Once PV = SP again, integral action stops ramping and simply holds the last value (Note 1 ).ĭerivative action interprets both steps as infinite rates of change, and so generates “spikes” (Note 3 ) at the leading and at the trailing edges of the step. Integral action ramps at a rate proportional to the magnitude of the input step, for as long as the PV is unequal to the SP. Proportional action directly mimics the shape of the input change (an up-and-down step). Responses to a momentary step-and-return input Note 1 : This is the meaning of the vertical-pointing arrowheads shown on the trend graph: momentary saturation of the output all the way up to 100%.

When combined into one PID output, the three actions produce this response: Derivative action interprets the step as an infinite rate of change, and so generates a “spike” (Note 1 ) driving the output to saturation. Since the input step holds a constant value, the integral action ramps at a constant rate (a constant slope). Integral action ramps at a rate proportional to the magnitude of the input step. Proportional action directly mimics the shape of the input change (a step). In order to quantitatively predict PID controller responses, one would have to know the values of all PID settings, as well as the original starting value of the output before an input change occurred and a time index of when the change(s) occurred. The illustrations of P, I, and D actions focus only on the shapes of the responses, not their exact numerical values.Īlso Read : Two out of Three Logic Analogy There is too little information given in each case to plot exact responses.

It should be noted that these graphic illustrations are all qualitative, not quantitative. an increase in process variable results in an increase in output). In each graph, the controller is assumed to be direct-acting (i.e. This section is divided into subsections showing P, I, and D responses for several different input conditions, in the form of graphs. A very helpful method for understanding the operation of proportional, integral, and derivative control terms is to analyze their respective responses to the same input conditions over time.

Pid control step labview