Stepper motor

Tuesday, 20 February 2007

A stepper motor, also called stepping motor, pulse motor or digital motor, is an electromechanical device which rotates a discrete step angle when energized electrically. Stepper motors are synchronous motors in which rotor’s positions depend directly on driving signal. Rotary moment is defined by magnetic energy and is proportional to the tooth number of the rotor. The main difference between the stepping motor and a general motor is that the stepping motor only powered by a fixed driving voltage does not rotate. A stepping motor exhibits excellent functions such as accurate driving, rapid stopping, rapid starting and the like. The stepper motor provides controllable speed or position in response to input step pulses commonly applied from an appropriate control circuit. Stepping motors are driven by a pulse signal. When a digital pulse signal is inputted into the stepping motor, the rotor of the stepping motor is rotated by a fixed angle, that is, a well-known stepping angle. Since the stepper motor increments in a precise amount with each step pulse, it converts digital information, as represented by the input step pulses, to corresponding incremental rotation. By increasing the rate of the step pulses, it is possible to increase the speed of the motor. Stepper motors provide many advantages over other types of motors, most notably the ability to rotate through controlled angles of rotation, called steps, based on command pulses from a driver circuit. The speed of stepping motors can be readily controlled based on the pulse frequency employed, enabling stepping motors to achieve variable speed synchronous movement of a load that is directly coupled to the drive shaft of the motor. A stepper motor can respond to a range of loads by providing a range of torque outputs at a fixed motor voltage for which the windings are sized. Thus, for each specific application of a stepper motor as an electromechanical actuator, the size of stepper motor selected is determined in large by the nominal operating load range to be encountered. Because the stepping motor is capable of receiving the digital signals to generate a corresponding angle variation in proportion to the total number of pulses, the stepping motor can be driven by an open loop without utilizing a feedback mechanism. Therefore, the stepping motor can be easily controlled when the stepping motor is driven to achieve a rotational speed within an acceptable range. Because stepping motors are driven by a simple control mechanism, the stepping motors have been applied on many control systems for controlling the velocity, the displacement, and the moving direction of loading devices.

Stepper motors are used in a wide variety of applications due to their low cost, ruggedness, simplicity of construction, and wide acceptance, among other factors. Stepper motors are widely used in applications wherein the position of a mechanical element must be readily and accurately adjusted over a predetermined range of possible positions. Stepper motors have proven very popular in modern, sophisticated mechanical equipment since, by varying the time between steps in a step table, a stepper motor may be driven through very flexible and highly precise velocity profiles necessary to implement complex mechanical functions such as those carried out by paper handling equipment. Stepping motors are extensively used in various fields, such as information devices and audio equipment including, a printer, facsimile, image scanner, copying machine, laser beam printer, CD-ROM, DVC. For instance, stepper motors are commonly used in devices such as desktop printers for a variety of purposes, including the feed of paper through the printer and the movement of a print head carriage across a paper path. Stepping motor modules are used particularly in printing and copying devices to drive transport drums, which transport the paper or forms that are to be printed through the printing or copying device. In these appliances, the motors are often controlled by microprocessors, which time the movement of the material along the assembly line and control other equipment based on the anticipated timed movement of the material along the assembly line. Small stepper motors have been utilized to drive a set of camera shutter blades. The number of pulses transmitted to the stepper motor determines the aperture achieved by the shutter blades. Such shutter blades are reasonably fast and simple to drive electronically. Because of its suitable performance characteristic for positioning control, stepping motors have been used in photo graphing electronics devices, such as a digital camera and video camera (camcorder) for adjustment of aperture, focus, and zoom. Compact electronic devices and information handling apparatuses, appropriate for portable use, have been recently developed, and miniature and light-weight stepping motors are widely employed in these devices. Timing devices such as an electronic timepiece or watch, and timing switches are typical of such electronic devices. In these timing devices, the energy generated by the movement of the user's arm is converted into electricity which is used to drive the stepping motor which moves the hands of the device. These timing devices operate without batteries and can continuously run off the energy generated by the user's movement. Recently, due to rapid development upon the photoelectronic technology, related photoelectronic products has become standard and required peripherals of the computer system. For example, an optical storage device is one of those popular apparatus. In controlling the sled of the optical storage device, control of the motor is particularly important. For the optical storage device and the related products such as the CD-ROM, the CD-R, the CD-R/W and the DVD, the stability during high-speed operation and the ability of accurate data-retrieving become crucially important to the control of the sled of the optical storage device. Compared with the DC motor, the stepping motor has a simpler and lower-cost driving mechanism and has the advantage upon circuitry design of the photo detection feedback and the magnetic detection feedback. Therefore, the stepping motor, replacing the DC motor, becomes more and more popularly as a sled control means for the optical storage devices. Stepping motors are also used factory automation (FA) equipment such as machine tools, automotive components, and a field of home appliances also employs a large number of stepping motors. For example, in controlling a throttle valve, the position of a valve head with respect to a valve seat must be adjustable over a range of head positions, typically from fully open to fully closed. Stepper motors are well-suited to providing such valve control. This extensive use of the stepper motors is mainly thanks to a low cost, and simple operation realizing a speed control or a positioning control.

A stepper motor relies upon a winding mounted on a stator to conduct or block current based on the position of the rotor. The stepping motor is caused to make a stepping rotation by changing instantaneously excitation currents for windings at each time when an external command pulse is given. A stepping motor maintains a very large static torque in the stopped position compared to other motors while being rotated at a given angle without feedback for detecting the position of a shaft and stopping at a considerably high precision rate. A stepper motor rotates by a fixed angle in every changed state of excitation in each phase of the motor by clock pulse signals, and suspends at a fixed angle if the state of excitation does not change. The stepper motor rotates by the fixed angle or by a fixed step according to inputted pulse signals. Therefore, the stepping motor does not require a separate position-maintaining mechanism such as electromagnetic brake and the rotation speed thereof is proportional to pulse rate. Stepper motors are generally constructed without the brushes commonly found in electric motors. In place of brushes, the stepper motor depends on switches to control the flow of electrical charge through a particular phase winding. Stepping motors can be viewed as electric motors without commutators. Typically, all windings in a stepper motor are part of the stator, and a rotor is either a permanent magnet or, in the case of variable reluctance motors, a toothed block of some magnetically soft material or a hybrid of both. All of the commutation is handled externally by a motor controller, and typically, the motors and controllers are designed so that the motor may be held in any fixed position as well as being rotated one way or the other. In stepper motors, current is applied to the individual coils in order to advance the stepper motor a desired number of steps. Variation of current and its polarity moves a rotor of the motor through the steps, or to a fixed position at a particular step. Depending on how the current is applied, the stepper motor can be caused to move in full steps, half steps, or even microsteps. Since the motor windings comprise a continuous coil of wire, they exhibit both inductive and resistive characteristics and an associated time constant related to the rise and decay of the applied current. To regulate current, stepper motor drivers periodically apply and remove voltage to the motor windings. Stepper motor drivers provide a step clock to activate circuitry in the driver electronics to sample and apply current to the windings or "phases" of the associated stepper motor. The amount of current to be applied is a direct function of the desired position of the motor shaft. For rotational motion, current is applied to opposing windings in a stepper motor in a quadrature manner. A linear stepping motor operates on the same electromagnetic principles as a rotary stepping motor. Linear stepper motors are used for positioning applications requiring rapid acceleration and high speed moves with low mass payloads. Mechanical simplicity and precise open loop operation are additional features of the stepper linear motor systems. Most stepping motors can be stepped at audio frequencies. With an appropriate controller, they may be started and stopped at controlled orientations. A stepping motor controller typically comprises an excitation signal generator, a switching circuit, a winding of the stepping motor, a PWM (pulse width modulation) constant current control circuit, a current sensor and a current setting circuit. Most stepper motors are operated in an open-loop configuration in which a position feedback device such as an optical encoder or resolver is unnecessary. Closed-loop configurations use feedback to sense rotor position via a conventional shaft encoder. The rotor position information may be utilized to produce each motor commutation. Most such configurations employ a fixed switching angle commutation.

Stepper motors generally have two phases, but three, four and five-phase motors also exist. A two phase stepping motor may described as comprising at least first and second coils perpendicularly oriented with respect to each other which are alternately driven with currents of opposite polarities. In a stepper motor having two phases, a drive circuit may cause electrical current to flow through the stator windings (phases) in accordance with the sine/cosine law. A two-phase stepping motor is mainly employed for use requiring a medium accuracy, while a three-phase stepping motor excellent in cost performance is employed for use requiring high-accuracy, low vibration and low noise. The three-phase machine comprises a cylindrical permanent magnet type rotor formed with multiple magnets in a cylindrical shape, or a hybrid type rotor having a permanent magnet held between two magnetic plates formed with multiple pole teeth, and a stator formed with pole teeth opposite the rotor surface. The structure of the stepping motor can be classified into a VR type (variable reluctance or variable magnetic resistant), a PM type (permanent magnet), and a hybrid (HB) type that combines the above two. A variable reluctance type stepping motor is driven by the attractive force between rotors that form teeth of the motor and stators of the magnetic poles. A permanent magnet type stepping motor is driven by the attractive force and repulsive force between a rotor formed by a permanent magnet having alternatively arranged N poles and S poles and stators of the magnetic poles. The hybrid stepping motor has a structure combining those of the VR type and PM type. The stepping motor having the permanent magnet rotor was accurately step-driven one step angle at a time by a driving pulse input from an outer part and an output shaft of the motor rotated as if intermittently driven. The permanent magnet stepping motor allows easy control of forward and reverse running operation and its physical size can be easily reduced depending on required driving torque. Use of a permanent magnet is widely diversified and in the use, a permanent magnet maintains an important position as a constituent element of an electronic apparatus, particularly, a small-sized motor, particularly as a rotor magnet. Permanent magnet stepper motors are currently used in a wide variety of apparatus including cameras, printers and scanners. Their ability to effect discrete and precise movement makes them the preferred choice for driving mechanical elements in this type of equipment. Hybrid stepping motors have been well known as actuators appropriated for highly accurate positioning movements. The hybrid stepping motors are widely used in various machine tools, e.g. with fully automated production lines, as well as computer related instruments including printers, plotters, facsimile machines, and disk drive units.

Related Information about Stepper motor