General model of intermittent control.

<p>The intermittent predictive controller is based on continuous control as a special case <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Gawthrop1" target="_blank">[12]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Loram6" target="_blank">[36]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Gawthrop2" target="_blank">[43]</a>, but generally the predicted system state is only used intermittently to update the time varying control signal sent from the generalized “hold” to the actuator. “Trig.” detects when the control trajectory is to be updated and this event trigger requires three conditions: (i) a single event must be detected (i.e. all events within the sampling delay (Δs) are considered as one), (ii) a minimum open-loop interval (Δ_OL) must have elapsed since the previous event and (iii) an error signal must exceed a threshold <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Gawthrop1" target="_blank">[12]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Loram6" target="_blank">[36]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Gawthrop2" target="_blank">[43]</a>. Scalar signals are represented by solid lines, vector signals are represented by dashed lines. The participant's neuro-muscular dynamics are modelled (linear) in the “NMS” block with input <i>u</i>(t). The linear external controlled system with output <i>y</i>(t) (represented by the “System” block) is driven by signals <i>u</i>_e(t) and <i>d</i>(t) representing the externally observed control signal and the disturbance signal. The state of the composite “NMS” and “System” blocks is estimated <i>x_o</i>(t) by the “observer” block. Sampling is preceded by an anti-aliasing low-pass filter “LP” of the subtracted set point disturbance <i>w</i>(t) and subject to an event delay “Δ_S” between event and sampling. The trigger for the sampling times <i>t</i>_i is provided by the event detector block labelled “trig”. Sampling <i>x</i>_w(t) takes place at discrete times ti. Sampled signals (represented by the dotted lines) are defined only at the sample instants <i>t</i>_i. The future state error <i>x</i>_p(ti) is provided by the “predictor” block. The various delays in the human controller are accounted for by a pure time delay of <i>t</i>_d represented by the “delay” block. The block labelled “hold” is a system-matched hold, that provides the delayed version of the continuous-time signal that is multiplied by the feedback gain vector <i>k</i> (block “State FB”) to give the feedback control signal <i>u</i>(t). This figure and its caption are based on <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002843#pcbi.1002843-Gawthrop1" target="_blank">[12]</a>.