Disk drive capable of autonomously evaluating and adapting the frequency response of its servo control system

In a disk drive comprising a sampled servo feedback control system having a sampling frequency and including (1) a plant which has a nominal plant frequency response and (2) a servo controller which comprises (i) a compensator with a nominal compensator frequency response and (ii) a gain element with a nominal open-loop gain, the compensator and gain element being designed to define the overall sampled servo feedback control system with a desired open-loop bandwidth frequency, a desired gain margin, and a desired phase margin which collectively provide for stable closed-loop operation given the nominal plant frequency response, a method of adaptively self-modifying the servo controller to compensate for deviations from the nominal plant frequency response owing to individual plant variations, the method comprising the steps of:

• autonomously determining a phase crossover frequency fPC by (1) measuring the sampled servo feedback control system's open-loop phase response to sinusoidal inputs at a plurality of discrete frequencies and (2) determining which of the plurality of discrete frequencies is associated with an open-loop phase response that is substantially equal to −180 degrees;
• autonomously determining a measured gain margin by measuring the sampled servo feedback control system's open-loop magnitude response to a sinusoidal input at the phase cross-over frequency fPC;
• adjusting the open-loop gain of the servo controller's gain element relative to the nominal open-loop gain if the measured gain margin is different than the desired gain margin to provide an adjusted gain margin that is substantially equal to the desired gain margin;
• and
• adjusting the compensator frequency response relative to the nominal compensator frequency response to compensate for the phase margin being moved relative to the desired phase margin by the step of adjusting the open loop gain and to provide an adjusted phase margin that is substantially equal to the desired phase margin.

In a disk drive having a sampled servo system having a sampling rate and a nominal bandwidth, wherein the sampled servo system comprises a plant and a servo controller that controls the plant using a compensator and a gain element with a nominal open loop gain, a method for adaptively modifying the servo controller to compensate for plant variations which are incompatible with the nominal gain and bandwidth comprising the steps of:

• implementing a self-generated bode plot to determine a gain margin and a phase margin, and if the gain margin is not greater than a predetermined minimum:
• adjusting the open loop gain of the servo controller to provide a gain margin which is greater than the predetermined minimum at a bandwidth which is different than the nominal bandwidth;
• and
• adjusting the compensator to provide a phase margin which is greater than a predetermined minimum.