Stepper motors are type of DC motors used for precise controlled operations. In this tutorial, we are looking into stepper motor basics.
What is a stepper motor
As the name implies, the stepper motor or simply step motor which is a brushless, synchronous DC motor, moves in discrete steps to complete its rotation. In general case, each step in a stepper motor moves 1.8 degree and hence it takes 200 steps for a rotor to finish a single rotation. The stepper motors are also available with step angles 30, 15, 5, 2.5 and 2. The stepper motor is operated based on the principle of electromagnetism and can be controlled with or without feedback. Since its possible to control the positioning and speed of the motor precisely with the help of sophisticated computer controlled systems, the stepper motors are preferred in many precision motion control applications such as robotics.
Stepper motor basics – How does a stepper motor work?
As in the case of any other motors, a stepper motor has a rotating part which is aptly called a rotor and a static part termed stator. The stator and rotor have magnetic poles and by energising the stator poles, the rotor moves in order to align with the stator. A rotor is a central gear-shaped piece of iron. Stator is a set of toothed electromagnets arranged around the central gear.
When the phase windings of the stepper motor are provided with a current, corresponding magnetic flux will be developed in the stator in the direction perpendicular to the direction of current flow. Electromagnets are energized one at a time. When one electromagnet is energised with the help of an external driver circuit or a microcontroller, the rotor shaft turns in such a way that it aligns itself with the stator in a position which minimizes the flux opposition. That means the electromagnet attracts the gear teeth by which the electromagnet is offset from the rest of the electromagnets. Because of this, when the next electromagnet is turned on, the first electromagnet gets turned off which results in the gear teeth getting attracted to the second electromagnet. Thus the rotor is made rotating in steps which is a integer determined by the angle of movement in each step.
Excitation or energizing modes of a stepper motor
By controlling the angle of each step, we can control the resolution and smoothness of rotation of the rotor. There are three modes of excitation.
Full step operation
In this mode the rotor moves through the basic angle of 1.8 degrees in a single step and thereby taking 200 steps to finish off a rotation. We can make this happen by energizing either only one phase of stator windings or two phases. Single phase on operation requires minimum amount of power from the driver circuit. In dual phase on operation, two phases are energized at the same time which results in increased torque and speed.
Half step operation
The rotor moves through half the base angle in a single step which results in improved torque than single phase full step operation. Also it doubles smoothness of rotation and resolution.
Microstepping
In micro step operation, the basic angle is divided into minute values even up to 256 times. Microstep operation is preferred where increased smoothness of rotation is required.
Types of stepper motor
Permanent Magnet Stepper
The typical step angles of a permanent magnet stepper motor is 7.5 degree to 15 degree. The rotor is magnetized by altering the north pole and south pole situated in a straight line parallel to the iron shaft of the rotor. An increased magnetic flux intensity is the result and hence the torque is considerably increased in this type of stepper motors. Permanent magnet stepper motors are having low resolution because of the higher values of step angles. They are also low cost motors.
Variable Reluctance Stepper
These are the earliest stepper motors and not commonly used nowadays. In this type of motors, the rotor consists of multi-toothed rotor made of soft iron and when the stator coils surrounding the rotor are energized using a dc supply, the rotor teeth are attracted towards the poles of the magnetized stator and thus the rotation takes place.
hybrid stepper motor
Since normal step angles of a hybrid stepper motor ranges from 3.6 degree to 0.9 degrees, the motor performance, torque and speed features of this type of motors are higher compared to a permanent magnet stepper motor. Since these kind of stepper motors comes up with the characteristics of both the permanent magnet type and they are called hybrid stepper motors. Rotor is multi-toothed as in the case of variable reluctance motor. The electromagnet is axially magnetized and concentric around the shaft.
Unipolar stepper motor and bipolar stepper motor
Another classification is based on the type of base winding arrangements in a two phase stepper motor. They are unipolar stepper motor and bipolar stepper motor. The main difference between a unipolar and bipolar stepper motors is that the former works on single polarity while the latter works on both positive and negative voltages. One other difference between unipolar and bipolar is that ib unipolar stepper motors, there is a wire required in each of the coils in unipolar motor in order to pass current to one or other end of the coil. This difference in direction makes two polarities of current instead of positive and negative polarities.
Due to the presence of extra wire in the middle of the coils, the unipolar stepper motor will produce half the amount of magnetic flux owing to the reduced current flow. Therefore bipolar stepper motor produces more torque than a unipolar stepper motor.
Stepper motor applications
As mentioned earlier, the stepper motors have found their applications in motion-controlled positioning system as it is easy to produce precise position control with the help of computer controlled stepper motors. They are widely used in biomedical equipment where precise and accurate position control is needed. They are also present in disc drivers, computer printers and scanners, intelligent lighting, camera lenses and various other common devices and equipment.
Because of their precision characteristic, stepper motors are preferred in robotics. 3D cameras, X Y Plotters, CNC and some other camera platforms also impart stepper motors because of its high reliability and precision.
Advantages and disadvantages of stepper motor
Advantages
Because of the precise increment of rotor movement, it is very easy to control the rotation speed precisely. This makes the stepper motor quite acceptable in robotics and automation applications. Simplicity of construction and low maintenance cost are other advantages. Stepper motor can easily get adjusted with any environment variations and it is a highly reliable machine. Other important and crucial feature is that it is highly sensitive to starting, stopping and reversal activities. This is also a welcoming characteristic. Since the torque at low speed is comparatively higher in stepper motors, they are preferred in applications where high torque is needed at low speed. The lack of brushes is an advantage as it increases the overall life of the motor. If the phase windings are energized, the rotor will produce maximum torque at standstill.
Disadvantages
One of the major disadvantage of this kind of dc motors is that it requires more amount of current than a normal dc motor. Though the torque is comparatively higher in lower speeds, it is very low in higher speeds and it is not really easy to operate at higher speeds. This is another crucial disadvantage. This kind of motors are relatively inefficient. Lack of feedback mechanism is another drawback as the feedback system is required to ensure safety.