Electricity: measuring and testing – Magnetic – Plural tests
Reexamination Certificate
2002-04-23
2003-12-09
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Magnetic
Plural tests
C324S229000, C324S232000, C324S243000
Reexamination Certificate
active
06661224
ABSTRACT:
TECHNICAL FIELD
The present invention concerns the technical field of the non-contact measurement of a value for first and foremost dimensions such as thickness and diameter of an electrically conducting, substantially non-magnetic object, based on electromagnetic induction. The present invention may also be used in order to measure dimensions as stated above and at the same time as electrical characteristics of the electrically conducting object such as electrical resistivity.
The present invention may be used during the manufacture of metal products such as plate, or strip, bar or tube where it is desirable to, measure dimensions of those products. It may further be used with measurement of dimensions in connection with pyro-metallurgical processes for production of metals. The present invention may also be used for measurement of dimensions of electrically conductive objects, including non-metallic objects, in another context such as with control of characteristics of metal parts and identification of objects that cannot be seen, such as metal objects in wood in connection with sawing and so on. The present invention may further be applied in applications according to the above where, at the same time, dimensions and electrical characteristics are desired.
BACKGROUND ART
A known method for non-contact measurement of the thickness of a plate to irradiate it with a radioactive radiation or with x-rays and then measure the absorption of radiation in the plate. This absorption is dependent, amongst others, on thickness of the plate and constitutes a primary measured value for the thickness. Variations in the materials composition and coatings on the surface of the material influence the absorption of radiation and reduces thereby the accuracy of such equipment. Further, the radiation used in such equipment necessitates health and safety measures.
It is known to measure the thickness of a strip or plate of an electrically conducting object with electrical induction methods. One or more transmitting coils produce a time-varying magnetic field which can penetrate into the electrically conducting object and there induce a current. These currents in their turn produce a magnetic field which in its turn induces a voltage in one or more receiving coils and there the induced voltage is used, after some signal processing, as a measure of thickness.
Such methods and devices are often based on sinusoidally varying magnetic fields where changes in amplitude and changes in phase caused by the object are measured. Both of these changes are influenced by at least three parameters in a measuring system, object position, electrical resistance of the object and thickness of the object, and therefore such systems, in their simplest forms, become fundamentally uncertain. Attempts have been made to solve that problem by introducing measurements at different frequencies in order, in a sense, to obtain even more measured parameters, but this has given the result that interpretation of the signal becomes greatly complicated and the sufficient measurement accuracy cannot be achieved.
The above problem has been solved by using a time-varying field which is characterised by a constant current supplied to a transmitting coil over a certain time period which is then suddenly cut off, as described in U.S. Pat. No. 5,059,902. By the use of this technique an induced signal in a receiving coil is measured during at least three time intervals, one directly after current cut-off, one directly after that and before changes in the magnetic field have had time to penetrate the object of measurement, and lastly during a time interval long after current cut-off when changes in the magnetic field have had time to penetrate the object of measurement. With help of at least these three measured values the thickness of the object can be calculated.
The above method has been shown to work well in many cases, but each of the three measured values include though a degree of uncertainty. Especially where it concerns the third measured value because speed of the object influences the measured value. Altogether it means that the accuracy of the measured value for thickness is not always as desired.
A method to measure thickness of a plate is shown in U.S. Pat. No. 5,059,902, with reference to
FIG. 14
, by means of two coils placed on opposites sides of the plate. The plate thickness is measured in that way by measuring the distance between a coil and surface of the plate for each side respectively during a time interval directly following the cut-off of current supplied to the coils. The difference between those distances coil to plate surface and the distance between the two coils is thus the thickness of the plate. Under ordinary demands for accuracy this meter works well, but with high demands for accuracy the natural variations in distance between the coils, for example due to temperature, is a considerable problem that reduces the usability of the meter.
One way to measure the dimensions of rod and similar products during manufacture has been described in U.S. applications Ser. Nos. 09/051333 and 09/051418. The methods have been shown to be useable for measurement in many production processes such as, for example hot rolling, and even here the methods have shown limitations where the demands for measurement accuracy and stability are very high. These limitations are in a context where measurement accuracy for those types of devices is determined by the accuracy of positioning of coils in the device and the resulting sensitivity for movement in the device.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method and a device for highly accurate inductive measurement of physical dimensions such as diameter or thickness of an electrically conducting object without influence of other varying parameters such as object position and the material parameters of the object. In order to produce highly accurate measurement in practice according to the above it is a precondition that the measured value is directly simply dependent on the dimensions of the object. Further, the measurement is carried out without influence from other material in the proximity of the object, such as water, oil and surface coatings. The geometric dimension is referred to in a particular predetermined direction. Thus, for example the thickness of a flat plate is referred to as a geometric dimension perpendicular to the plane of the plate, and a bars diameter as the geometric dimension perpendicular to the long axis of the bar.
This is achieved according to the present invention by the help. of appropriately shaped coils, transmitter coils, supplied with a substantially constant current producing a magnetic field in the object. An important feature of the present invention is that the field is directed so that it has a mean component, the sum of the field, which is generally perpendicular to the desired measurement direction. After a time sufficiently long enough that the field has stabilised, the current supply to the coil is cut off.
In a similar way as described in U.S. Pat. No. 5,059,902 the measurement is begun a time after that at which current supply has been cut off. However, in U.S. Pat. No. 5,059,902, measurement is started directly at the time that current supply is cut off. In the present invention, that starting time after which measurement is begun is determined from the time that it takes for the field outside the object to decay. After the field outside the object has decayed, the measurement sequence is begun and the integration of the signal carried out under a time it takes for the field in the measurement area, the field due to the field in the object material, to decay. A measurement signal which is an integral based a voltage due to the decay of the field in the object material is the basis for determining a dimension of the object. As further described below, that time period after the decay of the field outside the object may subsequently be divided into two time intervals from which measurement signals are
ABB AB
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Snow Walter E.
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