CAN on the entire 2012 robot

[JDNovak]

Team 16 has used CAN since 2010. We started with the 2CAN from the beginning and ran 10 Jaguars the first year and 11 Jaguars in 2011 and this year. The biggest issue the first two years was an initialization problem at boot up that was hard to detect but would render the robot motionless at the start of the match. This risk caused us to convert to PWMs at the end of last season.

This year we started again with CAN intent on finding the issue and solving it rather than avoiding it. We haven't had the problem with this year's firmware and CAN driver. We did have problems with the original 2CAN due to static but we were able to overcome that until we could get the redesigned 2CAN just before the start of the season.

There are tradeoffs with every system. Every time we build another robot we have cable issues until all marginal ones are found and replaced. After that we have had very little trouble. As several have alluded to, a cable problem usually takes the whole system down due to daisy chaning and failed termination. I intend to create a bus system that reduces this weakness similar to the ones referred to above. The 2CAN web page is invaluable in finding problems quickly. It can even be monitored through the router while driving to see motor loading and intermittant problems.

One side issue I would like point out is the Jaguar overcurrent protection vs Victors. I have had a few motors fail in the last year and a couple with very low resistance. The Jaguars shut down fast enough to keep the battery voltage above the reset point of the Crio and D-Link. I have heard of shorted motors on victors in at least a couple of cases that left robots dead while the system rebooted. Sometimes the quick overcurrent protection is worth something. We have had no problem with overcurrent shutdown on properly geared systems.

One of the reasons we continue to come back to CAN is the inherent current monitoring on every motor. One use is to protect motors in parallel. Our winch this year uses worm gearboxes in parallel to eliminate backdriving. if one motor doesn't drive the other is stalled and fails quickly. current monitoring allowed shutdown fast enough to avoid this problem.

As for PID control on board. the testing we have done has worked well but we always wind up back in the Crio because of lack of features in the Jaguar. Syncing two motors is a big issue. If there were some filtering options and the configuration was non-volitile I think we would be using it.

It will be interesting to see what transpires with TI backing out of manufacturing Jaguars but if we have to go back to PWM I'll be dissapointed.

My opinion is that a lot of the challenge of robot design is in finding how much technology you can incorporate into your design without making it the weak link. This about science and technology, isn't it?

[mikets]

Originally Posted by JDNovak

As several have alluded to, a cable problem usually takes the whole system down due to daisy chaning and failed termination. I intend to create a bus system that reduces this weakness similar to the ones referred to above.

Our team ran 7 Jags on a T-tap bus topology like the following:
http://www.chiefdelphi.com/forums/sh...t=99554&page=3
We never had any problem with it. We had a couple of bad CAN cables but they were quickly identified and replaced. Since they weren't daisy chained, only the bad ones showed up with problems, the others kept on working.

[enrique]

Hello all. This year team 1251 started off 100% CAN this year at the Orlando regional. Our robot worked in the pits, but would not work on the field. We switched to PWM, for the mean time and it started working. Later we found out our serial adapter was missing the resistor that it was supposed to have. My guess is that the missing resistor opened the CAN network to interference from the field. We then put the CAN network back on and half of the motors on it. 2 motors on the drive system where PWM and the other 2 where CAN. Our shooter motors where also 100% CAN. Everything worked fine, except we had 1 PWM pop out and one of the PWM drive motors stopped, even with glue on the connector. When we went to the regional in Fort Lauderdale, we switched 2CAN. I really was impressed with 2CAN. Plus overtheroad electronics provided good support. We did have some of the CAN network go out 1 time in Fort Lauderdale. I found it was a 12 pin connector we used from radio shack, built up corrosion and would cut power to a couple of the jaguars. The connector pins where black (Char). Removing this connector fixed the problem. I hear the current protection is being changed. I have to say, this year there where motors plugged in backwards (opposing). Each time this happened, the jaguars went to protect mode and we fixed it. In the past I have had victors blow, because of this. Of course, you should not wire the motor backwards, but its nice to know it will not cost you money if it happens. I have read some threads with people causing the jaguars to go into protect mode with correct wiring. I would venture to say the motor in some cases are working to hard and probably do not have a proper gearbox for the job. But as it stands, we now feel comfortable enough and experienced enough to go 100% CAN.

[Phalanx]

Hi everyone,

Team mercury 1089 has used CAN in a variety of different setups with varying results.

We used CAN over our entire robot in 2010, and while it worked well, it did suffer from "brown out" conditions which caused our robot drive to stop functioning. As long as we didn't brown out, it worked well.

This past year we used CAN and the PID built into the Jaguar to manage our shooter motor, but, we used PWM with JAG's to run the drive system. We did find a bug in the Labview Periodic Status Update VI that would throw an error, but once we knew the condition and the bug, we suppressed that error.

When the JAG browns out it "forgets" your CAN configuration settings(it still remembers it's ID number) and resets to PWM operation mode by default. We really need to write some code to detect the brown out condition and reinitialize the JAG with our settings.

[connor.worley]

Maybe I can provide some insight as to why a team wouldn't use CAN. My team chooses not to use CAN for two big reasons:

• We've had multiple Jags fail in testing
• We haven't had a Victor fail in testing or in our past 10 competitions (not sure about before then- that was before I joined)

The features CAN offers are appealing, but the implementation is prone to failure, at least in my team's case.

[mikets]

Last season, our team tried CAN for the first time. We read about the brownout problem and decided to do something about it before it happens to us. Our team uses Wind River C++. We wrote a wrapper CANJag class inheriting the CANJaguar class for detecting brownouts and restoring all the configurations if necessary. We never had any problem with CAN during the entire season. So we never really know if the code worked. After the season ended, we had a public event that required demo'ing our robot the entire day. It went very smoothly but at some point, our driver reported to me that he saw yellow warnings flying by the debugger saying something about brown out detected. I took a look and was delighted to see that was a warning in the new module detecting brownouts and restoring configurations. The driver didn't even notice anything wrong with respect to the driving. It just kept going normally (may be a little sluggish). It turned out, the driving demo non-stop for a few hours ran down the battery to a voltage level that started to cause brownouts. So we replaced with a fresh battery and everything was good again. If you want to look at how we implemented that class, you can access it here in case you want to port it to LabView (http://proj.titanrobotics.net/hg/Frc...rclib/CanJag.h).
Ideally, this code should be integrated into WPILib. This module basically shadows all the important configuration data. It overrides the CANJaguar::Set method so that before setting the motor power, it checks for brownout condition. If there is no brownout, no extra work done. But if it does detect brownout, all the shadowed configurations will be programmed back to the Jaguar before programming the motor power. The module also has an optimization to minimize unnecessary CAN traffic by checking if the caller is setting the same motor power over and over again (which is very common in your iterative robot loop). It will only send a CAN message to set power if the power is different from the last value.
Actually even better, if the CAN implementation of Jaguar or Talon makes configuration data non-volatile, then we don't need this code at all! But of course, shadowing values is still beneficial for CAN traffic optimization.

[techhelpbb]

Originally Posted by mikets

Last season, our team tried CAN for the first time. We read about the brownout problem and decided to do something about it before it happens to us....

I think this is excellent work and very valuable to the whole community but I've said that before.

I do want to point out that there's another issue with a brown out that may not be considered.

If you use encoders and the Jaguars brown out the pulse counts will no longer be accurate. In a velocity control sense where the system can move freely all the way around that's not a big issue. However, in a system with limits and no limit except the encoder readings that could be an issue to consider.

[mikets]

Originally Posted by techhelpbb

If you use encoders and the Jaguars brown out the pulse counts will no longer be accurate. In a velocity control sense where the system can move freely all the way around that's not a big issue. However, in a system with limits and no limit except the encoder readings that could be an issue to consider.

That's a fair scenario. I will modify the code to also restore the position count. That should take care of that scenario.

[techhelpbb]

Originally Posted by mikets

That's a fair scenario. I will modify the code to also restore the position count. That should take care of that scenario.

A modification like that is a great idea but there's something to consider with it:

The problem with quadrature and even single channel encoders is that if you fail to track their outputs momentarily you loose your accurate fix on their direction of rotation during the missing interval and the amount of rotation during that interval as well.

Absolute encoders can help because they produce directional information that is absolute to the encoder rotation/position. To a point that helps till the encoder has enough time to fully revolve and then that information will be wrong. So it gives you a little more buffer but I can see how a fast moving input to the encoder could quickly entirely revolve before a Jaguar resets to full operation.

So if the Jaguar browns out or looses power it'll cease to service the encoder interrupt and that information will be lost. Restoring the last known good value is a good starting point, but if the mechanism was influenced by gravity or momentum while the Jaguar was 'out to lunch' the old data might not be any more valid than the most current reading.

Here's a scenario:

An arm with shoulder. The shoulder has a limited range of rotation. The arm is exposed to additional load as the end effector holds an object. During the course of driving the arm tries to move the shoulder back to one limit. As it approaches that limit the forces increase and the Jaguar browns out. The routine sees the brownout and resets the encoder position to a value approaching that limit. However due to momentum the encoder is now at or past that limit. So the arm tries to move back to that limit. It's choices are: move a complete rotation which it can't so that's an overload or hit the hard stop trying to move to a fictional target and that's an overload. Either case there could be mechanical issues induced and more brown outs causing more and more attempts to reach a fictional target.

[mikets]

Yes, I understand. The software recovery solution in our CANJag wrapper class is not the answer to every brownout scenarios but I believe it will handle majority of the cases. For special case like you pointed out, you may need to do extra steps to ensure it still works after the recovery. For example, if your scenario does not tolerate any inaccuracy of the encoder due to loss of count during brownout, your code may want to also add a recovery calibration (e.g. detecting brownout and then move the arm to resting position with a limit switch, reset the encoder at the resting position, try the operation again). Since this is very scenario specific, it cannot be fixed in the CANJag wrapper generically. So it is up to your code to anticipate and recover from brownouts.

[Phalanx]

Something else you can consider in your mechanical design to go along with your brown out recovery software.

Typically we have a "RESET" button on our custom driver or arm operator station. It's function is to basically ignore just about everything except for it's "HOME" indicator and reset the manipulator to it's "HOME" position.

What we usually do is have a mechanical "HOME" for our manipulator that is a physical stop. Typically that "HOME" is monitored by a limit switch. So when the limit switch is engaged we know our manipulator is "HOME". When it's "HOME" we reset our encoder counts to zero, turn off it's motor, etc....

This does two things. It allows us to reset/recover our position, and a helps alleviate position drift.