CA2585830A1

CA2585830A1 - Microwave cavity load cell

Info

Publication number: CA2585830A1
Application number: CA 2585830
Authority: CA
Inventors: Stan Bleszynski
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-03-21
Filing date: 2007-03-21
Publication date: 2008-09-21

Abstract

The subject of the invention is a device to measure force, weight or pressure based on the principle of shifting the resonant frequency of a microwave cavity subjected to stress. Microwave Cavity Load Cell is a superior replacement for existing load cell technology based on resistive strain gauges.

Description

II I i Microwave Cavity Load Cell DESCRIPTION
Introduction Load cells are force sensors commonly used for measuring load in many applications such as weight scales, cranes, conveyor belt scales, stoves, storage tanks etc. The necessary requirements in the industrial applications are:
ruggedness, reliability accuracy, high dynamic range, longevity and low maintainance. Typical load cells utilise resistive strain gauge technology.
The set of resistive strain gauges (typically four to six) is glued onto a metal support element that carries the main stress of the device under load. The strain gauge converts material strain under stress into a change in resistance which is then sensed and amplified electrically. Resistive strain gauges are connected in bridge (differencial) configuration in order to eliminate the effect of temperature drift and to eliminate sensitivity to off-axial stresses (if necessary).

The existing strain gauge based design has the following drawbacks: - it relies on the gluing process. Gluing is not an easy process, especially if maintaining consistency in production is important. It has also some long-term implications for longevity and may be subject to breaking under thermal stresses.

The temperature span of existing strain gauges is also limited - due to the nature of glue (typ organic polimers) and thermal stresses.

The dynamic range of strain gauges is also limited to typically about 1:10000.
In contrast to the above, a microwave cavity-based load cell has been shown in prototype to exhibit a very good dynamic range of about 1:70000, that is an order of magnitude better. It is also inherently more robust due to the cavity design being purely a pasive bare metal system with no fragile sensing components placed inside. It does not rely on the quality of gluing which makes it not only easier to manufacture but also more consistent.

EMBODIMENT OF THE INVENTION

In one particular embodiment, the sensor consists of two toroidal microwave cavities coupled mechanically such that a mechanical load applied on the center axis shaft causes a reduction of the volume of one cavity and at the same time causes expansion of the volume of the second cavity. Both cavities are electrically coupled with two independent oscillators oscillating in the frequency range corresponding to the natural self-resonance of each cavity. When one , ,, , , 45 cavity compresses, its oscillation frequency changes correspondingly; when the second cavity expands its frequency changes correspondingly in the opposite direction. An electrical circuit mixes the signals from two cavity-coupled oscillators and produces the difference frequency that is related to the degree of the cavities' compression/expansion; thus it is also related to the applied force.
50 The relation is very close to linear (with some constant frequency offset).
Since the thermal expansion of the material expands both cavities by the same factor, the temperature-dependent frequency shift is subtracted (cancelled) and the difference frequency is temperature independent (but still retains the force-dependency). The sensor is thus temperature-compensated. The difference 55 frequency is measured by digital counting or other technique and converted to the required sensor output signal, which could be either an analog voltage output proportional to the force applied, or some digital output such as pulse-width or frequency encoded or digital data encoded.

A sample single cavity evaluation prototype (12cm diameter) was machined and tested. The resonant cavity frequency was 1.1GHz, Q=5000, the frequency shift was 4kHz/kgf (per kilogram-force), the frequency resolution was estimated to be 65 about 0.6kHz. Given that the prototype cell had an estimated maximal load capacity of 10ton, its dynamic range is estimated to be -1:70000.
Conclusions:

70 The laboratory tests of a microwave cavity load cell proved the concept and demonstrated feasibility of its operation.

Claims

1. A device composed in the shape of two hollow cavities coupled mechanically together, made out of metal that acts as resonators for electromagnetic wave in the microwave frequency range.

2. The device of claim 1 wherein only one cavity is subject to mechanical deformations under applied force or pressure, that result in the frequency shift of this cavity.

3. The device of claim 1 wherein the mechanical deformations under applied force or pressure produce opposite deformation on each cavity, such that if one cavity is compressed, the other one is expanded, producing a frequency shift in the opposite direction.

4. The device of claim 1 and 2, or 1 and 3 wherein the two resonant cavities are adjusted to operate at close frequencies and that the frequency shifts caused by their thermal expansions or contractions are the same or similar..

5. The device of claim 1 and 2, or 1 and 3 wherein the cavities are energized to resonate electrically by electrical circuits connected to the cavities through cables or waveguides or enclosed inside the cavities.

6. The device of claim 1 and 2,4,5 or 1 and 3,4,5 wherein the electrical circuits of claim 5 operate in such a way as to subtract the frequencies produced by the cavities, resulting in the subtraction of the common thermal drift of the resonant frequencies while preserving the frequency shift caused by mechanical deformation.

7. The device of claim 6 wherein the electrical circuit is outputting the subtracted frequencies of the cavities either directly or offset by a constant value, as the sensor output.

8. The device of claim 6 wherein the electrical circuit is converting the subtracted frequencies of the cavities into another form of analog output such as driven voltage source, current source, or current sink (such as for example 4-20mA).

9. The device of claim 6 wherein the electrical circuit is converting the subtracted frequencies of the cavities into another form of digital output for transmission over a network or over a wireless data channel.