The basic idea behind a PID controller is to read a sensor, then compute the desired actuator output based on a setpoint by calculating proportional, integral, and derivative responses and summing those three components to compute the output.
The PID controller receives the process variable (PV) and controls the manipulation variable (MV) in order to adjust the PV to match the set value (SV).
In the PLC, a PID function block and analog input/output modules are used for PID control (one PID loop). To configure two or more PID loops, two or more PID function blocks are used. However, the number of the PID loops configurable by one PLC is normally limited by the available number of analog inputs/outputs.
Please see Tech Note 7013 "PID Loop Configuration"
for more information.