Stepper Motors

• A stepper motor is an electro-mechanical device which converts electrical pulses into discrete mechanical movements.The shaft or spindle of a stepper motor rotates in discrete step increments when electrical command pulses are applied to it in the proper sequence.The sequence of the applied pulses is directly related to the direction of motor shafts rotation.


• The speed of the motor shafts rotation is directly related to the frequency of the input pulses. A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. It works on the principle of electromagnetism.


• Stepper motors have emerged as cost-effective alternatives for DC servomotors in high-speed, motion-control applications (except the high torque-speed range) with the improvements in permanent magnets and the incorporation of solid-state circuitry and logic devices in their drive systems.


• Today stepper motors can be found in computer peripherals, machine tools, medical equipment, automotive devices, and small business machines, to name a few applications.


• Stepper motors are usually operated in open loop mode.

Classification of stepper motor

• Permanent Magnet Stepper Motor.
• Variable Reluctance Stepper Motor.
• Hybrid Stepper Motor.

Features of stepper motors:

• There are lots of special features of stepper motor; some common special/custom features are shaft modifications, connectors, wire length, conduit boxes or terminal boxes, molded cables, and shielded cables.


• Operational speed is proportional to the input frequency


• High torque even at lower speeds


• Simple and low-priced control electronics


• High expected service life (brushless, therefore service life depends only on the bearings)


• Controlled by digital pulses for speed and position


• Step-accurate positioning without external linear encoder (open loop)


• The high stiffness enables the rotor to remain in its holding position without requiring a brake.

Construction of Permanent Magnet Stepper Motor:

• It is also called a ”Tin Can” or ”steel shell” motor. The permanent magnet stepper motor is used when low power consumption, precision and reliability are required.


• Permanent magnet stepper motors withstand harsh operating conditions.The permanent-magnet stepper motor operates on the reaction between a permanent-magnet rotor and an electromagnetic field. The rotor shown in Figure has a permanent magnet mounted at each end.


• The stator is illustrated in Figure. Both the stator and rotor are shown as having teeth. 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. Greater the number of teeth, smaller the step angle.

Functioning of Permanent magnet stepper motor:

• Stepper motors operate differently from normal DC motors, which rotate 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 micro controller. 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 thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So, 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.


• Each of those slight rotations is called a "step," with an integral (complete number) number of steps making a full rotation. In that way, the motor can be turned by a precise angle.


• The rotor and stator poles of a permanent magnet stepper are not teethed. Instead the rotor have alternative north and south poles parallel to the axis of the rotor shaft.


• When a stator is energized, it develops electromagnetic poles. The magnetic rotor aligns along the magnetic field of the stator.


• The other stator is then energized in the sequence so that the rotor moves and aligns itself to the new magnetic field. This way energizing the stators in a fixed sequence rotates the stepper motor by fixed angles and have a large stepping angle of 7.5° to 18°.


• As a permanent magnet stepper motor rotates, each pole of the stator must be capable of producing either a north or south magnetic pole, depending on the position of the rotor.As a result, the coils in the stator winding must be able to take bi-directional currents.


• The resolution of a permanent magnet stepper can be increased by increasing number of poles in the rotor or increasing the number of phases.

Characteristics of stepper motor:

Stepper motors are constant power devices. As motor speed increases, torque decreases. The torque curve may be extended by using current limiting drivers and increasing the driving voltage. Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. This vibration can become very bad at some speeds and can cause the motor to lose torque. The effect can be mitigated by accelerating quickly through the problem speed range, physically damping the system, or using a micro-stepping driver. Motors with a greater number of phases also exhibit smoother operation than those with fewer phases.

Stepper Motor Ratings and Specifications:

• Stepper motors nameplates typically give only the winding current and occasionally the voltage and winding resistance. The rated voltage will produce the rated winding current at DC: but this is mostly a meaningless rating, as all modern drivers are current limiting and the drive voltages greatly exceed the motor rated voltage.
• A stepper’s low speed torque will vary directly with current. How quickly the torque falls off at faster speeds depends on the winding inductance and the drive circuitry it is attached to, especially the driving voltage.
• Steppers should be sized according to published torque curve, which is specified by the manufacturer at particular drive voltages and/or using their own drive circuitry. It is not guaranteed that you will achieve the same performance given different drive circuitry, so the pair should be chosen with great care.

Advantags

• The rotation angle of the motor is proportional to the input pulse.
• he motor has full torque at standstill (if the windings are energized).
• 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.
• 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 dependant on the life of the bearing.
• The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
• It is possible to achieve very low speed synchronous rotation with a load that is directly coupled to the shaft.
• A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages

• More complicated driving algorithm and motor driver circuit is needed.
• Resonances can occur if not properly controlled.
• Not easy to operate at extremely high speeds.

Applications

• CD drive
• Printers
• In banking system for paper feeder
• Valve control
• As actuators for mobile phones camera modules