GB2261731A - Vibrating beam sensor - Google Patents

Abstract

The vibrating beam sensor 16 has coplanar beams, 20, 21, electrostatic driving means for driving at least one of the beams into flexural vibration in the common plane of the beams, the driving means being drive electrodes, 80, 81 and supply means for supplying a drive signal having an alternating component to each electrode, the signals to the at least two electrodes being in antiphase to each other, the sensor also having electrostatic detecting means for detecting changes in the frequency of the flexural vibrations of a beam in response to changes in an external variable, the detecting means having at least one detect electrode 82 adjacent to and spaced from the beam. <IMAGE>

Description

VIBRATING BEAM SENSOR The present invention concerns sensors and in particular sensors in which variations in a beam's natural resonant frequency caused by an external variable such, as temperature, air damping, mass loading or tension applied to the beam, is used to measure the variable.

Vibrating beam sensors are known and essentially comprise a beam or beams driven into flexural vibration in planes perpendicular to the plane of the beam or beams, the vibrational frequency changing in response to changes in external variables. These changes in frequency are measured to provide a measurement of the variables.

It is also known for a sensor to have two beams which are caused to vibrate in their common plane in anti phase. Because the vibrations induced by each beam in the mounting are equal and opposite, they cancel out. Thus very little energy is transferred to the mountings, and the sensor therefore has a high Q factor.

In sensors in which the beams are spaced well apart, adequate shielding is provided by the circuit board on which the sensor is mounted. However, the smaller the separation of the beams, the greater the pick up between the electrodes used to drive and detect the sensor vibration.

One object of the present invention is to provide a beam sensor in which some of the defects of prior art sensors are overcome.

According to the present invention there is provided a vibrating beam sensor comprising at least two coplanar beams, electrostatic driving means for driving at least one beam into flexural vibration in the common plane of the beams, the driving means comprising at least two drive electrodes and supply means for supplying a drive signal having an alternating component to each electrode, the signals to the electrodes being in anti phrase to each other, the sensor further comprising electrostatic detecting means for detecting changes in the frequency of the flexural vibrations of the beam in response to changes in an external variable, the detecting means comprising at least one detect electrode adjacent to and spaced from the beam.

In order that the present invention may be more readily understood, embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a side view of a known weighing system incorporating a vibrational beam force sensor, Figures 2 and 3 are plan views of a vibrational beam force sensor for use in the present invention, Figure 4 is a block diagram of a weighing system incorporating the force sensor of Figures 2 and 3 and for use in accordance with the present inventon, Figure 5 is a plan view of the force sensor, showing one embodiment of a driving and detecting arrangement for use in accordance with the present invention, Figures 6A, 6B and 6C are plan views of the driving and detecting arrangement shown in Figure 5 at various stages of its manufacture, Figure 7 is a cross-section of the driving and detecting arrangement shown in Figure 5 along the line x - x and Figure 8 is a plan view of an alternative embodiment of a driving and detecting arrangement for use in accordance with the present invention.

Referring now to Figure 1 of the accompanying drawings this shows what is known as a Roberval mechanism. A weighing pan 10 is supported above a load cell 11 by a pan support 12. The cell 11 is made from a suitable metal and essentially comprises a hinged parallelogram. Thus the cell 11 is mounted on a base plate 13 at one side only, the mounting being indicated at 14.

As can be seen the bulk of cell 11 projects cantilever fashion over the base plate 13. The pan support 12 is mounted on the side of the cell remote from mounting 14. The central area of the cell 11 is cut away to leave a space 15 which in this embodiment resembles a crude representation of the letter Z. A double beam sensor 16 is mounted in this space by respective top and bottom sensor fixings 17 and 18. When a load is placed on the weigh pan 10 the cell 11 flexes in response to the added weight and changes the tension in the force sensor. In operation of the weighing system the sensor 16 is driven into flexural vibrations and these are detected.

The frequency of the vibrations varies with the applied load so that a measurement of the load can be derived from the detected frequency.

Referring now to Figure 2 of the drawings this shows a plan view of sensor 16. The sensor is electrically conductive and is appropriately manufactured from a metal or metal alloy. Use of a metal provides a number of advantages. Firstly there is a large variety of alloys available which in turn allows a wide range of properties to be chosen. Secondly metal sensors can be manufactured using a large number of techniques, including machining, spark eroding, chemical etching and laser machining. Thirdly metal sensors are tough, robust and have good shock resistance.

The double beam sensor 16 shown in Figure 2 includes a strip having two beams 20, 21 of equal width. The beams 20, 21 are manufactured from a single piece of metal which has flanges 22 at each of its ends by means of which the sensor can be mounted in a load cell. As can be seen in Figure 2 the two beams 20, 21 extend from common beam mounting portions 23 having shoulders 24. The width of each shoulder 24 is approximately half of the space between the two beams.

A desired mode of vibration of this structure is shown in Figure 3, greatly exaggerated for clarity. The flexural vibration takes place in the common plane of the beams 20, 21.

Referring now to Figure 4 of the accompanying drawings this shows a general block diagram of a weighing system including a sensor of the kind shown in Figure 2. The sensor itself is shown at 40 and the drive and detect mechanisms at 41 and 42 respectively. The drive mechanism 41 is driven by a feedback oscillator circuit 43 which in turn receives the detected vibrational frequency of the sensor beam from detect mechanism 42. When the sensor is connected to oscillator circuit 43 the circuit oscillates at a frequency determined by the resonant frequency of the sensor. Thus the electrical signal that the oscillator circuit 43 feeds to drive mechanism 41 is counted in a frequency or period counter 44 against a clock signal generated by a clock circuit 47.The output of counter circuit 44 is a number related to the weight being measured as the resonant frequency of the sensor will vary with varying loads. A calculating circuit 45 translates the output of counter 44 into a weight reading which is then available for display by display 46 and/or for further processing such as taring, signal averaging or temperature correction.

Referring now to Figure 5, the sensor 16 is mounted over a printed circuit board (PCB) 53. Formed on the PCB 53 are a drive electrode 50 and a detect electrode 51. The drive and detect electrodes extend underneath and parallel to the beams 20, 21 respectively, the electrodes 50, 51 being adjacent to but spaced from the beams 20, 21. To excite the mode of vibration shown in Figure 3, a drive voltage consisting of an a.c. voltage at the resonant frequency of the sensor superimposed on a constant bias voltage is applied to the drive electrode 50. In this way, an electrostatic force is set up between the drive electrode 50 and the beam 20, the sensor 16 (and thus the beams 20, 21) being maintained at a constant potential. The horizontal alternating component of the electrostatic force causes the beam 20 to vibrate towards and away from the beam 21 in their common plane. A corresponding movement in the opposite direction the beam 21 is caused by a reactive force generated in the flange 23. The beams 20, 21 move as indicated by Figures 2 and 3.

The charge on the detect electrode 51, maintained at constant potential, varies due to the change in capacitance caused by the movement of the beams 20, 21 and thus has the same frequency as the mode of vibration. The detect electrode 51 forms part of a detect mechanism 42 which supplies the feedback circuit 43 to the drive mechanism 41. Thus by feeding the frequency signal derived from the varying charge on the detect electrode 51 back to the drive electrode 50, the sensor 16 is made to vibrate at its natural resonant frequency. A bias voltage is added to the drive mechanism alternating voltage so that the fundamental frequency of driving electrostatic force is that of the drive mechanism voltage. In addition, by adding the bias voltage, the magnitude of the driving force is increased.

Referring now to Figures 6A, 6B, 6C and 7 these show a preferred form of the electrostatic driving and detecting arrangement.

In this arrangement the detector and drive electrodes, 51 and 50 are made from a graphite-based conductive ink and have been deposited on a layer 55 of solder resist in turn deposited on the upper copper layer 54 of a PCB 53. This layer 54 is sufficiently close to the electrodes 50, 51 to act as a shield with the solder resist layer 55 acting as insulation.

To form the electrodes 50, 51 on the PCB 53, a copper coating is etched as shown in Figure 6A to produce the upper copper layer 54 and the drive and detect connections 58, 59. A solder resist layer 55 is then formed over the copper layer 54 in Figure 6B. The layer 55 has holes 56, 57 located over the ends of the connections 58, 59.

The detector and drive electrodes 51 and 50 are deposited over holes 57 and 56 respectively (Figure 6C). Each of electrodes 50, 51 extend through a hole 56, 57 and contacts a connection 58, 59, as shown in Figure 7. The electrodes 50, 51 are insulated from the copper layer 54 by the part of solder resist layer 55 deposited over the copper layer 54.

This arrangement removes most of the pick up through the PCB 53 leaving pick up through the air gap 60 as the dominant pickup mechanism. This enables separation of the electrodes 50,51 to be small relative to the thickness of the substrate. This in turn means that smaller structures can be used. In a typical embodiment the vibrating portion of the beam is between 5 and 50 millimetres long. The individual beams 20, 21 are of equal width and are between 0.5 to 2 millimetres wide.

Electrostatic driving and detecting arrangements have a number of advantages over known piezoelectric or optical arrangements.

An alternative preferred embodiment of an electrostatic drive and detection arrangement is shown in Figure 8. In this embodiment three electrodes 80, 81 are used to drive the beams 20, 21 and electrode 82 is used to detect their motion. The drive electrodes 80, 81 are coplanar and are adjacent to the beams 20, 21 but are spaced therefrom. The central electrode 81 is positioned adjacent the space between the beams 20, 21 and outer electrodes 80 are located on opposite sides of the two beams. The common plane of the drive electrodes 80, 81 is parallel to the common plane of the beams. Outer electrodes 80 are driven by the same signal. The central electrode 81 is supplied with an alternating signal of similar waveform but opposite phase to the waveform supplied to electrodes 80, and both of these waveforms are given DC voltage biases of the same polarity relative to the sensor 16.

The sensor 16 (and therefore the beams 20,21) is maintained at a fixed potential. The drive signals supplied to the drive electrodes 80, 81 produce electrostatic forces which alternately cause repulsive and attractive forces between the beams 20, 21 of the sensor 16.

The forces cause corresponding movement of the beams 20, 21.

As the detect electrode 82 is kept at a constant potential by suitable feedback circuits, the charge on the electrode 82 will vary with the change in capacitance due to the movement of the beams 20, 21.

The feedback oscillator circuit 43 supplies the signals to the drive and detect electrodes. This is a suitable arrangement for driving the mode shown in Figure 3. The shielding layer (not shown) formed between the electrodes reduces stray electrostatic coupling from drive to detect electrodes through the board, on which the electrodes are formed, to a level negligible compared with that through the medium above the board. However, the stray electrostatic coupling from central electrode 81 to electrode 82 and from outer electrodes 80 to electrode 82 are opposite in sign and therefore tend to cancel one another out.

Pick up can also be reduced by an appropriate choice of electrode geometry and of the relative amplitudes of the two drive signals.

The manner in which the electrodes are located on the substrate in any embodiment will be dependent on the nature of the latter.

It will be realised that whilst the foregoing description has been directed to the use of sensors in a weighing system, that this is not the only possible application of such sensors. Thus the sensors can be used in accelerometers, pressure sensors, hydrophones, thermometers, inclinometers and the like. The sensors may also act as compensating elements for other sensors. US 4947694 describes a force sensor in which electrostatic means are used to drive the coplanar beams of the sensor into flexural vibration in a plane normal to the common plane of the beams. It will be appreciated that electrostatic means could be provided to produce simultaneous flexural vibration of the beams in both the plane of the beams and normal thereto. If two measurands each affected the frequency of vibration of only one of the modes, a single sensor could be used to sense the two measurands simultaneously, using detect electrodes as described in US 4947694 and in the present application, formed on a single PCB.

Figures 6A and 7 of the drawings show the shielding layer 54 as part of a PCB 53. Alternatively the shielding layer be part of a thick or thin film substrate or may be a thin or thick film deposited on a thin or thick film substrate.

Claims (12)

1. A vibrating beam sensor comprising at least two coplanar beams, electrostatic driving means for driving at least one beam into flexural vibration in the common plane of the beams, the driving means comprising at least two drive electrodes and supply means for supplying a drive signal having an alternating component to each electrode, the signals to the electrodes being in antiphase to each other, the sensor further comprising electrostatic detecting means for detecting changes in the frequency of the flexural vibrations of the beam in response to changes in an external variable, the detecting means comprising at least one detect electrode adjacent to and spaced from the beam.

2. A vibrating beam sensor as claimed in Claim 1, wherein the drive electrodes are spaced one on either side of a beam to be driven.

3. A vibrating beam sensor as claimed in Claim 1 or 2, wherein the drive and detect electrodes are formed on the surface of an intermediate insulating layer, the insulating layer being located on the surface of a shielding layer of a conductive material.

4. A sensor as claimed in Claim 1, 2 or 3 wherein the strip has two beams and the driving means comprises three coplanar drive electrodes, the electrodes comprising a central electrode adjacent the space between the beams and outer electrodes located on opposite sides of the beams, the common plane of the electrodes being parallel to the common plane of the beams.

5. A sensor as claimed in any preceding claim, including a feedback oscillator circuit having an input to which the detecting means is connected and an output to which the driving means is connected, the circuit being operative for generating an output signal for driving the driving means.

6. A sensor as claimed in Claim 5 wherein the supply means is the feedback oscillator circuit.

7. A sensor as claimed in any of Claims 3 to 6, wherein the shielding layer is part of a printed circuit board.

8. A sensor as claimed in any of Claims 3 to 6, wherein the shielding layer is part of a thin or thick film substrate.

9. A sensor as claimed in any of Claims 3 to 6, wherein the shielding layer is a thin or thick film deposited on a thin or thick film substrate.

10. A sensor as claimed in any preceding claim having further means for driving at least one beam into flexural vibration in a plane normal to the common plane of the beams and further means for detecting changes in the flexural vibrations of the beams, in a plane normal to the common plane, in response to change in a further external variable.

11. A sensor as claimed in any preceding claim wherein said variable is the force applied to the sensor.

12. A sensor as hereinbefore described and illustrated in Figures 1 to 8.