Why ST robots have stepping motors.

Please see this excellent article

Strengths
  • For equivalent performance stepping motors are cheaper
  • Stepping motors are brushless motors so have longer lifetimes.
  • Being digital motors they can be positioned accurately without hunting or overshoot.
  • Drive modules are not linear amplifiers which means less heat sinks, higher efficiency, greater reliability.
  • Drive modules are less expensive than linear amplifiers.
  • No expensive servo control electronics are required since signals originate directly from the MPU.
  • Software is fail-safe. The MPU issues stepping pulses. If the software fails to work or crashes the motors stop.
  • Electronic drives are fail-safe. In case of drive amplifier failure the motors lock solid and will not run. When a servo drive fails the motor can still run, possibly at full speed.
  • Speed control is accurate and repeatable (crystal controlled).
  • Stepping motors will run extremely slowly if required.
  • All ST robots have encoder feedback which is compared to software motor counts. In the event of any error which cannot be corrected the system will halt. Thus the integrity of the system is much higher.
  • Stepping motors are low speed high torque devices so transmissions are shorter which means higher reliability, greater efficiency, less backlash and lower cost.

    Weaknesses and supposed weaknesses
  • Power-to-weight ratio is lower than DC motors.
  • Stepping motors are positional devices so cannot work with errors, For example will not slow down under excessive load but will stall. They cannot be used to exert a force independent of position.
  • At certain low speeds stepping motors can resonate resulting in loss of synchronism and stalling.
    how we overcome this
    Each stepping motor drive is individually microprocessor controlled and drives the motors pseudo-sinusoidally known as micro-stepping.
  • Stepping motors make more noise.
    how we overcome this
    Silent drives run the motors in micro-stepping mode throughout the speed range.
  • Continuous path is notoriously difficult with stepping motors.
    how we overcome this
    DSP hardware and software shares time between motors and simultaneoulsy ramps individual motors up or down as necessary. The system has the advantage of zero overshoot on corners - if the required change of direction cannot be achieved it is reported beforehand.
  • Old technology.
    Some critics have said that stepping motors are outdated. Far from it; they are a relatively recent invention which has had the benefit of considerable development and with fast digital processing their time has come. As a digital motor it is ideally suited to microprocessor control, with the benefit of accurate conformity to programmed speeds and speed/distance profiles. By contrast DC servomotors can be crude, approximate and even unstable.