E80 The Ultimate Adventure

Inertial Measurement Units

What is Inertial Measurement?

One of the first questions a beginning rocketeer asks after the first launch is, "Where did it go?" An inertial measurement unit (IMU) is used to help answer that question. A complete 6-DOF (Degree-Of_Freedom) IMU consists of three orthogonal accelerometers and three orthogonal gyroscopes. The orientation of the IMU is determined from the gyroscopes and the velocity and position of the IMU are determined by integrating the output of the accelerometers while correcting for the orientation. Most commercial and military aircraft have an IMU. These days the IMU is usually supplemented by a GPS (Global Positioning System) receiver and a magnetometer (an electronic compass)

IMUs suffer from accumulation errors and drift. Depending on the quality of the accelerometers and gyros, the errors can range from very small to substantial. IMUs can be very high-speed devices with update rates of 50 Hz of faster. GPSs are typically much slower, with update rates around 1 Hz. Combining IMU data with GPS and magnetometer data can correct for accumulation error and drift. In addition an absolute pressure sensor can be used to deduce altitude and be combined with the IMU data to increase the accuracy of the measurements. A Pitot-static tube can also be used to measure airspeed and add to the IMU data mix.

Whe have designed and flown one custom IMU on the Mudd III rockets. It was sampled at 200 SPS. We are in the process of designing a second-generation IMU that will be used with a GPS and a magnetometer to improve on the accuracy of the measurements.

Gen 1 IMU

The major electrical components used in the Gen 1 IMU are listed below. The schematics and PCB layouts were constructed in PCB Artist, a schematic capture and board layout package available for no cost from Advanced Circuits (if you would like the schematics or PCB layout in another form please contact Prof. Erik Spjut). The boards were fabricated by Advanced Circuits.There were four boards in total:

The accelerometers were arranged so that the ADXL78 measured acceleration along the axis of the rocket, and the ADXL320 measured the acceleration in two orthogonal radial directions. There were two IDG300s for angular rate measurements. They were arranged to provide redundant measurements along the axis of the rocket and single measurements along the same two orthogonal radial directions as the ADXL320. The main board also contained the altimeter pressure sensor (MPXAZ4115A) and the conditioning circuits for the thermistors (voltage divider followd by an op-amp buffer), the Pitot-static remote board, and the on-board sensors. The output levels were all in the 0 to 5 V range for interfacing with the R-DAS Tiny's 6 analog inputs.


Table 1 – Gen 1 IMU Major Parts
Part Description Part Number Manufacturer Voltage Req Current Req (mA) Physical Input Output Range Bandwidth or Response Rate
High-g MEMS accelerometer AD22279-A-R2 (ADXL78) Analog Devices 5 2 Accel. ±35 g 0.575 to 4.425 V 400 Hz
Dual axis accelerometer ADXL320 Analog Devices 3 to 5 1 Accel. ±5 g 0.6 to 2.4 V 2500 Hz
Dual-axis rate gyro IDG300 InvenSense 3.0 to 3.3 10 Ang. Vel. ±500°/s 0.5 to 2.5 V 140 Hz
Pressure transducer MPXAZ4115A Freescale 5 7 Pressure 15 to 115 kPa 0.2 to 4.8 V 1 ms
Diff. pressure transducer MPXV5050GP Freescale 5 7 Pressure 0 to 50 kPa 0.2 to 4.7 V 1 ms
Thermistors ATH10KR8 Analog Technologies     Temperature 10 kΩ nominal <1 s
Thermistors NTSD1XH103FPB50 Murata     Temperature 10 kΩ nominal <10 s
Precision quad op-amp AD8608 Analog Devices 5 1 Voltage 0 to 5 0.02 to 4.98 V 10 MHz
Precision 3 V reference REF193 Analog Devices 4 to 15 0.045 N/A 3.00 V N/A
Precision 5 V reference REF195 Analog Devices 6 to 15 0.045 N/A 5.00 V N/A

The company contacted to populate the main boards and the rate gyro daughter boards did a poor job even after repeated resubmissions for repair. Our final yield was two fully-functioning IMUs and six IMUs with one or more sensors malfunctioning.

Gen 2 IMU

The Generation 2 IMU is going to be part of the integrated avionics section. The new IMU board will be similar to the Gen 1 board but with a slightly different mix of sensors to save space and eliminate the two daughterboards. The new parts list (still under revision) is in Table 2. The dual axis accelerometer is being replaced with a triple axis accelerometer to get the direction normal to the chip, and there will be redundant measurements along the axis of the rocket, but with two different maximum ranges. In addition, one of the IDG300s is being replaced with a single-axis rate gyro with the axis normal to the plane of the chip.

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