PLC.PIDFF3

Structure of the Controller

 

The Base_PID component represents a control structure PI with implementation of reverse action. The controller has two working states available:

        Regulation: The output signal is driven by the PID function.

        Tracking: The output signal is set equal to the position request.

 

Equations

 

The two input signals to the controller are the setpoint and the controlled process variable:

 

PV.signal[1]

SP.signal[1]

 

The process input signal is normalized as a function of the maximum and minimum range of the process value defined by the user, and also the active setpoint signal:

 

The active setpoint is calculated as a function of the active setpoint speed:

 

An error signal, 'dev', is defined as the difference between these two normalized values.

 

The feedback into the integrator to avoid saturation is

 

 

The integral part of the output is calculated from

where:

����������� �is the 'dev' variable calculated previously

����������� is the gain of the integral action

����������� is the time of integral action

����������� �is the relation between the time of integral action and the time constant of anti-windup

 

The proportional action is proportional to the error between the normalized setpoint and the normalized input signal from the process.

Differential action is implemented as follows:

where:

�is a differential term that modiffies the value of the proportional gain.

The unlimited output is the sum of the proportional output, the integral output and the derivative output.

 

The variable 'sense' can be a positive number or a negative number. When 'sense'>0, the controller is direct-acting. In this case, the controller output decreases if the controlled variable increases. A typical case of direct-acting control is the temperature control of heat exchanger, i.e. if the outlet temperature increases, the controller closes the vapor inlet valve to reduce the heating power.

 

If 'sense'<0, the controller is inverse-acting, and the controller output increases if the controlled variable increases. A typical case is a level control where, if the level of liquid in a tank increases due to an increase of the inlet flow, the controller must increase the position of the tank exit valve to re-establish the level.

For more information, see http://j2eeps.cern.ch/wikis/display/EN/Controller+%28PID%29.