Rockwell Automation 1771-QB Linear Pos. User Manual
Page 23
Positioning Concepts
Chapter 2
2Ć7
Figure 2.7
Derivative Control
50039
Linear
Displacement
Transducer
Servo Valve
Desired
Velocity
Axis
Velocity
Command
s
Following
Error
dt
Position
Command
s
dt
K
Kp
D/A
Integrator
+
+
F
+
-
Actual
Position
Feed
Forward
+ +
s
dt
Integrator
I
K
dt
D
K
d
+
Derivative
Deadband
Most systems have friction and play in their mechanical linkages. These
characteristics can cause a cylinder to oscillate around a programmed
endpoint–especially if you use an integral term. You can use a deadband to
reduce these oscillations.
A deadband is an area surrounding the programmed endpoint where the error is
ignored. Outside the deadband, error is reduced by one half the width of the
deadband.
If you apply a deadband to an integral term, the integral output remains constant
while the axis is within the deadband. This reduces oscillations around the
endpoint. However, if the deadband is too large, it can also reduce the
positioning accuracy of the system.
PID Band
Integral and derivative control can cause undesirable results when the axis
moves from one position to another. The integral term can cause the axis to
overshoot the programmed endpoint. The derivative term opposes changes in
error, and thereby changes in position.