The Feedback Control Loop: Controller Characteristics (2)

Another, perhaps the most important, controller parameter is the control action, which is set as either ‘‘direct’’ or ‘‘reverse’’. If not set correctly, positive feedback in the control loop would result in unstable operation with the valve reaching a wide open or closed limit. By convention, if the valve position is to increase as the measurement increases, then the controller is considered ‘‘direct’’ acting.

By first determining the process action, then specifying the opposite controller action, the desired negative feedback loop is achieved. A typical flow loop is a good example as follows: the process action is ‘‘direct’’ because the flow increases as the valve position is increased, therefore the controller action should be specified as ‘‘reverse’’.

The actual output signal from the controller will further depend upon the specified failure mode of the valve. For example, a fail-closed valve will require an increase-to-open signal, whereas a fail-open valve will require an increase-to-close signal. Most industrial controllers will have a separate parameter to specify the required signal for the failure mode of the valve. In order to minimize confusion, rather than displaying actual output, most controllers display an ‘‘implied valve position’’, which indicates the desired position of the valve.

The response characteristics of a direct acting PID controller are shown in Figure 3.2. For illustrative purpose, a step change to the measurement is made and held constant without feedback. In response to this disturbance, the independent contributions of each controller mode are provided in Figures 3.2(A, B and C), and the combined PID response is presented in Figure 3.2(D). Note that the Proportional mode has an immediate effect on the output, as defined by its algebraic relationship. The Integral mode keeps changing the output at a constant rate as long as the constant error persists. The Derivative mode provides an initial exaggerated response, which decays rapidly since the measurement stops changing after the initial step disturbance.

Although there are many ways to implement PID modes into a controller, the ISA standard algorithm is an ideal, non-interacting combination of the modes. This algorithm is a relatively new standard, made feasible by digital implementation. Note that many previously published tuning guidelines have been developed based upon various analog implementations of an interacting, series combination of these modes.