GB2251488A

GB2251488A - Load cell and method and apparatus for driving a system at resonance

Info

Publication number: GB2251488A
Application number: GB9024934A
Authority: GB
Inventors: Borge Riis Jensen; Miles John Padgett; David Michael Coombs
Original assignee: PA Consulting Services Ltd
Current assignee: PA Consulting Services Ltd
Priority date: 1990-11-16
Filing date: 1990-11-16
Publication date: 1992-07-08
Also published as: GB9024934D0

Abstract

A load cell comprises a sealed vessel (3) containing a force transducer (2). The force transducer (2) comprises first and second parallel diaphragms (7, 8) having equal external surface areas and being connected to each other via force transmitting means. The arrangement of the parts is such that a load applied to one diaphragm is applied to the force transducers. The force transducer may be a vibratory device which is driven at resonance by apparatus which detects the phase difference between applied oscillatory signals and output oscillatory signals, and adjusts the frequency until they are 90 DEG out of phase. The resonant frequency of the force transducer will vary with applied load. <IMAGE>

Description

LOAD CELL AND METHOD AND APPARATUS FOR DRIVING A SYSTEM AT RESONANCE The present invention relates to a load cell for use in measuring loads such as weight and pressure. More particularly, the present invention relates to a load cell of the type including a force transducer contained in a sealed environment There are many reasons for providing a sealed environment for a force transducer. Certain materials which are suitable for force transducers are easily corroded or deteriorate in other ways in humid conditions.

Vibratory transducers function better in evacuated chambers since the presence of a gas environment may damp vibrations too quickly to enable them to be measured. A particular problem with load cells of which the force transducer is to be contained in a sealed environment is that of ensuring satisfactory transmission of an external force to the force transducer whilst eliminating the effects of a change in the atmospheric pressure surrounding the cell.

The present invention solves this problem in a simple and efficient manner by providing a load cell comprising a sealed vessel containing a force transducer, the sealed vessel comprising first and second parallel diaphragms having equal external surface areas and being connected to each other via force transmitting means, and wherein a load applied to the first diaphragm is transmitted to the force transducer.

When the load cell of this invention is being used under atmospheric pressure conditions, atmospheric pressure will act on the first diaphragm in addition to an applied load. However, atmospheric pressure will also act on the second diaphragm and the force on the second diaphragm will be equal and opposite to the force on the first diaphragm and will be transmitted to the first diaphragm via the force transmitting means so that the net effect of atmospheric pressure on the force transducer is zero.

In solving the aforementioned problem, the present invention has provided a load cell which is suitable for any force measurement applications,including some where atmospheric pressure is not a problem. For example, the load cell of the present invention can be used as a differential pressure meter since a difference in the pressure acting on the two diaphragms will cause a net force on one of the diaphragms which will be transmitted to the force transducer.

The force transducer is preferably an element which is arranged to be strained in response to a load applied to the diaphragms. The strain may then be measured by suitable strain gauges. Alternatively the force sensor may be of the vibratory type such as a stretched string or a tuning fork whose resonant frequency of vibration depends on the strain applied. The tuning fork may advantageously be manufactured from single crystal silicon, as will be described below. In the case of a vibratory sensor it is desirable to evacuate the vessel.

The applied load may be transmitted directly from the first diaphragm to the force sensor or it may be transmitted via a lever mechanism to reduce the force applied to the force sensor and avoid overload.

This invention also relates to a method and apparatus for driving a system to oscillate at resonance.

The use of vibratory devices such as tuning forks for load measurement has been proposed previously and drive arrangements have been devised for driving the devices at resonance. These previous drive arrangements have generally detected resonance from a peak in the output oscillations of the system, and typically include amplifier circuits whose gain is highest at resonance.

It is known that when a system is vibrating at resonance the output oscillations are 900 out of phase with respect to the input oscillations. The method according to this invention comprises comparing the phases of the input and output oscillations of the system and adjusting the frequency of the input until the input and output are 900 out of phase.

Thus, the present invention also provides apparatus for driving a system to oscillate at resonance comprising means for inputting oscillations to the system, means for detecting output oscillations of the system, means for comparing the phases of the input and output oscillations and means for adjusting the frequency of the input oscillations until they are 900 out of phase with the output oscillations.

The apparatus and method of the invention are particularly appropriate to an electronic arrangement in which the drive signals and output signals are applied and received respectively by electronic devices such as piezoelectric transducers. As will be explained below, the apparatus may advantageously comprise a phase-locked loop which enables the resonant frequency to be very simply detected.

Certain embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 is a cross sectional view of a first embodiment of load cell according to this invention; Figure 2 is a perspective view of a tuning fork force transducer for use in the load cell of figure 1; Figure 3 is a schematic diagram of the drive electronics used in conjunction with the tuning fork of figure 2; Figure 4 is a cross sectional view of a load cell according to a second embodiment of this invention; Figure 5 is a perspective view, partially cut away, of a weigh scale incorporating a load cell according to this invention Figure 1 illustrates a load cell 1 according to a first embodiment of this invention.The cell comprises a force sensor in the form of a tuning fork 2 supported in a sealed vessel 3 in a manner to be described below. The vessel comprises a cylindrical wall 4 having a first diaphragm, 5 covering its upper end and a second diaphragm 6 covering its lower end. The diaphragms 5, 6 are held in place by retaining rings 7, 8. The diaphragms are linked together by a load transfer strut which, in the illustrated arrangement, comprises first and second discs 9, 10 secured to respective diaphragms 7, 8 by means of screws 11, 12 and connected to each other by joining columns, two of which are indicated by numerals 13, 14.

The respective ends of the tuning fork are held in cups 15 and 16. One of the cups is secured to the lower disc 10 and the other is secured to a cross brace 17. The cross brace extends across the vessel through the load transfer strut and is secured to the cylindrical wall at each of its ends.

When a force is applied to the first diaphragm 5 in the direction of the arrow F shown in figure 1, this force is transmitted through the load transfer strut 9, 10, 13, 14, to the second diaphragm 6 and the tuning fork 2.

Since the cross brace, to which the upper end of the tuning fork is secured, is fixed with respect to the housing the result is to strain the tuning fork in its longitudinal direction. The strain causes a change in the resonant frequency of the tuning fork which can be measured by suitable circuitry to be described below, to provide a measure of the force applied to the first diaphragm.

It will be noted that the screws 11, 12, the tuning fork 2 and the cups 15, 16 are mutually aligned.

This alignment helps to ensure that forces are applied longitudinally to the tuning fork.

As explained above, one of the objects of this invention is to ensure measurement of force independently of variations in atmospheric pressure. Because the vessel 3 is sealed, atmospheric pressure will act on the diaphragms 5 and 6 as indicated by the arrows A indicated in Figure 1 tending to push the diaphragms inwardly.

Without the presence of the load transfer strut, atmospheric pressure acting on the first diaphragm 5 would add to the strain across the tuning fork due to the applied force F. However, since the diaphragms are linked together, an equal and opposite force is applied to the tuning fork due to atmospheric pressure acting on the second diaphragm 6. Hence the net force acting on the tuning fork due to atmospheric pressure is zero.

In the preferred embodiment of the invention the vessel is evacuated. This renders the load cell less susceptible to temperature variations. However, the use of a vacuum is not essential. This vessel may simply contain air or a ballast gas such as nitrogen.

It will be appreciated that the load cell illustrated in Figure 1 may be constructed in various ways.

The clamping rings may be secured to the cylindrical wall 4 using screws which may also secure the diaphragms 5, 6 in place. Seals may be provided between the wall 4 and the diaphragms 5,6 to prevent gas from escaping and/or entering. Seals 18 and 19 are illustrated between the screws 11 and 12 and the diaphragms 5 and 6. The columns 13 and 14 may be secured to the discs 9 and 10 by means of screws passing through the discs. The cross brace 17 may be supported in the vessel in a number of ways. One possibility is to provide an annular shelf on the inner surface of the chamber and weld the cross brace in place on the shelf.

In the the illustrated arrangement, the force sensor is a tuning fork although it will be appreciated that many other types of force sensor may be used, including other types of vibratory force sensors and sensing arrangements including strain gauges. The preferred form of force sensor for use in this arrangement is illustrated in Figure 2.

Figure 2 illustrates a double ended tuning fork comprising first second substantially planar pieces of silicon 21 and 22 each of which has a central portion 21a, 22a which forms a tine of the tuning fork and end portions 21b, 21c, 22b, 22c. The two portions 21 and 22 may be manufactured from single crystal silicon and are preferably cut from a wafer of the type used in the semiconductor industry. The tine portions 21a, 22b, may be formed by etching, advantageously carried out before the wafer or other substrate is cut into individual pieces 21, 22. The pieces of silicon are joined at their thicker end portions with the aid of a layer of glass 25 between their surfaces.

The tuning fork carries two piezoelectric transducer elements 23, 24 which may be formed on the silicon surface by deposition. One of these acts as a driver element and the other as a vibration sensor. Electrical connections 26, 27 are made with the transducer elements and the silicon surface using epoxy bonds 28, 29. The manner of manufacture of this type of tuning fork is described detail in our copending International Patent Application PCT/GB90/01361.

As explained above the ends of the tuning fork are held in cups 15 and 16. These cups completely surround the ends of the tuning fork enabling a secure bond to be made to both tines. The cups 15, 16 may be screwed to disc 10 and cross brace 17 respectively.

Figure 3 illustrates schematically the circuitry for driving the tuning fork. The circuitry is one embodiment of apparatus for driving a system at resonance according to the invention, utilising the method of the invention. Signals from the vibration sensor are applied to an amplifier 30. The contents of the amplifier are conventional and will not be described in detail herein.

The output, A is applied to the input of a phase detector 31 via resistor R1. The phase detector provides an output voltage which is dependant on the phase difference between the input waveform and a reference waveform applied to its other input. The output from the phase detector is applied to a voltage controlled oscillator (VCO) 32 which provides an oscillating output voltage whose frequency depends on the voltage applied to its input. This output voltage is applied to a counter (not shown) which determines the frequency of the output signal. The output from the VCO is also passed via a frequency divider 33 to the reference input of the phase detector 31. The divided output B is also applied to the input of a phase shifter and drive circuit 34 whose output is applied to the drive transducer of the tuning fork.The phase shifter and drive circuit integrates the input waveform so that the output signal is 900 out of phase with the input. The contents of the phase shifter and drive circuit are conventional and will not be described in detail herein.

The vibrator and drive element are positioned adjacent respective tines of the tuning fork as illustrated in Figure 2. When the tuning fork is vibrating at its resonant frequency the output signals from the fork will be 900 out of phase with respect to the input. Since the signal output to the fork is 900 out of phase with respect to the signal B applied to the phase shifter and drive circuit 34, when the tuning fork is vibrating at resonance the signal A obtained from the output of the amplifier 30 will be in phase with the signal B. The phase detector 31, VCO 32 and frequency divider 33 together form a phaselocked-loop.Thus, the effect of the phase detector 31 and VCO 32 is to continually adjust the frequency of the signal B input to the drive circuit 32 until the tuning fork is at resonance, at which point, the output from the phase detector 31 will correspond to zero phase difference and the frequency of the signal output from the voltage controlled oscillator will be constant. The frequency measured by the counter is compared (by means not shown) with the resonant frequency of the tuning fork under zero load to determine the amount of the load.

The circuitry illustrated in Figure 3 may be located inside the vessel 3 of figure 1.

Figure 4 shows a second embodiment of a load cell according to the invention. Like parts in Figures 1 and 4 are indicated by the same reference numerals increased by 100, and will not be described in detail hereinbelow.

One end of the tuning fork 102 is directly secured to the wall 104 of the vessel 103 via a flexible element 140 and the other end of the tuning fork is connected to a load transfer strut 141 via a lever 142 pivotably mounted on the load transfer strut, and a further flexible element 143 joining the lever 142 to the tuning fak mounting cup 116. The. other end of the lever 142 is attached to the load transfer strut via a further flexible element 144. The load transfer strut comprises a solid element joining the two diaphragms and having a hollow central portion having a projection 145, provided with a fulchrum element 146 for the lever 142.

The operation of the load cell 101 shown in Figure 4 is the same as that shown in Figure 1 except that a load on the diaphragm 105 is transmitted to the tuning fork via the lever 142 and hence reduced. Thus, larger loads can be measured than with the arrangement of Figure 1 without any additional strain on the tuning fork. The load induces a bending moment on the lever acting in the clockwise direction at the point of attachment of the flexible element 143 to the fork 102 which induces a strain in the tuning fork which can be measured as described above. It is important for the tuning fork to be flexibly mounted to ensure that the strain is directed along the axis of the tuning fork.

Figure 5 shows a practical application of a load cell according to this invention in a weigh scale. A weigh scale such as that illustrated would be suitable for weighing loads of the order of a few tens of kilograms.

In this arrangement a load cell 150 is surrounded by a protective shield 151 in the form of a cylindrical container. The base of the weighscale comprises a rectangular open topped container 152.

A rectangular scale pan 153 is provided for supporting a load to be weighed. The scale pan has stiffening ribs, one of which is indicated at 154. The scale pan 153 is supported over the load cell 150 by crossed springs 156,157 so that the load of the scale pan 153 is transmitted directly to the upper diaphragm of the load cell via the springs 156,157.

As shown in the drawings the cross springs 156 pass through slots 158 in the shield 151. The arrangement includes an overload protection mechanism for the cell comprising a threaded post 155 which passes through the shield 151. When a sufficiently large load is applied to the scale pan the springs will be deformed and the pan will rest on the top of the post 155 and thus relieve the load on the load cell In other words, the post and casing support the load so that the diaphragm of the load cell is not deformed further. The load at which the protection mechanism comes into play can be adjusted by rotating the threaded post 155 to adjust its position.

The above is just one of many possible applications for the load cell according to this invention The load cell is, for example particularly suitable for use as a differential pressure meter since different pressures on the upper and lower diaphragms will cause a strain across the force sensor.

Claims

CLAIMS:

1. A load cell comprising a sealed vessel containing a force transducer, the sealed vessel comprising first and second parallel diaphragms having equal external surface areas and being connected to each other via force transmitting means, and wherein a load applied to the first diaphragm is transmitted to the force transducer.

2. A load cell according to claim 1, in which the force transducer is arranged to be strained on application of a load to the first diaphragm.

3. A load cell according to claim 1 or 2, in which the force transducer is a vibratory device and further comprising means for causing the force transducer to vibrate at resonance.

4. A load cell according to claim 3, further comprising means for measuring the resonant frequency of the force transducer.

5. A load cell according to claim 1,2,3 or 4, n which the force transducer has the form of a tuning fork.

6. A load cell according to claim 5, in which the tuning fork is manufactured from single crystal silicon.

7. A load cell according to any preceding claim in which the vessel is evacuated.

8. A load cell according to any preceding claim, including means for transmitting a load applied to the first diaphragm directly to the force transducer.

9. A load cell according to any of claims 1 to 7, including means for transmitting to the force transducer a proportion of a load applied to the first diaphragm.

10. A load cell according to claim 9, in which the transmitting means comprises a lever arrangement.

11. A load cell substantially as hereinbefore described with reference to the accompanying drawings.

12. A weighing device including a load cell as claimed in any preceding claim.

13. A weighing device as claimed in claim 12, including means for transferring a load to be weighed to the diaphragm of the load cell and means arranged to support the load for preventing overloading of the diaphragm.

14. A method of causing a system to oscillate at resonance comprising applying oscillations to the system, detecting the resulting oscillations of the system, comparing the phases of the input oscillations and output oscillations and adjusting the frequency of the applied oscillations until the input and output oscillations are 900 out of the phase.

15. Apparatus for driving a system to oscillate at resonance comprising means for inputting oscillations to the system, means for detecting output oscillations of the system, means for comparing the phases of the input and output oscillations and means for adjusting the frequency of the input oscillations until they are 900 out of phase with the output oscillations.

16. Apparatus according to claim 15, in which the means for inputting oscillations to the system and the means for detecting output oscillations of the system are arranged respectively to receive and transmit electrical signals.

17. Apparatus as claimed in claim 17, in which the inputting means and the detecting means each include piezoelectric transducers.

18. Apparatus as claimed in claim 16 or 17, in which the phase comparison means and the frequency adjusting means comprise a phase detector in series with a voltage controlled oscillator.

19. Apparatus as claimed in claim 18, in which the phase detector compares the output signals with a reference signal derived from the voltage controlled oscillator.

20. Apparatus as claimed in claim 19, in which the phase of the output signal from the voltage controlled oscillator is shifted by 900 before being input to the system whereby at resonance the signals input to the phase comparator are in phase.

21. A load cell as claimed in claim 3, in which the means for causing the force transducer to vibrate at resonance comprise apparatus as claimed in any of claims 15 to 20.

22. Apparatus for driving a system at resonance frequency substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.