Tuning in a Distributed Environment

The tuning of a PID control loop should be based on an accurate knowledge of the control parameter response to a change in the associated manipulated process input. If the process response is determined using values communicated from a controller to a workstation, then these communications may impact the identified process response. For example, the control network may limit access by the workstations to 1 sec samples even thought the PID and its associated IO may be executing in the controller at a much faster rate e.g. 100 msec. Also, the timing associated with values seen at the workstation may vary or be delayed with communication loading i.e. communications jitter. The combined effect of communications sample rate, delay and jitter introduces a degree of uncertainty into the identification of the process response. For very slow processes, the error introduced in the identified process response may have little impact on the calculated tuning. However, for fast responding processes such as liquid pressure and flow loops such uncertainty can lead to unsatisfactory tuning. In general, to provide best tuning in a distributed environment it is necessary to capture the process response at the point of control.

The influence of communication sample rate, delay or jitter may be eliminated by capturing the process dynamic response as part of the function block execution. This approach also naturally allows the process response (control and manipulated parameter) to be collected at the block execution rate. Such functionality may be introduced into a control system as part of the PID block or as a modifier to the PID function block. The concept of a function block modifier is something that Dennis Stevenson and I developed and patented a few years ago. For controller based implementations, function block modifiers may be used to minimize controller memory requirements since the modifier is instantiate and exists only when the loop is being tuned.

When control is done in the field using fieldbus devices, then the sample limitations, communications delay and jitter from the controller to the workstation can be eliminated similar to controller based PID. This can be implemented by designing the autotune block modifier to work with the proxy that represents the field PID in the controller. However, to eliminate the delay and jitter introduced by communications between the field device and the controller, it is necessary to capture the process response at the field device. For example, the latest Emerson fieldbus devices provide this capability. Through the use of these techniques, it is possible to provide tuning support that is independent of where control is done.

If you would like to read more on this topic, then you may find it helpful to get a copy of the paper, Autotuning in Distributed Environment, Blevins, Wojsznis, Thiele, ISA TECN1999 conference. Copies of this paper can be downloaded through the ISA web site. Also, Aadditional information on how to minimize the impact of communications sampling, delay and jitter on PID tuning is contained in the following:

Minimizing Communications Delay/Jitter in PID Tuning