A stepper motor is an electric machine that rotates in discrete angular increments or steps. Stepper motors are operated by applying current pulses of a specific frequency to the inputs of the motor. Each pulse applied to the motor causes its shaft to the Nema 42 stepper motor causes its shaft to move a certain angle of rotation, called a stepping angle. Since the input signal is converted directly into a requested shaft position without any rotor position sensors or feedback, the stepper motor has the following advantages:
- Rotational speed proportional to the frequency of input pulses
- Digital control of speed and position
- No need of feedback sensor for open loop control Excellent acceleration and deceleration responses to step commands
The stepper drive motor has salient poles on both the stator and the rotor, andnormally only the stator poles hold the poly-phase windings called the controlwindings. Usually stepper motors are classified as:
- Active rotor (permanent magnet rotor)
- Reactive rotor (reluctance type)
- Hybrid motors (combining the operating principles of the permanentmagnet (PM) and reluctance stepper motor)
While each of these types of stepper motors has merit, hybrid stepper motorsare becoming more popular in industrial applications. In this chapter, we focus onthe principles and implementation of a hybrid stepper motor control system usingthe LF2407 DSP controller.
The operation of the stepper motor relies on the simple principle of magneticattraction. This principle states that opposite magnetic poles attract while like polesrepel each other. If the windings are excited in the correct sequence, the rotor will rotate following a certain direction. The basic operation of a stepper motor can beclassified generally as either full step mode or half step mode. These modes are discussed in detail in the following section using the simplified stepper motorconstruction shown in Fig. 8.1.
If none of the stator windings are excited, an attraction between the stator polesand rotor teeth still exists because the PM rotor is trying to minimize the reluctanceof the magnetic flux path from one end to the other. As a result, the rotor will tend to rest at one of the rest equilibrium positions. From Fig. 8.1, a rest position existswhen a pair of rotor teeth are aligned with two of the stator poles. In the case ofFig. 8.1, the rotor is aligned with pole 1 and pole 3 on the stator. There are a total of12 possible equilibrium positions for a 4-phase, 6-pole stepper motor. The force ortorque that holds the rotor in one of these positions is called the detent torque. The value of the detent torque is usually small because no current flows through thestator windings.
The stepper motor operation discussed rotates 300 per step. In the half step mode, alternately exciting one winding, then exciting two windings, will cause therotor to move through only 15 degree per step. Though there is a slight loss of thetorque while the single winding is being excited, half-step operation allows forsmoother operation at lower speeds and less overshoot at the end of each step. The excitation sequence of the stator windings in half-step mode is given in Table 8.3.During this operation, each switch between the two nearest modes will cause a 450 shift of stator field which results in a 150 rotation of the rotor. A total of 24steps are required for a complete revolution, double of what is required for full stepmodes.
Micro Step Mode
For the operating modes discussed previously, the same amount of current flows through the energized stator windings. However, if the currents are not equal, the rotor will be shifted towards the stator pole with the higher current. The amount of deviation is proportionate to the values of the currents in each winding. This principle is utilized in the microstep mode. During this mode, each basic full mode step can be divided into as many as 500 microsteps, providing the proper current profile is applied.