Abstract: The invention relates to a device to maintain the alignment of two articulated elongate members, particularly adjacent spans of a mobile irrigator or pivot, there being a flexible joint between the spans, connecting them together and having freedom of rotation about the x- and y- axis and limited freedom to rotate about the z-axis, the joint including a sensor device for sensing movement and converting same to electrical energy for feeding to a computer for correcting alignment. The sensor may be a Hall Effect device or an optical sensor device.