Why are Limit Switches outlawed with PWM, but not CAN?

[jhersh]

I’m guessing it’s similar reasoning that almost outlawed closed-loop control on the Jaguar entirely. It’s not founded in anything technical or safety related.

-Joe

[Al_Skierkiewicz]

@PhilBot

We’ve established that regardless of the control method you are using with the Jaguars (CAN or PWM), the Limit switch inputs perform the same control function… that is: they stop the motor running a specific direction if that limit switch circuit goes open-circuit.

The only difference between the two interfaces is that CAN ALSO lets the cRIO read the state of the switches,

Phil,
I am not a programmer so tender my answer with that. It is my understanding (from inspector training) that in the CAN implementation, the state of the limit must be read, passed on the CAN bus to the Crio which then generates motor command. Under FMS the Crio is controlled and the limit switch based commands are then enabled or inhibited by the FMS command. It is for this (and others) reason that we went through several firmware revisions in that first year. As each revision was released, it was necessary to determine that any and all motion could be stopped by the FMS. If it could not, then another release was generated. The safety of the participants, volunteers and field are still the highest priority in making decisions about the control system.

Please ask the Q&A so everyone can be satisfied.

[Alan_Anderson]

@Al Skierkiewicz

I am not a programmer so tender my answer with that. It is my understanding (from inspector training) that in the CAN implementation, the state of the limit must be read, passed on the CAN bus to the Crio which then generates motor command. Under FMS the Crio is controlled and the limit switch based commands are then enabled or inhibited by the FMS command.

I’m pretty sure your understanding is faulty. From everything I have seen in action and read in the available documentation, the limit switch action is entirely independent of any commands to the Jaguar, either by CAN or PWM, whether FMS is involved or not.

[dyanoshak]

@Al Skierkiewicz

Phil,
I am not a programmer so tender my answer with that. It is my understanding (from inspector training) that in the CAN implementation, the state of the limit must be read, passed on the CAN bus to the Crio which then generates motor command. Under FMS the Crio is controlled and the limit switch based commands are then enabled or inhibited by the FMS command.

The limit switch functionality is independent of commands; there is no way to override the limit switches in software.

There is a “Soft Limit” feature that uses the encoder or potentiometer to read the position, and stops the motors at a user defined position. However, motor movement will still be stopped by the hardware limit switch opening the circuit, even if it hasn’t reached the soft limit yet.

When using CAN, it is entirely up to the user to read the limit switch states from the Jaguar. The cRIO does not issue motor commands based on the limit switch states. The cRIO issues standard commands and the Jaguar follows them to the best of its ability. If the cRIO is telling the Jaguar “move forward!” it will try to. If, however, the forward limit switch input is open-circuit, the Jaguar will not move forward, despite the cRIO continuing to send a forward command.

@Al Skierkiewicz

It is for this (and others) reason that we went through several firmware revisions in that first year. As each revision was released, it was necessary to determine that any and all motion could be stopped by the FMS. If it could not, then another release was generated.

The multiple Jaguar firmware updates last year were to fix bugs (a power-up bug, PID bugs, etc.), none of which had to do with the limit switches or the FMS. Actually, the FMS doesn’t ever talk to a Jaguar; it tells the cRIO “Disable!”, the cRIO stops sending motor commands, and the Jaguar times out.

-David

[PhilBot]

@Al Skierkiewicz

Phil,
Please ask the Q&A so everyone can be satisfied.

I have posted the question.

Whether everyone will be satisfied is a completely different issue

If nothing else, it will be interesting to hear the reasoning behind this rule.

Phil.

[Al_Skierkiewicz]

David,
I was speaking of the updates in 2009 when this control system was first introduced.

[Joe_Ross]

@Al Skierkiewicz

David,
I was speaking of the updates in 2009 when this control system was first introduced.

There were no jaguar firmware updates in 2009 because no team had the capability of loading them.

[Al_Skierkiewicz]

OK,
There were Crio, driver’s station and Lab View updates in 2009, parts of which addressed the inability to turn off Jaguars reliably at the end of a match. Not only was it not possible to update the Jaguar, there was no way to check it since CAN was not legal in 2009 nor was it legal to connect to the CAN bus connector on the Jaguar.