Stepper motors are motors which is used to rotate through a specific angle. The speed of the rotation and and torque are secondary concern. The primary concern is that the stepper should rotates through the exact angle and then halt. Each rotation is referred to as a step, and the most common step angles include 30°, 15°, 7.5°, 5°, 2.5°, and 1.8°.
Stepper motors are widely used in electrical devices such as manufacturing robots, in 2D and 3D printers, CNC machines. An important advantage of using steppers is that the controller does not need to read the stepper's position to determine its orientation. If the stepper is rated for 2.5°, each control signal from microcontroller such as Arduino will turn the rotor through an angle of 2.5°.
In many applications, the step angle needs to be as small as possible. The smaller the motor's step angle, the better its angular resolution. Another essential figure of merit is torque, particularly the holding torque. A stepper is expected to maintain its position when it comes to a stop, and holding torque identifies the maximum torque it can exert to hold its position.
Modern steppers can be categorized into three types:
- Permanent Magnet(PM) stepper motor
- Variable Reluctance (VR) stepper motor
- Hybrid (HY) stepper motor
PM stepper motors have high torque but poor angular resolution. VR stepper motors have excellent angular resolution but low torque. The HY steppers combine the structures and advantages of PM and VR steppers and provide good torque and angular resolution.
Permanent Magnet Stepper motor
A permanent magnet stepper motor has a rotor in the inside with magnet mounted on its perimeter and has stator on the outside which is made up of number of electromagnets constructed by winding coils on slots. Below shows picture of a permanent magnet stepper motor.
The electromagnets on the stator are grouped such that when electric control pulses are from micrcontroller is received, the electromagnet form magnets with north and south poles. The magnetic action of attraction and repulsion between the rotor and stator causes the stator to rotate. By sending electric pulses alternatively between the different groups of the stator electromagnet, the rotor gets rotated in one direction. By changing the direction of the electric control signal the rotor can be rotated in the other direction. Permanent Magnet stepper motor is similar to inrunner brushless DC motors(BLDC). PM stepper motor are used in computer disk drives and printers.
Permanent Magnet stepper motor can be controlled using microcontroller or Arduino. Following are tutorials how to control Permanent Magnet Stepper motor.
- Control of Stepper Motor using Arduino
- Wireless Stepper motor control with Arduino
Variable Reluctance Stepper motor
Variable Reluctance (VR) stepper motors are a type of stepper motor that operates by the principle of magnetic reluctance. These motors use a rotor made of soft iron that has multiple teeth, and the stator has several winding phases. The rotor teeth align with the stator phases, and as the phases are energized, they create a magnetic field that attracts the rotor teeth, causing them to move. The rotation of the rotor is determined by the number of teeth and winding phases. Shown below is picture of VR stepper motor.
VR steppers have excellent angular resolution due to their design, with step angles as small as 0.36°. However, they have low torque and can't produce as much power as other types of stepper motors. VR steppers are commonly used in low-power applications such as in clocks, robotics, and small positioning systems. They are also known for their simplicity and low cost, making them a popular choice in various industries.
The step angle of VR stepper motor can be calculated using the formula below.
\(Step \space Angle = \frac{N_w-N_t}{N_wN_t} 360 ^\circ \)
where \(N_w\) is the number of windings in the stator and \(N_t\) is the number of teeths in the rotor.
For example in the above VR stepper motor figure, \(N_w=8\) and \(N_t=6\) and so the step angle is,
\(Step \space Angle = \frac{8-6}{8 \times 6} 360 = ^\circ 15^\circ\)
Hybrid stepper motor
A Hybrid stepper motor is a type of stepper motor that combines the best features of both Permanent Magnet (PM) and Variable Reluctance (VR) stepper motors. They have a rotor with teeth that align with the stator's teeth and use a combination of permanent magnets and magnetic reluctance to create rotational movement.
Hybrid stepper motors provide good torque and angular resolution, making them suitable for a wide range of applications. As in VR stepper motor, the Hybrid stepper motor has rotor with teeths to improve angular resolution and like PM stepper motor, the Hybrid stepper motor has rotor with magnets that provides the torque. Following shows diagram of a Hybrid stepper motor:
The construction of a Hybrid stepper motor allows for greater control and precision than other stepper motor types, making them a popular choice for many industrial and automation applications. They are commonly used in CNC machines, robotics, and other high-precision equipment that requires accurate positioning and control. An example of hybrid stepper motor is the Nema 17. Microcontrollers such as Arduino can be used to control stepper motor like Nema 17 as illustrated in the following tutorials:
- stepper motor speed control arduino
- Stepper Motor Speed and Direction Control with Potentiometer
The disadvantage of Hybrid stepper motor is that they are heavier than PM stepper motor and are more expensive.
