Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-05-11
2001-06-12
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S662000, C324S686000, C324S071100
Reexamination Certificate
active
06246244
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for measuring axial deviation in a taut wire alignment system. It also relates to a measuring device implementing this method and a taut wire alignment system including such measuring devices.
2. Description of the Related Art
In large instruments used in particle physics research, such as linear colliders, it is necessary to obtain a precise alignment of heavy elements such as magnet sections, over distances which can be as great as several hundred meters. For this purpose, there have been developed non-contact alignment systems making use of a wire stretched along the length of the elements to be aligned and devices for measuring deviation using optical or electrical techniques, in particular capacitive techniques. The present invention falls within the category of taut wire alignment systems using capacitive measuring techniques. In this context, a taut wire alignment system is already known in which there is sent along an electrically conductive wire a short pulse with a time constant of one nanosecond. This pulse induces electrostatic charges between two metal plates by influence. A measurement of charge difference provides quantitative information on the deviation of the wire with respect to the median plane between the two plates.
Another taut wire alignment system is known in which a tungsten wire of diameter 125 &mgr;m and having a resistivity of 40 &OHgr;m is subjected to a high frequency alternating voltage. Capacitive pick-up rings are disposed between guard segments connected to earth. These rings and the electronics are disposed in a screened case.
In these alignment systems, the conductivity of the taut wire is used for making measurements of the axial deviation, which excludes the use of materials having high electrical resistivity for making these wires.
SUMMARY OF THE INVENTION
The purpose of the invention is to overcome these disadvantages by proposing a method for measuring an axial deviation in an alignment system comprising a taut resistive wire which can, in particular, be made from a material having better mechanical characteristics than the materials currently used in taut wire deviation measuring devices.
According to the invention, the method comprises:
application of an alternating voltage signal to a measuring electrode provided with a guard electrode, this measuring electrode being disposed in a measuring plane on a holder attached to an element to be aligned and in the vicinity of the taut wire, this taut wire being dynamically connected to earth,
measurement by means of a capacitive bridge, comprising a first capacitor consisting of the measuring electrode and the wire and a second predetermined capacitor, in order to provide a measurement signal representative of an axial position of the measuring electrode with respect to the taut wire along a first
measuring axis normal to the said measuring plane, and
processing of this measurement signal in order to provide information representative of an axial deviation of the element to be aligned with respect to a predetermined alignment position.
Thus, with the measuring method according to the invention, unlike the methods of the prior art which necessitated the use of a sufficiently conductive wire, it becomes possible to use a wire made of materials having higher electrical resistivity but better mechanical characteristics, thus making it possible to apply higher tension to the wire which is therefore more taut. A significant improvement in the measuring accuracy is then obtained, whilst using a strong wire which can have a total resistance of several tens of k&OHgr;.
In a preferred form of implementation of the method according to the invention, first and second alternating voltage signals are respectively applied to first and second measuring electrodes respectively provided with guard electrodes and respectively disposed in first and second measuring planes located on either side of the reference axis, and the method furthermore comprises, for each measuring electrode, a capacitive bridge measurement to provide a signal representative of the capacity between the measuring electrode and the wire, and a differential processing of the said first and second signals representative of the capacities respectively corresponding to the first and second measuring electrodes in order to supply information representative of an axial deviation of the element to be aligned with respect to a predetermined positioning of alignment along a first measuring axis.
Preferably, the first and second measuring planes are substantially parallel, and there is obtained an axial deviation information on the element to be aligned with respect to a predetermined alignment position along a measuring axis normal to the said first and second measuring planes.
When it is necessary to measure an axial deviation along another axis, such as the vertical axis, the method according to the invention furthermore comprises at least one application of an alternating voltage signal to at least one other measuring electrode disposed in the vicinity of the taut wire in another measuring plane which is not parallel with the first measuring plane, a capacitive bridge measurement to provide at least one other measurement signal representative of an axial position of the other measuring electrode along another measuring axis normal to the other measuring plane, and a processing of this other signal in order to supply information representative of the axial deviation of the element to be aligned with respect to a predetermined alignment position along the said other measuring axis.
In a particular embodiment of the invention, the other measuring plane is not parallel with the first measuring plane and can, for example, be orthogonal to the latter.
According to another aspect of the invention, there is proposed a device for measuring an axial deviation in a taut wire alignment system, implementing the method according to one of the preceding claims, comprising a holder attached to an element to be aligned along a reference axis, characterized in that it comprises:
at least one measuring electrode provided with a guard electrode and disposed in a measuring plane located in the vicinity of a taut conductive wire connected to an earth,
means of generating and applying an alternating voltage signal to this measuring electrode, with respect to the guard electrode,
capacitive bridge means for supplying a measurement signal representative of an axial position of the measuring electrode with respect to the taut wire along a measuring axis normal to the said measuring plane, and
means of processing this signal in order to supply information representative of an axial deviation of the element to be aligned with respect to a predetermined alignment position.
In a preferred version of the invention, the measuring device comprises at least two measuring electrodes respectively placed in two substantially orthogonal measuring planes.
In order to obtain better natural accuracy, the measuring device according to the invention comprises at least two measuring electrodes disposed respectively in two substantially parallel measuring planes on either side of the reference axis.
In an effective embodiment of a measuring device according to the invention, the latter furthermore comprises a central recess for receiving the taut wire, around which are disposed several measuring elements, each measuring element comprising an insulating holder on which the measuring electrode is disposed in the form of a thin layer.
In a practical embodiment, each measuring element comprises a part made of insulating material comprising, on its outer face, the measuring electrode and attached to a metal holder serving as a guard for this measuring electrode and which can have various geometric shapes.
The means of generation and the capacitive bridge means can be housed inside the device, in the immediate vicinity of the measuring electrodes, and disposed at least partially inside a screened en
Leteurtre Patrick
Ossart Frédéric
Brown Glenn W.
Nanotec Solution
Young & Thompson
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