How Servo Motors Work: A Simple Guide

A servo motor makes things move with very exact movement. How does a robot arm know the right place to stop? How does an airplane move its flaps? I learned the answer was the servo motor. In this post, I will tell you what I learned about the way these great tools work. We will look at the feedback mechanism that lets them move so exactly. We will also look at their many industrial applications.

What is a Servo Motor, and Why is it Important?

Basically, a servo motor is a special kind of motor. It lets you control the exact spot of its turning part. You can control its angular or linear position. You can also control how fast it moves and speeds up, which are its velocity and acceleration. A regular DC motor just spins and spins. But a servo motor is different. It can be told to go to a certain spot and hold that position.

This skill comes from a smart setup called a closed-loop control system. It works like this: You tell the servo motor a position to move to. It moves there. Then, a sensor inside the motor tells a controller that it is in the right place. This process of always checking is why servos can be so exact. If you are building a robot, a CNC machine, or anything that must move to the right spot, you should learn how a servo motor works.

What’s Inside a Servo Motor?

If you open a servo motor, you will see a few main parts that work as a team. It is a small but smart tool.

A normal motor, like an AC or DC motor.
Some gears to control its speed and torque, which is its turning force.
A sensor that knows the position. This is usually a potentiometer or an encoder.
A control circuit, which is the control electronics. It reads the sensor and tells the motor what to do.

The motor gives the power. The gears give the motor more turning force, which is the torque of the motor. The sensor and control circuit act like the brains. Putting these parts together is what makes a servo mechanism work so well for robotics and automation. The motor shaft connects to the gears. The gears then connect to the turning part you see on the outside.

How Does the Closed-Loop Feedback Mechanism Work?

This is the most special part of how a servo motor works. The idea of a closed-loop control system is what makes servos different from other motors. Think about parking a car. You want to be one foot from the side of the road. You would look to see how far you are, and then you would fix your position. You are using your eyes to check and correct your work.

A servo motor does the same thing. The controller sends a message, or control signal, to the motor. This message tells it to move to a certain spot, which is the desired position. As the motor rotates, the sensor (the potentiometer or encoder) is always reading the position of the shaft. This information goes back to the controller. The controller looks at the current position and the desired position. If they are not the same, it keeps sending power to the motor. It stops when the two positions are the same. This system of always checking and fixing is the feedback mechanism. It helps the servo motor hold its position with great accuracy.

How Do We Control a Servo Motor?

I found out that you cannot just hook up a battery to a servo motor. It will not just start spinning. A servo motor needs a special set of instructions to understand orders. These instructions are called Pulse Width Modulation, or PWM.

A PWM signal is a group of short bursts of electricity. The servo motor expects to get one of these bursts, or a pulse, every 20 milliseconds. The length of the pulse is what matters, not how much electricity it has. The length of this electrical pulse tells the motor’s shaft where to turn.

A pulse that lasts for 1 millisecond usually tells the servo motor to turn all the way to one side (0 degrees).
A pulse that lasts for 2 milliseconds tells it to turn all the way to the other side (180°).
A pulse that lasts for 1.5 milliseconds tells it to move to the middle spot (90 degrees).

By sending the right electrical pulse, we can get very precise control of the motor shaft. Many fun projects use controllers like an Arduino. You can add a servo shield to these boards. This makes it easy to control multiple servo motors at once.

What Are the Main Kinds of Servo Motors?

When you look for a servo motor, you will see that servo motors come in a few main kinds. The two most common types of servo motors are AC servos and DC servos.

Part	AC Servo Motor	DC Servo Motor
Power	Alternating Current (AC)	Direct Current (DC)
How It Works	It uses an alternating current to make a spinning magnetic field.	It uses brushes and a part called a commutator, or it can be brushless.
Best For	Jobs that need very exact movements, industrial applications, and heavy things.	Simpler projects that cost less, like hobby robotics and small tools.
Good Things	Works very well, does not need much repair, has a high torque.	Easy to control, costs less money.
Bad Things	It is more complex and costs more.	It is not as efficient. The brushes can get old and cause wear and tear.

There are also linear servo motors. These move in a straight line instead of in a circle. There are also continuous rotation servos. These can spin all the way around like a normal motor, but you can control their speed. Picking the correct servo motor is based on what your project needs. Think about power, how exact it needs to be, and your budget. For example, big CNC machines often use servo motors that are strong AC servo motors.

What Is a Potentiometer in a Servo Motor?

In many regular servos, the position sensor is a potentiometer. This is especially true for servos used in hobby projects. A potentiometer is a kind of resistor that can change. It has a knob that you can turn. When you turn it, the amount of electrical resistance changes.

Inside the motor, the potentiometer is linked to the last gear. When the motor shaft turns, the potentiometer also turns. The control circuit passes a voltage through the potentiometer. Then it reads the voltage that comes out. The output voltage is different based on the shaft’s position. This tells the circuit the exact spot of the shaft at any time. This is a simple but good way for the servo system to get its feedback.

What Is an Encoder and a Resolver?

For tougher factory jobs, a potentiometer might not be exact enough. For these jobs, a servo motor will use an encoder or a resolver for its sensor.

An encoder is a sensor that changes movement into an electric message. It can tell the controller more than just the position. It can also report the velocity and direction of the motor shaft. There are two main types:

Incremental Encoder: This kind tells the controller about changes in position. You always need a starting spot for it to work from.
Absolute Encoder: This kind gives the exact angular position all the time. It knows the position even if the power is shut off and then turned back on.

A resolver is another kind of sensor. It is very strong and is often used in places that are rough or dirty. Just like an encoder, it gives feedback about the position of the shaft. Both of these sensors are much more accurate than a potentiometer. That is why you will see them in high-level robotics and automation machines.

Where Are Servo Motors Used?

After you learn what a servo motor is, you will start to see them in many places. The list of servo motor applications is very long. This is because so many things need to move in a very exact way.

Here are a few places servos are used:

Robotics: Every joint in a robotic arm might have a servo motor. This lets it control its position. This is true for a big industrial robot and for a small hobby robot. A gripper on a robot uses a servo motor to open and close with the right amount of force.
Industrial Applications: In factories, servo motors are used to run moving belts, machines that fill bottles, and CNC machines. Their speed and accuracy are what make factory automation possible.
Remote-Controlled Vehicles: A remote-controlled airplane uses servos to move the small wings. An RC car uses a servo to steer. This is a very common use in hobby applications.
Medical Devices: Servo motors are used in hospital tools like scanners and robots for surgery. In these tools, being exact is very important.
Common Electronics: You can even find a tiny servo motor in things like a camera. It works to make the picture clear by making small, exact changes.

From the first time people used steam to control the rudders on ships to today’s robotics, the main idea of the servo mechanism is still a great way to control a machine.

What is the Difference Between Servo Motors and Stepper Motors?

When I was first learning, I would often mix up servo motors and stepper motors. They are both used to control position. But they do their jobs in very different ways.

A stepper motor moves in small, fixed steps. It does not have a feedback mechanism. You tell it to move a number of steps, and it just does it. It is an “open-loop” system, which means it does