Design and implementation of a wheel speed measurement circuit using field programmable gate arrays in a spacecraft

Wheels are used for maintaining attitude of a spacecraft (S/C). An attitude and orbit control computer (AOCC) receives the current state of the S/C from various sensors and actuators and determines the control torques to be applied on the S/C for stability and proper maintenance of attitude. Resulting from various disturbances acting on the spacecraft in orbit, then will be a build up in attitude errors. The AOCC applies a correcting torque to the S/C by changing the wheel speed according to a pre-specified control algorithm to reduce attitude error. Depending on the S/C mission, the wheel speeds can vary from very low speed ( < 100 rpm) to very high speeds ( > 6000 rpm). So, accurate measurement of wheel speed is important for attitude control of a S/C. As field programmable gate arrays (FPGAs) are leading to compact realization of digital logic, they an finding increasing applications in spacecraft electronics which is guided by an overriding concern to minimize weight and volume. In this article, we discuss an FPGA based wheel speed measurement circuit and present the various results. (C) 1999 Elsevier Science B.V.