What is a stepper motor?
A stepper motor is an electromechanical device that converts a series of electrical pulses into angular displacements, which means that it is capable of rotating a number of degrees (step or half step) depending on its control inputs.
Stepper motors are ideal for building mechanisms where very precise movements are required. The main characteristic of these motors is the fact of being able to move them one step at a time for each pulse applied to it. This step can vary from 90 ° to small movements of 1.8 °, which is why these types of motors are widely used since they can be moved at the user’s request according to the sequence indicated by a microcontroller.
These motors have the ability to be locked into position if one or more of their coils is energized or totally free of the current.
The working principle is based on a stator built by several windings in a ferromagnetic material and a rotor that can freely rotate in the stator.
These different windings are fed one after the other and cause a certain angular displacement which is called «angular pitch» and is the main characteristic of the motor.
Stepper motor types
There are three types of stepper motors:
Variable reluctance stepper motor
This motor does not use a permanent magnetic field, as a result, it can move without limitations or without a stopping torque. This type of mounting is the least common and is generally used in applications that do not require a high degree of torque, such as positioning a jog lever.
It was developed in order to achieve smaller angular displacements than in the previous case, without for this reason the number of windings having to be considerably increased. The stator will have the usual cylindrical shape, generally containing a total of three windings distributed in such a way that there will be an angle of approximately 120 ° between two of them.
If the motor stator has three coils connected, with a common terminal, to all coils, it will probably be a variable reluctance motor. The common conductor is usually connected to the positive terminal and the coils are supplied in a consecutive sequence.
Permanent Magnet Stepper Motor
There are two types of permanent magnet motors that are the most widely used in robotics:
These motors usually have 5 or 6 output cables depending on their internal connection, they are usually 4 cables through which the pulses that indicate the sequence and duration of the steps are received, and the rest serve as motor supply. This type is characterized by being simpler to control.
For this type of motors, there are 3 operating sequences
With this sequence, the motor always advances one step at a time because there are always 2 coils activated, with this sequence a high step and holding torque is obtained
step by step normal sequence
Wave drive sequence (full step)
In this sequence, only one coil is activated at a time, causing the motor shaft to rotate towards the active coil. In some motors, this provides smoother operation, but otherwise, the holding and passing torque is less.
step by step wave drive
Half step sequence
In this sequence the coils are activated in such a way that the previous sequences are combined, the result obtained is a shorter step (half the step of the previous sequences), the first 2 coils are activated and later only 1 and so on. successively
step by step half step
This type of motors usually have 4 output cables, they need certain manipulations to be able to be controlled because they require the change of direction of current flow through the coils in the appropriate sequence to perform a movement, it is necessary to H-bridge for each motor coil, that is to say, that to control a 4-wire stepper motor (two coils), two H-bridges need to be used. This makes the controller card more complex and expensive. Its use is not as common as in the case of unipolar type.
step by step bipolar
Sequence to handle it
As said, these motors require the inversion of the current flowing through their coils in a certain sequence. Each polarity reversal causes the axis to move in one step. The direction of rotation is determined by the sequence followed.
bipolar stepper motor
Hybrid stepper motor
The term Hybrid Stepper Motor refers to a stepper type electric motor, the operation of which is based on the combination of the other two types of stepper motors, the Variable Reluctance Motor, and the Permanent Magnetization Motor.