This invention relates to the measurement of loads on wheels of a vehicle, in order, for example, to provide an input signal for initiating auxiliary equipment such as an active suspension control system for the vehicle, or for altering the threshold of a braking system for the vehicle, for example an anti-lock braking system.

In particular, the present invention relates to a system for providing measurements indicative of wheel load, as well as measurements indicative of the brake torque and/or the traction forces and/or the cornering force operating on a vehicle. It will be appreciated that these forces act on mutually perpendicular axes. Previously, the measurement of all three forces has required the use of multiple sensors.

US 5 127 277 shows such a sensor used to monitor the vertical load. Such a load measuring device monitors the load at the top end of a strut to which it is attached, typically a MacPhearson strut. Such a measuring device is susceptible to being effected by dampers and the like and so does not give data representative of true dynamic load. Such a measurement requires monitoring the dynamic forces on three mutually perpendicular axes.

A further disadvantage of this prior art occurs in its use in advanced ABS/traction and suspension system development where a measure of the wheel load or brake torque would be beneficial. When making such measurements it is essential that dynamic forces are monitored. Such known single axis systems give a quasi-static measure of wheel loads. While this has

some value it can still be affected by spring rates, dampers and the like and as such represents a compromise solution.

Thus there is a need in the art for a system which gives independent dynamic measurement of vertical, longitudinal and lateral forces acting on the unsprung means, that is the wheel/hub assembly of a vehicle. Solving this problem will provide information on the wheel load forces (acting vertically), the brake reaction/traction forces (acting longitudinally) and the slip forces (acting laterally) . This information can then be applied in advanced ABS/traction and active suspension control systems.

In EP 0 504 731, the vertical drag, brake torque and road surface friction are determined by way of a stress detection sensor located and secured in a hole in the vehicle axle, a detection signal from the stress detection sensor then being processed in a signal processing unit. The sensor is secured in the hole by means of a filler, the filler serving to locate the sensor and to protect it from the external environment.

This construction has the disadvantage that it requires a hole to be formed in the axle of the vehicle. Also, once embedded in the filler the sensor cannot be maintained or serviced in the event of malfunction.

According to the invention a load measuring device to be incorporated in a vehicle wheel load supporting unit measures the force acting on three mutually perpendicular axes.

Preferably the load measuring unit comprises a sleeve slidably mounted about the load supporting unit and supported in relation thereto by load detecting means.

The load detecting means comprises a load cell.

Preferably the load detecting means connects the bottom of the load supporting means to the bottom of the sleeve.

The load cell may be of cruciform configuration.

Preferably the load detecting means is provided with a plurality of strain gauges. Alternatively the load detecting means is provided with a plurality of screen printed piezo-resistive elements. In either case the strain sensitive elements are arranged in a suitable configuration from which the three force components can be obtained.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 shows in .section a known front drive suspension system;

Figure 2 shows in section a front drive suspension system incorporating a suspension strut according to the invention;

Figure 3 shows a section through a strut according to the invention;

Figure 4 shows a perspective view of a load cell for use with the strut of Figure 3;

Figure 5 shows a part section through a strut having a second version of a load cell in accordance with the invention;

Figure 6 shows a perspective view of a load cell for use with the strut of Figure 5;

Figure 7 shows a section through a strut having a third version of a load cell in accordance with the invention;

Figure 8 shows a perspective view of a load cell for use with the strut of Figure 7;

Figure 9 shows a section through a strut having a fourth version of a load cell in accordance with the invention; and

Figure 10 shows a perspective view of a load cell for use with the strut of Figure 9.

In Figure 1 a known front wheel drive suspension system is shown. The system includes a MacPhearson strut 1/hub assembly 2 and brake assembly 3. The three forces to be determined at the road/tyre interface, namely Fvert 5, Flong 6 (acting out of the page) and Flat 7 act through the hub assembly and are reacted by the suspension system.

Figure 2 shows how this arrangement may be modified in accordance with the invention. A sleeve 10 is fitted over the existing strut 11 so that it is attached to the bottom 12 of the existing strut 11

through a load cell 13. The sleeve 10 is slidably mounted at its top end. As can be seen forces Pvert 85, Plong 86 and Plat 87 will act through the load cell. The forces imposed on the vehicle body by the top of the strut are Rvert 95, Rlong 96 and Rlat 97.

The magnitude of these forces can be determined by taking moments about points A and B.

For example, for measurements representative of the vertical forces

P (vertical) = F (vertical)

For measurements representative of the lateral forces, taking moments about A

P (lateral) = R (lateral) * (h4-h3)

(h3-h2)

= F (lateral) * hi * (h4-h3)

(h4-hl) (h3-h2)

For measurements representative of the longitudinal braking forces

P (longitudinal) = F (longitudinal)*(h4-h3)x hi

(h3-h2) (h4-hl)

For measurements representative of the longitudinal traction forces

T (longitudinal) = F (longitudinal)

Where T (longitudinal) acting at 98 is the longitudinal force reaction when traction torque is applied to the wheel via the drive shaft and taking moments about B

R (longitudinal) = T (longitudinal) * (hO-hl)

(h4-hl) and taking moments about A

P (longitudinal) = R (longitudinal) * (h4-h3)

(h3-h2)

P (longitudinal) = F(longitudinal) *(h0-hl)* (h4-h3)

(h4-hl) (h3-h2)

Accordingly the three components can be calculated/extracted using any suitable electronic processing means.

The load cell may take a number of forms, examples of which are shown in Figures 3-10.

Figures 3 and 4 show a load cell 13 comprising a flat plate 15 attached around its circumference 16 to the sleeve 10 and through a control boss or column 17 to the existing strut 11. Four holes 18 are so positioned so as to concentrate the stresses acting on the plate 15 to enable the three independent forces to be determined.

To monitor these forces known strain gauges 21-28 may be bonded to the plate and arranged in three separate Wheatstone bridge formats. Each Wheatstone bridge represents a separate set of forces. A suitable arrangement would be for strain gauges 21 and 22 to

measure Plong, strain gauges 23 and 24 to measure Plat and strain gauges 25, 26, 27 and 28 to measure Pvert.

Alternatively, the plate 15 could be screen printed with piezo-resistive elements. Such a construction would be more robust and provide a lower cost unit.

A finite stress analysis of the plate shows that with strain gauges in this configuration the three forces can be determined independently both in magnitude and direction even when all three forces are acting concurrently.

In the embodiment of Figures 5 and 6, where appropriate, similar reference numerals are used to refer to similar parts.

In Figures 5 and 6, the load cell 13 is of cruciform configuration. In this arrangement the ends of the arms 45-48 of the load cell 13 are simply supported at their outer ends by support means 49 formed on the sleeve 10. The centre of the load cell 13 is rigidly attached to the existing strut 11 through a central boss or column 17. Each arm of the load cell acts as a cantilever with a direct load applied due to Pvert and end loads applied due to Plong and Plat.

This configuration also provides for independent measurement of the three forces, where strain gauges 31 and 32 measure Plong, strain gauges 33 and 34 measure Plat and strain gauges 35 to 42 Pvert.

Figures 7 and 8 show a modification to this embodiment, where appropriate similar reference numerals have been used to refer to like parts.

The ends of the arms of the load cell are once again supported by support means 49. However the arms thicken leading to a thicker, stiffer central section 50 of the load cell 13. This central section 50 then supports Pvert with the thinner sectioned supported outer ends of the arms being subject to similar stress levels in the longitudinal and lateral directions, thus improving the sensitivity when monitoring Plong and Plat.

Alternatively Pvert may be measured directly by way of the stresses in the central boss or column 17 itself.

Figures 9 and 10 show a further form of the load cell 13. Again, where appropriate similar reference numerals are used to represent similar parts.

As in the previous embodiments a plate 15 is connected to the sleeve 10 surrounding the strut 11. The plate is provided with four spaced columns 60, 61, 62, 63. A top ring 66 is supported by the columns.

The strut is provided with an abutment 70 in the form of a ring welded thereto. In use the top ring 66 locates against the abutment 70.

The four columns 60, 61, 62, 63 thus support the vertical load Pvert through the abutment 70 and also reacts to longitudinal (Plong) and lateral (Plat) forces.

Strain gauge elements 71-76 shown on one column 63, can be arranged as shown, where strain gauges 71 and 72 measure Pvert, and the other strain gauges 73, 74, 75, 76 measure Plong and Plat.

This arrangement may be repeated on the other three columns to compensate for eccentric loading and Wheatstone bridge temperature compensation.