CHR-6dm Attitude and Heading Reference System (IMU)

This Inertial Moment Unit (IMU) is a bunch of sensors  that are supposed to tell the robot’s brain the position and orientation of the robot. Typically solid state gyros, accelerometers and magnetometers are used. However, these sensors have varying degree of error, delay and accumulating delay and error. To deal with errors, the naive approach is to use a filter that smooths the signal but that approach creates delay. The textbook way of dealing with these limitations is to use a predictive algorithm that assumes that the measured object has body weight and inertia and in return provides a very low latency clean signal. Kalman Filter is the classic solution but more modern algorithms provide better results.

Instead of dealing with these problems, a recent development in IMU’s has spawned several small boards that integrate several sensors plus a small processor that does all the fancy math and sends out a low latency smooth signal in the form of a bit stream. All we lazy robot developers have to do is read the XYZ and yaw-pitch-roll data off the board’s serial connector.

To get the principal space position the IMU is once placed in  the head of our Jacobian1 robot.

The IMU that we are using is CHR-6dm Attitude and Heading Reference System.

” The CHR-6dm AHRS is a cost-effective

orientation sensor providing yaw, pitch, and

roll angle outputs at up to 300 Hz.  An

Extended Kalman Filter (EKF) combines data

from onboard accelerometers, rate gyros,

and magnetic sensors to produce yaw, pitch,

and roll angle estimates.

Communication with the CHR-6dm is

performed over a           L (3.3V) UART at 115200

Baud.  The AHRS can be configured to

transmit raw sensor in addition to angle estimates, and the transmission rate can be configured

in 1 Hz increments from 20 Hz to 300 Hz.  Alternatively, the CHR-6dm can operate in “silent

mode,” where data is transmitted only when specific requests are received over the UART.

Regardless of the transmission mode and rate, internal angle estimates are maintained at over

500 Hz to ensure long-term accuracy.

The CHR-6dm simplifies integration by providing a number of automatic calibration routines,

including rate gyro bias calibration, magnetometer hard and soft iron calibration, and

accelerometer “zeroing” to compensate for AHRS-platform misalignment.  All calibration routines

are triggered by sending simple commands over the serial interface.

The CHR-6dm AHRS is a cost-effective  orientation sensor providing yaw, pitch, and  roll angle outputs at up to 300 Hz.  An  Extended Kalman Filter (EKF) combines data  from onboard accelerometers, rate gyros,  and magnetic sensors to produce yaw, pitch,  and roll angle estimates.    Communication with the CHR-6dm is  performed over a           L (3.3V) UART at 115200  Baud.  The AHRS can be configured to  transmit raw sensor in addition to angle estimates, and the transmission rate can be configured  in 1 Hz increments from 20 Hz to 300 Hz.  Alternatively, the CHR-6dm can operate in “silent  mode,” where data is transmitted only when specific requests are received over the UART.   Regardless of the transmission mode and rate, internal angle estimates are maintained at over  500 Hz to ensure long-term accuracy.    The CHR-6dm simplifies integration by providing a number of automatic calibration routines,  including rate gyro bias calibration, magnetometer hard and soft iron calibration, and  accelerometer “zeroing” to compensate for AHRS-platform misalignment.  All calibration routines  are triggered by sending simple commands over the serial interface. …”

To test the IMU, we are using a RB110 Roboard and one of the first problems that we had, was that UART 3 and 4 were not found by Linux, so as temporary work around, we just disabled UART 0 and 1 in the BIOS and put the same IRQ interruption address of  0 and 1 , to 3 and 4.

To connect the IMU to the board, we have used the 5v from UART0 to supply the Vxx (see RB110 datasheet page 8 to 14) for the experience, once the power available on the Uart3 pins is Vxx.

On our first test, with Roboard and the IMU, is a little code running in the Roboard that unpack the data from IMU and just print it, with 115200 Baud of speed in UART3.