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Modefo's RC Helicopters . XHELI.COM . Autography FlightPower

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Radio - Servo - Gyro - Gov - Batt > FM or PCM
 
 
Mark Ryder
Administrator
Location: Encino, CA

OK vote your conscience.
09-05-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

Quote 
Absolutely false, a PCM rx is just as prone to interferrence as a PPM rx is. The difference being the PCM rx will ignore a corrupted signal whereas a PPM rx will try to interpret the signal the best it can.


First off PPM doesn't really intepret the signal, that suggests some sort of adaptive process going on, typically they just receive the signal apply a simple filter and feed it out to a 4015 with a suitable clock.

PCM devices generally have an input opamp where the output is the input to the encoder section, the encoder immediately runs a low pass filter on the the incoming analog signal with an active RC filter to eliminate VHF noise from being modulated down to the passband by the switched capacitor filter,.

From the active RC filter the analog singal is converted to a differential signal. from the this point all signal processing is done differentially, this allows processing of an analog signal that is twice the amplitude allowed by other methods, which significantly reduces the noise to both the inverted and non inverted signal paths, advantage is that noise from any power supplies (yes it happens due to some the ICs on the servos feeding a negative pulse back out on the supply line, (certainly futaba servos) unless cancelled by a suitable non IC method or perhaps with a large enough capacitor across the power lines, though the size of it would be unwieldy for most CC apps, which no servos i know of are fitted with, at least in the commerical hobby range)

This is certainly the design of the PCM hardware I use.

I'm not making a mountain out of a molehill either, I'm discussing it.

As for the bots generating metal to metal contact, do you think helis don't do that either, in other words they are equivalent, a heli and a bot are very similar in that they both generate a lot of noise. A bot sitting still or just moving around generally generates a huge amount of RF, the extra RF generated by them in combat is generally significantly less than what they put out by themselves. Since friction does not cause charge imbalance, and generally friction is the biggest side effect going on in combat, in relation to forces and depending on the design , charge imbalance won't significantly increase ( unless the design of the system is such that two nonconducting materials are constantly joining and unjoining between two opponents)

they are also loaded with charge imbalance that causes yet more intererfence, generally even more than the metal 2 metal EMF.

Its not that i don't see the value of it, i just don't think its a good indicator, theres so many other things to go wrong and cause the things you are seeing its too much of a general catchall.. Its about equivalent of using one of those LED bargraph voltage indicators, or an unloaded multimeter to test batteries (at least to me), I can't help it I'm an engineer

On what? Aerial bot competitions having been running for over 10 years. Autonomous heli competitions are all over the place, MIT has been running one for years too.

Battlebots barely scratches the surface.

If you want a system that offers the best of both worlds consider IPD.

Which is what i would choose for the poll, if it were a choice .
09-05-2001 Over year old.
 
 
RappyTappy
Elite Veteran
Location: Las Vegas, NV

What would PCM do?

Since we are talking here, I remeber a time when a friends was flying his raptor around of FM, when all of a sudden it started to go crazy, then he regained control, then it went crazy again. He finally regained control again acted fast and brought it down safely. During the post flight inspection I noticed his antenna was pulled to tightly and was freying right at the receiver. It would make a connection and then when we pulled it and playing with it, the heli would lose signal. Everytime the antena came back together the signal came back to the heli immediately. If it were PCM what would have happened? Would it have gone into failsafe and locked out? I know that if the antena had fully come out that PCM and FM would probably be out of luck, but in this situation what would PCM have done?

Chris
Rex 500
Burning them electrons :D

Forever Brothers
Mickey Tylo
09-05-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

very similar thing probably, the antenna is a key component, likely the signal was being broken intermittently, so depending on how the PCM receiver was setup, it would lockout or hold last good position.

It really depends on the frequency of the break, if it was <1 second(depending on the equipment, times vary) the PCM would filter out the bad signals, so at least the heli should stay fairly normal, doing what it was doing last. if greater, it would hold or go to failsafe positions. So you'd still get some control, it'd be stuttered a little, but we're talking ms.

What would not happen is that the heli would freak out, the servos won't move unless the command is valid, unless the timeout occours and then the failsafe kicks in. Theres not much you can do with PCM failsafe on a heli, throttle down seems a good idea, they gyro should hold the tail, but if you are doing a kaos or a death spiral you're out of luck

Certainly having the choice of trying to counteract the interference is a good thing compared to the lockout of the PCM receiver.

Has anyone ever actually experienced PCM lockout?
09-05-2001 Over year old.
 
 
Dragon2115
Key Veteran
Location: New England

[QUTOE]First off PPM doesn't really intepret...

No PPM isn't applying some AI and making it's own decisions. Maybe interpret was a poor choice of wording.

Great Charlie, we could sit down and compare schematics all day but we would accomplish what? Nothing. Obviously you work with electronics as have I for over 16 years some of which was in receivers and RF fuses.

Without going the nuts and bolts the bottome line is quite simply this, put a bad main shaft bearing in my heli and with PPM I'll know it's there. Do the same with your PCM system and you won't know about until you plow it into the ground because you got locked out in failsafe. I've seen it happen.

Quote 
As for the bots generating...


No, helis shouldn't be making the same kind of metal to metal contact. If they are then something is loose or mounted improperly.

Quote 
they are also loaded with charge imbalance...


Glad you brought this up. You're absolutely correct in this and it is mainly due to poor conductivity throughout the model. Plastic fantastics are known for it.

On a personal note however, my Raptor had a vicious tail kick when I first ran it. Turns out it was the tail drive belt building a static charge and then discharging causing the kick. By detecting the problem I was able to effect a solution by grounding the tailboom. Now the static generated by the drive belt never gets the chance to build to a level where it is going to effect my radio system. If I had masked this by using PCM the problem would still have existed and would have continued until it possibly could have caused damaged to the system.

Quote 
Its not that i don't see the value of it...


We'll just have to agree to disagree on this Charlie. I see no reason to excuse a bad setup by using something that will hide the symptoms from me until it's too late. If you're using PPM and your heli is acting up you've got something wrong, simple as that. Either go find the problem and solve it or prepare to pick up the pieces. Most people are content to focus on the mechanical aspects of their helis yet ignore the electrical aspects. Because they don't understand the problem or how to solve them they're perfectly happy to use something that will hide the problem from them. Hey, if they don't see a problem there must not be a problem right? If a heli is glitching constantly most people would rather spend money on a PCM receiver rather than track down the problem. Iow the quickest solution that requires the least amount of thinking or effort on their part. I see that as a crash waiting to happen.
09-05-2001 Over year old.
HOMEPAGE  
 
 
Dragon2115
Key Veteran
Location: New England

Quote 
Has anyone ever actually experienced PCM lockout?


Yes, I've seen it happen on 4 occassions. One was when the fun fly CD and I were on the same channel. I had a wild flight but landed. He locked out and crashed from 100 feet. Big bill for that SE he balled up.

Two occassions no source was ever identified. The end result was the same in both cases, two crashed helis. The last one was a bad main shaft bearing that had been damaged in a previous crash. The result was the same here too, a crashed heli.

There's a saying about PCM. "At least you can predict where the wreckage will be".
09-05-2001 Over year old.
HOMEPAGE  
 
 
Dragon2115
Key Veteran
Location: New England

And as for the autonomous heli competitions, I'm well aware of them. I just watched the German team kick the snot out of the competition recently on Discovery Channel when they flew over the test fire site, identified two victims, identified the fuel source, and even showed the temperature of the fire. Very impressive. I also fly with a guy who is a pilot Draper Labs in their Autonomous Aircraft program. They're using a Bergin Intrepid I believe. It's a boat with all the gear on it.And yes they have to use PCM because of all the different signals they're transmitting and electronics on the board. This also isn't the normal configuration for one of our model helis.

And this really isn't the same arena as Battlebots when it comes to generating spurious signals due to slamming into one another is it?
09-05-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

It would accomplish my point that PCM receivers have a better signal to noise rejection ratio. You cannot filter an analog PPM signal well, because you cannot do any real predicition, you can with a digitally encoded signal, since you know what the rejection limits are, and how to reshape the signal into a more usable stream, one of the key reasons to switch to a digital signal in any device is for fault tolerance, its amazing how much information you can recover from a bad signal.

Ok answer me this,(unless you don't want to play anymore , i've had four shots of coffee in my starbucks this morning so i'm all up and about, and this is very interesting area to me, debate for me makes me think about things more, learn more, and hopefully generates more knowledge) if PPM allows you to better control the craft during times of intereference, why does every industrial radio control system that i know of use PCM, why does robotic comabt use it?

Sure they might get more interference with PPM from the metal 2 metal and charge imbalance, but if both PPM and PCM suffer exactly the same from interference as you suggest, then why not go with the PPM given as you've said PPM is easier to control under interference than PCM.(since PCM supposedly allows no control (before lockout) )

And if the amount of interference is the same, then PCM would lockout, whereas PPM would not ? Especially given that you can reasonably shield a lot of interference that the device generates.

I would say the engine and bearings should be causing a fair amount of metal to metal contact, certainly dissimilar, temperatures varying (which is where the real problem lies), spinning shafts, servos etc etc.

Certainly when i put my scope on the bird it was very noisy,

I certainly agree with you that the PPM can be used to find fault, as you do, no argument there, i just don't know if i agree that its a good enough reason, or not. But thats by the by really, we all have our ways.

Whats interesting, is that you and I both seem to been on the same track, that its important not to ignore the electronic side of the heli, we just have different methods, all too commonly i hear people telling me how they've done it for N years and never had a problem, then watch as they crash every other week, or more. Wrapping antennas round engines, cutting them up, rolling them into a ball. Switching XTALs willy nilly.

I saw a rep fror a heli company crash his bird after losing radio, and after they were fiddling with the XTALs, but yet claiming it was a radio hit. TWICE!
09-05-2001 Over year old.
 
 
charliex
Heliman
Location:

I'm lost as too why you ask then , if you know about them then why say you hadn't seen them , its a flying bot ?

Quote 
And this really isn't the same arena as Battlebots when it comes to generating spurious signals due to slamming into one another is it?


Do you really think that ? I have to say the RC helis have been the closest thing i've come to in terms of noise and interference, since working with robotic systems , combat or otherwise. I don't think its the spurious signals that matter they are very short in duration, and as i said before are considerably less than the noise generated by the beast itself, at least in most cases. Its the equivalent of the "taik kick" you mention.
09-05-2001 Over year old.
 
 
RappyTappy
Elite Veteran
Location: Las Vegas, NV

I fought radio interference from another radio for 2 min and kept it in the air. Even though the servos are going through erradic phases, I still could control it every now and then, keeping it from going into the pits and cars, etc. Also, I wonder how many people really know about failsafe and how to program it. There was a study done on this at a field where a airplane went into failsafe on takeoff and it was programed wrong. It nosed up and turned right, headed for the pits, stalled out and went into a dive, the engine stayed at takeoff power. It came down and hit a little boy standing in the pits killing him. The AMA did a whole investigation on it and it was determined that more than half of the people who owned PCM radios didn't know how to program or they didn't have too much knowledge about their failsafe at all. Anyways, we are controling these machines with a signal. We lose the signal anytime or it is interupted, suddently our remote control becomes out of control. The best method is to stay away from bystanders and take every precaution assuming that something might go wrong every flight. Also, like I said, choose one(PCM or FM) and become biased about the other Happy Flying

Chris
Rex 500
Burning them electrons :D

Forever Brothers
Mickey Tylo
09-06-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

If the AMA did a study at the basin, there would be more than PCM newbies to worry about =)
09-06-2001 Over year old.
 
 
Dragon2115
Key Veteran
Location: New England

I'll play for awhile longer Charlie. Dinner won't be ready for awhile yet.

[QUOTE] Ok answer me this...

In industrial applications there is a good chance that the system will have to operate in a very electrically noisy environment where there are a lot of spurious signals being generated from various sources that might be as shielded as they're going to get. Loss of signal for even up to a few seconds is usually of less importance than is receiving and acting upon a wrong command and the resulting glitch to the system.

As for Battlebots, we've been through this. They are operating in a high metal to metal impact environment which will generate more spurious signals. Plus, during these impacts there is the very real chance of damaging a filtering cap on a motor or other system which will generate enough noise to effect their system. Take out the impacts and make sure the system is shielded properly and they would run fine on PPM.

[QUOTE]then why not go with the PPM given as you've said PPM is easier to control under interference than PCM.(since PCM supposedly allows no control (before lockout)

I never said there was a guarantee that you'd have better control. You don't know, but something is definitely better than nothing. It also depends on your priorities such as I decribed in industrial applications.

[QUOTE]And if the amount of interference is the same, then PCM would lockout, whereas PPM would not ?

PPM won't lockout unless the interferrence signal completely overpowers the desired signal. But to answer your question, it would depend on the input sensitivity of each receiver. But you knew that already.

[QUOTE] I would say the engine and bearings...

Not necessarily. A good bearing of any reputable quality shouldn't be producing any significant RF. If it is you bought cheap bearings. But you are right, every now and then you will see a model glitching because it's got a bad engine bearing. I've seen this on a plane that had been in storage and the engine bearings had some rust on them.

[QUOTE]Certainly when i put my scope on the bird it was very noisy,

Ah, but referenced to what, earth ground? Is so then of course it's going to appear noisy. If you're referenced to the models battery ground then you've got a noise source and if it's near our band of interest then you've got some work to do.

[QUOTE]I saw a rep fror a heli company crash...

I agree, and the crash I saw due to the bad bearing was a very similar case. The heli was glitching and the owner couldn't find the problem so he installed a PCM receiver. The problem went away or so he thought, until two days later he went into lockout and crashed. During the repair he found the bad bearing and replaced it. When it was fixed he tried using his PPM receiver and the glitch was gone.

[QUOTE]I'm lost as too why you ask then...

Vecause I took what you said as to imply that there were Battlebot style combat between aircraft. You got to admit, that would be a pretty short event. One crash and it's over. I guess deciding the winner would be who had the biggest or least amount of pieces afterwards.
09-06-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

Its a bit early for dinner..

The sticky point for me on the bots, is that if you have a collision/crash where so much RF is being generated a PCM will lockout and you won't have any control? Agreed ? You've then just lost the bout.

Since we all agree given significant noise a for N duration (where as N is a number somewhere around 1s) a PCM receiver will lockout, stopping the bot dead, or continuning where it was going depedant on the setup of the receiver.

Whereas with a PPM supposedly you can still control it somewhat, until it gets so bad its all noise. See what i'm saying ? Even if theres constant noise (but not so much its all noise) you can still wrestle with it, if you've seen the bot competitions you'll notice they aren't in that much control half the time anyway. this is what Chris seems to be saying, even when the birds flipping out hes still able to bring it in relatively safely, and its considerably harder to land a misbehaving heli, than drive a bot.

But as we know the reason is because the PCM tolerates noise better !

No i referenced the ground correctly, its possible the birds i tested were not setup correctly, but it seems you'd get noise from an engine running on a bench. More stuff to check out.

Servos generate a lot of noise too, give that with all the spinny bits etc, I'd be suprised to see no noise. I'm not saying it was significant enough to mess with the receiver, just significant enough to show up and mess with a badly setup receiver/antenna.

I think your friend with the bad bearing isn't a good argument against PCM receivers, more an argument of not checking things out properly and trying to fix things with shiny new upgrades instead of sound engineering principles, but who am i kidding.

Finding the problem is key, using the PPM receiver is a good way of early warning that something is going on, but theres a lot of work needed finding out what..

Perhaps a solution is to use a PPM receiver with some kind of interference indicator as well as the PCM. That way you get the best of both. You could package up a small low power TX/RX combo that mounts on the heli with a simple flight recorder and a counter.

Or just use IPD which is pretty much a mix.

They do have combat aircraft. i haven't seen heli ones, but plankers and others for sure, yes it would be a short fight, unless the rules said projectiles etc and no actual contact between craft, that'd be interesting, but expensive.

A couple of my friends are the hosts for TLC's robotica I'll mention it to them.

I think AMA disallows projectiles being fired IIRC, but TV can get away with anything.
09-06-2001 Over year old.
 
 
Dragon2115
Key Veteran
Location: New England

Well yes and no, the duration of the signal generated by the impact of a bot is more on the order of milliseconds. In PPM it wiil cause a glitch, in PCM you won't see it. It's of too short a duration to trip failsafe. The receiver will simply ignore the bad frame or two of data.

Actually PCM doesn't actually "tolerate" noise, it rejects it by ignoring the bad frame and uses the last known good frame of positioning data.

Using the term noise isn't really correct either. What the PPM receiver is reacting to is a valid signal but without the positioning data it needs to control the servos. It's only noise in the fact that we didn't send the signal from the tx, it's not background or ambient noise so to speak. Think of it more as poor image rejection.

And it's a bad idea to shut off your tx before your rx because the rx will respond to anything it receives now that the intended signal is no longer present which may end up damaging your servos.

As for the noise generated by a running heli, we're only interested in frequencies that will effect our control systems. If the noise isn't in our band of interest then it's not really of concern to us. You also have to remember that the frame of the heli is an antenna for some frequencies too. But as I said, they're generally not of concern to us because they're out of our band. Sure we could go nuts and try to shunt everything out of the heli frame and make it totally electrically quite, but what would be gained by it?

Actually a major source of electrical noise isn't from the moving bits in a servo but the clocks used throughout the system. Fast rise times on the leading edge of a clock pulse (or control signal for that matter) generate huge noise signals especially in the third harmonic. A lot of electrical enginneers don't realize, or trivialize, this when designing a system. You can still have the fast clock but by slowing down the rising edge a little you take a huge amount of power out of the harmonics. As long as you don't violate the slew rate of the component its going to it's not a problem. Generally the only time it's of concern is if it's an edge sensitive device.

If you want to get down to it most systems are an EMI nightmare due to improper trace placement, fast rising edges, poor ground planes and return paths, etc. Another thing that gets me tweaked is that for all the money we pay for these radio systems you'd think they could spend a couple of cents more and give us something other than the cheapest p.o.s. wire they could find. I'd be more than will to pay a few bucks more and get a system with good three conductor shielded wire running to the gyro and servos and a four contact connector to support it.

Quote 
I think your friend with the bad bearing isn't a good argument against PCM receivers, more an argument of not checking things out properly and trying to fix things with shiny new upgrades instead of sound engineering principles, but who am i kidding.


Sure, and you'd only be repeating what I told him. But as I said, most people given the choice, will choose the path that requires the least amount of thinking and effort on they're part. Hence the incessant rush to use a PCM rx instead of finding the problem.

Actually it's not a bad idea, if you have one available, to use a PPM rx to check things out and then go back to PCM if that is what you want to run full time.

As for air to air combat, it's done with 10 foot streamers on the end of 10 or 20 foot fishing line. To win you cut your opponents streamer. They do have mid airs from time to time and that's why you'll see them run cheap easy to fix or replace aircraft. They don't go out and try to intentionally mid air though. That gets expensive, fast. Not to mention dangerous from the falling debris.

Yes, AMA does forbid any projectiles or pyrotechniques unless there are special waivers for some demonstation for example.
09-06-2001 Over year old.
HOMEPAGE  
 
 
Dragon2115
Key Veteran
Location: New England

Btw, if you're interested in tracking down radiated emissions from your heli or it's electronics, (This will work with the engine running but can't be used when airborn.) you can get an Electric Field Probe from the Van Doren Co. that you can use with an oscilliscope. The part number is EWB-11 and it has a bandwidth of 60 Hz to 100 Mhz. Cost is under $50 if I remember correctly. Their phone number is 573-341-4097

They also make a magnetic field probe as well.
09-06-2001 Over year old.
HOMEPAGE  
 
 
RappyTappy
Elite Veteran
Location: Las Vegas, NV

Very interesting information. Dpwatson is probably wondering what the heck all the hubbub is about, but this is one of those topics that everyone has their opinions which is best. In the end it all comes down to this. We know that PCM works great, we also know that FM works great as well. Both will fly any helicopter, both will not make us better pilots and both will not keep us from crashing. PCM cost a little more and some people don't mind spending the extra money, while some do and don't see a reason for it and spend it on a FM system. The reason, pure choice or someone that taught them how to fly uses one or the other. PCM and FM have their flaws, so make you choice on one(PCM or FM) and be biased about the other Happy Transmitting!

Chris
Rex 500
Burning them electrons :D

Forever Brothers
Mickey Tylo
09-06-2001 Over year old.
HOMEPAGE  
 
 
charliex
Heliman
Location:

noise is the commonly used term though as in signal to noise ratio, you'll rarely high signal to interference quoted, interference is a better term, since noise is always present, but after all people always say "static electricity" which is wrong too, but you'll never get them to change !

as for the clocks, absolutely this is one of the reasons i avoid using microprocessors in my receiver circuits, that way i can avoid using clocks of any kind, since weight is the main issue in mine, i want to minimize the amount of components so can't apply any fancy shaping.

PCM will tolerate more noise/interference than a PPM though , the filtering used can get a much cleaner signal out. which is why i say it wil tolerate noise better, since it has a better domain in which to recover bad signals.

Its a bit like applying a schmitt trigger to a noisy squarewave.

Incidently a lot of people think that PCM lockout lasts for a long time, not so, it returns control immediately that the signal is good.

PPM signal-signal--%^%$-signal-%$^%$^%----signal-signal---signal-
HELI control-control-freakout-wont matter servos need to recover--pilot might recover-servos-recover-control

PCM signal-signal--%^%$#-signal-^%$^%$^%$^%----signal-signal
HELI control-control-ignore-control-ignore-hold-------control-control

Anyway we are now getting to the point where nothings really appearing thats new, time for some actual data and less theory
09-06-2001 Over year old.
 
 
Ken B
Elite Veteran
Location: Phoenix, AZ

OK, here's my experence.

I left my PCM TX on one day. Totally forgot to shut off. Well another guy who was on my channel flew his plane No Problem at all. He was on PPM.

So what's the point? First, I was was imperessed that he flew and had no trouble. Second if he found out I did this he'd kick my arse! Heck I'd kick my Arse for doing something that stupid. But it happens once in a while.

I fly PCM, because I want to know when I get a hit and the lockout feature. My Heli is set up to go to hover settings when it locks. THe idea is it won't go far away and it won't go in at top speed or go wizzing all over the place. The odds are better that it will pic a direction and give time to yell and tell everyone to look out. To me it's safer than a completly uncontrolled eggbeater thrashing every direction and know idea where it might go next.

Ken B
09-06-2001 Over year old.
HOMEPAGE  
 
 
Dyehard
Veteran
Location: Cedar Bluff, Va.

I don't expect to change anyones opinion when it comes to the great PCM/PPM debate, I never have before, but think about this. No one flys more than the top five in Fai at the Nats, they nomally fly over five gallons a week, every week. They will not fly anything that consistly causes crashs, you have no chance whatsoever of winning the Nats if you are spending all your time repairing helis instead of practicing. Flying 300 or so gallons a year, they have much more exposure to interferance, metal to metal or any other glitch causing source than anyone else, yet take a look at the top five at the Nats and see how many PPM systems you see. Personaly, I haven't had a single glitch using Futaba PCM systems for the last eleven years, every time I stick a PPM reciever in one of my helis, I start getting glitchs. A friend stuck a Hitec PPM reciever in a Competitor that has been flying for years on PCM with no glitches at all and the servos went nuts just sitting on the ground. I think I'll stick with my PCM's.
09-06-2001 Over year old.
 
 
gnichola
Senior Heliman
Location: Santa Rosa, CA. USA

Re: PCM vs. PPM (FM)

Quote 
Originally posted by Dragon2115

PCM - you don't see glitches or minor radio hits because the receiver goes into a "hold" mode ( 1.5 second precurser to failsafe) and ignores the inteferrence by holding the sevos at the last commanded position.
[/B]


Well, that may be true for some radios but on my JR 8103 you select hold or failsafe on a channel by channel basis.

But it seems to me the discussion is more about what happens when one or the other gets jammed, when it should be more focused on how hard it is to jam one vs. the other.

Later, Guy
09-07-2001 Over year old.
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