Commissioning Control Systems – High Order Process Response

The process response to a change in disturbance or manipulated input is often the result of many mechanisms working together and interacting: heat transfer, momentum, inertia, air capacitance in an actuator, and so on. The combined effect of these different mechanisms constitutes a higher order process response. However, response of a self-regulating process often closely (and, for most purposes, adequately) may be approximates a first order plus deadtime response. We address the topic of process characterization in Chapter 9 of Control Loop Foundation – Batch and Continuous Processes. In that chapter, one of the process examples shown as being best characterized by a first order plus deadtime response is a heater, as illustrated below.

For this example, the combined impact of the transport delay and heat transfer provides a first order plus deadtime response. In more complex processes, multiple mechanisms working together can sometimes combine to provide what may be approximated as a first order plus deadtime response, even where no transport or measurement delay is involved.

Many of the newer control systems used in the process industry include auto-tuning, the capability to automatically establish PID block tuning. Most of these tools assume that a self-regulating process response may be approximated as first order plus deadtime. On critical control loops where the very best close loop response is desired, then in some cases it is possible to improve control tuning by modeling the process response as higher order. One example of a product that may be used to characterize the response as higher order is the Emerson Entech Toolkit. Based on a series of step test, the process response may be identified as higher order as illustrated below.

The manufacture of such products often offer a training class that can be helpful in learning about the added capability this provides.