What Is The Difference Between Servo Motor and Stepper Motor? . Both servo motor and stepper motor are special electrical machines that can satisfy the basic requirements of a positioning application. Servo motors and stepper motors convert the input electrical energy into angular velocity or movement of the motor shaft. However, there are quite different from each other in the way they operate.
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In this article, we have highlighted all the differences between servo motor and stepper motor. But, before discussing the differences, let’s have some basic information about the servo and stepper motors so that it becomes easier to understand the differences between them.
What is a Motor?
A motor is an electrical machine that converts electrical energy into mechanical energy. A power source such as a generator or battery is used to supply electrical power and it converts it into rotational motion which can be converted into linear motion.
A motor consists of two main parts; stator and rotor. The stator is the stationary part while the rotor is the rotating part. A generic motor works on the principle of Fleming’s left-hand rule. Simply put, the stator produces a rotating magnetic field that interacts with the rotor’s magnetic field and starts rotation.
There are several types of motors mainly used for generating rotational motion. The Servo motor and stepper motor are used for precise positioning and control over its rotating shaft.
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What is a Servo Motor?
A servo motor is an actuator (rotatory or linear) which allows the precise control of angular or linear position, velocity and acceleration. The working of a servo motor is based on the principle of servomechanism.
Servo motor involves a closed loop control system, i.e. a feedback mechanism which allows the angular precision so that it can produce desired rotation according to the applied electric signal. Basically, the servo motor consists of an electric motor coupled to a sensor for position feedback.
The applied electric signal to the servo motor can be either an analog signal or a digital signal. When the electric signal is input to the servo motor, it produces a movement required to position the shaft corresponding to the electric signal. Here, an encoder is used as a feedback unit that provides information about the speed and position to a controller. Thus, the speed of the servo motor changes in proportion to the applied electric signal.
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What is a Stepper Motor?
A type of brushless DC electric motor which divides a full rotation of the shaft into a number of equal steps is known as stepper motor. The stepper motor uses open-loop control system, i.e. it does not have any feedback mechanism.
The rotor of a stepper motor is generally a permanent magnet rotor. When the stator winding of the stepper motor is energized from an external electric supply, it rotates the rotor discretely. This rotation of the rotor is due to magnetic interaction between the magnetic poles of the stator winding and the poles of the rotor.
Therefore, in case of a stepper motor, the electrical input to the stator winding of the motor produces an output in the form of a discrete angular rotation of the rotor. This discrete rotation is produced in a way that when the stator winding is energized, a step by step current flows through the winding sequentially, thereby magnetizing the stator. Consequently, the electromagnetic poles developed in the stator rotate the rotor in the steps.
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Difference between Stepper Motor and Servo Motor
Both stepper and servo motors are used in the same applications, i.e. positioning. However, there are many differences between a servo motor and a stepper motor that are listed in the following table
| Basis of Difference | Servo Motor | Stepper Motor |
|---|---|---|
| Definition | A type of electric motor whose operation is controlled with the help of servo mechanism is known as servo motor. | A type of brushless DC motor which divides the full rotation of shaft into a number of equal discrete steps is known as stepper motor. |
| Operation | The operation servo motor is continuous. | The operation of stepper motor is divided into discrete steps. |
| Control system configuration | Servo motor is configured in a closed loop control system. | Stepper motor has open loop control system |
| Feedback mechanism | The feedback mechanism exists in a servo motor. | There is no feedback mechanism in case of stepper motor. |
| Torque speed characteristics | Servo motor produces high torque at high speeds. | Stepper motors produce high torque at low speeds. |
| Suitability with fluctuating loads | Servo motors are not suitable for fluctuating loads. | Stepper motors can be used for fluctuating loads as well. |
| Suitability for low rigidity mechanism | Servo motors are not suitable for low rigidity mechanisms like belt and pulley mechanism. | Stepper motors are suitable for low rigidity mechanisms. |
| Hunting | In a servo motor, the hunting exists during stop position. | In a stepper motor, there is no hunting during stop position. |
| Design | Servo motors have relatively complex design. | The stepper motors are simpler in design. |
| Power consumption | Servo motors need relatively less power. | Stepper motors consume comparatively more power. |
| Efficiency | The efficiency of servo motor is comparatively high. | The stepper motors are relatively less efficient. |
| Noise | The noise in case of servo motor is comparatively less. | Stepper motors produce high noise. |
| Operating speed | Servo motors have high operating speed. | The operating speed for stepper motors is low. |
| Reliability | Servo motors are comparatively less reliable. | Stepper motors are more reliable than servo motors. |
| Tolerance towards moment of inertia | Servo motors have low tolerance towards moment of inertia. | Stepper motors have high tolerance towards moment of inertia. |
| Life span | Servo motors have short life span. | The life span of stepper motors is longer. |
| Prone to damages due to overload | The servo motors are more prone to get damaged due to overload. | Stepper motors are less prone to get damaged due to overload. |
| Responsiveness | Servo motors have low responsiveness. | The responsiveness for stepper motors is high. |
| Synchronous operation | The synchronous operation is relatively difficult in case of servo motors. | The synchronous operation is easier in case of stepper motors. |
| Need of tuning and gain adjustment | Servo motors require tuning and gain adjustment. | Stepper motors do not require tuning and gain adjustment. |
| Cost | The cost of servo motors is high. | The cost of stepper motors is comparatively less. |
| Applications | Servo motors are extensively used in remote controlled equipment, robots, antenna positioning systems, industrial automation, cameras, etc. | Stepper motors are used in 3D printers, welding machines, textile machines, medical equipment, etc. |
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Comparison Between Servo and Stepper Motors
Definition
- The term ‘servo’ refers to the configuration in which a motor is used in a closed-loop system providing continuous feedback of the position of the motor.
- The term ‘stepper’ refers to the movement of the motor in discrete steps controlled by applying pulses in a sequence.
Feedback System
- The servo motor has a closed-loop system that provides feedback using a position sensor. It constantly monitors the position and compares it with the input-applied signal for accurate positioning.
- The Stepper motor is a brushless DC motor in an open loop configuration having no feedback. The train of input pulses rotates the rotor in steps to acquire positioning.
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Torque
- The servo motor provides high torque at high speed. Therefore it is used in torque control applications.
- The stepper motor has high torque at low speed which reduces as the speed gradually increases.
Speed
- The servo motor provides a very high operating speed in the range of 5000 RPM.
- The stepper motor has a lower operating speed in the range of 1500 RPM.
Stability
- Servo motors are not stable due to the presence of hunting even at a standstill. It oscillates about its steady state position.
- The Stepper motor is very stable and must be used in applications that require holding the load steady in place.
Design
- Design-wise, the servo motor is complex due to the involvement of the gearbox and feedback system.
- The stepper motor has an open loop system therefore it is very simple.
Hunting
In motors, hunting is a phenomenon that causes the rotor to oscillate about a steady state position. It occurs due to sudden changes in the load. The rotor starts hunting for the new synchronous speed.
- The Servo motor is affected by hunting this is why the synchronous operation is difficult in the servo motor.
- There is no hunting in the stepper motor.
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Cost
- The servo motor is more costly as compared to the stepper motor due to the presence of a gearbox, encoder and feedback system.
- The stepper motor is cheaper for its open-loop and simple design.
Maintenance
- Due to the complex design of the servo motor including the gearbox and feedback mechanism, it requires frequent maintenance. On top of that the brushes also need replacement after some time.
- As the stepper motor is a brushless motor, the only part that needs maintenance is the ball bearings. Therefore it is maintenance-free.
Micro stepping
Micro stepping is a technique used in stepper motors to further reduce the step size. It increases the resolution of the motor by dividing the current between the two poles. However, micro-stepping reduces the torque of the motor.
Which Motor Fits Your Application the Best?
Stepper motors are a good candidate for applications that require simple motion and no feedback. Steppers are available in a large variety of sizes, which helps to accommodate for size constraints.
Servo motors aid applications where sound and vibration need to be minimal. Servo motors are better for complex systems that require precise feedback. Certain applications such as CNC machinery require high accuracy, where a servo motor is an optimal solution. Servo motors are more efficient than stepper motors, with efficiencies between 80-90%.
In overall performance, servo motors are best for high speed, high torque applications while stepper motors are better suited for lower acceleration, high holding torque applications. ISL Products can help tailor the correct motor for all applications, small or large.
The table below will help you determine stepper motor vs servo for your next application requirements.
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Frequently Asked Questions
Q: What is the main difference between stepper and servo motors?
A: The main difference lies in their control systems. Stepper motors typically operate in open-loop systems, moving in discrete steps without feedback, while servo motors use closed-loop control with continuous feedback for precise positioning and speed control.
Q: When should I choose a stepper motor over a servo motor?
A: Choose a stepper motor for applications requiring precise positioning at lower speeds, where cost-effectiveness is important, and the load is relatively constant. They’re ideal for open-loop control systems in devices like 3D printers and simple CNC machines.
Q: How do the torque characteristics of stepper and servo motors compare?
A: In speed-torque curves, stepper motors have higher initial torque but experience a significant drop-off as speed increases. Servo motors maintain a more consistent torque output across their speed range, making them better suited for high-speed or variable-load applications.
Q: What are the cost implications of choosing between stepper and servo motors?
A: Stepper motor systems typically have lower initial costs due to simpler construction and control requirements. Servo motor systems are more expensive upfront but may offer long-term cost benefits in energy efficiency and performance in certain applications.
Q: How does the integration complexity compare between stepper and servo motor systems?
A: Stepper motor systems are generally simpler to integrate, requiring less complex control electronics and wiring. Servo motor systems are more complex, requiring encoders, more sophisticated drivers, and tuning, but offer greater flexibility and performance in dynamic applications.
