Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
2001-03-28
2002-08-13
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
Reexamination Certificate
active
06431011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring an electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
2. Background Information
In a magnetic flow meter an electrical potential difference developed in the fluid is sensed by at least one pair of electrodes contacting the liquid and spaced apart from each other along a line that is generally orthogonal to both the direction in which the flow is being measured and a magnetic field produced by a magnet. The measured potential difference has a magnitude proportional to the flow rate of the fluid. As is known to those skilled in the art, the overall potential difference between two such electrodes, usually termed a voltage difference, has two major components: a) a flow-related voltage due to the flow of the fluid when acted upon by the magnetic field; and b) a net ‘drift voltage’, which is the sum of voltages due to all other factors, such as electrode polarization.
In prior art flow sensors of this type, alternating magnetic fields from electromagnets have generally been used to provide an alternating magnetic field. The alternating magnetic field facilitates signal amplification and processing that accepts flow-related electrode signals while rejecting electrode drift signals which would otherwise introduce serious measurement errors. However, generating those fields and processing the measured voltage signals requires sophisticated circuits and techniques which raise the cost of such sensors and limit their application.
An example of a radical departure from the prior art is found in my U.S. Pat. No. 6,085,599 in which I teach mechanical means to alternate the polarity of the magnetic fields. Those techniques provide practical ways of simplifying magnetic flow sensors and reducing their costs. However, the use of mechanical means to alternate the field polarity, even though this may be performed with a high degree of ruggedness and reliability, reduces the marketability of such an instrument The disclosure of U.S. Pat. No. 6,085,599 is incorporated herein by reference.
Another problem encountered in prior art magnetic flow sensors is that of entrapment of ferromagnetic debris. This is particularly true of arrangements using permanent magnets as in my U.S. Pat. No. 6,085,599. Such debris can change the magnetic flux distribution and thereby alter the calibration of the flow meter. Moreover, pieces of ferromagnetic debris can sometimes bridge the electrodes, which are normally electrically insulated from each other, producing a conductive path that may partially short out the electrode signals and thereby reduce the output voltage. Fine particles of debris can also form a film on normally insulating portions of the structure surrounding the electrodes and thereby shunt the electrode signals.
It is therefore an object of the invention to provide a practical magnetic flow sensor using stationary permanent magnets.
It has also been discovered that the methods of the present invention can be used with conventional magnetic flow sensors using electromagnets to improve their performance and such is therefore a further objective of the invention.
BRIEF SUMMARY OF THE INVENTION
The above and other objects are attained by magnetic flow sensors in accordance with various preferred embodiments of the present invention. In preferred embodiments the magnetic axis (i.e., the line extending from the south to the north pole) of a permanent magnet is oriented generally perpendicular to a direction of flow of a fluid. As is known in the magnetic flow metering art, the flux from a magnet arranged in this fashion generates, in the fluid, a voltage difference proportional to the flow rate of the fluid. In various embodiments of the invention this voltage difference is sensed by the use of a sensing head comprising a pair of electrodes (which preferably have the same size and shape and are made of the same material) which are spaced apart from each other along a line that is generally orthogonal to both a direction of flow and the magnetic axis.
The voltage indicative of flow rate is measured when the two electrodes of a pair are in an open-circuit state in which they are externally electrically connected to a high impedance voltage measurement circuit. In this open circuit state the electrode potentials are electrically influenced by electrode polarization and other measurement error-inducing factors that develop relatively slowly. In order to minimize measurement errors with these factors, sensors of some embodiments of the invention provide an operating cycle in which the two electrodes of a pair thereof are in a closed circuit state for most of the time, and are placed in an open circuit state only during a brief measurement interval portion of the operating cycle. When in the closed circuit state the electrodes may be short circuited to each other, connected to respective reference voltage sources (typically zero to a few tens of microvolts) or connected to a common potential such as ground. A major purpose of the closed circuit state, reducing drifts, is served by connecting the two electrodes together. Connection to other selected potentials, including ground, can provide compensation for minor drifts. The reference voltage sources include voltage levels which may be different for each electrode and which may even vary with the output flow rate signal from the flow sensor. In the closed circuit state, particularly during installation and set-up, the electrodes may be connected to alternating potentials having magnitudes as high as several volts and frequencies of several kilohertz in order to drive the electrodes quickly into a steady state condition. Periodically, each electrode pair may be switched from its closed circuit to its open circuit state for a brief time interval so that the flow-generated voltage difference then appearing at the electrodes may be detected and processed to provide an output signal representative of the flow rate of the fluid. During the open circuit portion of this duty cycle, drift inducing factors begin to cause drift signals to develop. However, they develop relatively slowly compared to the brief time interval required to detect the flow rate signal and thereby enable electronic processing to discriminate between the two. This method of flow rate detection thereby enables an extremely simple magnetic flow sensor to be made. In other cases, in which the flow rate signal is found to change slowly with respect to drift signals, the closed circuit state may comprise a smaller portion of the operating duty cycle and the open circuit state a correspondingly large portion of the duty cycle so as to allow the full magnitude of the flow rate signals to be detected.
As will be disclosed in greater detail hereinafter, the flow rate of a fluid can be sensed by arrays of sensing heads comprising two or more pairs of electrodes and at least one magnet having its magnetic axis oriented perpendicular to a direction of flow. Each of the sensing heads in an array, as recited above, comprises a pair of electrodes spaced apart from each other along a line generally orthogonal to both the direction of flow at that sensing head and to the magnetic flux. The sensing heads in an array thereof are spaced apart from each other along the flow path of the fluid. For example, two sensing heads can be spaced out along a section of pipe or tubing. The flow rate voltages from the plurality of heads can be polarized to be additive in the associated signal processing circuitry, which may be adapted to measure all the heads simultaneously, or which may measure the voltages one at a time in a sequential, scanning, fashion. Furthermore, because more than one pair of electrodes may be used with a single or with cooperative magnetic fields, the sensor can be configured as comprising paired arrays of electrodes that can be momentarily externally connected in differing combinations so as to provi
Fuller Benjamin R.
Kiewit David
Thompson Jewel V.
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