Journal papers by Gawthrop in 2010

[1] Peter J. Gawthrop and Liuping Wang. Intermittent redesign of continuous controllers. International Journal of Control, 83:1581--1594, 2010. [ bib | DOI ]
The reverse-engineering idea developed by Maciejowski in the context of model-based predictive control is applied to the redesign of continuous-time compensators as intermittent controllers. Not only does this give a way of designing constrained input and state versions of continuous-time compensators but also provides a method for turning continuous-time compensators into event-driven versions. The procedure is illustrated by three examples: an event-driven PID controller relevant to the human balance control problem, a constrained version of the classical mechanical vibration absorber of den Hartog and an event driven and constrained vibration absorber.

[2] Ian David Loram, Henrik Gollee, Martin Lakie, and Peter Gawthrop. Human control of an inverted pendulum: Is continuous control necessary? Is intermittent control effective? Is intermittent control physiological? The Journal of Physiology, 589:307--324, 2011. Published online November 22, 2010. [ bib | DOI ]
Human motor control is often explained in terms of engineering ″servo″ theory. Recently, continuous, optimal control using internal models has emerged as a leading paradigm for voluntary movement. However, these engineering paradigms are designed for high bandwidth, inflexible, consistent systems whereas human control is low bandwidth and flexible using noisy sensors and actuators. By contrast, engineering intermittent control was designed for bandwidth-limited applications. Our general interest is whether intermittent rather than continuous control is generic to human motor control. Currently, it would be assumed that continuous control is the superior and physiologically natural choice for controlling unstable loads, for example as required for maintaining human balance. Using visual manual tracking of an unstable load, we show that control using gentle, intermittent taps is entirely natural and effective. The gentle tapping method resulted in slightly superior position control and velocity minimisation, a reduced feedback time delay, greater robustness to changing actuator gain and equal or greater linearity with respect to the external disturbance. Control was possible with a median contact rate of 0.8±0.3 s-1. However, when optimising position or velocity regulation, a modal contact rate of 2s-1 was observed. This modal rate was consistent with insignificant disturbance-joystick coherence beyond 1-2 Hz in both tapping and continuous contact methods. For this load, these results demonstrate a motor control process of serial ballistic trajectories limited to an optimum rate of 2 s-1. Consistent with theoretical reasoning, our results suggest that intermittent open loop action is a natural consequence of human physiology.

[3] Liuping Wang, Peter Gawthrop, David. H. Owens, and Eric Rogers. Switched linear model predictive controllers for periodic exogenous signals. International Journal of Control, 83:848--861, 2010. [ bib | DOI ]
This article develops switched linear controllers for periodic exogenous signals using the framework of a continuous-time model predictive control. In this framework, the control signal is generated by an algorithm that uses receding horizon control principle with an on-line optimisation scheme that permits inclusion of operational constraints. Unlike traditional repetitive controllers, applying this method in the form of switched linear controllers ensures bumpless transfer from one controller to another. Simulation studies are included to demonstrate the efficacy of the design with or without hard constraints.

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