The Difference Between Servo and Stepper Motors

Servo and stepper motors have similar construction and share the same fundamental operating principle. Both motor types incorporate a rotor with permanent magnets and a stator with coiled windings, and both are operated by energizing, or applying a dc voltage to, the stator windings, which causes the rotor to move. However, this is where the similarities between servo and stepper motors end. The following will compare the differences between Stepper and Servo Motors, and when each technology is most appropriate for use in specific applications.

Closed-Loop vs. Open-Loop:

Stepper Motors are generally operated under open-loop control. Commands determine the specified movement of the Stepper Motor. In rare instances, Stepper Motors can stall or lose steps, due to resonance issues or unexpected force. While it is a rare occurrence, the possibility is a drawback for Stepper Motor technology. Stepper Motors can operate in a closed-loop configuration. However, this results in a costly system design.



A servo motor, on the other hand, runs on a closed-loop control. Although the servo also receives a command signal from its controller, just like a stepper does, the key difference is the servo motor has an onboard encoder that continuously communicates back to the controller. As it communicates, the servo motor is updating its position, communicating back its progress, and ultimately verifying that the final target position has been achieved.

A closed-loop stepper motor system, such as the HBS86 stepper motor drivers, may be the best option when the application requires improved energy efficiency and smoothness of operation, especially at high loads.

The HBS series offers an alternative for applications requiring high performance and high reliability when the servo was the only choice, while it remains cost-effective. The system includes a 2-phase stepper motor combined with a fully digital, high performance drive and an internal encoder which is used to close the position, velocity and current loops in real time, just like servo systems.

Feature

30-80V, 8.2A Peak, No Tuning, Nulls loss of Synchronization
Closed-loop, eliminates loss of synchronization
Broader operating range – higher torque and higher speed
Reduced motor heating and more efficient
Smooth motion and super-low motor noise
Do not need a high torque margin
No Tuning and always stable
High torque at starting and low speed, high stiffness at standstill
Lower cost

Speed and Power:

At high speeds, Stepper Motors typically have poor torque characteristics. Through microstepping, torque can be improved. However, unless Stepper Motors are used in closed-loop mode, they do not perform as well as Servo Motors.

Comparing similar sizes, Servo Motors can generate speeds and power anywhere between two and four times the speed of a Stepper Motor. Servo Motors operate under constant position  feedback (closed-loop), allowing for higher speed and greater reliability. Servo Motors perform under a closed-loop system, allowing the Servo Motor to attain higher peak torque capabilities.

Required Maintenance and Reliability:

Stepper Motors are brushless so they are not prone to wear and require no maintenance.

Servo Motors are available in brush-type or brushless options. Similar to steppers, brushless Servo Motors do not require maintenance. However, brush-type Servo Motors generally require a change of brushes every 5,000 hours. 

Accuracy and Resolution:

Stepper Motors generally produce 200 full steps, 400 half steps, and up to 25,000 microsteps per revolution. The specified location is not always achieved, due to the Stepper Motor’s open-loop nature, especially when operating under a load. To attain a smooth motion,microstepping is often used; however, it often results in less positional accuracy.

Servo Motor resolution is dependent upon the type of encoder used. Most encoders produce between 2,000 and 4,000 pulses per revolution, while some can produce up to 10,000 pulses  per revolution. nema 23 stepper motors can maintain positional accuracy due to their closed-loop operation.