Robots often need bumpers to protect themselves as well as people and furniture. Many robots use a round chassis design. Those and others can be protected using a normal 1/2" garden tube. A pressure sensor and a shut end could give an interrupt.
This approach goes a little beyond that:
I used a single sensor ultrasonic sensor SRF02 and fed it into a normal garden tube. The sensor detected the length of the tube, here 167cm. It detected the open end of the tube. Shutting the end with a thumb changed the result by about 3..5mm (theoretical lamda 1/4; reflection at the open or closed end..)
Pressing the tube mildly somewhere between the ends, also leads to an echo, that gives the exact information where the tube hit an obstacle. Round omnidirectional robots (omnibots) can calculate the angle (360°/tube lengt * measured lenth= angle in degree) and behave like a ping pong ball reflected on a wall. The tube can form a ring, without triggering the US-sensor, but pressing it will give an echo. It is not necessary to completely close the tube, a soft touch, maybe 2-4mm deep, is enough to cause an echo. (why ever). I used an simple versatile ATmega8 board with LCD display to show the length.
BTW: See also my fast long distance IR - bumber