The stepper motor belongs to the group of synchronous motors and has a rotatable motor element with a shaft, the rotor, and a non-rotating motor element, the so-called Stator. While the rotor acts as a permanent magnet, the stator consists of magnetic coils which are arranged in an offset manner and thus generate a magnetic field. Said represents the basis for the positioning capabilities of the motor. Makes the difference to the related servo motors clear. While the latter use different sensors for position measurement and subsequent feedback, the stepper motor, sometimes also called stepper, basically manages without them.
As a brushless synchronous motor, it is the basic Task and function of the Stepper motor, electrical impulses can be converted into mechanical movement. In this context, the rotatable rotor is moved stepwise, whereby the number of steps or positions per revolution can be defined. Accordingly, the step quantity of each stepper motor determines the angular change per step. If one revolution, corresponding to 360°, is divided into 200 steps, for example, the rotor moves at a constant rate of exactly 1.8° with each step – the most commonly used step angle in practice when using stepper motors. The regular change in position throughout is enormously significant and makes the motor what it is. It is the reason that no feedback signal is needed and the current orientation can instead be measured based on the given pulses.
To set the rotor or the shaft in motion, it is not enough to apply a constant voltage as in the example of DC motors. On the contrary – the individual magnetic coils of the stepper motor are specifically supplied with voltage. Intermittently suspended (unipolar) or alternately reversed polarity (bipolar). Versatile motor controls allow operation of unipolar as well as bipolar models.
Use of position encoders
While the stepper motor by definition does not have any special sensor technology for position control, position encoders are frequently mounted in practice. These measure the exact rotor position and help the control system to correct it directly in case of a possible misalignment. In the past, this method has become more relevant due to the problem of step loss. In this case, the stepper motor is overloaded – for example by an external load torque – so that the rotor sometimes skips several steps and its actual position can no longer be traced. This results in inaccurate positioning of the rotor. However, if a position encoder is present, it can record the misalignment exactly and provide the control unit with the basis for correction. In principle, it is advisable not to operate stepper motors permanently above their load limit.
Sizes and shapes of stepper motors
In addition to the size, which is often standardized by industry associations, it is also the design, which can differ from motor to motor. Although three designs – permanent magnet stepper motor, reluctance stepper motor and hybrid stepper motor – are usually listed, only one particular type dominates the market and is used in almost all applications today: the hybrid stepper motor. As a hybrid, this combines various advantages. Positive features of the further designs in themselves. Thus, it adopts the permanent magnet of the permanent magnet motor as rotor and combines it with soft magnetic toothed disks, which represent two poles. This ensures important properties such as small step angles, high torque and self-holding torque.
Areas of application of the stepper motor
Basically, the stepper motor is used when objects need to be precisely placed and aligned – for example, as part of automated processes. As a result, it is mainly used in the fields of robotics and precision mechanics. The fact that the so-called steppers are also extremely reliable drive components is further demonstrated by their use in space and aviation applications. Because in order to enable a water circuit on the International Space Station, ISS for short, specially manufactured gears from Harmonic Drive SE are used to control valves. As an essential basis of the drive serves thereby a step engine, which is characterised by uncomplicated control possibilities.
Stepper motors in everyday life
However, the areas of application for stepper motors do not only extend over large parts of the industrial sector – they are also used in private, partly everyday applications. For example, the motor takes over the positioning of the laser unit within optical CD, DVD, Blu-ray and burner drives, among other things. In printing devices such as the commercially available inkjet printer, it is the job of the stepper motor to precisely position the print head. Last but not least, a large number of people make use of it every day in their own cars. Here, the motor in question controls the flow of fuel when the vehicle is idling and allows the seat and mirrors, among other things, to be adjusted.
The spread of the stepper motor is strongly favored by ongoing efforts in technology and research. Here, aspects such as the miniaturization of a wide variety of components, computer control and cost reduction have developed from tendencies to real trends. Stepper motors meet these criteria due to low maintenance costs, variable sizes and defined control.