TRANSDUCER

Abstract: A non-contact transducer defining a potentiometer has an elongated anti-ferromagnetic substrate (1) carrying a lower ferromagnetic layer (2), an electrically insulating layer (3), an upper ferromagnetic layer (4) and an upper, non-magnetic, electrically conductive layer (5). Layers 1 to 4 define an SDT material. The layers 2 and 4 respectively define an electrically resistive track and an electrically conductive track. The transducer also includes a magnet (6) which is movable relative to the tracks. In use, electron tunneling is possible through a localised region of the layer (3) adjacent the magnet so that such region is rendered more electrically conductive than other regions of the magnetoresistive material.

Description: TRANSDUCER This invention relates to a transducer (or potentiometer) which may be used for indicating position, and is more particularly concerned with a non-contact transducer, i.e. a transducer which does not have a moving contact which is in sliding contact with a resistive track. Non-contact transducers have the advantage over contact-type transducers in that there is no wear of the electrically resistive track because there is no moving part in sliding mechanical contact with the track. One previously proposed type of non-contact transducer is an optical transducer or potentiometer (see for example WO 96/12304) in which a photoconductive layer is provided between a pair of mutually spaced electrically resistive tracks. In use, a concentrated beam of light from a light source is directed onto the photoconductive layer between the tracks so as to cause a localised region of such layer to become conductive and thereby locally bridge the tracks. Relative movement between the incident light beam and the tracks is effected to vary the overall electrical resistance of the tracks. This type of non-contact device, however, requires the provision of a light source and a beam- aiming and concentrating arrangement as well as some form of light seal to prevent external light from reaching the photoconductive layer. It is an object of the present invention to provide a constructionally simple type of non-contact transducer. According to the present invention, there is provided a non-contact transducer comprising a pair of electrical tracks which are spaced apart, and a member spaced from the tracks and mounted so that relative movement can be effected between the member and the tracks longitudinally of the direction of extent of the tracks, wherein the member comprises a magnet and wherein a magnetoresistive material is provided in the space between the tracks, the arrangement being such that, in use, a localised region of the magnetoresistive material adjacent the magnet is rendered more electrically conductive than other regions of the magnetoresistive material. The transducer according to the present invention can be used as a potentiometer whose electrical resistance varies depending upon the position of the magnet relative to the tracks or as a position transducer wherein the longitudinal position of the member relative to the tracks can be measured by reference to the electrical resistance of path defined by the tracks and the localised region of the magnetoresistive material. The magnetoresistive material may be a so-called Spin Dependent Tunnelling (SDT) material or it may possibly be a so-called Giant Magnetoresistive (GMR) material. SDT materials are produced by thin-film technology and consist of two upper and lower ferromagnetic layers (typically NiFeCo or CoFe) separated by an insulating layer such as Al 2 0 3 . The magnetic layers are deposited in a magnetic field to achieve the desired anisotropy. In an SDT material, conduction electrons flow from one magnetic layer to the other in a current perpendicular-to-plane (CPP) mode. In use, the lower magnetic layer has its magnetic moment pinned by anti-ferromagnetic coupling to a base layer. The upper magnetic layer has a magnetic moment that can be switched by an external magnetic field. When the magnetic moments of the upper and lower layers are anti-parallel, there is the least path for the SDT current and the electrical resistance between the two layers is at a maximum. When the upper layer is switched by the application of an external magnetic field, the path for the SDT current is increased and the resistance between the two layers is at a minimum. The SDT current (and hence the resistance) is related to the magnetic field applied and the degree of saturation. GMR materials are also produced by thin-film technology and consist of metallic films only a few nanometers thick made up of at least one pair of first and second ferromagnetic layers separated by a thin non-magnetic layer. When no external magnetic field is applied, the non-magnetic layer causes the magnetic layers to couple anti-ferromagnetically to each other with the magnetic moments of the first magnetic layer being aligned anti-parallel with the second magnetic layer. Conduction electrons spin polarised in the first magnetic layer are likely to be scattered when they reach the interface to the second magnetic layer where they meet conduction electrons with anti-parallel spin. This electron scattering gives the material a relatively high electrical resistance. When an external magnetic field is applied, the anti- ferromagnetic coupling is overcome and the magnetic moments in the two magnetic layers are now spin polarised in the same direction, resulting in a reduction in electron scattering and therefore a corresponding reduction in the resistivity of the GMR material. By multilayering, GMR materials can be made such that the change in resistance, ΔR/R, is as high as 50%. The electrical tracks are preferably constituted by an electrically conductive track and an electrically resistive track. The electrically resistive track may be defined by one of the ferromagnetic layers of the SDT or GMR material. The electrically conductive track may be provided by an electrically conductive layer extending over the other ferromagnetic layer of the SDT or GMR material. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawing which is a schematic longitudinal section through one example of a potentiometer according to the present invention. Referring now to the drawing, the potentiometer comprises an elongated anti-ferromagnetic substrate or base layer 1 upon which are deposited in turn, by thin film sputtering technology, a lower ferromagnetic layer 2, a thin ( approx. 2 nm ) electrically insulating layer 3, an upper ferromagnetic layer 4, and an upper, non-magnetic, electrically conductive layer 5. The layers 1 to 4 together define an SDT material. The anti-ferromagnetic base layer 1 serves to pin the magnetic moment of the lower ferromagnetic layer 2 so that it is anti-parallel to that of the upper ferromagnetic layer 4, as shown by the directions of the arrows in such layers in the drawing. The lower ferromagnetic layer 2 and the upper electrically conductive layer 5 respectively define an electrically resistive track and an electrically conductive track of the potentiometer. In this embodiment, the anti-ferromagnetic base layer 1 is formed of NiO, the layers 2 and 4 are formed of NiFeCo and CoFe respectively and typically have a thickness of 125 A, the layer 3 is of Al 2 0 3 , and the conductive layer 5 is formed of Cu, Al, Au or Ag. However, it is to be understood that any suitable materials may be chosen for such layers 1 to 5. The potentiometer further comprises a magnet 6 which is spaced by a narrow gap above the upper conductive layer 5 and which is mounted for sliding movement longitudinally of the tracks defined by the layers 2 and 5. Typically, the magnet 6 is formed of NdFeB, SmCo or AlNiCo. The magnet 6 is mounted for sliding movement in a housing (not shown) of the potentiometer and has an upwardly extending operating arm (also not shown) projecting through a slot in the housing so as to enable the magnet 6 to be moved longitudinally of the tracks. The resistive track defined by the lower ferromagnetic layer 2 has electrical contacts 7 and 8 at opposite ends thereof so as to enable a potential difference to be applied across such ends. The upper conductive layer 5 has a contact 9 at one end thereof to enable an output voltage signal to be taken. In use, at the locations along the layers 2 and 4 where the magnetic moments therein are anti-parallel, there is no path for the SDT current. However, the magnetic moment of the upper ferromagnetic layer 4 is switched in the locality of the magnet 6 so that the magnetic moments in the layers 2 and 4 at such locality are mutually parallel. Thus local conduction by electron tunnelling through the insulating layer 3 takes place. Accordingly, the voltage signal which appears at the contact 9 is dependent upon the position of the magnet 6 along the tracks.

Claims: CLAIMS 1. A non-contact transducer comprising a pair of electrical tracks which are spaced apart, and a member spaced from the tracks and mounted so that relative movement can be effected between the member and the tracks longitudinally of the direction of extent of the tracks, wherein the member comprises a magnet and wherein a magnetoresistive material is provided in the space between the tracks, the arrangement being such that, in use, a localised region of the magnetoresistive material adjacent the magnet is rendered more electrically conductive than other regions of the magnetoresistive material. 2. A non-contact transducer as claimed in claim 1 , wherein the magnetoresistive material is a Spin Dependent Tunnelling material or a Giant Magnetoresistive material. 3. A non-contact transducer as claimed in claim 1 or 2, wherein the electrical tracks are constituted by an electrically conductive track and an electrically resistive track. 4. A non-contact transducer as claimed in claim 3 when appended to claim 2, wherein the electrically resistive track is defined by one ferromagnetic layer of the Spin Dependent Tunnelling or Giant Magnetoresistive material. 5. A non-contact transducer as claimed in claim 4, wherein the electrically conductive track is provided by an electrically conductive layer extending over another ferromagnetic layer of the Spin Dependent Tunnelling or Giant Magnetoresistive material. 6. A non-contact transducer as claimed in claim 5, wherein the layers are provided upon an anti-ferromagnetic substrate. 7. A non-contact transducer as claimed in claim 1 , wherein the tracks are defined respectively by a ferromagnetic layer providing an electrically resistive track and a non-magnetic, electrically conductive layer providing an electrically conductive track, and wherein these layers are separated by (i) an electrically insulating layer sufficiently thin to permit localised electron tunnelling when the magnet is in its vicinity and (ii) another ferromagnetic layer adjacent to the non-magnetic, electrically conductive layer.

Inventors: ASHRAF MUFTI MOHMED, TEDSTONE SEAN FRANCIS, ASHRAF, MUFTI, MOHMED, TEDSTONE, SEAN, FRANCIS
Applicants: ASHRAF MUFTI MOHMED, PENNY & GILES ELECTRONIC COMPO, TEDSTONE SEAN FRANCIS, ASHRAF, MUFTI, MOHMED, PENNY & GILES ELECTRONIC COMPONENTS LIMITED, TEDSTONE, SEAN, FRANCIS
Application year: 1997
Main classification: G01D5
IPC codes: G01D5, H01F10