A method of assembling a position sensing arrangement for sensing the position of a revolute joint of a robot arm, the position sensing arrangement comprising a magnetic sensor assembly (11, 12) and a disc having an inner magnetic ring with j magnetic pole pairs and an outer magnetic ring with k magnetic pole pairs, where |j-k|>1 and wherein the shape and size of the magnetic poles on the inner and outer magnetic rings are the same, the inner radial boundary of the disc being constrained by a shaft of the robot arm and the outer radial boundary of the disc being constrained by a housing of the robot arm, the method comprising: determining a number of pole pairs of the inner magnetic ring p such that a centreline of the inner magnetic ring is separated from the inner radial boundary of the disc by at least the portion of the radial width of the magnetic sensor assembly (11, 12) which is disposed between the centreline of the inner magnetic ring and the inner radial boundary; determining a number of pole pairs of the outer magnetic ring to be an integer q such that the outer magnetic ring is radially separated from the inner magnetic ring by at least the circumferential length of each magnetic pole pair; and if p and q are co-prime: selecting j to be p and k to be q; and assembling the position sensing arrangement by: mounting the disc to the robot arm such that both the disc and the revolute joint are permitted to rotate about the same axis; and mounting the magnetic sensor assembly (11, 12) to the robot arm so as to enable detection of relative rotation of the disc and the magnetic sensor assembly. determining a number of pole pairs of the inner magnetic ring p such that a centreline of the inner magnetic ring is separated from the inner radial boundary of the disc by at least the portion of the radial width of the magnetic sensor assembly (11, 12) which is disposed between the centreline of the inner magnetic ring and the inner radial boundary; determining a number of pole pairs of the outer magnetic ring to be an integer q such that the outer magnetic ring is radially separated from the inner magnetic ring by at least the circumferential length of each magnetic pole pair; and if p and q are co-prime: selecting j to be p and k to be q; and assembling the position sensing arrangement by: mounting the disc to the robot arm such that both the disc and the revolute joint are permitted to rotate about the same axis; and mounting the magnetic sensor assembly (11, 12) to the robot arm so as to enable detection of relative rotation of the disc and the magnetic sensor assembly. selecting j to be p and k to be q; and assembling the position sensing arrangement by: mounting the disc to the robot arm such that both the disc and the revolute joint are permitted to rotate about the same axis; and mounting the magnetic sensor assembly (11, 12) to the robot arm so as to enable detection of relative rotation of the disc and the magnetic sensor assembly. mounting the disc to the robot arm such that both the disc and the revolute joint are permitted to rotate about the same axis; and mounting the magnetic sensor assembly (11, 12) to the robot arm so as to enable detection of relative rotation of the disc and the magnetic sensor assembly.