A servomotor is a combination of a motor and a position control circuit. Model-craft servos often are build using a brushed DC motor, driving a gearbox to reduce the speed of the outgoing shaft. Also a position sensor, mostly a potmeter is connected to the motor. The position sensor gives feedback on the actual position of the outgoing shaft. So the control circuit can move the shaft to the desired position. All this comes in a small casing complete with mounting holes. Making a similar, position control system yourself is a toff job. And still, servos are very  cheap.

servomotor meaning and application-field

According  https://www.merriam-webster.com/dictionary/servomotor the word servomotor comes from: French servo-moteur, from Latin servus slave, servant + French -o- + moteur motor, from Latin motor one that moves.

Servo mechanisms are applied in many mechanical, motor driven systems. Also in high advanced positioning systems in science and industry. In this article we discuss only the model-craft type of servo, being short for servo motor.

Connections

A servo has 3 electrical connections. The 2 power connections +6Volt and ground. And one control input. The driver chip inside the servo unit reads this input to know the desired position of the servo.

Typical specifications

You can buy servos having plastic gears but also with metal gears. The last ones being more robust and the first being more silent. However they are in general not the most silent solution.

Most model craft servo’s can move the shaft only 180 degrees. This is because they were originally designed to move the ruder of a model craft ship or aircraft. (This also explains why they were not designed to be silent.) But as the number of applications increased, also the demand for a wider angle arose. Nowadays you can buy servo’s that can rotate continuously (360 degrees).

Most model-craft servo’s are designed to work optimal on 6 Volt power supply. Mostly they will work at 5Volt as well. Keep in mind that when the motor voltage is lower the output torque will be smaller. Output torque is usually specified in N cm. Servos are sold in different sizes, often called standard, mini or micro. Different brands have shafts that look similar, but are often slightly different. Keep that in mind when mixing servo accessories. Or when you make something that fits tight on a servo shaft.

Controlling a servo position

A servo motor needs a digital input signal; A block wave repeating every 20 ms. The width of the pulse determines the desired position. Where 1.5 ms is the center position (remember; of the ruder). 1 ms fully CCW. and 2 ms maximal CW. These values however differ a bit per servo brand. (They are cheap, very easy to use, there should also be any disadvantage. Well here it is: they are not completely standardized). For the 360 degrees type, the pulse-width controls the speed instead of the position.

When controlling a servo from within the Arduino environment, there are many libraries available.