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.

The Advantages and Disvantages of Stepper Motors

A stepper motor is essentially a servo motor that uses a different way of motorisation. Where a servo motor uses a continuous rotation DC motor and integrated controller circuit, stepper motors utilise multiple toothed electromagnets arranged around a central gear to define position.

Stepper motors consists of two varieties; unipolar or bipolar. Bipolar motors are the strongest type of stepper motor and usually have four or eight leads. They have two sets of electromagnetic coils internally, and stepping is achieved by changing the direction of current within those coils. Unipolar motors, identifiable by having 5,6 or even 8 wires, also have two coils, but every one has a centre tap. Unipolar motors can step without having to reverse the direction of current in the coils, making the electronics simpler. However, because the centre tap is used to energise only half of each coil at a time they typically have less torque than bipolar.

Stepper Motors Design

Step motors are called “digital motors” because they move in steps, like the hands on a clock.  When the first coil is energized, the rotor teeth align with the teeth in the first stator winding and hold position. When the second winding is energized, the teeth in the rotor move slightly and align with the second stator winding and hold position.  The total movement in this example is one full step.

There are usually 200 steps per revolution, each step being 1.8°.The step motor was made possible by the development of electronic step   motor controllers. Electronics are required to energize the windings   with proper voltage and current, with the proper phase, in the right sequence, at the right time.  Controllers have evolved to be able to move step motors in as many as 20,000 steps per revolution, providing 100 times finer movement (0.018° per step).



Stepper Advantages:

Stepper motors offer several advantages over servo motors beyond the larger number of poles and easier drive control. The design of the Step motor driver offers a constant holding torque without the need for the motor to be powered.

The torque of a stepper motor at low speeds is greater than a servo motor of the same size. One of the biggest advantages of stepper motors is their relatively inexpensive and availability.

Stepper Limitations:

• Low Efficiency – Unlike DC motors, stepper motor current consumption is independent of load. They draw the most current when they are doing no work at all. Because of this, they tend to run hot.
• Limited High Speed Torque - In general, stepper motors have less torque at high speeds than at low speeds. Some steppers are optimized for better high-speed performance, but they need to be paired with an appropriate driver to achieve that performance.
• No Feedback – Unlike servo motors, most steppers do not have integral feedback for position. Although great precision can be achieved running ‘open loop’. Limit switches or ‘home’ detectors are typically required for safety and/or to establish a reference position.

Step motors can lose sync; that is, lose synchronization with the step pulses from the controller. In other words, step pulses from the controller are converted into power to the windings of the motor, but the motor does not rotate.  This will happen when the torque required to move the load exceeds the torque capability of the motor at the desired speed.

Fasttobuy supply both hybrid Stepper Motor and brushless AC servo motors and drives for machine automation. We do not supply DC brushed servomotors and drives which are older technology. Both steppers and brushless servomotors are similar in construction but the servomotors have feedback devices which enable closed loop operation. There are a lot of varying opinions about the pros and cons of either style of motor, so this can help you decide which to use. It isn't as easy as one being much better than the other. Overall machine functionality is highly determined by the controller and software and is more important than just comparing one style of motor with the other.