Your take on CAN...

[techhelpbb]

Originally Posted by JABot67

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?

When some or all of the Jaguars cut out, do they do it at the exact same moment? Could be an intermittent connection on the CAN bus. Are all the units you loose sequential in the CAN chain? Perhaps the forward and reverse shakes something loose.

When you tried the window motor did you use a fully charged battery? You know when you suddenly shift the direction of your possibly 130lb robot you have to fight all the mechanical momentum of the robot that was previously moving the other way. You absorb that extra force with a surge of power from your battery. The more you suddenly shift the direction as hard as possible the greater the surge. Hence why the automatic ramp helps because you slowly absorb that energy...not all at once. If you absorb too much energy (relate current to kinetic energy of electrons) all at once the potential energy of the battery (relate voltage to the potential number of electrons to move out of the chemical system in the battery) and then keep trying, your battery voltage will drop and that can cause a brownout which all or part of the system might notice and reset. On the other hand, that's a big rush of current and that current might trip the safety of the Jaguar or the circuit breaker as well. Of course it could be a combination of all those possible issues as well.

You know if there's nothing else wrong with the system you can write your own ramp in software.
You can even simulate the brake function in your own software.
The team I mentor does this frequently.