Abstract: A method and device for controlling a displaceable object operated by a motorized drive unit, for example a window pane, sliding roof or the like in a motor vehicle, and is particularly suitable for use as an anti-collision protector. By using surface waves in the area being monitored, it allows a simplified attachment of transmitter and receiver on the displaceable object. At least one mode converter is mounted in active connection with the area being monitored or with an area connected to this area wherein the mode converter (a) converts a volume wave running into the monitoring area (5,5') into a surface wave and/or (b) converts a surface wave running into the receiver into a volume wave. The mode converter is preferably constructed from several periodically arranged elements which are mounted along the propagation direction of the surface wave and have a division t which corresponds approximately to the wave length .lambda. of the surface wave.