Stepper motor: principle of operation, circuit, description, characteristics

In modern electrical engineering a lot of various devices are used, some of them are intended for automation of technological operations. So is the stepper motor. The principle of operation and the device of this device are described in the article.

What it is?

This is the name of the electromechanical device used to transmit the control signal to the mechanical motion of the rotor. Each movement ends with a fixation in a strictly defined position. The device is angular or linear. It is worth remembering that the stepping motor, the principle of operation of which will be described below, is a synchronous device.

Control systems with open circuit (without feedback)

Most often this equipment is controlled by a special electronic circuit. It feeds only from an AC source. Such motors are often used in circuits where speed control is required. This avoids the need for an expensive and complex feedback loop, and the motor protection becomes simpler (only need to provide for rapid de-energization).

This principle of operation is used in open-loop circuits. It should be remembered that this scheme (without a feedback loop) is economically advantageous, but it has a number of significant limitations.

Thus, the rotation of the rotor is quite unstable, vibrational, which is why the speed and other characteristics of motion can in no way be as accurate as they are in DC motors with a feedback loop. To expand the scope of the stepper motor, it is required to find ways to reduce vibration.

system configuration

In order to better understand the construction of the stepper motor and the principle of its operation, it is possible to consider the scheme of the functioning of the device under its control, which was used 20 years ago to produce punch cards. For this purpose, three- and four-phase SDs were universally applied. Now we will consider the scheme of the first.

We have already mentioned that the rotor of the motor rotates a certain distance in response to each control impulse. The value of this rotation is expressed in degrees and is called the step. The logic circuit is activated during the reception of the signal, after which it immediately determines the phase necessary for the activation. After that, it sends its signal to the inverter responsible for the current value that stepper motors use. The characteristics of this equipment involve the use of different types of control circuits. As a rule, the latter are mounted from widely used transistors, although comparatively recently integrated circuits were used for this purpose . With its high output potential, the required winding phase (the first, for example) automatically excites. If the potential decreases, the phase is automatically switched off. So the protection of the electric motor is realized.

Phases are denoted by ordinal numbers 1, 2, 3, etc. Either by letters A, B, C, etc. The latter option is only used in the case of some two-phase motors. Thus, only one phase of two, three or four available (depending on the type of engine) is excited at any particular time. In explaining the principles of operation of such a device, this circumstance is constantly mentioned, but it must be understood that this scheme is not at all an ideal control method.

Step and Increment

The simplest option is to feed single pulses from the control circuit. In this case, for example, the engine at a time turns the leading sprocket of the conveyor for some distance forward. It should be noted that when the massive mechanism is fed forward only by one step, the problem of vibration is further aggravated, and the considerable inertia makes itself felt.

In such cases, it is much more justified to use a stepper motor, which can make several movements for one control pulse. Also it does not hurt to use an asterisk with smaller teeth. By the way, every such movement is called an increment.

In the cases described by us, the increment is equal to one and several steps, respectively. After each cycle, the engine stops for a while, after which everything is repeated again. This is called incremental movement and incremental control, respectively.

If one motion is performed in several steps (as we discussed above), and there may not be any oscillations of the rotor. When the movement is one-step, the oscillations must be extinguished by means of a special electronic device. Generally stepping motors (the characteristics of which we are considering) belong to the science-intensive devices, for their work requires a lot of complex electronic "stuffing".

General principle of management

For one increment the number of steps is more than four times in some production lines, conveyors. When data from the storage device (internal flash memory, hard disk of the computer) is sent to the controller, they are performed block by block. Each of them contains a strictly defined number of symbols (32, 48 or 64), and in different systems and for different purposes of the device this figure can vary greatly.

Not surprisingly, in recent years, homemade products based on the Arduino microcomputer have become common. Stepper motor in this design is ideal, since in such a bundle it can be adapted both as a power plant for a toy, and for a rather complex industrial equipment.

The data block is transferred to the semiconductor memory on the controller before use, after which the movement will start according to the instructions that were written in the first information block (it is necessary to find out these characteristics before connecting the motor).

After executing the instructions, the system starts reading the second array of information. If each motion consists of many small steps, an additional cascade must be mounted in front of the main controller. Most often its functions are performed by the input controller. It sends data to the second control loop at some interval specified by the system (Arduino). Stepper motor in this case is protected from overloading by requests.

Some specificity of using the SD

We will tell you about some of the nuances of using stepper motors, and also give a definition of the terms often used in this field:

- A small step angle. As you already know, after each control pulse, the rotor of the motor rotates to a certain degree. The smaller the step, the higher the direct speed can be. It is important to know that stepper motors can very well provide a very small step. The step number in this case is the number of revolutions per step, and this value is very important for engineers. It is calculated according to the following formula:

S = 360 / θS, where S - step number, θ - step angle (angle of rotation).

In most cases, the stepper motor drive can perform 96, 128 or 132 steps per revolution. Four-phase models sometimes have a value of 200. Rare types of precision motors in one revolution only can make 500 or 1000 steps at once. However, for simple varieties this is unattainable, since their rotation angle is 90, 45 or 15 °.

- High accuracy of rotation speed. It is this parameter that determines the overall quality of the device. You already know that the operation of the stepper motor involves its stopping and fixing in a certain position after the execution of the data block. Of course, conventional mechanics unambiguously tells us that due to inertia, frictional force and other factors, all deviations from the given parameters are possible.

Fighting unwanted phenomena

The clearance between the rotor and stator teeth is always made minimal to increase the rigidity of fixation. The very positioning accuracy depends on the characteristics of only the inverter, since other factors affect it to a much lesser extent.

And now it is necessary to consider a number of important characteristics and concepts, such as the maximum static moment, the positions of the "dead" rotor, and the accuracy of positioning all these positions. For the definition of the above terms, there are at once two generally accepted common concepts.

Maximum static effect

As we have already said, he has two positions at once:

• Restraining. This is the maximum permissible effect that can theoretically be applied to the shaft of an already excited stepper motor without the appearance of motion.
• Fixing. Accordingly, it is also the maximum static effect that can theoretically be applied to the shaft of an unexcited motor without the occurrence of subsequent rotation.

The higher the holding torque, the lower the probability of positional errors caused by the unpredictable load (capacitors for motors failed, for example). A full fixing torque is possible only in those engine models in which permanent magnets are used.

Dead rotor positions

There are three positions at once, in which the rotor stops completely:

• Position of equilibrium. It completely stops the excited stepper motor.
• Fixation. Also the state in which the rotor stops. But this concept is used only for those engines in which the design has a permanent magnet.
• In modern models of stepper motors, which comply with all norms of environmental and energy security, when the rotor is stopped, the winding is completely de-energized.

About positioning accuracy

Finally, let's talk about the most important concept. It's about positioning accuracy. One can guess how important it is in the operation of complex industrial equipment. There are two important terms:

• Angular position error. It is defined as a positive or negative departure from the normative angular state, which is very often observed in cases of the transition of the rotor from one position to another. As a rule, inertia is to blame, as well as poor fitting of parts.
• Positioning accuracy. This is the maximum value of rotor angular position errors that occur during the entire period of stepping motion.

Important! Find regulatory information for each category of stepper motors can be found on the official page of their manufacturers, and from the reference documentation that is attached to these types of products. As a rule, the error value is in the range from +0.08 to -0.03 °. Simply put, the positioning accuracy is calculated as the sum of these two indicators: 0.08 ° + 0.03 ° = 0.11 °.

Thus, the stepper motor, the principle of operation of which we describe, refers to high-precision equipment.

High ratio of the electromagnetic moment to the moment of inertia

As you can imagine, from the stepper motor it is required to start the movement as soon as possible after the control pulse arrives at the controller. It should stop just as quickly, having high accuracy of positioning. If during the movement the sequence of control pulses is interrupted, the motor will stop operating at the position determined by the last pulse.

It should also be borne in mind that the ratio of the electromagnetic moment to the moment of inertia of the rotor in the SH should be much higher than that for conventional electric motors.

Stepping speed and pulse frequency

Since the speed of the SD actually represents the number of steps per unit of time, instead of the term "rotational speed" in specialized literature, it is often possible to find the definition of "stepping speed". Before connecting the motor, it is necessary to read about these nuances.

Since for most stepper motors this frequency is equal to the number of control pulses, one should not be surprised at its unusual designation in technical reference books. More precisely, for such motors, the unit of measurement is often a hertz (Hz).

It is important to understand that the stepping speed of the real rotor speed does not in any way reflect. Experts believe that there is no reason not to use in the description of stepper motors all the same number of revolutions per minute, which is used in describing the technical characteristics of conventional electric motors. The relationship between the actual rotation speed and its step analog is calculated by the following formula:

N = 60f / S, where n is the rotational speed, expressed in revolutions per minute; F - step speed; S is the number of steps.

By the way, how to determine the required capacitors for electric motors? Very simple! It is enough just to use this formula:

С = 66 · Рном

It is easy to guess that under Rnom is the nominal power of the electric motor in kW.

The simplest scheme of connecting the engine EM-178

And now we will consider the simplest connection of a stepper motor using the EM-178 model as an example, which is widely used in industrial printers.

Phase 0	White controller
Phase 1	Orange
Phase 2	The stepper motor is connected to the red controller
Phase 3	Connects to the blue connector
General "+" power supply	Brown controller

It is not possible to simply write down the work more simply because there are millions of very diverse models whose characteristics have significant differences.

Currently, various types of electric motors of this design are used. In this article we will discuss the most common.

Jet engines

It is this variety of devices that is widely used to this day. In fact, it's almost a standard three-phase motor, with six teeth on the stator. Simply put, every two prongs opposing each other belong to the same phase. A serial or parallel connection of their coils is used.

As for the rotor, only four prongs are located on it. Most often, stator and rotor manufacturers are made of soft magnetic material, but it is often possible to meet simply massive rotors made of ordinary metals. The thing is that there is only one important requirement for the substances that go into their production: they must ensure the best conductivity of the magnetic field. This is extremely important if we discuss the stepper motor: the principle of operation is directly related to the strength of the magnetic field.

Devices with permanent magnets

As a rotor a magnet of cylindrical shape is used, on the stator there are four teeth with an individual winding. In order to reduce the step angle more strongly, in these models of stepper motors it is necessary to increase both the number of poles of the rotor and the number of teeth on the stator. However, it should be remembered that both these parameters have rather strict physical limitations. In the last paragraph of our article there is information about their alternative design (bipolar stepping motor), but such models can not be found so often.

As we have already said, stepping devices with permanent magnets stop in a strictly fixed position even in those cases when the voltage from the windings is removed. In this case, the same locking mechanism that we discussed above is triggered is the fixation position.

The use of permanent magnets is justified from many points of view, but at the same time, their application can lead immediately to several problems. First, their price is far from affordable. By the way, how much is such a stepper motor? The price of models with permanent magnets exceeds 100 thousand rubles.

Secondly, the maximum density of the magnetic field may not be too high, since this value is limited by the magnetization of the carrier itself. Thus, relatively cheap permanent ferrite magnets do not allow obtaining more or less sufficient field strength. And what are the other types of electric motors working on this principle?

Hybrid installations

There is another type of stepper motor, partly using the same principle. Hybrid models work with both reactive and magnetic motors.

The rotor has almost the same design as the reactive SHD, but the windings are produced in a slightly different way. The fact is that at each pole the winding is only in one coil (three-phase SD). It is easy to guess that two coils are already wound in four-phase models. The winding is carried out according to the bifilar scheme. The peculiarity is that during excitation on the coils a magnetic field of different polarities is created (bipolar stepper motor).