 5 for how to operate the TCM with
TCM Studio, or Section 7 for how to operate the TCM using the PNI binary protocol.

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4.2 Installation Location

The TCM’s wide dynamic range and sophisticated calibration algorithms allow it to operate
in many environments. For optimal performance however, you should mount the TCM with
the following considerations in mind:

4.2.1 Operate within the TCM’s dynamic range

The TCM can be user calibrated to correct for static magnetic fields created by the host
system. However, each axis of the TCM has a calibrated dynamic range of ±125 µT. If
the total field exceeds this value for any axis, the TCM may not perform to specification.
When mounting the TCM, consider the effect of any sources of magnetic fields in the
host environment that, when added to Earth’s field, may take the TCM out of its dynamic
regime. For example, large masses of ferrous metals such as transformers and vehicle
chassis, large electric currents, permanent magnets such as electric motors, and so on.

4.2.2 Locate away from changing magnetic fields

It is not possible to calibrate for changing magnetic anomalies. Thus, for greatest
accuracy, keep the TCM away from sources of local magnetic distortion that will change
with time; such as electrical equipment that will be turned on and off, or ferrous bodies
that will move. Make sure the TCM is not mounted close to cargo or payload areas that
may be loaded with large sources of local magnetic fields.

4.2.3 Mount in a physically stable location

Choose a location that is isolated from excessive shock, oscillation, and vibration. The
TCM works best when stationary. Any non-gravitational acceleration results in a
distorted reading of Earth’s gravitational vector, which affects the heading measurement.

4.2.4 Location-verification testing

Location-verification testing should be performed at an early stage of development to
understand and accommodate the magnetic distortion contributors in a host system.

Determine the distance range of field distortion.

Place the compass in a fixed position, then move or energize suspect components
while observing the output to determine when they are an influence.

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Determine if the magnetic field is within the dynamic range of the compass.

With the compass mounted, rotate and tilt the system in as many positions as
possible. While doing so, monitor the magnetometer outputs, observing if the
maximum linear range is exceeded.

4.3 Mechanical Mounting

The TCM is factory calibrated with respect to its mounting holes. It must be aligned within
the host system with respect to these mounting holes. Ensure any stand-offs or screws used
to mount the module are non-magnetic. Refer to Section 3.2 for dimensions, hole locations,
and the reference frame orientation.

Note: Ensure that when attaching the TCM to the host system, the mounting method does not
introduce stresses on the board, as this can affect the performance of the accelerometer, and
therefore also negatively affect heading accuracy.

4.3.1 Pitch and Roll Conventions

The TCM uses a MEMS accelerometer to measure the tilt angle of the compass. This
data is output as pitch and roll data, and is also used in conjunction with the
magnetometers to provide a tilt-compensated heading reading.

The TCM utilizes Euler angles as the method for determining accurate orientation. This
method is the same used in aircraft orientation where the outputs are heading (also called
yaw or azimuth), pitch and roll. When using Euler angles, roll is defined as the angle
rotated around an axis through the center of the fuselage while pitch is rotation around an
axis through the center of the wings. These two rotations are independent of each other
since the rotation axes rotate with the plane body.

As shown in Figure 4-1, for the TCM a positive pitch is when the front edge of the board
is rotated upward and a positive roll is when the right edge of the board is rotated down.

Figure 4-1: Positive & Negative Roll and Pitch Definition

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4.3.2 Mounting Orientation

The TCM can be mounted in various orientations, as shown for the TCM XB in Figure
4-2. All reference points are based on the white silk-screened arrow on the top side of the
board. The orientation should be programmed in the TCM using TCM Studio or the
kSetConfig command. The default orientation is “STD 0°”.

Note: TCM XB is shown. The Z axis sensor and the connector are on the module’s top surface,
regardless of model.

Figure 4-2: Mounting Orientations

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5 User Calibration

The magnetic sensors in the TCM are calibrated at PNI’s factory in a magnetically controlled

environment. However sources of magnetic distortion positioned near the TCM in the user’s

system will distort Earth’s magnetic field and should be compensated for in the host system with

a user calibration. Examples of such sources include ferrous metals and alloys (ex. iron, nickel,
steel, etc.), batteries, audio speakers, current-carrying wires, and electric motors. Compensation
is accomplished by mounting the TCM in the host system and performing a user calibration. It is

expected the sources of magnetic distortion remain fixed relative to the TCM’s position within

the host system. By performing a calibration, the TCM identifies the local sources of magnetic
distortion and negates their effects from the overall reading to provide an accurate heading.

As with the magnetic sensor, the accelerometer in the TCM is calibrated at PNI’s factory. But
the accelerometer will gradually change over time, and the user either will need to periodically
perform a user accelerometer calibration or return the unit to PNI for recalibration. As a general
rule-of-thumb, the accelerometer should be recalibrated every 6 to 12 months. Unlike a
magnetic calibration, the accelerometer may be calibrated outside the host system.
Accelerometer calibration is more sensitive to noise or hand jitter than magnetic calibration,
especially for subsequent use at high tilt angles. Because of this, ideally a stabilized fixture
would be used for accelerometer calibration, although resting the unit against a stable surface
often is sufficient.

Key Points:

Magnetic calibration:

o Requires incorporating the TCM into the host system to compensate for magnetic

sourcing and distorting components with the user’s system.

o Allows for 4 different methods of calibration. Full-Range Calibration provides

the highest heading accuracy, while 2D and Limited-Tilt Calibration support a
limited range of motion during calibration. Hard-Iron-Only Calibration updates
just the hard-iron coefficients with a relatively easy procedure.

Accelerometer calibration requires rotating the TCM through a full sphere of coverage,

but the TCM does not need to be incorporated into the user’s system during calibration.

If the TCM will experience different states during operation, such as operating with a

nearby shutter sometimes closed and sometimes open, or operating over a broad
temperature range, then different sets of calibration coefficients can be saved for the
various states. Up to 8 magnetic calibration coefficient sets and 3 accelerometer
calibration coefficient sets can be saved.

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5.1 Magnetic Calibration

Two fundamental types of magnetic distortion exist, hard-iron distortion and soft-iron
distortion. A given component can exhibit both hard-iron and soft-iron distortions. These
distortions are reviewed in the ensuing paragraphs, and are followed by discussions on
temperature effects and other considerations. For more information on magnetic distortion

and calibration, see PNI’s white paper “Local Magnetic Distortion Effects on 3-Axis
Compassing” at PNI’s website (http://www.pnicorp.com/technology/papers).

Hard-Iron Effects

Hard-iron distortions are caused by permanent magnets and magnetized objects in
close proximity to the sensors. These distortions add or subtract a fixed component to
each axis of the magnetic field reading. Hard-iron distortions usually are unchanging
and in a constant location relative to the sensors, for all heading orientations.

Soft-Iron Effects

Magnetically “soft” materials effectively bend the magnetic field near them. These
materials have a high magnetic permeability, meaning they easily serve as a path for
magnetic field lines. Unlike hard-iron effects, soft-iron effects do not increase or
decrease the total field in the area. However, the effect of the soft-iron distortion
changes as the host system’s orientation changes. Because of this, it is more difficult
to compensate for soft-iron materials.

Temperature Effects

While the hard-iron and soft-iron distortion of a system may remain quite stable over
time, normally the distortion signature will change over temperature. As a general
rule, the hard-iron component will change 1% per 10°C temperature change. Exactly
how this affects heading depends on several factors, most notably the hard-iron
component of the system and the inclination, or dip angle.

Consider the example of a host system with a 100 µT hard-iron component. This is a
fairly large hard-iron component, but not completely uncommon. A 10°C
temperature change will alter the magnetic field by ~1 µT in the direction of the hard­iron component. Around San Francisco, with an inclination of ~60°, this results in up
to a couple of degrees of heading change over 10°C.

Consequently, no matter how stable a compass is over temperature, it is wise to
recalibrate over temperature since the magnetic signature of the host system will
change over temperature. The TCM helps accommodate this issue by allowing the
user to save up to 8 sets of magnetic calibration coefficient sets, so different
calibration coefficients can be generated and loaded at different temperatures.

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Calibration
Mode

Accuracy

Tilt Range
during Cal

Number of Samples

Minimum

Recommend

Allowable

Range

Full Range

0.3° rms

>±45°

12

10 – 18

2D Calibration

<2°

<±5°

12

10 – 18

Limited Tilt
Range

<2° over 2x tilt
range

±5° to ±45°

12

10 – 18

Hard Iron Only

Restores prior
accuracy

>±3°

6

4 - 18

Other Considerations

The TCM measures the total magnetic field within its vicinity, which is a
combination of Earth’s magnetic field and local magnetic sources and distortions.
While the TCM’s calibration algorithms can compensate for local static magnetic
sources, it is not possible to compensate for dynamic changes in the magnetic field.
Consequently, it is recommended to keep the TCM away from dynamic magnetic
fields. If this is not possible, then take measurements only when the state of the
magnetic field is known. For example, if an electric motor is nearby take
measurements only when the motor is off. Alternatively, different sets of magnetic
calibration coefficients can be generated in advance for various states and then called
when appropriate. Using the prior example, generate and use one set of coefficients
for when the motor is off and another set for when the motor is on.

The main objective of a magnetic user calibration is to compensate for hard-iron and soft-

iron distortions to the magnetic field caused by components within the user’s host system.

To that end, the TCM needs to be mounted within the host system and the entire host system
needs to be moved as a single unit during a user calibration. The TCM allows the user to
perform a calibration only in a 2D plane or with limited tilt, but provides the greatest
accuracy if the user can rotate through 360° of heading and at least ±45°of tilt.

The following subsections provide instructions for performing a magnetic calibration of a
TCM system. Several calibration mode options exist, as summarized in Table 5-1. To meet
the accuracy specification, the number of samples should be the “Minimum Recommended”
value, or greater. Calibration may be performed using Studio or using the PNI binary
protocol, and up to 8 sets of magnetic calibration coefficients may be saved. The
recommended calibration patterns described in the following sub-sections provide a good
distribution of sample points. Also, PNI recommends the location of the TCM remain fairly
constant while only the orientation is changed.

Table 5-1: Magnetic Calibration Mode Summary

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Before proceeding with a calibration, ensure the TCM is properly installed in the host
system, as discussed in Section 4. Also, the software should be properly configured with
respect to the mounting orientation, Endianness, north reference, etc.

Section 6.4 outlines how to perform a calibration using Studio, while Section 7.3.10 provides
a step-by-step example of how to perform a calibration using the PNI protocol.

5.1.1 Full-Range Calibration

A Full-Range Calibration is appropriate when the TCM can be tilted ±45° or more. This
method compensates for hard and soft iron effects in three dimensions, and allows for the
highest accuracy readings. The recommended 12 point calibration pattern is a series of 3
circles of evenly spaced points, as illustrated in Figure 5-1 and listed in Table 5-2. The
pitch used in the second and third circles of the calibration should at least match the
maximum and minimum pitch the device is expected to encounter in use.

Figure 5-1: 12 Point Full-Range Calibration

Note: While Figure 5-1 shows the location of the device changing, this is for illustration purposes and
it is best for the location of the device to remain constant while only the orientation is changed.

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Sample #

Yaw1

Pitch

Roll

First Circle

1

0°

±5°

30° to 40°

2

90°

±5°

-30° to -40°

3

180°

±5°

30° to 40°

4

270°

±5°

-30° to -40°

Second Circle

5

30°

> +45°

30° to 40°

6

120°

> +45°

-30° to -40°

7

210°

> +45°

30° to 40°

8

300°

> +45°

-30° to -40°

Third Circle

9

60°

< -45°

30° to 40°

10

150°

< -45°

-30° to -40°

11

240°

< -45°

30° to 40°

12

330°

< -45°

-30° to -40°

Table 5-2: 12 Point Full-Range Calibration Pattern

Footnote:

1. Yaw listings are not absolute heading directions but rather relative heading
referenced to the first sample.

5.1.2 2D Calibration

A 2D Calibration is intended for very low tilt operation (<5°) where calibrating the TCM
with greater tilt is not practical.

This procedure calibrates for hard and soft iron effects in only two dimensions, and in
general is effective for operation and calibration in the tilt range of -5° to +5°. The
recommended 12 point calibration pattern is a circle of evenly spaced points, as given in
Table 5-3.

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Sample #

Yaw

Pitch1

Roll1

1

0°

0°

0° 2 30°

max. negative

max. negative

3

60°

0°

0°

4

90°

max. positive

max. positive

5

120°

0°

0°

6

150°

max. negative

max. negative

7

180°

0°

0° 8 210°

max. positive

max. positive

9

240°

0°

0°

10

270°

max. negative

max. negative

11

300°

0°

0°

12

330°

max. positive

max. positive

Sample #

Yaw

Pitch

Roll

First Circle

1

0°

0°

0°

2

90°

0°

0°

3

180°

0°

0°

6

270°

0°

0°

Second Circle

7

45°

> +5°

> +5°

8

135°

> +5°

> +5°

11

225°

> +5°

> +5°

12

315°

> +5°

> +5°

Third Circle

13

45°

< -5°

< -5°

14

135°

< -5°

< -5°

17

225°

< -5°

< -5°

18

315°

< -5°

< -5°

Footnote:

Table 5-3: 12 Point 2D Calibration Pattern

1. For best results, the tilt experienced during calibration should match that experienced
in service. For example, if the TCM is restrained to a level plane in service, then
calibration should be in a plane, where “max. positive” and “max. negative” are 0°.

5.1.3 Limited Tilt Range Calibration

A Limited Tilt Range Calibration is recommended when 45° of tilt isn’t feasible, but >5°
of tilt is possible. It provides both hard-iron and softiron distortion correction. The
recommended 12 point calibration pattern given below is a series of 3 circles of evenly
spaced points, with as much tilt variation as expected during use.

Table 5-4: 12 Point Limited-Tilt Calibration Pattern

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Sample #

Yaw

Pitch1

Roll1

1

0°

max. negative

max. negative

2

60°

max. positive

max. positive

3

120°

max. negative

max. negative

4

180°

max. positive

max. positive

5

240°

max. negative

max. negative

6

300°

max. positive

max. positive

Note that a similar and acceptable alternative pattern would be to follow the
recommended 12 point Full-Range Calibration pattern, but substituting the >±45° of pitch
with whatever pitch can be achieved and the ±10° to ±20° or roll with whatever roll can
be achieved up to these limits.

5.1.4 Hard-Iron-Only Calibration

It is not uncommon for the hard-iron magnetic distortions around the TCM to change.
Some reasons for this include significant temperature change or temperature shock to a
system, as well as gradual aging of components. A Hard-Iron-Only Calibration allows
for quick recalibration of the TCM for hard-iron effects, and generally is effective for
operation and calibration in the tilt range of 3° or more (≥45° is preferred). The
recommended 6 point calibration pattern given below is a circle of alternately tilted,
evenly spaced points, with as much tilt as expected during use.

Table 5-5: 6 Point Hard-Iron-Only Calibration Pattern

Footnote:

1. For best results, the tilt experienced during calibration should match that experienced
in service. For example, if the TCM will be subject to ±45° of pitch and roll when in
service, then “max negative” should be -45° and “max positive” should be +45°.

5.2 Accelerometer Calibration

The TCM uses a MEMS accelerometer to measure the attitude of the compass. This data is
output as pitch and roll data. Additionally, the accelerometer data is critical for establishing
an accurate heading reading when the TCM is tilted, as discussed in the PNI white paper
“Tilt-Induced Heading Error in a 2-Axis Compass”, which can be found on PNI’s web site
(http://www.pnicorp.com/technology/papers).

The TCM algorithms assume the accelerometer only measures the gravitational field. If the
TCM is accelerating, this will result in the TCM calculating an inaccurate gravitational

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vector, which in turn will result in an inaccurate heading reading. For this reason, the TCM
should be stationary when taking a measurement.

As previously mentioned, PNI calibrates the accelerometer in its factory prior to shipment.
But over time the bias and offset of the accelerometer will drift. For this reason PNI
recommends the accelerometer be recalibrated every 6 to 12 months. The user may return
the TCM to PNI for accelerometer calibration, or the user may perform a user accelerometer
calibration. The remainder of this section covers the user accelerometer calibration.

5.2.1 Accelerometer-Only Calibration

The requirements for a good user accelerometer calibration differ significantly from the
requirements for a good magnetic calibration. Specifically, a good accelerometer
calibration involves the TCM experiencing a wide range of pitch and roll values,
preferably seeing both ±180° of pitch and ±180° of roll. Also, it is necessary for the
TCM to be very still during an accelerometer calibration. If possible, PNI recommends
using a fixture to hold the device during calibration, although resting the TCM on a hard
surface normally is sufficient.

The accelerometer either can be calibrated while mounted in the host system or it may be
removed and calibrated outside the system. The advantage of performing the calibration
while mounted in the host system is the user does not need to remove the TCM from the
system, which can be burdensome, and a simultaneous Mag-and-Accel Calibration may
be appropriate. The advantage of performing the calibration outside of the system is it
may be much simpler to obtain the desired range of pitch and roll.

Figure 5-2 shows the two basic starting positions for the recommended 18-point
calibration pattern. Starting with the TCM as shown on the left in Figure 5-2, rotate the
device about its z axis such that it sits on each of its 4 edges, taking one calibration
sample on each edge. Then place the TCM flat on the surface and take a calibration
sample, then flip it over (roll it 180°) and take another sample. Next, starting with the
TCM as shown on the right, take a calibration point with it being vertical (0°). Now tilt
the TCM back 45° and take another calibration point (+45°), then tilt the device forward
45° and take another calibration point (-45°). Repeat this 3-point calibration process for
the TCM with it resting on each of its 4 corners. Note that it is possible to perform an
Accelerometer Calibration with as few as 12 sample points, although it generally is more
difficult to obtain a good calibration with just 12 sample points. Also, the maximum
number of calibration points is 18.

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