Stepper Motors Working
Stepper motors contains of a permanent magnetic
rotating shaft, called the rotor, and electromagnets on the stationary portion
that surrounds the motor, called the stat-or Fig-01 illustrates one complete rotation of a stepper motor. At
position 1, we can see that the rotor is beginning at the upper electromagnet,
which is currently active (has voltage applied to it). To move the rotor
clockwise (CW), the upper electromagnet is deactivated and the right
electromagnet is activated, causing the rotor to move 90 degrees CW, aligning
itself with the active magnet. This process is repeated in the same manner at
the south and west electromagnets until we once again reach the starting
position.
In the above example, we used a motor with a resolution
of 90 degrees or demonstration purposes. In reality, this would not be a very
practical motor for most applications. The average stepper motor's resolution
-- the amount of degrees rotated per pulse -- is much higher than this. For
example, a motor with a resolution of 5 degrees would move its rotor 5 degrees
per step, thereby requiring 72 pulses (steps) to complete a full 360 degree
rotation.
There are several types of stepper motors. 4-wire
stepper motors contain only two electromagnets; however the operation is more
complicated than those with three or four magnets, because the driving circuit
must be able to reverse the current after each step. For our purposes, we will
be using a 6-wire motor.
we may double
the resolution of some motors by a process known as "half-stepping".
Instead of switching the next electromagnet in the rotation on one at a time,
with half stepping you turn on both electromagnets, causing an equal attraction
between, thereby doubling the resolution. As you can see in Fig-02, in the first position only the
upper electromagnet is active, and the rotor is drawn completely to it. In
position 2, both the top and right electromagnets are active, causing the rotor
to position itself between the two active poles. Finally, in position 3, the
top magnet is deactivated and the rotor is drawn all the way right. This
process can then be repeated for the entire rotation.
Unlike our example motors which rotated 90 degrees
per step, real-world motors employ a series of mini-poles on the stator and
rotor to increase resolution. Although this may seem to add more complexity to
the process of driving the motors, the operation is identical to the simple 90
degree motor we used in our example. An example of a multi-pole motor can be
seen in Fig-03. In position 1,
the north pole of the rotor's permanent magnet is aligned with the south pole
of the stator's electromagnet. Note that multiple positions are aligned at
once. In position 2, the upper electromagnet is deactivated and the next one to
its immediate left is activated, causing the rotor to rotate a precise amount
of degrees.
In
this example, after eight steps the sequence repeats.
The specific stepper motor we are using for our
experiments (ST-02: 5VDC, 5 degrees per step) has 6 wires coming out of the
casing. If we follow Fig-04,
the electrical equivalent of the stepper motor, we can see that 3 wires go to
each half of the coils, and that the coil windings are connected in pairs. This
is true for all four-phase stepper motors.
However, if you do not have an equivalent diagram
for the motor you want to use, you can make a resistance chart to decipher the
mystery connections. There is a 13 ohm resistance between the center-tap wire
and each end lead, and 26 ohms between the two end leads. Wires originating
from separate coils are not connected, and therefore would not read on the ohm
meter.
First Stepper Circuit
Fig-05
is the schematic of our first test circuit. The PIC's output lines are first
buffered by a 4050 hex buffer chip, and are then connected to an NPN
transistor. The transistor used, TIP120, is actually a NPN Darlington (it is
shown as a standard NPN).
Due to a inductive surge created when a coil is
toggled, a standard 1N4001 diode is usually placed across each transistor as
shown in the figure, providing a safe way of dispersing the reverse current
without damaging the transistor.
The TIP120 transistors do not need an
external snubbing diode because they have a built in diode. So
the diodes shown in the drawing are the internal diodes in the TIP120
transistors.
The simplest way to operate a stepper motor with a
PIC is with the full step pattern shown in Table 1. Each part of the sequence
turns on only one transistor at a time, one after the other. After the sequence
is completed, it repeats infinitely until power is removed.
Table 1
|
Q1
|
Q2
|
Q3
|
Q4
|
|
+
|
-
|
-
|
-
|
|
-
|
+
|
-
|
-
|
|
-
|
-
|
+
|
-
|
|
-
|
-
|
-
|
+
|
