Force sensing elastomer?

Your force sensing rubber would probably be best. However, methinks a rubber hose, run along the inside of the rectangular frame, and connected to an air pressure switch, would probably suffice.

Or, just some carbon doped polyurethane foam like the anti-static stuff used for shipping IC's.

I still think the rubber hose is easier and better, especially since it doesn't absorb water and is very easy to clean.

That's an interesting idea. Unfortunately the site is within a gimbaled structure which is subject to sudden movements - I would conjecture that distinguishing a small-area contact (say, one fingertip) from a false positive from a sudden motion of the hose (the hose pathway distorted as it turns) might be an iffy proposition. In fact it is this gimbaled structure that is the potential pinching device - probably not adequately explained before.

I'll look into those links, though, thanks!

-F

I'm wondering if a theremin-type circuit could be used, to respond to the capacitive coupling of a finger nearing or in the aperture.

We've used capacitive sensors in some applications. The difficulty in this application is that the framework is grounded and the hand/fingers are not. Hard to see how we could detect a finger/nail in the limited space without falsely responding to the mechanism itself which, when properly operating, has surfaces that get very close to one another.

-F very close. This is, after all, the potentially pinching mechanism.

Interesting product, but WAY to big for our device. The whole aperture is not much bigger than a fist. The aperture that potentially endangers is only big enough for a finger...so we're looking for something small.

-F

Capacitive sensing does work, Frank, even in environments such as yours. I do not have reference materials at hand, but see Cypress application notes concerning this.

Well, make a miniaturized version of it.

You can reinforce the corners with hard metal tubing, either internal or external. You won't have any sensing in those corners, but the rest of the hose should be unaffected.

If pinching is a problem, fill the hose with water which is incompressible. That should give it a more positive actuation.

Long ago, I designed a retrofit safety interlock system for what amounted to a stamping press. There were too many restrictions on running wires or conduit across the work area. I opted for two rubber bladders and some vinyl tubing with pressure switches normally used on an air compressor at the ends. Later, these morphed into solid state pressure switches. While not the most elegant solution, the kit was shipped for about 8 years until the press was deemed obsolete and an operator platform with built in conduit and hand sensors was delivered.

I'm not sure if a rubber hose would detect a single fingertip. Mine was initially detecting when the room door was slammed but that was with overkill sensitivity. If you can't get sufficient sensitivity with a pneumatic system, then the hydraulic equivalent will certainly be more sensitive. However, I see problems. One of the tricks I used with the pneumatic system was to pressurize the system to a few PSI above atmospheric. That allowed me to detect both falling and rising pressure, both of which were important. Falling pressure was a leak, which required an immediate system shutdown. Rising pressure was activation by the operators hands. That's easy to do with pneumatic, but not so easy with hydraulic, which would require a combination of hydraulics and pneumatics to do the same thing. It's not impossible, just more difficult.

Sounds like you had an excellent solution. Unfortunately my device is much smaller, with considerable dynamic control issues. This greatly reduces any fluid volumes (evaporation?) and makes hose robustness a potential problem. If it's anything like its predecessors, will be operated for many years by a parade of grad students and research techs - so it has to be pretty bulletproof.

I had a tentative solution now - using a cantilevered shim-stock material over a set of low-profile, low-force "tactile" switches, essentially a custom microswitch. My machinists will have to make a few more parts but in comparison with the rest of this device that's nothing! The total added mass is fairly low, and its overall simplicity is hard to beat. Hopefully getting the sheet stiffness right - stiff enough to reliably depress at least one of the switches without too much force applied - won't be too exacting.

Thanks everyone for your suggestions!

-F

Everyone that saw it hated it. However, nobody could offer a cheaper or better replacement, so it stayed. However, the rubber hoses were not the main issue. I initially had problems finding a suitable bladder for the operator to depress with both hands to operate the press. This was the best I could come up with in the mid 1970's:

I disguised and repurposed it as best I could, but its original function soon became obvious. The production version was somewhat more compatible with an industrial environment.

Rubber or vinyl hoses are quite liquid tight and nothing will evaporate through them. It's a closed system. You might get a leak or blow out if it over filled. If you use water, there won't be any hazmat issues.

Grad students are expendable. Salaried research techs are only slight less so. Both are guaranteed to break anything they touch, so I agree that it should be made reasonably bulletproof. The easiest way to do that is to give them something to break. Add some kind of easily broken mechanism, that does nothing useful, but will relieve them of their natural urge to break things. Once this urge is relieved, I suspect the rest of your device will be fairly safe.

I dunno. If it's made from shim stock in order or it to be sufficiently flexible to act as a hinge, it will not be stiff enough to operate a microswitch (or one of the cheap little switches found inside computah mice). It will also have very different travel distances between directly under the switch compared to between switches. However, if you use a real piano hinge type mechanism, this potential problem can be eliminated.

Beware of simple solutions to complex problems. They rarely stay simple.

Depending on the distance between switches, I believe you will not be able to make it work without adding stiffeners to the shim stock or a real piano hinge.

Good luck.

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A single LED / phototransistor pair across the gap?

Are you absolutely certain you cannot just cover the aperture with a plate to keep their fingers out?

shim metal sounds problematic, can you use something rigid? thicker metal or Heat formed PMMA perhaps?

how about a encircling spring that contacts the frame tubing when deflected? put a silicone rubber sheath over it if needed.

Thanks, I'll look into that.

We haven't figured out how cover the space that we need to - at least so far.

We have added a couple guard plates (one to the left, one to the right). Remaining problem areas are above and below, which have resisted this wonderfully simple and robust solution.