Technical data – Stepper Motors

Technical Data – Stepper Motors

Stepper motors operate differently from normal DC motors, which rotate when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a micro controller. To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth. When the gear’s teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a “step,” with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.

Stepper motors are constant power devices. As motor speed increases, torque decreases. The torque curve may be extended by using current limiting drivers and increasing the driving voltage.

Steppers exhibit more vibration than other motor types, as the discrete step tends to snap the rotor from one position to another. This vibration can become very bad at some speeds and can cause the motor to lose torque. The effect can be mitigated by accelerating quickly through the problem speed range, physically damping the system, or using a micro-stepping driver (such as how Delta Tau handles the control). Motors with a greater number of phases also exhibit smoother operation than those with fewer phases (there are 2, 3 and 5-phase variants, albeit 5-phase tyes made by Oriental Motor tend to have very specific uses and peculiar commutation requirements).

Steppers are generally commutated open loop, ie. the driver has no feedback on where the rotor actually is. Stepper motor systems must thus generally be over engineered, especially if the load inertia is high, or there is widely varying load, so that there is no possibility that the motor will lose steps. This has often caused the system designer to consider the trade-offs between a closely sized but more expensive servomechanism and an oversized but relatively cheap stepper.

Some stepper control incorporates a rotor position feedback (such as an encoder), so that the commutation can be optimised for torque generation according to actual rotor position. This effectively converts the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution. An advance on this technique is to normally run the motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large — this will allow the system to avoid hunting or oscillating, a common servo problem.

There are three generic types of stepper motors:

  • Permanent Magnet Stepper
  • Hybrid Synchronous Stepper
  • Variable Reluctance Stepper

Delta Tau’s motion controllers, such as the PMAC range, can control 2-phase and 3-phase stepper motors alongside other servo motor technologies. This can be useful in a variety of circumstances where a mixture of servo and stepper axes occur within a single machine.