Sensors

Goals

• Relays raw accelerometer and gyroscope data to controller board on CAN Bus

Grades and performance categories

The grade and performance list is organized from high-end expensive IMU systems that provide Inertial Navigation Systems to low-end accelerometer and gyroscope configurations:

• Marine and Navigation
• Tactical
• Industrial
• Automotive & Consumer

There are two grades applicable to our need: Automotive, and Industrial.

Automotive vs Industrial

Automotive grade sensors are typically sold as separate accelerometers and gryoscopes, so it is possible to combine many accelerometers and gyroscopes from different manufactures into one IMU. The main difference between an Automotive grade and Industrial grade sensor is in the sensor calibration. Without good calibration, the automotive grade sensor will create large errors in angular velocity rates and orientation data. Automotive grade sensors are not good for position tracking even with the aid of a GPS unit, due to its position drift errors. With good calibration it can be useful for angular velocity, and pitch/yaw/roll measurements.

IMU Measurement errors

The errors specific to angular velocity and orientation measurements are listed below:

Dynamic errors due to gyro scale factor

This is the main concern for using the automotive grade sensor in tracking the orientation of the car. The gyro scale factor correctly scales the measurement during the motion, and if the scale factor is off significant errors after the motion will be present. The automotive grade sensor is useless in inertial navigation systems and in tracking orientation without rigiourous and precise calibration. With proper calibration, it can perform basic navigation techniques. 

Dynamic errors due to gyro misalignment

This is the main concern for using an automotive grade sensor in tracking angular velocity. Any gryo misalignment will skew the angular velocity in all coordinate directions. Rotating the sensor in one axis for 360 degrees and placing it down will result in angular rates being recorded in the all other axis that are misaligned, even when there was no change in that axis.

Static errors due to accelerometer bias

The errors from accelerometer bias arise in automotive grade sensors due to imprecise calibration. The bias changes the measured acceleration vector to an incorrect direction, and after subracting the gravitational acceleration the resulting acceleration will result in errors in the pitch and roll.

For industrial grade sensors, these errors are due to temperature changes.

Static errors due to accelerometer misalignment

These are removed after a tumble test during the calibration procedure.The magnitude of error is of no concern here.

Static errors due to accelerometer scale factor 

The error in orientation is due to remaning quantities after the calibration process. A scale factor error in one axis will stretch a vector in that dimension, affecting the measured angle.

Background and strategic fit

The sensor board is used to obtain data and diagnostics of the motion of the car (i.e. pitch, yaw, roll). Additional features of the board are to be decided, based on what other metrics or data, such as humidity or temperature, is useful to have in the car. The sensors board will be connected to the main CAN Bus.

Assumptions

Requirements