Direct digital measurement of loop gain and phase in digital control systems

A method is presented for obtaining loop gain and phase measurements from digital control systems. Using a serial interface to a personal computer, samples of a variable frequency sine wave may be added to any signal within the digital controller. Resulting steady state loop disturbances in the outward and return directions can then be correlated using a discrete Fourier transform based method to obtain accurate gain and phase responses. This method is particularly suitable for PWM based digital control systems, where signals are not always smooth and well behaved. Results obtained in this way are presented from a digital current mode controlled current-fed inverter.