Process Control: Modeling, Design, and Simulation

M16.4. Performance of Digital Control Systems

The performance of a digital PID controller is rarely significantly different from that of a continuous PID controller, as long as the sample time is relatively short compared with the dominant process time constant and the desired closed-loop response time. A rule of thumb for digital control systems in general is that the sample time should be less than one tenth the dominant time constant. In practice, digital PID controllers have a sample time much shorter than that. Many distributed control systems have a sample rate that is several times per second, so the sample time is less than 1 second. This makes sense for flow control loops but is much shorter than necessary for other loops. The main disadvantages ...