You’ll understand in no time after reading this article! Stepper motor knowledge
Latest update time：2023-10-29
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This article will introduce you to the basic knowledge of stepper motors, including their working principles, construction, control methods, uses, types, and their advantages and disadvantages.
Basic knowledge of stepper motors
A stepper motor is a motor that rotates a shaft by stepping (that is, moving at a fixed angle). Its internal structure makes it possible to know the exact angular position of the shaft through a simple step count without the need for sensors.
This property makes it suitable for a variety of applications.
How stepper motors work
Like all motors, stepper motors have a fixed part (stator) and a moving part (rotor). The stator has gear-like protrusions wrapped with coils, while the rotor is a permanent magnet or variable reluctance iron core.
We’ll cover different rotor structures in more depth later. Figure 1 shows a cross-sectional view of a motor whose rotor is a variable reluctance iron core.
Figure 1: Cross-sectional view of stepper motor
The basic working principle of a stepper motor is: energize one or more stator phases, the current passing through the coil will generate a magnetic field, and the rotor will be aligned with the magnetic field; apply voltage to different phases in sequence, the rotor will rotate at a specific angle and Finally get where you need to be. Figure 2 shows how it works.
First, coil A is energized and generates a magnetic field, and the rotor is aligned with this magnetic field. After coil B is energized, the rotor rotates 60° clockwise to align with the new magnetic field. The same situation occurs after coil C is energized. The color of the stator teeth in the figure below indicates the direction of the magnetic field generated by the stator winding.
Figure 2: Stepper Motor Stepping
Types and construction of stepper motors
The performance of a stepper motor (whether resolution/step size, speed or torque) is affected by construction details, which may also affect how the motor is controlled.
In fact, not all stepper motors have the same internal structure (or construction), as different motors have different rotor and stator configurations.
rotor
There are basically three types of rotors in stepper motors:
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Permanent magnet rotor: The rotor is a permanent magnet that is aligned with the magnetic field generated by the stator circuit. This rotor guarantees good torque and has braking torque. This means that the motor is resistant, even if not very strongly, to changes in position whether the coil is energized or not.However, the disadvantage is lower speed and resolution compared to other rotor types. Figure 3 shows a cross-sectional view of a permanent magnet stepper motor.
Figure 3: Permanent magnet stepper motor
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Variable reluctance rotor: The rotor is made of 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 generally produces lower torque and has no braking torque.
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Hybrid rotor: This type of rotor has a special structure that is a mixture of permanent magnets and variable reluctance rotors. The rotor has two axially magnetized magnetic caps with alternating small teeth. This configuration gives the motor the advantages of both a permanent magnet and a variable reluctance rotor, especially high resolution, high speed and high torque. Of course, higher performance requirements mean more complex structures and higher costs.
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Figure 3 shows a simplified schematic diagram of this motor structure. After coil A is energized, a small tooth of the rotor N 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 teeth magnetized N. Although the working principle of a stepper motor is the same, the structure of the actual motor is more complex and has more teeth than shown in the figure. A large number of teeth allows the motor to obtain extremely small step angles, as small as 0.9°.
Figure 4: Hybrid stepper motor
stator
The stator is the part of the motor that is responsible for generating the magnetic field that the rotor aligns with. The main characteristics of the stator circuit are related to its number of phases, pole pairs and conductor configuration.
The number of phases is the number of independent coils, and the number of pole pairs represents the main pair of teeth 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).
Figure 5: Two-phase stator winding (left) and three-phase stator winding (right)
Figure 6: Two-phase unipolar stator (left) and two-phase bipolar stator (right). The magnetic field produced when a positive voltage is applied between A+ and A- is represented by the letters N and S.
Stepper motor control
We know from above that the motor coils need to be energized in a specific sequence to create a magnetic field that the rotor will align with. The devices that can provide the necessary voltage to the coil for the motor to operate properly are the following (starting with devices closer to the motor):
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Transistor Bridge
: A device that physically controls the electrical connection of a motor coil. A transistor can be thought of as an electronically controlled circuit breaker. When it is closed, the coil is connected to the power supply, and current flows through the coil. Each motor phase requires a transistor bridge.
Predriver
: A device that controls the activation of transistors, which is controlled by the MCU to provide the required voltage and current.
MCU
: A microcontroller unit usually controlled by a motor user program that generates specific signals for the pre-driver to obtain the desired motor behavior.
Figure 7 is a simple schematic diagram of the stepper motor control scheme. The pre-driver and transistor bridge can be contained in a single device, the driver.
Figure 7: Basic scheme of motor control
Stepper Motor Driver Types
There are a variety of different stepper motor drivers on the market with different capabilities for specific applications. But one of its most important features is related to the input interface. The most common input interfaces include:
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Step/Direction (Step/Direction) – Send a pulse on the Step pin, and the driver changes its output to cause the motor to perform a step. The direction of rotation is determined by the level on the Direction pin.
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Phase/Enable (Phase/Enable) – For the stator winding of each phase, Enable determines whether the phase is energized, and Phase determines the current direction of the phase.
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PWM – directly controls the gate signal of the upper and lower FETs.
Another important feature of a stepper motor driver is whether, in addition to controlling the voltage across the winding, it can also control the current flowing through the winding:
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Featuring voltage control, the drive regulates the voltage across the windings, producing torque and step speeds that depend solely on the motor and load characteristics.
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Current-controlled drives are more advanced because they can regulate the current flowing through the active coils, providing better control over the torque produced and thus the dynamic behavior of the entire system.
Unipolar/bipolar motor
Another characteristic that can have an impact on a motor's 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).
Stepper motors can control the direction of current flow in two different ways.
Figure 8: Controlling the direction of the magnetic field according to the direction of the coil current
In a unipolar stepper motor, a lead is connected to the center point of the coil (see Figure 9), allowing the direction of the current to be controlled with relatively simple circuits and components. The center lead (AM) connects to the input voltage VIN (see Figure 8).
If MOSFET 1 is on, current flows from AM to A+. If MOSFET 2 turns on, current flows from AM to A-, creating a magnetic field in the opposite direction. As mentioned above, this approach simplifies the drive circuit (requiring only two semiconductors), but has the disadvantage 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 strength is only used half when using all copper conductors. Additionally, these types of motors are more difficult to construct due to the greater number of motor input leads.
Figure 9: Unipolar stepper motor drive circuit
In a bipolar stepper motor, there are only two leads per coil, and to control direction, an H-bridge must be used (see Figure 10). As shown in Figure 8, if MOSFETs 1 and 4 are turned on, the current flows from A+ to A-; if MOSFETs 2 and 3 are turned on, the current flows from A- to A+, generating 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.
Figure 10: Bipolar stepper motor drive circuit
With the continuous advancement of technology, the advantages of unipolar motors have gradually weakened, and bipolar stepper motors have become the most popular motor type at present.
Stepper Motor Drive Technology
There are four main different drive technologies for stepper motors:
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Wave mode: Only one phase is energized at a time (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 positive flow; otherwise, it is called negative flow. Starting from the left side of the figure below, the current flows only in the forward direction in phase A, and the rotor, represented by the magnet, is aligned with the magnetic field it produces. Then, current flows forward only in phase B and the rotor rotates 90° clockwise to align with the magnetic field generated by phase B. Subsequently, phase A is energized again, but the current flows in the negative direction, and the rotor rotates 90° again. Finally, the current flows in the negative direction in phase B, and the rotor rotates 90° again.
Figure 11: Fluctuation pattern step
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Full-step mode: Both phases are always energized at the same time. Figure 12 shows the step-by-step steps of this drive mode. The steps are similar to the wave mode. The biggest difference is that in the full-step mode, because more current flows in the motor, the magnetic field generated is stronger, so the torque is also greater.
Figure 12: Full-step mode stepping
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Half-step mode is a combination of wave mode and full-step mode (see Figure 12). This mode doubles the step size (rotated 45° instead of 90°). Its only disadvantage is that the torque generated by the motor is not constant. The torque is higher when both phases are energized, and the torque is smaller when only one phase is energized.
Figure 13: Half-step mode stepping
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Micro-step mode: It can be regarded as an enhanced version of the half-step mode, because it can further reduce the step distance and has a constant torque output. This is accomplished by controlling the intensity of current flowing through each phase. Microstepping mode requires a more complex motor driver than other solutions. Figure 14 shows how microstepping mode works. Assuming that IMAX is the maximum current that can pass in a phase, starting from the left side of the diagram, in the first diagram IA = IMAX, IB = 0. Next, the current is controlled to achieve IA = 0.92 x IMAX, IB = 0.38 x IMAX, which produces a magnetic field that is rotated 22.5° clockwise compared to the previous magnetic field. Control the current to reach different current values and repeat this step to rotate the magnetic field 45°, 67.5° and 90°. It reduces the step size by half compared to half-step mode; but it can be reduced even more. Very high position resolution can be achieved using microstepping 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 smaller, the torque is smaller. This has the potential to cause lost steps, that is, the position of the rotor may not change even if the current in the stator winding changes.
Figure 14: Microstepping mode stepping
Advantages and Disadvantages of Stepper Motors
Now that we understand how stepper motors work, it will be helpful to summarize the pros and cons of each type of motor.
advantage:
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Thanks to their internal structure, stepper motors do not require sensors to detect motor position. Stepper motors move by performing "steps", so the motor position at a given time can be obtained by simply counting the number of steps.
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Additionally, stepper motors are very simple to control. It also requires a driver, but requires no complex calculations or adjustments to work properly. Compared with other motors, their control workload is usually very small. Moreover, if micro-stepping mode is used, position accuracy as high as 0.007° can be achieved.
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Stepper motors provide good torque at low speeds, hold position well, and have a long life.
shortcoming:
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Out of step may occur when the load torque is too high. Since the actual position of the motor is not known, this can have a negative impact on control. This problem is more likely to occur when using micro-stepping mode.
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Stepper motors always draw maximum current even when stationary, thus reducing efficiency and possibly causing overheating.
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Stepper motors have small torque and produce a lot of noise at high speeds.
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Stepper motors have low power density and low torque-to-inertia ratio.
All in all, stepper motors are the best choice when you need a low-cost, easy-to-control solution that doesn't require high efficiency and torque at high speeds.
To learn how to choose the right motor type for your project, and to learn more about the differences between stepper motors, brushed motors, and brushless motors, click here.
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