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VEX Competition Tips for Teams

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:

Excessive Current Draw

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

  • 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. The current draw must drop to near zero Amps before the main breaker returns power to all Motor Output Ports.
  • 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 cannot move—such as 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).
  • 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.


Many of these situations are worsened by repeatedly moving the motors from forward to reverse. This action 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 problem is to design the 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 behavior 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.


  • 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.
  • A description of all VEXnet lights and their meanings can be found on page 8 of the Cortex User Guide.

VEXnet Performance

  • Keep keys safe and marked by team at all times.
  • On the robot, make sure that the CPU VEXnet has a good signal path to the drive team. Putting your VEXnet behind or under a bunch of metal will impact performance. You are allowed to use a USB extension to place your key in a more optimal location.

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.
  • Once a 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.
  • Testing under load can be performed with the Battery Beak device from Cross the Road Electronics (be sure to purchase the Tamiya connector when purchasing to enable plugging into 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 accidentally plug the Rx Cable into the Serial Port. This can cause the VEX Controller to “lock-up” (stop responding) in a Program State and thus the controller will stop functioning. Ensure the cables are plugged into the proper ports. FIXME
  • 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 / disabling the VEX transmitter (joystick). It disables the robot by preventing the transmitter from radiating. When the Field Controller enables the robot, the transmitter is enabled, allowing 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 joystick, 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 and volunteers do not have the time to diagnose 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.1)

robotics_competitions/team_tips.txt · Last modified: 2017/09/07 02:46 by