Weighing scales – Plural distinct load holders with common evaluator – Load comparing type
Patent
1981-08-07
1983-09-20
Miller, Jr., George H.
Weighing scales
Plural distinct load holders with common evaluator
Load comparing type
177 1, 177210FP, G01G 1900, G01G 314
Patent
active
044050240
DESCRIPTION:
BRIEF SUMMARY
This invention relates to weighing devices. It is intended primarily for use in a moisture meter where it is important to have an accurately weighed sample. The proposed device allows this to be done in a receptacle that can form part of the meter itself.
It is also desirable to have a weighing device that is not dependent on counterbalancing weights or one where its orientation is important, as in most scales.
According to one aspect of the present invention there is provided a device for weighing a load of unknown mass comprising a first body of known mass, a second body of known mass adapted to carry a load of unknown mass, said bodies being supported so that for one mode of movement they are substantially unimpeded at least over a limited distance, spring means of known rate connecting said bodies so that their natural oscillations are in said one mode, and means for determining the resonant frequency of such oscillations and hence the mass of said load.
According to another aspect of the present invention there is provided a method of weighing a load of unknown mass comprising:
(i) supporting first and second bodies of known mass in such manner that for one mode of movement they are substantially unimpeded at least over a limited distance,
(ii) spring connecting said bodies so that the natural oscillations of the two bodies are in said one mode,
(iii) determining the resonant frequencies of said oscillations over a range of ladings of said second body,
(iv) lading said second body with a load of unknown mass,
(v) causing the body/spring assembly to oscillate in said one mode, and
(vi) determining from the resonant frequency of natural oscillations the mass of said load.
By natural oscillations is meant the usual movement of a spring mounted body, making most use of the spring characteristics. For example, with a coil spring this movement is longitudinal of the coil, although oscillations are possible transversely of this direction, by means of the spring flexing.
Conveniently, the vibrations will be initally generated by electro-magnetic means, such as a coil mounted on either body and co-operating with magnetised poles on the other body of the assembly. Pulses applied to this coil at a frequency approximating to the expected resonant frequency generate the oscillations, and when the pulses are stopped substantially free harmonic motion exists. The coil can be used in detecting the amplitude during intervals between pulses, and when a given amplitude is achieved the pulses may be stopped. This is advisable since frequency is affected by amplitude. Alternatively, the pulses may be stopped after a predetermined number. The coil can subsequently be used to detect the natural frequency of oscillation, which is a direct measure of the mass of the sample since the other masses and the values of the spring constants are known. A spring constant is a function of temperature, and a corrective factor can be introduced to allow for this, for example in the electronic processing of the detected oscillations, or use could be made of spring material with a very low temperature constant.
Preferably, the first body and the second body when laden are in the same mass range, and each body is supported solely by matching spring means from a base member. The bodies move in opposite directions and the components of forces in the springs in the direction of oscillation cancel out, thus isolating the oscillating system from the base member.
For a better understanding of the invention some examples will now be described, by way of example, with reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic sectional elevation of a weighing device,
FIG. 2 is a diagrammatic sectional elevation of a modified weighing device,
FIG. 3 is a diagram of a control and read-out circuit associated with such weighing devices,
FIG. 4 is a block diagram illustrating a microprocessor program,
FIG. 5 is a side view of a practical example of a weighing device,
FIG. 6 is a section on the line VI--VI of FIG. 5, and
FIG. 7 is a diagr
REFERENCES:
patent: 3322222 (1967-05-01), Baur
patent: 3555886 (1971-01-01), Thornton
patent: 4158395 (1970-06-01), Brown
Fraval Robert H.
Oakford Howerd P.
Miller Jr. George H.
National Research Development Corporation
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