Using an oversample technique to improve servo control performance

An analog-to-digital converter (DAC) is used to convert the calculated control value to a voltage signal in digital control systems. A DAC is considered as a zero-order hold function (ZOH). In general, digital control systems suffer a phase lag which originates from the ZOH. This phase loss often forces control engineers to either limit a control system's bandwidth or otherwise compromise the system's stability. In some other cases, more expensive equipment is added to increase the sample frequency in order to gain satisfactory performance. For many commercial applications, this alternative approach is not feasible. This article discusses three servo control schemes that use the simple proportional-integral-derivative (PID) control algorithm to reduce the phase loss and improve the bandwidth of digital servo systems. The study is done for disk drives but is also applicable to other industrial applications where cost and performance are concerned.