The present invention relates to a method of and a system for determining the weight of load on a vehicle, and in particular to an on-board system for determining the load weight and a method of determining the load weight using such a system.

It is highly desirable to know the actual weight of a load being carried by a vehicle, particularly a goods motor vehicle where the laden weight is critical and limited under law. Conventionally the weight of a load of a vehicle has been determined using a weighbridge over which the vehicle is driven, but such devices are expensive to install and to maintain. If a weighbridge is being used to determine the loading of a vehicle as it is being loaded, the vehicle will have to be repeatedly driven over the weighbridge to determine the changing weight of the vehicle and this will result in a co plexed time consuming loading procedure. Clearly it would be desirable for each vehicle to have some means for continually determining the loading. Various systems for determining the loading of goods motor vehicles having hydraulic suspensions have been proposed, and these generally measure the variation of pressure in the hydraulic suspension. However many goods motor vehicles have suspensions which use leaf springs mounted on the vehicle chassis to carry the axles of the vehicle.

The present invention seeks to provide a system for determining the weight of a load of a vehicle having axles connected to a chassis through a spring suspension, and the invention also seeks to provide a method of determining the weight of a load of a vehicle having a chassis supported on axles at a number of points

Accordingly the present invention provides a system for determining the weight of a vehicle having a

- 2 - suspension including spring support members, the system comprising a plurality of strain gauge sensors each for attachment to a respective spring support member of the suspension for producing an output signal related to the strain in that member; and signal processing means adapted to determine the weight of the vehicle and/or the load on • the vehicle from the said signals.

A preferred embodiment of .the invention provides a system for determining the weight of a vehicle having a suspension including spring supporting members, comprising a plurality of strain gauge sensors each for attachment to a spring support member of the suspension of the vehicle for producing an output signal related to the strain in that member; for each sensor signal processing circuitry for receiving the output of the respective strain gauge sensor and providing a processed output signal; a processor unit connected to receive the processed signals from the processing circuits and for determining from the processed signals the weight of the load on the vehicle; and a display for displaying the determined weight.

Preferably in addition there is a calibration unit which can be temporarily coupled to the processing unit to calibrate the system for a particular vehicle and a non¬ volatile memory in the processor unit for storing calibration data provided during the calibration process.

According to a second aspect the invention provides a method of determining the weight of the vehicle which has a sensor system with sensors connected to a plurality of points of suspension of the chassis of the vehicle on a number of axles, comprising the steps of:

(a) obtaining calibration data based on the outputs of the plurality of sensors with a known weight loading the vehicles;

(b) storing the calibration data;

(c) obtaining the outputs of the plurality of

sensors with the vehicle carrying an unknown weight; and

(d) determining from the obtain data and calibration data the weight of the load on the vehicle.

Preferably the step of obtaining calibration data comprises the steps of: obtaining a set of output data from the plurality of sensors with the known weight at a first position on the vehicle, and then moving the weight and establishing further calibration data. Preferably the steps of moving the weight and establishing the calibration ^' data are repeated until as many sets of calibration data as there are sensors have been obtained. Thus the calibration data will provide for a known weight as many sets of obtained sensor outputs as there

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are sensors so that the processing means can, for example by means of simultaneous equations, establish calibration information for each individual sensor. Thereafter the calibration data for each indivi dual sensor is stored and when an unknown weight is carried by the vehicle the processing means can combine the calibration data with the individual outputs the sensors and from the processed, combined data the weight of the load of the vehicle can be obtained.

A preferred embodiment of the invention will now be descr ibed by way of example and with reference to the accompanying drawings, wherein:

Figure 1 is a diagrammatic plan view of a vehicle showing the arrangement of sensors of a system according to an embodiment of the invention, on the suspension of the vehicle; Figure 2 is a side view of one axle and associated suspension of the vehicle of Figure 1, showing the positioning of a sensor on the suspension; and

Figure 3 is a block diagram of a circuit of the system according to the embodiment of the invention. Referr ing first to Figure 1, a vehicle 1 has a front axle

2 and rear axles 3 and 4 which carry a chassis 5 through leaf spring suspensions 6. The axles 2, 3 and 4 carry a road wheels 7 in known manner. The vehicle itself is quite conventional, and is for example a lorry for carrying goods. Each leaf spring suspension 6 has attached a strain gauge sensor 10.

As can be seen from Figure 2 each suspension 6 includes hangers 8 attached to the chassis 5 and a leaf spring 9 suspended from the hangers 8, and which is mounted on one of the axles 2, 3, 4. This form of leaf spring suspension is well known, and only the general principle is illustrated here. A strain gauge sensor 10 is mounted on an upper surface of each leaf spring 9 towards one end of the leaf spring. The strain gauge sensors are somewhat delicate and are therefore mounted in an environmental housing.

The strain gauges used in the embodiment are of the modern polyester film type and are compensated for temperature and

def l ect ions in all directions and have the potential for great accuracy. Typically they are approximately 1" diameter, and are rather fragile. The gauges 10 are glued to the prepared surface of the suspension spring 9 using an epoxy adhesive, and terminals are at tached. This assem bly is encapsulated using a silicon rubber compound and a protective metal cover attached^

The strain gauge sensors 10 are connected as part of an electronic system , as shown in Figure 3. The output of each strain gauge sensor 10 is connected to a head amplifier unit 11 which is mounted close to the sensor. The head amplifier unit 11 includes a preci s ion voltage amplifier, a precision ramp generator, and a comparator. The output of each head amplifier unit 11 is connected to a processor unit 12 by a balanced line 15. The head amplifier uni ts 11 convert the amplified output of the strain gauge sensors into a form acceptable for use by the processor unit 12. In the i l l ustrated embodiment the converter produces a fixed frequency pulse of variable duty cycle, with the duty cycle being proport ional to the measured strain. The processor unit 12 is connected to an alpha numeric display 13 which is mounted in a suitable position on the dashboard of the vehicle. A calibration unit 14 is temporarily connectable to the processor unit 12 to carry out the cali bration process which will be described below.

It will be appreciated that there are a num ber of var iat ions in the design of the system which could be made without depart ing from the scope of the present invention. Such variations incl ude, but are not limited to, for example the number of sensors provided to the system. It is preferred that each leaf spring of the suspension has a strain gauge sensor. Also, the conversion sect ion of the head amplifier unit may use an analogue-to-digital converter which produces a binary data output or an analogue current loop, which would necessitate the use of a data logger.

The general operation of the system and the principles on which that operation are based, will now be described. As acknowledged above, the strain gauge sensors 10 are of known type. Generally when a strain gauge is bonded to a surface,

if that surface is subject to a bending movement there is an electr ical resistance change in the gauge that is proportional to appl i ed strain. In the present invention, in principle the measure of the strain or change in strain is used to determine the applied weight. Evidently, it is important that relationship between change of resistance in the gauge of and a change of strain (weight) when the gauge is bonded to a suitable surface is known, and of course that the surface behave according to Youngs elast icity laws. The leaf springs 9 that form the suspension of many goods vehicles, because of their function, will obey, general ly, Youngs elasticity laws, and the load applied to the axles through will be proportional to the weight of the vehicle plus the load. In the described system, the outputs of sensors 10 will thus be related to the weight of the vehicle 1 and the load being carried by the vehicle. It is clearly necessary to proces the outputs of the strain gauges to obtain the weight of the vehicle and load, and hence load alone. The outputs of the strain gauges are however of a very low level and as the gauges are distributed all over the vehicle the outputs have to be amplified and converted to a suitable form and sent to the central processing unit .

Thus, the output of each sensor 10 is supplied to the adjacent head amplifier unit 11 where the signal is amplified by the precision amplifier and encoded into a fixed frequency pulse with the duty cycle being proportional to the output (strain) from the sensor 10. The encoded signal is fed down the balanced line to the processor unit 12. The processor unit is programmed to cont inual ly scan the outputs of the head amplifier units 11, so that a continually updated reading of the strain being detected by each sensor is obtained. The processor unit 12 uses the obtained strain readings and previously established calibration data to determine the weight of the load, and this is displayed on the alpha-numer ic display 13 in the cab of the vehicle. The general pr inciple of that determination, and the calibration of the system necessary to put into effect will now be described.

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As mentioned above the load amplified through the axles and detected by the strain gauges is proportional to the weight of the ^' vehicle plus load. So it is necessary to be able to eliminate the effects of the weight of the vehicle when determining the weight of the load. Further, it is not known how the weight of the vehicle is distributed on the suspension points, and the centre of gravity of the load cannot be assumed to be in a consistent posit ion in the load space of the vehicle. Thus sensors are appl ied to each point of suspension of the vehicle, and the weight of the load computed from their com bined outputs. Also the centre of gravity of the load and the load on each axle may be computed wi th the information to hand.

The operational principle and method of cali bration will now be described. First, to show the general principle, consider an i rregular shaped object of unknown mass ( weight ) and centre of gravity. If this object is supported at a number of suspension points ( in the present case the leaf springs of the vehicle ) then the sum of the masses supported at the suspension points is equal to the total mass of the object, e.g. consider a mass supported by six points then

M=M[A] -rMCB] -rMEC] -rMCD] -rMCE ] -τM[F ] where M = total mass of object M[A] = Mass supported by point A M[B] = Mass supported by point B etc.

Now if the suspension points are equipped with a weight sensing means (the sensors 10), then the mass supported by point a, M[A] can be represented by u(a), where "a" is a variable, (the output of the sensor) directly proportional to the mass supported at point A, and u is an unknown constant, (a cali bration factor ). Therefore total mass M(T) of the object is M(T) = u(a) + v(b) + w(c) + x(d) + y(e) + z(f ) ( 1 )

Where u, v, w, x, y, z are the constants of proportionality at points A, B, C, D, E, F, respectively. Obviously, if the variable quantities (a ) to (f ) are

obtained from the sensors the constants must be determined in order to determine the total mass M. This can be done applying a known mass M' to the system.

When mass M' is added to load M the load supported by each suspension point will increase, and will assume new values a + a ' , b + b', etc., where a', b', e' etc., represent the increase in load supported by points A, B, C, etc. thus the expression (1) becomes M + M' = u(a-r-a' ) + v(b+b' ) + w(c+c ^« ) + x(d+d') + y(e+e ^» ) + z(f+f' ) (2 )

By subtracting 1 from 2 the following expression is obtained.

M - u(a') + v(b') + w(c ^! ) + x(d' ) + y(e') + z(f').

Here we have an expression where M' is known, but u, v, w, x, y and z are unknown, and a', b', c', e', f , are measured values from the sensors (10 ).

If the position of the known mass M' is moved (i.e. on the vehicle) then the variables a ^τ , b', c', e', f, assume new val ues, but u, v, w, x, y, z, remain constant. By repeating the process of shifting the known calibration weight six times the data for a set of six equations with six unknown constants are obtained, i .e.

M = u(a' ) + v(b' ) + w(c') + x(d') + y(e') + z(f ).

M = u(a") + v(b") + w(e") + x(d") + y(e ") + z(f").

M = u(a"') etc. Where M' is the calibration weight applied to the object, a' etc. is the value of a, with the M' in position 1 a" is the value of a with M' in position 2, etc. etc..

In present cases the suspension points are the leaf springs on the vehicle axles, and M' is an accurately known test weight. The computer system stores the values a', d", a™, b', b" , etc. and by use of simultaneous equations (solved using Matrices) the constants can be given numerical val ues and held permanently in the memory system.

In practice the calibration will be as follows. The software in process or unit 12 contains a calibration routine ,

access i ble only by use of cali bration unit 14 which is as described a separate plug-in item.

To calibrate, the calibration unit 14 is plugged into the processor unit, to gain access to the calibration routine. The cal i brat ion procedure then commences. Firstly, the number of axles on the vehicle is entered, by use of the calibration unit, into the memory of the processor unit 12. Secondly, the weight of the vehicle empty is "fixed", by storing the relative readings of all sensors in the non-volatile memory of the processor unit 12. Then a known precision weight is placed in the vehicle load space and the readings of sensors stored or "fixed" as before, by use of the cal i brat ion unit. U nder instruction of the processor prompts, the weight is moved to another position in the vehicles load space, and once again the readings are " fixed" by use of the calibration unit 14.

This action of moving the weight is repeated for the number of sensors there are in the system , once this has been done, the microprocessor has in its memory all the data required to cal i brate the system. This is accomplished by use of the above-mentioned s imul taneous equations which form part of the processor program , whereby the calibration factor of each sensor is considered to be an unknown variable. The calibration factor for each sensor are retai ned in a non-volatile memory. In use to obtain the weight of a load on the vehicle the weight sensed by each sensor is determined using the appropriate cal ibrat ion factor. Then overall weight of vehicle and load is obtained and the known vehicle weight deducted to obtain the weight of the load. Addi t ional information which may be made available to the dr iver in the cab via the display 13, could include warning of an imbalanced load and also excess axle loading.

Some variation in the arrangement of the system of the descr i bed em bodiment may be necessary for some applications for example the case of articulated vehicle, the use of multiple

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connectors will be necessary in conjunction with a second processing unit mounted on the trailer to store the calibration factors unique to that particular trailer, and such variation is intended to be within the scope of the invention. It may prove necessary to re-calibrate the system per iodically to allow for "drift" in the strain gauges. This can readi ly be accomplished because of the straightforward use of the cali bration system and method described.