Electricity: measuring and testing – Magnetic – Displacement
Patent
1994-12-30
1996-06-25
Snow, Walter E.
Electricity: measuring and testing
Magnetic
Displacement
32420726, 324226, G01B 714, G01R 3300
Patent
active
055303463
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a magnetic measuring method for measuring the size and shape (profile) of a slot.
The invention also relates to an apparatus for determining the size and shape of a slot.
The method can be applied to measuring the thickness and, especially, the profile of a material web passing through a slot. As a special application the method can be used, among other things, for continuous measuring of the discharge opening (slice) of the head box of a paper machine.
DISCUSSION OF THE PRIOR ART
The size and profile of a slot can be measured with a known method based on eddy currents:
Let us suppose that the slot is limited underneath by an electrically conducting edge such as the lower lip 5 of the slot in FIG. 1. One measures at a high frequency (in the order of 100 kHz or more) changes in the inductance of a coil attached to the upper edge 2 of the slot. If a capacitor is connected in parallel with the coil, thereby forming a resonant circuit, one may measure changes in the resonant frequency of the circuit. The changes in the inductance or resonant frequency are proportional to the distance of the coil from the electrically conducting lower edge. A weakness of this method is its sensitivity to eddy currents induced into other electrically conducting structures surrounding the slot. For example, adjacent to the discharge opening of a paper machine there are often shield plates which in practice totally prevent the application of the eddy current method.
The size and profile of a slot can also be measured by means of a permanent magnet. As an example of this method, permanent magnets are attached at specified distances from each other to the lower edge 1 of the solution according to FIG. 1. The magnetic field created by them is measured by magnetometers attached to the upper edge 2 of the slot. The method based on permanent magnets has two essential weaknesses. Firstly, magnetization of permanent magnets is known to be dependent on temperature. If the method is required to be accurate and stable, the temperature of the permanent magnets must be stabilized or their magnetization measured must compensate for the changes in magnetization in the measurement results. Secondly, when operating with permanent magnetic fields disturbances caused by the environment are a problem. For example, an extra permanent magnet getting near to the magnetometers will distort the desired signal.
Known is also a method (the published FI specification 73831), wherein the thickness of a non-magnetic, especially moving material web is measured. In the method the material web moves touching a rotating non-magnetic cylinder to which a magnetic bar is connected. At the opposite side of the web the magnetic field caused by the bar is measured with a magnetoresistive sensor touching the material web. A weakness of even this method is the dependence of the signal on the magnetic properties of the bar in the cylinder, which may change because of changes in temperature, mechanical stresses etc. In addition, with one stationary magnetometer the profile of the material web can not be measured.
The profile of a slot can be measured by ultrasound for example as follows: At either side of the slot there is an ultrasound transmitter and receiver. Ultrasound propagates in the medium in the slot. By measuring the propagation time in the slot one may obtain the size of the slot at each ultrasound sensor when the propagation velocity of ultrasound in the medium in the slot is known. Propagation of ultrasound is dependent on the properties of the medium. For example, changes in temperature, pressure etc. of the medium must be compensated, which is an obvious weakness of the method.
The slot can also be monitored by means of optical methods. A weakness of these methods is, however, the dependence of the propagation of light on the optical properties of the medium. In addition, soiling of optical sensors in an industrial environment is a significant problem.
SUMMARY OF THE INVENTION
It is the purpose of th
REFERENCES:
patent: 4470013 (1984-09-01), Posseme 324207.26
Seppa Heikki J.
Sundstrom Mayri O.
Varpula Timo T.
Snow Walter E.
Valmet-Tampella Oy
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