Servo Vs. Stepper Motors: Which Is Best for Your CNC Application?

Industry Outlook Team | Wednesday, 24 November 2021

The perfect solution to any engineering deterrent is an absolute myth. A solution considered ideal for a specific case might be redundant for the other. This truism holds particularly for servo motors and stepper motors. While neither of them is the perfect solution for all situations, they both are implemented extensively in the industry.

Both stepper motor and servo motor are respectively effective and offer reliable power for an eminently successful system. The decision tree to choose between the two comprises several branches, including speed, acceleration, price target, and more. For instance, if you operate a small-scale business, a small milling machine might be preferable over a regular CNC machine.

Stepper Motor and Servo Motor: Definitions and Overview

Stepper Motor
A stepper motor is typically regarded as a type of brushless DC motor. Stepper motors provide a defined number of steps per revolution. For instance, a stepper motor with 200 steps/revolution will provide a position resolution of 1.8°/pulse (360°/200 pulses).

Every time the motor is pulsed by the stepper controller, it moves forward or in the reverse direction, depending on the number of received pulses. In the above example, 100 pulses would lead to the motor rotating 180°.

Servo Motor

Servo motors are controlled via feedback and defined in the tech jargon as an AC permanent magnet synchronous motor. The stator in these motors is similar to an induction motor; however, their rotors are manufactured using rare earth magnets.

This unique manufacturing approach removes the slip that is common in induction motors, allowing the servo motor to operate at a synchronous speed based upon the frequency and number of poles.

Stepper Motor and Servo Motor: Pros and Cons

Stepper Motor
Pros
● Stepper motors are stable and can drive a variety of frictional and inertial loads.
● Stepper motors require no feedback, implying that they also act as the position transducer.
● Stepper motors are relatively inexpensive relative to other motion control systems.
● Stepper motors are safe, considering that if a malfunction occurs, they come to a halt.
● Stepper motors have a long life, with bearings being the only wear-out mechanism.
● Stepper motors offer standardized frame size and performance.
● Stepper motors feature exceptional low-speed torque and can drive several loads without gearing.
● Stepper motors are overload safe and cannot be impaired by mechanical overloading.
Cons
● Stepper motors have low efficiency as they draw considerable power irrespective of the load.
● Torque falls rapidly with increasing speed.
● Stepper motors are less accurate with a yield of 1:200 at full load and 1:2000 at light loads.
● Stepper motors are more susceptible to resonances and thus require micro-stepping to operate smoothly.
● Stepper motors lack feedback to reveal missed steps.
● Stepper motors have a low torque to inertia ratio and cannot accelerate loads rapidly.
● A stepper motor will not "pick up" after momentary overload.
● Stepper motors offer minimal output power for size and weight.

Servo Motor
Pros
● Servo motors are constructed using rare earth permanent magnets, which offer top-notch torque density.
● Servo motors are the sought choice for very dynamic motor applications that require rapid acceleration and deceleration.
● Servo motors incorporate some form of feedback, be it a resolver or encoder, which is crucial to control rotor position and torque.
● Servo motors can provide momentary peak torque across their speed range.
● The peak torque offered by servo motors delivers an optimized motor selection.
● Servo motors operate at substantially greater speeds relative to stepper motors.
● Servo motors are ideal in situations that involve critical motion control applications, functional safety, and the handling of valuable products.
● Servo motors come equipped with absolute encoders, which help retain their position information despite the system getting powered down.
Cons
● Servo motors rotate as per the command pulses but often lag, which is not ideal for precision rotation control.
● Servo motors are expensive since they offer high torque overload.
● On stopping the motor, its rotor oscillates back and forth one pulse, which induces vibration.
● Servo motors are extremely power-hungry and consume substantial energy regardless of the operation.

Servo and Stepper Motors: A Detailed Juxtaposition
Here is a detailed tabular representation of the contrasting differences between servo and stepper motors:

Characteristics	Servo Motor	Stepper Motor
Affordability	Servo motors cost higher than a stepper motor system, besides offering an equal power rating.	Steppers are more economical than servo motors with similar power ratings.
Frame Sizes	Servo motors come in several frame sizes, ranging from small to large, with each capable of operating massive machines.	Stepper motors do not offer diversity in size selections but might be found in several NEMA frame sizes.
Reliability	Servo motors are typically reliable; however, it depends on the environment and the preservation of the motor.	Stepper motors are slightly more reliable because they do not require an encoder, which is more susceptible to failure.
Setup Complexity	Servo motors involve tuning the (PID) closed-loop variable circuit to receive the appropriate motor function.	Stepper motors are plug-and-play and require only wiring of the motor wires to the stepper motor driver.
Overload Safety	Servo motors are more susceptible to malfunction if overloaded mechanically.	Stepper motors are less likely to get damaged by mechanical overload.
Motor Life	The brushes on servo motors demand replacement every 2000 hours of operation. Even the encoders need timely replacements.	The bearing on stepper motors is the only wearing part, thereby giving the stepper motor an upper edge in terms of longevity.

Final Words

The stepper vs. servo motor might be a never-ending debate for CNC manufacturers; however, you can make the right choice as per your requirements. While servo motors deliver higher precision, stepper motors are relatively affordable and have a longer life.

If precision and high torque are not on your list of requirements, then consider the affordable choice, which is the stepper motor. However, if budget is not an issue and you can bear the constant replacement demands of a servo motor, then it might be the better option for you.

About the Author:
Peter Jacobs

Peter Jacobs is the Senior Director of Marketing at CNC Masters. He is actively involved in manufacturing processes and regularly contributes his insights for various blogs in CNC machining, 3D printing, rapid tooling, injection molding, metal casting, and manufacturing in general.