Electrical resistors – Mechanically variable – Movable contact electrically adjustable over length of...
Reexamination Certificate
2003-06-05
2004-09-07
Easthom, Karl D. (Department: 2832)
Electrical resistors
Mechanically variable
Movable contact electrically adjustable over length of...
C338S162000, C338S307000
Reexamination Certificate
active
06788187
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
The invention relates to an electromechanically controllable electric resistance element. With such an element a determined resistance, and therefore in electric circuits having a constant voltage, the current or a partial voltage can be selectively adjusted. The adjusted current and/or the partial voltage can be used as a correcting variable for an electric automatic regulation and control, respectively, e.g. of a servo drive.
With the corresponding known solutions a metal, graphite or an appropriately electric conductive composite material has been deposited upon a substrate consisting of a dielectric material, wherein for this screen printing techniques with a subsequent temperature treatment have been employed, for example. Along such an electrically conductive layer a metallic contact element is reciprocated. The contact element is pushed against the surface of this electrically conducting coating with a determined spring force. With these solutions, abrasion of the electrically conducting material due to wear also occurs which is caused by the reciprocation of such contact elements, which therefore leads to a change or the respective specific resistance during the operating period as well.
That's why in WO 00/44032 A1 it has been proposed to substitute the electrically conductive layer for a single layer which comprises at least silicon and a metal in addition to carbon similar to diamond, such that wear of the electrically conductive layer can be appropriately reduced without any additional lubricant as well.
In accordance with the teachings described in WO 00/44032 A1 such a layer is to be formed under vacuum, e.g. with an organosiloxane added, wherein in particular the content of the metal within the layer results in a reduction of the electric resistance, and the layer solely serves as an electric conductor between the mechanically movable metallic contact element and at least an electric contacting terminal.
However, the deposition of such a layer with conventional methods is only possible in a very difficult way, if at all, when a great number of such layers are to be obtained with reproducible electric properties.
If a great spreading of the electric properties of the appropriately made layers is permitted, however, it is urgently required both to calibrate each single layer equivalently and to accomplish expensive calibrations and compensations, respectively, such as with electronic means which increases the manufacturing effort of complete units adequately and influences the cost negatively.
Therefore, it is the object of the invention to provide an electromechanically controllable electric resistance element which can be economically manufactured, in particular with reproducible electric properties, and which comprises a high wear resistance and which achieves a high life period without any lubrication as well.
According to the invention this object is solved with a resistance element having the features of claim 1.
Advantageous embodiments and improvements of the invention can be achieved with the features included in the subordinate claims.
The electromechanically controllable electric resistance element according to the invention provides an electrically conductive layer per se known which is located and formed upon a dielectric substrate, respectively. The electrically conductive layer consists of a homogeneous material and is linearly shaped in a line form or is curved following a radius.
In accordance with the desired electric properties the layer is formed each with a predetermined width and thickness over the total length of the layer in order to ensure a constant specific resistance irrespective of which location a voltage tapping takes place with a contact element which is mechanically movable along the length of the layer. The thickness of the electrically conductive layer should be constant over the total length. In the normal case, this applies to the width of the electrically conductive layer as well. However, the width can also be varied over the length of the layer continuously or in bounces.
Of course, on such an electrically conductive layer there is at least another electric contacting terminal which is preferably located on a front end of the electrically conductive layer. It is possible to provide a second contacting terminal which therefore should be formed advantageously on the opposite front end of the electrically conductive layer.
The mechanically movable electric contact element is pressed with a predetermined pressing force orthogonally upon the surface of the electrically conductive layer, and is able to be moved in a translatory motion or following a circular path, as the case may be. Such a contact element can be formed from a resilient material, for example, which is bent at right angles towards the surface of the electrically conductive layer.
According to the invention, a wear resisting layer separating this layer and the mechanically movable contact element from each other is formed upon the electrically conductive layer, which is exclusively made of carbon similar to diamond and does also not contain any additional hydrogen, in contrast to the solutions known from the prior art. This wear resisting layer is in a contiguous contact with the mechanically movable contact element, and due to its mechanical properties, in particular the frictional behaviour and the achievable hardnesses, wear of the wear resisting layer does not occur although it is allowed to abandon lubricants.
It is necessary to form the wear resisting layer with a constant thickness such that, in each position of the contact element, it has a constant electric boundary resistance between the contact element and the electrically conductive layer which will be added as a constant value to the electric resistance which is determined by the effective conductor length of the electrically conductive layer between a contacting terminal and the respective position of the mechanically movable contact element.
In connection with relevantly suitable manufacturing methods which subsequently still are to be dealt with in more detail, a resistance element according to the invention can be manufactured in a great number of pieces which electric properties thereof can be maintained within close tolerance ranges in a reproducible manner, also with great numbers of pieces.
In addition to the already mentioned dimensional parameters for the electrically conductive layer with respect to the width and thickness thereof it is also allowed to influence the controllable resistance range of the resistance element by selecting an appropriate material for these electrically conductive layers.
Thus, the most different metals or metal alloys can be selected in order to provide low-impedance through high-impedance resistance elements.
However, it is also possible to employ an electrically conductive layer made of graphite carbon.
With definite combinations of material of the substrate and electrically conductive layer it may be favourable to form between them a so-called bonding agent layer, wherein in this case as well the conductivity of such a bonding agent layer should be considerably smaller than that of the electrically conductive layer. Therefore, the bonding agent layer should have insulating properties. Examples of suitable materials of such a bonding agent layer are Al
2
O
3
or carbon similar to diamond as well.
The less expensive polymer plastics can be employed as substrate materials. Even though it is a matter of most different plastics per se known such as for example PMMA, polycarbonate, polyimides, acrylics and others which can also contain filling materials, in particular fiber reinforcements.
Such a substrate can also be provided appropriately and employed in the form of a film with electrically conductive and wear resisting layers.
However, it is particularly advantageous to select a substrate with a surface which comprises a low surface roughness, if possible, since both the thickness of the electrically conductive layer an
Meyer Carl-Friedrich
Scheibe Hans-Joachim
Barnes & Thornburg
Easthom Karl D.
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
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