Woketman

My Feedback: (6)

Some quotes from same very learned guys on the "Beyond Profi 4000" e-mail list about the suitability of these X-Bee modules for radio control aircraft application. And before anyone says "this guy is biased against XPS", well, YEAH! I have this funny thing. I get mad when I am lied to and I pay for a product that does not perform as advertised. Just a quirk of mine I guess!

">Which brings up the obvious question, is it a problem with XPS or
>with Xbee ?

A friend of mine is an EE who has experience with the ZigBee modules and
says he would never use them in a dynamic environment like an airplane.
I dont know if ZigBee is the same as Xbee.

On a similar note, another friend, also an EE, has used the modules that
XPS uses and says he would never claim the range performance that XPS claims.

FWIW"


"For what it's worth, about 5 years ago I checked into a number of XPS like modules and had an extended dialogue with the engineering team on the subject and concluded that they were not suitable. As the distance and number of users rises, the retransmission count and time between transmissions rises at a power law meaning that the data packets will not be sent on an even frame rate and no guarantees could be made about the timeliness of the data.

Since then, I have had my doubts about some of the new modules on the market using similar technologies but every says how great they are but I am not jumping."

"Zigbee is the coalition that puts together the Zigbee wireless
standards followed by its members. XBee is a company that
manufactures Zigbee compliant equipment. So, based on your friends
comment, it does look really bad for my idea and a lot worst for
XPS..."

Gordito Volador

My Feedback: (1)

Wocketman,

We have the X-Bee Pro development kit here at our company, I bought it for the sole purpose of developing a wireless foot switch system to activate X-ray systems in the O/R. We needed something immune to electrical noise and also something that would not interfere with other equipment. I will have to honestly say that after watching the XPS scenario unfold, we put our project on hold for the time being. As owner of the company, I did not feel the need to bring down the wrath of the FDA and a multi faceted law suit if we didn't get it right the first time. This is no slam on X-Bee, just a FWIW based on our observations in the RC world.

Regards, Bill

Woketman

My Feedback: (6)

Sounds like a good, conservative path to take.

Gordito Volador

My Feedback: (1)

Yes, people get a little testy if you let an X-ray exposure (no matter how short or low energy) be released without a command to do so. I think that the X-Bee may still be great in this application, but I now need to know more about it's real world abilities. We will now have to write additional code to build in some more failsafes and interlocks of our own. Actually, Kiwi and X-Jet may have saved my bacon by demonstrating some of these issues.

XJet

Well as many know, I've been using XBee/pro modules in UAV applications for quite some time and I was one of the first to question the claims being made by XPS for their system.

The XBeePro module was designed for pretty static point-to-point or networked communications in relatively benign and static 2.4GHz environments.

This is exactly the opposite of what you need for RC model use where the receiver is highly mobile (constantly changing distance and orientation) and because you're flying up and out of ground-clutter, the RF environment can change dramatically in just a few short seconds.

The XBeePro modules use a single quarter-wave groundplane antenna. This is a big negative and something that XPS attempted to mitigate by claiming they actually used a patented eight-element antenna array (utter BS by the way). The very fact that they felt it necessary to engage in such snake-oil is an indication that they know full well the limitations the single quarter-wave whip has in an RC environment.

If you look at all the other 2.4GHz systems on the market you'll find that (with the exception of an old Assan receiver), they all have multiple antennas and/or multiple receivers.

This is very important to avoid several nasty characteristics of 2.4GHz RF...

1. Because the wavelength of 2.4GHz is so short, its signal is easily blocked by objects such as engines, bits of carbon fiber or other metalic objects likely to be found in models (especially large models). A single quarter-wave antenna can be blocked very easily.

2. Because there are many objects in the environment that can reflect a 2.4GHz signal you end up with something called multipathing. This is where the signal direct from the transmitter is cancelled out or distorted by a reflected signal that also reaches the receiver antenna. Multipathing can cause a complete loss of usable signal at surprisingly close range, because the strength of the reflected signal can be as strong as the direct signal, especially if the direct signal is blocked (as in point 1 above) but the reflected is not.

3. You'll probably recall that those old TV antennas that used to be on every house roof were mounted with their elements vertical or horizontal. Ever wondered why you had to get the correct orientation for a good signal? Orienting your antenna in the wrong plane (ie: vertical when it should be horizontal or v/v) would produce a snowy signal at best That's called cross-polarization. This is because radiowaves can be pretty two-dimensional. If your transmitter antenna is vertical then your receiver antenna should also be vertical to get the strongest signal. If you have a vertical transmitter antenna and a horizontal receiver antenna (or v/v) then the received signal can be reduced by up to 75%.

Now compare the XPS/XBeePro setup with something like FASST, Spektrum, etc.

With XPS you have a single transmitter antenna and a single receiver antenna. This makes it very easy for bits of airframe and engine to block the signal. It also means that multipathing can result in a loss or severe distortion of the received signal and it means that the chances of the receiver antenna being at the same angle as the transmitter one is very remote most of the time (thus a far weaker signal will be received).

However, if you have multiple receivers or antennas then you find that it's far less likely the signal will be blocked (at least one antenna/receiver will be visible at all times in a good installation), because the receiver antennas will be at 90 degrees to each other then the loss due to polarization issues will always be far less, and because the receivers/antennas are separated by a small distance, multipathing will no longer be an issue.

XPS's answer to all this is that their systems are bi-directional. If (as is extremely likely) data is lost because of cross-polarization, blocking or multi-pathing, the transmitter will simply keep re-sending the data until it's received.

While that can mitigate the problems associated with the XBee's very simple antenna setup, it's not without costs.

Firstly, it means that the receiver must also be a transmitter in order to send back an ACKnowledgement signal. That means it has to draw far more current than a regular receiver (which only listens). If you've got a borderline BEC, switch or battery, that ACK transmission can be the straw that breaks the camel's back and cause a reboot. It's for this reason that XPS recommend not using switches at all but that you plug your battery directly into your receiver and a great many problems have been caused by inadequate BECs or regulators with the XPS system.

Secondly, the brute force method of simply resending the data until it gets through doesn't always help. If the signal remains blocked then no matter how many times you resend, it's just not going to get through. Obviously it's better to have multiple receivers/antenna to ensure that the transmitter signal can be heard in the first place than to simply hope that the model changes position or attitude enough to allow the signal to get through before it impacts the earth.

And then there is the issue of frequency agility.

None of the other major RC makers has been willing to rely on a single fragment of the 2.4GHz band to provide a reliable connection between transmitter and receiver. They are clearly aware that because the 2.4GHz band is pretty much a "free for all" and since there are so many different and varied devices using this band, you have to be ready for an interfering signal to appear out of nowhere and right on top of the frequency you're already using.

Spektrum/JR have addressed this by using two parts of the band so as to provide redundancy. If one of their operating frequencies gets hit, the other will still provide reliable control.

FASST/Airtronics have opted to constantly hop across the entire band so that any interfering signal will have such a small impact that it will go unnoticed.

XPS is woefully inadequate in this regard.

If you're flying XPS and an interfering signal appears on the part of the band you're using you're stuffed. It has neither agility (the ability to change frequencies on the hop) nor redundancy (a second frequency to fall back on).

Again, this is down to the limitations of the XBeePro module. It's simply not designed to hop and negotiating a new operating frequency can only be done *before* interference strikes. If something gets turned on somewhere and starts clobbering the bit of the band you're using, you're stuffed.

I can see why XPS chose the XBeePro modules for their RC system. They're relatively cheap, a schoolboy could design the hardware and write the software and they work most of the time.

If you're flying relatively small, slow, cheap sports models in an environment that is a long way from potential interference sources then XPS will work just fine. The small size of the model reduces the chances of the signal being blocked by an engine or other airframe components, the relatively short ranges at which such models are flown reduces the effects of cross-polarization and the lack of buildings and other structures may well reduce the effects of multi-pathing.

However, if you throw an XPS system in a giant-scale model and fly it near to a population or industrial area then you are asking for trouble.

Likewise, if you use XPS in a very fast model then the fact that it relies on resending data rather than receiving it reliably has the potential to introduce a small latency that can be fatal to the model.

In short, XPS spotted the chance to make good money for little effort and took it. They can't be blamed for that.

But would *I* use an XBeePro-based RC system in anything faster or more expensive than a .40-sized trainer?

Hell no I've seen what happens to the latency and packet loss when you stick an XBeePro module in a UAV and fly it at varying attitudes/altitudes. It's not pretty.

dirtybird

My Feedback: (5)

What Xjet says is all true. There is one thing he left out, however. The most important figure of merit of a receiver is its sensitivity. JD claims his receiver is many db more sensitive than the others. What is needed is a good comparison test of the receiver sensitivity of the systems. If JD is correct all of the factors claimed by Xjet are insignificant.
I have several XPS receivers and transmitters. I have used them in all my airplanes for 1&1/2 years from a 40" electric to a 77" BME 100 powered Weeks Special. I have never had a glitch that was not caused by my own carelessness

XJet

Actually DirtyBird raises an important issue but it's not as beneficial as he suggests and far from mitigating the problems I describe, it simply creates a whole set of new ones.

In an ideal world, your receiver should be only as sensitive as it needs to be, certainly no less but also no more.

Why?

Well an excessively sensitive receiver amplifies not only the good signal you want to receive but also the noise that's out there.

This makes an extremely sensitive receiver easier to "overload" with a strong interfering signal.

In effect, if your receiver is *too* sensitive, strong interfering signals (even on another frequency) can just force their way through the "front end" and totally corrupt the "good" signal causing loss of control.

Indeed, it's been shown recently that the XPS system is far more sensitive to strong out-of-band signals than most other 2.4GHz systems.

What does this mean to someone flying XPS?

Well if there's a powerful transmitter on some other frequency (perhaps 2.3GHz radar etc) then the XPS receiver (due to its greater sensitivity) is more susceptible to being knocked out by that signal.

It's interesting to note that some folks have had difficulty using FPV systems (with onboard video at 2.4GHz or 900MHz) in conjunction with XPS because those transmitters have far more of an overloading effect on the XPS receiver than they do on FASST or Spektrum.

I'm also currently investigating another possible failure-mode for XPS which might explain another two very similar reports of crashes, involving a total of four models.

The problem with XPS is that if a receiver is unable to return an ACK signal to the transmitter, that transmitter will start using brute-force (ie: continuous resends) to try and get through.

Now if two XPS users are standing close together on the flight line and one goes into "brute force" mode trying to contact its receiver, that signal *may* interfere with the transmitter (which is also a very sensitive receiver) standing right next to it if they happen to have both chosen the same operating frequency at power-up. Everyone's seen the effect where a strong signal will cause a receiver not to work. Indeed, if you try to power-up an XPS system with the transmitter and receiver too close to each other they often won't bind. Now imagine that you've got a really strong signal right next to a really sensitive receiver trying to pick up a faint reply from a distant receiver. Lots of potential for problems there.

In effect, up to 10 XPS systems can share a part of the band (we'll call it a frequency) because each one only transmits for about 10% of the time. However, when a transmitter fails to receive the ACK from its receiver, it re-sends the data. If it still doesn't get an ACK it sends again. This means it starts using a *lot* more than its 10% of the timeline. It's quite possible that when two transmitters are close together and one is constantly resending (monopolizing the timeline) the strong signal makes it hard for the other to hear the ACK packets from its own receiver. In effect the excessive sensitivity of the XPS system means it's easier for a strong nearby signal to stomp all over the good signal.

There are now two separate reports where two people flying XPS at the same time have had both models affected almost simultaneously It's possible that interference caused this but the effect of the interference was magnified by the "brute-force" mode the transmitters went into effectively blocking each other.

I have a JR and a Futaba XPS system here. As soon as time allows I'll try out this theory (and until then it's only a theory) to see if it might be what happened in those cases.

So more sensitivity is not always better and it won't help one jot with multi-pathing (because both signals will be equally amplified), nor will it help with a lack of frequency agility and it will definitely increase the susceptibility to off-channel interference.

dirtybird

My Feedback: (5)

What you are forgetting is that a well designed receiver includes an automatic gain control(AGC)
The AGC adjusts the sensitivity of the receiver to what it needs to receive the signal it is tuned to. This reduces the receiver bandwith making it less susceptible to off channel interference.
I am sure all of you that have been in this hobby for awhile have noticed when you turn on your receiver without your transmitter it will get glitches from off frequency transmitters that happen to be on. Once you turn on your transmitter the glitches go away. This is because the receiver sees your signal and the AGC reduces the gain and bandwith of your receiver. This is why they all say to turn on your transmitter first.
Multipath and antenna orientation is a problem to all RF transmission systems. Its just as bad on 72 as it is on 2.4.
Just remember with a frame rate of 50cps only about 1 out og 5 frames need to get thru to maintain control.
With a more sensitive receiver the transmission path problems are reduced.

Woketman

My Feedback: (6)

Yeah, but we are talking about XPS here, not a well designed reciever, but a quick design designed to make a quick buck.

XJet

AGC only kicks in when the detector sees a sufficient signal strength to start winding back the gain.

In the case of a strong off-frequency interference source, the front end can be overloaded long before enough signal gets through to cause the AGC to kick in because the interfering signal is outside the pass-band of the IF strip.

However, what does happen is that the desired signal becomes grossly degraded because the front end is driven into a non-linear operating region of its semiconductor devices. Effectively the receiver becomes hugely desensitized while at the same time the noise levels seem to rise.

The result is that the receiver can't hear your transmitter and you crash.

I've worked on a lot of RF relay systems over the years and have had quite a bit of experience with desensitization. Thats when you learn about the truth of the claim that "your receiver should only be sensitive *enough* to do the job". Sometimes we had to spend a lot of time working hard to reduce the effects of desensitization to get these relay/repeater systems to work properly and it was always the ones with the lowest sensitivity that were easiest to set up. The were also the ones that usually gave the most consistent and reliable performance too.

dirtybird

My Feedback: (5)

I said a well designed receiver. It sounds to me like you have been working with poorly designed ones. In a well designed receiver the front end will not be overloaded.

rmh

I guess no one remembers the AGC setups which would "bounce" the model on a landing approach-
Thanks to the 2.4 Spektrum I use now - this stuf is a thing of the past -If it has AGC -I can't find it- I couldn't wait to give away/sell all that friggen 72/53 stuff -with a jillion persnippiters

XJet

Well you ought to have a play with the XBeePro modules (the subject of this thread) and see how they do/don't handle strong off-frequency signals you might be a little surprised at what passes for a receiver these days ;-)

Don't get me wrong the XBeePro is a brilliant module for what it's designed to do it's just that it's not designed for RC model applications and the number of "unexplained" crashes as a percentage of units in use seems to prove that when compared to the Futaba and Spektrum/JR offerings.

dirtybird

My Feedback: (5)

OK if you have the statistics to prove that last statement lets have them

XJet

I have a pile of emails here from people with reports of their own problems or problems that fellow modelers have had.

Some of those reports have also been posted to forums, many of them haven't.

I don't think I need to go through and do an analysis of those reports just to please you you'll find plenty of reports on the Net where people have had issues (except in the XPS forum on RCG where they often get removed pretty quickly).

I'm not attempting to change any of the FB's opinions that's a waste of time. All I'm doing is presenting my own observations and experiences (as a professional electronics engineer) when working with XPS modules and the scope of the problems the use of these modules has caused for some who used XPS gear.

People are free to buy and use what they want but I think it's only fair that they do so with a full awareness of the limitations of the gear they're buying.

Futaba has had some awful problems with their 2.4GHz systems, so have JR (a new one related to frayed wiring surfacing just recently). No manufacturer is perfect and no system is perfect but the difference is that at least the others are working hard to remedy their flaws, XPS simply tries to reinvent reality and claim "there is no problem". That is unfair on customers.

Woketman

My Feedback: (6)

Dirtybird, it seems that X-Jet has quite an extensive experience base on using the X-Bee modules for R/C purposes unrelated to XPS. Can you please tell us what your X-Bee module experience is? Outside of XPS use, I mean. Thanks.

dirtybird

My Feedback: (5)

I do not have experience with Xbee modules outside of RC. I have only my experience with XPS and have watched demos by JD.
But the title of this thread is the suitability of the module for RC use. I dont see where experience outside of that use can be relevant. In fact it might lead you to erroneous conclusions.
When I asked Xjet to produce statistics proving his statements he falls back on a bunch of emails. That hardly proves anything. He don't even tell you how many emails he has.
If Xjet has worked as a professional engineer as he claims I think he should spend more time in the lab providing back up to his statements than he does on the internet. Then I would be more likely to believe what he says.
In this country if you claim to be a professional enginer you had better be able to produce a PE license to prove it.

dirtybird

My Feedback: (5)

No I don't remember. In fact I never heard of it. But then there was a 20 year period when I turned my back on RC.
Please explain what you are talking about.

Woketman

My Feedback: (6)

Or perhaps JD's bullcr_p marketing has led you to some erroneous conclusions!!

The tons of e-mails, forum posts, and PM that we have all seen mean a HECK of a lot more than the anecdotal evidence the FanBoys use (like the secret Boeing test!!!) when you know the honesty and integrity of one of the modelers who lost his hard earned cash to XPS.

Face it: X-Jet is trying to lead you to a better, safer 2.4 GHz solution, but you simply demand to fly 2nd tier. At least he tried!

dirtybird

My Feedback: (5)

OK where are the tons of emails? To who are they addressed?
Give us the post threads and PM's you talk about.
Have you compared them to the threads and PM's complaining about Spectrum and Futaba?
Come on guys don't hold out on us!

Woketman

My Feedback: (6)

Go look at the FASST thread in the jets forum, almost no issues. Certainly far, far less than XPS has for many more users. Look at Jon, Dustflyer, the dude in LA, the European pylon guys, the Frenchies, the Graupner pilot with the large turbine powered airliner..... The XPS failure list is long and distinguished.

Woketman

My Feedback: (6)

And the PMs were PM: that means private. I assume they wanted it that way. Does not matter anyway, most of the PMs were covered in the forums too.

Face it: XPS is not suitable for anything bigger than a small foamie. That's the facts, Jack!

XJet

As I said, I'm not about to try and convince the XPS FBs (if the cap fits..) of anything. They have already made their decisions and feel hell-bound to stick with them. That's fine, it's their choice and it's a free country.

I think any intelligent person will look at the number of "unexplained" XPS incidents and compare it to the number of "unexplained" FASST, JR/Spectrum incidents then compare the relative market-share of the respective products. You don't have to be a rocket scientist to see that XPS problems are out of proportion to the number of systems actually in use (as opposed to the inflated claims of units-sold by JD).

Attend any major competition and Spektrum/JR and FASST systems outnumber XPS by a huge margin.

Also, I'm not aware of any organization or event that has banned the use of Spectrum/JR or FASST yet there appear to be a growing number of "XPS banned" situations appearing all round the world now.

That tells the intelligent person something very important (but means nothing to FBs).

dirtybird

My Feedback: (5)

Is that all? There are more complaints than that about Futaba and Spectrum right here on RCU.

DadsToysBG

My Feedback: (35)

I have followed all the threads on this topic in every forum and have never posted in any of them. most of the time the material is way over my head. We have many in our area flying 2.4, but I have only seen two on XPS and one of them is now afraid to use it in anything larger then 40 size, even tho he has not had any problems.
The one thing I have noticed is that almost every time someone question xjet or kiwi on XPS they have a AZ near their handle, and on one with any expertize has countered with proof.
Just my thought on the matter. Dennis