5-24 Operation
MN1957
www.baldormotion.com
5.8 Servo axis - tuning for velocity control
Drive amplifiers designed for velocity control incorporate their own velocity feedback term to
provide system damping. For this reason, KDERIV (and KVEL) can often be set to zero.
Correct setting of the velocity feed forward gain KVELFF is important to get the optimum
response from the system. The velocity feed forward term takes the instantaneous velocity
demand from the profile generator and adds this to the output block (see Figure 34). KVELFF
is outside the closed loop and therefore does not have an effect on system stability. This
means that the term can be increased to maximum without causing the motor to oscillate,
provided that other terms are setup correctly.
When setup correctly, KVELFF will cause the motor to move at the speed demanded by the
profile generator. This is true without the other terms in the closed loop doing anything
except compensating for small errors in the position of the motor. This gives faster response
to changes in demand speed, with reduced following error.
Before proceeding, confirm that the encoder feedback signals from the motor or drive
amplifier have been connected, and that a positive demand causes a positive feedback
signal.
5.8.1 Calculating KVELFF
To calculate the correct value for KVELFF, you will need to know:
The speed, in revolutions per minute, produced by the motor when a maximum demand
(+10 V) is applied to the drive amplifier.
The setting for
LOOPTIME
. The factory preset setting is 1 ms.
The resolution of the encoder input.
The servo loop formula uses speed values expressed in quadrature 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 3000 rpm when a maximum
demand (+10 V) is applied to the drive amplifier:
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 1 ms (0.001 seconds), so:
Revolutions per servo loop
=
50 x 0.001 seconds
=
0.05
3. Now calculate how many quadrature encoder counts there are per revolution. The
NextMove ESB-2 counts both edges of both pulse trains (CHA and CHB) coming from
the encoder, so for every encoder line there are 4 ‘quadrature counts’. With a 1000 line
encoder:
Quadrature counts per revolution
=
1000 x 4
=
4000
4. Finally, calculate how many quadrature counts there are per servo loop:
Quadrature counts per servo loop =
4000 x 0.05
=
200
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