How RC servos work

Your Reviewer

My Credentials
So who's doing this reviewing then?

Well I've been building and flying or driving radio controlled models for over 40 years and during that time I like to think I've built up a reasonable amount of knowledge.

I'm also a qualified electronics engineer who has worked in radio frequency, analog, digital systems and software for more than three decades. In fact I designed and built my first RC set back in 1969.

For the past nine years I've also been involved in the design and manufacture of some rather sophisticated engine technology and UAV flight control systems.

So, chances are I've been there, done that and have a huge pile of tee shirts to prove it.

Right now I'm heavily into 3D flying and enjoy all aspects of the RC hobby. I may be old but I don't feel it.

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How do RC Servos Work?

THE BASICS OF RC SERVO OPERATION

Pots and Amplifiers

Inside every servo is a tiny circuit board that contains a bunch of components.

It is the job of this circuit (which is called an amplifier) to convert the signal from the receiver into a signal that drives the servo's motor to position the output arm to the requested position.

Way-back, when proportional RC gear was first developed, there was only one kind of servo amplifier: the analog amp, but today we also have digital versions.

Standard/analog Servo Amplifiers

Modern receivers send a series of pulses to each servo. Those pulses vary in width from about 1 thousanth of a second (1mS) to two thousandths of a second (2mS) -- with the center-point being around 1.5mS.

These pulses are sent at a rate of about 50 per second and every time a pulse arrives in a standard/analog servo, the amplifier checks to see if the servo's output arm needs to be moved one way or the other.

If the amplifier decides that the servo arm does need moving because the transmitter stick has been moved then it sends a short burst of power to the motor in order to rotate the gears and (ultimately) the output.

For most applications, this works just fine but since the servo motor isn't being driven continuously (only for a moment every time a new pulse is sent from the receiver), the full torque potential and speed of the servo isn't fully realized.

Another issue with standard servos is that the torque tends to drop off quite dramatically as the difference between the requested position and actual position of the output arm gets smaller. In fact, when this diference is very small, the torque of the servo be insufficient to move the arm against a slightly binding linkage and the result will be a buzzing noise.

Digital Servo Amplifiers

Since the standard/analog servo amp was designed, electronics have moved on significantly and now manufacturers can put tiny computer chips called microcontrollers in servos.

These little computers can provide significantly improved speed, torque and accuracy.

They do this by allowing the servo's motor to be driven far more frequently than was the case before.

Instead of only driving the motor each time a pulse arrives from the receiver (a mere 50 times per second), they effectively remember the length of the pulse and then drive the motor almost continuously (or at a much higher frequency).

The result is that the motor produces more torque and can accelerate/stop more quickly.

Digital servos are often easily identified when running because of the different sound they make as a result of this increased motor-drive. Hitec digitals will "sing" at a high frequency and some others like Futaba and JR will "growl".

Which is best?

Clearly, because of their greater torque, accuracy and speed, digital servos are usually superior to standard servos but in many cases, such as sport models, that extra performance might not be worth the extra price.

Feedback Pots

So how does a servo know exactly where its output arm is so that it can command the motor to move it to the position commanded by the transmiter stick?

Well that's the job of the feedback potentiometer ("pot" for short).

The pot is just a tiny version of the volume control knob on older-type radios and TV sets. It's a variable resistor which can be used to create a voltage that changes as the servo's output arm moves.

That voltage can then be used by the servo amp to work out the exact position of the arm and decide whether it needs moving and if so, which way to drive the motor.

Good servos use high quality pots, cheap servos tend to use inferior ones and the quality of the feedback pot is very important to the accuracy and reliablity of a servo. When a pot becomes worn or dirty, the servo can jitter and become erratic in movement. Cheap pots may also be adversely affected by high-vibration environments.

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