Practical implementation issues for active control of large flexible structures

Abstract

Most active control strategies, whether designed in the discrete or continuous domain, will most likely be implemented using a digital control system. Therefore, it is important to study the effects of digital implementation on the desired control law. In this work, the effect of quantization due to the finite wordlength of microprocessors, analog-to-digital, and digital-to-analog I Introduction The next generation of spacecraft being investigated are much larger than current spacecraft. In consideration of the cost of transporting material into space, these spacecraft will most likely be lightweight, relatively flexible, and lightly damped. Spacecraft performance requirements, such as antenna shape control and pointing accuracy, demand that these flexible structures should not vibrate significantly due to external disturbances and pointing maneuvers. Although the passive damping found in these structures may be an important dissipative mechanism, the need to meet stringent spacecraft performance criteria may require the use of active vibration control Numerous control strategies for vibration control of Large Flexible Structures (LFS) have been proposed (Balas, 1982). These control strategies range from local rate-feedback control to more complicated controls, such as adaptive control (Auburn, 1978; Bar-Kana et al., 1983). Most active control strategies, whether designed in the discrete or continuous domain, will most likely be implemented using a digital control system. It is then important to study the effects of digital implementation on the desired control law. The most studied issue has been the effect of the sample period on the implementation of the control law. However, there are other practical issues that have not received much attention. These issues include the effects of microprocessor, analog-to-digital (A/D) converter and digital-to-analog (D/A) converter wordlength and the resulting quantization errors. The implementation of a control law often involves the differentiation and/or integration of sensor measurements. The accuracy of these calculations is influenced by both the sample period and wordlength of the digital controller. Historically, the sample frequency for active digital control has been relatively slow. For example, the hardware used by Finally, an active control experiment is reported on which utilizes the previously developed guidelines for control law implementation. The test structure is a cantilevered beam with a space-realizable force actuator attached at the tip II Digital Differentiation The explicit or implicit differentiation of sensor signals may be required to implement a given control law. For example, position signals may be differentiated to implement a rate feedback control law. This same control law could be implemented by integrating acceleration signals. One disadvantage of differentiation is the amplification of signal noise; an advantage is that differentiation avoids the DC bias problem typically encountered with integration. In this section, the effects of quantization, sampling period, and signal amplitude and frequency content on the results of digital differentiation are investigated. Real-time differentiation of a signal, f(t), can be approximated using a two-point backward difference approximation