Stepper Motor Working Principle

  • A stepper motor is a rotary-type electromechanical apparatus that transforms electrical digital input pulses into discrete mechanical movements. 
  • Unlike traditional motors that rotate continuously, stepper motors move in steps or increments, 
  • This stepper motor is suitable for applications requiring precise positioning and control.
  • It works on the principle of electromagnetism and is widely used in applications where precise control of angular or linear position is required. 

Construction of stepper motor

The construction of a stepper motor is quite similar to a DC motor. 

Construction:

  • A stepper motor consists of a rotor and a stator. 
  • The rotor is the part of the motor that rotates, and the stator is the stationary part. 
  • The rotor is typically equipped with teeth or cogs to provide discrete positions.

Coils:

  • The stator contains multiple coils, often arranged in pairs. 
  • These coils are usually wound around individual pole pieces. 
  • Stepper motors can have two, four, six, or more coils, and this determines their step resolution and performance.

Driver Electronics: 

  • To control a stepper motor, you need a driver or controller that generates the appropriate sequence of pulses to energize the coils in a specific order. 
  • This sequence determines the direction and distance of the motor’s movement.

Phases:

Stepper motors can have different numbers of phases, which dictate the number of trigger-able coil windings.

Working Principle of stepper motor 

  • The stepper motor works on the principle of Electro-Magnetism. 
  • The Stepper motor consists of a permanent magnet located in the middle & it will turn once upon acting as a force. 
  • This rotor is enclosed through a number of the stators wound through a magnetic coil all over it. 
  • The stator is arranged close to the rotor so that the magnetic fields within the stator control the rotor’s movement.
  • The magnetic field is developed within the stator when supply is given to the winding of the stator.
  • The stepper motor can be regulated by triggering every stator one by one. 
  • So the stator gets magnetized & works like an electromagnetic pole using repulsive energy on the rotor to move forward. 
  • Magnetizing & demagnetizing of the stator will shift the rotor gradually & allow it to turn through great control.
  • A Stepper motor consists of a rotor made up of a permanent magnet, whereas a stator is made up of electromagnets. 
  • The rotor starts moving within the rotating magnetic field of the stator, hence is the fundamental working principle of this motor
  • The poles of the stator & rotor don’t rely on the type of stepper motor. 
  • Soft iron is enclosed in this motor through the electromagnetic stators
  • Once the stator gets triggered the rotor rotates to line up itself with the stator, otherwise turns to have the least gap through the stator. In this manner, the stators are triggered in a series to function the stepper motor.

Components of a Stepper Motor

Stator: 

  • The Stator in the stepper motor is the stationary element.
  • It consists of multiple coils and defines the motor’s configuration to create magnetic poles. 
  • The stator generates a magnetic field necessary for the rotor to move.     

Rotor: 

  • The Rotor is the moving part of the stepper motor that consists of permanent magnets. 
  • The rotor aligns itself with the magnetic field of the stator making the motor move in steps.

Coils: 

  • The stator coils are wound with wires, and they develop magnetic poles when current flows through them. 
  • The arrangement of these coils and the sequence in which they are energized determines the characteristics and step size of the motor.

Permanent Magnets: 

  • In some stepper motors, the rotor contains permanent magnets that interact with the stator’s magnetic field. 
  • This interaction is what drives the motor to move in discrete steps.     

Poles and Pole Pairs:

  • The stator is designed with poles and pole pairs. Each pole has a specific magnetic polarity (north or south). 
  • The arrangement of these poles and the energization sequence of the coils determine the stepping sequence of the motor.

Rotor Position Sensor :

  • Some stepper motors may include position sensors to provide feedback on the rotor’s position. 
  • This feedback can be used for closed-loop control systems to improve accuracy.

Types of Stepper Motor

Stepper motors are classified into 

Permanent Magnet Stepper Motor

  • Permanent magnet type Stepper motor is the most common type of stepper motor. 
  • This type of stepper motor is also known as a tin-can/can-stack type stepper motor.
  • It consists of a Permanent Magnet (PM) in the rotor and works on the force of attraction or repulsion between the rotor Permanent Magnet and the stator electromagnets.
  • It offers low manufacturing costs, 
  • It has 48-24 steps.     
  • The motor is controlled by sending electrical pulses to the coils in a specific sequence. This sequence determines the direction and distance of each step. 
  • The stepping sequence can be full-step, half-step, or microstepping, depending on the application requirements.     

Variable Reluctance Stepper Motor

  • Variable Reluctance is the basic type of stepper motor
  • Variable Reluctance (VR) type stepper motors consist of plain iron rotors.
  • Its operation is based on the principle that minimum reluctance exists with the minimum gap.
  • Here in this type, the rotor points are attracted toward the magnet poles of the stator.
  • The angular position of the rotor depends on the reluctance of the magnetic circuit formed among the teeth of the stators & rotor.
  • These motors may have lower torque and speed capabilities compared to other types of stepper motors, such as those with permanent magnets in the rotor. They may not be suitable for high-performance or high-speed applications.     

Hybrid Synchronous Stepper Motor

  • Hybrid Synchronous stepper motors are a combination of permanent magnet (PM) and variable reluctance (VR) type stepper motors.
  • A Hybrid Synchronous Stepper Motor is the most popular type of motor since it provides better performance in terms of speed, step resolution, and holding torque.
  • This motor achieves maximum power in small package sizes.
  • These motors are used only for lower stepping angles of 1.5, 1.8 & 2.5 degrees.
  • It is much more expensive compared to permanent magnet stepper motors.      
  • Hybrid stepper motors are versatile and find applications in various industries, including robotics, CNC machines, 3D printers, and other motion control systems.     

Various Control Modes in Stepper Motor

Stepper motors can operate in different modes:

Full Step: 

  • In this mode, both coils are fully energized, causing the rotor to move to the next step. 
  • Full stepping provides higher torque but has a coarser step resolution.

Half Step: 

  • The motor alternates between full steps and intermediate positions, effectively doubling the step resolution compared to full-step mode. 
  • Half-stepping results in smoother motion but may have reduced torque.

Micro-stepping: 

  • Micro-stepping divides each step into smaller increments, allowing for very precise control and smoother motion. 
  • Micro-stepping can provide even higher resolution and reduced vibration, but it may reduce the motor’s torque.

Common Types of Stepper Motors

The two most common types are bipolar and unipolar motors.

Bipolar Stepper Motor: 

  • These have two coil windings, and the current can flow in either direction through each coil. 
  • Bipolar stepper motors require an H-bridge driver to reverse the direction of the current flow.

Unipolar Stepper Motor: 

  • These have multiple coil windings, and the current can only flow in one direction through each coil. 
  • Unipolar stepper motors are easier to control but are less efficient.

Commutation:

  • To make the motor step, you energize the coils in a specific sequence. 
  • This sequence is determined by the motor’s design and the desired direction of rotation. 
  • By energizing one coil or coil pair at a time, the rotor aligns itself with the magnetic field generated by the activated coils.

Step angle:

  • Stepper motors are characterized by their step angle which moves in discrete steps.
  • Each step of the stepper motor corresponds to a fixed angle of rotation or linear displacement. 
  • The step angle is determined by the motor’s design and the number of phases. A common step angle comprised of 
  • 200 steps per revolution 1.8 degrees per step.
  • 400 steps per revolution 
  • Smaller step angles result in finer control but require more steps to complete a full revolution.

Control:

  • To control a stepper motor, you use a stepper motor controller or driver. 
  • The controller sends a sequence of electrical pulses to the coils, causing the rotor to move in discrete steps. 
  • By controlling the number and timing of these pulses, you can accurately control the motor’s position and speed.

Open-Loop System:

  • Stepper motors are typically used in open-loop control systems, which means they don’t provide feedback about their actual position. 
  • This makes them less suitable for applications that require precise positioning under varying loads, as they may lose steps if overloaded.

Closed-Loop Control:

  • While stepper motors are often open-loop systems (no feedback on the rotor position), closed-loop control systems incorporate position feedback devices such as encoders or sensors. 
  • Closed-loop control adjusts the motor operation based on the actual position, providing better accuracy and the ability to compensate for missed steps or external disturbances.

Applications of Stepper Motor 

Stepper motors are popularly used in various applications where precise motion is required

  1. 3D Printing: Precise control of the print head’s position and movement in 3D printers.
  2. CNC Machines: Accurate positioning of the cutting tool in computer numerical control (CNC) machines.
  3. Robotic Systems: Joint control in robotic arms and mobile robots.
  4. Camera Autofocus: Lens positioning in cameras.
  5. Textile Machines: Control of thread and fabric movement in textile machinery.
  6. Consumer Electronics: CD/DVD drives, printers, and scanners.
  7. Automated Systems and Conveyors:Stepper motors are employed in automated systems for tasks such as material handling, sorting, and packaging. They provide accurate control of conveyor belts and other mechanisms in these systems.

Advantages of Stepper Motors

  1. Precise positioning and control.
  2. No feedback mechanism (encoders) is required, making them more straightforward and cost-effective in some applications.
  3. Good torque at low speeds.

Disadvantages of Stepper Motors

  1. Inefficient at high speeds.
  2. Can lose steps if overloaded or pushed beyond their capabilities.
  3. Typically less energy-efficient compared to other motor types.
  4. Complexity in control systems, particularly at high microstepping resolutions

Frequently asked Questions

How to Select a Stepper Motor?

  • The selection of a stepper motor for the specific application is essential to examine the torque-speed curve of the motor.
  • The torque-speed curve of the motor must match the requirements of the application.
  • These details are available from the motor designer.
  • The torque-speed curve is a graphical illustration of the motor torque at a defined speed. 

Is it possible to Interface Stepper Motor with 8051 Microcontroller?

Yes, the Stepper Motor can be interfaced with 8051 by using three modes like 

  1. Wave Drive, 
  2. Half Step Drive 
  3. Full Step Drive.

What is the Step for Each Revolution?

  • The steps for each revolution can be defined as the number of step angles required for a total revolution. 
  • The formula for this is 360°/Step Angle.
  • Steps per Second = [(Revolution per Minute * Steps per Revolution) / 60]

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