PID (FB)

 

FUNCTION_BLOCK PID

Represents a PID controller

Note

The PID controller itself measures the elapsed time between two calls, however with a maximum accuracy of milliseconds. This might lead to rough running in case of short cycle times: For example in case of a cycle time of 1ms the PID sometimes might measure 2 ms, sometimes 0 ms. So if possible, for such cases better use PID_FIXCYCLE, where the cycle time can be set precisely. See PID_FIX­CYCLE

Note

Consider that the controller parameters only get applied when used in the manual mode at a start, a reset or at a change down.

A PID controller continuously calculates an error value e(t) as the differ­ence between a desired set point and a measured process variable. The PID controller applies a correction based on proportional, integral, and derivative terms (sometimes denoted P, I, and D respectively) which give their name to the controller type.

PID_en.svg.png

As a PID controller relies only on the measured process variable, not on knowledge of the underlying process, it is broadly applicable. By tuning the three parameters of the model, a PID controller can deal with specific process requirements. The response of the controller can be described in terms of its responsiveness to an error, of the degree to which the system overshoots a setpoint, and of the degree of any system oscilla­tion. The use of the PID algorithm does not guarantee optimal control of the system or even its stability.

Y_OFFSET, Y_MIN and Y_MAX serve for transformation of the manip­ulated variable within a prescribed range. MANUAL can be used to switch to manual operation; RESET can be used to re-initialize the controller. In normal operation (MANUAL = RESET = LIMITS_ACTIVE = FALSE) the controller calculates the controller error e as difference from SET_POINT – ACTUAL, generates the derivation with respect to time and stores these values internally.

The output Y is the manipulated variable unlike the PD controller contains an additional integral part, and is calculated as follows: So besides the P-part also the current change of the controller error (D-part) and the history of the controller error (I-part) influence the manipulated variable. The PID controller can be easily converted to a PI-controller by setting TV=0. Because of the additional integral part, an overflow can come about by incorrect parameterization of the controller, if the integral of the error e becomes to great. Therefore for the sake of safety a BOOLean output called OVERFLOW is present, which in this case would have the value TRUE. This only will happen if the control system is instable due to incorrect parameterization. At the same time, the controller will be suspended and will only be activated again by re-initial­ization.

Note

As long as the limitation for the manipulated variable (Y_MIN, Y_MAX) is active, the integral part will be adapted, like if the history of the input values had automatically effected the limited output value. If this behaviour is not wanted, the following workaround is possible: Switch off the limitation at the PID controller (Y_MIN>=Y_MAX) and instead apply the LIMIT operator (IEC standard) on output value Y (see an example in the figure below).

Note

It is not necessary to readjust the controller parameters (KP, TN, TV) if the cycle time changes.

Temperature control with PID and LIMIT

See in the following figure a simple example of using the PID module for temperature control and in combination with the LIMIT operator. The input of the actual temperature is simulated by giving a constant value via ActualTemperature.

PIDExample.png

InOut:

Scope

Name

Type

Initial

Comment

Input

ACTUAL

REAL

Current value, process variable

SET_POINT

REAL

Desired value, set point

KP

REAL

Proportionality const. P

TN

REAL

Reset time I [sec]

TV

REAL

Rate time, derivative time D [sec]. If set to 0, then it works as PI controller

Y_MANUAL

REAL

Y is set to this value as long as MANUAL = TRUE

Y_OFFSET

REAL

Offset for manipulated variable

Y_MIN

REAL

Minimum value for manipulated variable

Y_MAX

REAL

Maximum value for manipulated variable

MANUAL

BOOL

TRUE: Manual: Y is not influenced by controller

FALSE: Controller determines Y

RESET

BOOL

TRUE: Set Y output to Y_OFFSET and reset integral part

Output

Y

REAL

Manipulated variable, set value

LIMITS_ACTIVE

BOOL

FALSE

TRUE: Y has exceeded the given limits Y_MIN, Y_MAX and is limited to these values

OVERFLOW

BOOL

FALSE

Overflow in integral part