Selecting Control Response

After the process characteristics are identified, the information is then used to calculate the recommended loop tuning. The tuning rules used to derive controller parameters from process characteristics can be chosen as appropriate for the specific process and the desired speed of response. As addressed in Chapter 4 of Advanced Control Foundtion – Tools, Techniques, and Applications, the response to a setpoint or load disturbance change is determined by the controller tuning and can be classified as follows:

  • Overdamped response – Controlled parameter is gradually restored without overshooting the setpoint value. In most cases an overdamped response is the best response for varying operating conditions that impact process gain and response time.
  • Critically damped response – Controlled parameter is restored in minimum time without overshooting the setpoint value. While this response minimizes the time to respond to a change in setpoint or process disturbance, unstable control can be observed with changing operating conditions that impact process gain or response time.
  • Underdamped response – Controlled parameter overshoots the setpoint value but eventually settles at the setpoint value. This response may minimize time to get back to setpoint but at the expense of stability and the controlled parameter overshooting to setpoint when responding to a change in setpoint or change in load disturbances.

Once the reset gain of the PID is set based on the identified process deadtime and time constant, it is possible to go from an overdamped response to a critically damped response or to an underdamped response by adjusting the PID proportional gain. Some on-demand tuning applications have a selection that changes the proportional gain to achieve a specified speed of response, for example, a fast, normal, or slow response.

The best tuning for a particular loop depends upon the control objectives. If the setpoint is constantly being changed, the loop’s response to setpoint changes would be a key factor in evaluating its performance. However, if the loop setpoint is always maintained at a fixed value, the response to disturbance inputs would be more important. If the response to both setpoint and disturbance inputs is important, tuning must be set to achieve a balance in control response. In many processes it is desirable to have an overdamped response. To achieve that response, the proportional gain used in the PID is typically much less than that required for an underdamped or critically damped response.