In an increasing number of high-standard industrial automation applications, technological advancements are changing the performance-cost ratio between stepper motors and servo motors.
With the adoption of closed-loop technology, lower-cost stepper motors are gradually penetrating application areas that were previously dominated by high-cost servo motors.
Stepper motors and servo motors
Traditionally, servo control systems are considered superior for applications requiring speeds exceeding 800 RPM and high dynamic response. Stepper motors, on the other hand, are better suited for applications with lower speeds, low to medium acceleration, and/or higher holding torque.
So what is the basis for this traditional view of stepper motors and servo motors?
structure
Stepper motors rotate 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 perform 100 step operations. Stepper motors use pulses to achieve incremental motion, enabling precise positioning without the use of any feedback sensors.
Figure 1. Mechanical structure of a hybrid stepper motor
Servo motors operate differently. They use a position sensor—an encoder—attached to a magnetic rotor that continuously detects the motor's precise position. The servo monitors the difference between the motor's actual and commanded positions and adjusts the current accordingly. This closed-loop system keeps the motor in the correct motion state.
Figure 2. Servo Motor
Simplicity and cost
Stepper motors are not only cheaper than servo motors, but also simpler to debug and maintain. Stepper motors are stable when stationary and can maintain their position (even under dynamic loads). However, for some applications with higher performance requirements, more expensive and complex servo motors must be used.
position
In applications requiring constant awareness of a machine's precise position, stepper motors and servo motors differ significantly. In open-loop motion applications controlled by stepper motors, the control system assumes the motor is always in the correct motion state. However, when a problem occurs, such as a component jamming causing the motor to stall, the controller loses its ability to determine the machine's actual position, resulting in misalignment. A servo motor's closed-loop system has the advantage of immediate detection if it becomes jammed. The machine stops operating and never loses its position.
Figure 3. Position feedback in a closed-loop servo system
Speed and Torque
The performance difference between stepper motors and servo motors stems from their different motor designs. Stepper motors have significantly more poles than servo motors, requiring far more winding current exchanges per full revolution. This results in a rapid decrease in torque as speed increases. Furthermore, stepper motors may lose speed synchronization once maximum torque is reached. For these reasons, servo motors are the preferred choice in most high-speed applications. Conversely, the higher pole count of stepper motors is advantageous at low speeds, where they offer a torque advantage compared to a servo motor of similar size.
Figure 4. As the speed increases, the torque of the stepper motor decreases.
Heat and energy consumption
Open-loop stepper motors use a fixed current and generate a lot of heat. Closed-loop control only provides the current required by the speed loop, thus avoiding the motor overheating problem.
A Comparison and Summary of Stepper Motors and Servo Motors
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 requiring low to medium acceleration and high holding torque, such as 3D printers, conveyors, and secondary shafts. Because stepper motors are less expensive, their use can reduce the cost of automation systems. Motion control systems that wish to utilize the characteristics of servo motors must demonstrate that these more expensive motors are worthwhile.
Figure 5. Driven by advancements in closed-loop technology, stepper motors are able to penetrate into high-performance, high-speed application areas previously exclusively belonging to servo motors.
Change mindset
What would happen if we could apply the advantages of closed-loop servo technology to stepper motors? Could we achieve performance similar to servo motors while maintaining the cost advantages of stepper motors?
By combining closed-loop control technology, stepper motors will become a low-cost, comprehensive product that combines the advantages of both servo motors and stepper motors. Because closed-loop stepper motors can significantly improve performance and energy efficiency, they can replace more expensive servo motors in an increasing number of high-standard applications.
Servotronix has embedded closed-loop control functionality into its stepIM integrated stepper motor.
Figure 6. Servotronix's stepIM product using closed-loop control
The stepper motor with integrated electronic control is equivalent to a two-phase brushless DC motor, capable of performing position loop control, speed loop control, DQ control, and other algorithms. A single-turn absolute encoder is used to achieve closed-loop commutation, ensuring optimal torque at any speed.
In this regard, we can consider stepIM products.
Low energy consumption and maintaining cooling
StepIM stepper motors are highly energy efficient. Unlike open-loop stepper motors that always operate on full current commands, which can lead to overheating and noise issues, the current of StepIM motors varies according to the actual motion conditions, such as during acceleration and deceleration. Similar to servo motors, the current consumed by these stepper motors is proportional to the actual torque required at any given time. Because the motors and integrated electronic control boards operate at lower temperatures, StepIM stepper motors can achieve higher peak torques comparable to servo motors.
Figure 7. Even at high speeds, the StepIM stepper motor requires less current.
Precisely match performance requirements
To ensure sufficient torque to overcome interference and avoid step loss, open-loop stepper motors typically require torque at least 40% higher than the application's requirements. Closed-loop StepIM stepper motors do not have this problem. When these motors stall due to overload, they maintain the load and do not lose torque. After the overload is removed, they resume operation. Maximum torque is guaranteed at any specified speed, while position sensors ensure no step loss. Therefore, the specifications of closed-loop stepper motors can precisely match the torque requirements of the application without requiring an additional 40% margin.
Open-loop stepper motors are prone to step loss, making it difficult to meet high transient torque requirements. StepIM closed-loop stepper motors, on the other hand, offer rapid acceleration, lower operating noise, and less resonance than traditional stepper motors. They can operate over a wider bandwidth and achieve superior performance.
cabinetless machine
stepIM integrates the drive control board with the motor, reducing the number of wires and simplifying implementation. Using stepIM, cabinet-less machines can be built.
Figure 8. Integrating electronic devices with stepper motors can reduce complexity.
Below, we examine the motion control performance in a wood cutting application, which typically uses servo motors; however, we will now use StepIM closed-loop stepper motors. (The following is a case study of a wood cutting application we studied. To achieve the required motion control performance, servo motors are usually used; now we will use StepIM closed-loop stepper motors.)
Closed-loop stepper motor in high-precision wood cutting machine
A global industrial automation company manufactures and sells hundreds of precision CNC machine tools annually for producing wooden window frames. This application requires precise synchronization and high torque, necessitating approximately 20-30 pneumatic and electronic servo motors per machine. The cost of these servo motors significantly increases the overall system cost.
Figure 9. Processing of wooden window frames
Because servo equipment is expensive, it significantly increases the overall cost of each machine. Furthermore, installing servo encoders in a single rack requires a large amount of additional cabling, increasing installation time and maintenance complexity.
After learning about Servotronix's closed-loop stepper motors (stepIM), the company was eager to determine if they were suitable for these woodworking machines.
The head of development said, "Cost has become an important factor in ensuring the competitiveness of our high-end machines globally. However, we cannot compromise performance, accuracy, and reliability under any circumstances."
If performance targets can be achieved using lower-cost closed-loop stepper motors, companies will use them to replace more expensive servo motors, thereby gaining an advantage in the market.
The company launched a pilot project to replace a servo motor on a machine with a Servotronix StepIM stepper motor. Everything else remained unchanged except for the motor – the motion controller and communication protocol (CANopen) were the same as before.
Furthermore, because the stepIM is integrated with the drive control board, fewer cables are required, resulting in significant simplification. With fewer cables, setup and maintenance are faster and easier.
Just a few months later, with excellent technical support from Servotronix, the company was able to build its own stepper machine with performance comparable to standard servo machines.
After achieving the project goals, the project manager was very pleased: "We can replace the servo motors on the target machine with the stepIM product. Although we still need to write some software and conduct some preliminary testing, we have achieved our expected goals in just a few months."
The next step is to determine whether the machine using a stepper motor can achieve the required accuracy, acceleration, energy consumption, and other key performance indicators.
After a month-long trial, the company determined that the machines using closed-loop stepper motors not only met all specifications and performance requirements, but also reduced the complexity and cutting cost of each machine by more than 5%. The company has now begun implementing stepIM on 300 new machines.
New opportunities
Closed-loop stepper motors have revolutionized the performance-to-cost ratio in many motion control applications. Thanks to their superior precision and energy efficiency, stepIM stepper motors can be used in fields previously dominated by more expensive servo motors. stepIM closed-loop servo motors are well-suited for multi-axis applications, positioning tasks with varying loads, and other applications requiring quiet operation, short settling times, and precise positioning.
Currently, Servotronix's stepIM closed-loop stepper motors are used in wood processing, medical, printing, dental grinding, solar energy, and many other high-standard applications that were previously exclusive to servo motors.
