Using a star configuation for Jaguars on CAN

[Al Skierkiewicz]

Dave,
There is a lot of confusion out there on these jacks including the Jaguar docs. The cheaper RJ11 jacks do not always have the depressions you speak of where the two outside contacts of a six position plug would normally be located. However, I have seen teams using the handset plug (which is narrower than a standard RJ11) this year. Specifying the six position connector prevents any ambiguity. Although one pin of the connector is labeled +5volts, that is not connected to any power supply internal to the Jaguar. If other sensors are used to that provide power on that pin, misalignment could cause some other serious issues. Also a crimper that is designed for four position plugs will not push in on the outer contacts of a six position plugs when assembling these plugs.

[EricVanWyk]

I've tried in the past, with varying degrees of success. There was a major push internally a few years back to launch FRC-CAN as a star network, but in the end it was determined to not be worth the trouble, in both the $$ sense and the confusion sense.

That is not to say that it is impossible, and I'd love to see your implementation.

One that showed promise was a passive star. 60 Ohm termination in the center, no termination on the end points, and equal length arms. The difficulty here is making sure that all devices are the same distance from each other (electrical length wise). I never got this to be quite good enough to launch to 2000 teams, but it might be do-able for the N teams with good enough mentors to compensate.

Another option is an active star. A CAN-hub has one PHY per branch, and a CPLD to act as a fancy OR-gate. Expensive!

Lastly, you could do a 'vampire tap' system. This is a trunk with lines hanging off of it. The maximum length of each tap is determined by the speed of the bus. I don't really like this option.

[Dale]

How important was it to have equal length arms? If that's vital that would be a deal killer for us since all that wire hanging around would start to look ugly real fast.

An active star module that allowed every jaguar to remain at it's default address of 0 would be very cool. You'd determine which address it had by the jack it was plugged into on the star module, just like PWM. The active module would handle translating the address from the cRio and rebroadcasting the signal to each device with the proper address. The advantage is then if a Jaguar fails, you just slap in a new one. I know that's just a fantasy, though.

[EricVanWyk]

Originally Posted by Dale

How important was it to have equal length arms? If that's vital that would be a deal killer for us since all that wire hanging around would start to look ugly real fast.

I'm not positive, but I think it is important. Only way to find out is to try (and measure signal integrity).

Since I changed jobs I don't have access to the RF / SI simulators. Perhaps one of our forum members could rig something up?

Originally Posted by Dale

An active star module that allowed every jaguar to remain at it's default address of 0 would be very cool. You'd determine which address it had by the jack it was plugged into on the star module, just like PWM. The active module would handle translating the address from the cRio and rebroadcasting the signal to each device with the proper address. The advantage is then if a Jaguar fails, you just slap in a new one. I know that's just a fantasy, though.

Costs associated are:
microcontroller + FPGA per board
CAN transceiver+ Connector + terminators per leg

Not something I can justify for the KoP. Also, it takes a bit of work to make sure you don't get the bus latched into a dominant bit. Lastly, propagation delay is a concern.

[apalrd]

It seems like a star configuration would negate the advantages of using CAN over PWM. Just a thought.

[NickE]

Originally Posted by apalrd

It seems like a star configuration would negate the advantages of using CAN over PWM. Just a thought.

In theory, you could still utilize the current sensing and PID features on the Jaguars.

[Dale]

In our case we needed to use CAN for current and voltage sensing throughout our robot. For example, our front ball magnet would capture a ball and then stall by design with the current held at a constant value so as not to smoke the motor. It could also sense if it had a ball by looking at the voltage being delivered to the motors.

Our kicker would vary it's kicking power by monitoring the current going to the winch as it's being cocked.

We'd love to use the daisy chain configuration if it were reliable. We might give it another whirl next season but be ready to fall back to PWM for speed controllers that don't need all the fancy features.

[apalrd]

Originally Posted by Dale

In our case we needed to use CAN for current and voltage sensing throughout our robot. For example, our front ball magnet would capture a ball and then stall by design with the current held at a constant value so as not to smoke the motor. It could also sense if it had a ball by looking at the voltage being delivered to the motors.

Our kicker would vary it's kicking power by monitoring the current going to the winch as it's being cocked.

Interesting uses for the current sensors. We used a broken-beam sensor and pulsed the roller, although we didn't have any problems with heating (with a FP). It would spin so much that the ball would begin climbing up the front of the machine and get carrying penalties, so we pulsed the roller to keep torque on it while not continuously forcing it. If we back up, the roller runs full.

We used a potentiometer for our kicker. The kicker motor is actually on a Spike, and Spikes don't have current sensors. We had no jaguars at all (all Victors and Spikes), so CAN wasn't a question we asked. (the answer would have been NO unless we needed the current sensors).

[biojae]

Originally Posted by apalrd

(the answer would have been NO unless we needed the current sensors).

The current sensors have a way of making themselves useful.

The original design (in software) of our kicker only used an encoder and a reed switch.
This software was buggy, sometimes it would not know when to return and continue applying power after it reached the hard stop. (I blame the encoder... )

Needing a quick fix, the current sensing feature of the kicking jag was used to terminate the kick if it was stalled (meaning it either reached the hard stop, or got stuck on something else ) .

This fix worked well enough that no more modifications were needed for the rest of the season.

And like Dale, our magnet uses the current to detect a ball.
(This is used mostly for auto)
It stops it if the ball gets too far in, but isn't needed as much to prevent blowing the motor. (Its a CIM)

The use of current sensing was an after thought, but because we had the capability, it was very useful.