Section 6 — Motion Control Tutorial
6 - 9
6.2.11 Load Capacity
There are two types of loads that are of interest for motion control applica-
tions: static and dynamic loads.
The
static
Load Capacity
represents the amount of load that can be placed
on a stage without damaging or excessively deforming it. Determining the
Load Capacity
of a stage for a particular application is more complicated
than it may first appear. The stage orientation and the distance from the
load to the carriage play a significant role. For a detailed description on
how to calculate the static
Load Capacity
, please consult the motion
control catalog tutorial section.
The
dynamic
Load Capacity
refers to the motor’s effort to move the load.
The first parameter to determine is how much load the stage can
push
or
pull
. In some cases the two values could be different due to internal me-
chanical construction.
The second type of
dynamic
Load Capacity
refers to the maximum load
that the stage could move with the nominal acceleration. This parameter is
more difficult to specify because it involves defining an acceptable
follow-
ing error
during acceleration.
6.2.12 Maximum Velocity
The
Maximum Velocity
that could be used in a motion control system is
determined by both stage and driver. Usually it represents a lower value
than the motor or driver are capable of. In most cases and in particular for
the ESP6000 controller card, the default
Maximum Velocity
should not be
increased. The hardware and firmware are tuned for a particular maximum
velocity that cannot be exceeded.
6.2.13 Minimum Velocity
The
Minimum Velocity
usable with a motion device depends on the motion
control system but also on the acceptable
velocity regulation
. First, the
controller sets the slowest rate of motion increments it can make. The
encoder resolution determines the motion increment size and then, the
application sets a limit on the velocity ripple.
To illustrate this, take the example of a linear stage with a resolution of 0.1
µ
m.
If we set the velocity to 0.5
µ
m/s, the stage will move 5 encoder counts in
one second. But a properly tuned servo loop could move the stage 0.1
µ
m in
about 20ms. The position and velocity plots are illustrated in Figure
6.2-12.
1s
position
velocity
average
velocity
Figure 6.2-12 — Position, Velocity, and Average Velocity
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