Engineering Notes by Oriental Motor

The Choice Between Servo Motors and Stepper Motors

[fa icon="calendar"] Nov 30, 2021 11:45:00 PM / by Johann Tang

Johann Tang

The best motor is the one that meets the application requirements.  However, when both servo motors and stepper motors satisfy the basic requirements of a positioning application, it's important to have a deeper understanding of the two technologies to make an informed decision.

In this blog post,  we will summarize the differences between servo motor systems and stepper motor systems, then show how advances in stepper motor technology are closing the gap.

Let's start with a quick comparison of the two motor technologies.

Stepper Motors

Servo Motors

2-phase bipolar stepper motors2-Phase Bipolar Stepper Motor Drivers

NX Series Tuning Free Servo MotorsAC Input Drivers

Pros

Pros

  • Good low speed torque
  • No tuning, no hunting
  • Compact motor size
  • Cost-effective
  • Good high speed torque
  • Peak torque available
  • Continuous Duty
  • Packed with functions
Cons Cons
  • Less high speed torque
  • Limited duty cycle
  • No peak torque
  • Typically runs in open-loop
  • Requires Tuning
  • Only works with feedback
  • Hunting
  • Cost

 

Although servo motors and stepper motors both use permanent magnets in the rotor and electromagnets in the stator to generate motion, and they both require drive circuits, there are fundamental differences in their design.  The next section will explain how these fundamental differences affect their performance.

 

Motor Design

A stepper motor can be commanded to move to a position, stop, then hold, while a servo motor needs to "hunt" for the target position with encoder feedback and use "servo lock" to generate holding torque.  This difference in performance comes from the existence or absence of an encoder and the number of poles. 

The poles discussed here are "magnetic poles" and can be defined as either north or south magnetic poles on the rotor.  These poles provide specific stable stopping points where the magnetic flux from the stator would interact with the rotor.  The number of poles also determines the number of times the motor windings have to be advanced in a full revolution.

To understand the difference in the number of poles between stepper motors and servo motors, we'll need to look deeper into each motor design.  The left image below shows the construction of a stepper motor, and the right image below shows a cross-sectional diagram of the rotor and stator.

 

Stepper Motor

Stepper motor design Stepper motor: phase excitation and teeth arrangement

 

A stepper motor design uses an axially magnetized rare earth permanent magnet that is sandwiched between two teethed rotor cups.  By axially magnetizing the permanent magnet, the teeth of the two rotor cups become magnetic poles of the opposite polarity.  A rotor cup can have either 50 or 100 teeth, and the two rotor cups are skewed at half a tooth pitch.  The teeth from both rotors are shown in blue and red in the right image above (if you were looking from the shaft side).  For a stepper motor, every tooth on both rotors becomes a pole.  

 

Servo Motor

 

Servo Motor Structure

 

A servo motor design (shown above) uses a radially magnetized rotor instead of teeth (shown below), which is the main reason servo motors have significantly fewer poles.  The low number of poles also requires the use of encoder feedback to minimize error, which increases the motor body length.

 

A servo motor design typically uses a rotor with 2~8 poles and a 3-phase stator (U, V, W).  Its rotor is radially magnetized with segmented permanent magnets instead of axially magnetized like a stepper motor. 

An example of a 4 pole rotor with a 6 pole stator design is shown on the right. 

As you can see, a servo motor offers significantly fewer poles than a stepper motor.

Servo motor rotor and stator

 

A servo motor requires more footprint due to the requirement of an absolute encoder.  The fewer number of poles gives it the ability to generate high speed torque.  A stepper motor can run in open-loop without an encoder, therefore it's more compact.  A servo motor relies on its encoder for stop accuracy while a stepper motor relies on the number of poles for stop accuracy.  We will explain more.

 

Stop Accuracy

For positioning applications, one of the primary requirements we have to satisfy is the motor's stop accuracy.  Both a stepper motor and a servo motor can stop accurately. 

A stepper motor's stop accuracy depends on the manufacturing quality of the windings (electrical) and teeth construction (mechanical), while a servo motor's accuracy depends on the assembly accuracy, encoder resolution, and algorithm.  

Remember that there is a very thin air gap between the rotor and stator, and the only friction is from its ball bearings.  Friction torque or gravitational load can alter the actual stop position, so there's a tiny bit of error as you move from position to position.  When we plot the errors that occur as the motors rotate 1 full revolution, they look like the graphs below.  

 

Servo Motor Stepper Motor
Servo motor stop accuracy Stepper motor stop accuracy

 

Notice that they both offer a stop accuracy of about +/-0.02°, which is under a stepper motor's typical repetitive stop accuracy specification of 3 arc minutes, or +/-0.05°.  While a servo motor can increase its stop accuracy by increasing its encoder resolution, a stepper motor offers better repeatability at 7.2° increments or almost perfect repeatability at 360° increments.  

 

The stop accuracy of a stepper motor is highly dependent on its winding characteristics, rotor construction accuracy, as well as the number of teeth/poles in its rotor.  The stop accuracy of a servo motor is dependent on assembly accuracy, encoder resolution, and operating algorithm.  In a way, you can say that a stepper motor is "mechanically designed" for positioning applications, and servo motors are "electrically designed" for positioning applications.

 
 
High Speed Performance

Servo motors are generally known for running higher speeds than stepper motors.  What this really means is that the servo motor will output more torque at a specified RPM than a stepper motor.  This difference in torque performance comes from the difference in pole count as well as winding inductance between servo motor and stepper motor design.

Remember that the number of poles also affects the number of times a motor winding needs to be advanced for a full motor revolution?  For a servo motor, it may take only 12 cycles to advance a motor a full revolution.  However, it takes 200 cycles for a 2-phase stepper motor.  At low speeds, this doesn't make a significant difference.  However, at high speeds, the driver will not be able to fully energize the windings.  Since current is proportional to torque, the speed torque curve drops at high speeds.

The performance between stepper motors and servo motors can be best shown by their speed-torque curves.  Here's an example comparing a NEMA 23 size stepper motor and a servo motor of similar size.

 

NEMA 23 Stepper Motor 200 W (1/4 HP) 60 mm Servo Motor
PKP266D42 stepper motor performance curve NX620 servo motor performance curve

 

The high starting torque from a stepper motor helps accelerate the load faster from a rest position, which is ideal for start/stop applications.  A servo motor's torque starts off lower in the continuous duty region, and the curve is flatter as it extends to high speeds.  A servo motor also offers a limited duty region, which provides maximum instantaneous torque that can be used for a short duration.

 

The high pole count of stepper motors allows them to generate high torque at low speed and run smoothly at low speed.  They can respond quickly, position accurately without an encoder, and easily generate holding torque.  However, due to high winding inductance, high pole count, and high L/R constants, the torque drops off in the high speed region.  The low pole count and low winding inductance of servo motors do not generate high starting torque but allow them to maintain its torque throughout its entire speed range.  Therefore servo motors can offer higher high speed torque.

 

Closed-Loop Feedback

By definition, a servo motor system has to operate with closed-loop control, and a stepper motor typically operates with open-loop control.  A servo motor uses feedback in order to control the motor's position, speed, or torque.  A stepper motor is commanded to move to a specific location in degrees without the need for feedback but could lose synchronism due to overload.

The addition of feedback to keep synchronism does complicate the driver design and increases the number of components.  In addition to a pulse generator, phase sequencer, and FET from a typical stepper motor system, a servo motor system also contains a rotor position counter, F/V converter, current amp, speed amp, position amp, and deviation counter.  All of these components are required to run the motor in a PID loop where the driver constantly computes errors and adjusts the proportional/integral/derivative gains for correction on the fly.  This is why servo motors are more expensive.  Servo motor systems are generally packed with functions like auto-tuning since closed-loop feedback enhances the motor's capabilities.  For example, the NX Series servo motor systems offer four operation modes: position control, speed control, torque control, and tension control.

 

Servo Motor System Stepper Motor System
Servo motor system Stepper Motor System - Open Loop

 

Along with closed-loop feedback, the motor's load-to-rotor-inertia-ratio also increases.   A stepper motor can handle about 10x its rotor inertia, while a closed-loop stepper motor can handle 30x its rotor inertia.

 

Stepper motors operate without the need for feedback, so they require fewer components to operate.  This is why they're more cost-effective.  Servo motors require feedback and operate in a PID loop, therefore more components are necessary.  Closed-loop systems can offer benefits that open-loop systems cannot.

 

Efficiency

Closed-loop feedback allows more efficient current control, which affects motor temperature and performance. 

In the graph below, we plot the temperature rise against the operating duty cycle [%] of a motor.  Notice how the temperature rises with operating duty.  This is important because the service life of a motor is determined by its bearing grease life, and the bearing grease life depends on temperature.

 

Stepper motors - operating duty vs temperature rise Grease life of motors

 

Most stepper motors use a current chopper driver technology, which provides a constant current supply regardless of load.  Current is proportional to temperature, so a stepper motor's duty cycle needs to be limited to about 50%.  A servo motor offers more efficient current control because it only uses the current it needs.  One thing that a stepper motor is better at is its ability to generate holding torque at zero speed.  Servo motors use more power to generate holding torque.

 

 

Efficient current control can also lead to other performance benefits, such as lower noise and vibration.  Make sure that your motor is properly sized.  Stepper motors tend to vibrate more when it's not properly sized.  Servo motors tend to "hunt" more if it's not properly sized or tuned.

 

Stepper motors typically use constant current chopper driving technology and operate on full current all the time.  Servo motors draw only the required current for the motion profile and the load.  Since the current is proportional to temperature, this results in higher duty cycles and longer service life for the servo motor.

The ability to only draw the required current is a  big advantage of servo motor systems that can also help increase service life, lower noise for certain applications, or lower power consumption.  For a stepper motor to offer the same kind of efficient current control, it would need more feedback, which requires closing the loop.  

 

Summary

To help with product selection, we have compiled a list of differences between servos and steppers.

Servo vs Stepper Motors

 

Final Thoughts...

Product selection is a constant balancing act between cost and performance.

Servo motors are definitely the powerhouse of electric motors.  If you have the budget, a servo motor system may present a one-for-all solution that may be excessive but will offer the best performance with some tuning.  For point-to-point applications that do not require feedback, a stepper motor system may be a way to simplify design and reduce cost.  

"Make good choices today so you don't have regrets tomorrow."

 

Is there an option between servo motors and stepper motors?

 

Topics: Stepper Motors, Absolute Positioning, Alphastep Hybrid Control, Servo Motors, Network, Service Life

Johann Tang

Written by Johann Tang

Johann Tang is a Product Specialist at Oriental Motor USA Corp. with over 15 years of knowledge and experience supporting applications of various types of electric motors, gearheads, actuators, drivers, and controllers.

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