Newport ESP6000, Руководство пользователя

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6.3.1 PID Servo Loops
The PID term comes from the proportional , integral and derivative gain
factors that are at the basis of the control loop calculation. The common
equation given for it is:
K e K e dt K
de
dt
p i d
• + + •
∫
where K
p
= proportional gain factor
K
i
= integral gain factor
K
d
= derivative gain factor
e = instantaneous following error
The problem for most users is to get a feeling for this formula, especially
when trying to
tune the PID loop. Tuning the PID means changing its three
gain factors to obtain a certain system response, a task quite difficult to
achieve without some understanding of its behavior.
The following paragraphs explain the PID components and their operation.
P Loop
Lets start with the simplest type of closed loop, the P (proportional) loop.
The diagram in Figure 6.3-2 shows its configuration.
e
Trajectory
Generator
Motion Controller
Servo
Controller
Driver
Motor
Encoder
Kp
Figure 6.3-2— P Loop
Every servo cycle, the actual position, as reported by the encoder, is
compared to the desired position generated by the trajectory generator.
The difference
e is the positioning error (the following error ). Amplifying it
(multiplying it by K
p
) generates a control signal that, converted to an analog
signal, is sent to the motor driver.
There are a few conclusions that could be drawn from studying this circuit:
• The motor control signal, thus the motor voltage, is proportional to the
following error.
• There must be a following error in order to drive the motor.
• Higher velocities need higher motor voltages and thus create higher
following errors.
• At stop, small errors cannot be corrected if they don’t generate enough
voltage for the motor to overcome friction and stiction.
• Increasing the K
p
gain reduces the necessary following error but too
much of it will generate instabilities and oscillations.
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