Nonlinear Servoanalysis of Human Lens Accommodation

Abstract
A servoanalysis of the feedback control system for lens accommodation has been based upon experimental evidence designed to explore the nonlinear properties. Because these nonlinearities are essential to the functioning of the system, a describing function approach was employed. Computation served to obtain open-loop characteristics. The transfer function of the linear part is F( s) = 4 ( 1 + 0.15s) e-0." s ( 1 + 2 [ 0.3( 0.08)] + ( 0.08s) 2) ( 4) and some assignments of portions of this equation to the physical elements of the lens system are suggested. The no-memory nonlinearity has general saturation characteristics; two related models, one with quite unusual aspects, are presented, and independent evidence for these adduced. The combined model accounts for large and small signal responses, stability characteristics, predicts certain noise spectral features of the openand closed-loop lens system. Phase discrepancies, possible sampled data properties, and the even-error-signal operator put forward crucial new experiments.

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