gapple12

I wanted to find out the difference between a regular or analog servo to a digital servo. I am moving up to large scale airplanes and am wondering the difference between them. Do you have to program the digital servos or do they work like regular ones? Thanks

rajul

My Feedback: (58)

I'm not sure if the jpg file will turn out well, but if you send me your email, I could send you the pdf file. It's a pretty good comparison of digital and analog servos

BarracudaHockey

My Feedback: (11)

http://www.futaba-rc.com/servos/digitalservos.pdf

There's the pdf.

Hitec's are programmable but its not really needed, from a user perspective they operate just like any other servo, they just do it better and consume a bit more power.

gapple12

Wow, that helps a lot, thanks for the reply...............

gapple12

One other question, PCM and PPM, what is the difference between them? Can you only use the digital servos with one of these features? Thanks for your help.....

BarracudaHockey

My Feedback: (11)

For the most part the servos don't know or care what type of modulation is used.

PCM digitally encodes each stick position then tags the frame with a check sum, if the reciever sees a good check sum a signal is passed to the servo, if not it drops it and goes on to the next one, enough bad frames and the servos either hold position or go to a predetermined fail safe position, this is known as pcm lockout. As soon as a good frame is recieved normal operation resumes. Since there's no variation in the signal, its either on (a 1) or off (a zero) there's no "glitching"

A ppm signal is analog, more susceptable to interference but it works well enough for most uses. Electric guys, those with electronic igntiion gas engines and helicopter with alot of moving metal tend to prefer PCM, though I've used ppm w/o issues in all 3 instances, so its up to you.

gapple12

Thanks BARRACUDAHOCKEY, great reply, helps a bunch. THANKS

eegeek

I don't totallyagree with what this article said. "Digital Servos Are The Only Solution! "<o></o>

<o></o>

No Way!!

The real reason why the digital servo has betteraccuracy, higher torque, and faster response is not due to the superiority of digital performance. It is due to the frequency at whichthe servois operating at. The analog servo runs 50Hz and the Digital servo runs 300Hz, so of course the analog servo wouldbeless accurate.The Analog servo wouldperform just the same as the Digital Servo if it were running at the same300Hz. The only difference would be thatthe analog servo wouldneed fewer parts, making it a cheaper solution.

BarracudaHockey

My Feedback: (11)

Most receivers only output 50 hz

eegeek

Yes the signal is sent 50Hz or 50 times per second, but that doesn’t mean that the servo can correct itself only 50 times per second. The digital servo receives information to it at 50Hz as well, but it corrects itself at 300Hz. If you want even better accuracy and faster response, all you have to do is keep on increasing the frequency at which the servo corrects itself ( through digital or analog means). The main downside is that there will be more switching losses and more losses through what is called the skin affect in the inductor (or motor). In other words, the servo will be consuming more power.

dirtybird

My Feedback: (5)

The frame rate of the transmitter determines the 50 cps update rate. With 72 Mhz radios there is no way to change that. They are limited by the bandwidth requirements.
With 2.4ghz it would be possible to increase the frame rate quite a bit but that would mean a redesigned servo that would respond to a much shorter pulse. You would then not be able to use it with older radios. Its not going to happen

JOHNS3D

My Feedback: (5)

you can use a programmer in a few analog servos but you cant do much on them...the digis are great and of course will cost you a lot of money(depends of which one), but i think you need to know if is necessary to get one for your airplane or not.... some big airplanes need it, some small planes doesnt.

BarracudaHockey

My Feedback: (11)

Actually it already has. There's several gyros and servos that are 760us center pulse.

dirtybird

My Feedback: (5)

760 us is only about 1/2 the current standard. That is quite a ways from a digital servo center pulse.
Even so you wont be able to use it with standard equipment.
I would think if the are going to make a non standard servo they should go at least 10 times better. Then you are going to have to redesign the transmission system to match it. It would take a brave manufacturer to go off alone like that. We had people that were happy to do such things when this hobby begin. They don't seem to be around anymore. The people that design our current systems are not known for innovation.

wantboost

do they mark them digital on the outside or u know by the model number?

Highflight

My Feedback: (4)

One more thing to add to the mix...

There's no doubting that the hobby is growing exponentially (pun intended ) toward 2.4ghz systems. And with the known voltage limitations that can result in drop-out of 2.4ghz receivers, most of what I read from the manufacturers suggests that we are all going to 6v RX packs as opposed to the common 4.8v RX packs in earlier 72mhz days.

Then I've come across some references that digital servos, many of them anyway, can't handle the extra voltage without damage.

So where does that leave us with having to balance our use of digital servos with the higher voltage of today's RX battery packs?

BarracudaHockey

My Feedback: (11)

6v wont hurt most digital servos. High frame rate devices such as gyros and tail rotor servos use a diode step down in line with that particular servo.

The chopper guys (the high end ones) are running 8v to their servos now.

The answer to your question though, is a regulator if your servos can't run at the 7v or so a 5 cell pack comes off the charger at.

Cliffmandude

BarracudaHockey

My Feedback: (11)

Thats not true. Digital servos produce full torque nearly instantly whereas analog servos are almost at the end of their movement before they reach full torque. What this means is the holding power of a digital servo is much better. They also center more acurately.

Keep in mind there's crap digital servos and very good analog servos. Also you'd need a plane or heli capable of seeing the advantage. Bombing around in a 40 size trainer you might never notice the difference.

Put them in a high performance airplane and the difference is like night and day.

onewasp

_________________________________

As "BarracudaHockey" has pointed out you really can't get much further from the truth than that which you have posted.

OldNick

A helpful reply indeed.

You can't argue with Cliffmandude. From what they gathered means that their statement is irrefutable.

Anyhoo....I was reading the thread and until Barracudahockey posted that last post I was pretty well gathering the same thing and Cliffmandude.

Sorry, but I was reading the thread to learn. I really did see abpout what CMD saw, until BH's last post (for which I am grateful). So your reply annoyed me a bit.

onewasp

It said what I wanted it to say.
BTW, you are on a very old thread.

If you were annoyed,.then you were annoyed.........soooo ?????.
It happens often in these various threads.

JNorton

My Feedback: (2)

The key is holding power at rest. Speed and torque are defined terms, the same for analog or digital servos. Where and how they are applied are not. BarracudaHockey and Onewasp are correct.
John

Chilll

I found this here, http://www.rchelicopterfun.com/rc-servos.html

UNDERSTANDING RC SERVOS
DIGITAL, ANALOG
CORELESS, BRUSHLESS


As we have briefly discussed, RC servos convert electrical commands from the receiver back into movement. A servo simply plugs into a specific receiver channel and is used to move that specific part of the RC model. This movement is proportional meaning that the servo will only move as much as the transmitter stick on your radio is moved.


Servo Basics

All RC servos have a three wire connector. One wire supplies positive DC voltage – usually 5 to 6 volts. The second wire is for voltage ground, and the third wire is the signal wire. The receiver “talks” to the servo through this wire by means of a simple on/off pulsed signal.

Sizes

Servos basically come in 3 different sizes (micro, standard, and giant or 1/4 scale) to accommodate the type of RC models they are being used in. There are slight variations depending on the specific application but for ease of explanation, these 3 sizes cover most of the RC servos out there.

Speed and Torque Ratings

Other than physical size, the next item that all RC servo specifications indicate is speed and torque.

Speed is a measurement of the time it takes the servo to rotate a certain number of degrees. This has been standardized in most specifications to 60 degrees; In other words, the time it takes the servo wheel to turn 60°. The smaller the number, the faster the servo is.

For example a 0.12 sec/60° servo rating means it will take 0.12 seconds to rotate the servo arm or wheel 60°. This would be twice as fast as a standard speed servo that is rated in the 0.24 sec/60° range. A RC helicopter tail rotor specific servo will have speeds as fast as 0.06 sec/60°.

Torque determines the maximum amount of rotational force the servo can apply. This specification is measured in ounces per inch (oz-in) or in kilograms per centimeter (kg-cm). The larger the number, the more force the servo can exert. A typical standard servo will have a torque rating around 40 oz-in. A high torque specific servo can have torque values well over 200 oz-in.

So what exactly does 40 ounce-inches mean?

Well if you had a servo arm that was one inch long on your servo it would be able to produce 40 ounces of pull or push force at the end of the servo arm before stalling. If you had a 1/2 inch servo arm what do you think the force would be? Yup, 80 ounces of force. How about a 2 inch arm, 20 ounces of force - easy huh?

I should also point out that both speed and torque specifications are usually given for the two common voltages used for receiver battery packs. 4.8 volts for a 4 cell battery pack and 6.0 volts for a 5 cell battery pack. This also translates over to the typical BEC's or voltage regulator outputs if that is how you power your on board electronics. Obviously the 6.0 volt packs give slightly higher speed and torque ratings.

Even higher voltage servos are starting to make their way into the market with ratings up to 8.6 volts. These servos offer even more speed and torque and will continue to grow in popularity as 2S LiPo RX battery packs become more and more popular so no voltage regulator will be required. Assuming of course your receiver will operate at these higher voltages but most of today's 2.4 GhZ receivers are able to handle it.

The limiting voltage factor in the RC heli world is generally the Gyro and or gyro servo, many of which are designed to operate at no more than 5 volts; but as I said, that trend is changing and it seems every new gyro/tail servo or electronic flybar system are now rated at 6 volts.

Digital Servos vs. Analog Servos

Now onto the real meat and potatoes of this RC servo discussion.

Up to just a few years ago, the only RC servos available were analog, but now we have digital. To answer the question of which is better for RC helicopters or planes and cars for that matter – let’s look at how each work and the choice will be pretty obvious.

First off, there is no physical or main component difference between a digital servo or analog servo. The servo case, motor, gears, and even the feed back potentiometer all have the same functions and operations in both types.

The difference between the two is in how the signal from the receiver is processed and how this information is used to send power to the servo motor.

Analog Servo Operation

An analog RC servo controls the speed of the motor by applying on and off voltage signals or pulses to the motor. This voltage is constant (the voltage of the receiver battery pack, voltage regular, or BEC to be exact - 4.8 to 6.0 volts).

This on off frequency is standardized to 50 cycles a second. The longer each on pulse is, the faster the motor turns and the more torque it produces.

This is the same way the speeds of most motors are controlled. For instance, if you have a ceiling or exhaust fan in your house that is controlled with a variable rotary dial speed switch; the fan motor is not given lower and higher voltages to adjust the speed.

The speed switch simply cycles the 120 volts to the fan motor on and off many times a second. The longer each on pulse is, the faster the fan runs. This is also the same way electronic speed controllers for electric RC helicopters , planes, cars, and boats work.

Now back to our analog RC servo. At rest, there is no voltage going to the motor. If a small transmitter command is given or some external pressure is applied to the servo horn forcing it off neutral, a short duration voltage pulse will be sent to the motor.

The larger the stick movement or potentiometer movement, the longer this "on" pulse will be in order to move the servo quickly to the desired position.

Remember me saying that these voltage pulses are sent 50 times a second. This means that in one second, there are 50 windows that last 20 milliseconds each (50x20 = 1000 ms = 1 second). The longer each on voltage pulse is in each of these fifty 20 millisecond windows, the faster the servo motor turns and the more torque it produces.

I just included this info for those of you who really want to know what makes a servo tick. You don’t have to understand all that however, as long as you understand that during small amounts of stick movement or when external forces are applied forcing the servo off its neutral or holding position; only a short duration voltage pulse will be sent to the servo motor every 20 milliseconds. With large stick movements, a long voltage pulse will be sent every 20 milliseconds to the servo motor.

As you can imagine, a short power pulse every 20 milliseconds doesn’t get the motor turning that quickly or allow it enough time to produce much torque. This is the problem with all analog servos; they don’t react fast or produce much torque when given small movement commands or when external forces are trying to push them off their holding position. This area of slow sluggish response and torque is called deadband.

Much of RC control, especially with RC helicopters is done with small quick stick movements moving the servo back and forth in very small increments. There are also many changing loads on the rotor system (both main and tail) that are always trying to force the servo off its hold position as well. Don’t forget about the gyro either. The new heading hold gyros or electronic flybarless systems are sending hundreds of small correction changes to the RC servos every second.

If I did a good job at explaining all this, you should realize by now that much of RC helicopter control and movement actually happens within the deadband area of an analog servo.

This is not really that big of deal for slow human response times, but as I mentioned, a problem for lightning fast gyros and electronic flybarless systems or advanced 3D pilots with cat like reflexes.

Digital RC Servo Operation

Digital servos to the rescue! Like I said before, a digital servo has all the same parts as an analog servo, even the three wire plug that plugs into the receiver is the same. The difference is in how the pulsed signals are sent to the servo motor.

A small microprocessor inside the servo analyzes the receiver signals and processes these into very high frequency voltage pulses to the servo motor. Instead of 50 pulses per second, the motor will now receive upwards of 300 pulses per second. The pulses will be shorter in length of course, but with so many voltage pulses occurring, the motor will speed up much quicker and provide constant torque.

Incidentally, if you have ever wondered why digital servos "sing" when very light force loads are placed on them, what you are hearing is the short high frequency voltage pulses acting on the motor.

The result is a servo that has a much smaller deadband, faster response, quicker and smoother acceleration, and better holding power. You can test this very easily by plugging in a digital servo and an analog servo to your receiver. Try to turn the servo wheel off centre on the analog RC servo.

Notice how you will be able to move it slightly before the servo starts to respond and resist the force - it feels a bit spongy.

Now do the same thing with the digital RC servo. It feels like the servo wheel and shaft are glued to the case – it responds that fast and holds that well.

Now nothing is perfect and this increase in speed, torque, and holding power does come with a small disadvantage. Power Consumption!

Yup, digital servos are power hungry. All those hundreds of power pulses per second use up more battery power than an analog servo would. This really is not that much of a problem these days since battery packs have at least double or triple the capacity of what the same size/weight pack had just a few years ago.

So yes, digital RC servos are much better than analog. You can still fly a RC helicopter with analog servos, but once you switch over to digital, you will likely never go back. Don’t forget about that heading hold gyro or electronic flybarless systems – they need to be paired with fast digital servos to work correctly – no exceptions!

One last point I should clarify with this whole analog/digital RC servo discussion. Remember those speed and torque specifications I talked about earlier... You will find analog servos that have better speed and torque ratings than some digital servos; so why not get one of them over a more expensive slower digital?

Remember, the analog servo is slow to respond and provides little torque during small, fast command inputs. Those good looking specifications are given at full stick movement when the servo has ramped up to full speed and torque. The slower spec digital servo in this case will still provide much more speed and torque where it is needed most.

Coreless & Brushless Servo Motors

Most low cost and standard servos (analog or digital) use what is called a 3 pole electric motor. This is just a standard 3-pole wire wound DC motor – the most common type of DC motor in existence. One step up from the 3 pole is the 5 pole servo motor. As you can imagine, two more wire windings will give a 5 pole motor quicker acceleration and more torque on start up.

You know by now the faster the servo ramps up to speed and the more torque it produces; the better it is for most applications. Well, improving the electric motor itself will produce more speed and torque too; coupled with digital technology, the resultant speed and torque are indeed impressive.

Coreless Servo Motors:

A standard 3-pole wire wound servo motor uses a steel core with wires wound around the core, this core is then surrounded by permanent magnets.

As you can imagine, the core and all that wire weighs a fair bit. When voltage is applied to turn the motor, it has to first overcome this weight to get things turning – it is slow to accelerate. Once up to speed, it also continues to turn for a while when the voltage is removed – it is slow to decelerate.

In a Coreless design, the heavy steel core is eliminated by using a wire mesh that spins around the outside of the magnets. This design is much lighter resulting in quicker acceleration and deceleration. The result is smoother operation, more available torque, and faster response time.

Brushless Servo Motors

The latest advancement is to use a small brushless DC motor in the servo. This is the exact same principle and has the same advantages as larger brushless DC motors that are used in electric RC helis, planes, and cars. There are no brushes that add drag or can wear out.

Brushless motors are more efficient, provide more power, speed, and acceleration - once again raising the bar on the level of speed, torque, response time, and smooth operation.

RC Servo Bearings, Metal Gears, & Water Resistance

These last few points are pretty basic.

Bearings:

You will notice when servo shopping, many specifications list if the servo has bearings and the number of bearings – usually 1 or 2. These bearings are used on the main servo output shaft instead of a simple bushing.

The advantages of having ball bearings on the output shaft in a servo are pretty much the same as I talked about in the Bearing Section of best RC helicopter features – less friction and slop. Because of the many vibrations and mechanical loads placed on the servo in RC helicopters, non bearing servos do develop slop very quickly and of course this makes control more sluggish and vague feeling.

Most quality RC servos, even lower cost ones these days will come with at least one bearing – this will be located on the servo output shaft where it exits the servo, this is where most of the side loads will occur. Better more expensive servos will use two bearings to further improve overall slop free performance.

Metal Gears & Metal Output Shafts:

With today's high torque and speedy digital servos, metal gears and output shafts are getting more and more common. They are a popular choice for several reasons, but strength is the obvious one. There are two downsides to metal gears however. They weight a little more than plastic or nylon gears and they wear out a little faster. Most servos have replaceable gear sets so you can easily replace the gears. The best metal gear servo's on the market these days are using titanium gear sets and this drops the weight down quite a bit. Water Resistant/Dust Proof RC Servos

Some servos are sealed to prevent water and dust from seeping inside. The case halves have gaskets, there is an o-ring around the output shaft, and there is silicone sealant where the wires exit the servo case.

Now this is obviously a good servo feature for RC boating or for your monster truck when you drive it through the old mud hole. It is however unlikely you will be flying your heli under the water, in the mud, or in a dust storm. If you are, you have bigger problems to deal with than contaminated servos.

That said - water resistant/dust proof RC servos do make sense with nitro RC helicopters or any nitro model that exposes the servo to nitro goo . If your RC servos are coated in nitro goo, it will eventually seep inside the servo. This won’t be a problem for the gears, but will play havoc with the circuit board and electric motor over time.