Quote:
Originally Posted by techhelpbb
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...which with the Jaguars your driver might mistakenly think means go faster (running you right back into the very overload you want to avoid and possibly pushing the breaker into opening).
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. 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.
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Thank you for giving us some detailed instructions. I will attempt to clarify our situation.
We use CAN and have been having a specific problem during our matches where some, most, or all of our Jaguars will cut out perhaps halfway to three fourths of the way through a match. This happens when we go from full forward to full reverse or vice versa.
We do believe this has something to do with the current spike that occurs when the Jaguars are trying to spin the motors in the opposite direction from where we're going. You said it was very possible to build a drivetrain that will draw more than 40 amps when switching directions. That may be so with our drivetrain, but we have tried switching directions on a single Window motor (with no load) attached to a Jaguar, and it still fails.
We set the Jaguars to brake mode in code, and we also tried setting the "automatic ramp" rate. This stops the errors from occurring; however, our drive becomes less maneuverable because when switching from forward to reverse, the robot's motors will still be running forward for a while. We have tried several ramp rate values, and none of them were good enough to be used in a match. (We got DQed in a match because we hit the opponents' bridge while they were balancing. Not sure if the lack of control caused that specific infraction, but there sure was a lack of control.)
Our drivetrain is a four wheel tank with AM supershifters. Nothing too ridiculous. We use encoders plugged into the Jaguars, but only use them in autonomous mode. Auton is not a problem for us.
Would a switch to PWM cables on Jaguars (perhaps with the jumpers set to ramp mode) allow us better control of the robot and at the same time allow us to compete for an entire match?
We just want to be able to go from full forward to full reverse and vice versa during a match without our speed controllers failing on us, in order to give our driver the slickest control possible. Is that too much to ask for?