Servomotors are used in everything from toys and drones to household items like DVD players. They come in a variety of configurations (the most common types are listed below); understanding the differences is the trick to figuring out which is right for your project.
In simple terms, servos are standalone electric motors that push and rotate parts in machines wherein a specific task and position need to be defined. When a servo is given a command, it moves to a position and holds there with resistive force. A servo uses either a rotary actuator or a linear actuator to control angular or linear positions through acceleration and velocity. They typically operate between 4.5V–6V, which run through power, ground, and control wires.
A servo’s DC motor rotates at a high RPM on low torque. The gears inside the servo convert the output to a much slower rotation speed but with more torque. This creates a large amount of force for a short period of time and is a perfect example of the basic law of physics: Work = force × distance.
The Three Main Types
Positional Rotation Servo — This variety rotates within a 180° range. It’s not designed to turn beyond its preset limits. Useful for limited-range applications like moving levers or steering linkages.
Continuous Rotation Servo — While levers are often used on standard servos, wheels and gears become more useful with this style, which can turn in any direction independently and continuously.
Linear Servo — This type offers more gears than the positional rotation servo, but is otherwise very similar. It uses a rack and pinion mechanism to change the output back and forth instead of circularly. This servo is rare, but can be found in larger hobby planes and robots.
Servos and PWM
A servo contains a small circuit board with a sensor that communicates its rotational position to an R/C controller, computer, or microcontroller. The resulting information is then translated into small electrical pulses with variable energy. Manipulating this energy, called pulse width modulation (or PWM), controls the position of the motor.
The internal components on a servo motor are what give it such useful versatility. Standardized case sizes and interchangeable accessory brackets minimize design complexities while providing interface options for almost any application. The small DC motor keeps the servo size minimal, while the controller board monitors motor position and offers user interface and control. A speed-reducing gear set allows for precision motor alignment with high levels or torque.
Analog vs. Digital Servos
Analog and digital servos look exactly the same. The difference is in the way they signal and process information.
Analog Servos operate based on on/off voltage signals that come through the PWM. When this type of servo is resting, the PWM is essentially off unless you transmit an action. Producing torque from the resting mode makes the initial reaction time sluggish, which can cause problems in advanced R/C applications.
Digital Servos use a small microprocessor to receive and direct action at high frequency voltage pulses. The digital servo sends 300 pulses per second, where the analog only operates at 50 pulses per second. These faster pulses provide consistent torque for quicker and smoother response times. This is a great benefit, but digital servos consume a lot more power.