This guide is part of our Industrial Automation Hub where you can discover more about AI, automation and control.
In this guide, we’ll gain a brief overview of how exactly a stepper motor works, before going on to look at some of the more common examples available on the market, and the numerous sorts of everyday roles they’re best suited to.
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.
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:
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.
A bipolar stepper motor has an onboard driver that uses an H-bridge circuit to reverse the current flow through the phases. By energising the phases while alternating the polarity, all the coils can be put to work turning the motor.
In practical terms, this means that the coil windings are better utilised in a bipolar than a standard unipolar stepper motor (which only uses 50% of the wire coils at any one time), making bipolar stepper motors more powerful and efficient to run. Although bipolar stepper motors are technically more complicated to drive, they tend to come with an inbuilt driver chip that handles the bulk of the necessary instructions and behaviours.
The trade-off is that they’re usually more expensive initially than standard unipolar versions because unipolar stepper motors don’t require the current flow to be reversed in order to perform stepping functions - this makes their internal electronics much simpler and cheaper to produce.
Hybrid stepper motors allow for yet more precision, through techniques such as half-stepping and microstepping. Microstepping is a way of increasing the fixed number of steps within a motor by programming a driver to send an alternating sine/cosine waveform to the coils. Doing this often means that stepper motors can be set up to run smoother and more accurately than in a standard setup.
Hybrid stepper motors usually have poles or teeth that are offset on two different cups around the outside of the magnet rotor. This also means steps and rotations can be more precisely controlled, as well as offering quieter operation, higher torque-to-size ratios and greater output speeds than standard stepper motors.
Stepper motors have a wide range of applications in numerous industries and disciplines, with some of the more common uses being:
In this section, we’ll look a little more closely at some of these everyday applications.
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 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.
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.
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