4.7 EDH0162En1040 – 06/99
MM4005 Motion Control Tutorial
4.2.6 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.
Fig. 4.4 — Effect of Stiction and Elasticity on Small Motions.
Fig. 4.4 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.
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 dis-
tance. The error plot will be similar to Fig. 4.5.
Fig. 4.5 — Error Plot.
Once the Minimum Incremental Motion is defined, the next task is to quan-
tify 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 µm resolution. If every time
we command a 1 µm 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 µm. If, on the other hand, the measured
motion is sometimes as small as 0.1 µm (a 90% error), we could not say
that 1 µm is a reliable motion step. The difficulty is in drawing the line
between acceptable 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 stan-
dards.
One way to solve the problem is to take a large number of measurements (a
few hundred at minimum) for each motion step size and present them in a
format that an operator can use to determine the Minimum Incremental
Motion by its own standards.
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