What is a Servo Motor and How Does it Work?

Welcome to our exploration of the magic behind motion in robotics, animatronics, and motorized prop projects! This magic is largely powered by an unsung hero – the servo motor. Servos are versatile, compact, and incredibly precise devices that convert electrical signals into controlled physical movement. Whether it’s an animatronic dinosaur moving its eyes, a robot performing delicate tasks, or a motorized prop bringing a stage to life, chances are, there’s a servo motor working tirelessly behind the scenes. This servo tutorial for beginners will demystify these impressive little devices, diving into what they are, how they work, and why they are the cornerstone of precise motion control in countless projects. So gear up for a fascinating journey into the world of servo motors!

What is a servo?

A servo, short for servomechanism, is a type of device used for precision control of angular or linear position, velocity, and acceleration. It usually consists of a motor coupled to a sensor for position feedback, which is then controlled by a servo controller.

The servo system is a closed-loop control system where the controller uses the feedback signal to accurately control the position or speed of the motor. This is accomplished by comparing the desired position or speed (the command signal) with the actual position or speed (the feedback signal), and then adjusting the motor’s operation to reduce the difference (the error signal).

Servos are widely used in systems like robotics, CNC machinery, automated manufacturing, and radio-controlled models for precise control of steering and propulsion systems. They can be based on different types of motors, such as DC motors, AC motors, or even stepper motors, depending on the requirements of the specific application.

How do Servo Motors Work?

An inside view of a servo showing the motor, circuit board, wiring and gears.
Photo credit: oskay
An inside view of a servo showing the motor, circuit board, wiring and gears.
Photo credit: Nick Ames

A servo motor is composed of several internal key parts that work together to control its motion. These components include:

  • Motor: This is the primary component that generates the motion in a servo system. It can be an AC or DC motor, depending on the type of servo.
  • Position Feedback Sensor: This device is typically either a potentiometer or an optical rotary encoder. It constantly monitors the position or speed of the motor, sending a feedback signal back to the control circuitry.
  • Gearbox or Gear Reduction System: A servo motor usually includes a gearbox, which slows down the motor’s speed while increasing its torque. The gear reduction system allows the servo to provide precise control over its output motion.
  • Control Circuit: The control circuit interprets the input signal, measures the feedback from the sensor, and controls the motor accordingly. It uses the difference between the input and feedback signals to adjust the power delivered to the motor, allowing it to maintain the desired position or speed.
  • Output Shaft: The output shaft is attached to the gearbox. It rotates or moves linearly (in the case of a linear servo) based on the commands given to the servo motor. The output shaft can be connected to a variety of horns and linkages, depending on the application.
  • Casing or Housing: All these components are typically contained within a protective plastic casing or housing, which can also include mounting points for attaching the servo to other systems or structures.
  • Power Supply: The power supply provides the electricity needed for the motor and control circuit to operate. The power supply can come from an external source, like a battery or an electrical outlet, and must be compatible with the servo’s requirements.
  • Connectors: Servos usually have connectors or leads for power, ground, and control signals. In many cases, these connectors conform to a standard that allows the servo to be easily integrated with other electronic devices and controllers.

Each of these parts plays an essential role in the functioning of a servo motor, and they all work together to provide precise, reliable motion control.

Servo motors operate based on a concept called closed-loop control, meaning they continually receive feedback and adjust their performance in real-time. Here’s a basic breakdown of how they work:

  1. Input: The process starts when the servo receives a command signal, typically a pulse-width modulation (PWM) signal. This signal indicates the desired position or speed for the servo motor.
  2. Processing: The servo’s control circuitry interprets the command signal. If the signal corresponds to a different position than the current one, the circuitry will activate the motor.
  3. Action: The motor turns until it reaches the position or speed dictated by the command signal. This rotation typically moves some external device or component, like the control surface on a model airplane or a robotic arm.
  4. Feedback: While the motor is turning, a sensor (usually a potentiometer or an encoder) is also monitoring its position or speed. This sensor sends a continuous feedback signal back to the control circuitry, indicating the motor’s current state.
  5. Correction: The control circuitry compares the feedback signal with the original command signal. If there’s a discrepancy (meaning the motor hasn’t yet reached the desired position or speed), the circuitry adjusts the motor accordingly. It increases or decreases the power to the motor, making it turn faster or slower.
  6. Continual Adjustment: This process of action, feedback, and correction continues as long as the servo is operating, allowing it to make continuous adjustments in response to changes in the command signal or external forces. This results in highly precise and reliable control of the motor’s position or speed.

The precise functioning can vary between different types of servos and different applications, but this gives you a general idea of how they work.

Types of Servo Motors

Servo motors come in several varieties, shapes and sizes which are typically categorized based on their range of motion. Here are the three main types:

Positional Rotation Servo Motors

A positional servo motor, often simply referred to as a servo motor, is a type of motor that can move to a specified position and hold that position. These motors are designed for precision control and are commonly used in robotics, CNC machinery, and radio-controlled models.

Example of a positional rotation servo motor.

The key characteristics of a positional servo motor include:

  • Positional Control: This type of servo motor can rotate to a specific angle based on the control signal it receives. The motor’s shaft can be positioned to specific angular positions by sending a coded signal to the motor.
  • Feedback Mechanism: The motor uses a feedback system, typically a potentiometer that provides a signal based on the shaft’s position. This feedback allows the control circuit to constantly monitor the current position of the motor and correct any deviations from the desired position.
  • Holding Torque: Once the motor has achieved the desired position, it will try to maintain this position even if external forces are applied. This is called holding torque.

Positional servo motors are generally limited in their rotation to less than a full circle, often being capable of somewhere between 0 and 180 degrees of rotation, though some models allow more or less than this. The exact position is determined by the duration of a pulse applied to the control wire, a method known as pulse-width modulation (PWM).

The advantages of positional servo motors include precise control, good holding torque, and relatively simple control mechanisms. However, they also tend to be more expensive than similar-sized motors that don’t offer positional control, such as basic DC motors.

Continuous Rotation Servo Motors

A continuous rotation servo is a variation of the standard positional servo motor. Unlike a standard servo, which is designed to rotate and hold its position within a certain range (typically 0 to 180 degrees), a continuous rotation servo can rotate freely in either direction indefinitely, similar to a regular DC motor.

Example of a continuous rotation servo motor.

The control signal, instead of setting the static position of the servo, is interpreted as the direction and speed of rotation. A pulse width near the mid-point (usually 1.5 ms) stops the motor, while pulse widths of 1 ms or 2 ms cause the motor to rotate clockwise or counter-clockwise at a speed proportional to the difference from the midpoint.

These types of servos are often used in mobile robotics for drive motors or any application where you need a full range of continuous rotation. They provide a simple and affordable solution for remotely controlling wheels or tracks. It should be noted, though, that while they provide convenience and ease of control, they don’t usually offer the same level of precise speed control or positional feedback as a proper DC motor and encoder setup.

Linear Servo Motors

A linear servo motor is a type of servo motor that moves in a straight line rather than in a rotational motion. Instead of rotating an output shaft, a linear servo motor pushes or pulls a load along a single axis. They operate on the same basic principles as a standard servo motor, using feedback from a position sensor to control the position of the actuator.

An example of a linear servo motor.

In a linear servo motor, a linear actuator (which is a motor that can create motion in a straight line) is combined with a control circuit and a position sensor. The control circuit receives an input signal indicating the desired position, and it uses this signal to control the actuator. The position sensor continually monitors the position of the actuator, sending a feedback signal back to the control circuit.

Linear servo motors can offer several advantages over rotational servos or other types of linear actuators. They can provide very precise control over position, speed, and force, and they can also have faster response times. These characteristics make linear servo motors well-suited for applications such as robotics, precision assembly, and automation systems.

However, linear servos tend to be more complex and expensive than their rotational counterparts, so they are typically only used when the benefits they offer are necessary for the application.

DC vs AC Servo Motors

Each servo type above is available as either a DC or AC motor option. The one you choose will depend on your project’s requirements but for small to mid-sized robotics or animatronics projects, you’ll most likely use DC servo motors most often.

  • DC Servo Motors: These are the most common type of servo motor and are used in applications requiring a motor with a low power rating. They use DC electric input to generate controlled rotational motion. DC servo motors are often found in radio-controlled devices and small robots.
  • AC Servo Motors: AC servo motors use alternating current to operate, which makes them suitable for high-power applications. They are frequently found in industrial machines and robotics where higher power and speed are needed.

How to Control a Servo Motor

Servo motors are controlled using a technique known as Pulse Width Modulation (PWM). This involves sending a series of pulses to the servo, with the width of each pulse determining the position of the motor. In order to send PWM signals to a servo you’ll need to connect it to either a dedicated servo controller or microcontroller with PWM capabilities like Arduino. We’ll cover how to connect and power a servo with Arduino in the next tutorial.

In the meantime, it’s important to have a general knowledge of how PWM signals control a servo’s position. Servo motors generally expect a pulse every 20 milliseconds or 50 Hz but many hobby servos work fine in a range of 40 to 200 Hz. When you send the servo a signal with a pulse width of 1.5 milliseconds (ms), the servo will move to the neutral position (90 degrees). The min (0 degrees) and max (180 degrees) position typically correspond to a pulse width of 1 ms and 2 ms respectively.

A diagram showing how PWM pulses control the angle and rotation of a servo motor.

The precise pulse widths and timings can vary depending on the specific servo motor. Therefore, you should always consult the servo’s documentation to determine the correct values.

Many modern microcontrollers and single-board computers have built-in support for PWM signals, making it relatively straightforward to control a servo motor. Typically, you would use a software library or module that provides high-level commands for setting the servo position, and the library would take care of generating the appropriate PWM signal.

Servo Motor Accessories

Servo motors, especially those used in robotics, RC (radio control) applications, and model building, often come with or require a variety of accessories to control movement and adapt to different use-cases. Here are a few common types of servo accessories:

Servo Horns or Arms

These are attachments that go onto the output shaft of the servo. They transfer the rotational motion of the servo to the device you’re controlling. Servo horns come in a variety of shapes and sizes, including single-sided, double-sided, and circular configurations, for different applications.

Servo Mounts & Brackets

These are used to securely attach the servo motor to a frame or casing. The design and size of these mounts and brackets depend on the servo motor’s size and application.

Servo Extensions & Cables

These are used when the cable of the servo motor is not long enough for your application. The extension is simply a plug-and-play device with a female end (to connect to the servo) and a male end (to connect to the controller).

Servo Controllers

These are electronic devices used to control one or more servo motors. They can often receive commands from a variety of sources, including RC receivers, computers, microcontrollers like Arduino, and even standalone control buttons and switches.

Power Supply

Some servo motors require more power than can be supplied by a basic microcontroller or RC receiver. In these cases, a separate power supply may be needed to provide the necessary power.

Servo Gear Sets

hese sets contain the gears needed to repair or upgrade the gear assembly inside a servo. They can be made of plastic or metal, with metal gears typically offering greater durability.

Servo Cases

These are replacement bodies for servos, often used in case the original casing gets damaged.

Servo Testers

These are small devices that generate control signals that can make a servo move to different positions. They’re typically used for testing and configuring servos before installing them into a system.

Heat Sinks

Some high-performance servos generate a lot of heat, and a heat sink can be attached to the servo to help dissipate this heat and prevent overheating.

Servo Safety Clips

These are used to secure the connection between the servo lead and the receiver to prevent them from getting detached during operation.

In wrapping up, we’ve journeyed through the intricacies of servo motors, discovering how they work in driving precision in robotics, animatronics, and motorized props. Their perfect blend of compactness, reliability, and control makes them the unsung heroes of the tech world, turning abstract electrical signals into tangible movement. Next time you marvel at a robot’s precise maneuvers, an animatronic’s lifelike movements, or a dynamic stage prop, remember the humble servo motor working behind the scenes to make it all possible.

Now that you have an understanding of the different types of servo motors and how PWM signals work to control them, let’s get hands on by connecting a micro servo to an Arduino and creating a few movements using code. If you’ve never coded anything before, don’t worry! I’ll show you how to hook up a servo to an Arduino, power it and write each line of code step by step.

Stop Scrolling. Start Creating.

Find out about new courses and live events!
Get my latest guides and reference materials.
Unsubscribe anytime!