The present invention relates to a method and apparatus for collision detection for robots using reduction of signal strength of an ultrasound signal to determine a collision event. In particular we describe a method and apparatus of collision detection in robots, such as robot lawn mowers and similar devices, which use a transmitter to transmit an ultrasound signal through a bumper to a receiver. A collision with the bumper causes a reduction in the signal strength and hence a reduction in the measured signal at a receiver compared with the expected signal. Ultrasound frequencies used are typically in the region of 40 KHz and the bumpers are constructed from either metal or polymers such as acrylonitrile butadiene styrene.
