Your take on CAN...

[techhelpbb]

Originally Posted by johnmaguire2013

I don't understand motors, electricity, etc. however I have a question on behalf of my team, and I think I'm wording this correctly.

Basically, we used CAN with Jaguars for our robot's drivetrain this year. Multiple times it stopped working during the match, and our team was seemingly able to determine that this was due to built-in protection on the Jaguar to stop, essentially, surges. It was happening when a large amount of voltage was sent through the Jaguar I believe, and our team said this wouldn't be an issue if we switched back to PWMs. It would usually happen if our driver was moving back and forth very quickly (e.g. the bridge.) Does anyone have an idea of what was happening or how this could be fixed?

I think you mean current not voltage (the battery voltage will generally get lower as you drive the robot).

There's a bunch of issues at work here:

1. The Jaguars and the Victors both reboot when the voltage of the battery gets too low (the battery can be too low because it needs to be charged, or because you are beating it down with heavy electrical loads). Victors, which use PWM only, will reboot fairly quickly and in some cases you might not even notice. The Jaguars using CAN can use quite a few modes and those modes have to be selected if they are not the defaults. If you use the Jaguars with PWM or the default mode on CAN (which performs a similar concept) when the voltage drops to a certain point (what is called a brown out) they come back up and just start working again (at least if you have the newest firmware they usually do). If you use another mode on CAN, then when they brown out you need to detect it and reset the mode. Problem is if you put an encoder on the Jaguar and it browns out...unless it's an absolute encoder...you'll loose track of the position of that encoder. Keep in mind that the brown out condition is not going to come back if the circuit breaker trips and hasn't yet reset.

2. You don't mention what you've got for a drive train. I assume you mean a drive train because of the way you've described it. However, it's very easy with the weight limits of the FIRST competition to build a drive train that draws more than 40A from the 40A breakers powering the CIMs. I assume you've put the CIMs in the drive train on the 40A breakers if not...that's probably gonna need to be changed. If you start to draw more than 40A from the breakers you'll start getting into the area where the circuit breakers are starting to trip (they take a while to trip) and fairly randomly the breaker will open cutting the power till it cools off (the longer you wait, the longer generally it will work after that...if you start doing it all over again shortly after the breaker cools, it's already pretty hot, so you'll probably cut out again shortly).

To add to this you've get the safety limits of the Jaguar. The Jaguars are designed to basically provide 40A. They'll provide 60A for a short period, but then they'll start to cut you back. The circuit breakers will also start to cut off at some point after 40A. So sooner or later you'll have the Jaguars cutting you back or the circuit breaker cutting you off for a bit, but one way or another if your drive train is quite often passing more than 40A of current you'll be running on the edge of failure.

Keep in mind the Victors won't make the breakers stop cutting off if you draw more than the 40A either. However, the Victors won't reboot into the wrong mode or cut you back. With the Jaguars your driver might mistakenly think being cut back means try to push to go faster (running you right back into the very overload you want to avoid and possibly pushing the breaker into opening). Remember that your drivers probably can't see the cut off in progress unless you report it, so they'll assume they are slipping or something mechanical like that.

A simplistic indicator that you're pushing too much current is the CIM motors will get quite hot. I warn you now that the CIMs can get hot enough to burn you so be careful if you plan on touching them to check that.

3. You might have a circuit breaker(s) in your power distribution board that are a little more 'touchy'. I've seen some cases where a drive train is marginally close to the current limits and variance in the performance of the circuit breaker causes the breaker to open. Usually the breaker is just doing what it should (in fact better than normal), but for this purpose it might be undesirable. Might want to try some other breakers it's quick and simple, but remember that if you don't quickly isolate a few 'touchy' breakers you are too close to the limitations of the system and you need to address that instead.

4. Are you sure you wired the effected part of the system correctly? Are the connectors firmly connected? Was the right gauge wire used? Are the screws properly tightened (be careful not to remove or strip them, sometimes when you remove the Jaguar screws you get some SWARF! from the bottom of the hole the screw was in loose in the Jaguar). If you can put a load (like a light bulb) on the wiring and check the voltage drop you can check to see if your wiring can handle the load (obviously there's more to this check than what I've written here...I'd ask a mentor or ask for more information if you want it).

5. Are you properly ventilating the Jaguars? Remember they have a fan inside them and it's important they get ambient temperature air flow. If you don't provide this you can cook the Jaguars and that's gonna lead to trouble.


What I wrote about is probably what they were trying to communicate.
Below are some more reasons your Jaguars might lock up or stop working for a bit.


6. If you're using encoders on the Jaguar then you might be loosing track of them. That can cause issues so you might want to think about how fast your encoders rotate versus their resolution, whether you properly installed the U.S. Digital encoder wheels, or if the encoder is electronically producing a clean signal. I won't wander too far off on this, it's a possible problem but I have no idea if you have encoders at all.

7. It appears some of the CAN bus connectors on some of the newer Jaguars might have some quality control issues. You might want to check the pins to make sure they make good contact. Also, it's easy to make bad CAN wiring, you want to check that because if you have bad CAN wiring it can stop several or all of the Jaguars from communicating and that might be intermittent.

8. You might be communicating too quickly on the CAN bus. The 2CAN is able to sustain a higher communications rate than the RS232 to CAN bridge in the black Jaguars. It's possible to overwhelm the communications process with CAN. Obviously if you're not properly communicating with the Jaguars they'll probably not do what you expect.

9. You might have set up your PID loops with poor choices for the relevant settings. This assumes you are using the PID functionality. It's possible that if the PID loop was setup properly your electrical system might not pass so much current, or the result will be the movements you expect. I can think of quite a few systems where a properly functioning PID loop might avoid an overload, especially systems with finite mechanical limits (like arms where if you don't stop you hit the mechanical limit and overload the motor trying to go past it).

There are 9 hints hope that helps.