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Motor overload is one of the most useful things that VEX coaches, mentors, and students 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.
Motor overload is often the result of a design flaw, so the sooner a team can identify the problem and figure out a remedy, the better; design problems take time and thinking to fix. (Overload can also be the result of something like the robot's wheels running while it's pinned against the field perimeter, but that's a more clear-cut, situation-specific cause.)
Some real-world examples of what motor overload looks like:
- Skyrise elevator lift goes up and down once, but then won't go up again.
- Robot sub-system looks a-OK, but when using 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 drives again fine after that.
- Robot drives fine for the first 30 seconds of the match, 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. The forklift may start working again after ~10 seconds of rest/non-use.
- Robot's 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.
Do any of the items in the list above sound familiar? If so, one has witnessed 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 the article 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 below on the left1) (stall torque), or the lower-right point of any of the lines in the graph below on the right2)—the point where torque is at its maximum, and speed is at 0.
Circuit Breakers, or PTCs
Each motor (and motor controller) has its own built-in “circuit breaker” at which point it stops working before this stall point, because the stall point is when internal damage occurs.
The circuit-breaker that's inside the VEX 2-wire 393 motor is a combination thermostat-and-resistor called a PTC (Positive Temperature Coefficient) component. The photo at right3) shows the actual DC motor that's inside the standard black-and-green VEX motor; this is the part that's actually driving the robot. (If a broken motor is available, teams should open one up and completely disassemble it—the gears from both sides, and this DC motor.) The PTC is the little yellow tab on top of the motor in the photo at right. Current flows through it on the way to the motor, and heats the PTC as it does. If the heat generated exceeds the heat lost to the surrounding environment, the PTC keeps heating up, causing its resistance to increase, which in turn causes MORE heating. This creates a vicious cycle: less and less current and voltage become available to the motor itself.4)
The PTC is actually a safety device that prevents the motor itself from overheating and becoming damaged. Once the PTC exceeds a certain temperature, it breaks the circuit/stops the flow of electricity into the motor, and the motor stops working. When the PTC cools sufficiently, the circuit will re-connect, once again allowing current to flow to the motor.
PTC Reset Needs Time
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 mentioned above, the system resetting itself won't really help—it'll just get you back to “works one time, then nothing after that” because it's the result of the robot's design being underpowered. If the motor overloaded because the robot was manipulating several external objects at once, after the system reconnects, the robot will be able to lift objects again, but lifting that same heavy weight will make it stall again. 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).
Below is a helpful description of what's happening behind-the-scenes in the reset process that explains a lot of the behavior robots exhibit after overload5):
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…
The major downsides to tripping the PTCs:
- Once it's tripped, the motor is completely out of commission for at least 3-5 seconds.
- After that 3-5 seconds, the motor is highly susceptible to tripping again, because it has not completely cooled back to its room-temperature state. If this occurs during a match, the drive team can only use the affected motor in a reduced way, probably for the remainder of the match.
- The PTC doesn't fully cool to its room-temperature state—and the motor doesn't return to full-capacity—for over 30 minutes! Here's the results from the jpearman thread linked above (paraphrased).
Testing a brand-new, never-been-tripped PTC component only [not attached to a VEX motor] with a 4 amp current:.
* Brand new: 59 seconds to trip at 4 amps
* After a 1-minute resting time, 29 seconds to trip it again
* 5-minute rest time, 40 seconds to trip
* 30 minutes rest time, 42 seconds to trip
Once a PTC is tripped, even after waiting a half hour, the component still does not have the capacity that it did before the first test, tripping after 42 seconds of use instead of running for almost a full minute. At a competition, a team has very little time between matches, and during that time, the robot is often also in use (practice field, testing in the pits, skills challenge); lunch break is probably the only time of the day where the robot is sitting unused for a half hour straight. Clearly, a competition robot does not want to trip a PTC, but making sure that won't happen starts early on in the design and build process!
Some competition teams hasten the cool-down process of the PTC with compressed air. After a stall has occurred, especially if another match is imminent, they remove the green cap from the back of the motor, as well as the back-side gears, and spray the motor with compressed air. Turning the can upside-down makes the air even colder, but should be used sparingly, as very cold air on a hot motor will lead to a shorter life span for the motor.
In lieu of compressed air, a team can simply open up the back and fan or blow on the insides of the motor to hasten cooling.
Some teams that use compressed air regularly don't even screw on the green-cap side of their motors, but rather hold them on with zip-ties, so they can quickly be cut to open up the motor, and then zip-tied back on for the next match.
Motor Stall/PTC Test at Competitions
To ensure that teams have not tampered with their PTCs (which would prevent them from stalling under a given load, or provide faster recovery), the VEX Worlds and some regional events have their inspection procedure include a PTC test. Judges choose one motor from the robot and connect it to their own Cortex that has a testing program installed on it (photo at right6)). They run the motor at full speed, and then force a stall by holding the moving part (such as a chassis wheel) in place with their hands.
From the PTC test Q&A:7)
Based on extensive in-house testing and discussions with the PTC manufacturer, the PTCs in the motor will return to high levels (95%) of performance within 15-20 minutes of being tested. They will be “as good as new” within one hour after a test. Inspectors will do their best to keep this impact to a minimum and fair to all teams at an event.
In other words, judges will attempt to perform the stall test with enough time for the student's robot to recover before the next match. However, at VEX Worlds 2017, not all division winners were alloted this full waiting period before the Finals Round Robin matches began.
Teaching Motor Overload
In the off-season, or when new students join the team, it's useful to have them do a little exercise from the vintage Carnegie Mellon Robot Academy website.8) 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 the little frame sits on the edge of a table with the crate hanging off the side, dangling from a string. Running the motor winds the string around the wheel and lifts the crate.
- Mentors: In real life, it was hard to keep the string from sliding off the wheel; try this out first and figure out what works best for teaching purposes.
- Alternatives to the little crate in the image are fine; this author made little weight “pouches” out of felt and filled with gravel. 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 that every trial will have the same power input, in lieu of using the analog joystick.
If a team is just investigating motor overload, attach weights to the bottom of the string, time how long it takes to raise up each weight to the table edge, and record both data on a piece of paper. Keep adding weights in steady increments until motor overload is reached so that the students can see for themselves what it looks like—what does the wheel actually do when it's maxed out? A few 1oz and 2oz weight pouches could be made so that when the motor gets near its limit, students can use smaller weight increments to zero in on the true overload weight.
If there are multiple groups of students doing this at once, when everyone is finished, compare each team's overload weight; discuss why they might not all be the same!
Gear Ratios Too
Mentors: If you are ALSO interested in teaching your team about gear ratios for simple and compound gears, this setup can be modified by adding more axles to the frame 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), each student team could modify their frame in a different way (different simple & complex gear arrangements), and then the students then rotate around the room to the different “stations” to run the experiment under each gear configuration.
Students should write down all of their data, and then compare their actual results at the different gear ratios to what would be expected (by multiplying their stand-alone motor overload weight by the gear ratio). Again inquire as to why they might not be exactly equal!
Depending on how long a team meetings is, this project could be split into 2 sessions, the first just looking at motor overload, and the second including the gear ratios. Seeing for themselves the difference in weight that could be lifted with different gear ratios, and seeing the unusual behavior of the motor when it gets near its maximum limit are invaluable for robot building.