The stepper motor belongs to the class of synchronous motors and consists of a rotating motor element with a shaft, the rotor, and a non-rotating element, the stator. While the rotor functions as a permanent magnet, the stator consists of electromagnetic coils in a circular offset arrangement that generate a magnetic field. It is this that gives the motor its ability to assume certain positions and also serves to distinguish stepper motors from the related servo motors. While the latter make use of various sensors for measuring and signalling positions, the stepper motor functions without these elements.

As a brushless synchronous motor, the fundamental task and function of the stepper is to transform electrical impulses into mechanical motion. The revolving rotor moves in steps, whereby the number of steps or positions per revolution can be defined. Accordingly, the number of steps that any particular stepper motor moves through determines the angular change per step. If one rotation of 360° is divided into, for example, 200 steps, the rotor moves through a precise of 1.8°  for each step – which is in practice the most frequently encountered step angle of a stepper motor. The regularity of its change in position is of enormous significance and is indeed the defining characteristic of this type of motor. It is the reason why no feedback signal is necessary, and the current orientation can be measured on the basis of the given number of pulses.

To set the rotor in motion, it is not enough to simply apply a constant voltage, as with the example of the DC motor. The individual magnetic coils of the stepper motor are supplied with a targeted voltage that is either periodically interrupted (unipolar) or its polarity is alternated (bipolar). Flexible motor controllers enable the operation of both unipolar and bipolar models.

Use of position transducers

While the stepper motor by definition does not possess any particular sensor technology to control its position, in practice, they are often used in conjunction with position transducers. These measure the exact position of the rotor and apply a direct correction to the controller in the event that its positioning has lost accuracy. This procedure has gained in importance due to the problem of step loss. This can occur when the stepper motor is overloaded, for instance, due to an external load torque. The rotor misses out several steps with the result that its position can no longer be accurately determined. However, if a position transducer is available, it is able to precisely record the positional inaccuracy and the controller is able to compensate accordingly and correct the motor’s position. In general, it is advisable not to operate a stepper motor above its load limit for any length of time.

Construction sizes and forms of stepper motors

Not only does the construction size vary from one motor, despite frequent standardisation by trade associations, but so does the form of the stepper motor. Although three different types – permanent magnet, variable reluctance and hybrid stepper motors – exist, it is the latter construction that dominates the market, and it is found nowadays in virtually all applications. The hybrid combines the various advantages and positive characteristics of the other construction types in a single unit. For instance, it uses a permanent magnet rotor arranged with magnetically soft toothed discs to form the two poles. This ensures that important features such as a small step angle, high torque and holding torque are maintained.

Applications of the stepper motor

In general, stepper motors are used wherever objects need to be precisely positioned and oriented, for instance in automated processes. Its main area of application is consequently in the various fields of robotics and precision mechanics. The stepper’s extreme reliability as a drive component is demonstrated by its use in the field of aerospace. For instance, specially produced gears produced by Harmonic Drive AG are employed in a valve controller on the International Space Station (ISS) to enable a water circulation system to operate. The basic essential component in this system is a stepper motor that is characterised by flexible controllability.

Stepper motors in everyday use

However, the application fields of stepper motors are not limited to the industrial sector, but can also be found in everyday domestic application. For instance, this type of motor is used to position the laser unit found in the optical drives of CD, DVD, and Blu-Ray players and burners. In standard retail ink-jet printers, a stepper motor is employed to precisely position the printer head. A further application is one that many drivers of motor cars use on a daily basis. A stepper motor is used to control the flow of fuel when the vehicle is in neutral and also enables the adjustment of, among other things, the seats and mirrors.

The spread of the stepper motor is strongly favoured by the constant aspirations in the fields of technology and research. Aspects such as the miniaturisation of a wide range of components, computer control, and cost reduction, have developed from small tendencies to major trends. Stepper motors satisfy these criteria due to their low maintenance costs, variable construction forms, and defined controllability.