A stepper motor is an electromechanical device which converts electrical pulses into discrete mechanical movements. It divides a full rotation into a number of equal steps. A stepping motor is driven by a series of electrical pulses, which are generated by the MCU in an NC system. Each pulse causes the motor to rotate a fraction of one revolution, called the step angle. The possible step angles must be consistent with the following relationship: ?=360ns Where ? = step angle (degrees), and ns, = the number of step angles for the motor, which must be an integer.
The angle through which the motor shaft rotates is given by Am=np? Where Am = angle of motor shaft rotation (degrees), np = number of pulses received by the motor. Advantages 1. The rotation angle of the motor is proportional to the input pulse. 2. The motor has full torque at standstill (if the windings are energized) 3. Precise positioning and repeatability of movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non-cumulative from one step to the next. 4. Excellent response to starting/stopping/reversing. . Very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing. 6. The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control. 7. It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft. 8. A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses. Disadvantages 1.
Resonances can occur if not properly controlled. 2. Not easy to operate at extremely high speeds. Mechanism DC brush motors rotate continuously when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first, one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth.
When the gear’s teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. When the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. The concept of stepper motor process is presented in figure 1. Figure 1. Animation of a simplified stepper motor Frame 1: The top electromagnet (1) is turned on, attracting the nearest teeth of the gear-shaped iron rotor. With the teeth aligned to electromagnet 1, they will be slightly offset from right electromagnet (2).
Frame 2: The top electromagnet (1) is turned off, and the right electromagnet (2) is energized, pulling the teeth into alignment with it. This results in a rotation of 3. 6° in this example. Frame 3: The bottom electromagnet (3) is energized; another 3. 6° rotation occurs. Frame 4: The left electromagnet (4) is energized, rotating again by 3. 6°. When the top electromagnet (1) is again enabled, the rotor will have rotated by one tooth position; since there are 25 teeth, it will take 100 steps to make a full rotation in this example. Types of Stepper Motor
There are three basic types of stepping motors: permanent magnet, variable reluctance and hybrid. This application note covers all three types. Permanent magnet motors have a magnetized rotor, while variable reluctance motors have toothed soft-iron rotors. Hybrid stepping motors combine aspects of both permanent magnet and variable reluctance technology. The stator or stationary part of the stepping motor holds multiple windings. The arrangement of these windings is the primary factor that distinguishes different types of stepping motors from an electrical point of view.
From the electrical and control system perspective, variable reluctance motors are distant from the other types. Both permanent magnet and hybrid motors may be wound using either unipolar windings, bipolar windings or bifilar windings. Permanent Magnet (PM) often referred to as a “tin can” or “canstock” motor the permanent magnet step motor is a low cost and low resolution type motor with typical step angles of 7. 5° to 15°. (48 – 24 steps/revolution) PM motors as the name implies have permanent magnets added to the motor structure. The rotor no longer has teeth as with the VR motor.
Instead the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft. These magnetized rotor poles provide an increased magnetic flux intensity and because of this the PM motor exhibits improved torque characteristics when compared with the VR type. Figure 2. Permanent Magnet Stepper Motor Variable-reluctance (VR) stepper motor has been around for a long time. It is probably the easiest to understand from a structural point of view. Figure 1 shows a cross section of a typical V. R. stepper motor.
This type of motor consists of a soft iron multi-toothed rotor and a wound stator. When the stator windings are energized with DC current the poles become magnetized. Rotation occurs when the rotor teeth are attracted to the energized stator poles. Figure 3. Cross-section of a variable reluctance (VR) motor Hybrid (HB) stepper motor is more expensive than the PM stepper motor but provides better performance with respect to step resolution, torque and speed. Typical step, for the HB stepper motor angles, range from 3. 6° to 0. 9° (100 – 400 steps per revolution).
The hybrid stepper motor combines the best features of both the PM and VR type stepper motors. The rotor is multi-toothed like the VR motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide an even better path which helps guide the magnetic flux to preferred locations in the air gap. This further increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM types. Figure 4. Cross-section of a hybrid stepper motor Application A stepper motor can be a good choice whenever controlled movement is required.
They can be used to advantage in applications where you need to control rotation angle, speed, position and synchronism. Because of the inherent advantages listed previously, stepper motors have found their place in many different applications. Some of these include printers, plotters, high-end office equipment, hard disk drives, medical equipment, fax machines, automotive and many more. Perhaps the best known application of stepper motor is in analog quartz watch. Figure 5. The coil of the stepper motor that powers the watch (shown on the top left)
References
Reston Condit and Douglas W. Jones (2004). Stepping Motors Fundamentals. Microchip Inc. Mikell P. Groover (2007). Automation, Production Systems, and Computer-Integrated Manufacturing, Third Edition. Prentice Hall, ISBN 0132393212 Bela G. Liptak (2010). Instrument Engineers’ Handbook, Fourth Edition, Volume Two: Process Control and Optimizatio. Taylor & Francis, ISBN 1420064002
