Servo motors are essential components in motion control systems, offering high precision and reliability across a wide range of applications, from robotics and automotive to industrial automation and consumer electronics.
There are two types of servo motors: positional rotation servo motors and continuous rotation servo motors. While both types serve distinct roles in motion control, understanding their differences is key to selecting the right motor for your specific needs.
This article will explore the fundamental differences between positional and continuous rotation servo motors, comparing their design, applications, performance characteristics, and suitability for different tasks. To aid in this comparison, we’ll also include a detailed chart and analysis.
Overview of Servo Motors
Servo motors are made to control torque, speed, and angular position precisely. They operate within a closed-loop control system, where feedback from the motor (typically from an encoder or potentiometer) is used to adjust the motor’s performance based on a command signal. This closed-loop system ensures that the motor performs as expected, allowing for fine motion control.
Servo motors can be divided into two main categories according to their capacity for rotation:
- Positional Rotation Servo Motors (also known as standard servo motors)
- Continuous Rotation Servo Motors
Positional Rotation Servo Motors
The range of motion that positional rotation servo motors can rotate within is usually between 0° and 180°, while some versions can reach 360°. The key feature of these motors is their ability to rotate the shaft to a specified angle and maintain that position until a new command is received. The motor’s position is controlled using Pulse Width Modulation (PWM) signals, which dictate the angle of rotation.
In a positional servo motor, the shaft is driven by a small DC motor, and the feedback mechanism, usually an encoder or potentiometer, constantly relays the shaft’s position to the motor controller. This allows the motor to move to the exact required angle and hold that position with high accuracy.
Applications
Servo motors with positional rotation are frequently employed in applications that call for exact control over angular movement. Some typical applications include:
- Robotics: For controlling robotic arms, grippers, or legs, where precise angular movements are essential.
- Modeling and RC Vehicles: Used to control steering, rudders, and other parts in remote-controlled cars, boats, and airplanes.
- Camera Systems: To provide fine control over the position of cameras in pan-and-tilt mechanisms, gimbals, and other motion-tracking systems.
- CNC Machines: For positioning tools and workpieces with high precision.
Performance Characteristics
| Precision | High precision, typically within 1° |
| Speed | Moderate speed, focused on precise positioning |
| Torque | Moderate to high (depends on size and design) |
| Load Capacity | Moderate (depends on motor size and application) |
| Control Method | PWM signal for angular position control |
| Power Consumption | Moderate (depends on position holding and load) |
| Cost | Generally higher due to precision and complexity |
| Applications | Robotics, RC vehicles, CNC machines, cameras |
Continuous Rotation Servo Motors
Unlike positional rotation servo motors, continuous rotation servo motors are designed for continuous rotation in either direction (clockwise or counterclockwise) and are often referred to as “standard servos with modified gearing.” Continuous rotation servos operate similarly to DC motors but with built-in control electronics that regulate speed and direction based on PWM signals.
When a PWM signal is input, the motor will rotate continuously in the direction dictated by the signal’s duty cycle. A 50% duty cycle PWM signal typically stops the motor (neutral position), while higher or lower duty cycles rotate the motor in either the clockwise or counterclockwise direction, respectively.
Applications
Continuous rotation servo motors are ideal for applications that require constant motion and do not need precise angular positioning. Some common applications include:
- Robotics: Used in driving wheels and other moving parts of robots, where continuous motion is needed, such as in mobile robots or vehicles.
- Conveyor Systems: For systems that need continuous and uninterrupted motion, such as in assembly lines or packaging machines.
- RC Vehicles: Used in wheels or other parts of remote-controlled cars, trucks, and boats.
- Camera Pan and Tilt Systems: For controlling the continuous movement of cameras without the need for precise angular control.
| Feature | Continuous Rotation Servo Motor |
| Movement Type | Continuous rotation (clockwise or counterclockwise) |
| Control Method | PWM signal for speed and direction control |
| Precision | Low (no precise positional control) |
| Speed Control | Proportional to PWM duty cycle (variable speed) |
| Torque | Moderate (decreases with speed) |
| Load Capacity | Low to moderate |
| Applications | Robotics (wheels, moving parts), Conveyor belts, RC vehicles, Camera pan systems |
| Cost | Generally affordable and simple |
| Power Consumption | Higher during continuous motion |
| Complexity | Simple control (PWM) |
Comparison: Positional vs Continuous Rotation Servo Motors
To better understand the key differences between positional rotation and continuous rotation servo motors, we can break down several aspects of performance, applications, and design characteristics. These important distinctions are summed up in the graphic below:
| Feature | Positional Rotation Servo | Continuous Rotation Servo |
| Movement Type | Rotates within a fixed range (0° to 180°) | Continuous rotation (clockwise or counterclockwise) |
| Control Method | PWM signal for angular position control | PWM signal for speed and direction control |
| Applications | Robotics, RC vehicles, CNC machines, cameras | Robotics (wheels), conveyor belts, RC vehicles, camera pan systems |
| Precision | High precision, typically within 1° | Low precision (no fixed positional control) |
| Speed Control | Limited speed control, focused on position | Full control over speed, proportional to PWM duty cycle |
| Torque | Moderate to high (depends on load and design) | Moderate torque, decreases with speed |
| Load Capacity | Moderate (depends on the motor size and design) | Low to moderate |
| Power Consumption | Moderate, depending on position holding and load | Generally higher during continuous motion |
| Use Cases | Precise control of angles, angular positioning tasks | Continuous motion, speed control tasks, simple directional control |
Detailed Analysis:
- Precision: Positional rotation servos excel in applications requiring high precision, offering repeatable positioning that is crucial for robotics and CNC systems. Continuous rotation servos, by contrast, are not designed to hold specific positions, which makes them unsuitable for applications where precision is a top priority.
- Speed and Motion: Continuous rotation servos provide flexibility in terms of continuous motion and speed control, making them ideal for driving wheels and moving parts. Positional rotation servos, however, focus on moving to specific angles and holding those positions, which means their speed is typically lower, and their motion is more discrete.
- Torque: Because their geared systems are built to give exact control over the motor’s movement, positional rotation servos usually have higher torque at low speeds. Continuous rotation servos, while providing continuous motion, offer less torque at higher speeds and are typically not designed to carry heavy loads.
- Cost and Complexity: Generally speaking, continuous rotation servos are simpler and more affordable than positional rotation servos. The latter’s complexity—due to the need for precise control and feedback mechanisms—adds to their cost. For many basic applications where precision isn’t crucial, continuous rotation servos offer an affordable and effective solution.
Choosing the Right Servo Motor for Your Application
The decision between positional and continuous rotation servo motors comes down to the specific requirements of your application. Here are some pointers to help you choose the best motor:
Use a positional rotation servo motor if:
- You must have exact control over the rotational angle.
- The application requires repeatability and accuracy, such as in robotics or CNC machining.
- You are building systems like camera gimbals or steering mechanisms in RC vehicles where precise angular control is essential.
Use a continuous rotation servo motor if:
- You require continuous motion, such as in a mobile robot or conveyor system.
- There is no requirement for exact placement; the main issues are directionality and speed control.
- You are building systems like wheels for robots or simple panning mechanisms where ongoing motion is needed.
Both positional rotation servo motors and continuous rotation servo motors have their unique strengths and limitations, making them suitable for different applications. Continuous rotation servos perform well in applications needing constant motion and speed control, while positional servos are best suited for jobs demanding exact control over angular position. As a servo motor manufacturer, understanding the differences between these two types of motors can help you make informed decisions based on your specific needs, whether you are building a robotic system, an RC vehicle, or an automated machine.