Table of Contents
Stepper Motors Overview
A stepper, or stepping motor converts electronic pulses into proportionate mechanical movement. Each revolution of the stepper motor's shaft is made up of a series of discrete individual steps. A step is defined as the angular rotation produced by the output shaft each time the motor receives a step pulse. These types of motors are very popular in digital control circuits, such as robotics, because they are ideally suited for receiving digital pulses for step control. Each step causes the shaft to rotate a certain number of degrees. A step angle represents the rotation of the output shaft caused by each step, measured in degrees. Figure 6-17 illustrates a simple application for a stepper motor. Each time the controller receives an input signal, the paper is driven a certain incremental distance. In addition to the paper drive mechanism in a printer, stepper motors are also popular in machine tools, process control systems, tape and disk drive systems, and programmable controllers.
Figure 6-17 Paper drive mechanism using stepper machine
The most popular types of stepper motors are permanent-magnet (PM) and variable reluctance (VR).
Permanent-magnet (PM) Stepper Motors
The permanent-magnet stepper motor operates on the reaction between a permanent-magnet rotor and an electromagnetic field. Figure 6-18 shows a basic two-pole PM stepper motor. The rotor shown in Figure 6-18(a) has a permanent magnet mounted at each end. The stator is illustrated in Figure 6-18(b). Both the stator and rotor are shown as having teeth. The teeth on the rotor surface and the stator pole faces are offset so that there will be only a limited number of rotor teeth aligning themselves with an energized stator pole. The number of teeth on the rotor and stator determine the step angle that will occur each time the polarity of the winding is reversed. The greater the number of teeth, the smaller the step angle.
Figure 6-18 Components of a PM stepper motor: (a) Rotor; (b) stator
When a PM stepper motor has a steady DC signal applied to one stator winding, the rotor will overcome the residual torque and line up with that stator field. The holding torque is defined as the amount of torque required to move the rotor one full step with the stator energized. An important characteristic of the PM stepper motor is that it can maintain the holding torque indefinitely when the rotor is stopped. When no power is applied to the windings, a small magnetic force is developed between the permanent magnet and the stator. This magnetic force is called a residual, or detent torque. The detent torque can be noticed by turning a stepper motor by hand and is generally about one-tenth of the holding torque.
Figure 6-19(a) shows a permanent magnet stepper motor with four stator windings. By pulsing the stator coils in a desired sequence, it is possible to control the speed and direction of the motor. Figure 6-19(b) shows the timing diagram for the pulses required to rotate the PM stepper motor illustrated in Figure 6-19(a). This sequence of positive and negative pulses causes the motor shaft to rotate counterclockwise in 90° steps. The waveforms of Figure 6-19(c) illustrate how the pulses can be overlapped and the motor made to rotate counterclockwise at 45° intervals.
Figure 6-19 (a) PM stepper motor; (b) 90 step; (c) 45 step.
A more recent development in PM stepper motor technology is the thin-disk rotor. This type of stepper motor dissipates much less power in losses such as heat than the cylindrical rotor and as a result, it is considerably more efficient. Efficiency is a primary concern in industrial circuits such as robotics, because a highly efficient motor will run cooler and produce more torque or speed for its size. Thin-disk rotor PM stepper motors are also capable of producing almost double the steps per second of a conventional PM stepper motor. Figure 6-20 shows the basic construction of a thin-disk rotor PM motor. The rotor is constructed of a special type of cobalt-steel, and the stator poles are offset by one-half a rotor segment.
The stator of a variable-reluctance stepper motor has a magnetic core constructed with a stack of steel laminations. The rotor is made of unmagnetized soft steel with teeth and slots. The relationship among step angle, rotor teeth, and stator teeth is expressed using the following equation:
Figure 6-21 shows a basic variable-reluctance stepper motor. In this circuit, the rotor is shown with fewer teeth than the stator. This ensures that only one set of stator and rotor teeth will align at any given instant. The stator coils are energized in groups referred to as phases. In Figure 6-21, the stator has six teeth and the rotor has four teeth. According to Eq. (6.6), the rotor will turn 30° each time a pulse is applied. Figure 6-21 (a) shows the position of the rotor when phase A is energized. As long as phase A is energized, the rotor will be held stationary. When phase A is switched off and phase B is energized, the rotor will turn 30° until two poles of the rotor are aligned under the north and south poles established by phase B. The effect of turning off phase B and energizing phase C is shown in Figure 6-21(c). In this circuit, the rotor has again moved 30° and is now aligned under the north and south poles created by phase C. After the rot or has been displaced by 60° from its starting point, the step sequence has completed one cycle. Figure 6-2 l(d) shows the switching sequence to complete a full 360° of rotation for a variable-reluctance motor with six stator poles and four rotor poles. By repeating this pattern, the motor will rotate in a clockwise direction. The direction of the motor is changed by reversing the pattern of turning ON and OFF each phase.
Figure 6-21 Variable-reluctance stepper motor and switching sequence.
The VR stepper motors mentioned up to this point are all single-stack motors. That is, all the phases are arranged in a single stack, or plane. The disadvantage of this design for a stepper motor is that the steps are generally quite large (above 15°). Multistack stepper motors can produce smaller step sizes because the motor is divided along its axial length into magnetically isolated sections, or stacks. Each of these sections is excited by a separate winding, or phase. In this type of motor, each stack corresponds to a phase, and the stator and rotor have the same tooth pitch.
Hybrid Stepper Motors
The hybrid step motor consists of two pieces of soft iron, as well as an axially magnetized, round permanent-magnet rotor. The term hybrid is derived from the fact that the motor is operated under the combined principles of the permanent magnet and variable-reluctance stepper motors. The stator core structure of a hybrid motor is essentially the same as its VR counterpart. The main difference is that in the VR motor, only one of the two coils of one phase is wound on one pole, while a typical hybrid motor will have coils of two different phases wound on one the same pole. The two coils at a pole are wound in a configuration known as a bifilar connection. Each pole of a hybrid motor is covered with uniformly spaced teeth made of soft steel. The teeth on the two sections of each pole are misaligned with each other by a half-tooth pitch. Torque is created in the hybrid motor by the interaction of the magnetic field of the permanent magnet and the magnetic field produced by the stator.
Stepper motors are rated in terms of the number of steps per second, the stepping angle, and load capacity in ounce-inches and the pound-inches of torque that the motor can overcome. The number of steps per second is also known as the stepping rate. The actual speed of a stepper motor is dependent on the step angle and step rate and is found using the following equation:
Figure 6-22 shows a plot of the relationship between pull-in torque versus pulses per second for a typical stepper motor. From this curve, it is apparent that torque is greatest at zero steps per second and decreases as the number of steps increases.
Figure 6-22 Torque versus steps per second for a stepper motor.
The direction of rotation is determined by applying the pulses to either the clockwise or counterclockwise drive circuits. Rotor displacement can be very accurately repeated with each succeeding pulse. Stepping motors are generally operated without feedback, which simplifies the control circuit considerably. One of the most common stepper motor drive circuits is the unipolar drive, shown in Figure 6-23. This circuit uses bifilar windings and four Darlington transistors to control the direction of rotation and the stepping rate of the motor.
Figure 6-23 Unipolar stepper motor drive.
Stepper motor drivers are available in half-step or full-step configurations. Full-step drivers are the simplest in design and have a control sequence of two on-time periods followed by two off-time periods. The half-step mode of operation provides a smoother, quieter performance with higher speed capability and efficiency. Figure 6-24(a) shows the switching sequence waveshapes of a typical stepper motor. Each stepper motor winding is energized one in every four input pulses. Consequently, the pulse train for each winding has a 25 percent duty cycle. The stepper motor output shown in Figure 6-24(b) has a step angle of 30°.
Figure 6-24 Switching sequence waveshapes.
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volt details for mae hy200 stepper motor
i need voltage detail for mae hy 200 stepper motor
- firstname.lastname@example.org - Mar 7, 2010
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