GB2064770A - Improvements relating to weighing devices - Google Patents

Abstract

A weighing device uses frequency measurement to determine weight. One body (3) is decoupled by springs (2) from the environment (1) and another body (8) is spring (9) connected to it, so that both bodies can oscillate in one mode in an effectively isolated system. The oscillations are electromagnetically generated, but when a certain amplitude is attained the drive is cut off and the system resonates freely. The frequency varies with the mass of the bodies, and so a load on one can be weighed by reference to the frequency, which can be detected through the electromagnetic means (12) which provided the initial drive pulses. <IMAGE>

Description

SPECIFICATION Improvements relating to weighing devices This invention relates to weighing devices. It is intended primarily for use in a moisture meter as described in our co-pending Application No. 7942431 but it can have other applications. It is important for this particular technique of moisture measurement to have an accurately weighed sample, and 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 counter balancing 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 an object of unknown mass comprising a first body of known mass supported in such manner that for one mode of movement it is substantially unimpeded at least over a limited distance, a second body of known mass adapted to carry an object of unknown mass, 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 object.

According to another aspect of the present invention there is provided a method of weighing an object of unknown mass comprising: (i) supporting a first body of known mass in such manner that for one mode of movement it is substantially unimpeded at least over a limited distance, (ii) spring connecting a second body of known mass to said first body 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 an object 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 object.

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.

Preferably, means will be provided for determining a given amplitude of oscillation and, when this is achieved, for activating the means for determining resonant frequency.

This is advisable since frequency is affected by amplitude.

Conveniently, the vibrations will be initially 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 approximately to the expected resonant frequency generate the oscillations, and when the pulses are stopped (at the given amplitude or perhaps after a predetermined number of pulses) free harmonic motion exists. The coil can be used in detecting the amplitude during intervals between pulses, and when they have been stopped it can also be used to detect the frequency. This 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.

For a better understanding of the invention one example will now be described, by way of example, with reference to the accompanying drawing in which: Figure 1 is a diagrammatic sectional elevation of a weighing device, Figure 2 is a diagrammatic sectional elevation of a modified weighing device, Figure 3 is a diagram of a control and readout circuit associated with such weighing devices, and Figure 4 is a block diagram illustrating a microprocessor program.

The device of Fig. 1 has a fixed outer casing 1 within which is suspended, by springs 2 attached to the wall of the casing, a structure 3 of known mass. The springs 2 are illustrative only of any means which can decouple the structure 3 from the casing 1 and its environment. In order to achieve accuracy, the structure 3 should not be in firm contact with its support, since the latter could then affect the resonant frequency of the system to be described.

The structure 3 has permanent magnets 4 (or a complete ring magnet) with annular pole pieces 5 and 6, the pole piece 6 being a disc of magnetically permeable materal with a central spigot 7 which forms an annular gap with the inner periphery of the pole piece 5. A receptacle 8 for a sample to be weighed is supported by the structure 3. This is done through the intermediary of springs 9 suspending a circular plate 10 from the centre of which a column 11 extends upwardly through the spigot 7 to carry the receptacle 8. A coil 1 2 is also carried by this column 11 and is located in the gap between the pole piece 5 and spigot 7. The sample to be weighed is put into an annular trough 1 3 around the periphery of receptacle 8.It will be distributed to keep the receptacle balanced with the column 11 upright, and generally the trough will be filled so that from the known volume once the weight is determined the density can readily be calculated. The receptacle assembly 8, 10, 11, 1 2 is of known mass and the springs 9 are of known rate so that from the resonant frequency of vertical oscillations the mass of any charge in the receptacle can be calculated.

For a weighing operation the receptacle is charged and pulses are fed to the coil 1 2.

This generates vertical oscillatory motion of both the receptacle 8 and the structure 3.

When this has stabilised or reached a given amplitude, the pulses are cut off, and the system achieves free harmonic motion. The springs 2 have little effect on this by virtue of their orientation, and the frequency depends mainly on the rate of springs 9 and the relative mass of the structure 3 and the filled receptacle 8. It also depends to some extent on the amplitude of the oscillations, and so for consistency and accuracy it is desirable to take the frequency measurement at a defined amplitude, which can be determined by mechanical electrical, electro-mechanical or optical methods. A particular one will be described later.

Different weights in the receptacle give different frequencies, and this relationship can be pre-calculated and/or determined by tests, and then calibrated for immediate read-out.

The free motion frequency achieved is most conveniently detected by using the coil 12 which is moving within a magnetic field.

Circuitry for achieving this will be described below.

As mentioned above, temperature affects the springs, and this can be allowed for electronically or by suitable selection of spring material.

A somewhat different device, but embodying the same principles, is shown in Fig. 2. A casing 21 suspends by means of springs 22 a structure 23. Within this is housed a magnet assembly comprising annular magnet 24 and pole pieces 25 and 26, the latter being a disc with a downwardly projecting central stem 27 forming an annular gap with the inner periphery of washer-like pole piece 25. A platform 28 is supported above the structure 23 by means of an S-shaped spring 29 spanning the structure and passing through a boss 30 centrally secured to the upper side of the pole piece 26. The connection between platform 28 and boss 30 is by a short column 31. A coil 32 mounted centrally of the base of the structure 23 projects upwardly into the gap between pole piece 25 and stem 27.

In Fig. 1 the coil is part of the variable mass, while in Fig. 2 it is part of the known mass. The receptacle in Fig. 2 is shown as a flat platform, and for many weighing purposes this is quite adequate. It will be understood that hollow receptacles could be used instead.

Also, instead of a coil/magnet system, the oscillatory drive could be generated by an electromagnet/magnet system.

Fig 3 shows a circuit for interfacing either of the weighing devices described with a microprocessor (not shown), for driving the initial oscillations and for obtaining usable pulses from the coil, here referenced 1 2 as in Fig. 1.

The microprocessor will provide negative going pulses at approximately the expected natural resonant frequency at input terminal 40. These pass through resistor 41 to a Darlington pair of PNP transistors 42, 43 which are in circuit across the supply voltage with the coil 1 2. With terminal 40 high (the off state) the transistors 42, 43 are switched off, but each pulse turns them on, generating a positive going pulse on the coil 12, and hence movement of the weighing device.

The coil 1 2 is connected to a first operational amplifier 44, which forms a high gain squaring circuit, converting the low amplitude sine wave signal from the coil into a square wave of the same period. Feedback resistor 45 limits the gain and increases the slew rate.

The output of the amplifier passes to a monostable (not shown as it has no material effect on the operation of the circuit) and thence to a terminal 46 at an input of the microprocessor.

The coil 1 2 is also connected to a second operational amplifier 47, the coil output being summed at point X with that of a small reference voltage from a divider chain. This is formed by potentiometer 48 and resistor 49, and the outputs are applied through respective resistors 50, 51 to the negative input of the amplifier. The positive input is grounded.

The voltage V applied to the divider chain is stabilised and the resistance values are R48 and R49.

When the voltage from coil 12 falls below -V. R48 R48+ R49 point X goes below ground potential and so the output of amplifier 47 goes high. This happens when the oscillations of the weighing device have reached a given amplitude the level of which is set by potentiometer 48. This output is to a terminal 52 at another input of the microprocessor. Of course, when input 40 is low, the coil voltage is high and blocks any high output from amplifier 47.

This circuit operates in conjunction with the weighing device and the microprocessor (MCP) program, which performs several functions in sequence as follows, and as illustrated diagrammatically in Fig. 4.

(i) The MCP sends an active low pulse of length equal to approximately one third of the unloaded weighing device resonant period to input 40. An oscillation is started in a "for- ward" direction by the coil 12. A corresponding pulse appears at terminal 46, but there cannot yet be a high output at 52 as coil 1 2 goes positive.

(ii) The MCP then waits for a period equal to half the length of that pulse, i.e. to just before the mid-point of the cycle of oscillation when the maximum output from the return movement of the coil occurs. When the device is loaded, the oscillations will be slower and so after a time equal to (1/3+1/6) of the unloaded period the loaded device will still have to attain its peak output on the return movement.

(iii) By this time terminal 52 could be high if the oscillation is sufficient to induce a negative voltage in coil 1 2 which outweighs the reference voltage. The MCP now checks for this high over a brief length of time, approximately 1 /8th of the unloaded resonant period, which will be sufficient even when the device is loaded since the periods will not vary to a large extent.

(iv) Whatever the amplitude reached, there will still be a pulse at terminal 46, induced by the return movement of the coil. If the required amplitude is not reached, the return to high of terminal 46 causes the MCP to return to stage (i).

(v) If the required amplitude is reached, the MCP, triggered from terminal 52, disables the drive pulse output to terminal 40 and counts up at a fixed rate for one or more periods of the weighing device as the device resonates freely, using the pulses at 46. Generally several periods will be counted and arranged, for greater resolution. Decay of amplitude, and hence a change in period, is negligible over say four periods.

The counting is carried out by the MCP which, to keep the count in the correct phase, is programmed (a) to loop while output 46 is low (b) to loop whilst counting while 46 is high (c) to loop whilst counting while 46 is low.

The instruction to count will occur when terminal 46 is low, during the induced pulse which sends terminal 52 high. However, this is an indeterminate point, and so the count should not commence immediately. Instead, the MCP waits until 46 goes high, counts during that phase and during the subsequent low phase and so a complete half period is ascertained. Preferably (b) and (c) are repeated to a total of eight times each so that four periods are counted.

Although the devices described have vertical oscillations, it should be understood that this is not essential and versions with horizontal or even inclined movement would be equally workable. Also it is not essential that the devices be held exactly in their intended attitude: they are tolerant of being held and operated in the hand.

Claims (12)

1. A device for weighing an object of unknown mass comprising a first body of known mass supported in such manner that for one mode of movement it is substantially unimpeded at least over a limited distance, a second body of known mass adapted to carry an object of unknown mass, 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 object.

2. A device as claimed in Claim 1, wherein means are provided for determining a given amplitude of oscillations and, when this is achieved, for activating the means for determing the resonant frequency.

3. A device as claimed in Claim 1 or 2, wherein pulse-operated electromagnetic means are provided for generating oscillations at a frequency approximating to the expected resonant frequency, there being means for blocking the pulses prior to the determination of the resonant frequency.

4. A device as claimed in Claims 2 and 3, wherein the electromagnetic means are arranged to produce, after the pulses are blocked, signals corresponding to the subsequent natural oscillations.

5. A device as claimed in Claim 4, wherin the electromagnetic means are arranged to produce, during a fraction of the pulse cycle when a pulse is not present, an indication of the amplitude of oscillation.

6. A method of weighing an object of unknown mass comprising: (i) supporting a first body of known mass in such manner that for one mode of movement it is substantially unimpeded at least over a limited distance, (ii) spring connecting a second body of known mass to said first body 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 an object 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 object.

7. A method as claimed in Claim 6, wherein said determination from the resonant frequency is carried out at a given amplitude of oscillation.

8. A method as claimed in Claim 6 or 7, wherein the oscillations are generated electro magnetically by pulses at a frequency approximating to the expected resonant frequency, these pulses being blocked prior to said determination.

9. A method as claimed in Claims 7 and 8, wherein the pulses are applied to electromagnetic means which also serve, after the pulses are blocked, to produce signals corresponding to the subsequent natural oscillations.

10. A method as claimed in Claim 9, wherein the pulses are applied over a fraction of a pulse cycle, and during the remaining fraction of that cycle the electromagnetic means serve to provide an indication of the amplitude of oscillation.

11. A weighing device substantially as hereinbefore described with reference to Fig.

1 or 2 and Figs. 3 and 4 of the accompanying drawings.

12. A method of weighing an object substantially as hereinbefore described with reference to Fig. 1 or 2 and Figs. 3 and 4 of the accompanying drawings.