5-16 Operation
MN1957
www.baldormotion.com
5.6.2 An introduction to closed loop control
This section describes the basic principles of closed loop control. If you are familiar with
closed loop control go straight to section 5.7.1.
When there is a requirement to move an axis, the NextMove ESB-2 control software
translates this into a demand output voltage. This is used to control the drive amplifier which
powers the motor. An encoder or resolver on the motor is used to measure the motor's
position. At specified intervals* the NextMove ESB-2 compares the demanded and
measured positions. It then calculates the demand needed to minimize the difference
between them, known as the
following error
.
This system of constant measurement and correction is known as closed loop control.
[
For the analogy, imagine you are in your car waiting at an intersection. You are going to go
straight on when the lights change, just like the car standing next to you which is called
Demand
. You're not going to race
Demand
though - your job as the controller
(NextMove ESB-2) is to stay exactly level with
Demand
, looking out of the window to
measure your position
].
The main term that the NextMove ESB-2 uses to correct the error is called
Proportional
gain
(
KPROP
). A very simple proportional controller would simply multiply the amount of
error by the Proportional gain and apply the result to the motor [
the further
Demand
gets
ahead or behind you, the more you press or release the gas pedal
].
If the Proportional gain is set too high overshoot will occur, resulting in the motor vibrating
back and forth around the desired position before it settles [
you press the gas pedal so hard
you go right past
Demand
. To try and stay level you ease off the gas, but end up falling
behind a little. You keep repeating this and after a few tries you end up level with
Demand
,
traveling at a steady speed. This is what you wanted to do but it has taken you a long time
].
If the Proportional gain is increased still further, the system becomes unstable [
you keep
pressing and then letting off the gas pedal so hard you never travel at a steady speed
].
To reduce the onset of instability, a term called
Velocity Feedback gain
(
KVEL
) is used.
This resists rapid movement of the motor and allows the Proportional gain to be set higher
before vibration starts. Another term called
Derivative gain
(
KDERIV
) can also be used to
give a similar effect.
With Proportional gain and Velocity Feedback gain (or Derivative gain) it is possible for a
motor to come to a stop with a small following error [Demand
stopped so you stopped too,
but not quite level
]. The NextMove ESB-2 tries to correct the error, but because the error is
so small the amount of torque demanded might not be enough to overcome friction.
This problem is overcome by using a term called
Integral gain
(
KINT
). This sums the error
over time, so that the motor torque is gradually increased until the positional error is reduced
to zero [
like a person gradually pushing harder and harder on your car until they've pushed it
level with
Demand].
However, if there is large load on the motor (it is supporting a heavy suspended weight for
example), it is possible for the output to increase to 100% demand. This effect can be limited
using the
KINTLIMIT
keyword which limits the effect of KINT to a given percentage of the
demand output. Another keyword called
KINTMODE
can even turn off integral action when it's
not needed.
* The sampling interval can be changed using the
LOOPTIME
keyword to either 1 ms or
2 ms.
Servo Systems Co. • 115 Main Road • P.O. Box 97 • Montville, NJ,
07045-0097 • (973) 335-1007 • Toll Free: (800) 922-1103
Fax: (973) 335-1661 • www.servosystems.com
