an amagnetic steel outer strip this outer strip protects the other strips against mechanical damage the encoder 3 is glued into the groove 11 after cutting it to the correct gap however a short length in the order of 1 mm to 2 mm of the groove 11 may be left uncovered by the encoder given that it is extremely difficult to butt joint the two ends of the strip the encoder 3 is cut without reference to the poles for example in the middle of a north pole or at the edge of a south pole thus it is not known if the alternation and the dimensions of the poles are complied with accordingly in and near the area with no encoder no reliable measurement is possible and the sensor 4 may falsify the position detection on passing over this area to overcome this problem the outer ring 2 can be equipped with two sensors 4 in respective radial holes 12 accordingly regardless of the angular position of the rings 1 and 2 at least one of the two sensors 4 is facing a portion of the groove 11 equipped with the encoder 3 the electronic system is then adapted to be able to recognise abnormal data from one of the sensors 4 due to its passage near the area of the.