If you connect a battery to a standard DC motor, it spins uncontrollably. While this is great for the wheels of a toy car, it's terrible for steering, moving a robotic arm, or adjusting the flaps on an RC airplane.
To move an object to a specific angle and keep it there, you need a Servo Motor. Servos trade continuous spinning for highly accurate rotational positioning. Let's open up the plastic casing and look at how they work.
What is Inside a Servo?
A servo is actually an assembly of four separate parts working together in a closed-loop system:
- A DC Motor: A small, fast-spinning electric motor provides the raw power.
- A Gearbox: A series of nylon or metal gears slows the DC motor down, drastically increasing the output torque (pushing power).
- A Potentiometer: This variable resistor is physically connected to the final output shaft. As the shaft turns, the resistance changes, allowing the servo to "know" its exact physical angle.
- A Control Circuit: A tiny circuit board that reads the potentiometer, reads the command signal from your Arduino, and drives the motor to correct any difference between the two.
The Magic of Closed-Loop Feedback
This setup is called a closed-loop feedback system. When you send a signal telling the servo to go to 90 degrees, the control circuit checks the potentiometer. If the potentiometer says the shaft is currently at 45 degrees, the circuit turns on the DC motor.
As the motor spins, the potentiometer turns. The circuit constantly monitors the changing resistance until it hits the value that corresponds to 90 degrees, at which point it instantly cuts power to the motor. If you try to push the servo arm away from 90 degrees with your finger, the potentiometer will detect the change, and the circuit will actively fight you to hold the commanded position.
Controlling with PWM
You communicate with a servo using a standard three-wire connection: Power, Ground, and Signal. The signal wire expects a Pulse Width Modulation (PWM) wave.
The servo doesn't care about voltage levels; it only cares about the duration (width) of the HIGH electrical pulse it receives every 20 milliseconds (50Hz).
Frequently Asked Questions
What is the difference between a standard DC motor and a servo motor?
A standard DC motor spins continuously when power is applied. A servo motor, however, uses closed-loop feedback to turn to a specific angle and hold that position. It contains a standard DC motor inside, but adds a gearbox, a position sensor, and a control circuit.
How do you control the angle of a servo motor?
Servos are controlled by sending a Pulse Width Modulation (PWM) signal. The width of the electrical pulse tells the servo which angle to hold. For example, a 1.5-millisecond pulse usually commands the servo to the center (90 degree) position.
Can a servo motor spin continuously 360 degrees?
Standard servos are physically limited to about 180 degrees of rotation by a mechanical stop on the main gear. However, you can buy "continuous rotation" servos where this stop is removed, allowing them to spin freely, though you lose exact positional control.
Why does my servo jitter and twitch?
Jitter usually happens if the servo is not receiving enough electrical current. Servos draw significant power when moving, often more than an Arduino pin or a standard USB port can supply. You should power servos from an external battery pack or power supply.
Conclusion
Servo motors brilliantly solve the problem of angular positioning by incorporating all the necessary feedback hardware inside a single, easy-to-use package. With just three wires and a few lines of code, you can start building articulated robots and precise steering mechanisms today.