Analog PID Controller: Basic Features

PID action can be performed using either analog or digital electronic circuits. In this article we look at the fundamental features of an analog PID controller.

To help us understand the basic operation of an analog PID controller, let’s consider the block diagram below:

In reference to the diagram above, the measured variable from the sensor is compared to the set point in the summing circuit, and its output is the difference between the two signals. This is the error signal. This signal is fed to the integrator.

When there is a change in the measured variable, the error signal is seen by the proportional amplifier, the differentiator, and the integrator – these three are used to implement proportional, derivative and integral action. The output from the amplifiers is combined in a summing circuit, amplified and then fed to the actuator motor to change the input variable.

Although the integrator sees the error signal, it is slow to react, so its output does not change immediately, but starts to integrate the error signal. If the error signal is present for extended period of time, then the integrator will supply the correction signal via the summing circuit to the actuator.

The circuit implementation of the PID controller is shown in the figure 1.1 below. Though each of the amplifiers is shown performing a single function, this has been used just as an illustration, in practice, there are a large number of circuit component combinations that can be used to produce PID action.

A single amplifier also can be used to perform several functions, which would greatly reduce the circuit complexity. In modern systems, the PID action can be performed in the PLC processor using digital techniques, and by the processor in a smart sensor.

Related articles:

• Digital PID Controllers
• How to Tune the PID Controller
• Basic Features of Modern PID Controllers
• The Performance Limits for PID Controllers
• Analog Proportional Controller