Fig.1 Digital Control System

There are many practical reasons why multi-rate digital control systems might be used. For example, many disk drives use sector-servo to control the read/write head position relative to track center lines. In a sector-servo system, every track is evenly divided angularly into sectors to store data; and position information is written at sector boundaries. In order to utilize as much disk surface as possible for data storage, the number of sectors on each track should not be too large. This limits the measurement sampling rate for digital control. In this case, digital control with a standard zero order hold may not be appropriate, and it has been suggested to update the control input at a higher rate. Although single-rate systems are easy to design and implement, it can be inefficient for systems with measurements at slow sampling rates due to intensive processing of data among other reasons. Another reason for using multi-rate control may be that the sampling rate is constrained by the computational speed of microprocessors for digital control. We can not assume that the most powerful digital signal processors can be applied for every application. ... More (Yuping Gu, 1999)