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This article will introduce you to the basics of stepper motors, including their operating principle, construction, control methods, uses, types and their advantages and disadvantages.
Stepper motor is a kind of motor that rotates the shaft by stepping (i.e. moving at a fixed angle). Its internal structure makes it possible to know the exact angular position of the shaft without a sensor, by simply calculating the number of steps. This feature makes it suitable for a wide range of applications.
Like all motors, stepper motors include a fixed part (stator) and a movable part (rotor). The stator has a gear-like protrusion around which a coil is wound, while the rotor is a permanent magnet or variable reluctance core. We will cover the different rotor configurations in more depth later. Figure 1 shows a cross-section of a motor with a variable reluctance core rotor.Translated with DeepL.com (free version)
The basic principle of operation of a stepper motor is that by energizing one or more stator phases, the current passing through the coils creates a magnetic field with which the rotor will align; by applying voltages to the different phases in sequence, the rotor will rotate by a specific angle and eventually arrive at the desired position. Figure 2 shows how this works.
First, coil A is energized and generates a magnetic field with which the rotor is aligned; when coil B is energized, the rotor is rotated clockwise by 60° to align with the new magnetic field; the same happens when coil C is energized. The color of the stator pinion in the figure below indicates the direction of the magnetic field generated by the stator windings.
The performance of a stepper motor (whether resolution/step, speed or torque) is affected by the details of its construction, which may also affect how the motor is controlled. In practice, not all stepper motors have the same internal structure (or construction), as the rotor and stator configurations vary from motor to motor.
There are basically three types of rotors for stepper motors: Permanent magnet rotor: the rotor is a permanent magnet, aligned with the magnetic field generated by the stator circuit. This type of rotor ensures good torque and has braking torque. This means that the motor resists (even if not very strongly) changes in position, regardless of whether the coil is energized or not.
However, the disadvantages are lower speed and resolution compared to other rotor types. Figure 3 shows a cross-section of a permanent magnet stepper motor.
The rotor is made from an iron core that is specially shaped to align with the magnetic field (see Figures 1 and 2). This type of rotor makes it easier to achieve high speeds and high resolution, but it usually produces low torque and has no braking torque.
This rotor has a special construction; it is a hybrid between a permanent magnet and a variable reluctance rotor. Its rotor has two axially magnetized caps and the caps have alternating small teeth. This configuration gives the motor the advantages of both a permanent magnet and a variable reluctance rotor, especially with high resolution, high speed and high torque. Of course higher performance requirements mean more complex structures and higher costs.
Figure 3 shows a simplified schematic of this motor structure. When the coil A is energized, one of the small teeth of the rotor N magnetic cap is aligned with the stator tooth magnetized as S. The rotor S magnetic cap is aligned with the stator tooth magnetized as S. At the same time, due to the structure of the rotor, the rotor S magnetic cap is aligned with the stator tooth magnetized as N. Although stepper motors operate on the same principle, actual motors are more complex and have a larger number of teeth than shown in the figure. The large number of teeth allows the motor to obtain very small step angles, as small as 0.9°.
The stator is the part of the motor responsible for generating the magnetic field with which the rotor is aligned. The main characteristics of the stator circuit are related to its number of phases, pole pairs, and wire configuration.
The number of phases is the number of independent coils, and the number of pole pairs indicates the major tooth pairs occupied by each phase. Two-phase stepper motors are most commonly used, while three-phase and five-phase motors are less commonly used (see Figures 5 and 6).
From the above we know that the motor coils need to be energized in a specific sequence to produce the magnetic field that the rotor will align with.
The devices that can provide the necessary voltage to the coils to allow the motor to operate properly are the following (starting with those closer to the motor): transistor bridge: the device that physically controls the electrical connections to the motor coils. A transistor can be thought of as an electrically controlled circuit breaker; it closes when the coil is connected to a power source for current to pass through the coil. A transistor bridge is required for each motor phase.
The device that controls the activation of the transistor, it is controlled by the MCU to provide the required voltage and current.
A microcontroller unit, usually programmed and controlled by the motor user, which generates specific signals for the pre-driver to obtain the desired motor behavior.
Figure 7 shows a simple schematic of a stepper motor control scheme. The pre-driver and the transistor bridge can be contained in a single device, the driver.
There are a variety of different stepper motor drivers on the market that have different features for specific applications. However, one of their most important features relates to the input interface, and several of the most common input interfaces include:
Step/Direction - By sending a pulse on the Step pin, the driver changes its output to make the motor perform a step, and the Direction of rotation is determined by the level on the Direction pin. Phase/Enable - For each Phase of the stator winding, Enable determines whether the phase is powered on, and phase determines the current direction of the phase. PWM - Direct control of the gate signal of the upper and lower tube FET.
Another important feature of a stepper motor driver is whether, in addition to controlling the voltage at both ends of the winding, it can also control the current flowing through the winding:
With a voltage control function, the driver can adjust the voltage across the windings, and the resulting torque and stepping speed depend only on the motor and load characteristics.
Current control drivers are more advanced because they can regulate the current flowing through the active coil, giving better control over the torque generated and thus better control over the dynamic behavior of the entire system.
Another property that can have an impact on motor control is the arrangement of its stator coils, which determines how the direction of the current changes. In order to achieve the movement of the rotor, it is not only necessary to energize the coil, but also to control the direction of the current, which determines the direction of the magnetic field generated by the coil itself (see Figure 8).
In a single-pole stepper motor, a lead is attached to the central point of the coil (see Figure 9), which allows the direction of the current to be controlled through relatively simple circuits and components. The central lead (AM) is connected to the input voltage VIN (see Figure 8).
If MOSFET 1 is on, the current flows from AM to A +. If MOSFET 2 is on, the current flows from AM to A-, creating a magnetic field in the opposite direction. As mentioned above, this method simplifies the drive circuit (only two semiconductors are needed), but the disadvantage is that only half of the copper conductors in the motor are used at one time, which means that if the same current flows through the coil, the magnetic field is only half as strong as it would be if all the copper conductors were used. In addition, because the motor input leads are more, this type of motor is more difficult to construct.
Stepper motors can control the direction of the current in two different ways.
In a bipolar stepper motor, there are only two leads per coil, and in order to control the direction, the H-bridge must be used (see Figure 10). As shown in Figure 8, if MOSFETs 1 and 4 are on, the current flows from A + to A-. If MOSFET 2 and 3 are on, the current flows from A- to A +, creating a magnetic field in the opposite direction. This solution requires a more complex drive circuit, but can maximize the use of motor copper to achieve maximum torque.
With the continuous progress of technology, the advantages of unipolar motors have gradually weakened, and bipolar stepper motors have become the most popular type of motors at present.
There are four main driving technologies for stepper motors: Wave mode: only one phase at a time is powered on (see Figure 11). For simplicity, if the current flows from the positive lead of A phase to the negative lead (for example, from A + to A-), we call it a positive flow; Otherwise, it is called negative flow. From the left side of the image below, the current flows forward only in phase A, while the rotor represented by the magnet is aligned with the magnetic field it generates. The current then flows forward only in the B phase, with the rotor rotated 90° clockwise to align with the magnetic field generated by the B phase. Subsequently, the A phase is energized again, but the current flows negatively, and the rotor rotates 90° again. Finally, the current flows negatively in phase B while the rotor rotates again by 90°.
Full step mode: Both phases are always powered on at the same time. Figure 12 shows the step-by-step steps for this driver pattern. The steps are similar to the wave mode, the biggest difference is that in the full-step mode, because the current flowing in the motor is more, the magnetic field generated is also stronger, so the torque is also larger.
The half-step pattern is a combination of the wave pattern and the full-step pattern (see Figure 12). This mode can reduce the step length by twice (rotation 45° instead of 90°). The only disadvantage is that the torque generated by the motor is not constant, and the torque is higher when both phases are energized, and the torque is smaller when only one is connected.
Can be seen as an enhanced version of the half-step mode, as it can further reduce the step distance and has a constant torque output. This is achieved by controlling the strength of the current flowing through each phase. The microstep mode requires a more complex motor driver than other schemes. Figure 14 shows how the microstep pattern works. Assuming that IMAX is the maximum current that can be passed in a phase, start from the left side of the diagram, where IA = IMAX and IB = 0 in the first diagram. Next, the current is controlled to reach IA = 0.92 x IMAX, IB = 0.38 x IMAX, which produces a magnetic field rotated clockwise by 22.5° compared to the previous field. Control the current to different current values and repeat this step, rotating the magnetic field 45°, 67.5°, and 90°. Compared with half step mode, it reduces the step length by half. But more can be cut. Very high position resolution can be achieved using the microstep mode, but at the cost of requiring more complex equipment to control the motor and producing less torque per step. The torque is proportional to the sine of the Angle between the stator magnetic field and the rotor magnetic field. Therefore, when the step distance is small, the torque is also small. This can result in a lost step, that is, even if the current in the stator winding changes, the position of the rotor may not change.
Now that we have understood the working principle of the stepper motor, it will be very helpful to summarize the advantages and disadvantages of various types of motors.
Thanks to its internal structure, stepper motors do not require sensors to detect motor position. Stepper motors are moved by performing a "step", so simply counting the number of steps can obtain the motor position at a given time. In addition, the stepper motor control is very simple. It also requires a drive, but does not require complex calculations or adjustments to work properly. Compared with other motors, its control workload is usually small. Furthermore, position accuracy up to 0.007° can be achieved if the microstep mode is used. Stepper motors provide good torque at low speeds, can also hold position well, and have a long service life.
May be out of step when the load torque is too high. Since the actual position of the motor cannot be known, the control will be negatively affected. This problem is more likely to occur when using microstep mode. Stepper motors always consume maximum current even when at rest, which reduces efficiency and can lead to overheating. Stepper motor torque is small, at high speed will produce a lot of noise. Stepper motors have low power density and low torque-inertia ratio. All in all, stepper motors are the best choice when you need a solution that is low cost and easy to control, without high efficiency and torque requirements at high speeds.
