Figure 6.3-3 - Pi Loop; Figure 6.3-4- Pid Loop - Newport ESP6000 User Manual

Section 6 — Motion Control Tutorial
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PI Loop
To eliminate the error at stop and during long constant velocity motions
(usually called steady-state error), an integral term can be added to the loop.
This term integrates (adds) the error during each every servo cycle and the
value, multiplied by the K
6.3-3).
Servo Controller
Trajectory
Generator
Motion Controller
The result is that the integral term will increase until it drives the motor
by itself, reducing the following error to zero. At stop, this has the very
desirable effect of driving the positioning error to zero. During a long
constant-velocity motion it also brings the following error to zero, an
important feature for some applications.
Unfortunately, the integral term also has a negative side, a severe de-
stabilizing effect on the servo loop. In the real world, a simple PI loop is
usually undesirable.
PID Loop
The third term of the PID loop is the derivative term. It is defined as the
difference between the following error of the current servo cycle and of the
previous one. If the following error does not change, the derivative term is
zero. Figure 6.3-4 shows the PID servo loop diagram.
Servo Controller
Trajectory
Generator
Motion Controller
The derivative term is added to the proportional and integral one. All three
process the following error in their own way and, added together, form the
control signal.
The derivative term adds a damping effect which prevents oscillations and
position overshoot.
gain factor, is added to the control signal (Figure
i
e
Driver
Kp
∫
e Ki
Figure 6.3-3 — PI Loop
e
Kp
∫
Driver
e Ki
de
Kd
dt
Figure 6.3-4— PID Loop
Motor
Encoder
Motor
Encoder
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