Servos 101

by FliteTest | April 22, 2015 | (29) Posted in Tips

WHAT IS A SERVO?

A servo (or servomotor) in the R/C industry is a small mechinism that turns the radio waves from your transmitter into kenetic energy to control your craft. 

They come in all shapes, sizes, brands, weights, types, and prices. We hope that by the end of this episode, you'll have the confidence to know exactly what kind of servo you need for your project. 

SIZES

When shoping for classes of servos you'll notice them labeled by grams. But what does that gram measure?

The gram is the weight of the servo itself (minus the wire) Here is a typical everyday 9-gram servo on a scale. We got a reading of 8 grams (probably rounded down).

This applies to 5-gram, 2.7-gram, and so on. 

TYPES

Above you will see a liner actuator servo. Instead of applying rotational motion these only move in a linear fashion. 

The reason you'll find these on a lot of smaller models is because they weigh significantly less than a standard servo. 

Blow that up a bit and you'll find something like this! This is another style of liner servo that trades speed for power.

If you took look inside you would find a threaded screw that is turned by a motor. Depending on the rotation, the rod either extends or retracts. 

THE GUTS

If you remove the bottom plate on a servo you'll see the control board.

This is the brain of your servo. The little black knob looking thing is a potentiometer (or pot). A potentiometer measures the variable resistance in voltage. A more common example of a pot would be the volume knob on your car stereo. 

This measurement information is send back to the board which then tells the servo which direction to turn, how fast to turn, where to center, etc. 

Remove the top...

...and you'll see the gear set. This is where the lack of torque is compensated by gearing up that energy. The motor can still rotate quickly and the gears do all the heavy lifting. 

On some models, underneath the top plate you'll see a bearing. This smooths out the rotation just like you would imagine.

If you ever see 'dual bearing' servos, that means exactly what it sounds like. There is the addition of another bearing underneath the gear shaft. 

The gears on most servos are made of nylon. Nylon has some self lubricating properties (although most companies still add a bit more just for insurance).

Because it is still a synthetic polymer it will never be as strong as titanium or metal gear servos, but will never be as expensive.

If you're trying to decide on wether to buy metal gear or not, it all depends on the project. If you are building a precision 3D plane or a tail mechinism for a tricopter, a metal gear is going to hold up to the pressure required for those designs.

For our foamies on the other hand, nylon gears work wonderfully and are dirt cheap! 

STANDARDIZATION

By now you would assume that all brands of servos have the same rules. This is ALMOST true. There are a few things that are unique to certain models and not all are created equal. 

As you see above, the splines are different sizes depending on which company you buy from. This servo arm does not line up with the horn. As a rule if you stick with the same company, all should fit perfectly. 

The same applys to servo wires. All models should have a signal wire, a positive, and a ground. If you look a bit closer the connector on the right has a little tooth sticking out on the side. 

This is commonly known as a Futaba style connector while the left is known as universal. To be honest there is no difference other than that little tab. If you decided to sand it off the connectors would be identical. 

SERVO MOTORS

Servo motors usually come in two flavors. Coreless and cored. These terms are pretty self explanitory, but what does that mean for performance?

The smaller motor is coreless. It has much less mass which give it the ability to react faster and weigh less. If you looked down the copper tube you would see that its hollow. 

This does not mean that coreless is better! This falls into the category of your projects needs. If you need razor sharp precision a coreless will give it to you. If you want more torque and power a cored will do the job. 

PULSE WIDTH MODULATION

How does a servo know how to do what you tell it? Well a servo knows how to speak electronic signal. The only way to really make sentences with an electronic signal is to use pulses. Think Morse code!

The amount of pulses being sent to the servo tell it what position to be in. 

If the pulses are coming in around 1 millisecond (or 1000 microseconds) it knows to rotate X degrees to the 1000us position. 

When you modulate that signal to something else it recognizes the change in pulse and knows to move to whatever position it hears. 

The direction in which it rotates depends on who you buy from! Rotation in not standardized between brands. 

TORQUE

If you look up the specs for servos you'll see torque measured in ounces per inch (oz-in). What does that mean?

This is one thing that IS somewhat standard between brands. To find the oz-in you measure out one inch from the center of the servo. However much pressure is applied from this point, in ounces, gives you your measurement.

Here we see that the servo is applying 3lb 4.7oz from a HALF inch. 

The mean weight applied between a few different servos of the same brand came out to 3lb 3.2oz. Divided by 2 to make up for that half inch from center, you'll find 25.6 oz-in.

The listed oz-in from this particular brand was 24.5 oz-in so it actually performs better than advertised! 

BROWN OUT

Have you ever been flying your plane a little too hard and had a loss of signal? The go-to excuse usually is, "It's my radios fault!" 

We say before you blame your radio we suggest you investigate into how much power you're drawing from your servos.

If you've never experienced a brown out, this is when the total milliamps being drawn from the servos is too much for your ESC's BEC to handle. The BEC then shuts down and will try and reboot, which effectively turns your plane off.

As an example, we hooked up way too many servos into this orange receiver. Keep an eye on the LED inside the receiver. 

Lights out! No light means no signal.

This can be avoided with some simple arithmetic. Find out how many milliamps your servos need and how much your reciever is rated for. This way you'll never overwork your gear. 

ANALOG VS. DIGITAL

By now you know how a servo works, how it talks to your radio, how they are measured; so the last decision you'll probably make is between digital and analog. What's the difference?

The ONLY difference between an analog and digital servo is the brain inside of it. Sans control board, they are identical. 

The control board inside will determine how fast it is able to think and how it can control the servo's motor.

An analong servo for example can make adjustments at around 50 cycles a second. 

A digital servo can make adjustments around 400 cycles a second! They react quicker and are far more accurate. 

THIS DOES NOT MEAN DIGITAL IS BETTER! We can't stress enough that your decisions should be based around what you are building.

You can absolutlely put digital servos onto one of our foamies and it will fly just as well as if it had analog servos. That extra cash you spent really only shows when you are putting it into projects that will truly benefit from the extra performance. 

We hope we helped you understand what all that silly math on the side of the box means!

At the end of the day if you are torn between a few brands, take a step back and ask what your plane really needs.

Don't bother spending a small fortune on the best-of-the-best if you are making a simple design.

On the flip side, don't go cheap if your craft is asking for precision and stability. 

We want to thank Little Bits for sponsoring this episode of Flite Test!

Little Bits are a modular electronic building kit that allows young minds to learn how electronics work and old minds to prototype ideas instantly with no waste!

They were so kind to not only sponsor this episode but provide a discount for our viewers! If you go to littlebits.com/flitetest they are offering $20 off your first kit! 

If you do end up integrating Little Bits into your planes we want to see! Take a picture and upload to social media with the tag #flitetest. 

 

COMMENTS

SP0NZ on April 22, 2015
Great episode guys! I really enjoyed the technical info on servos. I learned a lot. I hope you plan to make more 101 series episodes in the future. 5 stars.
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fishmanpuffer on April 23, 2015
Love the video gave it 5 stars. I wanted Peter to get back to that new magnetic servo from Hobby King and what was the all white plane in the back grounds looked pretty cool you have plans for that one........ also seen in the first brownout pic on the left of Peter
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PropSpinner on April 23, 2015
I think that's the Shinden.
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ChopperChopper on April 23, 2015
Very good boys - clear basic info and the BEST demo of digital versus analog I've seen. Brown Out demo could be an episode in itself! In a future episode you could explain the High Voltage (HV) servos that are becoming more common esp on big Helis.
You promised to come back to the Magnetic Induction Servos (at 2:15) but you didn't - please explain them too. Did it go more than 360 degrees like the old 'sail winch' servo I had in the eighties? Good info, well done.
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Milliester on April 23, 2015
Awesome Flite Tip guys. Peter was really helpful. Now I know a lot more about servos. :) Oh and by the way, what is that plane in the back that looks like the FT Viggen?
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willrocks on April 23, 2015
that is the shinden in the back

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Milliester on April 23, 2015
Ok, thanks
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Milliester on April 23, 2015
I think its coming out next month!!!
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SimplyDoug on April 23, 2015
Great information guys. It is always great to get the background on how it works. I think your heading "Pulse With Modulation" needs to be "Pulse Width Modulation" though.
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FliteTest on April 23, 2015
Good eye :) Thanks!
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CJGFX on April 23, 2015
Great episode guys. Just the right amount of tech info for the average hobbiest without being too confusing. Very useful information to bear in mind for future projects.....
Interesting teaser plane behind Peter, looking forward to seeing that one!!!! I really need to build the Canadair plane hanging behind Eric. They fly over us every summer and my son and I love watching them.
Good job again guys....
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banditjacksRC on April 23, 2015
EXCELLENT Episode! I highly suggest you guys do more episodes like this! Getting into the technical operations of the devices we use everyday. Loved it! Great job!
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Bayboos on April 23, 2015
Excellent, long overdue episode; hopefully the first of many. More of that pls.
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Gryf on April 23, 2015
Brilliant, guys! I really found this one fascinating. How about a 101 on ESCs?

Thanks a bunch,

Gryf
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MEbyGood on April 23, 2015
Nice episode. One IMPORTANT point about digital servos: They use much more current than analog servos of the same size, so it takes fewer of them to brown out a given power supply. It's the price of that extra precision and power. You need a generous BEC or battery to run digitals.
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UncleMoses on April 23, 2015
I had that exact question in mind because I thought I had read something about it somewhere, thanks for answering it.
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mkmitchell on April 23, 2015
Wonderful episode!

Thanks!
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Ano Pilot on April 23, 2015
Excellent, actually useful information and NOT the inane prattling of idiocy and ignorance. I might re-watch this episode a few times. Thanks Peter and Eric.

An episode or series of episodes on GLOW/GAS ENGINES AND PLANE SETUP PLEASE. I have actually added ic to my interests as a result of joining a good club, it looked such fun... or should I say 'awesome'. BUT I have found it difficult to find proper information on getting started. It seems to be assumed that 'everyone' knows. FT's blanket coverage of electric flight has really spoiled me on this one, I just assumed that information about ic aero-modelling would be AS readily available, but not any more.
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PropSpinner on April 23, 2015
Good job! I like your visual aids! 5 stars

Most ESCs and BECs will have in the specs the type and number of servo they'll support. It pays to read the manual!!!! Of course most of us want to get our new creation in the air as fast as possible. 😉😜 Sometimes you have to read those manuals multiple times because they have most likely been translated from some other language and some of the meaning has been lost.
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jrvalentin62 on April 23, 2015
+1 on MORE 101 type episodes. Wealth of knowledge shared. Thanks!!
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UncleMoses on April 23, 2015
Excellent job guys 5 stars, I found this to be a really useful episode as I find myself wondering what servos I should be using on certain projects of mine. Thank you.
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Yogenh on April 23, 2015
I like this a lot and found it very helpful. Now I know more on how to get the servos that I should have.
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Tom123 on April 27, 2015
Ft Foam soon?

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Pancho1 on April 29, 2015
Great video and I will be sharing this with our Civil Air Patrol cadets at our next Aerospace Education lecture.

I have a question regarding the brown out, and maybe it was addressed and I missed it, but say you are able to diagnose your brown-outs as being primarily caused by overloading your servos (and/or too many). What is the solution? My guess would be go to ESC with higher rated BEC but might the next size up servo also work? Seems like if you have brown-outs often, then the problem is probably the BEC component. But, if it only happens rarely, would the next size up servo work? I've not paid much attention to the BEC ratings of my ESC and a very quick look at HobbyKing 30A ESCs shows a variance from 1A-6A of the BEC/UBEC ratings. For most, if not all of my current planes, I'm probably fine - several FT foamies and a few trainers that came with most of the electronics, including the ESCs. I realize that it doesn't mean they are 100% reliable, but they were designed by someone with a lot more experience than me! But, if they do have brown-out problems, I now know of another possible cause and can probably even test for it....which for me would be to put in an ESC with a higher BEC rating. If I was more techie, I could probably set up a bench test with a multi-meter and diagnose it.

Thanks again for the content. You guys are informative and entertaining. There is lots of very educating content out there these days, but so many of the videos are so hard to watch. Obviously, you can do a LOT more on servos, hence the 101 title.
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pressalltheknobs on July 8, 2015
If your servos are causing a brown out, the solutions are..
1. Use less servos

2. Use servos that draw less current. Generally smaller ones use less current. Analog uses less current than digital.

3. Use a lower S battery. Linear BECs get less efficient with a higher input voltage so if you are pushing the plane with a higher s battery, say 3s when it came with a 2s, your BEC may not be able to supply enough current. Its max current may fall from 2 amps to 1 amp and consequently your servos may brown out.

4. Use a Battery Elimination Circuit (BEC) that can supply enough current for the servos you want to use.

The BEC is the regulated power supply circuit that provides the power for the receiver and servos on your plane. Commonly it is built into the motor speed controller (ESC) and can only supply 1 or 2 amps. You can either get an ESC with a higher rated BEC or you can use a separate BEC or you can use a separate 5v or 6v battery (eg 2s LiFe).

Generally Switching BECs provide more reliable power than linear BECs particularly when you are using 3S or greater batteries. Be careful of the terms UBEC or SBEC since they are often meaningless. Look for the terms "linear" or "switching". Linear BECs are generally fine for most small airplanes with 2 to 4 servos and up to 3s batteries. For larger airplanes using higher s batteries you want a switching BEC. For multi rotors there are other considerations so I won't discuss that here.
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ASW92 on April 30, 2015
If you turn on the subtitles automatically generated by YouTube, the result is absolutely hilarious and actually now that I think about it, vulgar at times. Just so those foreigners know that this is a hobby channel, not something else....
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Dirt Pilot on May 1, 2015
Great episode. I always learn a lot from watching your videos.
One thing I noticed, is that the torque doesn't actually vary with the servo arm length. The thrust in the push rod varies but the torque is a constant (see around about the 11:40 mark). Torque=Thrust x distance.
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Servos 101