Competition Radio Interference Solution?

[FIRST JerseyKid]

I was thinking about the IFI lib file. I know how at competitions there can be issues with robots in the pit using their radios interfering with robots on the field.

I was wondering why IFI doesn't add to it's .lib a check for a packet, that the field would transmit, indicating what teams are "in the round". If no packet is received don't do anything but if a packet is received than check the team numbers to the RC's team number and if the team isn't "in the round" than disable the radio.

Just an idea... What do you think?

[DonRotolo]

That might make it hard to use the radio, say, at your school.

And, if it can be disabled, it will be.

I think the issue is less being one of interference, and more one of safety. Pits are crowded, and the LAST thing anyone needs is one inadvertent motion by the robot, causing an injury.

Don

[Stuart]

the problem isnt on a logical layer but on a physical layer. while Im not sure how many channels are used for the first RC I am sure that it is less than 384(thats close to the number at nats). and youll just have teams transmitters stepping on the toes of the field transmitters.

in short that special packet you want to send would just be drowned out.

now what they could do is make the recievers dual band(75mhz and 900mhz) and have the field transmitters be one and the teams be another, but thats just over complicating things.

[Jon Jack]

the problem isnt on a logical layer but on a physical layer. while Im not sure how many channels are used for the first RC I am sure that it is less than 384(thats close to the number at nats). and youll just have teams transmitters stepping on the toes of the field transmitters.

in short that special packet you want to send would just be drowned out.

now what they could do is make the recievers dual band(75mhz and 900mhz) and have the field transmitters be one and the teams be another, but thats just over complicating things.

There still wouldn't be enough channels to allow everyone that's not in a round to run their robot via the radio.

If you just want to sit in you pit and test your robot, use a tether cable. If you're on a practice field and want to drive around a far easier solution than making switching to dual band radios would be to buy a long tether cable from sandrag (http://www.chiefdelphi.com/forums/member.php?u=2022).

[JohnBoucher]

From the 2007 Manual.

3.14.3 Pit and Competition Safety
Robot Operation: Operate robots on tether only in Pit area.

I like the suggestion of the long teather for pratice field.

[Mark Rozitis]

the problem isnt on a logical layer but on a physical layer. while Im not sure how many channels are used for the first RC I am sure that it is less than 384(thats close to the number at nats). and youll just have teams transmitters stepping on the toes of the field transmitters.

in short that special packet you want to send would just be drowned out.

now what they could do is make the receivers dual band(75mhz and 900mhz) and have the field transmitters be one and the teams be another, but thats just over complicating things.

In news I work with a lot of radio receivers and transmitters and wireless mics and all that....from a radio operations perspective I would simply have a set of frequencies/channels set aside for use in the pits only...and clearly posted, those of course would not be used out on the field and that would solve the problem of units keying up ontop of each other, I suppose another solution would be to add CTCSS/DCS squelch and you can look that up on www.radioreference.com but still even with PL/CTCSS tones you can knock someone else off the air by stepping on their channel if you are too close....

just an idea.....separate and set aside channels for the pit would be my fix.

[Andy Grady]

Does anyone know what the frequency range of the radio's is? I am assuming it carries a VHF range similar to that used on wireless microphones at the events...another problem which may or may not become more of an issue. I just remember last year at Championships having to switch mics and change frequencies often as my microphone kept taking frequency hits and cutting out. I wonder if this problem is cause for some of the unexplained robot issues we see at offseasons and beyond.

[Mark Rozitis]

Does anyone know what the frequency range of the radio's is? I am assuming it carries a VHF range similar to that used on wireless microphones at the events...another problem which may or may not become more of an issue. I just remember last year at Championships having to switch mics and change frequencies often as my microphone kept taking frequency hits and cutting out. I wonder if this problem is cause for some of the unexplained robot issues we see at offseasons and beyond.

All depends on what wireless mic system you are using, alot of the Sony camera mics are in the 700 mhz range, mine is the senheisser G2 kits and i'm in the 518.mhz range+ and then I guess there is some stuff up in the 900mhz range now....

when you say "hits", unless I can hear it, it is hard to say if perhaps you're mic was just going a touch out of range or perhaps the signal was multipathing which at such low power than these units run it actually happens.

if a robot or other radio keyed up on you're mic freq you should have been knocked off the air good! not just hits here and there :)

another wireless mic can also knock you off the air. I use this site in canada to look up frequencies http://sd.ic.gc.ca/pls/eng_alpha/web_search.licensee_name_input

you might have a similar site in the states with the FCC, Industry Canada is our version of the FCC.

[Alan Anderson]

just an idea.....separate and set aside channels for the pit would be my fix.
There aren't enough channels to do that.

The radio modems provided to us have 40 channels. They are apparently not quite selective enough to be able to use adjacent channels at close range without risking interference; I think I remember someone saying that only 14 of the channels are really usable at the same time in the same area. The user-selectable channels are 04, 13, 22, 31, and 40 (a spacing of 9), with 01 added a couple of years ago.

Each field requires six channels, one for each robot. Some regionals have two fields. Championship has four fields running simultaneously, but the opposite corners are probably far enough away that interference isn't a real problem between them.

However, there are dozens to hundreds of robots in the pits. You do not want radios interfering with each other when teams are trying to work on their robots. The "no radios in the pits" prohibition is a good safety rule even if you can keep field interference from being an issue.

[Andy Grady]

if a robot or other radio keyed up on you're mic freq you should have been knocked off the air good! not just hits here and there :)


The mic that i use personally is a Shure brand radio. According to the website you posted, it ranges in the low VHF range of around 152.820 MHz. I know from my work expertise in radio that alot of mobile radio frequencies operate at this range. In a large city like Atlanta, its very possible for alot of frequencies to be jumping all over each other. When you have many VHF frequencies popping up in a small band of frequencies, they tend to raise the noise floor on the VHF spectrum, particularly around the interupting frequency. Often this will cause for radios to squelch off the tones. More than likely, the problems with the microphones at the competitions has more to do with that than robot frequencies. But chances are that the robot radio's frequency range would be in a different area than that of wireless mic's or mobile radios would carry.

Also keep in mind that power of a signal can make a big difference in how a piece of audio or radio equipment squelches. Often if you have two frequencies sitting on top of each other (and they have to be directly on top to really cut something out alot) there will be massive amounts of intermodulation. Instead of the absolute cutoff like you are suggesting would happen with a robot frequency sharing a mic frequency, what you would get would be the higher power frequency winning over, with alot of interuptions, though not quite a complete cutoff. This is an effect we often run into with pager frequencys intermodding over our company radio frequencies. I find that it is very rare for two frequencies to be quite that dead on to be that destructive to a signal.

I am still curious however to find what frequency the radios run at. There are tons of radio signals that run rampid in a city...any one of them could cause an issue. Not to mention noise problems from exsisting robot radios that may have damaged parts from all the collisions robots take.

Essentially...radio control is a plecomplicated thing!

[DonRotolo]

You do not want radios interfering with each other when teams are trying to work on their robots. The "no radios in the pits" prohibition is a good safety rule even if you can keep field interference from being an issue.

Absolutely right, Alan.
Again, folks: It's not logistics - interference can be addressed. It is a safety issue, plain & simple.

Don

[Dave Flowerday]

I am still curious however to find what frequency the radios run at.
The radio modems used up through last year were 900MHz (who knows, they may be different this year). Same band as 900MHz cordless telephones.
It is a safety issue, plain & simple.I won't disagree that the rule is probably in the interest of safety, but from a technical perspective interference from other robots shouldn't be a safety concern really. There's several mechanisms built in to make sure that interference from other robots doesn't inadvertently operate your robot: 1) RCs will only respond to data coming from an OI with their team number, and 2) each data packet is protected with a checksum and if that is not valid it is discarded. With these two safeguards in place it should be essentially impossible for multiple robots on the same channel to interfere with each other and mis-operate each other's robot. If this were not true then I would expect FIRST to take this one step further and confiscate each team's radio modems to guarantee no interference (like they did back in the days of the RNets).

[Billfred]

There's several mechanisms built in to make sure that interference from other robots doesn't inadvertently operate your robot: 1) RCs will only respond to data coming from an OI with their team number, and 2) each data packet is protected with a checksum and if that is not valid it is discarded. With these two safeguards in place it should be essentially impossible for multiple robots on the same channel to interfere with each other and mis-operate each other's robot. If this were not true then I would expect FIRST to take this one step further and confiscate each team's radio modems to guarantee no interference (like they did back in the days of the RNets).1293 had an experience with this during build in 2005. We had both Ockham and Hockham (the practice robot) running at the same time. Ockham had its team number set to 1293, Hockham was set to 1294--but we had forgotten to switch the channel on one of them. Both would barely even twitch while on the same channel (we finally came to our senses and flipped one around).

While it would seem to be good that neither robot moved on the same channel (given that the field channels are inaccessible by teams), I fear for the time when someone pegs the joystick and the other team using that channel powers down the robot.

[Kingofl337]

Based on the trend we are seeing with most devices. They will probably be around 2.4GHz range.

[Pavan Dave]

That might make it hard to use the radio, say, at your school.

And, if it can be disabled, it will be.

I think the issue is less being one of interference, and more one of safety. Pits are crowded, and the LAST thing anyone needs is one inadvertent motion by the robot, causing an injury.

Don

This happened at our pits at LSR without the radio when someone forgot to turn off the OI and when we went to make an adjustment, someone leaned over the OI and pushed a Joystick and the robot fell off the cart.

Think of what would happen if someone forgot to turn the OI and RC off when it was radio. At least when there is a tether it is a bit safer and helps prevent some of the stuff that happened.

Pavan.

[Astronouth7303]

This happened at our pits at LSR without the radio when someone forgot to turn off the OI and when we went to make an adjustment, someone leaned over the OI and pushed a Joystick and the robot fell off the cart.

Of course, the way to deal with that is to build a cart such that the wheels don't touch anything solid. (Highly recomended.)

IMHO, the minor convenience afforded by being off the tether is offset by the technical complexity of such a system. The fields last year had enough issues without the pits (the Ypsilanti regional comes to mind). Even with Hatch Technologies out of the picture (IIRC), I think the entire control system needs less complexity, not more.

[Andy Grady]

The radio modems used up through last year were 900MHz (who knows, they may be different this year). Same band as 900MHz cordless telephones.


900 MHz radios are widely used in the electrical industry. As a matter of fact, most of our radio controlled devices out in the field use 900 MHz frequencies to operate. In a big city, there may be a 900 repeater at the top of a large building (say for instance the Westin Building) that would be sending out signals at fairly high power. It would be very easy to cause intermod with a robot signal which isn't even close in terms of power to a repeater. You would need some sort of PL or similar encoding to prevent massive issues, but you would still be at a high risk for intermod.

I think what it comes down to is...radio frequencies are everywhere, we might just have to live with the nuances of interference for a while.

Dave, do you remember when RNETS would cause for many rematches in FIRST as a result of the radio being on the same frequencies as Orlando International Airport? God I miss the old days.

[Dave Flowerday]

Dave, do you remember when RNETS would cause for many rematches in FIRST as a result of the radio being on the same frequencies as Orlando International Airport? God I miss the old days.
Absolutely. Anyone who complains about the current radio control system clearly never experienced the days of RNets. I remember that back in those days every field had a frequency analyzer sitting next to it with someone watching it during each match to scan for interference, and I remember lots of rematches (and more importantly, several matches that I'm sure we lost due to interference that they did not replay :mad:).

To be honest I'm quite amazed that the current radio system seems to work so well at so many different competitions over the course of so many years. Like Andy said before, radio control is hard to get right, and between IFI and the makers of the radio modems I think they succeeded.

[Jimmy Cao]

I was wondering why IFI doesn't add to it's .lib a check for a packet, that the field would transmit, indicating what teams are "in the round". If no packet is received don't do anything but if a packet is received than check the team numbers to the RC's team number and if the team isn't "in the round" than disable the radio.

Just an idea... What do you think?

I believe that IFI included something that would compare the team numbers on your OI with the RC. It should only let you control the robot in competition mode when the 2 numbers match, if i'm not mistaken.

[mtaman02]

We all know that the IFI Control Systems alows -X- amount of channels to be used by the teams and the rest to be used by the Field Control System. My next quetstion is why can't more channels be added to the current set that already exsists (instead of 40 channels why not make it 80 or more. Is there a reason behind only having 40 channels to use with only 5 / 6 of them to be selected by the team. I know the system is complex already but how about a Tri-Band System with one band for the Pits another for Competition and the last to be used for Team use and then 40 channels for each.

I remember that back in my days of being in H.S. that our robots use to go nuts reguardless of having different channels & team numbers set and no one at the helm, As a matter of fact the robots use to react to Walkie Talkie uses back in my old H.S. (which is why the Walkie Talkies are not allowed at competitions - since robots do tend to stop performing / preform poorly during a match when that device is in use.)

[Al Skierkiewicz]

OK, some weird data coming through in this thread.
The 900 MHz band that the radio modems use in a part of the shared Amateur and FRS band just above 900 MHz. Although close to cell phone frequencies they are not exactly crossing but, any high power transmitter close to or adjacent to your frequency can produce spurious signals on your frequency. Common wireless microphone frequencies in this country are designed to operate in the VHF TV band or on the now unused channels at the high end of the UHF band, ch 65-69. I have seen some in the 900 MHz band but they are pretty scarce at this point. However, second and third harmonic radiation from these radios can occur in the 900 MHz range. In particular, wireless microphones in the presence of metal produce out of band response as the RF is affected by the rectification of the metal to metal contact. Often, hanging jewelry, metal belts, even metalized fabric places havoc with wireless mics. (not to mention brush noise from DC motors) Regardless of the error checking that takes place there is still so much in the way of interference that can affect modem communications and since we can control it by not using the radios in the pit let's do it.
I am a big fan of adding a tether extension so that the tether can be connected to the outside of the robot, not directly to the RC. (Don't use the locking hardware for this when you connect to the outside tether.) When using the tether in the pit, should the robot act erratically, you just yank the tether out. Robot comes to an immediate stop, simple and effective. Also highly recommended is the use of a disable switch on the OI port. These are easy to build and you need one to enable the robot and autonomous mode during build. Info is on the IFI website.
If you think you can get away with using your radio in the pit, be advised that the IFI monitor at the field table, can tell every robot that is fired up and what the status of things like battery, backup batt, etc. They know when you are sleeping and they know when you're awake.

[Astronouth7303]

OK, some weird data coming through in this thread.

Too much metal in the server.