Comparative Analysis of Stepper Motors and Servo Motors

In an increasing number of demanding industrial automation applications, technological advances are changing the performance-cost ratio between stepper motors and servo motors.

After adopting closed-loop technology, closed-loop stepper motors provide users with excellent precision and efficiency, achieving the performance of servo motors while having the low price advantage of stepper motors. Lower-cost stepper motors are gradually penetrating into application areas that were originally dominated by high-cost servo motors.

Comparison between stepper motors and servo motors

According to traditional thinking, servo control systems perform better in applications that require speeds over 800 RPM and high dynamic response. Stepper motors are more suitable for applications with lower speeds, low to medium acceleration, and high holding torque.

So what is the basis for this traditional view on stepper motors and servo motors? Let's analyze it in detail.

1

Structure

Stepper motors move in steps, using magnetic coils to gradually pull a magnet from one position to the next. To move the motor 100 positions in any direction, the circuit needs to step the motor 100 times. Stepper motors use pulses to achieve incremental movement, which can achieve precise positioning without the use of any feedback sensors.

A servo motor uses a different approach to motion. It uses a position sensor – an encoder – attached to a magnetic rotor to continuously detect the motor's exact position. The servo monitors the difference between the motor's actual position and the commanded position and adjusts the current accordingly. This closed-loop system keeps the motor moving in the correct direction.

2

Simplicity and cost

Stepper motors are not only cheaper than servo motors, but are also easier to commission and maintain. Stepper motors are stable at rest and can maintain position (even with dynamic loads). However, if some applications require higher performance, more expensive and complex servo motors must be used.

3

position

In applications where it is important to know the exact position of a machine at all times, there is an important difference between stepper motors and servo motors. In open-loop motion applications controlled by stepper motors, the control system assumes that the motor is always in the correct motion state.

However, if a problem occurs, such as a motor stalling due to a stuck component, the controller will not know the actual position of the machine, resulting in a loss of position. The closed-loop system of the servo motor itself has an advantage: if it is stuck by an object, it will be detected immediately. The machine will stop operating and will never lose position.

4

Speed ​​and torque

The performance difference between stepper motors and servo motors stems from their different motor designs. Stepper motors have many more poles than servo motors, so the number of winding current exchanges required for a full rotation of the stepper motor is many times greater, resulting in a rapid drop in torque as speed increases.

Additionally, if the maximum torque is reached, the stepper motor may lose its speed synchronization capability. For these reasons, servo motors are the preferred solution in most high-speed applications. In contrast, the higher number of poles in a stepper motor is advantageous at low speeds, where the stepper motor has a torque advantage over a comparably sized servo motor.

As the speed increases, the torque of the stepper motor decreases

5

Heat and energy consumption

Open-loop stepper motors use a fixed current and dissipate a lot of heat. Closed-loop control only provides the current required by the speed loop, thus avoiding the motor heating problem.

Comparison Summary

Servo control systems are best suited for high-speed applications involving dynamic load changes, such as robotic arms. Stepper control systems are better suited for applications that require low to medium acceleration and high holding torque, such as 3D printers, conveyors, and secondary axes. Because stepper motors are less expensive, they can reduce the cost of automation systems when used. Motion control systems that need to take advantage of the characteristics of servo motors must justify the higher cost of these motors.

Driven by advances in closed-loop technology, stepper motors are able to penetrate high-performance, high-speed applications that were previously the exclusive domain of servo motors.

Stepper motor with closed-loop technology

What if the advantages of closed-loop servo technology could be applied to stepper motors?

Can we achieve the cost advantages of stepper motors while achieving performance similar to that of servo motors?

By combining closed-loop control technology, stepper motors will become a low-cost integrated product that combines the advantages of servo and stepper motors. Because closed-loop stepper motors can significantly improve performance and energy efficiency, they can replace more expensive servo motors in more and more high-standard applications.

Below we take the stepIM integrated stepper motor with embedded closed-loop control function as an example to analyze the performance and advantages and disadvantages of stepper motors using closed-loop technology.

After the integrated electronic control, the stepper motor is equivalent to a two-phase brushless DC motor, which can perform position loop control, speed loop control, DQ control, and other algorithms. A single-turn absolute encoder is used to achieve closed-loop commutation, ensuring that the best torque can be achieved at any speed.

Low energy consumption and stays cool

StepIM stepper motors are energy efficient. Unlike open-loop stepper motors that always operate at full current, which can cause heating and noise issues, the current of stepIMs varies according to the actual motion, such as during acceleration and deceleration.

Similar to a servo, the current drawn by these stepper motors at any given moment is proportional to the actual torque required. Because the motor and integrated electronics run cooler, stepIM stepper motors can achieve higher peak torques similar to servo motors.

StepIM stepper motors require less current even at high speeds

Precisely matching performance requirements

To ensure that there is enough torque to overcome disturbances and avoid losing steps, open-loop stepper motors usually have to ensure that the torque is at least 40% higher than the value required by the application. Closed-loop stepIM stepper motors do not have this problem. When these stepper motors reach a stall state due to overload, they will continue to maintain the load state and will not lose torque. After the overload condition is eliminated, they will continue to run.

At any given speed, maximum torque is guaranteed, while position sensors ensure that there is no step loss. As a result, the size of the closed-loop stepper motor can be precisely matched to the torque requirements of the application, without the need for an additional 40% margin.

For open-loop stepper motors, it is difficult to meet high transient torque requirements due to the risk of losing steps. stepIM closed-loop stepper motors can achieve very fast acceleration, run quieter, and have less resonance than traditional stepper motors. They can operate at a higher bandwidth and achieve excellent performance.

Cabinet-less machine

stepIM integrates the drive control board with the motor, reducing the number of connections and simplifying the implementation plan. Using stepIM, cabinet-less machines can be built.

Integrating the electronics with the stepper motor reduces complexity

Conclusion

Closed-loop stepper motors have changed the performance-cost ratio in many motion control applications. Because of their excellent precision and energy efficiency, stepIM stepper motors can be used in areas that were previously dominated by more expensive servo motors. The success of stepIM closed-loop stepper motors has also given us a possibility - low-cost stepper motors can replace high-cost servo motors.