Page 688 - Mechatronics with Experiments

674 MECHATRONICS
1 2 3 4 1 2 3 4 5 6 7 8
Phase 1 Phase 1

Phase 2 Phase 2

Full step current, 2-phase on Half step current
(a) (b)

1 2 3 4 5 6 7 8 9
Phase 1 current: zero

Phase 2

Phase currents in microstep mode
(c) (d)
FIGURE 8.50: Phase currents in a stepper motor in different operating modes: (a) full step
mode, (b) half step mode, (c) modified (increased current during alternate half steps) half step
mode, and (d) microstepping mode.

is OFF (i.e., both S1 and S2 OFF, or both S3 and S4 OFF). The corresponding stable rotor
positions are shown in the figure. However, notice that since the magnetic force pulling the
rotor is provided by only one phase, the holding torque of the motor at these switch states
is less than (approximately 1∕2) that of the holding torque at the full-step mode. This mode
of switching power transistors at the drive is referred to as the “half step” mode. We can,
therefore, energize the motor in alternate modes (one full step and one half step) in every
other step in order to increase its positioning resolution (from 4 stable positions to 8 stable
positions). The current pattern in the phase windings in full and half step modes is shown
in Figure 8.50. In order to make sure the torque capacity is similar at all steps, the current
can be increased during the half step mode compared to the current during full step mode
(Figure 8.50c). The step size is defined as the smallest rotor position change that can be
achieved by switching the current from one state to another state in the stator winding using
full step and half step modes. For this simple concept stepper motor, step size is
360 ◦
= = 45 (8.265)
step
8
The standard hybrid permanent magnet (PM) step motor operates in the same way as
our simple model, but has a greater number of teeth on the rotor and stator, giving a smaller
basic step size. The rotor is in two sections axially. The two sections of the rotor, both
sections with teeth, are separated by a permanent magnet (Figure 8.47). North and south
poles are magnetized axially, so the N-pole is on one side and the S-pole is on the other side
along the shaft. Hence, one side is magnetized as the north pole, the other side is magnetized
as the south pole. Furthermore, there is a one-half tooth pitch angular displacement between
the two sections (north and south sections) on the rotor. Let us consider an example hybrid