Page 686 - Mechatronics with Experiments

672 MECHATRONICS
AC power source

DC power Stepper
supply motor

Step
Switch
Controller Translator
set
Direction R

FIGURE 8.48: Stepper motor control system components: step motor, DC power supply,
power switch set, translator, controller.

coil. The stator and rotor have teeth. For a given state of a stator current, the rotor moves
to align its teeth with those of the stator by the natural tendency to minimize reluctance
to magnetic flux. The air gap between the stator and rotor represents the resistance to
the magnetic flux. The smaller the air gap is, the less the resistance to magentic flux,
hence the higher the torque capacity of the motor. A typical air gap in stepper motors is
in the range of 30–125 μm. The components of a stepper motor control system are shown
in Figure 8.48. A given switch state of a stator represents a magnetic flux field. There is
a corresponding stable rotor position for each stator phase current condition. The “switch
set” block represents the power transistors. The “translator” block represents the logic
block which determines the order and time the power transistors should be switched based
on a planned motion which is generated by the controller. It is the responsibility of the
translator block or the controller block to limit the maximum switching frequency so that
the rotor is not left behind to the point of missing a step. Similarly, the translator block
should minimize operating at a switching frequency range which might excite the natural
resonance frequency of the stepper motor. Today, stepper motor sizes are standardized and
the most common ones are NEMA 17, NEMA 23, NEMA 34, and NEMA 42.
A given phase current condition generates a certain stable rotor position, not con-
tinuous torque. Normally the drive or the controller does not need position feedback. But
position feedback is commonly used as a way to detect possible step loss and compensate
for it when necessary.
The translator, switch set, and DC power supply blocks are collectively called the
“drive.” Drive controls the current in each phase. Velocity or position feedback is used to
control the speed or position of the motor.

8.6.1 Basic Stepper Motor Operating Principles
Let us consider the operating principles of a basic stepper motor. In our basic stepper motor,
the rotor has one north and one south pole permanent magnet. The stator has four-pole,
two-phase winding with four switches (Figure 8.49). At any given time either switch S1 or
S2, and S3 or S4 can be ON to affect the polarity of electromagnets. For each switch state,
there is a corresponding stable rotor position. In this concept figure, a stepper motor with
unipolar winding is shown where each coil is center tapped to the ground.