Retrieve Limit Switch State on CAN Speed Controller

Technical Programming
,
1

Similar to this post (Spark Limit Switches) from back in 2017, I’m curious if anything has changed now that many (probably most) teams are using CAN-controlled speed controllers.

It seems like it should be fairly simple for a TalonSRX / TalonFX / SparkMax to provide programmers a flag to be able to tell if a limit switch attached directly to their breakout has been tripped, however, I have not been able to find any information about if this is possible in their user docs.

There are many cases where this would be useful, IE, not only prevent a mechanism from overrunning its bounds, but also 0 the encoder if the bound is reached.

Something like _talon.getForwardLimitSwitch() as a boolean seems like it would make total sense to have.

2

Taking a quick peak at the Rev Robotics Library API it appears this does exist now.

There are getForwardLimitSwitch and getReverseLimitSwitch​ methods that return CANDigitalInput objects which have a boolean get method.

So you could write something like the following

sparkMax.getForwardLimitSwitch(CANDigitalInput.LimitSwitchPolarity.kNormallyOpen).get()

I don’t see anything immediately for the CTRE Pheonix Library, but I’ll take a deeper look in a bit.

2 Likes
3

For CTRE, its a bit more interesting, but it does appear you can get the limit switch values:

There is a method in the TalonSRX class called getSensorCollection which then has boolean isFwdLimitSwitchClosed and isRevLimitSwitchClosed methods.

So you could write something like the following

talonSRX.getSensorCollection().isFwdLimitSwitchClosed()
2 Likes
4

That is awesome, thank you!

I must not have dig deep enough to find it in there. Though honestly, I know a lot more about the electrical than I do about the code! :sweat_smile:

5

Worth noting that the Talon supports rezeroing the encoder in hardware when a limit switch is asserted. I don’t think the Spark Max does though.

6

Oh snap really? How do you configure that?

My thought was to just try to have a command triggered when the limit switch was hit that reset the encoder value. Or have something in periodic watch the value of the limit switch.

7

The CANDigitalInput class and the optional factory functions for it have existed since the library’s creation for the 2019 season. v1.5.4 deprecates the CANDigitalInput constructor.

8

configClearPositionOnLimitF/R will do the trick.

It’s also available in the configAll API if you’re using it already.

1 Like
9

Does any of that invalidate my answer? I didn’t test anything, just posted my findings from the API Docs.

I don’t know what existed when, I simply assumed based on the prior thread and context from the OP’s initial post that something must have not-existed at some point.

10

I don’t believe so.

11

This topic was automatically closed 365 days after the last reply. New replies are no longer allowed.