Baldor MN1903 5.4.2 Anintroduction to closedloop control

6. Toremove the demand and stop the test, ty pe:

STOP.0

Thiss hould cause the demand produced at

theDemand 0 output to bec ome 0V.

5.4.2 Anintroduction to closedloop control

Thiss ection describes the basic principles of clos ed loop control. If you are familiar with

closed loop control go straight to section 5.5.1.

Whenthere is arequirement to mov e an axis, the NextMove PCI control softw are translates

thisinto a demand output voltage.This isus ed to control the drive (servo amplifier) which

powers the motor.An enc oder or resolver on the motor is used to measure the motor’s

position.E very 1ms* (adjustable using the LOOPTIME keyword) theNex tMove PCI compares

thedemanded and measur ed positions. It then calculates the demand needed to minimize the

differencebetween t hem, known as the following error.

Thissystem of constantmeasurementand correction is knownas closed loop control.

[Forthe analogy, imagine you are in y our car waiting at an intersection. You are going to go

straight on when the lights change, just like the car standing next to y ou which is called

Demand.You’re not going to race D emand though - your job as the controller (NextMove PCI)

isto stay ex actly level with Demand, looking out of the window to measure your position ].

Themain term that the N extMove PCI uses to correct the error is calledPr oportional gain

(KPROP). A ver y simple proportional controller would simply multiply the amount of err or by

theProporti onal gain and apply the result to the motor [ the further Demand gets ahead or

behindyou, the mor e you press or release the gas pedal ].

Ifthe Propor tional gain is set too high overshoot will occur, resulting in the motor vibrat ing back

andforth around the des ired position before it settles [ you press the gas pedalso hard y ou go

rightpas t Demand. Totry and st ay level you ease off the gas, but end up falling behind a little.

Youkeep repeating this and after a few tr ies you end up level with Demand, travelling at a

steady speed. This is what you wanted to do but it has taken y ou a long time ].

Ifthe Propor tional gain is increased still further, the system becomes uns table [ you keep

pressing and then letting off the gas pedal so hard you never travel at a steady speed].

Toreduce the onset of instability, a term called Velocity Feedback gain (KVEL) is us ed. This

resists rapid movement of the motor and allows the Proportional gain to be set higher before

vibrations tarts. Another term called Derivative gain (KDERIV) can als o be used to give a

similareffect.

WithPropor tional gain and Velocity Feedback gain (or Derivative gain) it is poss ible for a

motortocometoastopwithasmallfollowingerror[Demandstoppedso you stopped too, but

notquite level ].

TheNex tMove PCI tries to correct the error, but because the error iss o small the amount of

torquedemanded might not be enough to ov ercome friction.

*The1ms s ampling interval can be changed using the LOOPTIME keyword to either 500µs or

200µs.