Rotational Displacement


Rotational displacement of the platform (read : roll, pitch, and yaw) is a necessary aspect of motion to measure in order to determine where the platform is "looking." To accomplish this task, an intertial measurement unit was selected, specifically, the 9DOF Razer IMU from sparkfun. A sparkfun product was selected based on previous positive experiances with community and company support. The interial measurement unit included a suite of sensors, each 3-axis, that included accerlerometers, rate gyroscopes, and magnetometers. From this information, however, we needed compute actual angular position, necessiating the conversion of the IMU to a Altitude, Heading and Reference System (AHRS), with the design of having the IMU output roll, pitch, yaw, and a mangetic heading. As the IMU had an on-board Atmel processor to read the sensnors and output the measured data, we had to implement new firmware. The AHRS reference code proved to be a major design hurdle, as the necessary computations were complex and unfamiliar. Nevertheless, sufficient open-source example code lead to the development of functional firmware for the IMU, enabling it to output roll, pitch , yaw, and magnetic heading.

Currently, the at this point in development, a problem with accurate data from the IMU has arisen that was not foreseen in the initial system development. That is, the IMU is mounted in close proximity to the stepper motors, which generate strong magnetic fields during operation. Necessarily, in order to compensate for magnetic drift, the IMU has a magnetometer to produce accurate heading data, which is becoming distorted when the IMU is mounted in its final position. Current plans to get around this issue are to case the motors is mu-metal, a compound with a very high magnetic permeability to shield the IMU from the effects of the motors. Initial attempts to rid ourselve sof this issue through hard and soft iron compensation proved ineffective due to the transisent nature and non-linear nature of the fields being generated. In short, the low-frequency magnetic field must be directed or attenuated away, or a single-axis, true interial reckoning system will need be implemented, which, lacking a concrete reference, is much less accurate than the IMU over long periods of time.

Linear Displacement

Linear displacment, measured from a GPS, has so far been unimplemented, though hardware suitable for the task has been selected. The desire for the project to track near-earth objects has expanded the requirements of accuracy on linear displacement, with the GPS unit under consideration no longer appropiate. In order to track near-earth objects, the GPS unit will require centimeter level accuracy, achievable through a waver (military grade equipment), or further integration work with the IMU (not sure if possible.)