Patent ID: 8278945

Claim:
An inductive position detector comprising an antenna device and a target device, wherein: the antenna device comprises a plurality of antenna windings, the target device comprises a passive resonant circuit formed by a target winding in series with a capacitor; one of the antenna and target devices defines a measurement path and the antenna and target devices are arranged for relative movement along the measurement path; the antenna device is configured to induce an alternating current in the passive resonant circuit, in use, such that the induced alternating current in the passive resonant circuit induces an alternating signal in the antenna windings by virtue of the mutual inductance of the antenna windings and the passive resonant circuit, which signals are indicative of the relative position of the devices along the measurement path; the target winding comprises a first portion and a second portion in series with the first portion and the antenna windings comprise a first antenna winding arranged to interact with the first portion of the target winding and a second antenna winding arranged to interact with the second portion of the target winding; at least one of the first portion of the target winding and the first antenna winding is configured such that the distance of the winding to the measurement path in the direction transverse to the measurement path varies with distance along the measurement path according to a first predefined relationship such that the mutual inductance of the first portion of the antenna winding and the first target winding is a function of the relative position of the devices on the measurement path; at least one of the second portion of the target winding and the second antenna winding is configured such that the distance of the winding to the measurement path in the direction transverse to the measurement path varies with distance along the measurement path according to a second predefined relationship such that the mutual inductance of the second portion of the target winding and the second antenna winding is a function of the relative position of the device on the measurement path; and the rate of variation of the transverse distance of the winding to the measurement path with distance along the measurement path is greater according to the first predefined relationship than according to the second predefined relationship.