Abstract
The availability of multiple inputs (plants) can improve output performance by conveniently allocating the control bandwidth among them. Beyond that, the intervention of only the useful plants at each frequency implies the minimum control action at each input. Secondly, in single input control, the addition of feedforward loops from measurable external inputs has been demonstrated to reduce the amount of feedback and, subsequently, palliate its sideband effects of noise amplification. Thus, one part of the action calculated by feedback is now provided by feedforward. This paper takes advantage of both facts for the problem of robust rejection of measurable disturbances by employing a set of control inputs; a previous work did the same for the case of robust reference tracking. Then, a control architecture is provided that includes feedforward elements from the measurable disturbance to each control input and feedback control elements that link the output error to each control input. A methodology is developed for the robust design of the named control elements that distribute the control bandwidth among the cheapest inputs and simultaneously assures the prescribed output performance to correct the disturbed output for a set of possible plant cases (model uncertainty). The minimum necessary feedback gains are used to fight plant uncertainties at the control bandwidth, while feedforward gains achieve the nominal output response. Quantitative feedback theory (QFT) principles are employed. An example illustrates the method and its benefits versus a control architecture with only feedback control elements, which have much more gain beyond the control bandwidth than when feedforward is employed.
Highlights
Uncertainties such as nonmeasurable disturbances or unavoidable simplifications in plant modelling justify feedback control loops, which, by permanently supervising the output, can correct its deviation from the reference or track reference changes
Better performances of the output response are linked to larger control bandwidths, which are provided by larger gains of feedback controllers
The control action is univocally conditioned by the plant frequency response and the desired performance, but its convenient allocation between feedback and feedforward control actions can prevent excessive sensor noise amplification linked with feedback gain
Summary
Uncertainties such as nonmeasurable disturbances or unavoidable simplifications in plant modelling justify feedback control loops, which, by permanently supervising the output, can correct its deviation from the reference or track reference changes. The control action is univocally conditioned by the plant frequency response and the desired performance, but its convenient allocation between feedback and feedforward control actions can prevent excessive sensor noise amplification linked with feedback gain. These facts, which are common knowledge in single-input control, have not yet been fully exploited in multi-input control. In [10,11], robust design methods of the feedback controllers distributed the frequency band among the most favourable plants to minimise the control action at each input (any number of inputs were possible) while achieving the desired performance of the output response. Remarkable application fields include bioprocesses [14,15], thermal systems [18], medical systems [19,20], scanner imaging [21,22], massive data storage devices [23,24], fuel engines [25] and electrical vehicles [26], robotics [27,28,29], and unmanned aerial vehicles [30]
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