【正文】 ered, the motor exhibits some magic resistance to turning. ? Variablereluctance (VR) stepper motor — Unlike the PM stepper motor, the VR stepper motor does not have a permanentmag and creates rotation entirely with electromagic forces. This motor does not exhibit magic resistance to turning when the motor is not powered. What is Inside? 畢業(yè)設(shè)計（論文） 31 Generally, a stepper motor consists of a stator, a rotor with a shaft, and coil windings. The stator is a surrounding casing that remains stationary and is part of the motor housing, while the rotor is a central shaft within the motor that actually spins during use. The characteristics of these ponents and how they are arranged determines whether the stepper motor is a PM or VR stepper motor. Figure 2 and Figure 3 show an example of these internal ponents. Figure 2. Permanent Mag (PM) Stepper Motor Taking a closer look, the rotor in PM stepper motors is actually a permanentmag. In some cases, the permanent mag is in the shape of a disk surrounding the rotor shaft. One arrangement is a magic disk which consists of north and south magic poles interlaced together. The number of poles on the magic disk varies from motor to motor. Some simple PM stepper motors such as the one in Figure 2 only have two poles on the disk, while others may have many poles. The stator usually has two or more coil windings, with each winding around a soft metallic core. 畢業(yè)設(shè)計（論文） 32 When electrical current flows through the coil windings, a magic field is generated within the coil. The metallic core is placed within the coil windings to help channel the electromagic field perpendicular to the outer perimeter of the magic disk. What is Inside? Depending upon the polarity of the electromagic field generated in the coil (north pole, out of the coil, or south pole, into the coil) and the closest permanent magic field on the disk, an attraction or repulsion force will exist. This causes the rotor to spin in a direction that allows an opposite pole on the perimeter of the magic disk to align itself with the electromagic field generated by the coil. When the nearest opposite pole on the disk aligns itself with the electromagic field generated by the coil, the rotor will e to a stop and remain fixed in this alignment as long as the electromagic field from the coil is not changed. VR stepper motors work in a very similar fashion. Figure 3 shows some of the physical details that characterize its operation. In a VR stepper motor, the surrounding coils that are physically located opposite of each other are energized to create opposite magic fields. For example, in Figure 3a), coil C produces a southpole magic field, and coil C produces a northpole magic field. The magic fields produced by the coils pass through the air gap and through the metallic rotor. Because the magic fields attract each other, the metallic rotor spins in a direction that brings the nearest edges (2 and 4) of the rotor as close as possible to the pair of energized coils (C and C). Like the PM stepper rotor, the VR stepper rotor will remain aligned to the coils as long as coils C and C are energized and the magic fields are not changed. To move to the next state and continue this rotation, coils C and C must be deenergized, while coils A and A must be oppositely energized to attract rotor edges 1 and 3 respectively. The 畢業(yè)設(shè)計（論文） 33 same process occurs with coils B and B to attract rotor edges 2 and 4 respectively, and so on. Figure 3 shows how the rotor spins as the coils are energized and deenergized. This is an example of a 3phase VR motor. Figure 3. How the Variable Reluctance (VR) Rotor Spins From the examples discussed earlier, we can see that if the electromagic fields in both the PM and VR stepper motors are turned on, off, and reversed in the proper sequence, the rotor can be turned in a specific direction. Each time an electromagic field bination is changed, the rotor may turn a fixed number of degrees. As these state changes in electromagic fields take place more rapidly, on the order of milliseconds, the rotor can rotate faster, smoother, and sometimes more quietly. Because of the mechanical limitations of the system, the rotor can only rotate effectively up to certain speeds. An external device, such as an HCS12 microcontroller (or, MCU), is very good for controlling the electromagic sequences by directing the flow of current through the coil windings. To do this, software can be written and loaded into an HCS12 MCU. Waveforms that can Drive a Stepper Motor Stepper motors have input pins or contacts that allow current from a supply source (in this applicationnote, a microcontroller) into the coil windings of the motor. Pulsed waveforms in the correct pattern can be used to create the electromagic fields needed to drive the motor. Depending on the design and 畢業(yè)設(shè)計（論文） 34 characteristics of the stepper motor and the motor performance desired, some waveforms work better than others. Although there are a few options to choose from when selecting a waveform to drive a two phase PM stepper motor, such as fullstepping or microstepping, this application note focuses on one called halfstepping. A graph of the waveform is given in Figure 4. In Figure 4a), four signals are shown. These signals can be produced by a dedicated stepper driver or a microcontroller. Each signal (a, a, b, b) is applied to a coil terminal. Because each coil has two terminals ， two signals must work together to drive a single coil. If we consider terminal a as a positive reference, then the bination of signals a and a cause the coil to see an effective signal A, shown in Figure 4b). Likewise, signal B in Figure 4b) is produced by bining signals b and b from Figure 4a). It is worth noting that the individual waveforms (a, a, b, b) directly from