Strapdown IMU

Current version

The current version of the IMU is built with three Murata ENC-05E angular rate sensors scavenged from older versions of the Gyration GyroPoint mice. The analog signals are filtered and amplified by a quad opamp in the sensor package, with a circuit borrowed from the GyroPoint mice.

There is also room for two ADXL202-EB accelerometers, although only the pitch and roll unit is currently installed. Also planned is a small one axis magnetometer and temperature sensor to better calibrate the gyros.

The schematic for the filter is:

                   ____
                  |    \
  Reference ------| +   \
                  |      >----+------  Output
  Gyro -/\/\/\-+--| -   /     |
         R1    |  |____/      |
               |              |
               |     R2       |
               +---/\/\/\/----+
               |              |
               +----||--------+
                    C1
R1 is 10k
R2 is 47k Ohm
C1 is 0.47 uF

The "real world" data from the sensors is fed into a Kalman filter. This combines the various inputs and their estimates of the current attitude to produce a better estimate with much of the noise filtered out. Here is our overview of Kalman filtering and our source code to perform the math. It currently takes place on the ground station, but once we have a faster MCU or the embedded Linux system this will be moved to onboard the aircraft.

Onboard version

We had a three axis version built on the custom PCB using two MG100's and two ADXL202-EB's. This worked fairly well, although some RFI and ADC sensitivity issues prevented it from detecting small rotations. The IMU above amplifies the signal to help with that problem. The other issue is that the circuit board needed to be properly aligned, which caused some problems with the smaller of the two prototypes. The separate IMU can more easily be positioned and "strapped down" to keep the reference frame fixed.

Two-axis version

Our previous version was a two-axis strapdown unit that consisted of a Gyration MG100 (pictured on the left) combined with a ADXL202-EB. The MG100 is a small, 2cm x 2cm x 2cm and low power (20 mA @ 3.3V) two axis rate gyroscope. It has low drift and temperature compensation, but high precession errors under G loads. Easy interfacing via analog voltage sampling. $58 in single unit quantities (including a full PS/2 mouse and parts!). It is the silver components pictured to the left on the factory board and below on the right mounted in the custom two-axis IMU. Source code for sampling the MG100 is available.

The rate integration isn't very accurate, so we augment it with a dual axis accelerometer. The ADXL202 (+/-2) is an inexpensive MEMS device ($30 in single unit quantities). It is the small board on the left of the IMU pictured to the right. Source code for sampling the ADXL is available, too.

Just a reminder -- all our source is available.

If you're interested in other AHRS / INS options, here is a short overview of different possibilities.