Stepper Motor: Types, Uses, Working Principles
Table of Contents
What is Stepper Motor
Stepper Motor Basics
A stepper motor is an electric motor whose primary characteristic is that its shaft rotates in steps,. Or by shifting by a predetermined number of degrees. This feature, which is made possible by the motor’s internal design, enables users to determine the precise angular position of the shaft by counting how many steps have been completed without the use of a sensor. It is also suitable for a variety of applications thanks to this feature.
Stepper Motor Working Principles
Stepper motors, like all electric motors, have a stationary component (the stator) and a moving component (the rotor). While the rotor is either a permanent magnet or an iron core with variable reluctance, the stator has teeth on which coils are wired. Later, we will delve further into the various rotor structures. Figure 1 depicts a drawing of a motor section with an iron core rotor that has a variable reluctance.
Figure 1: Cross-Section of a Stepper Motor
The basic working principle of the stepper motor is the following. The current flowing in the coil creates a magnetic field by energising one or more of the stator phases,. And the rotor aligns with this field. The rotor can be rotated by a specific number of turns to reach the desired final position by supplying various phases in the proper order. An illustration of the working principle is shown in Figure 2. The rotor is initially aligned with the magnetic field created by coil A when it is first powered up. The rotor rotates 60 degrees clockwise to line up with the new magnetic field when coil B is powered on. The same thing occurs when coil C is powered on. The stator teeth’s colours in the images show the stator winding’s generated magnetic field’s direction.
Figure 2: Stepper Motor Steps
Stepper Motor Types and Construction
Construction details have an impact on a stepper motor’s performance in terms of resolution (or step size), speed, and torque. These details may also have an impact on the motor’s controllability. In actuality, since there are various rotor and stator configurations,. Not all stepper motors have the same internal structure (or construction).
For a stepper motor, there are basically three types of rotors:
- Permanent magnet rotor: A permanent magnet serves as the rotor, which aligns with the magnetic field produced by the stator circuit. This answer ensures both a good torque and a detent torque. This means that regardless of whether a coil is energised, the motor will resist a change in position, albeit weakly. This solution’s disadvantages include its slower speed and lower resolution when compared to other kinds. A section of a permanent magnet stepper motor is depicted in Figure 3.
Figure 3: Permanent Magnet Stepper Motor
- Variable reluctance rotor: The rotor is made of an iron core, and has a specific shape that allows it to align with the magnetic field (see Figure 1 and Figure 2). With this solution it is easier to reach a higher speed and resolution, but the torque it develops is often lower and it has no detent torque.
- Hybrid rotor: This type of rotor is a hybrid of permanent magnet and variable reluctance versions and has a specific construction. The rotor is axially magnetised and has two caps with alternating teeth. With this setup, the motor can benefit from the high resolution, speed, and torque of both the permanent magnet and variable reluctance versions. A more complex construction is necessary to achieve this higher performance, which increases the cost. A simplified illustration of this motor’s structure is shown in Figure 3. An N-magnetized cap tooth lines up with an S-magnetized stator tooth when coil A is energised. The S-magnetized tooth of the stator aligns with the N-magnetized tooth at the same time because of the rotor’s structure. Although the stepper motor’s operating principle is the same, real motors have a more complex structure and more teeth than the one in the illustration. The motor can achieve a small step size, down to 0.9°, thanks to its large number of teeth.
Figure 4: Hybrid Stepper Motor
The component of the motor called the stator is in charge of producing the magnetic field that the rotor will align with. The stator circuit’s primary attributes are the number of phases and pole pairs, as well as the wire arrangement. While the number of pole pairs shows how many primary pairs of teeth are occupied by each phase, the number of phases represents the number of independent coils. The most popular stepper motors are two-phase, while three-phase and five-phase motors are less frequently used. (see Figure 5 and Figure 6).
Figure 5: Two-Phase Stator Winding (Left), Three-Phase Stator Winding (Right)
Figure 6: Two-Phase, Single-Pole Pair Stator (Left) and Two-Phase, Dipole Pair Stator (Right). The Letters Show the Magnetic Field Generated when Positive Voltage is Applied between A+ and A-.
Stepper Motor Control
As we’ve seen in the past, the motor coils must be energised in a particular order in order to produce the magnetic field that the rotor will align with. In order to properly operate the motor, the coils must be supplied with the necessary voltage using a variety of devices. As we move away from the motor and towards the devices:
- A transistor bridge is the device physically controlling the electrical connection of the motor coils. Transistors can be seen as electrically controlled interrupters, which, when closed allow the connection of a coil to the electrical supply and thus the flow of current in the coil. One transistor bridge is needed for each motor phase.
- A pre-driver is a device that controls the activation of the transistors, providing the required voltage and current, it is in turn controlled by an MCU.
- An MCU is a microcontroller unit, which is usually programmed by the motor user and generates specific signals for the pre-driver to obtain the desired motor behavior.
Figure 7 shows a simple representation of a stepper motor control scheme. The pre-driver and the transistor bridge may be contained in a single device, called a driver.
Figure 7: Motor Control Basic Scheme
Stepper Motor Driver Types
On the market, various stepper motor drivers with various features for various applications are offered. The input interface is one of the most crucial features. The most typical choices are:
- Step/Direction – By sending a pulse on the Step pin, the driver changes its output such that the motor will perform a step, the direction of which is determined by the level on the Direction pin.
- Phase/Enable – For each stator winding phase, Phase determines the current direction and triggers Enable if the phase is energized.
- PWM – Directly controls the gate signals of the low-side and high-side FETs.
Another important feature of a stepper motor driver is if it is only able to control the voltage across the winding, or also the current flowing through it:
- The driver only controls the voltage across the winding when using voltage control. Only the characteristics of the motor and the load affect the torque generated and the speed at which the steps are carried out.
- In order to better control the torque generated and, consequently, the dynamic behaviour of the entire system, current control drivers are more sophisticated. They regulate the current flowing through the active coil.
The placement of the stator coils, which govern how the current direction is changed, is another aspect of the motor that has an impact on control. It is necessary to control the direction of the current, which determines the direction of the magnetic field produced by the coil itself, in order to move the rotor, in addition to energising the coils. (see Figure 8).
In stepper motors, the issue of controlling the current direction is solved with two different approaches.
Figure 8: Direction of the Magnetic Field based on the Direction of the Coil Current
In unipolar stepper motors, one of the leads is connected to the central point of the coil (see Figure 9). This allows to control the direction of the current using relatively simple circuit and components. The central lead (AM) is connected to the input voltage VIN (see Figure 8). If MOSFET 1 is active, the current flows from AM to A+. If MOSFET 2 is active, current flows from AM to A-, generating a magnetic field in the opposite direction.
As pointed out above, this approach allows a simpler driving circuit (only two semiconductors needed), but the drawback is that only half of the copper used in the motor is used at a time, this means that for the same current flowing in the coil, the magnetic field has half the intensity compared if all the copper were used. In addition, these motors are more difficult to construct since more leads have to be available as motor inputs.
Figure 9: Unipolar Stepper Motor Driving Circuit
In bi-polar stepper motors, each coil has only two leads available, and to control the direction it is necessary to use an H-bridge (see Figure 10). As shown in Figure 8, if MOSFETs 1 and 4 are active, the current flows from A+ to A-, while if MOSFETs 2 and 3 are active, current flows from A- to A+, generating a magnetic field in the opposite direction. This solution requires a more complex driving circuit, but allows the motor to achieve the maximum torque for the amount of copper that is used.
Figure 10: Bi-polar Stepper Motor Driving Circuit
With technology progress, the advantages of unipolar are becoming less relevant, and bi-polar steppers are currently the most popular.
Stepper Motor Driving Techniques
There are four different driving techniques for a stepper motor:
- In wave mode, only one phase at a time is energized (see Figure 11). For the sake of simplicity, we will refer to the direction of the current as positive if it is moving from the + lead to the – lead of a phase (for example, from A+ to A-), and negative if it is moving in the opposite direction. The rotor, represented by a magnet, is aligned with the magnetic field it produces and the current is only flowing in phase A in the positive direction starting from the left. The rotor then spins 90 degrees clockwise to align with the magnetic field produced by phase B, and it only flows in phase B in the positive direction in the following step. Later, phase A is energised once more, but this time the rotor is spinning at a 90° angle and the current is flowing in the opposite direction. In the last step, the current flows negatively in phase B and the rotor spins again by 90°.
Figure 11: Wave Mode Steps
- In full-step mode, two phases are always energized at the same time. Figure 12 shows the different steps of this driving mode. With this mode, the motor is able to produce a higher torque because more current is flowing through it and a stronger magnetic field is being created. The steps are similar to those in wave mode with the main difference being that this mode.
And Figure 12: Full-Step Mode Steps
- Half-step mode is a combination of wave and full-step modes (see Figure 12). Using this combination allows for the step size to be reduced by half (in this case, 45° instead of 90°). The only drawback is that the torque produced by the motor is not constant, since it is higher when both phases are energized, and weaker when only one phase is energized.
Figure 13: Half-Step Mode Steps
- Microstepping can be seen as a further enhancement of half-step mode, because it allows to reduce even further the step size and to have a constant torque output. This is achieved by controlling the intensity of the current flowing in each phase. Using this mode requires a more complex motor driver compared to the previous solutions. Figure 14 shows how microstepping works. If IMAX is the maximum current that can flow in a phase, starting from the left, in the first figure IA = IMAX and IB = 0. In the next step, the currents are controlled to achieve IA = 0.92 x IMAX and IB = 0.38 x IMAX, which generates a magnetic field that is rotated by 22.5° clockwise compared to the previous one. This step is repeated with different current values to reach the 45°, 67.5°, and 90° positions. This provides the ability to reduce by half the size of the step, compared to the half-step mode;. But it is possible to go even further. Using microstepping helps reaching very high position resolution,. But this advantage comes at the cost of a more complex device to control the motor,. And a smaller torque generated with each step. Indeed, the torque is proportional to the sine of the angle between the stator magnetic field and the rotor magnetic field;. Therefore, when the steps are smaller, the torque is smaller. This may lead to missing some steps, meaning the rotor position does not change even if the current in the stator winding has.
Figure 14: Microstepping
Stepper Motors Advantages and Disadvantages
Now that we understand the working principles of the stepper motors, it is useful to summarize their pros and cons compared to other motor types.
- Stepper motor control is also not too difficult. The driver is necessary for the motor to function, but no intricate calculations or tuning are necessary. In comparison to other motors, the control effort is generally lower. You can achieve high position accuracy using microstepping, up to about 0.007°.
- Stepper motors do not need a sensor to determine the motor position because of the way they are built internally. Since the motor moves in “steps,” you can determine its position at any given time by simply counting these steps.
- Stepper motors offer good torque at low speeds, are great for holding position, and also tend to have a long lifespan.
- If the load torque is too high, they risk missing a step. Since it is impossible to determine the motor’s actual position, this has an adverse effect on control. Stepper motors are even more prone to this problem when microstepping is used.
- Stepper motors have low torque and become pretty noisy at high speeds.
- These motors always drain maximum current even when still, which makes efficiency worse and can cause overheating.
- Finally, stepper motors have low power density and a low torque-to-inertia ratio.
To summarize, stepper motors are good when you need an inexpensive, easy-to-control solution. And when efficiency and high torque at high speeds are not necessary.
Stepper Motor Applications (applications of stepper motor)
Due to their properties, stepper motors are used in many applications. Where a simple position control and the ability to hold a position are needed, including: