Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1998-05-28
2001-02-13
Strecker, Gerard (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S207170, C324S229000, C324S232000, C324S243000
Reexamination Certificate
active
06188217
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a measurement device based on electromagnetic induction for non-contacting determination of the dimension of tubes, rods, beams, etc., of electrically conductive material. The invention may also be used for measurement on products made of graphite, electrically conductive ceramic, or the like.
The measurement device can suitably be used when manufacturing the above-mentioned products for continuous monitoring during the manufacturing process. The measurement device can also be part of a sensor in equipment for controlling the final dimension of rolled products. This results in a reduction of the rejection rate, etc.
The measurement device is based on the measuring principle which is described in U.S. patent application Ser. No. 09/051,333.
BACKGROUND OF THE INVENTION
One known method for non-contacting measurement of positions and various dimensions, such as height and width, of tubes, rods, beams or like products is to use optical methods based on shading or reflection of beams, or by processing of images taken by a video camera.
The environment in many manufacturing processes, include, above all, pollution and often high temperatures, which deteriorate the reliability and the accuracy of such equipment.
It is previously known to measure the dimensions and position of an electrically conductive measuring object by means of inductive methods. In that case, a transmitter coil is used which generates a time-varying magnetic field which induces currents in the conductive measuring object. These currents generate a magnetic field which, in turn, induces a voltage in a receiver coil. This voltage is dependent, among other things, on the shape, the conductivity, and the magnetic permeability of the measuring object, as well as on the geometrical conditions. From this voltage, under certain conditions, geometrical measures, such as distance and position of the measuring object, may be calculated. For generating a time-varying magnetic field, sinusoidal currents in the transmitter coil may be used, as described in U.S. Pat. No. 4,475,083, or a constant current which is suddenly interrupted, such as described in U.S. Pat. No. 5,059,902, may be used. The latter method is more robust from the point of view of a measurement technique and facilitates the separation of different properties of the measuring object. One problem with these measurement devices, however, is to determine the dimensions of the measuring object when its position is changed.
U.S. Pat. No. 5,270,646 discloses a method of arranging coils so as to measure the width of a strip. However, this technique can only be used for a strip of relatively limited width. Further, it is assumed for a correct function that the edge of the strip is substantially plane. For many applications, the accuracy is not sufficient, primarily when there are large distances between the strip and the measuring coils, which is due to difficulties in correctly compensating for variations in the distance.
Common to prior art devices for inductive measurement of distance, thickness and other dimensions of electrically conductive objects, derived therefrom, is that the transmitter and receiver coils are arranged with the same symmetry axis or are located on different sides of the measuring object. It also occurs that the same coil is used as transmitter and receiver coil. The magnetic field generated by the transmitter coil then becomes substantially perpendicular to the surface of the measuring object at the measuring point, or at least has a large component towards the surface of the measuring object. This results in currents and magnetic fields from different depths into the measuring object contributing to the measurement signal which thus becomes both material-dependent and dependent on the thickness and shape of the measuring object in a relatively large region around the location where measurement is to take place.
SUMMARY OF THE INVENTION
The invention comprises a U-shaped structure, in the following referred to as a “U sensor”, with two branches and one retaining part arranged therebetween. During measurement, the measuring object is placed, as far as possible, centrally in the inner opening of the U sensor. The measuring principle is based on at least one transmitter coil and one receiver coil being fixed in the U sensor. These are to be placed in relation to each other such that the magnetic field generated by the transmitter coil is substantially parallel to the surface of the measuring object at a conceived measuring region. The receiver coil is placed such that a conceived field line which originates from the transmitter coil touches the measuring region at a measuring point and such that the field line in its extension reaches the receiver coil. This is achieved by a substantially symmetrical loca- tion of the coils in relation to the measuring point in question and where the transmitter coil, the measuring point and the receiver coil, as far as possible, lie on one and the same circular arc and where the circular arc relative to the measuring region is curved outwardly.
One advantageous method of supplying a transmitter coil is described in U.S. Pat. No. 5,059,902. It describes supply with a constant current which has a sufficient duration for the magnetic field to be regarded as quasi-static.
The voltage induced in the receiver coil as a function of the time after the constant current in the transmitter coil has been interrupted, at a time t1, comprises
a brief voltage pulse S
1
rapidly diminishing up to a time t2 and induced by the rapidly decreasing magnetic field in the air between the coils and the measuring object, and
a voltage pulse S
2
diminishing considerably more slowly from the time t2 and relating to the magnetic field within the measuring object which decreases slowly due to the skin effect.
On the basis of the above-mentioned symmetrical location of the coils relative to the measuring region and the location of the coils and the measuring point on the same circular arc, the position of the measuring region on an extended line between the center of the circular arc and the measuring point can be determined. The position or the distance from a fixed reference point on the line, for example the center of the arc, is determined as a linear combination of the integral of the voltage pulse S
1
between t1 and t2 and the integral of the voltage pulse S
2
between t2 and t3, where t3 is determined as follows:
M
=
a
·
∫
t1
t2
S1
·
ⅆ
t
+
b
·
∫
t2
t3
S2
·
ⅆ
t
The time t3 is preferably chosen such that t3-t2 is of the same order of magnitude as the time difference t2-t1 and where the coefficients a and b are chosen after calibration with measuring objects made from materials with different electrical conductivity such that the difference in M between the materials becomes as small as possible. If the time difference t3-t2 is chosen equal to 2(t2-t1), a and b will be substantially equal and no calibration according to the above need be made.
To be able to measure a plurality of different dimensions of the measuring object, for example height and width, a plurality of associated transmitter and receiver coils according to the invention are placed around the measuring object. The supply current of the transmitter coils is then normally interrupted at different times in order not to influence each other.
One advantage of a sensor according to the invention is that it is not particularly sensitive to deviations from the central location of the measuring object according to the above. This means that the sensor provides relevant measures of the change of the position of the measuring region if the change is smaller than 10-30% of the distance between the measuring region and any of the coils.
REFERENCES:
patent: 4475083 (1984-10-01), Linder
patent: 4605898 (1986-08-01), Aittoniemi et al.
patent: 4866424 (1989-09-01), Parks
patent: 5059902 (1991-10-01), Linder
patent: 5270646 (1993-12-01), Kihlberg et al.
patent:
Asea Brown Boveri AB
Pollock, Vande Sande & Amernick, R.L.L.P.
Strecker Gerard
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