YachtBot's AHRS (attitude and heading reference system) option includes a 9 degrees of freedom (9DOF) AHRS inertial and magnetometer system which provides GPS time synchronised orientation estimations as well as raw kinematic  sensor data at rates up to 200Hz. 

GPS rate HDGM (magnetic heading) information, from the orientation estimation, is available for visualisation on the YachtBot web portal or via the Igtimi API for cloud  based data access. 

Full rate orientation and kinematic data may be logged in a NMEA format compatible CSV logfile.

Orientation estimation

The goal of orientation estimation is to report the attitude of YachtBot relative to the frame defined by the earth's surface and magnetic north. Orientation can be  reported as Euler angles or Quaternions and the "zero" orientation is when YachtBot's mounting frame of reference (see Mounting options) is flat to the earth and pointing north. 

The orientation estimation uses an algorithm that includes Kalman filtering techniques. Raw sensor data input is oversampled, and final estimations low pass filtered to  remove unwanted impulse noise and vibration. 

Mounting options

YachtBot can be mounted in several different orthogonal orientations which are configured via the  

IMU AXES [DECK/BOW/STERN/PORT/STARBOARD] 

 command. The argument describes the surface of the object that the rear surface of YachtBot is to be mounted against. The GPS antenna should always be pointing up, except for DECK mount, where it should be pointing towards the bow. The option only affects the orientation data, not the kinematic data.

Raw kinematic data

Linear acceleration and gyro information may be logged. Linear acceleration can be logged with, or without the static gravity vector included. This is not affected by the mounting option and will use the natural axis of the IMU. For details about the natural axis refer to the attached image on mounting options.

Calibration and accuracy

The AHRS has been factory calibrated to correct for linear and non-linear errors in the sensors. Each YachtBot passes calibration testing if 4-points static heading errors are  less than 0.5 degrees in a ferritic-free environment. 

 

Zero point offset

First order Euler angle offsets can be set using the  

IMU PYR

command, and can account for any fixed offset errors due to remaining zero offsets in the orientation  estimation, or due to mounting alignments. Note that a first order correction is only suitable for small corrections. Where large mounting errors are present, post-processing rotation of the orientation estimation is recommended. Note that the Euler angle orientation estimation outputs with non-orthogonal mounting are still valid,  but the parameters no longer map directly to the physical properties expected of Pitch/Roll/Yaw. 

 

Drift and zeroing of the reference frame 

YachtBot estimates reference vectors that define the orientation of the earth frame. These reference vectors are determined at power-up and continuously corrected. To 

ensure that the reference vectors are correctly determined, it's important that YachtBot should be powered up while stationary. 

Magnetic declination 

Orientation is measured relative to magnetic north. The angle between magnetic north and true north is called magnetic declination. Where yaw/heading needs to be 

referenced to true north, using NOAA's empirical declination model is recommend. 

http://www.ngdc.noaa.gov/geomag/geomag.shtml 

Live Data 

Magnetic heading (HDGM) is transmitted live to the YachtBot server directly using the Yaw field from the Euler angle output. To enable HDGM output in YachtBot, Euler 

angle messages must be enabled and the mounting configuration correctly configured. The maximum rate for HDGM data is defined by the GPS rate, and it is 

recommended to set the Euler angle message rate at the same rate as, or multiple of, the GPS rate. An example enabling the Euler message for HDGM is:  

IMU MESSAGE EULER 1 log 1 live

 which would set the eular rate to that of the IMU.

Logfile 

AHRS data is logged to <Stream_ID>.IMU.NMA. The Steam ID is a monotonically increasing number which is incremented each power cycle. 

An example of the logfile is;  

$PGSNC,24391,2013-11-28T21:40:57.886Z,DC-DK-AADK,0xa2*71
$PIIMU,24390,0x1,-6.0,-139.1,-0.2,*00
$PIIMU,24500,0x1,-5.9,-139.0,-0.2,*04
$PIIMU,24750,0x1,-6.0,-138.9,-0.1,*02
$PIIMU,25000,0x1,-6.0,-138.8,0.0,*2C
$PIIMU,25250,0x1,-5.9,-138.8,0.1,*20 

  

The fields are; 

$PGSNC	Header indicating Proprietary GNSS SyNC message
24391	System timer in milliseconds
2013-11-28T21:40:57.886Z	ISO8601 timestamp
DC-DK-AADK	Device Serial number
0xa2	Device Stream ID, in hex
*71	NMEA compliant CRC

 

$PIIMU	Header indicating Proprietary Inertial message from an IMU
24390	System timer in milliseconds
0x1	Message identifier (Euler angles)
-6.0	Pitch
-139.1	Yaw
-0.2	Roll
	Blank field to assist CSV processing
*00	NMEA compliant CRC

For detail on the message types, values, field order and sign conventions, see the appendices below.

Configuration 

The following is a typical configuration block for the AHRS. Note that the term IMU is used to reference configuration of the AHRS system in the configuration. 

## IMU
##
#
IMU rate 4
IMU message euler 4 log
IMU message quaternion log
#IMU message gyro log
#IMU message linearg log
IMU pyr 0 0 0
IMU on
IMU axes deck
IMU start

 In words;

"Configure the IMU to log Euler angles at 1Hz, and Quaternions at 4Hz. Raw sensor data for gyro and linear accelerometer is disabled. No zero offset corrections are 

set and the unit is expected to be mounted in the DECK orientation with the back of the device on a flat deck and GPS antenna pointing towards the bow". 

Note that the order that some of the commands are issued is important. AXES and START commands must come after ON. 

Commands 

Arguments in square brackets are optional. A forward slash inside square brackets means "or". 

For example, [DECK/BOW] means the argument is optional, or can be either DECK or BOW. 

 

IMU RATE [<rate>]

 Read (no args), or set, the native (maximum) query rate for IMU in Hz.

 

IMU MESSAGE <type> [<divider>] [log]

Read (no args) or set message types to query IMU for. The message type is requested at a rate defined by <rate>/<divider>. If [log] is set, the message will be logged to the IMU logfile.

Default argument values are <divider> = 1 and log disabled. 

 

IMU MESSAGE <type> off

Disable querying IMU for message <type>. This is the default setting for all message types.

 

IMU EULER PYR [<pitch> <yaw> <roll>]

Read (no args), or set, offsets that are added to Euler angles from the orientation estimation.

  

IMU AXES [DECK/BOW/STERN/PORT/STARBOARD]

 Read (no args), or set, the installation orientation of YachtBot on the object to be tracked. The argument describes the surface of the object that the rear of YachtBot is to be mounted against. The GPS antenna should always be pointing up, except for DECK mount, where it should be pointing forward.

Logfile Synchronisation

For IMU logfile sync see Synchronisation mode

Appendix: Conventions and units

• Axis system is left handed with Z forward/north, Y up, and X right. 

• Euler angles (more accurately Tait-Bryan angles) are reported in Pitch-Yaw-Roll order. 

• Quaternions reported in XYZW order. 

• Linear accelerations are reported in XYZ order. 

• Gyro is reported in XYZ order. 

• Units

○ Euler angles; degrees 

○ linear acceleration; g 

○ Gyro; rad/s 

Appendix: Signs 

• Axis and natural sign means behaviour of the field with respect to the frame of YachtBot. 

NEMA means the sign convention for where the data is translated to standard NMEA0183 messages created by YachtBot. For example where NMEA data is output 

on the YachtBot serial output

• Logfile means as recorded by the $PIIMU message. Not to be confused with NEMA. 

• API means as the data is recorded in Igtimi's API for use with YachtBot. 

Field	Axis and natural sign	NMEA	Logfile	API/YachtBot
Euler: Pitch	+ve for clockwise rotation looking back down x to origin	-180 to +180 +ve bow up	-180 to +180 +ve bow down	-
Euler: Yaw	+ve for clockwise rotation looking back down y to origin	0-360 +ve turn right	-180 to +180 +ve turn right	0-360 +ve turn right
Euler: Roll	+ve for clockwise rotation looking back down z to origin	-180 to +180 +ve = port up	-180 to +180 +ve port down	-
Linear/linearg	All fields +ve when accelerating back down axis to origin	-	As natural	-
Gyro	All fields +ve for clockwise rotation looking back down axis to origin	-	As natural	-

Denotes fields where the natural sign has been reversed to match NMEA standards 

Appendix: Data types 

Type	Description	Message identifier
Euler	Orientation as Euler angles	0x1
Quaternion	Orientation as Quaternions	0x0
Linearg	Raw (oversampled) linear accelerometer data	0x27
Linear	As above, without component of gravity	0x29
Gyro	Raw gyro data	0x26