Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-04-19
2001-06-12
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207120, C324S207240
Reexamination Certificate
active
06246230
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a noncontacting displacement sensor with a measuring coil to which alternating current can be applied, the measuring coil having at least two voltage taps, with an electrically and/or magnetically conductive measuring object, and with an evaluation circuit for evaluating and, if need be, determining an output voltage corresponding to the position of the measuring object with respect to the voltage taps.
A noncontacting displacement sensor of the kind under discussion is also disclosed in WO 94 03778 and corresponding U.S. Pat. No. 5,629,619. The measuring coil of the known sensor has plurality of voltage taps that are arranged at defined intervals, and it is enclosed in a casing. The measuring object is a ring that surrounds the casing in spaced relationship and is displaceable along the casing wall. With the use of the known displacement sensor it is possible to determine the position of the ring with respect to the voltage taps arranged on the measuring coil. The presence of the ring between two voltage taps is effective on the impedance of the measuring coil segment that extends between these voltage taps and on all such “partial” impedances of segments of the measuring coil that follow in one direction. This means, that depending on the location of the measuring coil in which the ring is positioned, a varying quantity of partial impedances are influenced. With the known displacement sensor, voltages that can be tapped on successive segments of the measuring coil are added. In each instance, subtotals are formed which differ in the increasing number of considered voltage values. These subtotals are again added to an output voltage that may then be clearly related to the position of the ring.
The measuring object of the known displacement sensor is located outside of the coil casing. As a result, the measuring object is relatively exposed and unprotected. Furthermore, the known displacement sensor requires relatively much space, so as to ensure a mobility of the measuring object on the outer side of the coil casing.
It is therefore the object of the invention to describe a noncontacting displacement sensor of the kind under discussion that is of an as compact design as possible and requires for itself only relatively little space.
SUMMARY OF THE INVENTION
The above and other objects and advantages of the present invention are achieved by the provision of a non-contacting displacement sensor having a measuring coil to which an alternating current can be applied. The measuring coil has at least two voltage taps, and an electrically and/or magnetically conductive measuring object is arranged for displacement in the interior of the coil. An evaluation circuit is provided for evaluating and determining an output voltage that corresponds to the position of the measuring object. Also, the total impedance of the measuring coil is independent of the position of the measuring object.
In accordance with the invention, it has been recognized that partial impedances of segments of the measuring coil can be influenced not only by a ring surrounding the measuring coil as a measuring object, but also via a measuring object of an electrically and/or magnetically conductive material that extends in the interior of the coil. With this arrangement, it is possible to expose the measuring object in a purposeful manner to certain forces or pressures that are to be determined, whereas it is shielded against other undesired influences. Furthermore, the arrangement of the measuring object in the interior of the measuring coil results in an extremely compact construction of the sensor. In the displacement sensor of the present invention, the total impedance of the measuring coil is independent of the position of the measuring object, so that the current flowing through the measuring coil is dependent only on the applied voltage and not on the position of the measuring object. Accordingly, in the ideal case, the output voltage of the displacement sensor according to the invention is likewise dependent only on the position of the measuring object.
As regards a simple evaluation of the output voltage, it is especially advantageous when the positions of the voltage taps are selected such that in the absence of the measuring object, it is always possible to tap substantially the same voltage between two adjacent voltage taps. This will lead in the case of a measuring coil with a constant coil cross section over its length and uniform coil winds, to an equidistant arrangement of the voltage taps.
As regards a simple and reliable evaluation of the output voltage, the dimensioning of the measuring object should also be adapted to the spacing between the adjacent voltage taps. In an advantageous manner, the measuring object should maximally extend over the spacing between two adjacent voltage sections, so that it is simple to localize between voltage taps.
Advantageous materials for the measuring object include all materials known from practice, which can be used to realize coil cores. Especially suitable are ferromagnetic materials and materials with a low resistivity.
Depending on its field of application, the displacement sensor of the present invention may be realized with different measuring coils, i.e., with different coil geometries. Thus, it would be possible to use as a measuring coil, for example, an elongate coil in the form of a cylindrical coil or a coil with a square cross section. However, it would likewise be possible to use a toroidal coil as a measuring coil.
In an advantageous embodiment of the displacement sensor according to the invention, the interior of the measuring coil accommodates a coil tube that extends preferably over the entire length of the measuring coil. The coil tube again accommodates the measuring object for displacement therein. On the one hand, the coil tube serves to stabilize the measuring coil, since same can be wound practically around the coil tube. On the other hand, it also serves as a guideway for the movement of the measuring object. This permits reducing in an effective manner the mechanical stress of the measuring coil by the measuring object. The coil tube should be made of a material that ensures an electric insulation between the measuring coil and the measuring object. For this reason the material should be nonferromagnetic and have a high resistivity. Suitable therefor is stainless steel with corresponding properties, but also plastic or glass may be used.
Depending on the case of application, the displacement sensor of the present invention may be provided with a casing which either encloses only the measuring coil with the measuring object, and advantageously shields same even against interfering influences, or also encloses and protects in addition the evaluation circuit.
Likewise, with respect to a simple evaluation of the output voltage of the displacement sensor according to the invention, alternating current is to be applied to the measuring coil symmetrically and oppositely directed. For example, it is possible to apply as alternating current a sinusoidal voltage or even a square-wave voltage. Basically, it is possible to determine the output voltage in three different ways.
A first possibility consists of determining respectively the voltage between two voltage taps of the measuring coil. These voltage values must then be added in the sequence that is predetermined by the arrangement of the corresponding coil segments, namely, for the time being, to all possible subtotals that result in the case of this sequence. The output voltage will then result as a sum of these subtotals. In this procedure, only voltages are acquired that can be tapped, i.e., measured between adjacent voltage taps. With the aid of the evaluation circuit, the output voltage is computed from these voltage values, and only by evaluating the thus- computed output voltage is it possible to determine the position of the measuring object.
In a second variant of the displacement sensor according to the invention, the voltages are
Alston & Bird LLP
Micro-Epsilon Messtechnik GmbH & Co. KG
Patidar Jay
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