Figure 6.6-9 - Single Phase Energization; Figure 6.6-10 - External Force Applied; Figure 6.6-11 - Unstable Point - Newport ESP6000 User Manual

This driving method is called mini-stepping or micro-stepping. For each step
command, the motor will move only a fraction of the full step. Motion steps
are smaller so the motion resolution is increased and the motion ripple
(noise) is decreased.
The ESP6000 drivers use the mini-stepping technique to divide the full step
in ten mini-steps, increasing the motor's resolution by a factor of 10.
However, mini-stepping comes at a price. First, the driver electronics are
significantly more complicated. Secondly, the holding torque for one step is
reduced by the mini-stepping factor. In other words, for a ×10 mini-stepping,
it takes only 1 / of the full-step holding torque to cause the motor to have a
10
positioning error equivalent to one step (a mini-step).
To clarify what this means, let's take a look at the torque produced by a
stepper motor. For simplicity, consider the case of a single phase being
energized (Figure 6.6-9).
Figure 6.6-9 — Single Phase Energization
Once the closest rotor tooth has been pulled in, assuming that we don't
have any external load, the motor does not develop any torque. This is a
stable point.
If external forces try to move the rotor (Figure 6.6-10), the magnetic flux
will oppose the forces. The more teeth misalignment exists, the larger the
generated torque.
Figure 6.6-10 — External Force Applied
If the misalignment keeps increasing, at some point, the torque peaks and
then starts diminishing. When the stator is exactly between the rotor teeth,
the torque becomes zero again (Figure 6.6-11).
Figure 6.6-11 — Unstable Point
Section 6 — Motion Control Tutorial 6 - 2 5
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