In many countries there are regulations defining the maximum loads which may be carried by road transport vehicles and in order to enforce these regulations, transport vehicles are obliged to pass through checking stations on major highways and if the vehicle is found to be overloaded the driver or owner is fined. Often, these fines are set in proportion to the excess weight carried by the vehicle. Unfortunately, the driver is not able to accurately judge the weight of his load without passing over a weighbridge and must usually rely upon the customer for whom he is carrying to advise him of the actual load weight and therefore, as the fee for cartage is set on the basis of load weight, the weight is often underestimated. As a result, the driver will often unknowingly overload his vehicle, possibly by as much as several tonnes, and with fines being set at rates in the order of one dollar per excess kilogram the driver may face extremely large fines. In the past, devices have been proposed which measure the load carried by a vehicle and provide an indication to the driver, however, these prior art devices have been relatively expensive and therefore not popular with truck owners.

According to a first aspect, the present invention consists in a transducer for measuring displacement in a suspension system, said transducer comprising a transformer having a moveable core, the windings of said transformer being provided with first mounting means and the core of said transformer being provided with second mounting means such that movement of the first mounting

means with respect to the second mounting means produces a corresponding change in the signal induced in the secondary winding of the transformer from the primary winding of the transformer. According to a second aspect, the present invention consists in a load indicating device wherein the transducer is responsive to deflection in a load suspension system to produce an electrical signal which is related to said deflection, signal characterizing means adapted to convert said electrical signal to a signal which is substantially proportional to a load causing said deflection and indicating means responsive to said proportional signal to indicate the weight of said load. In a preferred embodiment of the invention a hydraulic cylinder is used as the movable core of the transformer, the piston of the hydraulic cylinder being connected via a piston rod to a housing adapted to move over the cylinder as the piston moves in the cylinder, the ^* transformer windings being mounted internally of the housing such that movement of the cylinder in the housing varies the amount of coupling between the primary and secondary windings of the transformer by varying the amount of the cylinder extending into the transformer.

Preferably the signal fed to the primary winding of the transformer is a square wave and the output of the secondary winding is fed to a rectifier to provide a d.c. signal, the level of which varies in response to the movement of the core with respect to the transformer windings, and a diode function generator is used to convert the rectified signal into a signal proportional to the load acting on a suspension system. The output of the function generator can either be used to directly drive an analog meter or alternatively an analog to digital converter may be used to provide a signal suitable to drive a digital display.

Embodiments of the invention will now be described by way of example, with reference to the accompanying... drawings in which:

Figure 1 schematically illustrates a leaf spring vehicle suspension system to which a device according to the present invention can be fitted;

Figure 2 illustrates the suspension system of Figure 1 when viewed from the rear;

Figure 3 schematically illustrates a torsion bar suspension system to which a device according to the present invention may be fitted;

Figure 4 schematically illustrates a walking beam suspension system to which a device according to the present invention may be fitted; Figure 5 schematically illustrates a four spring suspension with short rockers to which a device according to the present invention may be fitted;

Figure 6 schematically illustrates a deflection measuring transducer in accordance with an embodiment of the present invention;

Figure 7 illustrates a block diagram of an embodiment of the present invention; and

Figure 8 graphically illustrates the operation of a diode function generation used in the system illustrated in Figure 6.

Turning now to Figure 1 and 2 of the drawings, a typical leaf-spring suspension system generally indicated by the numeral 10 comprises a plurality of spring leaves shackled together and pivotably connected at one end 12 to the chassis 13 of the vehicle. An axle 14 is connected to each set of springs 11 by shackles 15, and a transducer 16 is provided to produce an electrical signal representative of the deflection in the set of springs 11.

Figures 3, 4 and 5 schematically illustrate alternative forms of suspension to which the present

invention may be fitted. In Figure 3, a torsion bar . suspension system, generally indicated by the numeral.20, comprises a torsion bar 21 connected at either end to a chassis 23 and linkages 25 connecting each of a pair of axles 24 to the torsion bar.

In Figure 4, a walking beam suspension system with a rigid beam, generally indicated by the numeral 30, comprises a rigid beam 31 pivotably mounted at its centre 32 to a leaf spring assembly 33. Axles 34 and 35 are mounted on each end of the beam 31 with the axle mountings forming two sides of a parallelogram.

In Figure 5, a tandem axle suspension system of a type known as four spring suspension with short rockers is generally indicated by the numeral 36. In this system the axles 34 and 35 are located in the fore and aft directions by radius arms 37 and 38 and each axle is suspended under its own leaf spring assembly- 71, 72. Each of the leaf spring assemblies are fixed at ^' one end 73 and connected to a short rocker arm 74 at the other. In each of Figures 3, 4 and 5 a transducer 16, similar to the transducer of Figures 1 and 2, is provided to produce an electrical signal representative of the deflection of the suspension system. In Figure 4 the transducer is connected between the chassis and the pivot point 32 of the walking beam, however, in Figures 3 and 5 the transducer is connected between the chassis and one or other of the axles and when these forms of suspension are used, typically one transducer would be fitted to the forward axle on one side of the vehicle and one transducer would be fitted to the rearward axle on the other side of the vehicle.

Referring to Figure 6, the transducer used to measure the deflection of a vehicle system comprises a transformer 41 formed on a hollow circular former mounted within an upper housing 42 of a shock absorber like assembly having

a hydraulic cylinder 43 arranged to move axially within the upper housing 42 when the piston 44 moves within.an inner chamber 45 of the cylinder. The transformer 41 is magnetically isolated from the housing 43 by a 0.12mm brass shim shield. The piston 44 is fitted within non-return valves 46 comprising a sealing washer located above a set of transfer passages, which allow oil to flow upwardly through the piston but prevents downward oil flow there through. This ensures that the oil level in the upper portion of the inner cylinder 45 is maintained at the level of the overflow ports 47, such that the rod 48 is kept lubricated in the region where it passes through the seal 49. Oil overflowing out of the inner cylinder 45 flows into an outer cylinder region 50 where it remains until drawn into the lower portion of the inner cylinder 45 through a lower vent 51 which may also be fitted with a non-return valve comprising a seal 55 and a retaining cage 56. Attachment points ^* 52 and 53 for the transducer are located at either end thereof and provision may also be made at one end for length adjustment in order to allow fitting to a variety of vehicle suspension systems.

The cylinder 43 acts as a core for the transformer 41 and therefore the amount of signal induced into the secondary winding of the transformer will be approximately proportional to the amount of core present within the transformer windings. When the assembly 16 is fitted to a vehicle and its length adjusted such that, with the vehicle unladen, the cylinder 43 is located just above the lower extremity of the transformer 41, the transducer will be able to detected variations in load by measuring the degree to which the cylinder 43 extends into the centre of the transformer 41.

In order to ensure correct positioning of the transformer 41 relative to the piston 44, the transformer is movably mounted in the housing 42 to enable adjustment

during installation. Further adjustment may also be. provided by way of threaded mounting points (not shown) at either end of the transducer.

Turning to Figure 7, a square wave signal source 61 is connected to the primary winding 41a of the transformer 41 and the signal coupled to the secondary winding 41b of the transformer is varied depending on the amount of the cylinder 43 which is located within the transformer. The output of the secondary winding 41b is then rectified using a precision rectifier 62 and the output of the rectifier is filtered with a low pass filter 63. The output of the filter 63 is then added to signals from similar circuits connected to the suspension of the remaining wheels of the vehicle to produce the total load indicating signal which is then calibrated in a diode function generator to produce a signal 66 which is approximately proportional to the total load of the « vehicle. The load signal 66 is then fed to either an • analog meter 67 or to an analog to digital converter 68 which produces a digital representation of the load signal, suitable to drive a digital display 69. In the preferred embodiment the analog to digital converter will be realised using a voltage controlled oscillator and a set of decade counters the outputs of which are fed to the digital display. Alternatively, commonly available panel mounted voltage meters may be used to indicate the load digitally. In a preferred embodiment of the invention switching means may be provided at the inputs of the adder 64 to enable various combinations of wheel loading to be displayed on the display 67 or 69.

Referring to Figure 8, a graphical representation of the transfer function of a diode function generator, such as that used in the system of Figure 6, is illustrated. Diode function generators are used to produce straight line approximations to curved transfer functions and

achieve this result by the use of multiple feed-back paths around an amplifier, each feed-back path defining a. portion of the transfer function, the various feed-back paths being disabled by diodes which become reverse biased during those portions of the transfer function in which they are not required. Such circuits are well known and may be implemented with any number of straight line segments, depending on the degree of accuracy required. It will be recognised that the diode function generator of the present system performs two functions, these being to remove any non-linearities caused by the transducer 16 and to counteract the non-linear relationship between load and deflection of the suspension system.

While the load indicator system of the present invention does not provide high accuracy in its measurement of the load, it provides a reasonable indication of the load carried by a vehicle, the indication being sufficiently accurate to determine whether the vehicle is overloaded, and the system has the added advantage that it is far less expensive than prior art arrangements and therefore will be more readily accessible to the majority of truck operators.

It will be recognised by persons skilled in the art that numerous variations and modifications may be made to the invention as described above without departing from the spirit or scope of the invention as broadly described.