Servo or Step Motor?

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om_tech_support_JT
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Servo or Step Motor?

Postby om_tech_support_JT » Fri Aug 17, 2012 3:28 pm

Q: I've been using servo motors for years for various positioning applications such as belt conveyors and ball screw lifts. I have a request from a customer to redesign, simplify, and reduce the cost of a machine, so I am searching for alternatives. I don't have much knowledge on the other types of motors in the market since most of my experience are with servo motors. Can you tell me if there's an advantage to using other types of motors, such as steppers?

A: The ideal motor choice ultimately depends on your application requirements. Generally, servo motors are better for very fast and long moves while a step motor is better for quick and short moves. For applications where positioning accuracy is critical, step motors may be a better choice because it takes less time to settle on its target position while servo motors need to "hunt" back and forth until it settles on its target position. The toothed rotor from the step motor allows the operator to command it to go to a specific target position. The servo motor is like a 3-phase AC motor with an encoder. It doesn't have a toothed rotor, so it has to "hunt" until the positioning error is minimized. Because a servo motor has a built-in encoder, the servo motor controller will also need to be more complex in order to interpret and convert the encoder signals. For this reason, a typical step motor system has much less components than a step motor system. If a step motor is sized right to the application requirements, even without an encoder to close the loop, the step motor would not miss steps. We also offer closed-loop step motor systems if you want the peace of mind of verifying if a move has completed or not.

Servo vs stepper system configuration, components.jpg
Servo vs stepper system configuration, components.jpg (124.75 KiB) Viewed 1949 times

Even if you install encoders to make a closed-loop step motor system, it will still be simpler than a servo motor system. See below.

Servo vs stepper closed-loop components.jpg
Servo vs stepper closed-loop components.jpg (79.9 KiB) Viewed 1949 times

In comparison with set up time, the typical servo motor system would take much longer than step motors since there's typically a PID loop that requires tuning. This tuning process can be quite complex and time-consuming. If you decide to change the load after you have tuned the servo motor system, you will have to tune it again to get the best performance out of the motor. You do not have to do the same with a step motor system. Our NX series servo motor systems offers an auto-tuning feature which shortens the set up time immensely.

From the performance side, the more compliant your mechanical set up is, the worse the servo motor will perform. I will show a few graphs below that depict this performance difference between servo motors and step motors in both compliant (belt & pulley), and rigid (ball screw) systems. I have decided to use one of our closed-loop Alphastep series motor systems to do a fair comparison with a servo motor system. Our Alphastep series as well as our AR series utilize a proprietary Keep-in-Step technology to continuously correct its position on the fly and keep synchronization with the input pulses.

Screw, Rigid, Short Move Comparison.jpg
Screw, Rigid, Short Move Comparison.jpg (92.54 KiB) Viewed 1949 times

The above shows a comparison of the Alphastep with 4 servo systems on a screw application. All five systems were timed for different move lengths and their values plotted. Because of the lack of gain settings, plus the fact that the motor has a high torque/low rotor inertia, the Alphastep can outperform servos for short moves.

Screw, Rigid, Long Move Comparison.jpg
Screw, Rigid, Long Move Comparison.jpg (94.57 KiB) Viewed 1949 times

The above shows that longer moves prove more difficult for the Alphastep. Because of the high pole count, the torque of the Alphastep drops off quicker as speed increases. Servo systems are good for applications that require a very high speed.

Belt, Compliant, Short Move Comparison.jpg
Belt, Compliant, Short Move Comparison.jpg (89.53 KiB) Viewed 1949 times

Highly compliant systems are a problem for servos. Because of the compliance, the ringing or overshooting has to be tuned out, and that results in delays. The Alphastep is great for highly compliant systems as evidenced by this short move.

Belt, Compliant, Long Move Comparison.jpg
Belt, Compliant, Long Move Comparison.jpg (90.72 KiB) Viewed 1949 times

Even in long moves the alpha step can outperform servo systems in a compliant system. This is due to the fact that the gain settings have to be set low and that results in a long delay time.

If you do not require as much positioning accuracy, or if you actually use servo motor systems for speed control applications, we can offer our highly efficient, continuous duty brushless motor systems. Our brushless motor systems all include a feedback device on the motor to close the loop. Our BX series offers encoder feedback as well as both speed control and positioning control capabilities. The rest of our brushless motor systems utilize less expensive hall-effect sensors to maintain accurate speed regulation, and they can all stop instantaneously to provide crude positioning if needed. Holding torque can be provided by a brushless motor with an electromagnetic brake.

For an overview of all of our step motor options, please click here:
http://www.orientalmotor.com/products/a ... index.html

For an overview of our tuning-free servo motor systems, please click here:
http://www.orientalmotor.com/products/s ... index.html

For an overview of all of our brushless motor systems, please click here:
http://www.orientalmotor.com/products/b ... index.html

If you have any questions, please contact our knowledgeable technical support group.

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