 Unipolar
stepping motors, both Permanent magnet and hybrid stepping motors with 5
or 6 wires are usually wired as shown in the schematic in Figure 1.2, with
a center tap on each of two windings. In use, the center taps of the windings
are typically wired to the positive supply, and the two ends of each winding
are alternately grounded to reverse the direction of the field provided
by that winding. An animated
GIF of figure 1.2 is available.
The motor cross section shown in Figure 1.2 is of a 30 degree per step permanent
magnet or hybrid motor -- the difference between these two motor types is
not relevant at this level of abstraction. Motor winding number 1 is distributed
between the top and bottom stator pole, while motor winding number 2 is
distributed between the left and right motor poles. The rotor is a permanent
magnet with 6 poles, 3 south and 3 north, arranged around its circumference.
For higher
angular resolutions, the rotor must have proportionally more poles. The
30 degree per step motor in the figure is one of the most common permanent
magnet motor designs, although 15 and 7.5 degree per step motors are widely
available. Permanent magnet motors with resolutions as good as 1.8 degrees
per step are made, and hybrid motors are routinely built with 3.6 and
1.8 degrees per step, with resolutions as fine as 0.72 degrees per step
available.
As shown in the figure, the current flowing from the center tap of winding
1 to terminal a causes the top stator pole to be a north pole while the
bottom stator pole is a south pole. This attracts the rotor into the position
shown. If the power to winding 1 is removed and winding 2 is energized,
the rotor will turn 30 degrees, or one step.
To rotate the motor continuously, we just apply power to the two windings
in sequence. Assuming positive logic, where a 1 means turning on the current
through a motor winding, the following two control sequences will spin
the motor illustrated in Figure 1.2 clockwise 24 steps or 4 revolutions:
Winding 1a 1000100010001000100010001
Winding 1b 0010001000100010001000100
Winding 2a 0100010001000100010001000
Winding 2b 0001000100010001000100010
time --->
Winding 1a 1100110011001100110011001
Winding 1b 0011001100110011001100110
Winding 2a 0110011001100110011001100
Winding 2b 1001100110011001100110011
time --->
Note that the two halves of each winding are never energized at the same
time. Both sequences shown above will rotate a permanent magnet one step
at a time. The top sequence only powers one winding at a time, as illustrated
in the figure above; thus, it uses less power. The bottom sequence involves
powering two windings at a time and generally produces a torque about
1.4 times greater than the top sequence while using twice as much power.
The step positions produced by the two sequences above are not the same;
as a result, combining the two sequences allows half stepping, with the
motor stopping alternately at the positions indicated by one or the other
sequence. The combined sequence is as follows:
Winding 1a 11000001110000011100000111
Winding 1b 00011100000111000001110000
Winding 2a 01110000011100000111000001
Winding 2b 00000111000001110000011100
time --->
|
