Getting Started
MN1258V4 08/2000
53
4.4.4
System Gains for Velocity Control
Velocity controlled drives incorporate the velocity feedback term in the amplifier which provides
system damping and therefore it is usually sufficient to have
KVEL = KDERIV = 0
on the
controller. Usually, the value of the proportional gain,
KPROP
, will be less than with an equivalent
current controlled system. Often a fractional value of proportional gain gives the best response. For
example
KPROP=0.1
compared with
2
for current controlled drives.
Correct setting of the velocity feed forward gain,
KVELFF
, is important to get maximum response
from the system. This is best performed using a storage oscilloscope (or the NextMove WorkBench)
to record the tachometer output for fast point-to-point moves. This enables velocity/time profile for
the motor to be seen in order to monitor actual acceleration and overshoot.
Referring to the servo loop block diagram, the velocity feed forward term is a block, which takes the
instantaneous speed demand from the profile generator and adds this to the output block. Because
KVELFF
is a feed forward term, a very important difference between this and the other terms exists.
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 the other terms are set-up correctly.
In practice however, a very high value of
KVELFF
is of no benefit to system performance. Instead, it
should be set such that a demand of
x
RPM from the profile generator results in a demand output to
the velocity drive, which gives x RPM on the motor shaft (a 1:1 relationship).
When set-up correctly,
KVELFF
will cause the motor to move at the demand speed from the profile
generator. This is true without the PID terms in the closed loop doing anything except compensating
for small errors in the position of the motor due to analog drift. This gives faster response to
changes in demand speed, with lower following errors.
Example calculation of KVELFF:
In order to calculate the correct value for
KVELFF
, you need to consider the workings of the servo
loop closure algorithms. In the servo loop, speeds are expressed in quadrature counts/servo loop
closure time. For instance, a speed of 100 is 100 counts every 1ms with NextMove PCI (note that
the default loop closure time is 1ms).
In this example the velocity of the servo is 3000RPM with a +10V input, and the encoder has 1000
counts per revolution.
At 3000RPM we require an analog voltage of +10V. 3000RPM relates to:
3000
60
50
=
revs per second
Summary of Contents for NextMove BX
Page 1: ...MN1258V4 08 2000 NextMove BX Installation Manual for Mint v4 Issue 4 0...
Page 2: ...NextMove BX Installation Manual for Mint v4 ii MN1258V4 08 2000...
Page 4: ...NextMove BX Installation Manual for Mint v4 iv MN1258V4 08 2000...
Page 8: ...NextMove BX Installation Manual for Mint v4 viii MN1258V4 08 2000...
Page 12: ...NextMove BX Installation Manual for Mint v4 xii MN1258V4 08 2000...
Page 18: ...NextMove BX Installation Manual for Mint v4 6 MN1258V4 08 2000...
Page 22: ...NextMove BX Installation Manual for Mint v4 10 MN1258V4 08 2000 Figure 3 3 Board Settings...
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