Self-oscillation method for measuring transfer functions

THE person analyzing or synthesizing a closed-loop control system often has need of sinusoidal transfer function data. If the data are to be obtained directly, ordinarily a measurement is made of both phase and magnitude of the sinusoidal response to a sinusoidal driving force. A low variable-frequency generator of sufficient power is required, as well as a means of measuring phase and magnitude of the sinusoidal response quantity. The phase-angle measurement is often troublesome and rather inaccurate at the low frequencies which are commonly employed in the closed-loop control system. Sometimes nonlinearities further aggravate the problem.