Newport ESP6000, Руководство пользователя

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Section 6 — Motion Control Tutorial 6 - 5
6.2.6 Minimum␣ Incremental Motion
The Minimum Incremental Motion is the smallest motion that a device can
reliably make, measured with an external precision measuring device. The
controller can, for instance, execute a motion equal to the
Resolution (one
encoder count) but in reality, the load may not move at all. The cause for
this is in the mechanics.
Figure 6.2-4 shows how excessive stiction and elasticity between the
encoder and the load can cause the motion device to deviate from ideal
motion when executing small motions.
Load
Motor/Encoder
Elasticity
Stiction
Figure 6.2-4 — Effect of Stiction and Elasticity on Small Motions
The effect of these two factors has a random nature. Sometimes, for a small
motion step of the motor, the load may not move at all. Other times, the
accumulated energy in the
spring will cause the load to jump a larger
distance. The error plot will be similar to Figure 6.2-5.
Error
(motion
increments)
–1
1
Position
Motion steps
Figure 6.2-5 — Error Plot
Once the Minimum Incremental␣ Motion is defined, the next task is to
quantify it. This is more difficult for two reasons: one is its random nature
and the other is in defining what a
completed motion represents.
Assume that we have a motion device with a 1
µ
m resolution. If every time
we command a 1
µ
m motion the measured error is never greater than 2%, we
will probably be very satisfied and declare that the Minimum Incremental
Motion is better than 1
µ
m. If, on the other hand, the measured motion is
sometimes as small as 0.1
µ
m (a 90% error), we could not say that 1
µ
m is a
reliable motion step. The difficulty is in drawing the line between accept-
able and unacceptable errors when performing a small motion step. The
most common value for the maximum acceptable error for small motions is
20%, but each application ultimately has its own standards.
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