Baldor MN1903 5.7 Servo axis - tuning for velocity control

5.7 Servo axis - tuning for velocity control

Drives designed for velocity control incorporate their ownveloc ity feedback term to provide

systemdamping.For this reason,KDERIV (and KVEL) can be set to zero.

Correctsetting of the velocity feed forward gain KVELFF is important to get the optimum

response from the system.The velocity feedforward term takes theinstantaneous velocity

demandfrom the profile gener ator and adds this to the output block (see Figure 11).

KVELFFis outside the closed loop and therefore does not have an effect on sy stem stability.

Thismeans thatthe ter m can be increased to maximum without causing the motor to osc illate,

providedthat otherterms ares etup correctly.

Whensetup correctly, KVELFF will cause the motor to move at the speed demanded by the

profilegenerator. This is true without theother terms inthe c losed loop doing anything except

compensating for small errors in the position of the motor. This gives faster res ponse to

changes in demand speed, with reduced following error.

Tocalculate the correct value for KVELFF,you will need to know:

HThespeed, in r evolutions per minute, produced by the motor when a maximum demand

(+10V)is appliedto the driv e.

HThesetting for LOOPTIME. The fac tory preset setting is 1ms.

HThenumber of encoder lines f or theat tached motor.Baldor BS Mmot ors use either 1000

or2500 line encoder s.

Theserv o loop formula uses speed values expressed in quadrat ure counts per servo loop.To

calculate this figure:

1. First, divide the speed of the motor, in revolutions per minute, by 60 to give the number of

revolutions per second. For example, if the motor speed is 3000rpm when a maximum

demand(+10V) is appliedto the driv e:

Revolutions per second = 3000/ 60

=50

2. Next, calculate how many revolutions will occur during one servo loop. The factory preset

servo loop time is 1ms (0.001 seconds), so:

Revolutions per servo loop = 50x 0.001 seconds

=0.05

3. Nowcalculate how many quadr ature encoder counts t here are per revolution. The Nex tMove

PCIcounts bothedges of bothpulse trains (CHAand CHB) comingfrom the encoder, so for

every encoder line there are 4 ‘quadrature counts’. With a 1000 line encoder:

Quadrature counts per revolution = 1000x4

=4000

4. Finally,calculate how manyquadr ature counts there are per servo loop:

Quadrature counts per servo loop = 4000 x 0.05

=200