Hybrid Stepper Motor

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.



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 (φ-1, φ-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.

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 φ-1 stator is temporarily energized north top, south bottom.
The top φ-1 stator teeth align north to rotor top south teeth.
The bottom φ-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 φ-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 φ-2 stator teeth are not aligned with the rotor teeth when φ-1 is energized. In fact, the φ-2 stator teeth are offset by one-quarter tooth. This will allow for rotation by that amount if φ-1 is de-energized and φ-2 energized. Polarity of φ-1 and drive determines direction of rotation.



Hybrid stepper motor rotation (Figure above):

Rotor top is permanent magnet south, bottom north. Fields φ1, φ-2 are switchable: on, off, reverse.
(a) φ-1=on=north-top, φ-2=off. Align (top to bottom): φ-1 stator-N:rotor-top-S, φ-1′ stator-S: rotor-bottom-N. Start position, rotation=0.
(b) φ-1=off, φ-2=on. Align (right to left): φ-2 stator-N-right:rotor-top-S, φ-2′ stator-S: rotor-bottom-N. Rotate 1/4 tooth, total rotation=1/4 tooth.
(c) φ-1=reverse(on), φ-2=off. Align (bottom to top): φ-1 stator-S:rotor-bottom-N, φ-1′ stator-N:rotor-top-S. Rotate 1/4 tooth from last position. Total rotation from start: 1/2 tooth.
Not shown: φ-1=off, φ-2=reverse(on). Align (left to right): Total rotation: 3/4 tooth.
Not shown: φ-1=on, φ-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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