Baldor NextMove BX, Руководство по установке

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NextMove BX Installation Manual
54
MN1258V4 08/2000
Now the number of quadrature counts per loop closure time can be calculated by using the
expression:
speed in rev per sec * encoder line count * 4
number of loop closures per second
The factor of 4 is included since the controller counts every edge of the pulse train coming from the
encoder
A and B channels, which gives four times better resolution than the number of lines.
50 1000 4 1000 200 * * /
=
quadrature counts per servo lop closure time
The DAC output has a resolution of 12 bits over the range -10V to +10V, the10V = 2048
counts.
The feed forward term is therefore given by:
KVELFF
= =
2048
200
10 24 .
Increasing
KVELFF
above the calculated value will cause the controller to have a following error
ahead of the desired position. Decreasing
KVELFF
below this value will cause the controller to have
a more normal following error behind the desired position. The calculated value above should give
zero following error in normal running.
It is possible to investigate the effect of velocity feed forward gain by jogging the motor at constant
speed and monitoring the axis following error for different values of
KVELFF
. When attempting this,
make sure that there is zero integral gain since this will cause the following error to tend to zero
under steady state conditions, thereby negating the effect of changes in
KVELFF
.
4.4.5 Eliminating Steady-State Errors
In systems where precise positioning accuracy is required, it is often necessary to position to within
one encoder count. Proportional gain,
KPROP
, is not normally able to achieve this because a very
small following error will only produce a small demand for the amplifier which may not be enough
to overcome mechanical friction (this is particularly so for current controlled systems). This error
can be overcome by applying some integral gain.
The integral gain,
KINT
, works by accumulating following error over time to produce a demand
sufficient to move the motor into the zero following error position.
KINT
can therefore also
overcome errors caused by gravitational effects, such as vertically moving linear tables, where with
current controlled drives, a non-zero
command output is required to achieve zero following error.
Particular care is required when setting
KINT
since a high value can cause instability during moves.
The effect of
KINT
should be limited by setting the maximum range of the integration (
KINTLIMIT
)
to the minimum value that is sufficient to overcome friction or static loads.