A stepping motor is an electric synchronous motor that makes a revolution with a number of equivalent equivalent displacements. The length of the elementary segment depends on the accuracy with which the rotor is positioned as desired. Separately, the minimum displacement is called pitch.
The principle of operation of stepper motors, a variety of
A stepper motor, complete with a driver, converts the number of incoming pulses into a given angular displacement of the shaft. The device is interfaced with digital technology, the control signal is often analog. The inputs of the windings are visited by the sine wave of the desired phase. The driver, receiving a digital signal on the contacts, decodes the wave, generates the necessary motor control signals. One, two, three, four phases. Determined by the design, the needs of technology.
A feature of a stepper motor is the shape of a steel rotor. Equipped with poles, underlined by placing on the tip of a sharp or blunt tooth. Dead metal, attracted by the stator coils. It is characterized by a certain magnetization of the residual kind, caused by the action of the field. Precise positioning of the stator poles provides the stepping motor with a unique property: precise positioning in the angle of rotation of the shaft. There are exceptions to the rule, which are discussed below.
Stepper motors are used by the industry, digital technology - where you need to ensure accurate positioning of the shaft. Some sources date the invention to the middle of the XIX century, the first information was leaked to specialized journals in the 20s of the XX century. Speech on the three-phase jet stepper motor. The initial use has traditionally become a military one: on the ships of the royal fleet of Great Britain, nodes sent torpedoes to the right direction. Later, the technology migrated, visiting the US Army.
The first open patent was obtained for a device with a rotor, a 32-tooth stator by Scottish engineer Walker in 1919.The device is designed to work with three-phase voltage. Today, stepper motors are found in personal computer hard drives, automated assembly lines. The key advantages are low cost, ease of positioning. Alternatives not invented. The devices have been used since about the 70s of the 20th century; they form four main groups:
- Permanent magnet stepper motors.
- Hybrid synchronous motors.
- Valve jet engines.
- Laveta Stepper Motors.
Poles of various windings, for example, unifilar, bifilar( see. Inductance coil).In the first case, the rotor makes turns unidirectionally, if not to provide additional switching of phases. The bifilar motor performs the reverse by simply applying voltage to other pairs of contacts. At each pole, the thread of the wire is wound, forming two coils. The design is such that the signs of the fields are opposite. Provides a simple organization of the reverse. Similar schemes are visible on the example of the washing machine drum drive engine.
The world practice has adopted standardized marking of the indicated types of devices:
- Red, yellow - the first winding.
- Black, orange - the second winding.
- Winding with central common output. Red, black, red with white - the first winding. Green, white, green with white - the second winding.
- Double pole winding. Red, red and white - the first pair of the first winding. Yellow, yellow and white - the second pair of the first winding. Black, black and white - the first pair of the second winding. Orange, orange and white - the second pair of the second winding.
Each winding is capable of forming several poles. To activate the reverse of bifilar stepper motors, another pair of contacts is switched. And if a forming controller is needed to form a reverse rotation of unifilar varieties, it is permissible to use an ordinary contactor here.
Modes of operation of stepper motors
The products operate in several modes:
- The full step is implemented by alternating the supply of control voltages in phases. The standard number is 200 movements per revolution.
- In the half step mode after activating one phase, the state remains unchanged the on time portion of the next one. It turns out that two poles act on the tooth simultaneously. The shaft freezes, fixing the intermediate position. Then the first phase disappears, the rotor makes a half step ahead. Despite the less developed torque, the regime is more widely used by the industry, due to the reduction of the level of vibrations.
- Micro-stepping modes are considered to be expert know-how by the developments of specific manufacturers. The mode fills a special chip that generates control voltages so that the shaft positioning accuracy is in the area of the hundredth step( 20,000 movements per revolution).Such refinements are needed in microelectronics, the emergence of the need for subtle technical solutions among industrial conveyors is not excluded. The driver generates more than 50 thousand cycles of control voltages per revolution.
Permanent magnet stepper motors
The engine can be found in the washing machine's pump. For example, a block that removes water from a tank after washing, between separate stages of a cycle. The speed of rotation of the shaft is small, the rotor in the composition contains a permanent magnet, the pitch is large. Assume 45 degrees. A voltage is alternately applied to the stator windings, creating a rotating magnetic field. The permanent magnet of the shaft follows the changes in the intensity vector.
Let's call the advantages of stepper motors simplicity, low cost. Permanent magnets are often used by printers. Difference from other stepping motors: the rotor is devoid of teeth, there are few poles. There are two, stator coils - 4, each movement of the shaft rotates 90 degrees. It requires 4 phases shifted relative to each other by 90 degrees. The driver is simply implemented using capacitors.
Due to the low speed, the engine develops high torque( loading paper from the printer tray).
Hybrid synchronous motors
Hybrid synchronous motors are used by industry because of the development of high torque, they keep static load well. The shaft is still represented by a permanent magnet, supplied with teeth, on the stator many poles. The type of engines provides high speeds of rotation. Each step in the standard version is equal to 1.8 angular degrees( 200 steps / revolution).Specialized versions are produced:
- 0.9 degrees( 400 steps / revolution).
- 3.6 degrees( 100 steps / turn).
Valve stepper motors
The main difference between valve motors is the absence of heavy permanent magnets. Due to which rigid fixation of the position does not occur in the presence of high accuracy. Engines are ideal for watching film slides. Relatively smooth, precise movement is ideal for the occasion.
The rotor is lightweight, steel, has pronounced, relatively few teeth. The pitch is average, for example, for three phases, 12 poles will be 15 degrees. The distance between the poles is 30 degrees. The shaft takes intermediate positions in cases when two adjacent phases are activated simultaneously. The alternation corresponds to a conventional industrial network( for example, 400 volts).
The main feature of valve engines is a relatively small number of blunt teeth. High positioning accuracy is not expected. To implement advanced algorithms apply complex drivers.
Stepper Motors The Laveta stepper motors are sometimes used with electric clocks. Designed to work with a single phase signal. Due to the possibility of miniaturization, Laveta engines will serve as the executive part of the wristwatch. The device name got the name of the inventor - engineer Marius Laveta.
In 1936, a graduate of the Higher School of Electricians designed the engine, which brought worldwide fame. The stator looks like an electric motor with split poles. One coilThe poles are formed by single turns of a relatively thick copper wire located on the magnetic core, creating the desired phase EMF.Induced currents provide the right torque. The propagation delay of the magnetic field across the core is used to shift the phase by 90 degrees, simulating a two-phase voltage. The rotor is represented by a permanent magnet.
Designs are readily used by home appliances( blenders, mixers).The difference between the Laveta engines is that, thanks to the teeth, the shaft is fixed with a certain pitch. The characteristic movement of the second hand becomes possible. Like most stepper motors, the variant is not intended to work in reverse.
Parameters of stepper motors
The individual parameters of stepper motors are critical when choosing an appropriate controller that generates control voltages:
- Inductance. A high value of the parameter is usually in low-speed engines with a clear torque. With increasing number of revolutions of the shaft, the equipment parameters will certainly deteriorate. With low inductance, the current causes a fast response, which is required in drives for reading optical discs.
- The current consumption affects the rigidity of switching between adjacent steps. Smoother mode requires reduced parameter. High current consumption increases torque. Thus, the correct choice of parameters loads the shoulders of the designer.
- The limiting level of operating temperatures for stepper motor is small. The upper limit is in the region of 90 degrees Celsius. Overheating is possible at high torques with significant current consumption. For unloading, a hold mode is sometimes used when the shaft stops for a while.
stepping motor drivers In a global sense, there are three groups of stepper motor control drivers:
- Unipolar form current pulses of one direction. Simple, unpretentious method, use reduces torque by 40%.Experts explain the phenomenon of the impossibility of simultaneously feeding all the windings that are able to participate in the movement. The technique is suitable for low operating speeds.
- Extinguishing resistor drivers are now considered obsolete. Allowed to squeeze out of the engine maximum speed. A large amount of energy is released by heat on the damping resistors.
- Bipolar drivers are popular today. Ignoring the complexity of the design, high efficiency is achieved. Each driver contains a shaping unit composed of four transistors. Power is supplied, bypassing the diodes, a feedback signal is removed from the resistor. The voltage reaches a certain level, the necessary keys are opened to reduce. The shape of the signal takes a sawtooth shape, the engine with a high constancy supports the specified power.