Measuring loads on vehicle wheels

Abstract: A load measuring device (5) is incorporated in a damper or suspension strut of a vehicle. The device (5) comprises an annular plate (15) which is interposed between axially spaced opposite faces (10,9) of a thrust bearing (8) and an abutment (6) for the adjacent end of the spring (7), and the plate (15) is supported from the opposite faces at at least three angularly spaced locations (16) of which a location on one side of the plate (15) is disposed substantially at the mid-point in the circumferential length between a pair of locations for the opposite side of the plate (15). Bending moments in the plate (15) are monitored by thick film technology to give a direct reading of load.

Description: This invention is concerned with a built in load measuring device for the measurement of loads on the vehicle wheels in order, for example, to provide an input signal for initiating operation of ancillary equipment such as an active suspension control system for the vehicle, or for altering the threshold of a braking system for the vehicle, suitably an anti-lock braking system. Most known applications have been concerned with load monitoring on commercial vehicles using conventional load cell/strain gauge arrangements. In another construction displacement transducers have been employed to monitor the height between an axle and the chassis of a vehicle as a measure of vehicle load. Known vehicle load measuring system are not normally arranged such that a direct measurement can be achieved. The accuracy of such systems is therefore often affected by spring rates and characteristics of dampers and the like. This means that individual vehicle calibration procedures will be required which is not a very practical proposition in a large scale automotive market. We are aware of DOS 3726146 which discloses a load measuring device incorporated in a damper or suspension strut of a vehicle. In the construction described in DOS 3726146 the load measuring device is of ring-like configuration and is clamped in surface contact between the frame of a vehicle and a securing flange on the damper or strut by means of the conventional mounting bolts which pass through openings in the device. The device comprises a hard disc washer, and a second stiff plate for introducing a force to a piezo-resistive thick film resistor arranged between them. It is essential therefore that an appropriate torque is applied to each bolt to ensure that the device is correctly calibrated. In addition the accuracy of the device may be affected, in use, by temperature distortion. It is known from US 3 743 041 to provide a load measuring system incorporating a device in a vehicle wheel load supporting unit of a vehicle, the device producing a signal indicative of the load on the individual axles. The device disclosed comprises a transducer beam assembly in which first and second support members are secured in a spaced apart relationship to the spring and axle, respectively, of a vehicle such as a semi-trailer rig. A beam is positioned between the support members, and is adapted to be subjected through spaced pairs of elongate fulcrum pins to bending moments which can be measured by measuring means responsive to such bending in order to provide a load measurement signal. Our invention is a load measuring device incorporated in the top of a damper unit or suspension strut unit of a wheel of a vehicle as a self-contained assembly, whereby the device comprises at least one beam of a load cell subjected to bending moments, measuring means adapted to provide a load measurement signal in response to the bending moments, whereby the wheel acts directly on the load cell, and the load cell comprises a disc including the at least one beam as a central beam which is supported between projections so arranged that the at least one central beam is subjected to a three-point loading with the vehicle load being transmitted through the projections to cause said bending moments to be set up in the at least one central beam. The moments may be monitored by use of thick film technology by means of which the emitted signals can be processed by electronic circuitry associated with the load measuring device. Piezo-resistive strain sensing elements may be provided to monitor the stress set up in the beam due to the vehicle loading. When such elements are provided they may be arranged in a typical Wheatstone Bridge configuration. Our load measuring devices can be easily installed in damper units or suspension struts without the need for addition calibration or adjustment during installation, and the devices can be used in front or rear suspension assemblies. Some embodiments of our invention are illustrated in the accompanying drawings in which:- Figure 1 is a section showing the upper end of a suspension strut unit for a front wheel of a vehicle; Figure 2 is a section substantially on the line 2-2 of Figure 1; Figure 3 is a plan of the load cell; Figure 4 is a view similar to Figure 2 but showing a modified load cell; Figure 5 is a view of a screen printed Wheatstone Bridge circuit for the load cell of the unit of Figures 1-4; Figure 6 is a section through the upper end of another suspension strut unit for a rear wheel of a vehicle; and Figure 7 is a plan of the load cell of Figure 6. The suspension unit illustrated in Figures 1-3 of the accompanying drawings supports the load on the front wheel of a vehicle through a spring/damper assembly 31, a thrust bearing 32, and annular spacer 33, a thrust button 34 mounted in a recess in the upper end of the spacer 33, a thrust button 35 received in a counterbored recess 36 in the upper end of the spacer with the button 35 in abutment with a face defining the base of the counterbored recess 36, a load cell 37, an upper thrust button 39, a load cell housing 40 containing the load cell 37, a strut top mounting plate 41, a strut top mounting rubber 42, and so on to the vehicle body 43. The load cell 37 comprises a circular disc 44 provided with a pair of parallel machined slots 45, 46 extending in the same direction to define a central, cantilever, type beam 47. The disc 44 is keyed in the housing 40 against relative rotation by means of a dowel pin 49. The thrust buttons 35 and 39 are spaced from adjacent sides of the disc 44 by pairs of projections 50, 51, and 52, 53. The projections 50, 51 are spaced from each other on a line parallel with, and spaced between, the slots 45 and 46, and the projections 52 and 53 are relatively close together, lying at the centre of the beam on a second line normal to the line of the projections 50 and 51 with the projections 52 and 53 disposed on opposite sides of the second line. The arrangement of the projections forms a three-point loading beam, and the vehicle load is transmitted through the projections to cause bending moments to be set up in the beam 47. The space 48 in the housing 40 which surrounds the load cell 37 is filled with a suitable low modular putty compound. This retains the load cell 37 in position. A thick film screen printed Wheatstone Bridge circuit, illustrated in Figure 5, is applied to the load cell 37 to monitor the stresses set up in the beam due to the vehicle loading, and the putty filling the space 48 also acts to protect the printed circuit. Specifically the stresses are monitored by piezo-resistive strain sensitive elements 60, 61, 62, 63, from which the signals are fed to output connections 64, 65, 66 and 67 through conductive tracks. The arrangement also gives the facility of balancing the bridge by means of a balancing resistor, suitably laser trimmed. This is achieved by connecting a terminal pad 68 from the output connection 67, to terminal pads 69, 70 and 71, by wire bonding or by soldering a wire jumper to the terminal pads 69, 70, 71. In a modification the balancing resistors may be of the discrete surface mounted type. The balancing resistors are positioned in an area of low stress in the beam 47 and are thus not effected substantially by stresses induced in the beam 47 by the vehicle loading. Preferably the balancing resistors have the same temperature co-efficient of resistance (TCR) as the resistors 60, 61, 62, 63 to give an improved operating temperature performance for the whole assembly. In a modification shown in Figure 4, the load cell 37 comprises a substantially square element. As illustrated in Figures 6 and 7 the load cell described above can also be used with a rear suspension assembly. In this construction the housing 40 is carried from a cup 72 clamped by a flange at its lower end between a lower resilient mounting block 73 and an upper mounting block 74. The construction and operation of the embodiment of Figures 6 and 7 is therefore the same as that of Figures 1-4, and corresponding reference numerals have been applied to corresponding parts.

Claims: A load measuring device incorporated in the top of a damper unit or suspension strut unit of a wheel of a vehicle as a self-contained assembly, whereby the device comprises at least one beam (47) of a load cell (37) subjected to bending moments, measuring means adapted to provide a load measurement signal in response to the bending moments, whereby the wheel acts directly on the load cell (37) and the load cell (37) comprises a disc (44) including the at least one beam (47) as a central beam (47) which is supported between projections (50,51,52,53) so arranged that the at least one central beam (47) is subjected to a three-point loading with the vehicle load being transmitted through the projections (50,51,52,53) to cause said bending moments to be set up in the at least one central beam (47). A load measuring device according to claim 1, characterised in that piezo-resistive strain sensing elements (60,61,62,63) are arranged in a Wheatstone Bridge configuration to monitor the stress set up in the beam (47) due to the vehicle loading. A load measuring device according to claim 2, characterised in that the Wheatstone Bridge is balanced by a balancing resistor. A load measuring device according to claim 3, characterised in that the balancing resistor is of the discrete surface mounted type. A load measuring device according to claim 3 or claim 4, characterised in that the balancing resistor is positioned in an area of low stress in the beam (47). A load measuring device according to any of claims 3-5, characterised in that the balancing resistor has the same temperature co-efficient of resistance as the elements (60,61,62,63). A load measuring device according to any preceding claim, characterised in that the device is incorporated in a housing (40). A load measuring device according to claim 7, characterised in that the device is incorporated in a unit for a front wheel of a vehicle with the housing (40) disposed between a thrust bearing (32) and the body (43) of the vehicle. A load measuring device according to claim 7, characterised in that the device is used with a unit for a rear wheel of a vehicle, with the housing (40) carried by resilient mounting blocks (73,74) at the upper end of the unit.

Inventors: APPLEYARD MICHAEL, FORRESTER JOHN STUART, APPLEYARD, MICHAEL, FORRESTER, JOHN STUART
Applicants: LUCAS IND PLC, LUCAS INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1990
Main classification: G01G19
IPC codes: B60G17, G01G19, G01G3