Has anyone considered using the Trinamic drivers to do sensorless dual endstops? For those who aren’t aware, some of the stepper drivers produced by Trinamic can sense the feedback from the force being exerted on a stepper (like running into something).
Sensorless Endstops?In 3D printing, this is usually used to allow the printer to do it's homing without mechanical endstops (it can detect when it bumps into the frame). I'm wondering about the feasibility of doing this with the Lowrider/MPCNC. First, the accuracy of the stall detection is good to 1 stop at best and 4 at worst. Assuming the worst case scenario of 4 stops, that's 1/50mm at our current 200 steps/mm X/Y settings, or less than one thou. This is actually quite a bit worse than mechanical endstops, but I suspect still way smaller than a laundry list of other factors that could affect accuracy. Of note: The additional cost of the drivers is more than just adding physical endstops, but would be simpler to set up (I think).
Stall Detection?Currently, Marlin only uses the stall feedback for sensorless homing, but one interesting safety consideration is the detection of stalls. If the router got "hung up" on something due to being too deep, hitting a clamp, etc. then the router could theoretically detect the event and shut down the operation, potentially preventing a fire. This wouldn't be trivial. The loads present in a CNC are way higher than something like a 3D printer. I suspect they're pretty dynamic too. DOC, speed, etc. would all change the range of force which would be considered "normal". I do wonder, however, if you could measure the feedback for the first X inches of cut, use it to set a baseline, and then use that baseline in concert with an acceptable buffer amount and perhaps a bit of hysteresis to detect problems. This definitely depends on just how dynamic the cutting forces are though, which is something I'm currently ignorant to. You could absolutely start passively measuring feedback throughout normal usage, though, and find out how feasible this is. If your spindle is relay controlled then you could pretty easily turn it off. If not then you could just raise it to ZMax.
The recently released TMC2209 can handle the full current of our steppers, as can the older TMC5160. I snagged 5 of them as well as an SKR Pro board which should let me do dual endstops, laser cutting/etching, and digitally controlled air assist. I won’t have it for 4-6 weeks though so I’ll probably do my initial build with my spare SKR 1.3 and DRV8825s. I’m a programmer by trade and maintain some Trinamic Marlin firmware for the Ender 3 so I think I can handle the code side.