Operational basis for crossover model conformance during compensatory tracking

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Conference Proceeding

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AIAA Scitech 2021 Forum



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The Crossover Model (CM) describes operator response over a broad range of conditions, but there is little justification for why the human adopts the CM other than this conforms to sound servo mechanical design. Human-in-the-loop operation introduces two phenomena that are absent from automatic control: 1) Substantial and varying time delay; 2) Dynamic variations in compensation due to tradeoff between mental workload and performance. The author has previously demonstrated that compensatory tracking workload was related to the product of error rate and stick rate, and that the mathematical equivalence of this product is error rate shaped by pilot compensation. The current work demonstrates that this equivalence is invariant to the forcing function when the operator provides compensation such that the open-loop system roughly behaves like an integrator in the vicinity of the crossover frequency. A technical approach called the Equivalent Crossover Model System (ECMS) is employed allowing accurate measurement of open-loop (display-pilot-vehicle), magnitude ratio slope at, phase margin (PM), and the first and second derivatives of PM. ECMS does not presume a form for the pilot or vehicle dynamics, rather it models the aggregate open-loop response using the Crossover Model (CM) modified with a lead-lag term. A skilled pilot will primarily adjust workload and performance by changing, which in turn affects open-loop phase margin (PM). Previous to this work there has been little quantitative validation of the inferentially based assumption that handling qualities are heavily influenced by open-loop PM behavior. With ECMS it is shown that operator workload is highly correlated to the complexity of PM slope with respect to frequency, as well as its derivative. This complexity depends on operator compensation and vehicle dynamics in the vicinity of and highlights the importance of designing vehicle and display dynamics that require minimal compensation by the operator.


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