9.
B addition - What is PID controller (regulator)
9.1
What is PID controller
As opposed to stepper motors which are,
we can say, controlled "blindly", servo
drives work in so called closed loop, so
when they control a motor they check if
its position is compliant with the set
position. If actual rotor position differs
from the expected one, there is a current
correction entered to overcome the
existing error. If the motor overtook the
position that was set, it just slows down
or speeds up to catch up if the set
position overtook the actual position. It
is just like driving a car we would like to
catch up another car next to us. If it is running away we step on the gas and speed up and if we are
too far we let up on the gas to slow down. What happens if the car we want to align with doesn't
make it any easier and every moment speeds up or slows down, moreover we are already nervous
and we overreact stepping on the gas or brake to the limit time and time again? As a result, we will
still pass the chased car, being most of the time far away from it. As a result, we will still pass the
chased car, being most of the time far away from it. Translating this analogy to a CNC machine, it
would appear that axes do not follow the set motion path and a lot of machining inaccuracies would
arise.
Servo drives require fast and precise correction to the position error. Going back to the cars
comparison, the point is that the driver from the chasing car should be as experienced as possible, so
he could predict behavior of the second running away car and to react precisely as the situation
evolves. In servo drives PID controller is the driver. The controller is a mathematical algorithm that is
responsible for motor reaction on deviations from set position. The PID name derives from the
following terms of the controller.
•
P
roportional
•
I
ntegral
•
D
erivative
46
simDrive™ - AC Servo Drive User Guide
