[EricH]

Most of those questions will depend on the ref and on what exactly is happening.

1) Accidental hit to opposing robot: Provided you aren't inside their frame perimeter, it'll almost certainly be incidental contact, no penalty. If it's inside frame perimeter, but does no damage and is accidental, most likely no penalty.

2) Intentional hit to opposing robot: <G48> doesn't really say, except for the case of damaging or intentional contact inside the frame perimeter, which is a penalty plus a yellow (also note that if it's egregious, yellow or red could be pulled). Probably OK otherwise, but watch out for <G48>.

3) Your robot's arm takes damage from the contact: The damage is your penalty, along with whatever else you get (see above). If it's obvious that your opponent is trying to damage it, they might get a penalty/yellow card.

4) An opponent comes under your arm and hits your bumper: No penalty. They're not in your frame perimeter. They're inside your playing configuration, but that's not a penalty.

In other words, you're probably OK unless you're doing damage to an opponent's robot. But, I'm not a ref, I'm not on the GDC, and CD isn't an official source, so your mileage may vary greatly.