The great debate: Stepper or Servo?

Many engineers may stumble upon this question when specifying a motor for an application; should I use a Stepper or a Servo motor?

Both stepper and servo motor are excellent but, what’s really important is understanding their capabilities.  The simplest way to understand the difference is to consider speed. Servo motors  are better for high speed applications while stepper motor are better for low speed applications. Let’s take a look at a simple example of belt drive application.

beltdriveforservoarticle

MSA Belt Drive Actuator

From the example above, let’s assume below are our requirements:
Travel requirement: 2500mm/sec
Stroke: 2 meter
Load: 1 Kg

Motor Sizing Example Results: (Sizing result is based on pulley diameter on the belt drive system)

Before you can select a stepper or a servo system, you’ll first need to know what the motor requirements are for your application. You’ll need to calculate inertia, required torque, and required speed. You will also need to add safety factors for the required torque calculation.

As an example, let’s say the required speed for this application is 2871 r/min, and the required torque is 3.117 lb-in, or 48.87 oz-in. Based on these needs, let’s compare a Nema 23 size servo and stepper motor side by side.

Stepper_Server_Comparison

As you can see from the speed toque curve above, Stepping motor would have a higher starting torque at low speed, but as speed increases, the torque curve falls.  Comparing it with the servo motor torque curve, the servo motor might not have a high starting torque, but a servo system maintain its torque quite well at higher speed, which is why for a high speed positioning application a servo system would be preferred.

Correct Servo Stepper ImageCorrect Servo Stepper Image2

By plotting on the charts where our application falls on the torque curve, we can highlight the difference in system performance for our high speed application. This identifies that the best choice for our application would be the Servo motor.  Using a stepper would leave no safety factor for the application as a servo system would.  As listed above our torque requirement is 48.87 oz-in.

If you have any additional questions regarding stepper or servo motors, the MISUMI Engineering Team is always ready to help and available at engineering@misumiusa.com.

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  • jbay88

    It looks like you’ve compared the 48V servo torque-speed curve with the 12V stepper torque-speed curve. At 48V and 2800 rpm, the stepper puts out nearly 50 oz-in. It’s certainly still less than the servo at that speed, but a significant improvement relative to the 12V curve that you’ve highlighted.

    • Kimmah Shah

      You’re right! Thanks for pointing that out for us! Our engineer Vick mistakenly used the 12VDC instead of 48VDC. The correct information is now noted in the article. I apologize for the inaccuracies. The advantage of using a servo over a stepping motor would be that a servo motor would be able to maintain torque at higher speed, hence suitable for applications that requires rapid long distance positioning.

    • misumiusa

      You’re right! Thanks for pointing that out for us! Our engineer Vick mistakenly used the 12VDC instead of 48VDC. The correct information is now noted in the article. I apologize for the inaccuracies. The advantage of using a servo over a stepping motor would be that a servo motor would be able to maintain torque at higher speed, hence suitable for applications that requires rapid long distance positioning.

  • Jean-David Godin

    The servo motor is a double torque version which is about twice the price than a regular servo motor. Taking this into consideration plus a 48V stepper, the best choice shouldn’t be the stepper ??

    • misumiusa

      With regards to your response, stepping motor part number STM23IP-3EE compared to Servo motor SMT23375DT are very similar in size. Please see table below to see specifications side by side.

      From the table above, we can see that it may look like the stepper motor would have a superior performance compared to the servo. However, when we look at the high speed toque curve, a Servo system can maintain a fairly constant flat torque curve compared to a stepper. (Please refer to stepping motor (48VDC) VS Servo Torque curve above).

      STM24IP-3EE
      Continuous Torque – 200 oz in
      Peak Torque – 255 oz in
      Encoder Resolution – 4000
      Step Size – up to 50,000/rev
      Closed Loop – Yes (Stall prevent)
      Mounting – Nema 23 *(8mm shaft)
      Motor Length – 125.5 mm

      SMT23375DT
      Continuous Torque – 83 oz in
      Peak Torque – 157 oz in
      Encoder Resolution – 4000
      Step Size – 4000 (same as encoder count)
      Closed Loop – Yes
      Mounting – Nema 23
      Motor Length – 115.3mm

      Which one is better??

      To answer this question, BOTH systems are good depending on your application. If your application required low speed and high torque, stepper would be a good candidate. (Cost effective)
      If your application requires high speed and high torque, servo would be your choice of system. TM24IP

  • Adisharr

    I wouldn’t have necessarily used the Animatics Smartmotor as a good comparison because it has quite a bit more overall capability than the stepper drive shown.

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