[OCCRA]: VEX Competition Tips - from VEX website

VEX Competition Tips
One of the most frustrating things that can happen to a team is losing control of their robot during a match. Please consider the following things that have been seen to cause problems at competition:
Antenna Placement
It is critical that the VEX Receiver Antenna (yellow wire) is free of any metal shielding. Many teams lace their Antenna Wire through the VEX metal structure of their robot. Some teams even coil their Antenna and tie it underneath their robot! Simply unwrapping the Antenna Wire from any metal framework and stretching it across open space will often cause robot problems to disappear.

Note: For best performance the small yellow Antenna Wire on the VEX Receiver should be mounted vertically using the Antenna Mount and Sleeve away from any metal structure.

Excessive Current Draw
Be careful when designing robots with multiple motors under simultaneous or heavy loading. There are three main points to consider:

  1. Most teams forget that if you draw more than 4 Amps combined from all the Motor Output Ports on the VEX Controller, there is a likelihood that the VEX Controller will be over the internal current limit (main thermal breaker). This over-current state will cause ALL Motor Outputs to lose power, have erratic operation and/or stop functioning completely.

When enough motors are in a stall condition, the total instantaneous surge of current from the motors can reach high enough to trip this limit. This may not happen immediately but could occur after 15-60 seconds of operation as the main internal thermal breaker heats up.

The main thermal breaker in the robot limits total output current to 4.0 Amps continuous, about 5 Amps for < 10 seconds, and 7 Amps for < 2 seconds (these numbers are approximate, not a specification). Once the main breaker is pushed into shutdown and held there from high current draw, no motors/servos will function properly until the current is reduced. Once the current draw drops below 4 Amps, the main breaker returns power to all Motor Output Ports.

  1. There is also a 1 Amp current limit internal to EVERY VEX Motor and Servo. When a VEX Motor is stalled, it draws about 1.5 Amps. (An example of a stalled state is when you are signaling the VEX Motor to go Full Forward but the Motor can not move - pushing against another robot.)

If the stalled state is held for about 10-20 seconds (not a specification), the Motor will begin to lose power. (Note: The trip point will decrease as the motor temperature increases. It will drop to about 1.1 Amps.) Once the motor is returned to neutral, normal operation will return in about 1-3 seconds at reduced power. Once a motor has heated up and tripped its internal breaker, maximum output power will be reduced until the motor is given a chance to cool down (this can take several minutes).

  1. A Battery that is undercharged or damaged will not be unable to sustain high current loads for extended periods of time. Under these conditions, the Battery Voltage will drop significantly during periods of high-load (like the cases listed above) and if it drops low enough, it will cause the VEX Controller to reset. If this happens, the VEX Controller will begin the Autonomous routine the same way it does at the start of the match - as soon as it sees a valid receive signal from a power-on state.

Note: Many of these situations are worsened by repeatedly moving the motors from forward to reverse. This results in subjecting the motors to a repeated stall load. Unfortunately this is often the natural reaction for drivers when their robot is acting erratically.

The best way to prevent this is to design your robot such that these loads are minimized.

If something like the failures above occur during a match: it may be best to stop for a few seconds to let the breakers cool, and then attempt to continue by driving slow and steady without unnecessary acceleration and turning. Decrease the load on the motors as much as possible to let things cool down.

Damaged Motors
The gears inside the VEX Motor can become damaged if they are excessively loaded or subjected to shock-loading. In some cases damaged and broken gears can cause a robot to behave erratically. This may look like the symptoms of a control problem when in reality the problem is 100% mechanical! If Motors are old or have been highly stressed it may be wise to consider replacing the gears, after all, that is why a spare set comes with every Motor.

Microcontroller LEDs
Ensure that all the lights on the VEX Microcontroller are clearly visible. These lights are invaluable when it comes to diagnosing problems on the field as they will display the “status” of the Robot Microcontroller. Here is what to look for:

  1. Power LED - This should be “On” and Green during the match. If it is flashing red then the battery is very low or weak/damaged.

  2. PGRM Light - This LED should not be “On” during a match. If it flashes Red, it indicates a problem with the code. If it remains “On” Green, it indicates the VEX Controller is “locked-up” in a programming mode. This “locked-up” state will require a download of Master and User Code to fix the issue.

  3. Microcontroller “Eye” - Fast flashing indicates the robot is in Autonomous Mode. Slow flashing indicates the robot is being controlled by the Operator.

  4. Rx1/Rx2 - These lights show that data is being received from a Transmitter on this port; if these are not “On” during a match there is a communication problem provided there is a corresponding Transmitter being used. Note: These lights will not be on during Autonomous Mode.

Transmitter Performance
Although the Transmitters are pretty rugged it is possible to damage them. A damaged VEX Transmitter Antenna may cause a reduction in performance; these can be easily replaced by un-screwing them (spares are available for purchase). Also, ensure the Antenna is securely screwed-on and fully extended for maximum range. Only about 2 sections on the Antenna on the VEX Transmitter should be extended during VEX competitions for best results.

A good performing Transmitter fully extended should be able to control a robot about 100 feet away. If a system cannot do this there is probably something wrong; it may be a good idea to change the Transmitter, replace the Receiver, or relocate the Receiver Antenna (see above description of Antenna Placement).

Battery Power
No team wants to hear the referee or volunteer say that their control issues are caused by a low Battery, especially when the Battery is fresh off the charger, but this is a common cause of problems.

There are some instances where the VEX 7.2V Batteries are charged incorrectly. If the 7.2V Battery is not fully seated in the VEX Charger Cradle before charging, it will be charged as a 9.6V Battery. This will cause the 7.2V Battery to be overcharged and can damage the Battery. Once the Battery is damaged, it may never charge correctly again. Ensure that all Batteries are charged properly.

Measuring voltage with a voltmeter, or the transmitter voltage monitor, is not a sufficient test for battery health; to do this properly the Battery Voltage must be tested under load.

Ni-Cads also 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). In this situation, the Battery appears to be fully charged but seems to discharge quickly after a brief period of operation. For a more detailed description, go to http://en.wikipedia.org/wiki/Nickel-cadmium_battery. If a Battery has been around for a while it may be a good idea to do a few deep discharges prior to your next tournament.

Note: Always remember to seat the Batteries in the VEX Charger Cradle to properly charge the VEX Batteries.

Ensure all Cables are Connected
Many teams have to be reminded to plug in their Rx Cable(s) prior to a match. While the refs and volunteers try to look for this on the field, it is the team’s responsibility to make sure the cables are plugged in correctly.

Some teams will plug the Rx Cable into the Serial Port. This can cause the VEX Controller to “locked-up” (stop responding) in a Program State and thus the Controller will stop functioning. Ensure the cables are plugged into the proper ports.

If the Cable is unplugged to run the robot on tether, ensure that someone reconnects it!

Field Control
Most teams do not have a clear understanding of how the Field Controller operates; they think it sends signals to the robot. The only thing the Field Controller does is control the enable / disable of the Robot by enabling and disabling the VEX Transmitter. It disables the robot by preventing the Transmitter from radiating. When the Field Controller enables the robot, the Transmitter is enabled which allows it to radiate.

The Autonomous Mode of the Microcontroller is triggered immediately after a valid signal is received by the VEX Receiver/Controller from a power-up state. Once the remote controlled portion of the match has begun, the Field Controller does nothing except to disable all the Transmitters at the end of a match.

When disconnecting the Field Control Cables, be careful to gently squeeze the release tab to prevent damage to the VEX Transmitter, or Field Controller Cable.

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/Volunteers 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. Matches are typically only replayed in the event of a clear field failure. It is the responsibility of every team to prevent Robot Failures which will result in a loss of functionality.

Now i know that OCCRA is over and that it was a blast and all… now my school usually has a mini vex “battle royal” with vex’s. In doing this last year we accidently learned alot. when you said the stuff about the vex motors and servos stalling and such when 2 robots are trying to push each other. The IS a way aroung that, we found out accidently. you can make your vex run insanly slow(1 to 3 or 2 ratio) but we were messing around during this season of OCCRA and noticed it could do alot more than we thought it would. you could stick you foot out and it would push you across the room. so i think he is right about most of what he said but there aer ways around alot of the problems. You should know that if you have a motor ratio for speed trying to ram a motor ratio with tourque you should know that the tourque will win every time. The vex batteries made 100% of the difference during the last few tournements we had to have them charged at 7.6-7.8 volts exactly otherwise we wouldnt make the turn. Yes i realize that there is a 1 amp limit to each motor and it goes down as you add several motors. ( I found out during the seasen where i got the birght idea to max out the motor limit) Damaged motors are also a very annoying and big problem that we took forever to realize. We even gave it a name left motoritis (dont ask me why it was always the left) and we couldnt cure it untill the end of the season when we tried a fresh and brand new motor. we had sevral unmarked bad motors from last year, if you even think about using a motor or servo at least think about spinning a shaft in it first and look for that distint sound. it can save you months of grief.