Newport MM4006, User Manual

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MM4006 M o t i o n Control Tutorial
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 every servo cycle and the value,
multiplied by the Ki gain factor, is added to the control signal (Fig. 4.15).
Trajectory
Generator
Motion Controller
Servo Controller
•
f c • x K
Fig. 4.15 — PI Loop.
Motor
y D r i v e r
▶
Encoder y
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 desir-
able 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 destabi-
lizing 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 dif-
ference 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.
Trajectory
Generator
Motion Controller
Servo Controller
Fig. 4.16 — PID Loop.
Encoder
Motor
Fig. 4.16 shows the PID servo loop diagram. 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.
N e w p o r t . 4 . 1 5 E D I - 1 0 1 8 1 E n 1 0 2 0 — 03/03