a known manner the operation of this sensor and target assembly is described below when the target 14 is rotated arrow f fig1 by the camshaft 16 the sensor 10 senses a sequence of variations of the magnetic field representing the length l of the teeth passing in front of it and their spacing s the curve obtained in this way is shown in fig2 in this figure the horizontal axis shows the angles u03b1 of the engine cycle varying from 0 u00b0 to 360 u00b0 and the vertical axis shows the value b of the sensed magnetic field the field being normalized as a function of the gap in the case shown in fig2 all the teeth have the same height and the target has no geometrical defect consequently the maximum field b max dent sensed by the sensor during the passage of each of the teeth is the same for all three teeth in the illustrated example the value of this field is 1 fig2 shows three passages of teeth the first two being relatively close together with the first wider than the second while the passage of a third tooth is narrower and farther from the second tooth this corresponds to the geometry of the target shown in fig1 if the geometry of the target is perfect as in the case of fig2 there is a.