Stepper motor is a brushless DC motor in which each rotation (revolution) is divided into a certain number of steps depending on the motor structure. Typically, a full 360° shaft rotation is divided into 200 steps, which means that a single step is performed every 1.8°. There are also motors where a single step is performed every 2; 2.5; 5, 15 or 30°.
The described functionality is possible due to the special construction of the stepper motor, which will be discussed below. Due to the fact that the full revolution of the shaft is divided into certain discrete sections, the stepper motor does not rotate smoothly, but it makes steps, skipping the intermediate states, and therefore the operation of the stepper motor is a source of characteristic sound and vibration.
Nowadays, stepper motor drivers are made on the base of digital circuits that control the drivers increasing the output load capacity. Usually, the drivers contain microcontrollers, but not necessarily, because such a driver can be constructed based on gates and flip-flops. The method of controlling the motor depends on its type, the number of phases and on the use of feedback. In some controllers, the current flowing through the windings can be adjusted using the PWM waveform, while the direction of rotation and the control of steps is carried out through square waveforms. However, when using a specific model of motor and its driver, their datasheets should be consulted to determine the correct method of control.
The windings loaded on the controller outputs are coils with a certain inductance and capacitance. Their reactance increases with frequency, which limits the flowing current and the maximum switching frequency. When it comes to selecting a motor for a specific application, there are always some trade-offs that need to be considered, for example, maximum angular velocity and required torque in relation to resolution. The article provides basic information on the working principle of bipolar and unipolar stepper motors and on most important things to consider when choosing a motor for a given application.
Stepper motor – how does it work?
The stepper motor consists of the rotor and the stator. The stator is a stationary part, while the rotor, which is mounted on the shaft with bearings, rotates following the rotating magnetic field created around the stator. The stator – made of steel or other metal – is the frame for a set of electromagnets, namely the coils mounted in specific places around the rotor. When current flows through the stator coils, the magnetic field is created around them. Particular magnetic fluxes have a direction and intensity depending on the amperage and direction of the current flowing through a given coil.
When the coil is powered, the resulting electromagnet is attracted by a magnet (a tooth) mounted on the rotor, which is displaced by some offset in relation to it. Then the rotor and the shaft rotate by the angle at which it is least repelled by the magnetic flux or the resultant of many fluxes. After moving by one offset, another electromagnet (coil or coils) on the stator switches on, and the rotor is pulled to its new position. By switching subsequent coils, the rotor can make further steps forwards or backwards, or make a complete or partial rotation with the shaft.
According to the description above, one can imagine a stepper motor as a series of electromagnets that attract the rotor magnet. However, it is much more complex, because the magnet is attracted by the resultant field around the electromagnet assembly. It allows not only the full-step operation mode but also half-step operation (a step divided by 2) or less, which is called micro-step operation.
Stepper motor – full step operation mode
The working principle of the stepper motor in the full-step mode is illustrated in Figure 1. In this mode, the motor rotates by the angle - dependent on its construction, e.g. 1.8°. It is easy to count that in this case, it is necessary to take 200 steps (200×1.8°=360°) per full revolution.
The shaft travel is performed after a switch-on of one or two coils. Single-coil operation requires minimum driver power. In 2-phase operation, two opposite coils are powered, and twice as much power is required, but the speed and torque also increase.
Stepper motor – half-step operation mode
The principle of motor operation in the half-step mode is shown in Figure 2. As the name implies, this mode enables a division of the discrete rotor’s pitch by 2 and rotating by half the nominal angle for a single movement. In the example above, a single step will be made in 0.9° increments, while the number of steps per full revolution will increase to 400.
The half-step mode requires the alternate power switch to the two phases (coils). It results in an increase of torque in comparison to the 1-phase mode, “smoother” motor work and doubled angular resolution.
Fig. 2. Working principle of half-step motor operation with 2-phase power supply
Stepper motor – micro-step mode.
In the micro-step mode, the nominal pitch is divided into even shorter sections than in the half-step mode. The maximum division factor is 256. The individual positions of the rotor are obtained by means of the resultant magnetic flux of the coils supplied by a step waveform. Micro-step operation is preferred in applications where “smooth” motor operation and/or high positioning accuracy are required.
When the motor is used in the micro-step mode, attention must be paid to the application's requirements regarding the rotational speed of the motor. As mentioned, the inductive reactance of the coil increases with the keying frequency of the current in the motor windings. Higher rotational speed requires more frequent switching, and then – higher frequency of windings keying. This results in an increase of the coil impedance and a decrease of the average current flowing through the windings. The important thing for the motor working is that the torque decreases as the current in the coils decreases, which may lead to oscillation, stopping the rotor or skipping steps by the motor, and consequently, by the element of the machine driven by the motor. Therefore, when the motor is used in the micro-step mode, the user must pay special attention to its datasheet. The sheet probably includes a graph showing the value of the torque as a function of the frequency of the current flowing through the coil.
Types of stepper motors
A stepper motor is not particularly complicated in terms of construction than a typical brushed DC motor, but must be constructed more precisely. Modern BLDC motors are very similar to the stepper motors with permanent magnets, and they are controlled in a very similar way.
The basic criteria that distinguish the types of stepper motors are their construction and the number of phases required to power the coils. Depending on the construction, individual types of stepper motors differ in purpose (target application), resolution, and the achieved torque.
Permanent magnet stepper motor
A permanent magnet motor could have two coils (electromagnets) forming the four variable poles, and the rotor of the motor would be radially magnetized. A change in the rotor position would be caused by the change in the current direction in the coils, changing the magnetic poles. With the appropriate change of the current direction, the rotor would turn by 90°. A single step of such a motor, although useful in some applications, would be very large and imprecise. Therefore, the really existing permanent magnet motors have more rotor poles and several magnets mounted on the rotor in order to increase the number of steps and positioning precision.
Typically, Permanent magnet stepper motors move by 7.5° to 15° per step, which adds up to 48 to 24 steps per full revolution. Magnetized rotor poles increase the magnetic induction, and therefore permanent magnet motors have a high torque. Simple construction results in a moderate price of the motor and rather low resolution.
Variable reluctance stepper motors
Variable reluctance motors were among the first stepper motor models. Today they are rarely used. In this type of motor, the rotor consists of a number of teeth made of soft iron. When the stator coils are powered by a direct current, the rotor tooth is attracted by the magnetic field. Thanks to sequential switching, the rotor rotates by the angle determined by the motor structure.
Motors of this type, although simple in construction and easy to control, have a low resolution and low torque.
Hybrid stepper motors
The hybrid motor is one of the most widely used types of stepper motors in the industry. It has a high resolution – its rotor makes steps every 0.9° to 3.6° (from 400 to 100 steps per revolution). This type of motor outperforms other types in terms of reliability, torque, holding torque and achieved rotational speed.
The rotor of the hybrid motor is composed of permanent magnets, but unlike the models discussed above, the magnets are not mounted radially but magnetised axially. Typically, the rotor consists of two oppositely magnetised rings placed on the motor shaft. Each ring has slotted grooves that form the rotor teeth.
Unipolar and bipolar motors
Another classification of stepper motors is based on the winding type in 2-phase motors. According to the classification, the motors are divided into unipolar and bipolar motors. The main difference is that the unipolar motor works with one current (voltage) polarity, while the bipolar motor works with two polarities, which means that the direction of the current flow in the coil is variable. Another difference is that the motor coils must be connected to allow the transfer of power from the end of one coil to the beginning of the other. This method of connection allows the use of current (voltage) of one polarity. The differences in the construction of both types of motors are shown in simplified figures 3 and 4.
A bipolar motor has a higher torque than a unipolar motor, but on the other hand, controlling it is more complicated.
Advantages and disadvantages of stepper motors
The main advantages of the stepper motors are precision of operation and easy control of the rotor position and its rotational speed. The result can be achieved with a relatively uncomplicated construction and low cost of the final solution. The torque is very high, while the rotational speed is low. The motor is brushless and therefore very durable and reliable. Another important feature is easy control of the motor: quick start due to high torque, easy stop due to high holding torque and the ability to quickly change the direction of rotation. Easy shaping of start and stop characteristics is also crucial for many applications.
One of the main disadvantages of a stepper motor is its high energy consumption. The motor requires the power supply while performing moves as well as while stopping. The torque of the motor is the highest with relatively low rotational speed (RS) and it decreases with high RS. As mentioned, the torque is highly dependent on the amperage of the current flowing through the coils, and the current depends on impedance, which increases with higher switching frequency. This is the reason why it is impossible to achieve high rotational speed and at the same time retain the torque and the ability of the motor to “bear” the required load.
If the torque is insufficient, lost motion (and skipped steps) occurs. Therefore, a feedback mechanism is needed for reliable motor control. It may be made based on an encoder or other type of sensor. The mechanism “ensures” the motor that it made the required number of steps.
The control of stepper motors
The good practice of drive construction is to design them as a whole set, i.e. the motor and the controller because the parameters of both affect the work of the entire drive system. In short, even the best motor cannot work properly without the appropriate controller and vice versa. The drive performance depends deeply on the choice of the motor and controller set.
The main trend in the development of stepper motors is to make them less inert while providing a higher resolution (number of steps), torque and power efficiency. This is the reason why are there so many modifications of the motors, along with the basic types described above. The modifications are aimed to improve the mentioned parameters. The motors may also differ in terms of the number of coils, and therefore, the controlling algorithm.
Many designs of stepper motor controllers may be found on the Internet. From the simplest ones, with a potentiometer for rotational speed regulation and a rotation direction change button, made from discrete components (gates, flip-flops and transistor switches) to the most sophisticated ones, e.g. based on specialised integrated circuits drivers and DSP processors. However, such designs are useful rather for amateur or experimental applications than for the industry. In the latter case, it is better to choose comprehensive, ready-made solutions from trusted manufacturers.
For non-professional applications, the motor controller based on Arduino can be constructed, along with an appropriate amplifier or a motor driver. The choice of a board (expansion module) depends on the motor used.
A bipolar motor must be supplied with the current that can flow in two directions. For the diversion of the magnetic flux in the core, a single bistable switch should be used, made of alternately switched transistors (a half-bridge system). A unipolar motor must be controlled by the current flowing in only one direction. Therefore, a single transistor switch per one coil is sufficient. It is easy to understand that unipolar control requires a smaller number of transistor switches, but we must remember that only half of the windings work in a given moment. Hence, a unipolar motor produces lower torque than a bipolar one. Bipolar motors require a more complex system of control, but many manufacturers offer specialised integrated circuits which include two complete transistor bridges, overheat, overcurrent and overvoltage protection circuits, as well as logic gates which facilitate the motor control. Such integrated circuits are offered by STM, Toshiba Electric and others.
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The control of motors working in a half-step or micro-step mode is much more complex. It requires the ability to manipulate the magnetic fluxes so that the resultant field makes the rotor move by a part-step, not by the whole step.
Nowadays, stepper motors are used in many different devices which require high precision of movement control and accurate determination of position. Therefore, they are mainly applied where a very precise control of movement and positioning is demanded, because an appropriate device and software can be easily produced with the aid of a computer and a controller. Stepper motors are also used in biomedical apparatus, computer disc drivers, printers, scanners, camera-controlling smart-light systems, in the control of engine-regulating elements, in robotics, in 3D printers and scanners, XY plotters, CNC machines and other devices. The most popular devices that employ the use of stepper motors are printers: from older types of dot-matrix printers to modern 3D printers, whose mode of operation is quite dissimilar from traditional printing.
Stepper motor applications are very well-known these days, and they are easy to use by hobbyists who use the motors for e.g. CNC devices or 3D printers. Stepper motors are easy to control by Arduino with an appropriate attachment (e.g. integrated circuit shield L293D). It opens up a whole gamut of options for building many interesting applications. They can be further used in various branches of industry, in housekeeping or in private hobbyist workshops.