Why Using a Stepper Motor?

A stepper motor is an electromechanical system which is transducing an electrical signal into a mechanical one. It is designed to accomplish a discrete movement (notion of step) and reach a precise position.

The movement is achieved through the use of a magnetic field provided by coils and sensed by magnets. Indeed, when one of the coils is energized, a magnetic field is created and, if the energy is supplied cyclically (by means of input pulses), then the magnetic field will vary. When a magnet is placed in this varying magnetic field, it will position itself in the lowest energy state (equilibrium), therefore providing motion. This principle is used in the stepper motor, composed of a fixed part (the stator) made of cyclically energized coils and a moving part (the rotor) made of ferromagnetic material or magnets.

So why using a stepper motor?

Stepper motors do not operate as DC or brushless motors. They have no integrated electronics, no brushes and can be controlled in open loop. To get a quick idea, stepper motors are very often considered as particular brushless motor that can be controlled without feedback in open loop. Thus, even though stepper motor may look a bit more difficult to understand technically, they have the advantage of being very simple to control and need no encoder or special driver to monitor the position of the rotor.

Stepper MotorsDC MotorsBrushless Motors
Position with feedbackNot really an option (cost)High resolution depending on encoderHigh resolution depending on encoder
Positioning w/o feedbackGood
24 steps/rev
High torque
6 steps/rev
Moderate torque
Cost (components)Motor, 100%
Encoder 40%
Driver chip, 5%
Enc. Interface, 20%
Controller, 40%

Total 145% (without encoder)
total 205% (with encoder)
Motor, 60%
Encoder 40%
Driver, 10%
Enc. Interface, 20%
Controller, 40%

Total 170%
Motor, 150%
Encoder 40%
Driver, 20%
Enc. Interface, 20%
Controller, 60%

Total 290%
Limited to max. boost running torque
Very high
Boost operation reduces brush life
Very high
Top Speed<15000rpm<15000rpm>15000 rpm
Life TimeVery goodLimited by wear of the brushesVery good
StabilityVery high, inherentƒ(Enc.res; controller)ƒ(Enc.res; controller)
ReliabilityHigh (no brushes/ no Hall sensors)Low (brushes)High (no brushes)
Table 1: summarizes the differences between those 3 kinds of motor.

Stepper motors are suitable for applications where compact and robust solutions are required. They develop their maximum torque at stand-still which makes them naturally suitable to hold a position. The external commutation ensures that the speed is perfectly constant even if the load varies. Thanks to the absence of any electronic component, stepper motors run where the hall sensors or encoders of other type of motor find their limit: high/low temperatures, external noise disturbances, etc.

Compared to a DC motor, a stepper motor is also much easier to use for positioning application as the notion of step enables the user to know the precise position or displacement of the rotor without feedback: it runs in open loop. Stepper motors are as a matter of fact frequently used when the following application requirements are specified:

  • Repetitive positioning tasks with high accelerations (i.e. XYZ of machine tools).
  • Whenever the settling time must be short and with repeatable discrete positions.
  • Whenever open loop (absence of electronics) makes sense (e.g. for noise immunity).
  • For back and forth motions.
  • Frequent “start/stop” operation.
  • Whenever the duty cycle is relatively small (Time ON << Time OFF).
  • Whenever the actual position must be held with high torque.
  • Whenever the actual position must be held when no current is applied (thanks to the residual torque).
  • Whenever long life times is required (i.e. using the brushless design).
  • Whenever minor speed variation under load are not allowed (peristaltic pumps, XYZ of machine tools).
  • For most small consumer electronic devices, such as hard disc drives, ink jet printers, cameras.

By contrast ATO stepper motor focuses on the following market segments:

  • Optical systems (e.g. zoom, focus, pan/tilt, and filter positioning, microscope stages) Photonics (e.g. laser tuning, laser scanner).
  • Telecommunications (e.g. variable optical amplifiers (VOA)).
  • Medical (e.g. pumps, DNA analyzers).
  • Instrumentation (e.g. gas analyzer).
  • Aerospace & Defense (e.g. UAVs).

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