Building or purchasing: A robust 250mm x 250mm flat square button

[rs20]

I'm interested in setting up speed climbing start and stop pads (basically, big buttons), preferably as close as possible to the official IFSC regulations. In summary, the key requirements are:

• Can withstand a large human jumping on/punching the button repeatedly
• The "sensitive area" (i.e. the button itself) should be a 250mm x 250mm square (+/- 5%)
• Overall thickness < 20mm
• Actuated by at most 5daN (i.e., 50 N or ~5 kgf)

However, I can't find the right search terms to find or buy something with such specific specifications (short of getting the overpriced complete systems from IFSC-certified suppliers), and building something that needs to be that robust seems daunting. Any ideas of how you would reasonably affordably buy or build this item would be most greatly appreciated!

[RoGeorge]

The closest search term I know would be (anti) "vandalism buttons", and find a model that can fit under your platform.

[artag]

If you want a readymade switch, a doorbutton for use at a disabled entry might (with an enlarging plate fixed to it) work, or a machine operators pedal.

I think I'd ignore the switch part initially and just start with a metal plate in a welded frame (some existing frame like one for a buried hydrant might work) with positive stoips and springs so the plate returns to the unpressed state and cannot be overpressed.

Then, add switches (or other actuator) to measure the position of the movable plate, such that the plate is not relying on the switch spring, and the plate movement cannot overstress the switch. Use, for instance, lever microswitches where overtravel will just temporarily bend the lever, not overtravel the switch button. Don't  forget to make sure it will still operate at least one switch if the plate is pressed at an edge only, rather than the centre.

Pneumatic buttons or touch switches may also be a possibility if the regulations permit. Touch switches are not always reliable.
 

[rs20]

I noticed the plates on the ones at the olympics translated evenly, rather than pivoting around the springs opposite the side they were pressed. I wonder if that can only be achieved with torsion bars (e.g. the anti-roll bars in your car, or the thing in your keyboard spacebar that causes the whole thing to move up and down even if you press one end of it).

I don't have access to welding gear btw 

[FrankT]

Have you considered strain gauges?  Effectively you'll be building a set of digital bathroom scales, but it would work as a switch when the weight/force measured exceeds a maximum.

[artag]

You could do the parallel-movement action by having a T-shaped button (in 3D, ie support in the centre) with the support in a very precise bearing such as a linear slide. A car's brake slave cylinder might do. The essential thing is to stop 'crabbing' which is caused by the shaft tilting slightly in the bearing and creating a high friction hotspot. This requires either a very low-friction or a very long bearing.

https://www.manholecovers.co.uk/catalogue/block-pavior-covers/ or https://www.ebay.co.uk/itm/274876384290 can give you a nice heavy welded frame.

I like FrankT's idea though. just get some cheap strong digital scales and hack the electronics. You might need to strengthen the endstop.

[EPAIII]

I do not know just how the official buttons are constructed, but speaking with some mechanical knowledge as well as some knowledge in fabrication, I have to say that making a 250mm button that moves in a parallel manner when force is applied to any arbitrary area of that button is going to be a difficult task. Not impossible, but difficult. Some devices achieve this to a limited degree. For example, the space bar on the keyboard I am using attempts this but is only partially successful. I can see a noticeable angle in it's orientation when I press it at either end. So, it works, but is only partially successful at it.

So I would abandon the idea of any requirement that the button must move in a parallel orientation. Somewhat parallel, perhaps, but not completely so.

I would construct the button itself from a type of plastic that can withstand abuse. Nylon, for one, comes to mind. Near the four corners I would place bolts with springs around them. The bolts would enter from the rear through holes in the framework. The four springs would be selected to provide the needed resistance to support the button while allowing it to be activated by the specified force. I would have those springs inside some counterbores in either the button or the supporting frame or both. The bolts would be selected and adjusted to provide only a small amount of movement, perhaps 2 to 5 mm.

Since the button plate is allowed to tilt at an angle, the switch must activate with different amounts of travel. This must be taken into account. One method would be to place multiple switches, wired in parallel, perhaps at the four corners. That would work and it also adds redundancy. But it is more complex and will cost more. And they would still need protection against high forces when there is over-travel so that complexity is needed four times.

 At the center of the supporting frame I would place a single, good quality switch which has some allowance for over-travel with no damage. The micro switch with a spring arm is one way. Another would be to use a spring between the switch and the button. Either approach would work. You could even fabricate a spring style, contact but a commercially available switch would be far more reliable. The position of the switch along with the spring would be adjusted to allow positive activation no matter where the button is pressed.

By using rather large screws/bolts at the corners the device should be strong enough for human abuse. I would try 6 mm bolt size at first and move up from there if problems are found. A high grade bolt would help. The springs should have a small movement vs. their overall length and diameter for long life.

[ve7xen]

I believe the IFSC timing pads have no or very limited travel. I'm not sure what sensing mechanism is used, but since it is intended to be force-based, probably something similar to safety switch mats or pressure sensitive mats for security systems. I wonder if a typical flex sensor is sensitive enough to detect the bending of a relatively thin HDPE or similar sheet mounted in a rigid frame, but this would have nonlinear sensitivity across the pad.

I have seen break-beam type timing pads too, but I don't think these are compatible with the IFSC requirements.

[KeepItSimpleStupid]

Look at FSR's: https://www.tekscan.com/fsr-standard-and-custom-force-sensitive-resistors

Also look at the way a digital bathroom scale is designed.  Possibly use one, but limit the travel.

[NiHaoMike]

What about the sensors in dance gaming mats? They're specifically designed to be stomped on several times per second.

[EPAIII]

What about capacitance switches? NO moving parts! I believe there are IC for implementing them.

[rs20]

Thanks for the responses everyone. Just taking notes for myself, I noticed that the switch used at this competition (nice closeup at timestamp: https://youtu.be/hRdxL6MYv6Q?t=2443 ) does indeed have (limited) travel, and appears to have a design similar to EPAIII's description:

I would construct the button itself from a type of plastic that can withstand abuse. Nylon, for one, comes to mind. Near the four corners I would place bolts with springs around them. The bolts would enter from the rear through holes in the framework. The four springs would be selected to provide the needed resistance to support the button while allowing it to be activated by the specified force. I would have those springs inside some counterbores in either the button or the supporting frame or both. The bolts would be selected and adjusted to provide only a small amount of movement, perhaps 2 to 5 mm.