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Abstract
In studies of human balance, it is common to fit stimulusresponse data by tuning the timedelay and gain parameters of a simple delayed feedback model. Many interpret this fitted model, a simple delayed feedback model, as evidence that predictive processes are not required to explain existing data on standing balance. However, two questions lead us to doubt this approach. First, does fitting a delayed feedback model lead to reliable estimates of the timedelay? Second, can a nonpredictive controller provide an explanation compatible with the independently estimated time delay? For methodological and experimental clarity, we study human balancing of a simulated inverted pendulum via joystick and screen. A twostep approach to data analysis is used: firstly a nonparametric modelthe closedloop impulse responseis estimated from the experimental data; second, a parametric model is fitted to the nonparametric impulseresponse by adjusting timedelay and controller parameters. To support the second step, a new explicit formula relating controller parameters to closedloop impulse response is derived. Two classes of controller are investigated within a common statespace context: nonpredictive and predictive. It is found that the timedelay estimate arising from the second step is strongly dependent on which controller class is assumed; in particular, the nonpredictive control assumption leads to timedelay estimates that are smaller than those arising from the predictive assumption. Moreover, the timedelays estimated using the nonpredictive control assumption are not consistent with a lowerbound on the timedelay of the nonparametric model whereas the corresponding predictive result is consistent. Thus while the goodness of fit only marginally favoured predictive over nonpredictive control, if we add the additional constraint that the model must reproduce the nonparametric time delay, then the nonpredictive control model fails. We conclude (1) the timedelay should be estimated independently of fitting a low order parametric model, (2) that balance of the simulated inverted pendulum could not be explained by the nonpredictive control model and (3) that predictive control provided a better explanation than nonpredictive control.
Original language  English 

Pages (fromto)  131146 
Number of pages  16 
Journal  Biological Cybernetics 
Volume  101 
Issue number  2 
DOIs  
Publication status  Published  1 Aug 2009 
Keywords
 Human balancing
 System identification
 Predictive control
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 1 Finished

Intermittent Predictive Control of Man and Machine
Lakie, M.
Engineering & Physical Science Research Council
15/07/09 → 14/07/12
Project: Research Councils