Endress+Hauser PROline Prosonic Flow 93 Function Manual page 192

6 PID function block (PID controller)
192
6.2 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.
æ 1
⋅ ----------------- - ⋅
= +
y GAIN e
è
RESET
y = Manipulated variable calculated from the PID algorithm.
GAIN = Proportional gain factor (P-term)
RESET = Time constant for the integral function (I-term)
RATE = Time constant for the derivative action (D-term)
e = Control deviation (e = setpoint value - PV)
F = Disturbance variable (F = FF_VAL · FF_GAIN)
The OUT output value results following checking of the OUT_HI_LIM and OUT_LO_LIM
range limits, (see Page 211).
The PID function block consists of a proportional term, an integral term and a derivative
term. The manipulated 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 terms are used in the calculation of the manipulated variable as
follows:
• Proportional term:
The proportional term reacts directly and immediately to a change in the setpoint
value SP or the process variable PV (controlled variable). The manipulated variable is
modified by the proportional factor GAIN. This corresponds to the control deviation
multiplied by the gain factor. If a controller works only with the proportional term, then
the control has a lasting control deviation.
• Integral term:
The control deviation arising from the calculation of the manipulated 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 manipulated 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:
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
manipulated variable is calculated as a function of the change in the control deviation.
PROline Prosonic Flow 93 FOUNDATION Fieldbus
∆e
ö
ò
⋅ ∆t ⋅ ----- -
+ +
e RATE F
ø
∆t
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