What is a stepper motor?

A stepper motor is a type of brushless synchronous DC motor that, unlike many other standard types of electric motors, doesn’t just rotate continuously for an arbitrary number of spins until the DC voltage passing to it is shut off.

Instead, stepper motors are a type of digital input-output device for precision starting and stopping. They’re constructed so that the current passing through it hits a series of coils arranged in phases, which can be powered on and off in quick sequence. This allows the motor to turn through a fraction of a rotation at a time - and these individual predetermined phases as what we refer to as ‘steps’.

A stepper motor is designed to break up a single full rotation into a number of much smaller (and essentially equal) part-rotations. For practical purposes, these can be used to instruct the stepper motor to move through set degrees or angles of rotation. The end result is that a stepper motor can be used to transfer minutely accurate movements to mechanical parts that require a high degree of precision.

Stepper motors are typically digitally controlled, and function as key components in an open-loop motion-control positioning system. They’re most commonly used in holding or positioning applications where their ability to assert much more clearly defined rotational positions, speeds and torques make them ideally suited to tasks demanding extremely rigorous movement control.

How do stepper motors work?

In a normal brushed DC motor, voltage is applied to terminals which in turn causes a wire coil to rotate at speed inside a fixed magnet housing (the ‘stator’).

In this setup, the spinning wire coil (the ‘rotor’) effectively becomes an electromagnet, and turns rapidly at the centre of the motor based on the familiar principle of magnetic attraction and repulsion. A combination of brushes (electrical contacts) and a rotary electrical switch known as a commutator allows the direction of the current running to the wire coil to be alternated quickly. This creates continuous unidirectional spinning of the rotor coil for as long as the assembly is being fed with sufficient voltage.

A potential downside of this type of motor is that it spins continuously and for an arbitrary number of rotations until power is cut off. This makes it very hard to control the exact stopping point of the motor, rendering it unsuitable for applications requiring greater precision control. Manually controlling the on/off flow of power to the motor can’t give you the required start-stop precision for performing minutely accurate movements.

In a stepper motor, the setup is quite different. Instead of a wire coil rotor spinning inside a fixed housing of magnets, stepper motors are built with a fixed wire housing (the stator in this case) arranged around a series of ‘toothed’ electromagnets spinning at the centre. The stepper motor converts a pulsing electrical current, controlled by a stepper motor driver, into precise one-step movements of this gear-like toothed component around a central shaft.

Each of these stepper motor pulses moves the rotor through one precise and fixed increment of a full turn. As the current switches between the wire coils arranged in sequence around the outside of the motor, the rotary part can complete full or partial turns as required, or it can be made to stop very abruptly at any of the steps around its rotation.

Ultimately, the real strength of a stepper motor versus normal DC brushed motors is that they can quickly locate themselves to a known and repeatable position or interval, and then hold that position for as long as required. This makes them extremely useful in high-accuracy applications such as robotics and printing. Learn Engineering have created the below video that demonstrates how a stepper motor works:

Types of stepper motor

There are numerous stepper motor types sold, and knowing what each of the different varieties do will help you decide which sort is best suited to the application you have in mind.

What is a stepper motor used for?

Stepper motors have a wide range of applications in numerous industries and disciplines, with some of the more common uses being:

• Computing
• Robotics
• Cameras
• Printing and scanning, including in 3D printers
• Process automation and packaging machinery
• Positioning of valve pilot stages for fluid control systems
• Precision positioning equipment

In this section, we’ll look a little more closely at some of these everyday applications.

Stepper motors for 3D printers

Common 3D printer parts lists almost always include a stepper motor of some description. This is because the use of a stepper motor in a 3D printer is an highly accurate and cost-effective way of being able to perform very precise, accurate actions and rotations while the printer is attempting to translate information from digital scans into physical 3D objects.

Stepper motors and drivers in 3D printers allow for tightly controlled movement along both the X, Y and Z axis, either separately or concurrently, meaning that extreme accuracy of movement and positioning is achievable without the need for encoders and other additional software or sensors.

Most 3D printers will incorporate multiple stepper motors - they’re typically found in both the build platforms themselves and the filament extruders, where they’re used to help pull in filament and control the consistent, even supply of material to the machine throughout the full duration of a print run.

Stepper motors for CNC

Stepper motors are an alternative option to servo motors for powering most types of CNC machinery. CNC applications include a very wide range of manufacturing processes in which pre-programmed computer software controls the operation and physical movement of machine tools in factory and fabrication settings.

While stepper motors in CNC applications are often seen as a more ‘budget’ alternative to servo motors, this is an oversimplification based on knowledge of older technologies that isn’t always strictly accurate today. Stepper motors are indeed typically less expensive than servo motors for the same power, but modern versions tend to be just as versatile. As a result, stepper motors are far more commonly available, and found in a much wider range of machines and systems, from machine tools to desktop computers and automobiles.

CNC stepper motors also have one very key advantage over servo motors in that they don’t require an encoder. Servo motors are inherently more complex to understand and operate than stepper versions, and part of this complexity is the fact that they include an encoder, which is more prone to failure than most components in the otherwise reliable servo motor. Stepper motors don’t need an encoder, theoretically giving them even greater reliability than servos.

Furthermore, the fact that stepper motors are also brushless (unlike servo motors) means that they won’t require regularly scheduled replacement, provided their bearings remain in good working order.

Stepper motors for Raspberry Pi

Stepper motors are an extremely common peripheral for adding to Raspberry Pi single-board computing modules for home enthusiast users teaching themselves the fundamentals of basic computer programming skills.

Raspberry Pi starter kits are usually sold in a very barebones configuration, with the idea being that the individual user will add whatever additional parts they like to their system in the order they choose to learn about them, adding to their skill set by learning to control new components using programming languages such as Python.

Among the Raspberry Pi user community, learning to manipulate and control small, inexpensive stepper motors is very commonly seen as a logical next step after learning to control LED on/off cycles and other simple switch or buzzer types. In effect, by linking a couple of these stepper motors in sequence, home hobbyists can begin to create a simple and programmable robot.

Many suitable types of stepper motors are available on the UK market for this type of application, starting from extremely inexpensive 5V versions that are easy to interface with headers on the Raspberry Pi motherboard.

Stepper motors for cameras

Stepper motors are widely used across a range of different applications in high-end camera technologies. They’re used both to control extreme precision internals, such as in-lens autofocus and aperture settings, as well as in the housings and external mechanics of security cameras and remote monitoring systems.

In particular, stepper motors and motorised camera sliders allow for very smooth operation of camera-positioning rigs, meaning that footage taken from security devices can be kept reliably free from potentially problematic image distortion caused by physical motion of the camera around its field of view.

Stepper motors provide several other attractive features for use in camera and video surveillance positioning systems, including full torque at standstill, extremely precise and immediate response times for all movement inputs, consistent repeatability of predetermined movements, and simple open-loop controls defined by fixed step sizes.

Summary

Stepper motors are an incredibly versatile, reliable, cost-effective and accurate way of controlling precise motor movements, allowing users to increase the dexterity and efficiency of programmed movements across a huge variety of applications and industries. As such, they form an important and widely used subset within the much broader category of automation and control gear.

With so many stepper motor brands, sizes, torque ratings, design styles and intended applications on sale in the UK and worldwide, it’s vital to figure out precisely which configuration is best suited to what sorts of user environments when planning a purchase.

Popular stepper motor brands

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