Transition Networks
S4224 Web User Guide
33558 Rev. C
Page 364 of 669
Servo
Parameters
The default clock servo uses a PID regulator to calculate the current clock rate using the formula:
clockAdjustment = OffsetFromMaster/ P co Integral(OffsetFromMaster)/ I co Differential
OffsetFromMaster)/ D constant
A Proportional - Integral - Derivative controller (PID controller) is a control loop feedback mechanism
widely used in industrial control systems. A PID is a commonly used type of feedback controller. A PID
controller calculates an "error" value as the difference between a measured process variable and a
desired setpoint. The PID controller tries to minimize the error by adjusting the process control inputs.
The PID controller calculation involves three separate constant parameters: the Proportional, the Integral,
and the Derivative values (
P
,
I
, and
D
). These values can be interpreted in terms of time, where:
P
depends on the present error,
I
depends on the accumulation of past errors, and
D
is a prediction of future errors, based on current rate of change.
The weighted sum of these three actions is used to adjust the process via a control element such as the
position of a valve, or the amount of power supplied to a heating element. The Proportional, Integral, and
Derivative terms are summed to calculate the output of the PID controller.
By tuning these three parameters in the PID controller algorithm, the controller can provide control action
designed for specific process requirements.
The Servo Parameters are explained below.
Display
If
True
then
Offset From Master
,
MeanPathDelay
and
clockAdjustment
are logged on the debug
terminal.
P-enable
If
True
the
P
part of the algorithm (
P
roportional) is included in the calculation.
I-Enable
If
True
the
I
part of the algorithm (
I
ntegral) is included in the calculation.
D-enable
If
True
the
D
part of the algorithm (
D
erivative ) is included in the calculation.
'P' constant
The
P
roportional value [
1
-
1000
]. The
P
roportional value makes a change to the output that is
proportional to the current error value.
'I' constant
The
I
ntegral value [
1
-
10000
]. The
I
ntegral is the sum of the instantaneous error over time and gives
the accumulated offset that should have been corrected previously.
'D' constant
The
D
erivative value [
1
-
10000
]. The
D
erivative of the process error is calculated by determining the
slope of the error over time and multiplying this rate of change by the derivative gain. The derivative
term slows the rate of change of the controller output.