Tips for being on the field

Joe Perrotto posted the following on Chief Delphi about things that we should all think about.

Tips for being on the fieldJoe Perrotto FRC #0365 (Miracle Workerz)
As teams prepare for the Vex Championships in a few weeks, the FTA’s from the FTC World Championships want to share some learnings that can make your experience a more enjoyable one. One of the most frustrating things that can happen to a team is losing control of their bot during a match. Before you blame the field, interference, or crystals, consider the following:

Antenna Placement: It is critically important that you keep your antenna wire free of any metal shielding. We found that a high percentage (>20%) of teams laced their antenna wire through the Vex metal. One team even coiled their wire and tied it underneath their bot! Then teams wonder why they lose control during a match. We took one robot that was having problems, unwrapped their antenna from the metal framework, stretched the wire across open space and miraculously all their control problems disappeared.

Excessive Current Draw: Most teams forget that if you draw more than 4 amps there is a likelihood that the controller will reset and restart in autonomous mode. This happens each time you exceed four amps. As robots get more complicated their use of motors is increasing with some bots using the full complement of 10 motors/servos. The problem typically does not reveal itself in the pit, since the drive motors aren’t under the same load as when they are pushing bots on the field. At least one bot with this problem would stop and restart during the match and appeared to be running without operator control.

Damaged Motors: The gears inside the motor can become damaged if they are excessively loaded. We had one robot that had difficulty moving during a match. The signals were strong and there was no obvious interference. When checked later in the pits we found that that three of the four drive motors had stripped gears and the fourth was badly damaged. Once the gears were replaced, the bot ran fine. If your motors are old or have been highly stressed you should consider replacing the gears, after all, that is why a spare set comes with every motor.

Watch the Lights: Make sure the lights on your controller are easily visible. These lights are invaluable when it comes to diagnosing problems on the field. Here is what to look for:

Power light: should be on during the match, if flashing red then battery is very low
*]PGRM: This light should not be on during a match, if it flashes or remains on it indicates problem with code
*]Eye": Flashing indicates the robot is in autonomous mode.
*]Rx1/Rx2: Data is being received from the transmitter, if these are not on during a match you have communication problems
**Controller Performance: **Although the transmitters are pretty rugged it is possible to damage them. At the FTC Championship we used a spectrum analyzer to measure transmitter power and we found 5 or 6 bad transmitters that had intermittent or weak signals. There was even one team that pulled their antenna completely out of the transmitter then tried to push it back in and run in a match. Obviously you are taking your chances when you do this. Not many teams have access to a $50,000 spectrum analyzer to check their transmitter performance but there is a way you can test your transmitter. A good performing transmitter should be able to control a robot 100 feet away. If your system cannot do this, you may want to consider changing the transmitter, replacing the receiver(s), or relocating your antenna.controllers.

Battery Power: No team wants to hear the ref or FTA say that their control issues are caused by low battery, especially when the battery just came off charge, but this is a common cause of problems. Measuring voltage with a voltmeter, or the transmitter voltage monitor, is not a sufficient test for battery health, you really need to test for voltage under a load. Ni-Cads have a problem called voltage depression or lazy battery effect. This is caused when a battery is used for only a short time and then put back on charge for a long period, precisely what most teams do after a match. The battery appears to be fully charged but seems to discharges quickly after a brief period of operation. For a more detailed description go to If your batteries have been around for a while you may want to consider doing a few deep discharges prior to your next tournament. Using the wrong charging port in the Vex battery charger has also been identified as a problem. You should not use the 9.6 v port to charge the 7.2 v robot batteries. This overcharging can damage the batteries.

Plug in your Rx cables: I have lost count of the number of teams I have had to remind to plug in their Rx cable(s) prior to a match. While the refs and FTA’s try to look for this on the field it is your responsibility to make sure the cables are plugged in correctly. I have also seen many bots with the Rx cable plugged into the serial port. This makes the robot do strange things during a match, very few of which will help you win the match.

Interference: During the FTC Championships we continuously monitored the signal traffic using two spectrum analyzers. During that time the only interference we found was a constant signal on crystal 74’s frequency, and an intermittent signal between crystal freqencies 76 and 77. We removed these crystals from use. At no time did we see anything else that would interfere with the robot operation. While this does not guarantee there will be no interference issues during the Vex Championships it should encourage teams to look elsewhere when or if control problems crop up.
Pull the plug**: Most teams do not have a clear understanding of how the field operates, they think it sends signals to the robot. In actuality all it does is short a pin in the transmitter preventing it from radiating. The short is removed briefly to start the autonomous mode. Once the remote controlled portion of the match has begun the field controller does nothing. So if you are experiencing trouble controlling your bot the first thing you should do is disconnect the cord connecting your transmitter to the field controller and leave it out. Be aware that the only thing that will stop your bot with the cord disconnected at the end of the match is your Gracious Professionalism.This probably won’t solve your control issues but it will confirm that the field is not the cause of your problem. By the way, when you pull the plug, please squeeze the connector and pull the cord gently, we have had many cords damaged by teams yanking the cord out.

As you can see, there are many possible causes for a robot to lose control or not function as designed. It is not always easy or quick to determine a cause. Unfortunately with the pressure to keep matches on schedule Refs/FTAs do not have the time to diagnose the robot problems. Unless there is clear evidence of a field failure, the Refs will ask the teams to leave and try to solve the problem in the pits.

I was one of the inspectors for the NJ regional and we made an effort to unwind the antenna wires on each robot we looked at. One team came back and said it was the first time their robot had worked well.

I can confirm the 4 amps motor statement, if the motors start pulling more than 4 amps the controller is not happy. Remember that current is a factor of load, holding the robot in the air and the wheels moving may only draw .5 amps. Pushing against Team 82 “Marvin” will cause a stall load, max current draw for the motor, and that will be much higher than the normal current. Current gets to 4 amps, you reset and it’s game over.

The only thing I can add to Joe’s message is to check the transmitter voltage. The display tells you the voltage, make sure it’s over 9.8 volts.

Are there new chargers that have specific 9.6v and 7.2v ports? My chargers are ambidextrous and can take either battery in either slot. This is also stated in the Inventors Guide insert for the battery packs.

Is he just reminding folks to fully insert their battery packs into the cradle before attaching the connector, or am I missing something?

  • Dean

He’s reminding people to put the battery in the charger the right way. There has been lots of problems (and lots of threads) about the 7.2 volt battery being charged at 9 volts.

Right - I’ve seen all those threads. His exact wording:

doesn’t really remind people to fully seat their batteries. And it seems to suggest that the ports are not equivalent, which contradicts everything I’ve heard and read, and is why I posted my Q.


  • Dean