AHRS using DCM filter.

No direct gyro bias estimation, but also compensates for attitude drift. Uses GPS speed for heading. Compensation of centrifugal force via GPS speed (to fly in circles with a fixedwing). Careful, it doesn't handle all BODY_TO_IMU rotations (mounting positions) correctly!

The algorithm was developed by William Premerlani and Paul Bizard. The algorithm is also used in the AHRS systems of the AdruIMU. The name DCM for the algorithm is really a misnomer, as that just means that the orientation is represented as a DirectionCosineMatrix (rotation matrix). But since people already know it under that name, we kept it.

Example for airframe file

Add to your firmware section: This example contains all possible configuration options, not all of them are mandatory!

<module name="ahrs_float_dcm">
  b'<define name="USE_MAGNETOMETER_ONGROUND" />\n    '
  b'<define name="USE_AHRS_GPS_ACCELERATIONS" />\n    '
  b'<define name="ACCEL_WEIGHT_FILTER" value="8" />\n    '
  b'<define name="ACCEL_WEIGHT_BAND" value="1." />\n  '
  b'<configure name="USE_MAGNETOMETER" value="FALSE" />\n    '
</module>

Module configuration options

Configure Options

name: USE_MAGNETOMETER value: FALSE
Description: set to FALSE to disable magnetometer

Define Options

name: USE_MAGNETOMETER_ONGROUND value: None
Description: use magnetic compensation before takeoff only while GPS course not good
name: USE_AHRS_GPS_ACCELERATIONS value: None
Description: enable forward acceleration compensation from GPS speed
name: ACCEL_WEIGHT_FILTER value: 8
Description: adjust accel drift heuristic filter (default 8, 0 to disable filter)
name: ACCEL_WEIGHT_BAND value: 1.
Description: band size of accel filter: 1. means that there is no correction when accel magnitude exceeds +/- 0.5G around 1G (normal flight)

Files

Source Files

Raw ahrs_float_dcm.xml file:

<!DOCTYPE module SYSTEM "module.dtd">
 
<module name="ahrs_float_dcm" dir="ahrs" task="estimation">
  <doc>
    <description>
      AHRS using DCM filter.
      No direct gyro bias estimation, but also compensates for attitude drift.
      Uses GPS speed for heading.
      Compensation of centrifugal force via GPS speed (to fly in circles with a fixedwing).
      Careful, it doesn't handle all BODY_TO_IMU rotations (mounting positions) correctly!
 
      The algorithm was developed by William Premerlani and Paul Bizard.
      The algorithm is also used in the AHRS systems of the AdruIMU.
      The name DCM for the algorithm is really a misnomer, as that just means that the orientation is represented as a DirectionCosineMatrix (rotation matrix).
      But since people already know it under that name, we kept it.
    </description>
    <configure name="USE_MAGNETOMETER" value="FALSE" description="set to FALSE to disable magnetometer"/>
    <define name="USE_MAGNETOMETER_ONGROUND" description="use magnetic compensation before takeoff only while GPS course not good"/>
    <define name="USE_AHRS_GPS_ACCELERATIONS" description="enable forward acceleration compensation from GPS speed"/>
    <define name="ACCEL_WEIGHT_FILTER" value="8" description="adjust accel drift heuristic filter (default 8, 0 to disable filter)"/>
    <define name="ACCEL_WEIGHT_BAND" value="1." description="band size of accel filter: 1. means that there is no correction when accel magnitude exceeds +/- 0.5G around 1G (normal flight)"/>
  </doc>
 
  <settings>
    <dl_settings>
      <dl_settings NAME="AHRS">
        <dl_setting MAX="100" MIN="0" STEP="1" VAR="imu_health" shortname="health" module="modules/ahrs/ahrs" />
        <dl_setting MAX="100" MIN="0" STEP="1" VAR="renorm_sqrt_count" shortname="err_norm" module="modules/ahrs/ahrs" />
      </dl_settings>
    </dl_settings>
  </settings>
 
  <dep>
    <depends>ahrs_common,@imu,@gps</depends>
    <provides>ahrs</provides>
  </dep>
 
  <makefile target="!sim|fbw">
    <configure name="USE_MAGNETOMETER" default="0"/>
    <define name="USE_MAGNETOMETER" cond="ifeq (,$(findstring $(USE_MAGNETOMETER),0 FALSE))"/>
    <file name="ahrs_float_dcm.c"/>
    <file name="ahrs_float_dcm_wrapper.c"/>
    <test firmware="fixedwing">
        <define name="PRIMARY_AHRS" value="ahrs_dcm"/>
        <define name="AHRS_TYPE_H" value="modules/ahrs/ahrs_float_dcm_wrapper.h" type="string"/>
    </test>
    <raw>
ifdef SECONDARY_AHRS
ifneq (,$(findstring $(SECONDARY_AHRS), dcm float_dcm))
# this is the secondary AHRS
$(TARGET).CFLAGS += -DAHRS_SECONDARY_TYPE_H=\"modules/ahrs/ahrs_float_dcm_wrapper.h\"
$(TARGET).CFLAGS += -DSECONDARY_AHRS=ahrs_dcm
else
# this is the primary AHRS
$(TARGET).CFLAGS += -DAHRS_TYPE_H=\"modules/ahrs/ahrs_float_dcm_wrapper.h\"
$(TARGET).CFLAGS += -DPRIMARY_AHRS=ahrs_dcm
endif
else
# plain old single AHRS usage
$(TARGET).CFLAGS += -DAHRS_TYPE_H=\"modules/ahrs/ahrs_float_dcm_wrapper.h\"
endif
    </raw>
  </makefile>
</module>