Pid Function Block Equation - Endress+Hauser iTEMP TMT 165 FOUNDATION Fieldbus Betriebsanleitung

FOUNDATION Fieldbus temperature transmitter
Endress+Hauser

11.7.2 PID Function Block equation

The PID Function Block (PID controller) can be used for various automation strategies.
The function block has a flexible control algorithm that can be configured differently
depending on the application.
• y = Actuating variable calculated from the PID algorithm.
• GAIN = Proportional gain factor (P-term)
• RESET = Time constant for the integral action in seconds (I-term)
• RATE = Time constant for the derivative action in seconds (D-term)
• e = Control deviation (e = setpoint value - PV)
• F = Disturbance variable (F = FF_VAL • FF_GAIN)
The output variable OUT results following checking of the range limits OUT_HI_LIM and
OUT_LO_LIM (see page 152).
The PID Function Block consists of a proportional term, an integral term and a derivative
term. The actuating variable is calculated on the basis of the control deviation between
the setpoint value SP and the process variable PV (controlled variable).
The individual PID actions are used in the calculation of the actuating variable as
follows:
• Proportional term - GAIN- :
The proportional term reacts directly and immediately to a change in the setpoint
value SP or the process variable PV (controlled variable). The actuating variable is
modified by the proportional factor GAIN, which corresponds to the control deviation
multiplied by the gain factor. If a controller works only with the proportional term, then
the control has a remaining control deviation.
• Integral term - RESET - :
The control deviation arising from the calculation of the actuating variable using the
proportional term is integrated via the integral term of the controller until this control
deviation is negligible. The integral function RESET corrects the actuating variable
depending on the size and duration of the control deviation. If the value for the integral
function RESET is set to zero then the controller works with pure I-control.
The influence of the integral term on the control is greater if the value of the integral
function RESET is reduced.
• Derivative term - RATE - :
In the case of control sections with long delays, e.g. temperature controls, it is useful
to use the derivative term RATE of the controller. Using the derivative term RATE the
actuating variable is calculated as a function of the change in the control deviation.
11.7.3 Selecting the operating mode
The operating mode is set via the parameter group MODE_BLK (see page 151). The
PID Function Block supports the following operating modes:
– AUTO – MAN
– OOS – CAS
– RCAS – ROUT
11 Appendix
137