CAPACITATIVE WEIGHT SENSOR

Abstract: A load sensor as disclosed which comprises an elongate hollow body 11, preferably formed as a metal extrusion and which forms a first capacitative plate 21. A second parallel capacitative plate 15 is formed on an insulating body which is held in place relative to the extrusion by a resilient support 16. Application of a load to the extrusion causes variation in the gap between the plates of the capacitor, thus affecting the capacitance in a manner which is measured and processed to provide an output signal which is directly proportional to the applied load.

Description: In a parallel plate capacitor, a pair of mutually insulated electrodes, each mounted on a support, face each other across a gap. If a load is applied to one support so as to cause it to flex towards the other support, the plate separation will decrease, thus increasing the capacitance as a function of the force applied. An example is given in DE-A-3 426 165 which discloses a load-measuring device in the form of a parallel plate capacitor of which one plate is resiliently deformable, as acknowledged in the preamble portion of Claim 1 below. If the capacitor however is a strip capacitor, elongated in one dimension of the plate to at least, say, twice the other dimension, and if the load is applied only over a region of the length of the strip, the capacitance will vary as a function of the pressure and the area over which it is applied. Such capacitors are subject to non linearities, which lead to inaccuracy and to contamination if foreign matter can gain access to the gap between the electrodes. The invention overcomes these problems by providing a capacitor as claimed in Claim 1 (sensor Claim 16). Preferably the wall is of conductive material and constitutes the first electrode. The body is preferably in one piece, preferably an extrusion, preferably of metal, preferably of aluminium. If the body is made of more than one piece, the pieces are preferably welded or glued together along an interface extending along the length of the strip, and the pieces are preferably of metal, preferably of stainless steel. When the hollow body is of conducting material, the second electrode is preferably mounted on an insulator mounted within the hollow body. The second electrode may be conducting film deposited on an insulating substrate. The substrate may be provided with conductive film divided longitudinally into three parallel strips, of which the centre strip forms the second electrode; the other two strips may engage shoulders on the hollow body projecting from either side of the first electrode by a distance equal to the desired inter-electrode gap. When the second electrode is a conductive film deposited on a substrate, means may be provided to press the substrate towards the first electrode, preferably using a resilient material, preferably comprising a spring strip and/or a pad of resilient resin or polymer material. Preferably the capacitor is connected to an electronic circuit to convert the capacitance to an output reading of desired form, and this electronic circuit is preferably mounted within the same hollow body. This provides protection of the electronic circuit, and keeps the connecting cables between the electrodes on the electronic circuit as short as possible. Preferably the electrodes are divided into mutually insulated portions spaced along the length of the strip. The capacitances of separate portions can be used to give weight readings from corresponding regions along the length of the strip. Connections can be made between the individual portions and an electronic circuit at one end of the strip of conductors contained within the hollow body, and when a substrate is provided to support the second electrode, the connections to the electronic circuits may constitute conductive material deposited on the side of the substrate remote from the second electrode. It is known to use a capacitor having parallel plates which are relatively moved in response to the depression of a weighbridge and hence derive a weight value from the capaciatance variation. A weighbridge weighs an object statically and requires a pit under the roadway for the weighbridge table supports. It is therefore hightly inconvenient for use in temporary locations. According to Claim 18 a method of measuring the weight of an object moving at a constant velocity across the capacitor is provided. Thus the frequency is directly proportional to the applied pressure. A count of the signal cycles thus is proportional to the total force applied by the object as it passes over the strip. An example of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross-section through a strip capacitor according to one embodiment of the invention, Figure 2A, 2B, and 2C are plans of strip capacitors attached to a road surface and Figure 3A, 3B, and 3C are corresponding side elevations of the capacitors of Figure 2. Figure 4 is a cross section through a strip capacitor according to a second embodiment of the invention. In Figure 1, a strip capacitor 11 comprises a hollow aluminium extrusion 12 containing within its hollow bore 13 a printed circuit board 14 bearing on its upper surface an etched copper film 15 and a resilient pad strip 16 pressing the printed circuit board 14 upwards within the bore. The upper surface 21 of the hollow bore has a central recess 22 between side shoulders 23, the recessed surface forming the upper electrode of the capacitor. The copper film on the printed circuit board is etched to form three parallel strips running lengthwise along the capacitor strip. The central strip 15 forms the lower electrode of the capacitor. The two side strips 15, are pressed into engagement with respective shoulders 23 of the aluminium extrusion by the resilient pad 16. A stainless steel spring strip or resilient filler could be used in addition or in substitution and the filler could be a resilient resin or polymer either injected directly into the hollow bore below the printed circuit board substrate or injected into tubes running below the substrate. Assuming that the copper film 15 deposited on the printed circuit board 14 is uniform across the width of the board, the air gap of the capacitor is accurately controlled to be equal to the depth of the recess and is independent of any variation in copper film thickness which may occur between different components. The extrusion 12 provides an electro-magnetic shield for the capacitor as well as a physically protective shield against impact damage. When a body such as a vehicle wheel passes in contact with the upper surface of the extrusion, the portion of the aluminum extrusion above the recess is deformed to reduce the air gap and therefore increase the capacitance of the capacitor which is connected to a capacitance measuring circuit 31 at one end of the extrusion within its bore. The capacitance of the sensor C is related to the air gap clearance width d such that C/Co = (l - w/L) + w/L (do/d) (1) Co = No-load capacitance do = No-load air gap clearance w = tyre contact width L = sensor length The air gap clearance varies linearly with the applied pressure P so that d = do - P/k where k = spring stiffness of strip The strip is connected into an oscillator circuit which is designed to generate a frequency which is inversely proportional to the capacitance C f = fo.Co/C where fo = the no-load oscillator frequency. Assuming that the change in air gap clearance is small so that d>. 0.95do, equations 1-3 can be combined to give f = fo - (fo/do.kL)Pw Thus the frequency varies linearly with Pw, the product of contact pressure and contact width. In order to determine the total wheel force, the frequency is integrated over the period of the tyre contact by summing cycles of the oscillator with a counter. The difference between this sum and the sum with no load is directly proportional to the instantaneous wheel force. The oscillator forms part of an electronic circuit contained within the hollow body housing the strip capacitor so that the electronic circuit is protected from damage and electro-magnetic interference, and the connecting cables are as short as possible. Connecting cables from the end of the strip lead to a counter which counts the cycles generated by the oscillator. The extrusion is approximately 9mm high, 27mm wide and 1.5mm long. In the arrangement shown in Figures 2A, 2B, 2C and 3A, 3B and 3C and these strips extend transversely across a road. The extrusion is sufficiently flexible to conform to any curvature in the road surface. The connecting cables from the end of the strip lead to a data logging device including the frequency counter and mounted at the side of the road. In Figures 2A and 3A, a single capacitor strip 11 is glued or othewise secured to road surface 60. In Figures 2B and 3B, several, in this case three capacitor strips 11 are mounted within a polymer tile 32 which itself can conform to any curvature in the road surface. The leading and tailing edges of 33 of the tile are chamfered. The three strips provide separate weight readings, and can be used to ascertain whether the vehicle forces are changing as the vehicle crosses the tile. In Figures 2C and 3C, the sensor strip is mounted within a recess 34 cut into the road surface, the remainder of the recess being filled with resin or a polymer. The resin transmits the weight of the wheel to the upper wall of the hollow body and supports the hollow body from below efficiently. Although the cutting of a recess 34 in the road surface involves extra effort, this method of mounting is preferred in some applications since the wheel is not deflected upwards by the strip sensor which might result in an inaccurate reading. The components of the sensor are fed into the bore of the extrusion from one end. Alternatively, the bore could be defined by two pieces, a base and a cover, in which the case the components could be placed in the base and the cover subsequently secured, for example by welding or glueing with a suitable adhesive. Stainless steel would then be suitable material for the body. A second embodiment of the invention is shown in Figure 4, placed on a road surface 160, in which like parts, to those of the embodiment of Figure 1, are indicated by like reference numerals with the addition of 100. In this embodiment, the printed circuit board 114 is pressed into engagement with side shoulder 123 by means of a resilient elongate strip, 150 shown in lateral cross section which is preferably formed from a spring material, preferably stainless steel. The aluminium extrusion 112 is surrounded by a polymer encapsulation 152.

Claims: A capacitor having opposed first and second mutually insulated electrodes within a hollow body (12), the first electrode (21) being deformable in response to an applied load and thereby moveable relative to the second electrode (15) to change a dielectric gap therebetween, characterized in that the capacitor is a strip capacitor, the hollow body (12) being elongated and having a bore (13) throughout its length defined on one side by a resiliently deformable wall (21) which supports the first electrode (21), the second electrode (15) being disposed within the bore (13) facing the first electrode (21). A capacitor as claimed in Claim 1 wherein the wall (21) is formed of conductive material and constitutes the first electrode (21). A capacitor as claimed in Claim 1 or Claim 2 wherein the hollow body (12) is formed in one piece. A capacitor as claimed in Claim 3 wherein the body (12) is formed as an extrusion. A capacitor as claimed in Claim 1 or Claim 2 wherein the body (12) is formed of more than one piece, the pieces being welded or glued together along an interface extending along the length of the strip. A capacitor as claimed in any one of Claims 1 to 5 wherein the second electrode (15) is mounted on an insulating member (14) within the hollow body (12). A capacitor as claimed in Claim 6 wherein the insulating member (14) engages shoulders (23) on the body (12) projecting from either side of the first electrode (21) by a distance equal to the desired interelectrode gap. A capacitor as claimed in Claim 6 or Claim 7 wherein the second electrode (15) comprises conducting film (15) deposited on an insulating substrate (14). A capacitor as claimed in Claim 8 wherein the substrate (14) is provided with conductive film divided longitudinally into three parallel strips (15) of which the central strip (15) forms the second electrode. A capacitor as claimed in Claim 9 as dependent on Claim 7 wherein the non-central strips (15) engage said shoulders (23). A capacitor as claimed in any one of Claims 6 to 9 further comprising means (16) for pressing the insulating member (14) towards the first electrode (21). A capacitor as claimed in Claim 11 wherein said means (16) is formed from resilient material. A capacitor as claimed in Claim 12 wherein said resilient material comprises a spring strip (150) or a pad of resilient resin or polymer material (16). A capacitor as claimed in any one of the preceding claims wherein the electrodes are divided into mutually insulated portions spaced along the length of the strip. A capacitor as claimed in any one of the preceding claims, of constant cross-section throughout its length. A sensor comprising a capacitor as claimed in any one of Claims 1 to 14 and a processing circuit (31) for converting the capacitance to an output reading. A sensor as claimed in Claim 16 wherein the processing circuit (31) is mounted within the hollow body (12). A method of measuring the weight of an object moving at a constant velocity transversely across part of the length of a strip capacitor according to Claim 1 of which the first electrode (21) deflects with respect to the second electrode (15) linearly with applied contact pressure, comprising counting the cycles of a signal generated at a frequency which is inversely proportional to the change in capacitance of the capacitor upon deflection, said signal frequency being directly proportional to the applied contact pressure and that part of the length of the capacitor which receives the said applied contact pressure. A capacitor according to Claim 1, in which the second electrode (15) is formed as a first portion of a conducting film (15) disposed on an insulating member, the conducting film including second portions (15) non-contiguous with the first portion (15) and the second portions being arranged to abut supports (23) connected to the first electrode (21).

Inventors: CEBON, DAVID, COLE, DAVID JAMES, HARDY, MICHAEL STUART ANTHONY, NEWLAND, DAVID EDWARD
Applicants: LYNXVALE LIMITED
Application year: 1989
Main classification: G01G7
IPC codes: G01G7