What is a stepper motor?
A Stepper Motor or a step motor is a brushless, synchronous motor which divides a full rotation into a number of steps.
The main difference between them and all the other motors, is the way they revolve. Unlike other motors, stepper motors does not continuously rotate! Instead, they rotate in steps (from which they got the name). Each step is a fraction of a full circle. This fraction depends mostly from the mechanical parts of the motor, and from the driving method. The Stepper Motors therefore are manufactured with steps per revolution of 12, 24, 72, 144, 180, and 200, resulting in stepping angles of 30, 15, 5, 2.5, 2, and 1.8 degrees per step.
The stepper motors also differs in the way they are powered. Instead of an AC or a DC voltage, they are driven (usually) with pulses. Each pulse is translated into a degree of rotation. For example, an 1.8o stepper motor, will revolve its shaft 1.8o on every pulse that arrives. Often, due to this characteristic, stepper motors are called also digital motors.
The stepper motor can be controlled with or without feedback.
A very basic stepper motor
As all motors, the stepper motors consists of a stator an a rotor. The rotor carries a set of permanent magnets, and the stator has the coils. The very basic design of a stepper motor would be as follows:
There are 4 coils with 90o angle between each other fixed on the stator.
The way that the coils are interconnected, will finally characterize the type of stepper motor connection. In the above drawing, the coils are not connected together. The above motor has 90o rotation step. The coils are activated in a cyclic order, one by one.
The rotation direction of the shaft is determined by the order that the coils are activated. The following animation demonstrates this motor in operation. The coils are energized in series, with about 1sec interval. The shaft rotates 90o each time the next coil is activated:
To rotate the stepper motors require an electronic driver. You can not use the stepper motor simply providing voltage as a DC motor.
There are various ways that the coils are energized, and the results on the motors shaft.
Wave drive or Single-Coil Excitation
The first way is the one described previously. This is called Single-Coil Excitation, and means that only one coil is energized each time. This method is rarely used, generally when power saving is necessary. It provides less than half of the nominal torque of the motor, therefore the motor load cannot be high.
This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle.
Full step drive
The second and most often used method, is the Full step drive. According to this method, the coils are energized in pairs.
According to the connection of the coils (series or parallel) the motor will require double the voltage or double the current to operate that needs when driving with Single-Coil Excitation.
Yet, it produces 100% the nominal torque of the motor.
This motor will have 4 steps per full cycle, that is the nominal number of steps per cycle.
This is a very interesting way to achieve double the accuracy of a positioning system, without changing anything from the hardware!
According to this method, all coil pairs can be energized simultaneously, causing the rotor to rotate half the way as a normal step. This method can be single-coil or two-coil excitation as well.
The following animations make this clear:
With this method, the same motor will have double the steps per revolutions, thus double the accuracy in positioning systems. For example, this motor will have 8 steps per cycle!
Microstepping is the most common method to control stepper motors nowadays. The idea of microstepping, is to power the coils of the motor NOT with pulses, but with a waveform similar to a sin waveform. This way, the positioning from one step to the other is smoother, making the stepper motor suitable to be used for high accuracy applications such as CNC positioning systems. Also, the stress of the parts connected on the motor, as well as the stress on the motor itself is significantly decreased. With microstepping, a stepper motor can rotate almost continuous, like simple DC motors.
The waveform that the coils are powered with, is similar to an AC waveform. Digital waveforms can also be used. here are some examples:
Powering with digital signal
Powering with high resolution digital signal
The microstepping method is actually a power supply method, rather than coil driving method.
Therefore, the microstepping can be applied with single-coil excitation and full step drive.
The following animation demonstrated this method:
Although it seems that the microstepping increases the steps even further, usually this does not happen. In high accuracy applications, trapezoidal gears are used to increase the accuracy. This method is used to ensure smooth motion.
Stepper motor types
Permanent Magnet Stepper Motor (PM)
The first and most basic type of stepper motors is the Permanent Magnet (PM).
The rotor of the PM motor carries a permanent magnet with 2 or more poles, in a shape of disk. The operation is exactly the one described above. The stator coils will attract or repulse the permanent magnet on the rotor and will generate the torque.
Here is a sketch of a PM motor:
PM stepper motors have usually step angle from 45o to 90o.
Variable Reluctance Stepper Motor (VR)
The VR motor does not have a permanent magnet on the rotor. Instead, the rotor is made of soft iron, and performs a teethed disk like a gear.
The stator has more than 4 coils. The coils are energized in opposite pairs, and will attract the rotor.
The lack of a permanent magnet has a negative affect on the torque that is significantly decreased. But it has a great advantage. These motors have no detent torque.
The detent torque, is the torque generated by the rotor permanent magnets that are magnetized to the stator's armature, when no current flows within the coils.
You can easily understand what this torque is, if you try to rotate an unconnected stepper motor by hand (NOT a VR stepper). You will feel the distinctive "clicks" of each step of the motor. Actually, what you feel is the detent torque that pulls the magnets against the armature of the stator. Here is an animation of a VR stepper motor in operation:
VR stepper motors have usually step angle from 5o to 15o.
Hybrid Stepper Motor
The hybrid stepper motors are named so, because they combine the characteristics from both VR and PM stepper motors. They have excellent hold and dynamic torque, and very small step angles, from 0.9o to 5o, giving them A+ in accuracy.
Their mechanical parts can rotate at high speeds relatively to the other stepper motor types. This is the type of motor used for high end CNC and robots. The major disadvantage is the cost.
A typical 200 steps per revolution motor, will have 50 North and 50 South poles, with 8 coils (4 pairs). Because such a magnet cannot be manufactured, an elegant solution has been given.
There are actually 2 separate disks, each one with 50 teeth. A permanent cylindrical magnet is also used. The disks are welded one on the North and one on the South pole of the permanent magnet. Thus, one disk has North pole on its teeth and the other South.
The trick, is that the disks are placed in a way that if you look them from above, you will see one disk with 100 teeth! The hills of the first disk, are aligned with the valleys of the other disk.
A permanent magnet with 50 North and 50 South poles is not possible to be manufactured...Therefore two disks are placed on top and bottom of a cylindrical permanent magnet
The hills of one disk are aligned with the valleys of the other. If you look the disks from above, it will be like looking a 100-teethed disk with 50 north and 50 south poles! An elegant solution!
The following animation shows a hybrid stepper motor with 75 steps per cycle (5o per step). Worth to notice that the 6 coils are in pairs of two, each one with its opposite coil.
Although someone would expect to find these pairs with angle difference of 60o, it is not so.
If we suppose that the first pair is the most top and most bottom coil, then the second pair is with angle difference of 60+5o from the first, and the third 60+5o from the second.
This angle difference is the reason why the motor moves! Full and half stepping can be applied, as well as single-coil excitation for power saving.
In this animation i use full step drive. With half step drive, the steps are increased to 150!
Don't try to follow the coils to see how it works. Just focus on one coil and wait.
You will notice that, whenever this coil is actuated, there are 3 North poles (red) 5o back, that are pulled to the rotation direction, and another 3 South poles (blue) 5o front that are pushed to the rotation direction.
The coil that is actuated is always between the North and South poles.
Stepper motors are actually multiphase motors. The more the coils, the more the phases. The more the phases, the smoother the operation of the motor and the higher the price.
The torque is irrelevant to the number of phases. The most common type of stepper motor is the two-phase.
Two phases, is the minimum number of phases needed for a stepper motor to operate. What you need to make clear here, is that the number of phases does not necessarily set the number of coils.
If for example each phase has 2 coil pairs, and the motor is a 2-phase motor, the number of coils will be 8. That has to do only with the mechanical characteristic of the motor.
To simplify things, i will explain the simplest 2-phase motor with one coil pair per phase.
There are basically 3 different connection types for a 2-phase stepper motor. The coils are interconnected and according the connection, a different number of wires are used to connect the motor to the controller.
This configuration is the most simple. 4 wires are used to connect the motor to the controller. The coils are internally connected either in series or parallel. This is an example of a bipolar stepper motor:
The motor has 4 terminals. The two yellow terminals (the colors i use are NOT according to standards!!!) are for powering the horizontal coils, while the two purple terminals are for powering the vertical coils.
The problem with this configuration is that, if someone wants to change the magnetic polarity, the only way to do this is by changing the current direction. This means that the driver circuit will have to be complicated, for example with a H-bridge.
In a bipolar motor, a common wire is connected to the point where the two coils are connected together:
With this common wire, the magnetic poles can now easily be changed. Suppose for example that we connect the common wire to the ground.
By powering once the first end of the coil and once the other end, the magnetic poles are changed. This means that the circuit for a bi-directional motor application is very simply, usually with only two transistors per phase. A major drawback is that, each time, only half of the available coil windings are used. This is like the motor is driven with single-coil excitation.
Therefore, the torque generated is always about half the torque that would have be generated if both coils were powered.
In other words, unipolar motors needs double the space as a bipolar motor, to provide the same torque.
The unipolar motor can be used as a bipolar motor, simply by leaving the common wire unconnected.
Unipolar motors may have 5 or 6 terminal wires. The drawing above demonstrates a unipolar motor with 6 wires.
There are situations though, that the two common wires are internally connected.
In this case, the motor will have 5 wire terminals.
This is the most flexible stepper motor in terms of connection modes. All coils have wire terminals for both sides:
This motor can be connected with any connection possible.
It can be connected as a 5 or 6 leads unipolar, as bipolar with series windings, as bipolar with parallel windings, or as bipolar with single winding per phase for lower current applications.