A Control Moment Gyro for Dynamic Attitude Control of Small Satellites

Control Moment Gyroscopes (CMGs) are not often considered for use on small satellites and, as a result few small satellite missions have implemented CMGs as on-board actuators. There are many reasons for this, but mainly this is due the complexity of the mechanical and control system needed to implement an effective CMG, and also because off-the-shelf CMG systems are generally made for the larger satellite market. .CMGs offer many advantages over reaction wheel systems. When used on a small satellite, a CMG based control system can provide the ability to perform very fast slew maneuvers, making CMGs very attractive to high resolution small satellite imaging missions. The CMG described in this paper incorporates two motors; a Brushless DC Motor (BLDC) and a Stepper Motor. The BLDC provides an efficient means of driving the inertia disk to store the angular momentum, whilst the stepper motor provides precision gimbal control. In order to keep mass and power consumption low, both motors are controlled from a single FPGA. The FPGA runs all associated commutation, speed and position control for both motors and also provides the command and telemetry interface to the rest of the spacecraft. The resulting system is a compact, power efficient design that is ideal for small satellites. INTRODUCTION With advances in computer aided design tools and electronics technologies, there are now opportunities to make feasible previously overly challenging concepts. One of these concepts is the use of Control Moment Gyros (CMGs) on small satellites. Up until recently, CMGs have only been used to control the attitude of large spacecraft and space infrastructure such as the International Space Station. However, it is now possible to develop effective, small CMGs for use on small satellites. Large torque amplification and momentum storage capacity are the two basic properties that make control moment gyroscopes (CMGs) superior when compared to the reaction and momentum wheels. Compared with reaction wheels, CMGs are relatively lightweight and they have a capability to generate higher torque levels per unit kg. This makes CMGs a very promising actuator for future small satellite missions. This paper describes the development of a single gimbal CMG, designed specifically for small satellite missions. The CMG development was led by Turkish Aerospace Industries with Clyde pace Ltd as a contractor to develop the electronics. This paper describes the control and interface electronics, demonstrating how the use of relatively new technologies to space, such as FPGAs, can enable more control to be performed locally, without incurring the mass and volume penalties which would normally preclude these control techniques being used on small satellites.