What is a servo?

What is a servo and how does it work?

Servos are the main driving force behind many robot and other machine movements where position control is essential.  But what exactly is it, and how does it work?

A servo basically consists of two simpler components: a DC motor and a potentiometer.  A DC motor is a electric motor, which is driven by direct current and a potentiometer is a varying resistor with a sliding contact that forms a voltage divider.

Here is a picture of what a servo looks like on the inside.  It is clear a servo could also include a gearbox (to assist the DC motor in driving a load) and some electronics to convert the feedback from the potentiometer into a desirable and clean signal.

The unaltered signal from the potentiometer is an analog signal and can usually be fed directly into a controller through a analog input.

The operation of this device is pretty simple.  The DC motor is given an electrical signal to make it turn (24Vdc in most small applications - giving it negative 24Vdc will make it turn in the opposite direction of when it gets positive 24Vdc), and the potentiometer will turn with the motor (usually connected to the motor shaft with some gears) to give an indication of the position of the motor shaft.

Some applications needs the 'zero' position of the servo to be at the centre point of it's range while other applications will use the servo from it's lower limit to its upper limit, which will be determined by the minimum and maximum values of the potentiometer.  Some servos may also have a mechanical limit, in which case the electronics will be designed to stop the power to the motor when it stalls against the  mechanical limit.

As with many other devices in electronics, there are many shapes, sizes and designs when it comes to servos, but they are all based on the same principles outlined here.  Servos should not be confused with encoders.  See more about encoders in: "Instrumentation: How does an encoder work?"

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- Jurie
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