I was just wondering what limit switched teams have been using that worked really well for teams in the past.
During Infinite recharge we used some very flimsy limit switches on our intake system and would nearly be broken and had to be replaced after a few matches. We were just connecting them straight to the talon srx and have plan to continue using them I just want to see what options have been success for other teams as we’ve only used the very small flimsy ones we currently have (Don’t know the name of the currently) and we have some bigger industrial ones that we use but sometimes feel a little unnecessary as they are so big.
We like the ones with rollers.
They work well when contacting a sloping trigger surface, which we often make from plastic and mount to the moving element with screws through slots, so the trigger point can be fine tuned.
If you’re not dead-set on mechanical switches, I highly recommend Littelfuse’s Hall-effect switches. Since they’re non-contact, if you mount them correctly, there’s no way for the mechanism to smash the switch. This is the model that we used on our 2018 elevator, 2019 climber arms, and 2020 turret: https://www.digikey.com/product-detail/en/littelfuse-inc/55110-3H-02-A/55110-3H-02-A-ND/565512
Edit: also, they run on 5 V and have open-collector outputs, so they can interface directly to a Talon SRX or Spark Max.
Funny how they don’t work more than once or twice when used as the hard stop… IIRC 330 had something like that happen in the shop quite a few years back, and it may or may not have triggered a hunt through the FP gearboxes for a replacement gear…
Plus: you can see the circuitry, has convenient LED status light just like the Littelfuse above
Minus: Requires a magnet, some work off of steel alone. Straight conformal-coated PCB, protect it yourself & run your own cable instead of getting a nice plastic case with a convenient pigtail. (Actually could be a plus: It doesn’t look like it could be a hardstop, so your designers won’t use it as one)
Also gonna throw a non-contacting limit switch in the mix, we’ve found these super easy to work with. And if you’re in a pinch it’s real easy to make a hole and mount wherever necessary: https://www.digikey.com/product-detail/en/littelfuse-inc/59065-010/59065-010-ND/43966
Another advantage of this style is that the sensor is threaded, and the two included nuts allow you to adjust the distance between sensor and magnet. This makes designing their mounting location less critical, and makes it easy to adjust as mechanisms get abused in season.
It would be nice to have an LED on them, but we’ve got a breakout board for the SRX to provide that for us now.
I try to advocate for non-contacting switches …always. But we did end up using normal contacting limit switches on our hatch panel manipulator in 2019 (couldn’t get consistent detection of clear/colored areas of the panel with IR sensors).
We used these …because they had one feature we desperately needed. They are red and black.
They were likely to see nearly the full brunt of the robot crashing into the wall repeatedly to acquire panels, so we specifically designed their mounting surface with a profile that protruded slightly past the edge of the limit switch body so all this force would be directed into the panel manipulator and not the switches themselves. …because you never want to use a limit switch as a hard stop.
We played ~116 matches plus our in shop practice time w/ these cheap-o switches. I think we had one fail in season.
We started using the rev magnetic limit switched in 2019 and have loved them since. Very easy to use and the blue led is super nice. Magnetic Limit Switch - REV Robotics
Seconding the Rev mag switch mentioned above. Comes with a magnet, LED indicator, nice housing, and good price. Easy to connect. We’ve had good experience with them.
While bulkier and a tad bit heavier, I’m a big fan of using whisker switches in applications you can fit them. Their design inherently discourages the usage of them as hard stops, and also basically reduces the odds of a mechanical failure in the switch to close to zero. You can still have some contact issues with cheaper switches, but that’s true of any limit switch variation.
We’ve purchased them from Automation Direct (PDV!) and Amazon before, but they’re also available from McMaster, Grainger, Digikey, and a number of other vendors.
They’re a decent option, quite bulky.
Technically still vulnerable to acceleration/vibration false trips as well, though I’m not sure how likely that is if you’re not pushing robot performance to hard physical limits the way 971 does.
I put some on a work setup where the omnidirectional trip was helpful for an off-axis movement in a HALT tester, and started failing the springs around… 200,000?.. it still mostly worked, just progressively jankier from about 200k on. Replaced somewhere well clear of 300k. In retrospect I could have put more effort into getting the mounting right to at least put less stress on the spring, or put the sensor on-axis and use a more durable conventional switch.
(HALT testing is a “test to failure” condition, our objective was >40k cycles across 4 samples, and we failed between 15k & 30k a few times…)
But yeah - they’re plenty strong for FRC, and the omnidirectional trip is sweet.
Inductive sensors make good contactless limit switches. Example: https://www.amazon.com/Taiss-NO（Normally-LJ12A3-4-Z-inductive-Proximity/dp/B073XD44CW#ace-g9859629705
Very easy to mount and adjust. Triggers when steel comes close. They work with aluminum as well but lose range. Has a little LED to tell you if it’s triggered. I like them more than hall effects because they’re more adjustable and don’t need a dedicated magnet. You definitely cannot use them as hard stops.
Downside is that most are 12V operation. Make sure you get a 5V one and check the output.
These aren’t limit switched per se but can be hooked up and/or powered by a Talon using the proper breakout board. The key is being powered by 5V. I can’t stress enough, generally speaking, any open input (i.e. the analog input and the limit switch inputs) on a Talon can be used as a DIO and read by the RoboRio. These are pretty inexpensive too.
Minor note on the non-contact ones - depending on which ones you get and how cheap they are, there may be some built-in hysteresis and debouncing . This may be unacceptable in some applications.
Source: Tried to use one on our 2019 robot’s arm to detect the up position. Ran two runs, one really really slowly to check the encoder position where the sensor tripped. Second one at full speed. On the fast run, there was 200ms between where the sensor tripped, and where the encoder values showed we’d crossed the threshold. Noticeable overshooting was happening, so we ended up scrapping it. We were using no-name amazon cheapos that looked like these, but no idea if they actually were. Those quote a ~7ms response time (? maybe?) so I"m guessing those are fine. But… buyer beware, it’s a thing.
On the other hand, of course, there may be other situations in which bounce is a lot worse than hysteresis (like if you’re counting items). It’s just one of those things where you have to understand the device and your requirements.
We have used the magnetic switches from bimba magnetic pneumatic cylinder kits for a couple years. Nicely sealed magnetic switch that doesn’t need power so you can connect them directly to limit switch contacts on a CTRE encoder or VP encoder with out needing a wire from somewhere else or breakout board.
We always use it with a ring magnet like the one WCP sells for their sensors, so we can easily bolt the magnet to the elevator or other moving item.
