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How the RC Servo Motor Works?

By May | Published on Sep 14,2015

Since the RC servo motor is widely used in our daily life and there are many question asked by lots of people, the purpose of this article is to give an overview of how RC servo motor work.

RC servo motors are controlled by sending them a variable wide pulse. Whenever the signal pulse exists on the signal line, the RC servo motor will keep the shaft angular position after it has rotated to that position. When the signal pulse changes, the angular position of the shaft will change accordingly.This can be useful because there is the RC servo motor system which makes the servo motor so well-known against other actuators. Servo can mean that you can set and forget it, and it will adjusts itself by continued operation through feedback.

An RC servo motor is composed of several main parts: the RC servo motor and gearbox, a position sensor, an error amplifier and RC servo motor driver and a circuit to decode the requested position. Picture below contains a block diagram of a typical RC servo motor unit.

The radio control receiver system produces a pulse of variable length about every 20 milliseconds. The pulse is normally between 1 and 2 milliseconds long. The length of the pulse is used by the servo to decide the exact position it should reach. The control pulse is fed back a pulse width to voltage converter. This circuit allows a capacitor to charge at a constant rate even through the pulse is high. When the pulse becomes low the charge on the capacitor is fed to the output via a suitable buffer amplifier, which generates a voltage related to the length of the applied pulse.

The circuit is tuned to cause a useful voltage. The output voltage is buffered, and therefore no significant will decay the length of time between pulses which is not critical between controlled pulses. The sensor is to read the current rotational position of the RC servo motor output shaft. This is usually a potentiometer that will generate a voltage related to the output shafts of its absolute angle.

The potentiometer then feeds its current value into the Error Amplifier which will contrast the current position with the commanded position from the pulse width to a voltage converter.

The error amplifier, an operational amplifier has negative feedback, trying to reduce mostly the difference between the inverting and non-inverting inputs by driving its output in the correct direction. The error amplifiers output pulse is either negative or positive voltage showing the difference between its inputs. The greater the difference is, the greater the voltage is. The output of error amplifier is going to drive the motor. If positive, the motor will turn in one direction or it will turn to another direction. This allows the error amplifier to narrow the difference between its inputs to let the RC servo motor to run the commanded position.

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