I Think I’ll Just Wing It. Pt.6 – Servos 101

Servos. They articulate movements in assembly lines, robots, satellites, radio control, planes, cars, boats, helicopters and about anything you can imagine that needs something moved remotely. The Latin translation is “slave” and that is essentially their function.

A Size For Every Application.
I have always thought the word Servo sounded like a breakfast cereal.  New Nabisco Serve-O’s now with fortified teeth decaying sugar. The cranky hobbyists didn’t like my sense of humor when they apprehensively asked for the correct “server” for their project when I was in the hobby shop. I would respond by asking them if they wanted Pentium or AMD. Maybe I do not know a good way to help a brother out to stop calling something the wrong name but I would let them know it was a “Servo” trying hard to not be the know it all ass-hat hobby guy but sometimes all good intentions…

Some common park, and indoor flyer servos

My intentions really were to fill gaps in knowledge about what servo to buy and that brings me to the purpose of this post, if you ever have found yourself looking for a servo or may in the future I hope to leave you with something to work with.

What you get when you buy a servo, a variety of control horns and screws
 Arms, Horns, Wheels and rubber dampers to keep nitro engine vibration away from the servo

Generally kits, plans and ready to go RC stuff will have some reference to what servo you might need for a Radio Control project be it air, and sea or robot. There might be a size requirement, torque or speed needed and as you will learn there are multitudes and combinations of all those items.

This is a continuous 360-degree servo with a geared wheel for some type of robotic use. Notice the brand “Fitec”? The biggest servo company is “Hitec” and right here is a great example of why to not trust equipment or their claims when it is evident they are trying to free ride on a namesake

To start let us look at some servo facts:

Most servos move 180 degrees but there are others that move 360 degrees and ones with a continuous movement that are for winding up a cord on a sailboat’s mainsail. there are special movement ones for Aircraft with retracts. Robot hobbyists also have some that are specific to their uses.

The servo cases are made from plastic and in some cases metal with a built-in heat sink to dissipate the motor heat. Some servos are available as waterproof.

Here is a servo with the bottom half of the case made from aluminum to dissipate heat from it operating

Servos have 3 wires consisting of a +, –  DC voltage and signal wire. One end of this has a 3 conductor plug that plugs into the receiver or robot servo board. The voltage is supplied here as well and it is typically 4.8 Volts DC up to 7.2 Volts DC on some select servos. Always check manufactures voltages to be sure so you know what voltage to supply it.

Red is +voltage Brown is – Voltage and yellow-orange is the signal wire

Another thing to note is that manufacturers like to make you guess, hook the servo up backward and buy another one.  So watch the colors and read the little paper in 50  languages that came with the servo. Unless they have red and black with some other color you can usually trust red is + and black is –

The wiring connects to a control board inside the servo and it controls the motor via the pulses received through the signal wire. The motor is geared to reduce the RPM of the motor at the output shaft of the servo where the servo control horn connects.

Here you can see the internal gears, this is one of the servos I will use
On the bottom inside is the electronics that control the motor, this is an analog servo

Higher duty servos are available with metal gears and most decent servo manufactures sell internal replacement gears. There are some with carbon composite gears lighter than metal and Delrin with strength in between the two.

You can see the Metal output gears here and one has a metal case. These are both examples of a low profile servo where you might need space below where the servo mounts. Notice the black one has a Brushless motor this is more efficient motor providing serious torque and speed.

The servo output shaft is splined so servo horn will lock in place when pressed on and it followed up with a screw in the center to hold it on. Be aware manufactures all have different splines there is no standard. Makes me mad people new to the hobby will struggle with this issue since manufacturers do not make this very clear.

Shaft Splines
Adjustable Servo Horn
Aftermarket servo control horns or arms, very stiff aluminum sometimes with machine thread, great for cars and helicopters

There are two types of servos, Analog and digital. I will try and explain the difference.

First, both types receive a signal that is a PWM or Pulse Width Modulation.  

Every 20 milliseconds a pulse is sent to the servo to tell it what position the servo should be in to match what you are doing with the sticks or controller via radio waves. So if you are moving the radio stick control you set up to control the rudder and you want it to move right the radio sends pulses in those 20-millisecond window frame to the servo to move it. The PWM frequency is 50hz. A longer pulse moves the motor farther but the transit time to get there is limited by that 50hz.

Think of a ceiling fan, especially the variable ones.  You turn it on and it takes a moment to spool up to speed, or if is variable there is a moment to see the increased speed you requested.  This situation can be bad in servos. You give input to stick, the frame rate is 50hz, the servo responds and moves but the amount of power or torque it can have is limited because it doesn’t “spool up”.  The feel at the sticks is spongy.

Here are the 180 degrees of travel, the splines are offset so you never get it dead center and equal points on each side but no worries the radio transmitter takes care of centering this up

So what if we increase that frequency by changing the tiny electronics inside the servo, change the circuit from analog to digital. Take out the transistors and replace them with the logic chips and small processors. Now the frequency is 300hz and the speed is increased along with a new ability to develop the needed torque. The mechanics are the same but we just improved it by talking to it faster.

So now you are thinking why would you ever use an analog servo since digital is so much better, well cost is one big factor the second is they just are not needed in most cases.

I’ll give some examples:

The plane I am building is fine with analog servos, it is a foam plane and while I intend on making it rigid there is going to be some flex in the plane and the mechanics. It is a fun park flyer, not a precise pattern plane or competition 3d plane. I don’t need overly snappy controls and the added expense would be lost on this airframe.

If I was building a helicopter I would not attempt it without digital servos and some really fast ones. The tail collective pitch the main head and the collective pitch, I want instant control as fast as possible input and transit time. I would also opt for metal gear servos which are more resistant to stripping.

RC Monster or stadium trucks, including buggies I would want some serious torque for the steering servo to move those giant tires around same with a rock crawler. Metal gears are a must here. Nitro vehicles through an analog cheap servo are just fine for the throttle control.

On aircraft, the name of the game is weight and weight to thrust ratio so keep the servos light but with enough balls to move a rudder or elevator under a load of flying.  There is a lot of air going over those surfaces adding forces on them the servo has to work against to move it or in some cases resist moving it too far. Nitro planes have no need for digital control of the throttle just like the land vehicles.

With all this said when it comes to choosing a servo:

  1. Consider the physical size.
  2. Speed
  3. Voltage (see the torque and speed increase with voltage increase on some models)
  4. Tourque

Take a look here: Servo City

You can see just one company has a ton of offerings. The servo I am using is comparable to the Hitec HS-55 on that page. Now let’s get one thing straight Spec reading is misleading. In the past, I have weighed parts, ran a test with watt meters on the current drain and all I can say is consider them rough guidelines. For instance, some companies weigh the servo without wires and plug, with or without control horn and the screw that holds it on.  So in an attempt to market it as lighter, it is the same and even heavier in some cases after the required hardware and wiring is added to use it. The other thing I have found is a servo advertised as metal with only one metal gear.

When it comes time to program the transmitter I will go into some more detail but a few things to know once it is all set up.

Here is a high-quality servo from Hitec notice the ball bearing where the output gear exits the case with titanium and brass gears

In the programming of most modern transmitters, there is the ability to reverse the direction of the servo, set the endpoint limit of travel each way and reduce the overall travel. There is also a function called exponential, this is a way to achieve a different feel of response and also tame down something a bit too lively. Let’s say you set your exponential at 80% now the center of the stick will be dead in the first 20% of its movement and this keeps you from twitching and overcorrecting. If it was set at 10% you would have to almost bury the stick one way to get the servo to move. The other function common on transmitters is “Dual Rate” or even Triple rates meaning a switch on the radio can be set up to have more limited movement of the travel. You may be able to get a huge amount of deflection of your rudder or elevator but do you need that much? Set up for lower rates is a great idea to make that first take off so you don’t stall or drive it into the ground right off. Once in the air, you can flip the switch for full rates and go all Chuck Yeager.

I try to get what is called “Three Mistakes High” meaning get up at least 100 feet or so in the air before you summon your inner Top Gun and you have some time before you meet terra firma to correct.

Radio brands have some different ways and nomenclature of going about these functions so always RTFM(Read The F%@**ing Manual). on the radio.

With all this talk of analog and digital, I don’t want to leave out the good old mechanical advantage of how the servo is installed.

 If the pushrod is attached to hole 1 the servo will move the least to move the surface the most

Typically Servo control horns and surface control horns have multiple holes to attach the pushrod. In basic terms at the servo the farther out the hole is from the output of the servo the more leverage you have, and at the control surface, the opposite is true. There are limits to this, of course as the horn moves it is at an arc and at some point, the lever arm can get too long. Also at the surface control horn if is too close to the surface itself and needs a big movement the pushrod can run into the surface itself bending to try and push it further.

More times than not when I have encountered a servo not doing what it is supposed to do it was the mechanical setup of pushrod and linkages. Friction and any loss of rigidity in the pushrod is the enemy.

I couldn’t make a small post on this subject I know it is a lot to read but I hope I shed some light on a device used a lot in a lot of places in our hobbies. I am going to finish this off with an example of another use of a servo on a little robot I built using Arduino, some stepper motor drivers, one servo and an ultrasonic sensor.

– Hobby Dude


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