Radio Cut-Out

[Dan 550]

All hear this: PWM outputs can interfere with the robot's radio signal on a sporadic basis when the standard, low-insulated PWM cables are bound together with wire ties, especially when forming a loop. We all know that that electrical lines cause really nasty radio static in our car radios, but who's really thinking about things like radio interference when you have 20 minutes until the Fed-Ex truck is supposed to pick up your crate and you're still working? Well, to make sure your controller doesn't go into 'No Data/Radio' or even 'Disabled' mode or start behaving erratically, avoid looping PWM cables together and try to seperate your radio and all other cables from each other. Just some help from someone who learned it the hard way at the J&J regional today.

[Greg Ross]

I'm really curious how you were able to determine that you had a problem with radio interference, and how you knew that the coils were your problem.

[Al Skierkiewicz]

I am interested in this as well. The inteface to the radio modem is RS422, which uses a "balanced" wire pair for both transmit and receive. This serial protocol was developed for high speed data transfer with good noise immunity. The PWM cables do radiate though, so I am interested in a full assessment.
Teams have reported problems with radios in the past and many of them are due to common problems with mounting the modem. i.e. mounting deep inside the robot, antenna mounted against metal(especially the end of the antenna) broken wires in the serial cable and pinched or crimped wires at either connector.
Good Luck

[Greg McCoy]

Yeah, it seems highly unlikely that a PWM lines would interfere balenced radio line. If the radios aren't mounted upright with both antennas vertical, performance can suffer.

[Al Skierkiewicz]

There appeared to be problems with radio interference in Chicago at the MMR. Motorola did bring out a spectrum analyzer and we were able to trace some of the problems and did find some signals in band that were not modems. We were able to work with Innovation First reps and found no real threat to competition links from outside interference. Although some signals were close to modem frequencies, the modems seemed to be stable. We continued to monitor periodically throughout the comptetition and only found problems with teams who were drawing large currents and pulling the battery down below operating voltage for the modem and/or robot controller. It was noted however, that during inspection, teams were told to power up without tether to check operation of the light and it was found that some teams were still using radios in the pits to check operations. There should not be any problems with that but it is not 100% safe. (Some other team might unknowingly take control of your robot) Always use the tether when in the pits to prevent accidents and keep the competition going for all involved.
Good Luck All

[Nate Smith]

Originally posted by Al Skierkiewicz
There should not be any problems with that but it is not 100% safe. (Some other team might unknowingly take control of your robot)

As far as radio interference from other teams goes...only if someone is on the same channel AND set to the same team number will they "take control" of your machine...if they are simply on the same channel, you will experience radio cutout, due to the fact that if the RC recieves a data packet on the right frequency, but wrong team number, it goes into a "no data" state, just like if you had unplugged your OI.

[Al Skierkiewicz]

Nate, that's true but in our experiments we have found that stray packets do enter into the stream from other robots. I would prefer the team next to me in the pits is dead sure they have control of their robot. I don't want to be the unsuspecting person bent over a robot with a hot iron in my hand when my neighbor's robot goes for a joy ride. (I suspect you wouldn't want to be holding the solder for me in the same scenario.) Also a possibility (it happens) is someone tripping over the power supply for the OI while the robot is up.
Let's all agree that it is not good engineering practice and leave it at that.
Good Luck All

[Dave Flowerday]

Just to elaborate, the problem we've seen is that while someone else cannot directly control your robot (because of the team number protection), another signal on the same channel can interfere with your signal by corrupting some of the bits in the datastream. We've seen direct evidence of this at Motorola when we had last years robot powered up on the same channel (but with a different team number). In our case it usually shows up as a "twitching" of the drive modules which can certainly be dangerous. The combination of the team number and the checksum that the robot controller does provides very good protection (it's light years ahead of the old RNets) but it's not 100% foolproof.