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The hybrid stepper motor combines features of both the
variable reluctance stepper and the permanent magnet stepper to produce
a smaller step angle. The rotor is a cylindrical permanent magnet,
magnetized along the axis with radial soft iron teeth (Figure below).
The stator coils are wound on alternating poles with corresponding
teeth. There are typically two winding phases distributed between pole
pairs. This winding may be center tapped for unipolar drive. The center
tap is achieved by a bifilar winding, a pair of wires wound
physically in parallel, but wired in series. The north-south poles of a
phase swap polarity when the phase drive current is reversed. Bipolar
drive is required for un-tapped windings.
Hybrid stepper motor.
Note that the 48-teeth on one rotor section are offset by half a
pitch from the other. See rotor pole detail above. This rotor tooth
offset is also shown below. Due to this offset, the rotor effectively
has 96 interleaved poles of opposite polarity. This offset allows for
rotation in 1/96 th of a revolution steps by reversing the field
polarity of one phase. Two phase windings are common as shown above and
below. Though, there could be as many as five phases.
The stator teeth on the 8-poles correspond to the 48-rotor teeth,
except for missing teeth in the space between the poles. Thus, one pole
of the rotor, say the south pole, may align with the stator in 48
distinct positions. However, the teeth of the south pole are offset from
the north teeth by half a tooth. Therefore, the rotor may align with the
stator in 96 distinct positions. This half tooth offset shows in the
rotor pole detail above, or Figure below.
As if this were not complicated enough, the stator main poles are
divided into two phases (?-1, ?-2). These stator phases are offset from
one another by one-quarter of a tooth. This detail is only discernable
on the schematic diagrams below. The result is that the rotor moves in
steps of a quarter of a tooth when the phases are alternately energized.
In other words, the rotor moves in 2×96=192 steps per revolution for the
above stepper.
The above drawing is representative of an actual hybrid stepper
motor. However, we provide a simplified pictorial and schematic
representation (Figure below) to illustrate details not obvious above.
Note the reduced number of coils and teeth in rotor and stator for
simplicity. In the next two figures, we attempt to illustrate the
quarter tooth rotation produced by the two stator phases offset by a
quarter tooth, and the rotor half tooth offset. The quarter tooth stator
offset in conjunction with drive current timing also defines direction
of rotation.
Hybrid stepper motor schematic diagram.
Features of hybrid stepper schematic
(Figure above)
- The top of the permanent magnet rotor is
the south pole, the bottom north.
- The rotor north-south teeth are offset by
half a tooth.
- If the ?-1 stator is temporarily
energized north top, south bottom.
- The top ?-1 stator teeth align north to
rotor top south teeth.
- The bottom ?-1' stator teeth align south
to rotor bottom north teeth.
- Enough torque applied to the shaft to
overcome the hold-in torque would move the rotor by one tooth.
- If the polarity of ?-1 were reversed, the
rotor would move by one-half tooth, direction unknown. The
alignment would be south stator top to north rotor bottom, north
stator bottom to south rotor.
- The ?-2 stator teeth are not aligned with the rotor teeth when ?-1
is energized. In fact, the ?-2 stator teeth are offset by one-quarter
tooth. This will allow for rotation by that amount if ?-1 is
de-energized and ?-2 energized. Polarity of ?-1 and
drive determines direction of rotation.
Hybrid stepper motor rotation sequence.
Hybrid stepper motor rotation
(Figure above)
- Rotor top is permanent magnet south,
bottom north. Fields ?1, ?-2 are switchable: on, off, reverse.
- (a) ?-1=on=north-top, ?-2=off. Align (top to bottom): ?-1 stator-N:rotor-top-S, ?-1' stator-S:
rotor-bottom-N. Start position, rotation=0.
- (b) ?-1=off, ?-2=on. Align (right to left): ?-2
stator-N-right:rotor-top-S, ?-2' stator-S: rotor-bottom-N. Rotate
1/4 tooth, total rotation=1/4 tooth.
- (c) ?-1=reverse(on), ?-2=off. Align (bottom to top): ?-1 stator-S:rotor-bottom-N, ?-1' stator-N:rotor-top-S.
Rotate 1/4 tooth from last position. Total rotation from start:
1/2 tooth.
- Not shown: ?-1=off, ?-2=reverse(on). Align (left to right): Total rotation:
3/4 tooth.
- Not shown: ?-1=on, ?-2=off (same as (a)). Align (top to
bottom): Total rotation 1-tooth.
An un-powered stepper motor with detent torque is either a permanent
magnet stepper or a hybrid stepper. The hybrid stepper will have a small
step angle, much less than the 7.5o of permanent magnet
steppers. The step angle could be a fraction of a degree, corresponding
to a few hundred steps per revolution.
Summary: hybrid stepper motor
- The step angle is smaller than variable
reluctance or permanent magnet steppers.
- The rotor is a permanent magnet with fine
teeth. North and south teeth are offset by half a tooth for a
smaller step angle.
- The stator poles have matching fine teeth
of the same pitch as the rotor.
- The stator windings are divided into no
less than two phases.
- The poles of one stator windings are offset by a quarter tooth for
an even smaller step angle.
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