Lab Puzzle at EC3

So, what were we hunting at the EC3 VEX lab? Better answers are those with some reasoning on why we were using this.

The perfect star!

An undamaged star will have higher resistance as compared to one that is broken and shorter.
And stars are in short supply…

We’d need an instrument to find and undamaged star; true. But no…

Looks like current draw since you are in line with the ground wire. Interested in the results.

Red wire, actually, but Kirchoff told us he was okay with the setup.

I wonder what the kids were measuring. And why? Only two lab days this week, with a competition last weekend and another one on Saturday. Hmm…

Just to move it along:

Though things went well for them at the tournament on Saturday, Team 6135K enountered an inssue in the last two rounds of the competition. Before the next tournament, they wanted to find and fix the problem.

I see the red wire now looking at the picture on my desktop. I couldn’t make it big enough on my phone last night. And the multi-meter is set on amps, so I’m definitely guessing current draw on motors. I would have used gator clip adaptors, just to make sure it has a good connection. We seem to experience more problems during semi-finals and finals just because the motors are getting used so much compared to qualifiers.

Alright, my knowledge of circuits is limited to textbook, not practical circuitry, but I’ll give it a go.

Multimeter in series: Measuring current of some sort.
Multimeter set to what looks like Amps: agrees with above
Experienced a problem: Needs to be solved through wiring of some sort ->motors dying, PTC trip, disconnect, etc.

My guess is that the problem you experienced is that your motors were drawing too much current, and thus tripped the cortex PTCs. You are measuring the current draw to more evenly distribute it among the PTCs.

You’ve started to tease the thread loose now. Just pull it and follow for a bit.

More hints:

  1. The problem persisted even after the match.
  2. This is an experienced team, and they’ve built robots and competed before.
  3. They’ve been running a power expander (and thus two batteries) all season.
  4. Balancing is not the issue.

I would almost say with as many motors we have tied together for our arms this year, trying to pinpoint one motor causing issues in a cluster of 4 to 6 tied together. Since taking an arm apart and testing them without load is quite different than directly after a match under load.

The only variable from the hints is how long did it persist after a match? Is that why they want to test it right away with an inline extension? (I’m curious as well now.)

Obvious, isn’t it?

(read " =>" as “implies”.)

Experienced team => they’ve solved stalling before.
Existence of jury-rigged monitoring cable => they haven’t had to resort to this in the past.

Established implications:
They are measuring current of some sort.
They have encountered some rare error that they have never encountered before.
The problem is electrical.
This error was catastrophic enough to require immediately jury-rigging this tool.
Current measurements are required to determine the source of the problem and/or how to fix it.

Emphasis mine.
This cable is designed to monitor current over time, not to take a single measurement.

So we’ve established that you are monitoring current during robot performance, but I haven’t the faintest idea what the problem is or why you need current measurements.

Rember, @creatorthomas said

and I confirmed. If you look at other online evidence, you can tell it’s likely a 6 motor tower/arm; I’ll give you that.

An integrated ammeter is not the first thing you think of when working on an arm stall, and this is an experienced team. They’ve had time to work on it.

So, to the question: What was NOT causing the arm stall? What are those things one would check before deciding you needed this tool?

That is, what did they rule out?

Why does this remind me of Hannibal Lector in Silence of the Lambs? I’ll keep thinking it over though.

My guess is they are having an issue with the mc29 motor converters that is giving you them inconsistent current. Listen to each mc29 and if you can hear any that click, feel warm, rattle, they could be the culprit. Also try shaking each, as we had one mc29 with an issue where it would stop moving if you shook it.
The other issue could be that you have a motor where the insulation ripped where the wires enter the case, which can create a short circuit and trip the circuit breakers.

Good. I hope some others are as well. Maybe things will progress after a day of VEX competition.

I’m happy you’re interestested; cool! The idea here is that given the picture and information from the initial post you can build a series of inferences that tell you a lot about a technical problem team 6135K encountered. With thought, reflection, and (if needed) a bit of research, you should be able to say what kind of a problem it was, and what the team did before they used this tool.

Why play, or at least follow along? When it’s done, the thread might show a bit about how to think about engineering problems. Or just problems in general.

Maybe that goal won’t be met; we’ll see.

At the end, I’ll provide a writeup for what I think someone should be able to puzzle out from the information given.

More logical deductions/things that were clearly stated:
The problem lies within a motor of the 6 motor arm, and at least (and most likely) 1 but not all of the motors are problematic.
This is a motor specific problem; anything that would affect more than one motor is not the problem.
Whatever the problem was, it affected how much current was drawn by a motor.
This is not a common problem; that is to say, it was not simply not having enough torque, it was not a cortex PTC trip, it was not a motor short.
It is more difficult/time consuming to take the motors off and individually test them than to jury-rig an ammeter.
This problem is not always present, rather, it was only present under heavy load. (Inferred from the fact that this is a monitoring cable, and that the problem presented itself at competition and not in practice before.)

I’m going to wildly guess that one of the motor PTC’s had gone bad in some capacity, and thus that motor browned out much earlier. However, I have no idea if that even matches the inferences I’ve made (affects current draw, etc.).

However, maybe someone with a little more experience in obscure motor problems can make something of my logical conclusions.

I just determined while reading the post above that I’m now too tired to parse complex sentences, and worried the simple ones I’m trying to write in response will be wrong. We attended yet another tournament today. It was a short drive this time (only 71 miles each way; yay!) but still a grueling day.

Back at it tomorrow…


True within what you know. However, it is standard practice in EC3 (and many other labs) to use Y cables on pairs of motors driving structural-joined members. Things like arms and lifts. Note that it’s necessary to do that if you want to use 12 motors, which is something you should assume likely if pneumatics aren’t mentioned. In addition, it is standard practice in some labs/programs/teams to avoid ports 1 and 10 (the H-bridge ports) if possible. This makes the use of Y cables even more likely. So, given that it was a lift, it was likely that the problem motor or motors was connected via a Y cable. Keep that in mind when someone describes a problem they’re having.

Yes. Implicit from the tool they used.

Very good. All those things are true, and all those things are reasonable inferences from the very small amout of details available.
This was a zebra, not a horse. Look up “when you hear hoofbeats…”
You know this is a zebra, because if it was a horse, it wouldn’t be a puzzle, it wouldn’t have taken much time to fix, the problem would be common and easy to find.

But this is presented as a puzzle, and the tool used to diagnose the issue was not a common one. So the problem isn’t a common one. Despite hearing hoofbeats, we had to think of zebras.

Yes, but… Is it possible to individually test motors without taking them off? Is that something an experienced builder would do? If that had produced the answer, would you need an ammeter?

Not in evidence. That is, there isn’t enough information to make that assumption.

You haven’t built quite enough foundation just yet to make that assumption. You are on the edge of a couple of useful details. You should consider the ways in which a motor or motors can fail to provide enough motive force to make the lift work, or at least to work well in competition.

If you researched using the team number (6135K) and the timing, (puzzle posted Feb 10, about a problem solved that week) and knowing they were in a tournament when the problem arose, what would you find?