[ksafin]

A really popular method in my team is a spring.

If your grabbing mechanism, say, a claw, is attached to a spring that's hooked up to a stationary point at the top of your bot, then:

1) When the claw grabs a regular ring, it will expand by X centimeters.
2) When it grabs a weighted ring, it will expand by 3X centimeters.

You'll be able to see it very easily.