Pid Servo Loops; Figure 6.3-2- P Loop - Newport ESP6000 User Manual

6 - 1 2
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com

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
p i
where K = proportional gain factor
p
K = integral gain factor
i
K = derivative gain factor
d
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.
Trajectory
Generator
Motion Controller
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
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
much of it will generate instabilities and oscillations.
de
+ •
d
dt
e
Kp
Servo
Controller
Figure 6.3-2— P Loop
) generates a control signal that, converted to an analog
p
gain reduces the necessary following error but too
p
Motor
Driver
Encoder