Torque and Speed Considerations Of Stepper Motor

Stepper Motor or Servo Motor; Which Should it be?Each technology has its niche, and since the selection of either of these technologies for a given application affects its chance of success, it is important for the machine designer to select the best motor-drive system for the application, while considering the technical advantages and disadvantages of both. It is all about utilizing these technologies with balance to achieve the desired process performance for a given machine design while balancing cost versus the capability of the required mechanism.

Machine designers shouldn’t limit utilization of steppers or servos by a predetermined mindset or comfort level, but learn where each technology works best for controlling a specific mechanism and process to be performed.

The issue of whether to use a stepper or servo is dictated by the application in most cases. Steppers are typically sized for twice their continuous requirements for additional acceleration and/or deceleration torque capability or for a required peak torque.

In contrast, servo motors are generally sized for the specific application velocities and torques for maximum intermittent acceleration/deceleration, holding torque (nema23 stepper motor), and continuous RMS requirements over the complete motion profile.

In general, if an application requires high-throughput, high-speed, and high-bandwidth capability for disturbance correction, and/or high-rpm, with or without tight coordination between axes, servo motors are the best option. If point-to-point position performance and rpm requirements are modest, (as a function of the process loads and expected disturbances) steppers may be a better choice. Additionally, when the loads are within reason, a stepper’s ability to hold position (using holding torque with power and detent torque without power) can be an advantage.

Speed-torque curves highlight the difference between stepper and AC permanent magnet (PM) servo motors of equal volume (Figure A). Steppers typically generate higher continuous torque at lower speeds than servo motors. However, servo motors produce intermittent peak torques in this same low-speed range and produce peak and continuous torques over a much wider-higher speed range.

Automated machine axes for adjustment and setup as well as video axes for inspection, are applications that stepper systems fulfill well. Steppers are especially ideal for these type axes because they tend to be easier to design into control systems and less expensive on initial setup. When an axis for a given setup can be locked into place they are less expensive to operate (e.g. optional ON/OFF reduced power mode). Additionally, when properly applied, steppers are less prone to failure because of their simplistic open-loop control, which only requires winding-to-drive match-up versus motor-drive-to-mechanism tuning required with feedback circuits of a closed loop stepper system.

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.

Assuming the desired process can be accomplished with either a stepper or servo motor solution, with the repeatability, accuracy and flexibility requirements, for present and future needs, the remaining considerations would likely be environment, life expectancy, operating noise, and/or energy utilization.

When specific requirements allow for either technology, one should utilize discernment and foresight, with the specific knowledge of the considered working process or job to be performed, possible future needs, and the designer’s machine experience.

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