A boom sensor (1, 101) for measuring the extent of extension of a boom of a lifting device, the boom sensor (1, 101) including a base (4, 104) mounted to the boom, the base (4, 104) being attached to a first part of the boom, a second part of the boom adapted to move relative to the first part of the boom as the boom extends or shortens. The boom sensor (1, 101, 201) further includes at least one wheel (6, 8, 106, 108) adapted to rotatably ride the boom, at least one biasing device (10a, 10b, 110) adapted to apply bias to the wheel 6, 8 to urge it into substantially constant contact with the boom; and at least one wheel position sensor (20, 22, 120, 122). In operation, the wheel (6, 8, 106, 108) contacts the second part of the boom, the wheel (6, 8) is movable relative to the base 4, 104 against the biasing device (10a, 10b, 110), pushing the wheel (6, 8, 106, 108) onto at least one boom contact surface of the second part and the wheel position sensor (20, 22, 120, 122) detects the angular position or the movement of the wheel (6, 8, 106, 108) to determine the extent of pay out or retraction of the boom.
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