Why ST robots have stepping motors.
Our unique technology using micro-stepped motors, encoders, DSP control and polyurethane drives is how we can make very low cost robots with real industrial performance and durability.
StrengthsFor equivalent performance stepping motors are cheaper
Stepping motors are brushless motors with much longer lifetimes.
Being digital motors they can be positioned accurately without hunting or overshoot. The software takes care of mechanical characteristics.
Drive modules are not linear which means less heat, higher efficiency, greater reliability.
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 failure the motors lock solid and will not run. When a servo drive fails the motor can still run, possibly at full speed.
All ST robots have watchdog encoders. Encoder counts are compared to software motor counts and in the event of an error the system will halt. Thus the integrity of the system is much higher.
The above 3 features mean ST robots meet category 3 safety in which no fault of any kind can compromise safety.
Speed control is accurate and repeatable (crystal controlled).
Stepping motors will run extremely slowly if required.
Stepping motors are low speed high torque devices so transmissions are shorter which means higher reliability, greater efficiency, less backlash and lower cost. It is this characteristic that makes steppers ideal for robotics since most robot motions are short distance requiring high accelerations to achieve short cycle times.
Criticisms and our answers
Power-to-weight ratio is lower than DC motors.
Our answer: Most robot motions are not long range high speed (and therefore high power) but typically comprise short distances stopping and starting. With high torque at low speeds they are ideal for robotics. (However when high speed is truly required the R17HS model has high power servos.)
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.
Our answer: Robots are positional devices, designed to move into precise positions without error. In the event of a stall or collision the watchdog encoders report the error and stop the robot. The advantage is that once stalled there is no attempt at further motion which again increases safety.
At certain low speeds stepping motors can resonate resulting in loss of synchronism and stalling.
Our answer: Each stepping motor drive is individually microprocessor controlled and drives the motors pseudo-sinusoidally known as micro-stepping. Micro-code watches for resonance and automatically shifts the phase and current to compensate.
Stepping motors make more noise.
Our answer: Silent drives run the motors in micro-stepping mode throughout the speed range as can be seen in the videos.
Continuous path is notoriously difficult with stepping motors.
Our answer: DSP hardware and software shares time between motors and simultaneously 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.