3 identical machines, two losing power, one not

We have three identical machines. One of the machines will run 7 two minute matches without blinking. Two of the machines will start to peter out close to the first two minute mark.
Currently running four 393’s for the mecamun’s four wheel drive and four additional 393’s for the lift and intake.
Put power expander on one of the machines losing power and it still begins to lose driving power before the two minute mark. We put two of the four wheels on the expander.
Again, one of the three runs like crazy for 7 consecutive two minute matches.
Any thoughts?

First off, I would recommend you put all your wheels on one circuit breaker. We caused a cascade effect last year on the robot when I did what you’re describing, and it ended up losing us more than a few matches. Wheels on one, Intake on one, Lift on one. That’s how I would recommend you do it.

Second, see if all the batteries are fully charged, or at least at the same voltage. Different battery levels have very different effects.

Third, check for friction on the machines losing power, or loose chain. Any loss or transference of power is bad.

If none of that is the problem, I’m out of ideas. Sorry.

When you say circuit breaker, are you saying the same battery? You state three different “breakers”… wheels, lift and intake? Are we not only allowed two batteries?

Also, I just weighed all three machines…almost exact using the wrestling teams digital scales.

Friction seems to not be a problem. I did purchase some non-aresol lube last evening to try that. The only place where there is movement is between the axle and the bearings. There appears to be no pinching or squeezing the chain sprocket or spacers.

Sorry if this seems a bit simple, but have you ensured the code on these machines are identical? Also, try swapping out the microcontrollers on each machine with each other to try to eliminate them as the problem.

I’m a high school student with a very basic knowledge of how this works. I’m sure someone else can do a better job, but this is how I understand it.

A circuit breaker is designed to prevent too much power from being drawn. Too much power can lead to motors overheating and catching fire or something bad. I don’t really know. There are two circuit breakers within the Cortex, one is on ports 1-5 and the other is on ports 6-10. Because you are using a Power Expander, you have access to another one. So I would put my lift on ports 1-5, the Base on 6-10, and the Intake on the Power Expander (just as an example).

Last year, I put one of the base motors in Port 5, and the Lift on 1-4. When that circuit breaker trips, it disables power flow to the motors on those ports. This led to the base using only 3 wheels to drive. That caused the base to not have enough power to move, causing the 5-10 breaker to trip. When the other breaker (1-5) recovered, the sole wheel moving was unable to move the base, and tripped the breaker again. Ad infinitum.

I’m sure there’s something wrong there, but that was basically what happened last year. And why I made my recommendation to you.

The cortex has protection built in to stop the motors using too much current from the battery. It use devices known as polyfuses (PTC’s, search the forum for lots of information on these). A 4A fuse protects motor ports 1 through 5, a second fuse protects ports 6 through 10. The polyfuse becomes open circuit when it is hot due to large currents, it will reset after a few seconds of the motors are stopped. The power expander also has a 4A PTC so in theory we can use a maximum of 12A continuous current over the three devices.

I should add that each motor also has a PTC device, these are of much lower rating and will go open circuit if the motor is stalled (or run under high load) for too long. Most of the time when a robot runs intermittently the problem is overloaded motors causing PTC over current protection to be activated.

When checking the wiring of my motors, why would I have some motors with the red wire on the left when looking at the blue clip, metal showing through and notch on left and some with the red wire on the right with the clip in the same orientation?
We’ve had to flip these and plugged the motor controller 29 so the clip notches match up to each other and the wires are red to black from controller to motor.

The orientation of the connection between the motor controller and motor simply flips the polarity of the motor (the direction it turns). This should not cause any of the problems you have mentioned.

When you say the robots “pitter out” do you mean that they completely stop functioning, or that only certain parts of the robot stop working? Does the cortex flash abnormal lights when this happens? If so, what are they? Also, does this only occur during matches when you are connected to a field control system (at competition or the handheld field control switch), or does it only occur when you are simply driving your robot around practicing, or both?

The puttering out occurs when practicing in our field. The drive train motors start losing strength sometime after 1 minutes and 45 seconds. I have not keyed in on the LED’s on the Cortex…I will watch for that tonight.
Interestingly, on the one machine that seems to run and run, the polarity of the wires and orientation to the clip is the same on all four drive motors. On the two machines that peter out, there are two drive train motors on each that we’ve had to switch the polarity due to the orientation of the red versus black.

Since you say it is a decline in motor strength and not a quick, sudden “shutoff,” I do not suspect it is the cortex resetting or the connection between the controller and cortex being lost.

Instead, I would agree with the others that a PTC is tripping. To test this, drive the robot until it stops working. Then, turn your controller off for a few seconds (2 or 4) and turn it back on. If the robot works again, most likely for only a short amount of time, this is indicative of a PTC tripping. This is because the breakers have a reset time of approximately 5-10 seconds. This isn’t a solution to your problem, it only helps identify for sure what the problem is.

Also, when your robot stops working, do all of the motors lose strength or only a select few? If so, which ones stop working? Also, what ports are your motors plugged into (please indicate which ones are on the power expander as well).

Friction. Keep looking – you have a motor working extremely hard because of high friction.

Clarification: This is not always true, but it is true about 95% of the time I’ve seen things like this. Check for binding axles and chains that are too tight. A chain should have a little slack in it or it squeezes the axles towards one another causing huge amounts of friction.

The motors are not suddenly quieting…they are slowly puttering out and yes…when we do stop for a number of seconds, the motors do then work again for a short while.
The motors are in ports 2,3,4 and 5. The machine that we’ve put an expander on, we placed the motors on the left side of the machine into ports 2 and 4. For the expander install, we ran the motor cables to the output and extension cables from the input on the expander to the 2 and 4 port on the cortex.
The motors that seem to be tripping quicker are the rear motors of the drive train and in ports 4 and 5. We are running mecanum wheels.

Hey Rick…we’ll keep checking. The chains were made to the identical length from machine to machine. To add a link to the two machines that are failing would make the chain way looser than the machine that is running like a deer.
We do have a encoder on each of the axles coming out from the motors. All drive trains are build with the exact number of spacers, collars, washers, chain length, etc, etc. Even turning the wheels by hand, all three seem to be very smooth.

You might have tried this already – disconnect your drive motors from the motors (either take off the chain or loosen the lock collars and slide the axle shaft out of the motor) and spin the wheels. Normally, they would spin a long time with one flick with the finger. Like I said, this is almost always a friction problem, but sometimes it isn’t.

I saw a motor once that was binding internally and causing problems like this. You cannot always tell about motor problems by turning them by hand, you have to wire them up. Have you tried swapping a couple of motors around and watching the results, or putting new drive motors on your problem robot? You can also put the robot up on blocks and run all the drive motors until the battery weakens. Watch to see if one wheel stops before the others and then check out that part of the drive train.

There must be some sort of mnemonic for this:

Identify that there is a real problem

IIICT is not much of an acronym, though. :slight_smile:

Okay, so all the motors work properly and there is minimal friction? Try this:

If there are different drivers for the different robots, try getting the driver(s) from the robot that does work to test out the other two. It could be that there is quite a load on the motors as it is and it’s at that point where the driver needs to be gentle with the controls. As I’m sure you know, going straight from full speed forwards to full speed backwards isn’t very nice on the motors, so it could be a driving technique issue.


Move 2 of the motors to ports 6-9. This will split the load between 2 different PTC’s in the Cortex that supply 1-5 and 6-10.

Also George’s comment about driving style is an important point.

So you should be able to run this type of drive all day without problems, I know I can with this robot that sounds like it has an identical configuration to yours.

So we need to figure out what is different, most likely candidates are weight and friction. Both will cause the motors to run under too much load and trip either the PTC inside the motor or the PTC in the cortex.

Have a look at the information in this post as background.

Also perhaps this original thread (which everyone keeps reading but was based on the original VEX spec, not the revised spec from July 2012)
Motor torque speed curves
and the revised version with updated data.
Motor torque speed curves - REV2

So what do these graphs tell us, I’m reposting the 393 data here for reference.

The first column is the motor speed, this is the measured speed of the motor when it is commanded to run at maximum, ie. we send a command value of 127. With no load on the motor it will run at approximately 100rpm (the motors all vary slightly, VEX spec is 20% difference between motors). At this speed the motor uses almost no current and therefore no power. As soon as the motor experiences a load it will slow, as the motor slows it draws more current and uses more power.

The internal PTC inside the motor is a HR30-090, the hold current for this device is 0.9A (it is guaranteed not to trip at this current at 25 deg C), the trip current for the device is 1.8A (minimum current at which it will trip at 25 deg C). These are not exact numbers and in practice and the actual current and time the device takes to trip can vary quite a lot. Looking at the table of data, column 5 shows motor current at a given motor speed. Notice that 0.9A is used at about 85 rpm, 1.8A used at about 65 rpm. This is the sweet spot for operating the motor, run the motor slower than 65 rpm and you will trip the PTC it’s just a question of when. Remember these numbers are for when the motor is being sent the maximum control value, the motor is running slower than it’s no load maximum because it is doing some work moving the robot, lifting it’s arm etc.

This was all a long way of asking that you measure the speed of the motor (effectively the wheels if directly driven). If the motor is running at 65 rpm or slower then internal PTCs will trip. A heavy robot can cause this, friction can cause this, over optimistic gearing can also cause this but I’m assuming that you are 1:1 due to the mecanums.

We are not 1:1. We are currently running a 18 tooth sprocket off the motor and a 12 on the wheels. What’s strange is that one of the three machines was running like crazy with that configuration…maybe driver???
I am going place two of the drive train motors and one lift and one intake motor on ports 2,3,4 and 5 and the other side of the machine on ports 6,7,8 and 9 as one poster suggested.
We may have to go back to 1:1 and deal with the slower speed of the machine. Or, maybe lighten a bit with aluminum to get to a 2 minute period of good running.

We did what you suggested Rick and there is indeed friction but it does not seem overly excessive. Minus the motor, we have the chain / sprockets (18 on the motor and 12 one the wheel), encoder and bearings. With those components the free spin of the wheel seems pretty good. I’m gonna split the drive train motors, lift motors and intake motors so they are divided among the Breakers within the Cortex.

This may sound crazy, but check to make sure all of the wheels are spinning at the same speed. I have seen bases accidentally put together with 2 high speed and 2 high torque motors, and when they end up on opposing sides it is not very apparent. If this is the case you could be stalling the high speed as they oppose the high torque (which win every time) and then add a huge amount of friction to the motors that did not stall as quickly.

While this is unlikely with new motors it is certainly plausible if you are reusing motors from last year, especially if they were rebuilt with high speed gears accidentally being used or not labeled as such.