Showing papers by "N.H. McClamroch published in 2001"

Development of air spindle and triaxial air bearing testbeds for spacecraft dynamics and control experiments

Abstract: The dynamics and control of spacecraft have been widely studied because of their technological significance. In the classical case the spacecraft is assumed to consist of a single rigid body with three-axis torque inputs and with attitude and rate sensing. In practice, however, the situation may be far more complex. For example, any component of the spacecraft that deforms relative to other components will entail a change in the spacecraft mass distribution; in effect, the spacecraft becomes a multibody system. Similarly, structural flexibility and fuel slosh give rise to vibrational degrees of freedom. Spacecraft control is also exacerbated by sensor and actuator nonlinearities. Traditional actuation devices such as thrusters, reaction wheels, momentum wheels, and control moment gyros entail amplitude and rate saturation constraints, gyroscopic coupling, and coupling between translational and attitude dynamics. Additional difficulties arise when accounting for gravitational effects and external disturbances. All of these issues have technological implications. A fundamental difficulty associated with spacecraft technology is the fact that ground-based testing must occur in a 1-g environment whereas the hardware will operate under zero-g conditions. Consequently, spacecraft control engineering must depend on first-principles analysis as well as extrapolation from 1-g testing. The purpose of this paper is to describe a laboratory-based testbed that will be used to explore various issues and concepts in spacecraft dynamics and control. This testbed is based on a triaxial air bearing to allow experiments involving large-angle, three-axis motion. As a precursor to this testbed, we have also developed an air spindle testbed which allows single-axis rotation.

Equations of motion for the triaxial attitude control testbed

Abstract: The triaxial attitude control testbed has been developed as part of a research program on spacecraft multibody rotational dynamics and control. In this paper, equations of motion are derived and presented in various forms. Actuation mechanisms are incorporated into the models including: moment actuators that are fixed to the triaxial base body, as well as reaction wheel actuators and proof mass actuators that are fixed to the triaxial base body. The models also allow incorporation of unactuated auxiliary bodies that are constrained to move relative to the triaxial base body. The models expose the dynamic coupling between the rotational motion of the triaxial base body, the relative or shape motion of the auxiliary degrees of freedom, and dynamics associated with actuation mechanisms.

Translational and rotational spacecraft maneuvers via shape change actuators

Abstract: This paper treats the simultaneous control of spacecraft translational and rotational maneuvers in a fixed plane using linear proof mass actuators. A crucial assumption is that the total linear and angular momenta are zero. We first study a fully actuated spacecraft using three actuators and then we study an underactuated spacecraft using two actuators. In both cases, appropriate equations of motion axe derived. For the fully actuated case, it is shown that feedback linearization can be used to achieve arbitrary base body maneuvers under a mild geometric assumption on the three slots. For the underactuated case, nonlinear control theory is used to show that the base body translation and rotation is controllable via two independent proof mass actuators; an open loop maneuver strategy is developed. Examples are given to demonstrate several planar maneuvers.

Sensitivity and robustness properties in the preview control of linear non-minimum phase plants

Abstract: Non-minimum phase zeros emit the achievable performance and robustness of a feedback control loop. These limitations can be expressed as an upper limit on the bandwidth that may be achieved without necessarily incurring a large peak in closed loop sensitivity. In the case of preview control, it is known that advance (or anti-causal) knowledge of the reference to be tracked permits accurate, rapid tracking over bandwidths which exceed the normal performance limits. The first aim of this paper is to explain this apparent paradox. In addition, we briefly discuss sensitivity and robustness issues for preview control systems.

Air spindle attitude control via proof mass actuators

Abstract: Attitude control of a single-axis air spindle testbed using two proof mass actuators is studied by means of analytical models and laboratory experiments. A key feature of the air spindle testbed is that ideally there is no external moment; hence the total angular momentum can be assumed to be conserved. After a brief hardware description of the air spindle testbed and proof mass actuators, the equations of motion are derived, assuming the total angular momentum is zero. It is shown that arbitrary platform reorientation can be achieved using periodic proof mass motions. Two proof mass actuator control strategies are constructed based on piecewise constant motions and sinusoidal motions. Experimental results are summarized and are compared with simulation results.