I think that motor overload is one of the most useful things that VEX coaches and mentors can learn to identify. Why? Because it often presents as a “mystery problem”: it’s intermittent, or it appears after a period of time in which the motor has been operating just fine.

And it’s often the result of a design flaw, so the sooner you can help your team identify the problem and figure out what needs to be done, the better. Design problems take time and thinking to fix. (It can also be the result of something like the robot’s wheels running while it’s pinned against the Starstruck fence, but that’s a more clear-cut, situation-specific cause.)

I asked the Facebook World Coaches Association for real-world examples of what this looks like, and here are their (any my) examples:

  • Skyrise elevator lift goes up and down once, but then won’t go up again.
  • Robot sub-system looks a-OK, but when you use the joystick or online window controls, nothing happens.
  • Nothing But Net flywheel works for a while and then just stops.
  • Robot drives fine until the student driver tries to push a Starstruck cube and then the robot can’t move at all for a short while, and then drives again fine after that.
  • Robot Drives fine for the first 30 seconds of the match then crawls to a stop and periodically comes back to life.
  • Forklift can pick up 3 Starstruck stars and lift half-way, but then the joystick controls have no effect after the half-way mark; sometimes the forklift then falls to the ground. Sometimes the forklift will start working again after 10 seconds or so of sitting on the ground.
  • Robot grabber arm has picked up Starstruck star and is 3/4 of the way to putting it over the fence, and the arm just stops; every few seconds it moves a few inches and then stops again.

Circuit Breakers

Do any of the items in the list above sound familiar? If so, you are witnessing motor overload! Motor overload occurs when the robot’s actions are asking too much of the motor—putting on too much load in one way or another. As described in my earlier post about how motors work, there is an upper limit to a VEX motor’s capability, which can be represented either by the upper-right point in the graph on the left (stall torque), or the lower-right point of any of the lines in the graph on the right—the point where torque is at its maximum, and speed is at 0.

torque-vs-current-draw2 voltage-vs-speed

Each motor (and motor controller) has its own built-in “circuit breaker” at which point it stops working before this stall point, when internal damage occurs (it saves us from our own stupidity, when applicable). IN ADDITION, as I mentioned in a post from earlier this year (“Motor Ports: Spread Out the Load“), Ports 1 through 5 have a separate, “parent” circuit breaker, and Ports 6-10 have their own as well. So even if you are not placing excessive demands on a single motor, but rather are placing high demands on many motors at the same time, you could trip the cortex’s circuit breaker and then have ALL of your motors in those ports stop working at the same time.

Reset*

Once a motor overloads, the system may reset itself after a few seconds, depending on the source of the problem. In the case of the Skyrise elevator lift, the system resetting itself won’t really help—it’ll just get you back to “works one time, then nothing after that.” If your motor overloaded because you were manipulating several external objects at once, after the system wakes up again, you will be able to lift objects again, but lifting that same heavy weight will probably get you back where you just were. That’s the difference between a design flaw (not enough motors powering the lift) and a robot capabilities problem (can lift 2 Starstruck stars, but not 3).

Sometimes our team has found that the system will not reset itself until you turn the cortex off & on again; sometimes you need to do cortex & joysticks too. This situation is the worst possible option during a tournament because your robot (or robot part) is basically dead for the rest of the match; with the reset-itself situation, you’re out of commission (or partly so) for a few seconds and then can get back to work, maybe at a lower capacity than before, as described by the examples in the bullet list above.

In all cases, you’d like your team to avoid motor overload at all costs, and it needs to start pretty early on in the design and build process!

* A very helpful description of what’s happening behind-the-scenes in the reset process is from Todd Ablett on the Facebook Coaches group, that explains a lot of the behavior robots exhibit after overload:

* The 393 motors use a PTC (positive temperature coefficient) breaker to limit current in high load settings…this breaker trips and needs a reset time of about 4 seconds…if you try to use it before the PTCs cool and reset, they will trip almost immediately. Most of the time if you just count to 4 the motors will run after that. Also once the PTCs have tripped they run about 90% of their original levels until they have completely cooled. So typical failure is, motors get into a state where they can be overloaded (see the many great examples above), the PTCs trip and then if the driver does not let them reset, the motors will trip and trip and trip…

Teaching This to Your Team

gearbox-construction5a

Source: Carnegie Mellon Robotics Academy (c) 2005

Now that you can identify motor overload, it’s time to get your team on board too. In the off-season, or when you have new kids joining your team, it’s useful to have them do a little exercise that I found on the vintage Carnegie Mellon Robot Academy website. It requires first building a little frame with a motor and wheel on it, something like what’s shown in the image at left. In this exercise you put the little frame on the edge of a table with the little crate hanging off the side, dangling from a string. Running the motor winds the string around the wheel and lifts the crate. Here’s the CRMA PowerPoint presentation from 2005 with all the steps (no longer on the CRMA website, sadly).

NOTES:

  • In real life, it was hard to keep the string from sliding off the wheel; try this out yourself first & figure out what works best.
  • I didn’t bother building the little crate shown here; I made hokey little weight “pouches” made out of felt and filled with gravel from my back yard. I weighed the gravel so that the weights were all, say 4oz or 6oz, and each had a little string on the top that could be used to hook onto the string with a loop of velcro.
  • Make a joystick digital button set at some medium-level power so you know you’ll have the same power input every time, in lieu of using the joystick (which has analog possibilities).

If you’re just investigating motor overload, have your kids attach weights to the bottom of the string, and time how long it takes to raise up each weight to the table edge and record both data on a piece of paper. Have them keep adding weights in steady increments until they reach motor overload so that they can see for themselves what this looks like—what does the wheel actually do when it’s maxed out? You could even make a few weight pouches in 1oz and 2oz so that when they get near the limit they can use smaller weight increments to zero in on the true overload value.

If you have multiple groups of kids doing this at once, when everyone is finished, have them compare what their overload weight is; discuss why they might not all be the same!

Gear Ratios Too

If you are ALSO interested in teaching your team about gear ratios for simple and compound gears, you can modify this setup by adding more axles to the contraption on the table and gearing up or down, and running the experiment again. Since adding gears, axles, shaft collars, etc., takes a fair amount of time (especially for novices), what I did was that I had each group of kids modify theirs in a different way, and then the kids rotated around the room to the different “stations” to run the experiment when it’s geared up, when it’s geared down, when there’s a compound gear, etc.

Have the kids write down all of their results and then compare their actual results at the different gear ratios to what would be expected (by multiplying their stand-alone motor value by the gear ratio). Again inquire as to why they might not be exactly equal!

Depending on how long your meetings are, you could easily split this up into 2 sessions, the first just looking at motor overload, and the second about the gear ratios. I thought that it really helped for my kids to see for themselves the difference in weight that could be lifted with the different gear ratios, and see the unusual behavior of the motor when it gets near its maximum limit.

TTFN

Well, that’s it for my excessive verbiage for this Thanksgiving weekend. I can’t think of any other mechanics topics to go on and on about, but I do take requests in case there’s something you’d like to ask. Our email is renegaderobotics@gmail.com.

Update 2/23/17

Thanks to a 2012 post from jpearman on the VEX Forum. For those who are interested in some technical specs on the load that can be placed on a power expander, please read this excellent post. jpearman did extensive testing on a batch of resistors similar to the ones used in the VEX power expander, and you can check out his results to see how much current they can draw before they trip their internal circuit breaker and turn your motors off. It’s a great resource to keep in mind!

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