Reference guide

Cluster (Dashboard) Using S12HZ256 as a Single-Chip Solution Designer Reference Manual, Rev. 0
Freescale Semiconductor 13
Chapter 3
Stepper Motor Drive Theory
3.1 Introduction
The stepper motor (MMT) is an excellent choice for cluster applications. This application employs high
current drives suited for pulse width modulator (PWM) motor control, which supports sine and cosine
drive, using the 9S12HZ256 device. Software running on the 9S12HZ256 device implements:
• Micro-stepping to achieve precise linear position
• Reduced noise
• Vibration during stepper motor running.
3.2 Stepper Motor
In theory, although a micro-stepping controller offers hundreds of intermediate positions between steps,
but it has some drawbacks. It is worth noting that micro-stepping does not generally offer great precision.
This is because of both linearity problems and the effects of static friction.
The utility of micro-stepping is limited by several considerations:
1. The angular precision achievable with micro-stepping will be limited if there is any static friction in
the system.
2. It involves the non-sinusoidal character of the torque versus shaft-angle curves on real motors.
Sometimes, this is attributed to the detent torque on permanent magnet and hybrid motors. But, in
fact both detent torque and the shape of the torque versus angle curves are products of poorly
understood aspects of motor geometry. Specifically the shapes of the teeth on the rotor and stator,
these teeth are almost always rectangular.
3. Problems arise because most applications of micro-stepping involve digital control systems. Thus,
the current through each motor winding is quantized, controlled by a digital-to-analog converter
(PWM). Furthermore, if typical PWM current limiting circuitry is used, the current through each
motor winding is not held perfectly constant, but rather, oscillates around the current control
circuit's set point. As a result, the best a typical micro-stepping controller can do is approximate the
desired currents through each motor winding.
In view of these considerations, note that the drive stepper motor running smoothly in micro-stepping
depends on the motor characteristics. The number of the steps in micro-stepping selection is also based
on the stepper motor. We chose 24 micro-stepping / step in this reference design manual as an example.