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Operation 5-11MN1904
5.5 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 5.6.1.
When there is a requirement to move an axis, the NextMove BX
II
control software t ranslates this
into a demand output voltage. This is used to control the drive (servo a mplifier) which powers the
motor. An encoder or resolver on the motor is used to measure the motor’s position. Every 1ms
(adjustable using the LOOPTIME keyword) the NextMove BX
II
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 theanalogy, imagineyou are inyour car waiting at anintersection. Youare goingto 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 BX
II
)istostayexactly
level with Demand, looking out of the window to measure your position ].
The main term that the NextMove BX
II
uses to correct the error is called Proportional gain
(KPROP). A very simple p roportional controller would simply multiply the amount of error by the
Proportional gain andapply 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 t ry 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, travelling a t a steady
speed. This is what you wanted to do but it has taken you a long time ].
If the Proportional gain i s increased still further , the system becomes unstable[ you keeppressing
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 m ovement 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 m otor
tocometoastopwithasmallfollowingerror[Demandstopped so you stopped t oo, but not quite
level ]. The NextMove BX
II
tries to correct the error, but because the error is so small the amount
of torque demanded might not be enough to overcome friction.
In this situation, a term called Integral gain (KINT) can be used. This sums the e rror 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 t urn off int egral action when it’s not
needed.
The r emaining gain t erms are Velocity Feed forward (KVELFF) and A cceleration Feed
forward (KACCEL).