Siemens Milltronics SF 500 - Derivative Control (Pre-Act or Rate), D; Feed Forward Control, F

7ML19985CN01 Milltronics SF 500 - INSTRUCTION MANUAL Page 59
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PID Control
Derivative Control (Pre-Act or Rate), D
The D term on the SF 500 influences the control output based on changes in the
magnitude and direction of the change in error. If there is a constant error, the D term has
no effect. As the error gets larger, the D term combines with the P term to make the SF
500 control output respond faster. When the error is getting smaller, the D term acts to
reduce the amount of control output to help prevent overshooting the set point. In
general, a higher P term requires a larger D term.
• allowable input range: 0.000 to 1.000
• typical operating range: 0.010 to 0.100
• default value:0.050
The result of the derivative action is that it can make a system more responsive and more
stable at the same time.
Feed Forward Control, F
The F term is used to adjust the control output based on a setpoint change. The use of
this term can make the system reach the new setpoint faster. If the term is not used, the
system responds using the P, I, and D terms only. The difference between the new
setpoint and the process variable is the error and the control algorithm responds to
eliminate this new error.
When the F term is used and a new setpoint is entered, a proportion of the difference
between the new setpoint and the process variable is automatically added on to the
control output. This shifts the process variable closer to the new setpoint faster than
using the P, I, and D terms alone. This is done on a one-time basis.
• allowable input range: 0.000 to 1.000
• typical operating range: 0.250 to 0.550
• default value: 0.300
Decreasing
Error
Increasing
Error
Decreasing
Error
Increasing
Error
Direction of
Proportional
Action
Direction of
Derivative
Action
Error
+
-
Setpoint