19 Stepper Motors I : Understanding Precise Movement

Think of a motor that doesn’t spin continuously, but moves in precise, controlled steps. This is the magic of stepper motors, behind everything from 3D printers and robotic arms to disk drives and camera lenses. Unlike their DC motor cousins, stepper motors excel in controlled, step-by-step movements, making them ideal for applications demanding high accuracy.

The Heart of Stepper Motors: Electromagnetism in Action

A stepper motor is a fascinating interplay between electromagnetism and mechanical design. It consists of two main parts:

• Rotor: This is the rotating part of the motor, typically made of a permanent magnet with teeth or notches machined around its circumference.
• Stator: The stationary part of the motor, housing electromagnets strategically arranged around the rotor.

The magic happens when we energize the stator’s electromagnets in a specific sequence. This creates a rotating magnetic field that “drags” the permanent magnet rotor along, causing it to rotate in distinct steps. Each step corresponds to a single electrical pulse applied to a particular electromagnet.

Think of it like a game of follow the leader. The rotating magnetic field is the leader, and the permanent magnet rotor, with its defined teeth, is compelled to follow in its footsteps, aligning itself with the ever-shifting magnetic pull.

The beauty of stepper motors lies in their precise control. By carefully controlling the timing and sequence of these electrical pulses, we can dictate the number of steps the motor takes, the direction it rotates (clockwise or counter-clockwise), and even the speed of rotation. This makes them ideal for applications like:

• 3D Printers: Precise movement of the printing head for creating intricate designs.
• CNC Machines: Controlled movement of the cutting tool for precise machining operations.
• Disk Drives: Accurate positioning of the read/write head on the disk surface.
• Camera Lenses: Controlling the zoom and focus mechanisms for sharp images.

Beyond the Basics: Different Types of Stepper Motors

While the basic principle remains the same, there are different types of stepper motors with their own unique characteristics. Permanent Magnet Variants, and Load Effects Stepper motors are electromechanical devices widely used for precise control of rotational motion. They are characterized by their ability to move in discrete steps, making them ideal for applications requiring accurate positioning and speed control. In this reading material, we’ll delve into the types of stepper motors, focusing on permanent magnet stepper motors, and explore how varying loads affect their performance.

Here’s a quick glimpse at the types of stepper motors:

Figure 1: Stepper Motor (“Stepper motor” by pl:Wikipedysta:Harkonnen2, oryginał: pl.Wikipedysta:Ras is licensed under CC BY-SA 4.0.)

Permanent Magnet (PM) Stepper Motors:

• Permanent magnet stepper motors are among the most common types of stepper motors.
• They consist of a rotor with permanent magnets and a stator with multiple windings, typically arranged in a toothed configuration.
• By energizing the stator windings in a specific sequence, the magnetic field interacts with the permanent magnets, causing the rotor to move in discrete steps.

Variable Reluctance (VR) Stepper Motors:

• Variable reluctance stepper motors operate based on the principle of magnetic reluctance.
• They feature a rotor with teeth and a stator with salient poles.
• As the stator windings are energized, the rotor aligns itself to minimize the magnetic reluctance, resulting in precise step movements.

Hybrid Stepper Motors:

• Hybrid stepper motors combine the features of both permanent magnet and variable reluctance designs.
• They offer improved performance, higher torque density, and smoother operation compared to other stepper motor types.

Permanent Magnet Stepper Motors:

A permanent magnet stepper motor is a type of stepper motor that utilizes a permanent magnet in the rotor. It operates based on the attraction or repulsion between the permanent magnet rotor and the stator electromagnets. When electrical pulses are applied, the rotor moves in discrete steps, providing precise control over its position. This type of motor offers advantages such as compact size, low losses, and low maintenance due to the absence of external excitation. However, it is limited in power and torque capabilities compared to other types of stepper motors. Additionally, it provides lower efficiency and draws substantial power regardless of load, leading to energy wastage.

Figure 2: Inside a Permanent Magnet Stepper Motor (Explain That Stuff, https://www.flickr.com/photos/euphy/8587102970)

The advantages and disadvantages of using a permanent magnet stepper motor over other types of stepper motors are as follows:

Advantages:

• Compact Size: Permanent magnet stepper motors are compact and small in size, making them suitable for various applications where space is limited[1].
• Low Losses: Due to the absence of any external excitation, the losses are less, resulting in higher efficiency[1].
• Low Maintenance: The absence of any external excitation also leads to lower maintenance requirements, reducing overall operational costs[1].
• Control Flexibility: They can be connected to an external circuit to control the speed of the motor, providing flexibility in operation[1].
• Wide Speed and Torque Range: Permanent magnet stepper motors can be operated in a wide range of speeds and torques, offering versatility in different applications[1].
• Lower Cost: They are typically less expensive than hybrid stepper motors, making them a cost-effective choice for various applications[3].

Disadvantages:

• Limited Power Applications: Due to limitations in the permanent magnet, they cannot be used for high-power applications[1].
• Limited Torque: The torque produced by permanent magnet stepper motors is limited, which may restrict their use in certain high-torque applications[1].
• Limited Life: The life of a permanent magnet is limited, which can affect the overall lifespan of the motor[1].
• Low Efficiency: Stepper motors, including permanent magnet types, provide low efficiency and draw substantial power regardless of load, leading to energy wastage[4].

Effect of Load on Stepper Motors: The performance of stepper motors can be affected by variations in load, including changes in torque requirements and mechanical resistance. Increasing the load on a stepper motor can lead to:

• Reduced speed and acceleration: Stepper motors may struggle to overcome higher torque demands, resulting in slower movement and longer acceleration times.
• Increased power consumption: Stepper motors may draw more current to maintain torque levels, leading to higher power consumption and potential overheating.
• Loss of steps: Excessive loads can cause stepper motors to lose steps, resulting in inaccurate positioning and compromised performance.

Selecting the Right PM Stepper Motor:

By understanding the different types of PM steppers and how load affects their performance, you can select the right motor for your specific application. Consider factors like:

• Required torque: Choose a motor with sufficient torque to handle the expected load at the desired speed.
• Desired speed: Select a motor capable of achieving the targeted rotational speed while maintaining accuracy under load.
• Positioning accuracy: Consider the motor’s step angle and choose one that offers the necessary level of precision for your application.
• Cost and size: PM steppers come in various sizes and price ranges. Find a balance between these factors that best suits your needs.

In summary, stepper motors play an important role in various applications requiring precise control of rotational motion. Understanding the different types of stepper motors, including permanent magnet variants, and how load variations affect their performance is essential for students and engineers working with electromechanical systems. By optimizing stepper motor selection and operation, engineers can ensure reliable and efficient performance in their applications.

Deepen your understanding: Watch the accompanying lecture video to delve deeper into the concepts covered in the reading.

References:

[1] Monolithic Power Systems, “Stepper Motors: Basics, Types, Uses,” [Online]. Available: https://www.monolithicpower.com/stepper-motors-basics-types-uses

[2] Omega Engineering, “Stepper Motors,” [Online]. Available: https://www.omega.co.uk/prodinfo/stepper_motors.html

[3] J. R. Hendershot and T. J. E. Miller, *Permanent Magnet Motor Technology: Design and Applications*. CRC Press, 2009. [Online]. Available: https://www.amazon.com/Permanent-Magnet-Motor-Technology-Applications/dp/1420064401

[4] Wikipedia, “Stepper motor,” [Online]. Available: https://en.wikipedia.org/wiki/Stepper_motor

[5] Kalias Gold Medal, “PERMANENT MAGNET MOTORS,” [Online]. Available: http://kaliasgoldmedal.yolasite.com/resources/SEM/PERMANENT%20MAGNET%20MOTORS.pdf