What is a stepper motor?
A stepper motor is a type of brushless DC motor that divides one complete rotation into multiple equal steps. It can be commanded to move to a specific position without any position sensor for feedback, and remain at that position as long as the motor is carefully chosen in terms of torque and speed 1.
On the other hand, switched reluctance motors (SRMs) are large stepper motors with a reduced pole count. They often use closed-loop commutation
Operational basics
When DC voltage is applied to the terminals of brushed DC motors, they rotate continuously. The characteristic of a stepper motor is to convert a series of input pulses (usually square wave pulses) into precisely defined increments of shaft position. Each pulse moves the axis a fixed angle.
A stepper motor actually has multiple “toothed” electromagnets arranged around a central gear-like piece of iron. The electromagnet is powered by an external driver circuit or microcontroller. To make the motor shaft turn, an electromagnet is first powered, which magnetically attracts the teeth of the gear. When the teeth of the gear are aligned with one electromagnet, they are slightly offset from the next electromagnet. This means that when the next electromagnet turns on and the previous one turns off, the gear rotates slightly to align with the next electromagnet. This process is repeated for each step, and each rotation is called a “step” and has an integer number of steps for the rotation. In this way, the motor can rotate to a precise angle.
The circular arrangement of electromagnets is divided into groups, each group is called a phase, and there are equal numbers of electromagnets in each group. The number of groups is chosen by the designer. The electromagnets of each group are interleaved with those of other groups to form a uniform arrangement. For example, if a stepper motor has two groups identified as A or B and there are ten electromagnets in total, the grouping method will be ABABABABAB.
Electromagnets in the same group are all energized together. Therefore, stepper motors with more phases usually have more wires (or leads) to control the motor.
What is the most popular type of stepper motor?
There are three main types of stepper motors:
- Permanent magnet stepper
- Variable reluctance stepper
- Mixed sync stepping.
Permanent magnet motors use permanent magnets (PM) in the rotor and operate by attraction or repulsion between the rotor PM and stator electromagnets.
The pulses move the rotor in discrete steps (clockwise or counterclockwise). If power is maintained during the last step, a solid pawl will remain at that axis position. The brake has a predictable spring rate and specified torque limit; if the limit is exceeded, slip will occur. If the current is disconnected, a smaller pawl remains, thus maintaining the shaft position unaffected by springs or other torques. Stepping can then be restored while reliably synchronizing with the control electronics.
Variable reluctance (VR) motors have a plain iron rotor and operate on the principle that minimum reluctance occurs with minimum gaps, so the rotor points are attracted to the stator poles. Hybrid synchronization is a combination of permanent magnet and variable reluctance types to maximize power at a smaller size.
Two phase stepper motor
Solenoid coils in two-phase stepper motors come in two basic winding arrangements: bipolar and unipolar.
Unipolar motor
Unipolar stepper motors have one winding with a center tap for each phase. For each direction of the magnetic field , each part of the winding is switched on. Because in this arrangement the magnetic poles can be reversed without switching the direction of the current, the commutation circuit can be made very simple (e.g., a single transistor ) for each winding. Typically, given a phase, the center tap of each winding is common: three leads are given per phase, compared to six leads for a typical two-phase motor. Typically, these two phase common points are connected internally, so the motor only has five leads.
A microcontroller or stepper motor controller can be used to prime the drive transistors to operate in the correct sequence, and this ease of use makes unipolar motors popular among hobbyists; they may be the best way to obtain precise angular motion. Cheapest way. For the experimenter, the windings can be identified by touching the terminal wires together in a PM motor. If the terminals of the coil are connected, it will be difficult for the shaft to rotate. One way to differentiate between the center tap (plain wire) and the coil end wire is to measure the resistance. The resistance between the regular wire and the coil terminals is always half the resistance between the coil terminals. This is because the length of the coil is twice as long between the ends and only half as long from the center (plain wire) to the end. A quick way to determine if a stepper motor is working properly is to short out every two pairs and try to rotate the shaft. Whenever higher than normal resistance is felt .
Bipolar motor
Bipolar motors have only one winding per phase. In order to reverse the magnetic poles, the current in the windings needs to be reversed, so the driver circuit must be more complex , usually in an H-bridge configuration (but there are several off-the-shelf driver chips available to turn this into an A-type). There are two leads per phase, none of which are common.
The typical drive mode of a two-coil bipolar stepper motor is: A + B + A- B-. That is, drive coil A with a positive current, and then remove the current from coil A. Coil B is then driven with a positive current and the current is removed from coil B. Then drive coil A with a negative current (by switching the wires, flipping the polarity with an H-bridge, for example) and remove the current from coil A. Coil B is then driven with a negative current (same polarity as coil A’s flip); the cycle completes and starts over.
In some drive topologies, stiction effects have been observed using H-bridges.
Dithering the step signal at a higher frequency than the motor’s responsiveness will reduce this “stiction” effect.
They are more powerful than unipolar motors of equal weight due to better utilization of windings . This is due to the physical space occupied by the windings. A unipolar motor has twice the number of wires in the same space, but uses only half as many wires at any point in time, giving it an efficiency of 50% (or approximately 70% of the available torque output). Although driving bipolar stepper motors is more complex, the abundance of driver chips means this is difficult to achieve.
8-lead steppers are like unipolar steppers, but the leads are not connected internally to the common of the motor. This motor can be wired in several configurations:
- Unipolar.
- Bipolar type with series winding. This results in higher inductance but lower current per winding.
- Bipolar parallel winding. This requires higher current, but the performance is better as the winding inductance is reduced.
- Two-phase, single winding per phase. This method will run the motor on only half of the available windings, which will reduce the available low-speed torque, but require less current
High phase number stepper motor
Polyphase stepper motors with many phases tend to have lower vibration levels. Although they are more expensive, they do have higher power density and are generally better suited to the application with the appropriate driver electronics.
Drive circuit
The performance of a stepper motor depends largely on the drive circuit. If the stator poles can be reversed faster, the torque curve can be extended to higher speeds, the limiting factor being the combination of winding inductance. To overcome the inductance and switch windings quickly, the drive voltage must be increased. This further leads to the necessity to limit the current that these high voltages may induce.
The back EMF of the motor is another limitation that can often be compared to the effects of inductance. As the motor rotor rotates, a sinusoidal voltage is produced that is proportional to the speed (step rate). This AC voltage is subtracted from the voltage waveform that can be used to cause a change in current.
L/R driver circuit
L/R driver circuits are also called constant voltage drivers because a constant positive or negative voltage is applied to each winding to set the step position. However, it is the winding current, not the voltage, that applies torque to the stepper motor shaft. The current I in each winding is related to the applied voltage V through the winding inductance L and the winding resistance R. The resistor R determines the xxx current according to Ohm’s law I = V / R. The inductor L determines the xxx change rate of the current in the winding dI/dt = V/L according to the inductance formula. A voltage pulse produces a rapidly increasing current as a function of inductance. This will reach the V/R value and remain the same for the rest of the pulse. Therefore, when controlled by a constant voltage driver, the stepper motor’s xxx speed is limited by its inductance, because at a certain speed, the voltage U changes faster than the current I can handle. Simply put, the rate of change of current is L/R (for example, a 10 mH inductor with a 2 ohm resistance will take 5 ms to reach about 2/3 of xxx torque, or about 24 ms to reach 99% of xxx torque). In order to obtain high torque at high speed , a large driving voltage with low resistance and low inductance is required.
Using L/R drivers, low voltage resistive motors can be controlled with a higher voltage driver simply by placing an external resistor in series with each winding. This wastes power in the resistor and generates heat . Therefore, despite being simple and cheap, it was considered a low-performance option.
Modern voltage mode drives overcome some of these limitations by approximating the sinusoidal voltage waveform to the motor phase. The amplitude of the voltage waveform was set to increase with pace. If properly tuned, the effects of inductance and back- EMF can be compensated, resulting in superior performance relative to current mode drivers, but at the expense of the simpler design effort (tuning process) of current mode drivers.
Chopper drive circuit
Chopper drive circuits are called controlled current drivers because they generate a controlled current in each winding rather than applying a constant voltage. Chopper drive circuits are most commonly used on two-winding bipolar motors, where the two windings are driven independently to provide a specific motor torque CW or CCW. On each winding, the “mains” voltage is applied to the winding as a square wave voltage. For example 8 kHz. The winding inductance smoothes the current which reaches a level according to the duty cycle of the square wave. Relative to the winding circuit, bipolar supply (+ and -) voltages are usually supplied to the controller. Therefore, a 50% duty cycle results in zero current. 0% results in full V/R current in one direction. 100% produces full current in the opposite direction. The controller monitors this current level by measuring the voltage across a small sense resistor in series with the winding . This requires additional electronics to sense the winding current and control the switching, but it allows the stepper motor to be driven at higher speeds with higher torque compared to L/R drives. It also allows the controller to output a predetermined current level rather than a fixed one. Integrated electronics for this purpose are widely available.
Phase current waveform
Stepper motors are polyphase AC synchronous motors that are ideally driven by sinusoidal currents. The full-step waveform is a rough approximation of a sine wave and is the reason why motors exhibit so much vibration. Various drive techniques have been developed to better approximate sinusoidal drive waveforms: these are half-stepping and micro-stepping.
Wave drive (one phase on)
In this driving method, only one phase is active at a time. It has the same number of steps as a full-step drive, but the motor’s torque will be xxx lower than rated. rarely use. The animated character shown above is a wave driven motor. In the animation, the rotor has 25 teeth and requires four steps to rotate one tooth position. Therefore, there will be 25×4 = 100 steps per revolution, and each step will be 360/100 = 3.6 degrees.
Full step drive (two phases open)
This is a common method of driving a motor in full steps. Both phases are always on, so the motor will provide its xxx rated torque. When one phase is closed, the other phase is open. Waveform drive and single-phase full step are the same, the number of steps is the same, but the torque is different.
Half step
In half steps, the drive switches between two-phase on and one-phase on. This increases angular resolution . The motor also has less torque (about 70%) in the full-step position (only single phase open). This can be mitigated by increasing the current in the active winding to compensate. The advantage of half-stepping is that the driver electronics do not need to be replaced to support it. In the animation shown above, if you change this to half steps, it will take 8 steps to rotate 1 tooth position. Therefore, there will be 25×8 = 200 steps per revolution, and each step will be 360/200 = 1.8°. The angle of each step is half of the full step.
Microstepping
What is commonly referred to as microstepping is usually sine cosine microstepping, where the winding current approximates a sinusoidal AC waveform. Chopper drive circuits are a common method for implementing sine and cosine currents. Sine-cosine microstepping is the most common form, but other waveforms are also available. Regardless of the waveform used, as the microstep size gets smaller, the motor will run smoother, thereby reducing resonance in anything the motor may be connected to, as well as in the motor itself. Resolution will be limited by mechanical stiction, backlash, and other sources of error between the motor and the end device. Gear reducers can be used to increase positioning resolution.
Reducing the step size is an important function of stepper motors and the fundamental reason for using them in positioning.
Example: Many modern hybrid stepper motors are rated such that each full step of travel (for example, 1.8 degrees per full step or 200 full steps per revolution) will be within 3% or 5% of every other full step of travel of the motor. Operates within its designated operating range. Several manufacturers have shown that their motors can easily maintain an equal 3% or 5% of the step stroke size as the step size is reduced from full steps to 1/10 steps. Then, as the microstep divisor increases, the step size repeatability decreases. With reduced large step sizes, it is possible to issue many microstep commands before any movement occurs at all, and then movement can “jump” to a new position.
Theory
The pull-out torque of a stepper motor is measured by accelerating the motor to the desired speed and then increasing the torque load until the motor stalls or loses steps. This measurement can be made over a wide speed range and the results used to generate a dynamic performance curve for the stepper motor. As explained below, this curve is affected by drive voltage, drive current and current switching technique. Designers can include a safety factor between the rated torque and the estimated full-load torque required by the application.
Braking torque
When not driven electrically, synchronous motors using permanent magnets have a resonance position-maintaining torque (called braking torque or cogging, sometimes included in specifications ). Soft iron reluctance cores do not exhibit this behavior.