In my post, A Day in the Life of a Tournament, I make reference to a “PTC Test” as part of the robot inspection process at tournaments. I started to include this explanation in that post, but it got too long for an already-too-long post! So here it is, in more detail.

What Does the PTC Test Test?

DC motor inside VEX motor

Inside of a VEX motor, showing the yellow PTC.

PTCs are the little circuit breakers inside each motor (the yellow tab in the image at right). If you’re unfamiliar with this part, please see my post on Motor Overload for details.

Some teams in past competitions have tampered with their motor’s PTC so that it won’t trip, and their robot will not stall if they put a large load on it (but their competitors’ will). Of course they run the risk of damaging their motors by doing so, but that’s a risk they were theoretically willing to make.

<R15> of the In the Zone game manual states “Parts may NOT be modified…” and lists PTCs specifically as falling under this rule. <R21> further states that teams intentionally circumventing these rules to gain an advantage over other teams may be disqualified from the current event and/or future events.

What Happens in the Test?

Inspector is holding the wheel of this clawbot still with her/his hand (upper left), while running the motor by pressing the button on their cortex device (lower right).

In the PTC test, the inspectors choose one motor from each of the robot’s subsystems (chassis, manipulator, etc.). In turn, the students unplug the motor from the cortex and the inspector plugs it into a cortex of their own (left), and then presses the button to run the motor. While it’s running, they hold the moving part still with their hand for 10 seconds, forcing a stall. If the motor does not stall after 10 seconds and the inspector can feel the motor still trying to run, then they know that the motor has been tampered with, and the team will most likely be immediately disqualified. (The test may be re-run at the inspector’s discretion.)

Through their own (presumably extensive) testing of this process, VEX has determined that there are no “false positives” to this test. That is, a legal, unmodified motor cannot fail this test by accident or fluke. See the official VEX document on this test, used by tournament organizers.

Recovery Time

The rub here is that it takes motors quite a while to recover from a stall (see my previous post on motor overload) and get back up to full capacity. The PTC testing document referenced above indicates that a motor will be back up to 95% capacity after 15-20 minutes and “as good as new” within 1 hour. (Separate testing done by jpearman, published on the VEX Forum, indicates that the recovery times may be longer than this.) Referees are supposed to give teams a suitable break between the test and their next match, and are supposed to  give all teams a similar amount of resting time. A review of VEX Forum posts from Worlds 2017 indicates that this does not always happen the way it’s supposed to.

When & Where Is This Test Run?

At VEX Worlds—just like every other competition—one of the team’s first tasks is to head to inspection. This year there were several inspection “stations” that the team moved along, each testing a separate component, with one station devoted to the PTC Test (I say “this year” because that’s the only time we’ve been at Worlds).

At VEX Worlds, this test was run AGAIN on all teams that won their division playoffs and who were headed to the round-robin finals. Teams were not allowed to take their robot out of the area of the division playoffs, and were administered the PTC test again. Then they took the robot straight to the round-robin in the arena under supervision to make sure that motor modifications were not made in between those locations. All this was to ensure that every robot in the finals was on a level playing field.

My team has only encountered this test at Worlds and once at CA State Championships. In Northern California, I have never seen this test applied otherwise. Coaches in other areas report that in their region, they encounter this test more often.

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