Today’s disk drives use closed-loop, digital control systems based on position information that is encoded onto the disks during the manufacturing process. A variety of analysis and design techniques are used to achieve the performance and stability criteria of the feedback control system, and to predict the required characteristics of the control systems for future products.

For more information, please see the attached tutorial. A basic introduction to feedback control systems is presented, and in particular, how they are implemented in HDDs. The implications of open loop, closed loop, and error rejection transfer functions are explained. Commonly used terms such as bandwidth, gain margin, and phase margin are defined, and their limiting factors are discussed. A review of basic control design tools such as lead-lag filters, peak and notch filters, and phase stable design, as well as more sophisticated techniques such as active damping, milliactuators and microactuators, and acceleration feedforward, are also presented.