Baldor NXE100-1608Dx, Руководство по установке

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Operation 5-25 MN1941
5.7.2 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 5.8.1.
When there is a requirement to move an axis, the NextMove e 100 control software translates this
into a demand output voltage (or just a numerical value over EPL). 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
e 100 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 e100) is to stay
exactly level with Demand, looking out of the window to measure your position ].
The main term that the NextMove e 100 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 e 100 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
CONTROLRATE
keyword - see the Mint help
file.