Brian J. Hamilton

Momentum-Control System Array Architectures

This chapter provides the analysis tools and fundamental theory for the design of an array architecture consisting of momentum devices. First, the properties of the actuator alignments and their effect on shaping the performance envelope of the momentum-control system are discussed. A survey of common array types for RWA, CMG, and mixed arrays follows. A discussion of performance metrics and methods used to optimize the array architecture concludes the chapter.

Requirements Development for Momentum Control Systems

This chapter addresses the subject of how to flow spacecraft agility requirements down to an array of momentum devices and provides guidelines for choosing the appropriate technology. We shall see that the design trades in selecting and sizing momentum devices span orders of magnitude in performance, complexity, and cost. Therefore, to arrive at an efficient and cost-effective design, it is important not to over-specify the momentum-system requirements.

Dynamics of Momentum-Control Systems

It is not enough to treat the momentum system as a black box, simply receiving power and control signals and imparting torque or momentum, as if it were any other element in a feedback-control block diagram. A momentum system offers unique opportunities to achieve robust and lightweight spacecraft designs, but taking advantage of these opportunities requires careful attention to rigid- and flexible-body dynamics. This chapter provides the foundational equations of motion for spacecraft with momentum-control devices and summarizes important flexible effects that drive the design of spacecraft that incorporate these actuators.

Inner-Loop Control of Momentum Devices

Implicit in a discussion of how to command the devices in momentum control system is the assumption that the devices faithfully track those commands. And yet, momentum devices exhibit several significant nonlinearities and can experience significant disturbances and errors. For this reason, momentum devices almost always include “inner loops” applying feedback control to the quantities of interest to ensure that those commands are tracked.

Modeling Simulation and Test Beds

Ground-based validation of spaceborne momentum control systems and the attitude control systems that depend upon them offers a unique set of challenges. Computer simulations must include a variety of nonlinear phenomena found in momentum devices, which fundamentally limits the way these simulations can be architected. And because momentum systems rely on the conservation of angular momentum for proper operation, hardware-in-the-loop test facilities must reproduce the dynamics of a free body with great precision.

Motors in Space

All momentum devices require a spin motor of some type to actuate the rotor spin axis. CMGs also require a motor on the gimbal axis. The torque and speed requirements for the gimbal motor design are typically much different from those for the rotor. These issues, among others, motivate the following discussion of motors commonly employed in spaceborne momentum devices. The objective of this chapter is to familiarize the reader with some of the key considerations with the goal of informing the analysis, implementation, and operation of momentum-control systems and the spacecraft that use them. Motor design is a broad specialty field that demands much more depth than offered here.

Singularities of Control Moment Gyroscopes

A book on spacecraft momentum control systems would not be complete without a discussion of geometric singularities inherent in CMGs. Therefore, this chapter starts with a discussion of common singularities associated with DGCMGs (e.g., gimbal lock) and then moves on to the more troublesome singularities associated SGCMGs. The mathematical structure of these singularities and how it relates to difficulties in avoiding them is also discussed in a way made more accessible to the reader not familiar with the subject matter. Also discussed is the location of singularities associated with an array of CMGs by way of a three-dimensional surface in the momentum space. This chapter concludes with some brief discussion on techniques to perform zero-momentum spin up of an array of CMGs in the presence of singularities at zero momentum.