Evaluating Control Performance

A plant operator is usually responsible for managing one or more process areas, with (potentially) hundreds of control loops and process measurements in each area. The operator’s job is made easier if the control loops in these process areas are in automatic mode and can compensate automatically for disturbances and maintain the operating conditions the operator specifies through the control loop setpoints. Thus, the operator will place a control loop in a manual mode of operation only if there is a measurement or control problem that prevents the control loop from satisfying operational or performance requirements. Therefore, in evaluating measurement and control performance, a good starting point is to quantify current and average control loop utilization in each process area of the plant.

As addressed in Chapter 3 of Advanced Control Foundation – Tools, Techniques and Applications, most performance monitoring tools are designed to provide both an instantaneous value and an historic view that summarizes control loop utilization by process area. If the utilization is lower than some established limit, the user can select a detail view of the process area that shows the utilization for each control loop. Control utilization can be determined by comparing the Actual and Normal mode attributes of the mode parameter. The mode parameter of the PID block determines the source of the block setpoint and the source of its output. This parameter is a critical part of the operator’s interface to the PID block and consists of four mode attributes:

  • Target – The mode of operation requested by the operator
  •  Actual – The mode of operation that can be achieved based on the target mode and the status of the PID block inputs
  • Permitted – The mode(s) available to the operator for a given application
  • Normal – The normal operating mode of the PID

The Actual mode attribute can take on a value other than that specified by the Target mode attribute. This occurs when an internal condition or input status indicates that the mode of operation requested by the operator through the Target mode attribute cannot be achieved. The workshop on evaluating control system performance in Chapter 3 of Advanced Control Foundation – Tools, Techniques and Applications illustrate this behavior. The solution for this workshop may be viewed on the book’s web site.  Under normal operating conditions, the Target and Actual mode attributes match. When the Actual mode attribute changes to Local Override (LO), the output tracking or an internal application such as autotuning is setting the block output. Similarly, if the path to the process is lost, the Actual mode attribute changes to Initialization Manual (IMan). When the Actual mode attribute of a PID is IMan, the path to the process is incomplete. The path to the process can be incomplete if a downstream block is taken out of Cascade mode. The path to the process can also be incomplete because of a physical condition such as the hardware failure of a downstream block or an actuator failure that is manipulated by the PID.