LOAD CELL

Abstract: A load cell (1) has a load sensing member (2) on wich are mounted strain gauges comprising circuit elements (18, 19) of a thick film circuit of which a surface portion of the member constitutes a substrate. The circuit elements are arranged in concentric circular arrays (18, 19) and are oriented so as to be responsive to radial compression or tension due to bending of the surface portion of the member. The load cell can be used in a suspension unit of an automotive vehicle to measure a component of load directed at right angles to the surface portion.

Description: "LOAD CELL" This invention relates to a load cell and in particular but not exclusively to a load cell for a suspension unit of an automotive vehicle. It is known to provide load cells for obtaining load measurements in the form of electrical signals either for monitoring the load in structural members or to generate feedback signals as part of a control circuit. The suspension units of automotive vehicles have been provided with such load signals as part of active suspension systems in which the instantaneous load experienced by each suspension unit of the vehicle is detected to allow optimum control of actuators determining suspension characteristics. Typically in such arrangements a load sensing member is connected between first and second load bearing structures and strain gauges are bonded to the member such that load transmitted between the structures via the member results in strain being observed in the member which can be sensed by means of the gauges to provide an output signal representative of the load. Typically it is required to sense a component of load in a specific direction and in order to minimise interference from stresses associated with unwanted components it is known to provide an array of strain gauges which are accurately and symmetrically positioned. The output of the gauges is then processed typically using a bridge circuit. The need for accurate bonding of an array of strain gauges to the load sensing member has hitherto resulted in such load cells being expensive. It has also proved to be difficult to manufacture load cells which are insensitive to loads in directions other than the specific direction of interest. According to the present invention there is disclosed a load cell comprising a plurality of strain gauges mounted on a load sensing member whereby in use the strain gauges facilitate measurement of strain produced in the member in response to an applied load, wherein the strain gauges comprise circuit elements of a thick film circuit of which a surface portion of the member constitutes a substrate. An advantage of such a load cell is that a thick film circuit printing technique may be utilised to accurately and repeatedly position the circuit elements on the load sensing member. Preferably the surface portion of the load sensing member is annular so as to extend radially between an outer annular portion of the member connected in use to a first load bearing structure and an inner annular portion of the member connected in use to a second load bearing structure, the circuit elements being oriented so as to be responsive to radial compression or tension due to bending of the surface portion whereby the load cell is adapted for measurement of loads comprising components of force directed along a load axis at right angles to the surface portion. Preferably the circuit elements are substantially unifor ally distributed circumferentially around the annular surface portion. Preferably the circuit elements are arranged such that a radially outermost annular array of circuit elements is provided adjacent to the outer annular portion and a radially innermost annular array of circuit elements is provided adjacent to the inner annular portion of the member. The outer and inner arrays may thereby be subjected to strain of opposite sense i.e. compression/tension, such an arrangement lending itself to the use of a detector circuit in which extraneous effects such as temperature change can be cancelled by for example using a bridge circuit. Preferably the radially outermost array comprises first and second chains of circuit elements, the circuit elements of each of the first and second chain being respectively connected in series and circumferentially distributed such that the radially outermost array is populated circumferentially with circuit elements alternately from the first and second chains, and wherein the radially innermost array comprises third and fourth chains of circuit elements, the circuit elements of each of the third and fourth chain being respectively connected in series and circumferentially distributed such that the radially innermost array is populated circumferentially with circuit elements alternately from the third and fourth chains. Advantageously the load cell comprises connection means operable to connect the circuit elements in a detector circuit in which the aggregrate resistance values of the first, second, third and fourth chains constitute first, second, third and fourth arms of a Wheatstone bridge circuit. In a preferred embodiment the circuit elements comprise thick film resistors. Such resistors can be printed in a single printing operation so as to have substantially uniform characteristics of resistivity and physical size. The size and position of each resistor is inherently accurately and repeatedly defined by the screen printing operations of standard thick film printing circuit techniques. A preferred embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings of which:- Figure 1 is a plan view of a load cell in accordance with the present invention; Figure 2 is a schematic circuit drawing of a bridge circuit for use with the load cell of Figure 1; and Figure 3 is a sectioned elevation of the load cell of Figure 1 fitted to a suspension unit of a vehicle. A load cell 1 shown in Figure 1 comprises a load sensing member 2 which is annular in shape and formed from steel plate of 6.4 mm thickness with inner and outer radii of 38 and 82 mm respectively. An outer annular portion 3 of the member 2 is provided with circumferentially spaced holes 4 by means of which the outer annular portion is bolted to a first load bearing structure 5 in use as shown in Figure 3. In the example of Figure 3 the first load bearing structure 5 is a suspension mount for an automotive vehicle suspension unit. The member 2 has an inner annular portion 6 which is provided with circumferentially spaced holes 7 by means of which the inner annular portion is bolted in use to a second load bearing structure 8 as shown in Figure 3. In the example of Figure 3 the second load bearing structure 8 supports a damper of a vehicle suspension unit such that a rod 9 of the damper transmits predominantly vertical loads to the second load bearing structure 8 which is transmitted to the first load bearing structure 5 through the load sensing member 2. In the example shown in Figure 3 the second load bearing structure 8 is a rigid steel disc located concentrically within the load sensing member 2 and the first load bearing structure 5 is a rigid annular steel plate located peripherally and concentrically with respect to the load sensing member 2. A thick film printed circuit 10 is fabricated on the member 2 such that an annular surface portion 11 of the member 2 constitutes a substrate of the circuit, the annular surface portion extending coaxially with and radially intermediate the inner and outer annular portions 6 and 3 of the member 2. In accordance with standard thick film circuit fabrication techniques, the thick film printed circuit 10 is built up from successive layers applied to the substrate surface portion 11, the initial layer being an adhesion layer upon which a buffer layer 12 is printed. A further layer comprising first conductive tracks 13 is printed on the buffer layer 12 and these tracks are partially overlaid by first and second dielectric layers 14 and 15 respectively. A further layer comprising second conductor tracks 16 is printed over the first and second dielectric layers 14 and 15 and extends into contact with the first conductor tracks 13 to provide an interconnecting conductive network for circuit elements 17 which are printed as a further layer on the buffer layer 12 adjacent to the dielectric layers 14 and 15. In Figure 1 successive cutaways are provided to schematically illustrate those features which are added at successive printing stages during fabrication. The circuit elements 17 each comprise a square area of resistive material, each being printed at the same time in a single printing operation such that a uniform thickness of resistive material constitutes each circuit element, the net result being first and second annular arrays of circuit elements 18 and 19 respectively, each array constituted by sixty identical resistors which are uniformly circumferentially spaced. The first array 18 of circuit elements is ^ radially outermost and located adjacent to the outer annular portion 3 of the load sensing member 2 and the second array 19 of circuit elements is radially innermost so as to be positioned adjacent to the inner annular portion 6 of the load sensing member 2. As can be seen in Figure 3 the first and second arrays 18 and 19 are located at positions such that vertical loading of the second load bearing structure 8 along a central load axis 20 will result in deformation of the surface portion 11 which constitutes the substrate } upon which the arrays 18 and 19 are fabricated. In Figure 3, an upwardly directed load along the load axis 20 will deform the load sensing member 2 such that the first array of circuit elements 18 is placed in compression and the second array 19 of circuit elements is placed in tension. Each of the circuit elements 17 thereby acts as a strain gauge in that the resistance of each square element of resistive material as measured in a radial direction will depend upon the extent to which it is subjected to compression or tension, the effect of compression being to decrease resistance and the effect of tension being to increase resistance. The first and second conductor tracks 13 and 16 provide interconnection between the circuit elements such that an electric current in a radial direction is maintained through each of the circuit elements. In the first array 18 of circuit elements, the sixty circuit elements are connected in first and second chains 21 and 22, each of which chains consists of thirty circuit elements connected in series. The circuit elements of the first and second chains 21 and 22 are each distributed around a complete circumference and are circumferentially distributed such that the array is populated with circuit elements alternately from the first and second chain. Circuit elements of the first and second chains 21 and 22 are in other words interdigitated around the circumference of the thick film circuit 10. The second array 19 of circuit elements similarly comprises third and fourth chains 23 and 24 respectively which are interconnected and interdigitated in like manner. The first, second, third and fourth chains 21, 22, 23 and 24 respectively present aggregate resistance values Rl, R2, R3 and R4 respectively which are sensed by connection to output terminal pads 25, 26, 27 and 28 by means of a detector circuit 29 illustrated in Figure 2. The detector circuit 29 is a conventional Wheatstone bridge circuit in which a source voltage is applied between the supply terminals 30 and 31 respectively and an output voltage is sensed between output terminals 32 and 33. In the example of an upward load being applied to the load axis 20 in the arrangement of Figure 3, the first array 18 of circuit elements is placed in compression thereby decreasing the resistances Rl and R2 whereas the second array 19 is placed in tension thereby increasing the resistance R3 and R4. An output voltage will then appear across output terminals 32 and 33 which is proportional to the resistance change in each array and to a good approximation is proportional to the strain experienced by each of the circuit elements 17. By suitable calibration the output signal may thereby provide a signal which is representative of the applied load. The sensitivity of the load cell 1 will depend upon the stiffness and hence the thickness of the load sensing member 2. Preferably as many as possible circuit elements should be printed on the thick film circuit 10 subject to the practical limitation of < circuit printing which typically require a minimum size of 1.25 mm square. The values of Rl, R2 , R3 , R4 are determined by the number of circuit elements in each chain and the resistivity of the paste used in printing the circuit elements. It is usual to use paste with the resistance of 10 kilohms per square as this provides a gauge factor of between 12 and 14. To reduce the bridge resistance to preferred levels it may be desirable to use pastes having a lower ohmic value such as 100 ohm paste thereby producing a gauge factor of approximately 2. This is equivalent to the gauge factor of metal foil gauges. The load cell 1 is insensitive to components of load transverse to the load axis 20 because of the distributed nature of the circuit element 17 constituting each resistance Rl, R2, R3 , R4 of the detector circuit. If for example a transverse component of load is applied in a direction from left to right as viewed in Figure 3 such that the second load bearing structure 8 is urged to the right relative to the stationary first load bearing structure 5, the effect on the circuit elements constituting the first chain 21 will be that those elements on the right-hand side of the load axis 20 will experience a degree of compression in the radial direction and those to the left will experience a degree of tension. The nett effect on the aggregate resistance Rl of the first chain 21 will be zero however. Similarly each of the second, third and fourth chains 22, 23 and 24 will experience similar compression and tension but with no nett effect on each of the resistance values R2 , R3 , R4. There will therefore be no consequent contribution to the output voltage of the detector circuit 29. Insensitivity to such transverse forces depends on their being sufficient numbers of circuit elements in each chain for the resistance values of the circuit elements to be evenly circumferentially distributed and also will depend upon the extent to which the circuit elements represent identical performance characteristics. It is however well within the capabilities of conventional thick film circuit techniques to achieve a high degree of uniformity of such circuit elements and for the position of the respective elements to be accurately and repeatedly defined. The load sensing member may be constructed from materials other than steel and in particular alternative metals such as aluminium may be used.

Claims: CLAIMS : 1. A load cell comprising a plurality of strain gauges mounted on a load sensing member whereby in use the strain gauges facilitate measurement of strain produced in the member in response to an applied load, wherein the strain gauges comprise circuit elements of a thick film circuit of which a surface portion of the member constitutes a substrate. 2. A load cell as claimed in claim 1 wherein the surface portion of the load sensing member is annular so as to extend radially between an outer annular portion of the member connected in use to a first load bearing structure and an inner annular portion of the member connected in use to a second load bearing structure, the circuit elements being oriented so as to be responsive to radial compression or tension due to bending of the surface portion whereby the load cell is adapted for measurement of loads comprising components of force directed along a load axis at right angles to the surface portion. 3. A load cell as claimed in claim 2 wherein the circuit elements are substantially uniformally distributed circumferentially around the annular surface portion. 4. A load cell as claimed in claim 3 wherein the circuit elements are arranged such that a radially outermost annular array of circuit elements is provided adjacent to the outer annular portion and a radially innermost annular array of circuit elements is provided adjacent to the inner annular portion of the member. 5. A load cell as claimed in claim 4 wherein the radially outermost array comprises first and second chains of circuit elements, the circuit elements of each of the first and second chain being respectively connected in series and circumferentially distributed such that the radially outermost array is populated circumferentially with circuit elements alternately from the first and second chains, and wherein the radially innermost array comprises third and fourth chains of circuit elements, the circuit elements of each of the third and fourth chain being respectively connected in series and circumferentially distributed such that the radially innermost array is populated circumferentially with circuit elements alternately from the third and fourth chains. 6. A load cell as claimed in claim 5 comprising connection means operable to connect the circuit elements in a detector circuit in which the aggregrate resistance value of the first, second, third and fourth chains constitute first, second, third and fourth arms of a heatstone bridge circuit. 7. A load cell as claimed in any preceding claim wherein the circuit elements comprise thick film resistors. 8. A load cell as claimed in any preceding claim wherein the load sensing member is formed of steel plate.

Inventors: HARDEN PAUL, LITTLE PETER JAMES, HARDEN, PAUL, LITTLE, PETER, JAMES
Applicants: HARDEN PAUL, IST LAB LTD, LITTLE PETER JAMES, IST LABORATORIES LTD.
Application year: 1993
Main classification: G01L1
IPC codes: G01L1