Advanced Parallel I/O - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 350

N

C4

C4

Rotor

Rotor

S

N

N S N S N S

S

C1

C2

C2 C1

N

C3

C3

Step 1 S

Step 2

S

C4

C4 Rotor

Rotor

N

S

S N S N S N

N C1

C2

C1

C2

S

C3

C3

Step 4

Step 3

N

Figure 7.38 ■ Full-step counterclockwise operation of step motor

proximity are energized, there is a resultant magnetic field whose center will depend on the rela-
tive strengths of the two magnetic fields. Figure 7.39 illustrates the half-stepping sequence.
The step sizes of stepper motors vary from approximately 0.72 to 90 degrees. However, the
most common step sizes are 1.8, 7.5, and 15 degrees. The steps of 90 or 45 degrees are too crude
for many applications.
The actual stator (the stationary electromagnets) of a real motor has more segments on it
than previously indicated. One example is shown in Figure 7.40. The rotor is also a little bit dif-
ferent and is also shown in Figure 7.40.
In Figure 7.40, the stator has eight individual sections (coils) on it and hence the angle
between two adjacent sections is 45 degrees. The rotor has six sections on it and hence there
are 60 degrees between two adjacent sections. Using the principle of a Vernier mechanism, the
actual movement of the rotor for each step would be 60-45 degrees, or 15 degrees. Interested
readers should try to figure out how these sections are energized to rotate the motor in the
clockwise and counterclockwise directions.

7.11.2 Stepper Motor Drivers
Driving a stepper motor involves applying a series of voltages to the coils of the motor. A sub-
set of coils is energized at the same time to cause the motor to rotate one step. The pattern of
coils energized must be followed exactly for the motor to work correctly. The pattern will vary
depending on the mode used on the motor. A microcontroller can easily time the duration that
the coil is energized, and hence control the speed of the stepper motor in a precise manner.
The circuit in Figure 7.41 shows how the transistors are used to switch the current to
each of the four coils of the stepper motor. The diodes in Figure 7.41 are called fly back