Minimum Incremental Motion; Figure 6.2-4 - Effect Of Stiction And Elasticity On Small Motions; Figure 6.2-5 - Error Plot - Newport ESP6000 User Manual

Section 6 — Motion Control Tutorial
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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
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.
(motion
1
increments)
–1
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 1µm resolution. If every time
we command a 1µm 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 1µm. If, on the other hand, the measured motion is
sometimes as small as 0.1µm (a 90% error), we could not say that 1µm 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.
Elasticity
Error
Figure 6.2-5 — Error Plot
Motor/Encoder
Motion steps
Position
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