Abstract: Analysis of a linear time-invariant system's sampled output data can identify the unknown continuous-time transfer function of the system. The method excites the system with two orthogonal sinusoids and constructs a linear complex equation that relates the sampled output points to the unknown values of the transfer function. This method can identify the continuous-time dynamics of a hard disk drive system in which the number of servo bursts and the spinning speed of the disk fix the sampling frequency of the output. In the hard disk case the input excitation signal is passed through a specially designed pre-filter and the sampled output data are modified to account for the control input and external disturbances. The method can also be used to reconstruct the output of the system even in the presence of aliasing provided the input excitation can be approximated by a sum of sinusoids with known frequencies.

Abstract: A servo control apparatus and method controls systems at least partially on the basis of an observable variable that has an absolute value functional relationship with the controlled variable and does not change sign for positive and negative variations from a nominal value. When applied to the positional control of an object, the control system observes a value of a position error signal and maps that signal to two different potentially correct displacement values. Two estimators within the control system are initiated, one using the positive displacement and the other using the negative displacement, and the two estimators each predict the future position of the object and the corresponding position error signal for each estimated position. A new position error signal is detected and compared to the two estimated position error signals.

Abstract: A servo control apparatus and method controls systems at least partially on the basis of an observable variable that has an absolute value functional relationship with the controlled variable and does not change sign for positive and negative variations from a nominal value. When applied to the positional control of an object, the control system observes a value of a position error signal and maps that signal to two different potentially correct displacement values. Two estimators within the control system are initiated, one using the positive displacement and the other using the negative displacement, and the two estimators each predict the future position of the object and the corresponding position error signal for each estimated position. A new position error signal is detected and compared to the two estimated position error signals.