Inexpensive actuators are often more rugged than costly alternatives, a particularly attractive feature for military applications or operations in harsh environments. A common drawback of inexpensive actuation is variability in response that contributes to tracking error. This can be ameliorated without substantial expense by the application of large feedback that not only enhances tracking performance and rejects disturbances, but also reduces the sensitivity to parameter variations of systems in the forward path. However, component imperfections in the inexpensive actuator often result in behaviors more parlous than a slow shift in frequency response modulus for which feedback easily compensates in the form of multiple, uncertain nonlinearities. While standard mild-feedback systems are intrinsically robust to such nonlinearities, the stability of large feedback systems is threatened by such characteristics. This eliminates from consideration the usage of imperfect actuators in linear, large feedback applications. The implementation of multiple-path nonlinear dynamic compensation, however, allows this combination. Such a strategy is the focus of this discussion. The features of the staged loop recovery system with reduction compensation in the quiescent condition are compared to those of an alternative approach of strict modulus reduction via nonlinear compensation for stability retention. The superiority of the loop recovery system is illustrated using closed-loop data taken during experiments on a parallel kinematic machine using inexpensive actuation.
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