The present invention relates to a directional obstruction detector, especially for a robotic apparatus (101). We describe a relative motion detecting device for detecting relative motion in a two dimensional plane between a first element and a second element of a robotic apparatus, said motion resulting from an external contact force transmitted to said first element, said device comprising a plurality of Hall effect sensors (208) and at least one magnet (207), said Hall Effect sensors (208) being located on a first of said first element or said second element, said magnet being located on a second of said first element or said second element, wherein a locus of points through which a first of said magnet or said Hall effect sensors may in use pass defines a generally spherical segment and a second of said magnet or said Hall effect sensors are spaced from said spherical segment.
