Oyostepper

Accuracy and Resolution of Stepper Motor

Stepper motor systems have a difference between their theoretical and actual resolution. For example, a two-phase, full stepping, 1.8° step-angle motor may have 200 possible positions in one revolution (360°/1.8°), but whether or not it’s achieved depends on how the motor was sized for the application. The same is true of half stepping and microstepping motor drive modes. A 1.8° microstepper, though specified as having ten microsteps per each full step, cannot necessarily find any position within 0.18°.

Additionally, several commanded microsteps may be required before there is enough torque build-up to overcome friction and load inertia. In a real-world situation, the motor could easily jump one or more microsteps beyond the number commanded and stabilize there. When positioning-resolution requirements need to exceed 200 steps per revolution, steppers may utilize a feedback encoder to achieve upwards of 1000 steps/rev. Five-phase motors and microstepping motors (with caution) can also improve on the steps/rev.

Servo motor resolution is theoretically infinite, but in closed-loop stepper motor operation, system positioning depends primarily on the resolution of the feedback device be it a sine encoder, resolver or a digital (TTL) type encoder. Today’s high resolution feedback devices can approach between 221 [2,097,152] to 228 [268,435,456], counts per motor revolution, plus the optional multi-turn capability (typically up to 4096 turns). Multi-turn capable feedback devices are available for an axis’ absolute position on machine power-up eliminating the initial axis power-up homing cycle.

Repeatability
Servo motors are extremely repeatable because they run closed loop. But steppers can be just as repeatable in many applications, especially when running in one direction. However, when an Idle Current Reduction (ICR) mode is utilized and/or the load increases (e.g. as during direction reversal) and exceeds the capability of the stepper the situation changes. Similar to how a gearbox must take up backlash, the stepper must catch up to system command. During the first move in a new direction, motor accuracy is affected, because the stepper is overcoming inertia and friction (effects of the load). Once that happens, the system regains its specified repeatability, but it may have lost or gained actual position steps over those commanded.

Input Power
A linear step motor is equivalent to an inductor in series with a resistance and as a result, the current that produces torque requires time to rise. This time, limits the speed for a given voltage, so increasing the motor’s speed in a given application, may require higher voltages.

A servo system works similarly, but working within its capability envelope, the drive’s control loops will present the required voltage and current to the servo motor to meet the demand of the load relative to its command and feedback error. In contrast, when a servo motor system is forced to work (for whatever reason) outside its operational envelope, even for a millisecond, it is no longer under control and thus, not operating as a servo.

Conclusion

Both technologies are a clear choice in today’s mechatronic machine designs. However, once the advantages and disadvantages of servo and stepper motor systems are clearly understood, especially relative to the process or work to be performed, the best selection for a given application becomes much clearer.

http://mihuos23.blogcu.com/the-advantages-of-stepper-motor-systems/35313471

http://obd2ware.soup.io/post/677298698/Power-Off-Brakes-for-NEMA-17-and