Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-06-27
2004-12-28
Nolan, Jr., Charles H. (Department: 2854)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S654000
Reexamination Certificate
active
06836128
ABSTRACT:
The invention relates to an inductive path sensor, for example a valve sensor, for determining the position of an influencing element, for example a valve rod or piston, and a process for determining the position of an influencing element with an inductive path sensor.
Path sensors for determining the position of an influencing element, which are often also called position sensors, are known in a host of embodiments and for a host of applications. These path sensors can on the one hand be classified according to whether the motion of the influencing element to be monitored is primarily linear motion, thus a distance is to be acquired by the path sensor, or the motion of the influencing element is first of all circular motion, so that the path sensor monitors or establishes the angle of rotation of the influencing element. Path sensors which detect an angle of rotation are often also called angular resolvers.
In addition, path sensors can also be classified according to their physical operating principle. For example, inductive, capacitive or optoelectronic path sensors are known.
The subject matter of this invention is an inductive path sensor, especially one with which linear motion of an influencing element, for example a distance, can be measured. These known inductive path sensors have several coils, of which at least one coil is made as the primary coil and at least another coil is made as a secondary coil. The coils are generally built according to the transformer principle, so that one secondary coil is located laterally adjacent to one primary coil at a time. The inductive coupling between the middle primary coil and the two laterally arranged secondary coils is changed by the position of the influencing element which is located in the area of the cylinder axis of the cylindrical coil system and is made for example as a magnetically conductive rod. These inductive path sensors are known from DE 43 37 208 A1 and DE 19632 211 A1.
But in the known inductive path sensors it is a disadvantage that on the one hand the structural length of the path sensor is distinctly longer than the maximum distance of the influencing element which can be monitored, so that at a give path length to be monitored a path sensor up to 100% longer is required. This is undesirable especially where only a limited installation space is available. On the other hand, in the known inductive path sensors the attainable measurement accuracy is often not sufficient or it can only be increased by increased circuitry cost.
Thus, the object of this invention is to make available an inductive path sensor which has a structural length as small as possible and in addition enables measurement accuracy as high as possible. In addition, the object of this invention is also to devise a process with which the position of the influencing element can be precisely and reliably acquired within a housing by means of an inductive path sensor.
The aforementioned object is achieved as claimed in the invention by an inductive path sensor with several coils arranged in succession, with at least one capacitor, with at least one amplifier element, with at least one changeover switch and with an evaluation unit, one coil at a time and the capacitor or a capacitor forming an oscillating circuit, and one oscillating circuit and the amplifier element or an amplifier element forming an oscillator, the individual coils and the individual oscillators being chosen in succession by the changeover switch and the evaluation unit measuring the change of impedance of the coil chosen by the changeover switch or of the oscillating circuit chosen by the changeover switch depending on the position of the influencing element relative to the respective coil.
By using several successive coils, the coils being located in succession in the direction of the position of the influencing element to be ascertained, and the evaluation unit measuring the change of impedance of each coil and each oscillating circuit in succession depending on the position of the influencing element by the changeover switch, an inductive path sensor can be implemented with a structural length which is only slightly greater than the total length of the distance to be monitored.
It was stated above that the evaluation unit measures the change of the impedance of each coil or each oscillating circuit. Preferably the evaluation unit measures the change of the frequency of each coil or each oscillating circuit depending on the position of the influencing element. But in addition it is also possible for the evaluation unit to measure the change of the inductance of the coil or of the oscillating circuit or the change of the amplitude of the oscillating circuit as a function of the position of the influencing element.
If according to the preferred embodiment of the invention the evaluation unit measures the change of frequency, generally the change of frequency of the oscillating circuit is measured depending on the position of the influencing element. But at least theoretically it is also possible for the change of the frequency to be measured solely by the coil, inasmuch as each actual coil in addition to the primarily characteristic inductance also has an ohmic resistance and several parasitic capacitances. Thus the actual coil has a natural resonant frequency which is determined by the inductance and the parasitic capacitances of the coil. But generally the change of frequency of the oscillating circuit consisting of a coil and an additional capacitor is measured by the evaluation unit.
According to one preferred embodiment of the invention, each oscillating circuit has the same, especially the identical capacitor, and each oscillator has the same, especially the identical amplifier element. In other words, the inductive path sensor does have several successive coils, but only one capacitor and also only one amplifier element, for example an operational amplifier. The individual oscillating circuits thus each consist of the same capacitor and a coil chosen by the changeover switch. This has first of all the advantage that only a few components are required for the inductive path sensor, by which it can be built both economically and also with a further reduced structural size. In addition, when the individual oscillators are each composed of the same capacitor and the same amplifier element, measurement signal adulterations due to component variations do not occur, as would be the case when using several capacitors or several amplifier elements.
If it was stated above that the inductive path sensor according to one preferred embodiment has only one capacitor and only one amplifier element, it is of course only meant that the individual coils are always interconnected via the changeover switch to the same capacitor or to the same amplifier element, by which a number of oscillators corresponding to the number of coils is accomplished. Of course it is also possible for the amplifier element to be implemented in terms of circuitry by two or more amplifier elements, for example by two operational amplifiers. Likewise the capacitor of the oscillating circuit can be implemented in terms of circuitry by several capacitors.
Influencing the coil or the oscillating circuit depending on the position of the influencing element is theoretically based on three different physical effects which differ greatly in their action, depending on what type of influencing element is used.
Within the framework of this invention the influencing of the impedance of the oscillating circuit by the influencing element based on the transformer principle is preferably evaluated. The physical effect called the transformer principle here is based on that coil of the oscillating circuit generating an alternating electromagnetic field which induces a voltage in the adjacent body—the influencing element—first of all according to Faraday's Law. When using an influencing element of a material with relatively high conductivity the induced voltage leads to current flow in the influencing element. This current res
i f m electronic GmbH
Nixon & Peabody LLP
Nolan, Jr. Charles H.
Safran David S.
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