AHRS400 Series User's Manual
AHRS400 Series User’s Manual
Models…
AHRS400CAAHRS400CBAHRS400CC(DMU-HDX-AHRS)
Revision A, March 2002
Document 7430-0004-01
Crossbow Technology, Inc., 41 E. Daggett Dr., San Jose, CA 95134
Tel: 408-965-3300, Fax: 408-324-4840
email: [email protected], website: www.xbow.com
©2001-2002 Crossbow Technology, Inc. All rights reserved. Information
in this document is subject to change without notice.
Crossbow and SoftSensor are registered trademarks and DMU is a
trademark of Crossbow Technology, Inc. Other product and trade names
are trademarks or registered trademarks of their respective holders.
AHRS400 Series User’s Manual
Table of Contents
1
Introduction...................................................................................................... 1
1.1
The AHRS Series Motion and Attitude Sensing Units.................... 1
1.2
Package Contents ................................................................................... 2
2
Quick Start ....................................................................................................... 3
2.1
GyroView Software ............................................................................... 3
2.1.1
GyroView Computer Requirements........................................... 3
2.1.2
Install GyroView........................................................................... 3
2.2
Connections............................................................................................. 3
2.3
Setup GyroView..................................................................................... 4
3
2.4
Take Measurements ............................................................................... 4
AHRS Details .................................................................................................. 5
3.1
AHRS Coordinate System.................................................................... 5
3.2
3.3
3.4
3.4.1
3.4.2
3.4.3
Connections............................................................................................. 5
Interface ................................................................................................... 7
Measurement Modes.............................................................................. 8
Voltage Mode ................................................................................ 8
Scaled Sensor Mode ..................................................................... 9
Angle Mode.................................................................................. 10
3.5
Commands............................................................................................. 11
3.5.1
Command List............................................................................. 11
3.6
Data Packet Format.............................................................................. 14
3.7
3.8
3.9
Timing.................................................................................................... 16
Temperature Sensor............................................................................. 16
Analog Output ...................................................................................... 17
3.10 Magnetic Heading................................................................................ 18
4
AHRS Operating Tips.................................................................................. 19
4.1
Mounting the AHRS ............................................................................ 19
5
4.2
AHRS Start Up Procedure .................................................................. 20
Appendix A. Mechanical Specifications................................................... 21
5.1
AHRS400CA Outline Drawing ......................................................... 21
5.2
AHRS400CB Outline Drawing ......................................................... 22
5.3
AHRS400CC Outline Drawing ......................................................... 23
6
Appendix B. AHRS Output Quick Reference ......................................... 24
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6.1
6.2
Analog Output Conversion................................................................. 24
Digital Output Conversion.................................................................. 24
7
Appendix C. Hard and Soft Iron Calibration ........................................... 25
7.1
Description ............................................................................................ 25
7.2
Command List...................................................................................... 26
8
9
Appendix D. AHRS Command Quick Reference................................... 27
Appendix E. Warranty and Support Information .................................... 28
9.1
Customer Service ................................................................................. 28
9.2
Contact Directory ................................................................................. 28
9.3
Return Procedure.................................................................................. 28
9.3.1
Authorization ............................................................................... 28
9.3.2
9.3.3
9.3.4
Identification and Protection..................................................... 29
Sealing the Container ................................................................. 29
Marking......................................................................................... 29
9.3.5
Return Shipping Address........................................................... 29
9.4
Warranty................................................................................................ 29
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About this Manual
The following annotations have been used to provide additional
information.
X NOTE
Note provides additional information about the topic.
þ EXAMPLE
Examples are given throughout the manual to help the reader understand the
terminology.
P IMPORTANT
This symbol defines items that have significant meaning to the user
M WARNING
The user should pay particular attention to this symbol. It means there is a
chance that physical harm could happen to either the person or the
equipment.
The following paragraph heading formatting is used in this manual:
1 Heading 1
1.1 Heading 2
1.1.1 Heading 3
Normal
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1
Introduction
1.1 The AHRS Series Motion and Attitude Sensing Units
This manual explains the use of the AHRS400 Series of products, nine-axis
measurement system designed to measure stabilized pitch, roll and yaw
angles in a dynamic environment.
The AHRS is a nine-axis measurement system that combines linear
accelerometers, rotational rate sensors, and magnetometers. The AHRS
uses the 3-axis accelerometer and 3-axis rate sensor to make a complete
measurement of the dynamics of your system. The addition of a 3-axis
magnetometer also allows the AHRS to make a true measurement of
magnetic heading.
The AHRS is the solid-state equivalent of a vertical gyro/artificial horizon
display combined with a directional gyro.
The AHRS series units are low power, fast turn on, reliable and accurate
solutions for a wide variety of stabilization and measurement applications.
All AHRS products have both an analog output and an RS-232 serial link.
Data may be requested via the serial link as a single polled measurement or
may be streamed continuously. The analog outputs are fully conditioned
and may be connected directly to an analog data acquisition device.
Crossbow Technology DMUs employ onboard digital processing to
compensate for deterministic error sources within the unit and to compute
attitude information. The DMUs accomplish these tasks with an analog to
digital converter and a high performance Digital Signal Processor.
The AHRS uses angular rate sensors and linear acceleration sensors that are
micro-machined devices. The three angular rate sensors consist of vibrating
ceramic plates that utilize the Coriolis force to output angular rate
independently of acceleration. The three MEMS accelerometers are surface
micro-machined silicon devices that use differential capacitance to sense
acceleration. Solid-state MEMS sensors make the AHRS both responsive
and reliable. The magnetic sensors are state-of-the-art miniature fluxgate
sensors. Fluxgate sensors make the AHRS sensitive and responsive, with
better temperature performance than other technologies such as magnetoresistive sensors.
The AHRS400 Series of products utilize a sophisticated Kalman filter
algorithm to allow the unit to track orientation accurately through dynamic
maneuvers. The Kalman filter will automatically adjust for changing
dynamic conditions without any external user input. No user intervention
or configuration is required at power-up.
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The AHRS should not be exposed to large magnetic fields. This could
permanently magnetize internal components of the AHRS and degrade its
magnetic heading accuracy.
1.2 Package Contents
In addition to your DMU sensor product you should have:
•
1 CD with GyroView Software
GyroView will allow you to immediately view the outputs of the
AHRS on a PC running Microsoft® Windows™. You can also
download this software from Crossbow’s web site at
http://www.xbow.com.
•
1 Digital Signal Cable.
This links the DMU directly to a serial port. Only the transmit,
receive, power, and ground channels are used. The analog outputs
are not connected.
•
1 DMU Calibration Sheet
The Digital Calibration Sheets contains the custom offset and
sensitivity information for your AHRS. The calibration sheet is
not needed for normal operation as the AHRS has an internal
EEPROM to store its calibration data. However, this information
is useful when developing your own software to correctly scale the
output data. Save this page!
•
1 DMU Data Sheet
This contains valuable digital interface information including data
packet formats and conversion factors.
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2
Quick Start
2.1 GyroView Software
Crossbow includes GyroView software to allow you to use the DMU right
out of the box and the evaluation is straightforward. Install the GyroView
software, connect the DMU to your serial port, apply power to your unit and
start taking measurements.
2.1.1
GyroView Computer Requirements
The following are minimum capabilities that your computer should have to
run GyroView successfully:
•
CPU: Pentium-class
•
RAM Memory: 32MB minimum, 64MB recommended
•
Hard Drive Free Memory: 15MB
•
Operating System: Windows 95, 98, NT4, 2000
2.1.2 Install GyroView
To install GyroView in your computer:
1. Insert the CD “Support Tools” in the CD-ROM drive.
2.
3.
Find the GyroView folder. Double click on the setup file.
Follow the setup wizard instructions. You will install GyroView
and a LabView 6 Runtime Engine. You will need both these
applications.
If you have any problems or questions, you may contact Crossbow directly.
2.2
Connections
The DMU is shipped with a cable to connect the DMU to a PC
communications port.
1. Connect the 15-pin end of the digital signal cable to the port on the
AHRS.
2. Connect the 9-pin end of the cable to the serial port of your
computer.
3.
The additional black and red wires on the cable supply power to
the AHRS. Match red to (+) power and black to (-) ground. The
input voltage can range from 9 - 30 VDC at 275 mA for the
AHRS. For further information, see the specifications for your
unit.
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M WARNING
Do not reverse the power leads! Applying the wrong power to the AHRS
can damage the unit; Crossbow Technology is not responsible for resulting
damage to the unit.
X NOTE
The analog outputs from the AHRS are unconnected in this cable.
2.3
Setup GyroView
With the AHRS connected to your PC serial port and powered, open the
GyroView software.
1. GyroView should automatically detect the AHRS and display the
serial number and firmware version if it is connected.
2. If GyroView does not connect, check that you have the correct
COM port selected. You find this under the “DMU” menu.
3.
4.
5.
Select the type of display you want under the menu item
“Windows”. Graph displays a real time graph of all the AHRS
data; FFT displays a fast-fourier transform of the data; Navigation
shows an artificial horizon display.
You can log data to a file by entering a data file name. You can
select the rate at which data is saved to disk.
If the status indicator says, “Connected”, you’re ready to go. If
the status indicator doesn’t say connected, check the connections
between the AHRS and the computer; check the power; check the
serial COM port assignment on your computer.
2.4 Take Measurements
Once you have configured GyroView to work with your AHRS, pick what
kind of measurement you wish to see. “Graph” will show you the output
you choose as a strip -chart type graph of value vs. time. “FFT” will show
you a real-time fast Fourier transform of the output you choose.
“Navigation” will show an artificial horizon and the stabilized pitch and roll
output of the AHRS.
Let the AHRS warm up for 30 - 60 seconds when first turned on. This
allows the Kalman filter to estimate the rate sensor biases. Now you’re
ready to use the AHRS!
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3
AHRS Details
3.1 AHRS Coordinate System
The AHRS will have a label on one face illustrating the DMU coordinate
system. With the connector facing you, and the mounting plate down, the
axes are defined as:
X-axis – from face with connector through the DMU
Y-axis – along the face with connector from left to right
Z-axis – along the face with the connector from top to bottom
The axes form an orthogonal right-handed coordinate system. An
acceleration is positive when it is oriented towards the positive side of the
coordinate axis. For example, with the AHRS sitting on a level table, it will
measure zero g along the x- and y-axes and +1 g along the z-axis.
Gravitational acceleration is directed downward, and this is defined as
positive for the DMU z-axis.
The angular rate sensors are aligned with these same axes. The rate sensors
measure angular rotation rate around a given axis. The rate measurements
are labeled by the appropriate axis. The direction of a positive rotation is
defined by the right-hand rule. With the thumb of your right hand pointing
along the axis in a positive direction, your fingers curl around in the
positive rotation direction. For example, if the AHRS is sitting on a level
surface and you rotate it clockwise on that surface, this will be a positive
rotation around the z-axis. The x- and y-axis rate sensors would measure
zero angular rates, and the z-axis sensor would measure a positive angular
rate.
The magnetic sensors are aligned with the same axes definitions and sign as
the linear accelerometers.
Pitch is defined positive for a positive rotation around the y-axis (pitch up).
Roll is defined as positive for a positive rotation around the x-axis (roll
right). Yaw is defined as positive for a positive rotation around the z-axis
(turn right).
The angles are defined as standard Euler angles using a 3-2-1 system. To
rotate from the body frame to an earth-level frame, roll first, then pitch, and
then yaw.
3.2
Connections
The AHRS400CA has a female DB-15 connector, whereas AHRS400CB
and AHRS400CC have a male DB-15 connector. The signals are as shown
in Table 1.
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Table 1. AHRS Connector Pin Out
Pin
Signal
1
RS-232 Transmit Data
2
RS-232 Receive Data
3
Positive Power Input (+Vcc)
4
Ground
5
X-axis accelerometer Analog voltage1
6
Y-axis accelerometer Analog voltage1
7
Z-axis accelerometer Analog voltage1
8
Roll rate analog voltage2
9
Pitch rate analog voltage2
10
Yaw rate analog voltage2
11
NC – factory use only
12
Roll angle/X-axis magnetometer scaled analog voltage3
13
Pitch angle/Y-axis magnetometer scaled analog voltage3
14
Yaw angle/Z-axis magnetometer scaled analog voltage3
15
NC – factory use only
Notes:
1. The accelerometer analog voltage outputs are the raw sensor output.
These outputs are taken from the output of the accelerometers.
2. The rate sensor analog voltage outputs are scaled to represent °/s. These
outputs are created by a D/A converter.
3. Actual output depends on DMU measurement mode. These outputs are
created by a D/A converter.
All analog outputs are fully buffered and are designed to interface directly
to data acquisition equipment.
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The serial interface connection is standard RS-232. On a standard DB-25
COM port connector, make the connections per Table 2.
Table 2. DB-25 COM Port Connections
COM Port Connector
Pin #
Signal
DMU Connector
Pin #
Signal
2
TxD
2
RxD
3
RxD
1
TxD
7
GND*
4
GND*
*Note: Pin 4 on the DMU is data ground as well as power ground.
On a standard DB-9 COM port connector, make the connections per Table
3.
Table 3. DB-9 COM Port Connections
COM Port Connector
Pin #
Signal
DMU Connector
Pin #
Signal
2
RxD
1
TxD
3
TxD
2
RxD
5
GND*
4
GND*
*Note: Pin 4 on the DMU is data ground as well as power ground.
Power is applied to the DMU on pins 3 and 4. Pin 4 is ground; Pin 3 should
have 9-30 VDC unregulated at 275 mA. If you are using the cable supplied
with the DMU, the power supply wires are broken out of the cable at the
DB-9 connector. The red wire is connected to VCC; the black wire is
connected to the power supply ground. DO NOT REVERSE THE POWER
LEADS.
The analog outputs are unconnected in the cable Crossbow supplies. The
analog outputs are fully buffered and conditioned and can be connected
directly into an A/D. The analog outputs require a data acquisition device
with an input impedance of 10kΩ or greater for DAC outputs and relatively
higher input impedance for raw analog outputs.
3.3 Interface
The serial interface is standard RS-232, 38400 baud, 8 data bits, 1 start bit,
1 stop bit, no parity, and no flow control.
Crossbow will supply DMU communication software examples written in
LabView. Source code for the AHRS serial interface can be obtained via
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AHRS400 Series User’s Manual
the web at http://www.xbow.com. The source code has a .vi file format and
requires a National Instruments LabView 5.0 or newer license to use.
The DMU baud rate can be changed per the following procedure:
1. Start with the DMU connected to the serial interface, with
your software set to the default baud rate of 38,400.
2.
3.
4.
Send the ASCII character “b” (0x62 hex) to the DMU. In a
terminal program like Windows HyperTerminal or ProComm,
this means simply type the letter “b”. The DMU is case
sensitive. The DMU will respond “B” (0x42 hex).
Now change the baud rate of your terminal software.
Send the ASCII character “a” (61 hex). The DMU will detect
the character and automatically match the baud rate your
software is using. Upon successful operation, the DMU will
return the character “A” (0x41 hex) at the new baud rate.
5. You can now use the DMU at the new baud rate.
The new baud rate setting is not permanent; therefore, this process must be
repeated after any power reset.
3.4 Measurement Modes
The AHRS400 Series is designed to operate as a complete attitude and
heading reference system. You can also use the DMU as a nine-axis sensor
module. The AHRS can be set to operate in one of three modes: voltage
mode, scaled sensor mode, or angle (VG) mode. The measurement mode
selects the information that is sent in the data packet over the RS-232
interface. See the “Data Packet Format” section for the actual structure of
the data packet in each mode.
3.4.1
Voltage Mode
In voltage mode, the analog sensors are sampled and converted to digital
data with 1 mV resolution. The digital data represents the direct voltage
output of the sensors. The data is 12-bit, unsigned. The value for each
sensor is sent as 2 bytes in the data packet over the serial interface. A single
data packet can be requested using a serial poll command or the DMU can
be set to continuously output data packets to the host.
The voltage data is scaled as:
voltage = data*(5 V)/212
where voltage is the voltage measured at the sensor, and data is the value
of the unsigned 16-bit integer in the data packet. Note that although the
data is sent as 16-bit integers, the data has a resolution of only 12 bits.
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The DMU rate sensor, magnetometer, and angle analog outputs are not
enabled in this mode. Only the linear accelerometer analog outputs on pins
5 - 7 are enabled because these signals are taken directly from the
accelerometers. See the “Analog Output” section for a complete description
of the analog outputs.
3.4.2 Scaled Sensor Mode
In scaled sensor mode, the analog sensors are sampled, converted to digital
data, temperature compensated, corrected for misalignment, and scaled to
engineering units. The digital data represents the actual value of the
quantities measured. A calibration table for each sensor is stored in the
DMU non-volatile memory. A single data packet can be requested using a
serial poll command or the DMU can be set to continuously output data
packets to the host. The data is sent as signed 16-bit 2’s complement
integers. In this mode, the DMU operates as a nine-axis measurement
system.
The scaled sensor analog outputs are enabled in this mode. Note that
stabilized pitch, roll, and yaw angles are not available in scaled sensor
mode. See the “Analog Output” section for a complete description of the
analog outputs.
To convert the acceleration data into G’s, use the following conversion:
accel = data*(GR * 1.5)/215
where accel is the actual measured acceleration in G’s, data is the digital
data sent by the DMU, and GR is the G Range for your DMU. (The data is
scaled so that 1 G = 9.80 m s -2 .) The G range of your DMU is the range of
accelerations your DMU will measure. For example, if your DMU uses a ±
2 G accelerometer, then the G range is 2.
To convert the angular rate data into degrees per second, use the following
conversion:
rate = data*(AR*1.5)/215
where rate is the actual measured angular rate in °/sec, data is the digital
data sent by the DMU, and AR is the Angular rate Range of the DMU. The
angular rate range of your DMU is the range of angular rates your DMU
will measure. For example, if your DMU uses ± 150 °/s rate sensors, then
the AR range is 150.
To convert the acceleration data into Gauss, use the following conversion:
mag = data*(MR*1.5)/215
where mag is the actual measured magnetic field in Gauss, data is the
digital data sent by the DMU, and MR is the Magnetic field Range of the
DMU. MR is 1.25 for the AHRS.
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The AHRS Kalman filter is not enabled in scaled sensor mode. Therefore,
the rate sensor bias will change slightly due to large changes in temperature
and time. If the unit is changed from angle to scaled mode, the last
estimated rate sensor bias values are used upon entering scaled mode.
3.4.3
Angle Mode
In angle mode, the DMU acts as a complete attitude and heading reference
system and outputs the stabilized pitch, roll, and yaw angles along with the
angular rate, acceleration, and magnetic field information. The angular rate,
acceleration, and magnetic field values are calculated as described in the
scaled sensor mode.
The DMU analog outputs are enabled in this mode, including stabilized
pitch, roll, and yaw angles.
The Kalman filter operates in angle mode to track the rate sensor bias and
calculate the stabilized roll, pitch, and yaw angles.
In angle mode, the DMU uses the angular rate sensors to integrate over your
rotational motion and find the actual pitch, roll, and yaw angles. The DMU
uses the accelerometers to correct for rate sensor drift in the vertical angles
(pitch and roll); the DMU uses the magnetometers to correct for rate sensor
drift in the yaw angle. This is the modern equivalent of an analog vertical
gyro that used a plumb bob in a feedback loop to keep the gyro axis
stabilized to vertical. The DMU takes advantage of the rate gyros’
sensitivity to quick motions to maintain an accurate orientation when
accelerations would otherwise throw off the accelerometers measurement of
the DMU orientation relative to gravity; the DMU then uses the
accelerometers to provide long term stability to keep the rate gyro drift in
check.
The AHRS400CA and AHRS400CB use a sophisticated Kalman filter
algorithm to track the bias in the rate sensors. This allows the DMU to use
a very low effective weighting on the accelerometers when the DMU is
moved. This makes the DMU very accurate in dynamic maneuvers. Unlike
other Crossbow DMU systems, the user does not need to set an erection
rate.
The AHRS outputs the stabilized pitch, roll and yaw angles in the digital
data packet in angle mode. To convert the digital data to angle, use the
following relation:
angle = data*(SCALE)/215
where angle is the actual angle in degrees (pitch, roll or yaw), data is the
signed integer data output in the data packet, and SCALE is a constant.
SCALE = 180° for roll, pitch and yaw.
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3.5 Commands
The AHRS has a simple command structure. You send a command
consisting of one byte to the DMU over the RS-232 interface and the DMU
will execute the command.
X NOTE
The DMU commands are case sensitive!
GyroView is a very good tool to use when debugging your own software.
GyroView formulates the proper command structures and sends them over
the RS-232 interface. You can use GyroView to verify that the DMU is
functioning correctly. GyroView does not use any commands that are not
listed here.
X NOTE
Certain combinations of characters not listed here can cause the unit to enter
a factory diagnostic mode. While this mode is designed to be very difficult
to enter accidentally, it is recommended that the following command set be
adhered to for proper operation.
3.5.1
Command List
Command
Reset
Character(s) Sent
R
Response
H
Description
Resets DMU to default state
Command
Voltage Mode
Character(s) Sent
r
Response
R
Description
Changes measurement type to Voltage Mode.
DMU outputs raw sensor voltage in the data
packet.
Command
Scaled Mode
Character(s) Sent
c
Response
C
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Description
Changes measurement type to Scaled Mode.
DMU outputs measurements in scaled
engineering units.
Command
Angle Mode
Character(s) Sent
a
Response
A
Description
Changes measurement type to Angle (VG) Mode.
DMU calculates stabilized pitch and roll. Also
outputs sensor measurements in scaled
engineering units.
Command
Polled Mode
Character(s) Sent
P
Response
none
Description
Changes data output mode to Polled Mode.
DMU will output a single data packet when it
receives a "G" command.
Command
Continuous Mode
Character(s) Sent
C
Response
Data Packets
Description
Changes data output mode to Continuous Mode.
DMU will immediately start to output data
packets in continuous mode. Data rate will
depend on the measurement type the DMU is
implementing (Raw, Scaled or Angle). Sending
a "G" will return DMU to Polled Mode.
Command
Request Data
Character(s) Sent
G
Response
Data Packet
Description
"G" requests a single data packet. DMU will
respond with a data packet. The format of the
data packet will change with the measurement
mode (Raw, Scaled or Angle). Sending the
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DMU a "G" while it is in Continuous Mode will
place the DMU in Polled Mode.
Command
Query DMU Version
Character(s) Sent
v
Response
ASCII string
Description
This queries the DMU firmware and will tell you
the DMU type and firmware version. The
response is an ASCII string that describes the
DMU type and firmware version.
Command
Query Serial Number
Character(s) Sent
S
Response
Serial Number Packet
Description
This queries the DMU for its serial number. The
DMU will respond with a serial number data
packet that consists of a header byte (FF), the
serial number in 4 bytes, and a checksum byte.
The serial number bytes should be interpreted as
a 32-bit unsigned integer. For example, the serial
number 9911750 would be sent as the four bytes
00 97 3D C6.
Command
Request Auto Baud Rate
Character(s) Sent
b
Response
-
Description
This starts the auto baud rate detection process.
This will allow you to change the DMU baud rate
from its default. This change will not affect the
default settings.
1. Start with communications program and
DMU at same baud rate.
2. Send "b" to the DMU. The DMU will
respond with “B”.
3. Change the baud rate of your
communications program.
4. Send "a" to the DMU. The DMU will
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respond with "A" at the new baud rate
when a successful detection of the new
baud rate is completed.
3.6 Data Packet Format
In general, the digital data representing each measurement is sent as a 16-bit
number (two bytes). The data is sent MSB first then LSB.
In voltage mode, the data is sent as unsigned integers to represent the range
0 – 5 V.
In scaled and angle mode, the data generally represents a quantity that can
be positive or negative. These numbers are sent as a 16-bit signed integer in
2's complement format. The data is sent as two bytes, MSB first then LSB.
In scaled and angle mode, the timer information and temperature sensor
voltage are sent as unsigned integers.
The order of data sent will depend on the selected operating mode of the
AHRS.
Each data packet will begin with a header byte (255) and end with a
checksum. The checksum is calculated in the following manner:
1. Sum all packet contents except header and checksum.
2.
3.
Divide the sum by 256.
The remainder should equal the checksum.
X NOTE
The header byte FF will likely not be the only FF byte in the data packet.
You must count the bytes received at your serial port and use the checksum
to ensure you are in sync with the data sent by the DMU. This is especially
critical when using the continuous data packet output mode.
Table 4 shows the data packet format for each mode.
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Table 4. AHRS400 Series Data Packet Format
Byte
VG Mode
Scaled Sensor Mode
Voltage Mode
0
Header (255)
Header (255)
Header (255)
1
Roll Angle (MSB)
Roll Angular Rate (MSB)
Roll Gyro Voltage (MSB)
2
Roll Angle (LSB)
Roll Angular Rate (LSB)
Roll Gyro Voltage (LSB)
3
Pitch Angle (MSB)
Pitch Angular Rate (MSB)
Pitch Gyro Voltage (MSB)
4
Pitch Angle (LSB)
Pitch Angular Rate (LSB)
Pitch Gyro Voltage (LSB)
5
Heading Angle (MSB)
Yaw Angular Rate (MSB)
Yaw Gyro Voltage (MSB)
6
Heading Angle (LSB)
Yaw Angular Rate (LSB)
Yaw Gyro Voltage (LSB)
7
Roll Angular Rate (MSB)
X-Axis Acceleration (MSB)
X-Axis Accel Voltage (MSB)
8
Roll Angular Rate (LSB)
X-Axis Acceleration (LSB)
X-Axis Accel Voltage (LSB)
9
Pitch Angular Rate (MSB)
Y-Axis Acceleration (MSB)
Y-Axis Accel Voltage (MSB)
10
Pitch Angular Rate (LSB)
Y-Axis Acceleration (LSB)
Y-Axis Accel Voltage (LSB)
11
Yaw Angular Rate (MSB)
Z-Axis Acceleration (MSB)
Z-Axis Accel Voltage (MSB)
12
Yaw Angular Rate (LSB)
Z-Axis Acceleration (LSB)
Z-Axis Accel Voltage (LSB)
13
X-Axis Acceleration (MSB)
X-Axis Magnetic Field (MSB)
X-Axis Mag Voltage (MSB)
14
X-Axis Acceleration (LSB)
X-Axis Magnetic Field (LSB)
X-Axis Mag Voltage (LSB)
15
Y-Axis Acceleration (MSB)
Y-Axis Magnetic Field (MSB)
Y-Axis Mag Voltage (MSB)
16
Y-Axis Acceleration (LSB)
Y-Axis Magnetic Field (LSB)
Y-Axis Mag Voltage (LSB)
17
Z-Axis Acceleration (MSB)
Z-Axis Magnetic Field (MSB)
Z-Axis Mag Voltage (MSB)
18
Z-Axis Acceleration (LSB)
Z-Axis Magnetic Field (LSB)
Z-Axis Mag Voltage (LSB)
19
X-Axis Magnetic Field (MSB)
Temp Sensor Voltage (MSB)
Temp Sensor Voltage (MSB)
20
X-Axis Magnetic Field (LSB)
Temp Sensor Voltage (LSB)
Temp Sensor Voltage (LSB)
21
Y-Axis Magnetic Field (MSB)
Time (MSB)
Time (MSB)
22
Y-Axis Magnetic Field (LSB)
Time (LSB)
Time (LSB)
23
Z-Axis Magnetic Field (MSB)
Checksum
Checksum
24
Z-Axis Magnetic Field (LSB)
25
Temp Sensor Voltage (MSB)
26
Temp Sensor Voltage (LSB)
27
Time (MSB)
28
Time (LSB)
29
Checksum
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3.7 Timing
The maximum AHRS data update rate is 70 samples per second.
In some applications, using the DMU’s digital output requires a precise
understanding of the internal timing of the device. The processor internal to
the DMU runs in a loop - collecting data from the sensors, processing the
data, and then collecting more data. The data is reported to the user through
a parallel process. In continuous mode, the system processor activity is
repeatable and accurate timing information can be derived based purely on
the overall loop rate.
The unit goes through three processes in one data cycle. First, the sensors
are sampled. Second, the unit processes the data for output. After
processing the data, the DMU will make another measurement while
presenting the current measurement for output. Third, the unit actually
transfers the data out; either over the RS-232 port, or onto the analog
outputs.
In the case of the analog output, the data is presented immediately on the
analog output pins after the data processing step is over. In the case of the
digital data, the data is transferred only if the previous data packet is
cleared. The DMU continues to take data, so that in practice, roughly every
third measurement will be available over the RS-232 interface.
A time tag is attached to each data packet. The time tag is simply the value
of a free running counter at the time the A/D channels are sampled. The
clock counts down from 65535 to 0, and a single tick corresponds to 0.79
microseconds. The timer rolls over approximately every 50 millis econds.
You can use this value to track relative sampling time between data packets,
and correlate this with external timing.
3.8 Temperature Sensor
The DMU has an onboard temperature sensor. The temperature sensor is
used to monitor the internal temperature of the DMU to allow for
temperature calibration of the sensors. The temperature sensor is specified
to be within ± 2% accurate over the DMU operating temperature range.
The DMU reads and outputs the temperature sensor voltage with 12-bit
precision.
The DMU will output the temperature sensor voltage in the digital data
packet scaled as follows:
Vtemp (V) = data * 5/4096
where data is the 16-bit unsigned integer sent as the temperature
information in the data packet. (The DMU uses two full bytes to express
the data, but it is really scaled to 12 bits.)
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Calculate the temperature with the following calibration:
T (°C) = 44.4 (°C/V) * (Vtemp (V) – 1.375 V)
The DMU temperature sensor is internal to the DMU, and is not intended to
measure the ambient temperature. The internal temperature of the DMU
may be as much as 15°C higher than the ambient temperature.
3.9 Analog Output
The AHRS400 Series of products provide nine fully conditioned analog
outputs; of these, six are output voltages created by a DAC in the DMU.
The analog signals can be connected directly to an ADC or other data
acquisition device without further buffering. The input impedance of any
data acquisition device should be greater than 10 kΩ for the DAC outputs
and relatively higher impedance for raw analog outputs. The circuit diagram
for the raw accelerometer outputs (Pin 5, 6 and 7) is shown below:
The DMU must be set to scaled sensor measurement mode or angle
measurement mode to enable the analog signals.
The analog outputs from the accelerometers are taken directly from the
sensor through a buffer. They are “raw” in the sense that they do not
represent a calculated or calibrated value. You will need the zero bias point
and scale factor given on the DMU calibration sheet to turn the analog
voltage into an acceleration measurement.
To find the acceleration in G’s, use the following conversion:
accel (G) = (Vout (V) – bias (V))*sensitivity (G/V)
where accel is the actual acceleration measured, Vout is the voltage at the
analog output, bias is the zero-G bias voltage, and sensitivity is the scale
factor in units G/volts. This applies only to the signals on pins 5, 6, and 7.
For example, if the x-axis of your accelerometer has a zero-G bias of 2.512
V, a sensitivity of 1.01 G/V, and you measure 2.632 V at the analog output,
the actual acceleration is (2.632 V – 2.512 V)*1.01 G/V = 0.121 G.
The analog outputs for the angular rate signals are not taken directly from
the rate sensors; they are created by a D/A converter internal to the DMU.
The output range is ± 4.096V with 12-bit resolution. The analog data will
represent the actual measured quantities, in engineering units, not the actual
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voltage at the sensor output. To convert the analog output to a sensor value
use the following relation:
rate = AR *1.5 * Vout (V) / 4.096 V
where rate is the actual measured rate in units °/s, AR is the angular rate
range of the sensor and Vout is the measured voltage at the analog output.
For example, if your DMU has a ±100 °/s rate sensor, and the analog output
for that sensor is –1.50 V, the value of the measurement is 100 (°/s)*1.5*(1.50)/4.096 = -54.9 °/s.
In scaled measurement mode, pins 12 – 14 represent the magnetic vector
measured by the DMU. To convert the voltage to magnetic field in Gauss,
use the following relation:
mag = MR *1.5 * Vout (V) / 4.096 V
where mag is the magnetic field measured along that axis, MR is the
magnetometer range, and Vout is the voltage measured at the analog output.
MR is 1.25 for the AHRS.
In angle mode, the AHRS outputs the pitch, roll, and yaw angles on pins 12
- 14. The analog outputs are created by the D/A. The voltage output will
be in the range ± 4.096 V. The output is scaled so that full scale is 180° for
both roll and yaw. Pitch is scaled so that full scale is 90°. To convert the
voltage to an actual angle, use the following conversion:
angle = FA * Vout (V) / 4.096 V
where angle is the actual pitch, roll or yaw angle in degrees, FA is the fullscale angle, and Vout is the analog voltage measured. FA is 180° for roll
and yaw; FA is 90 for pitch.
3.10 Magnetic Heading
Magnetic north is the direction toward the magnetic north pole; true north is
the direction towards the true North Pole.
The AHRS yaw angle output is referenced to magnetic north. The direction
of true north will vary from magnetic north depending on your position on
the earth. The difference between true and magnetic north is called
declination or magnetic variance. You will need to know your declination
to translate the AHRS magnetic heading into a heading referenced to true
north.
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4
AHRS Operating Tips
4.1 Mounting the AHRS
The AHRS should be mounted as close to the center of gravity (CG) of your
system as possible. This will minimize any “lever effect.” If it is not
mounted at the center of gravity, then rotations around the center of gravity
will cause the DMU accelerometers to measure an acceleration proportional
to the product of the angular rate squared and the distance between the
DMU and the CG.
The DMU will measure rotations around the axes of its sensors. The DMU
sensors are aligned with the DMU case. The sides of the DMU case are
used as reference surfaces for aligning the DMU sensor axes with your
system. You should align the DMU case as closely as possible with the
axes you define in your system. Errors in alignment will contribute directly
to errors in measured acceleration and rotation relative to your system axes.
The DMU should be isolated from vibration if possible. Vibration will
make the accelerometer readings noisy and can, therefore, affect the angle
calculations. In addition, if the magnitude of the vibration exceeds the
range of the accelerometer, the accelerometer output can saturate. This can
cause errors in the accelerometer output.
The AHRS400 Series should be isolated from magnetic material as much as
possible. Magnetic material will distort the magnetic field near the AHRS,
which will greatly affect its accuracy as a heading sensor. Because the
DMU is using Earth's weak magnetic field to measure heading, even small
amounts of magnetic material near the sensor can have large effects on the
heading measurement.
"Bad" materials include anything that will stick to a magnet: iron, carbon
steel, some stainless steels, nickel and cobalt. Use a magnet to test
materials that will be near the AHRS. If you discover something near the
DMU that is magnetic, attempt to replace it with something made from a
non-magnetic material. If you cannot change the material, move it as far as
possible from the DMU. Even small things, such as screws and washers,
can have a negative effect on the AHRS performance if they are close.
AHRS can correct for the effect of these magnetic fields by using hard and
soft iron calibration routine as explained in Appendix C.
"Good" materials include brass, plastic, titanium, wood, and some stainless
steels. Again, if in doubt, try to stick a magnet on the material. If the
magnet doesn't stick, you are using a good material.
DO NOT try to stick a magnet to the AHRS. We have removed as much
magnetic material as possible from the unit, but we could not make the unit
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AHRS400 Series User’s Manual
completely non-magnetic. You can permanently magnetize ("perm up")
components in the AHRS if you expose the unit to a large magnetic field.
You can use a demagnetizer (tape eraser) to demagnetize the DMU if it gets
“permed.” Follow the instructions for your demagnetizer.
The DMU case is not weatherproof. You should protect the DMU from
moisture and dust.
þ EXAMPLE
4.2 AHRS Start Up Procedure
As an example, look at how the DMU might be used on an airplane.
Assume AHRS is mounted on a small twin-prop plane and will be used to
record the plane's attitude during flight. Flights will be 2 – 6 hours long.
The AHRS is mounted near the CG of the plane, and is connected to a
laptop serial port during flight.
Page 20
1.
Turn on power to the DMU and let it warm up 5 – 10 minutes.
Power can be on to all electronics, but the engines should be
off.
2.
3.
Start the engines.
Perform hard iron and soft iron calibration routines (Appendix
C).
4.
5.
Start data collection.
Proceed with flight.
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5
5.1
Appendix A. Mechanical Specifications
AHRS400CA Outline Drawing
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5.2
AHRS400CB Outline Drawing
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5.3
AHRS400CC Outline Drawing
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6
Appendix B. AHRS Output Quick Reference
GR is the G-range of the accelerometers. For example, if your DMU has ±
2 G accelerometers, GR = 2.
RR is the rate range of the rate sensors. For example, if your DMU has ±
100°/s rate sensors, RR = 100.
6.1
Analog Output Conversion
Accelerometer
Use sensitivity, offset from
calibration sheet. Output is raw
sensor voltage.
Pin 5
Pin 6
Pin 7
X axis accelerometer, raw
Y axis accelerometer, raw
Z axis accelerometer, raw
Rate Sensor
Rate (°/s) =
Vout (V) * RR * 1.5/4.096
Pin 8
Roll rate sensor
Pin 9
Pitch rate sensor
Pin 10 Yaw rate sensor
Magnetometer (Scaled Mode)
Mag (Gauss) =
Vout (V) * 1.25 * 1.5/4.096
Pin 12 X axis magnetometer
Pin 13 Y axis magnetometer
Pin 14 Z axis magnetometer
6.2
Roll, Pitch, Yaw (Angle Mode)
Angle (°) = Vout (V) * FA/4.096
Pin 12 Roll Angle FA = 180
Pin 13 Pitch Angle FA = 90
Pin 14 Yaw angle FA = 180
Digital Output Conversion
Data is sent as 16-bit signed integer for all but Temperature. Temperature
sensor data is sent as unsigned integer.
Roll, Pitch, Yaw (Angle Mode)
Acceleration
Accel (G) = data * GR * 1.5/215
Angle (°) = data * 180/215
Rate
Magnetic Field
15
Rate (°/s) = data * RR * 1.5/2
Mag (Gauss) = data * 1.25 * 1.5/215
Temperature
Temperature (°C) =
[(data * 5/4096) – 1.375]*44.44
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7
Appendix C. Hard and Soft Iron Calibration
7.1 Description
The AHRS400 Series of products use magnetic sensors to compute heading.
Ideally, the magnetic sensors would be measuring only earth's magnetic
field to compute the heading angle. In the real world, however, residual
magnetism in the AHRS itself and in your system will add to the magnetic
field measured by the AHRS. This extra magnetic field will create errors in
the heading measurement if they are not accounted for. These extra
magnetic fields are called hard iron magnetic fields. In addition, magnetic
material can change the direction of the magnetic field as a function of the
input magnetic field. This dependence of the local magnetic field on input
direction is called the soft iron effect. The AHRS can actually measure any
extra constant magnetic field that is associated with the AHRS or your
system and correct for it. The AHRS can also make a correction for some
soft iron effects. The process of measuring these non-ideal effects and
correcting for them is called hard iron and soft iron calibration. Calibration
will help correct for magnetic fields that are fixed with respect to the
AHRS. It cannot help for time varying fields, or fields created by parts that
move with respect to the AHRS.
The AHRS accounts for the extra magnetic field by making a series of
measurements. The AHRS uses these measurements to model the hard iron
and soft iron environment in your system. The correction algorithm is twodimensional. You start the magnetic calibration by sending the "s"
command. The AHRS will use all subsequent measurements to model the
magnetic environment. You should make at least one complete turn, with
your system basically level. For example, in an airplane, do a circle on the
taxiway. Multiple turns will slightly improve the estimates, but more than 3
turns is usually not helpful. At the end of this time, send the "u" command
to end the magnetic calibration process. The AHRS will calculate the hard
iron magnetic fields and soft iron corrections and store these as calibration
constants in the EEPROM.
To clear the hard iron calibration constants, send the “h” command. The
AHRS will set the hard iron offset corrections to zero. To clear the soft iron
calibration constants, send the "t" command. The AHRS will set the soft
iron correction parameters to zero. This is useful to see the performance of
the bare AHRS in your system.
For best accuracy, you should do the calibration process with the AHRS
installed in your system. If you do the calibration process with the AHRS
by itself, you will only correct for the magnetism in the AHRS itself. If you
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AHRS400 Series User’s Manual
then install the AHRS in a vehicle (for instance), and the vehicle is
magnetic, you will still see errors arising from the magnetism of the vehicle.
7.2
Command List
Command
Start Soft Iron Calibration
Character(s) Sent
s
Response
S
Description
This command starts the soft iron calibration.
The AHRS will remain in calibration mode until
it receives the "u" command. While in
calibration mode, the AHRS should be rotated
through at least one complete turn (360° of
rotation) with the system basically level.
Command
End Soft Iron Calibration
Character(s) Sent
u
Response
U
Description
This command ends the soft iron calibration
process. The AHRS will store the soft iron
calibration constants in its EEPROM. The
calibration constants will be applied to all
subsequent magnetic measurements.
Command
Clear Hard Iron Calibration
Character(s) Sent
h
Response
H
Description
This command clears the hard iron calibration
constants stored in the AHRS EEPROM. The
calibration constants will be set to zero.
Command
Clear Soft Iron Calibration
Character(s) Sent
t
Response
T
Description
This command clears the soft iron calibration
constants stored in the AHRS EEPROM. The
calibration constants will be set to zero.
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8
Appendix D. AHRS Command Quick Reference
Command
(ASCII)
Response
Description
R
H
Reset: Resets the DMU firmware to default operating
mode of Voltage Mode and Polled operation.
r
R
Change to Voltage Mode.
c
C
Change to Scaled Sensor Mode.
a
A
Change to Angle Mode (VG Mode).
P
None
Change to polled mode. Data packets sent when a G
is received by the DMU.
C
None
Change to continuous data transmit mode. Data
packets streamed continuously. Packet rate is
dependent on operating mode. Sending "G" stops
data transmission.
G
Data
Packet
Get Data. Requests a packet of data from the DMU.
Data format depends on operating mode.
S
ASCII
String
Query DMU serial number. Returns serial number as
32-bit binary number.
v
ASCII
String
Query DMU version ID string. Returns ASCII string.
b
Change
baud rate
Autobaud detection. Send "b"; DMU will respond “B”;
change baud rate; send "a"; DMU will send "A" when
new baud rate is detected.
s
S
Start Soft iron calibration. DMU should be rotated
through at least one complete turn (360° of rotation)
with the system basically level.
u
U
End Soft iron calibration. Calibration is saved in
EEPROM.
h
H
Clear hard iron calibration.
t
T
Clear Soft Iron Calibration
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9
Appendix E. Warranty and Support
Information
9.1 Customer Service
As a Crossbow Technology customer you have access to product support
services which include:
9.2
•
Single-point return service
•
Web-based support service
•
Same day troubleshooting assistance
•
Worldwide Crossbow representation
•
Onsite and factory training available
•
Preventative maintenance and repair programs
•
Installation assistance available
Contact Directory
United States: Phone: 1-408-965-3300 (7 AM to 7 PM PST)
Fax: 1-408-324-4840 (24 hours)
Non-U.S.:
9.3
Email: [email protected]
refer to website www.xbow.com
Return Procedure
9.3.1 Authorization
Before returning any equipment, please contact Crossbow to obtain a
Returned Material Authorization numb er (RMA).
Be ready to provide the following information when requesting a RMA:
•
Name
•
Address
•
Telephone, Fax, Email
•
Equipment Model Number
•
Equipment Serial Number
•
Installation Date
•
Failure Date
•
Fault Description
•
Will it connect to GyroView?
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9.3.2 Identification and Protection
If the equipment is to be shipped to Crossbow for service or repair, please
attach a tag TO THE EQUIPMENT, as well as the shipping container(s),
identifying the owner. Also indicate the service or repair required, the
problems encountered, and other information considered valuable to the
service facility such as the list of information provided to request the RMA
number.
Place the equipment in the original shipping container(s), making sure there
is adequate packing around all sides of the equipment. If the original
shipping containers were discarded, use heavy boxes with adequate padding
and protection.
9.3.3 Sealing the Container
Seal the shipping container(s) with heavy tape or metal bands strong enough
to handle the weight of the equipment and the container.
9.3.4 Marking
Please write the words, “FRAGILE, DELICATE INSTRUMENT” in
several places on the outside of the shipping container(s). In all
correspondence, please refer to the equipment by the model number, the
serial number, and the RMA number.
9.3.5
Return Shipping Address
Use the following address for all returned products:
Crossbow Technology, Inc.
41 E. Daggett Drive
San Jose, CA 95134
Attn: RMA Number (XXXXXX)
9.4
Warranty
The Crossbow product warranty is one year from date of shipment.
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Page 29
Crossbow Technology, Inc.
41 E. Daggett Drive
San Jose, CA 95134
Phone: 408.965.3300
Fax: 408.324.4840
Email: [email protected]
Website: www.xbow.com
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