WO1993018375A1 - Mobile mass measurement apparatus using work and impulse - Google Patents

Abstract

An apparatus for measuring the mass of e.g. a moving motor vehicle comprises first means (1-5; 1, 2, 4, 5, 31) for measuring the accumulated work energy or impulse done during movement of the vehicle over a measurement period and providing an accumulated first work/impulse signal (W, I) representative thereof; velocity means (7, 11, 12; 11, 121, 122) for measuring the velocity of the object and providing first and second velocity signals (V) representative of the velocity of the object at the beginning and end of the measurement period respectively; store means (6) for storing said first work/impulse signal and velocity signals; and processor means (6) for deriving a mass signal representative of the mass of the object, as a function of said accumulated first work/impulse signal and velocity signals measured over at least two measurement periods (a, b, c, d).

Description

MOBILE MΑSS MEASUREMENT APPARATUS USING WORK AND IMPULSE.

Technical Field

The present invention relates to the measurement of mass or weight of a moving object such as a vehicle or ship.

Background Art

The conventional way to measure vehicle mass for example, is to weigh it on weighbridge so measuring the force exerted on the mass by gravitational acceleration. This requires the vehicle to be taken to a weighbridge.

Disclosure of the Invention

The present invention seeks to provide an improved method and apparatus for measuring the mass of an object such as a vehicle.

Accordingly, the present invention provides a method of measuring the mass of an object comprising: causing the object to move over a first measurement period whereby said object moves over a first distance; measuring the accumulated first work energy or impulse done during movement of the object over said first distance to provide a first work energy or impulse value; causing the object to move over a second measurement period whereby said object moves over a second distance; measuring the accumulated second work energy or impulse done during movement of the object over said second distance to provide a second work energy or impulse value; measuring the velocity of the object at the beginning and end of each of said first and second distances; deriving a value for the mass of the object as a function of the measurements of said velocities and said first and second work energy or impulse values; wherein during movement of the object over at least one of said first and second distances the object undergoes a change of momentum.

The present invention also provides an apparatus for measuring the mass of a moving object comprising: first means for measuring the accumulated work energy or impulse done during movement of the object over a measurement period and providing an accumulated first work/impulse signal representative thereof; velocity means for measuring the velocity of the object and providing first and second velocity signals representative of the velocity of the object at the beginning and end of the measurement period respectively; store means for storing said first work/impulse signal and velocity signals; and processor means for deriving a mass signal representative of the mass of the object, as a function of said accumulated first work/impulse signal and velocity signals measured over at least two measurement periods.

The present invention relates to the measurement of mass of a body undergoing a change of linear velocity and/or the measurement of rotational moment of inertia of a rotating body undergoing a change of angular velocity, through the accurate measurement of both the change of velocity and the accumulated work" which caused it, or the change of angular velocity and the accumulated work which caused it, over a period of measurement. The average force or average torque is calculated from the measurement of aggregated mechanical work energy over a measurement distance. Alternatively, average force value can be obtained from the measurement of aggregated impulse over a time period. The work method is much superior in most land applications but the impulse method can be used to advantage depending on the constraints imposed by the mass measurement.

The physical formula: Force = Mass x Acceleration appears to offer an easy way for mass measurement if the measurement of force F and acceleration are undertaken simultaneously as shown in Figure 1. This method is in fact very difficult to use in practice since the measured force F illustrated in Figure 1 is greater than the accelerating force. It has also to overcome the combined friction and resistance force Ff and the weight force component FG. Since neither Ff nor FG can be easily measured at random locations in a journey an instantaneous application of the simple physical formula above is not practicable.

Where a force is exerted in order to accelerate a mass over a distance of travel, some work energy is transformed into increased kinetic energy of the mass . The measurement of this work energy and change of velocity can therefore reveal the magnitude of the mass. Other forces involved in resisting the motion, such as friction drag, also consume work and without relationship to the mass, and this work must be discounted from the total applied work to allow the measurement of the mass. Gravitational forces due to road gradient are relate to the magnitude of the mass and they also consume work which must be discounted from the measured applied work.

The present invention overcomes these difficulties. It offers an apparatus and method for an accurate measurement of mass of a vehicle with or without its cargo, through measurements undertaken over a distance or over a time period. The prime measurements undertaken in a preferred method of mass measurement according to this invention are:

1) Distance travelled by vehicle from a particular starting point.

2) Vehicle velocity at start and end of the measurement distance or the measurement period. and either:

3a) Force or torque propelling the vehicle sampled at pre- determined short distance intervals and added together over the measurement distance to give a value of the total accumulated work energy used to propel the vehicle over the measurement distance. or:

3b) Force or torque propelling the vehicle sampled at pre- determined short time intervals and added together over the measurement period to give a value of the total accumulated impulse quantity used to propel the vehicle over the measurement period.

The novelty of the measurement technique according to this invention lies in:

1) Obtaining a measurement of the average propulsion force or average propulsion torque over a measurement distance by first measuring the aggregated work energy produced by the force, or by the torque and dividing it by the distance.

2) Cancelling out unknown forces by undertaking two consecutive measurement journeys to allow the subtraction of forces which remain substantially the same for both journeys. The forces deliberately chosen to cancel out typically include wind resistance, rolling resistance and gravity forces. The method of cancelling out gravity forces is applied in a novel way as follows:

3a) Forces generated by unknown terrestrial elevation changes between the start and end of the measurement distance are cancelled out by conducting a second measurement journey over practically exactly the same measurement distance and over practically exactly the same geographical location as the first measurement distance.

3b) Alternatively, using a vehicle of known mass to conduct a measurement of the change of elevation over a particular measurement distance in a given location which can be used to obviate second measurement journeys on subsequent occasions.

Typical applications for the measurement of the mass of vehicles Include: single self propelling vehicles such as a car or a lorry; towed vehicles such as an articulated truck, an aeroplane being towed on the ground or a ship being towed on water. Further application to vehicles coupled in series such as a train is also described. The measurement can be carried out when the vehicle is unladen or laden with cargo, and the mass of the cargo, can be done by subtraction.

The present invention enables the mass of a vehicle to be measured with an instrument situated either 'on board' the vehicle whilst it is on the move or alternatively on board a second detachable towing vehicle.

The impulse based method of mass measurement uses a similar measurement method where impulse quantities are accumulated instead of work quantities but the sampling of values of force or torque, during the journey, is triggered not by distance intervals as is the case for the measurement of work but by time intervals. The impulse based method is only accurate for measurement movements which do not involve a change of elevation in the direction of gravitational acceleration, hence an important application for it is the measurement of mass of ships floating on calm water.

As many vehicles contain both rotating and non rotating masses, for example a train has rotating wheels as part of its total mass, the sum of the two types of inertia masses are determined by the method of measurement according to this invention. In other words the total mass moved linearly as well as the rotating masses multiplied by their effective radius of gyration. The mass of rotating wheels is measured twice, once as a translationally accelerating mass and again as a rotationally accelerating mass. This gives an inertia mass value Mi greater than the static mass Mw of the vehicle but the extra gyration mass MG can be allowed for or neglected, according to the values of rotating mass inertia relative to the total mass being measured. For instance, the measurement of inertia mass of an unladen vehicle can be subtracted from the measurement of inertia mass of the same vehicle when laden to yield the measurement of the static mass of the payload alone.

The equations governing the relationships on which the apparatus and method are based use the following terms:

W = Total Work Energy measured.

Wk = Work energy used to change kinetic energy.

Wf = Work energy used to overcome all unmeasured friction resistance forces opposing motion.

WG = Work energy used to overcome gravity force opposing

motion. (negative value applies in reverse direction) Mw = Total weight-revealed mass, or static mass of vehicle. Mi = Total inertia-revealed mass of vehicle including 'Gyration Mass '.

MG = 'Gyration Mass Equivalent' or rotating mass multiplied by its radius of gyration. Mc = Static mass of cargo within Mi .

Ff = Mean effective friction force or resultant force causing friction resistance in direction against motion, averaged over the journey.

g = Gravitational acceleration.

Z = Gravitational elevation.

ΔZ = Change in elevation in gravitational direction.

V = Vehicle velocity

Vm = Arithmetical average velocity over measurement journey. X = Position of vehicle along measurement journey measured from a set origin.

The measurements which are taken in order to calculate the mass of an object such as a vehicle are taken over one or two stages of movement. Where one stage is used it is divided into two phases of movement (two phase) . Where two stages are used, each stage may be divided into one (two stage single phase) or two (two stage two phase) phases of movement. Where two phases are used, the second may follow on continuously from the first phase, or not, as desired.

For each phase, the object is moved over a distance (X₂-X₂ ) . The term "distance" as used herein refers both to the length of travel and the path of travel covered by a moving object during a measurement period. Thus, distances which are describes as being the same are of the same length of travel and over the same path.

Measurements may be taken at the beginning, the end and/or over the duration of a phase (distance), and the phrase "measurement period" as used herein refers to a selected distance or a selected period of time (which thereby sets the length of travel covered) over which the measurements are taken.

The relationship of the above terms to the phases is indicated by the following subscripts: subscript

1 = start of a phase

2 = end of a phase

a = Denotes a first phase of a first stage.

b = Denotes a second phase of a first stage.

c - Denotes a first phase of a second stage.

d = Denotes a second phase of a second stage. superscript:

^* Denotes a known or predetermined value.

Referring to Figures 2 and 3, the equations governing the relationships used in the measurements are presented as work quantities integrated over a distance (X₂-X₁) using the integral:

Mi = Mw + MG ........ (1) Wk = (1/2).Mi (V² ₂-V² ₁) =Mi . Vm. (V₂-V₁) ........ (2) Wf = Ff. (X₂-X₁ ) ........(3) WG = Mw. g. (Z₂-Z₁) ........(4) W = Wk + Wf + WG ........(5)

A two phase measurement is over over two successive distances (X_2a-X_1a) and (X_2b-X_1b) and is used to cancel out friction resistance work Wf (see Figure 2).

Two stage single phase measurements are taken twice over the same distance, say (X_2a-X_1a), firstly through phase (a), and then through phase (c) by returning the vehicle to the starting point at X_1a for the second stage. Such a compound measurement is used to cancel out gravitational elevation work. It can also cancel out friction resistance with only two measurements and is useful when friction resistance is not very sensitive to vehicle velocity.

A two stage, two phase measurement is one in which the second stage is a repeat of the first stage with phases (c) and (d) being over the same distances as phase (a) and (b). It cancels out elevation work for each phase pair (a) with (c) and (b) with (d) and cancels out friction resistance for phase pairs (a) with (b) and (c) with (d). In this case four measurements are taken for the same purpose but the first stage can effectively cancel out wind resistance, which is velocity sensitive, by operating phases (a) and (b) at similar velocities which can be higher in the first stage, if it is the acceleration stage, than in the second stage. In the second stage, which effectively cancels out elevation work, little acceleration is required and both phases (c) and (d) can be conducted at similar but lower velocities than phases (a) and (b) .

Looking firstly at a two phase measurement Ff, the mean effective friction force averaged over each phase may be safely regarded as having the same or nearly the same value over both measurement phases. Using Equations (2) to (5) gives:

W_a - WG_a = (1/2) .Mi . (V² _2a-V² _1a) + Ff. (X_2a-X_1a) . . .. .. .. (6a)

W_b - WG_b = (1/2 ) .Mi . (V² _2b-V² _{1 b}) ⁺ Ff. (X_2b-X_1b) . . .. .. .. (6 b)

Where W_a is the work quantity measured over phase (a) and WG_a is the work consumed against the force of gravity over phase (a) if the moving mass increases its elevation. W_a has a positive value when the mass accelerates and a negative value when the mass decelerates. Similarly WG_a has a positive value when the mass increases its elevation and a negative value when the mass decreases its elevation.

Eliminating Ff between Equations (6a) and (6b) gives an expression for Mi :

Alternatively, for a two stage single phase measurement we may cancel out both WG and Ff. (X₂-X₁) from Eq. (6a) and Eq.(6b) to give:

which can be further simplified if we start both measurement periods from a standing start so that V1=0.

If the distances covered in the phases of a two phase measurement, for example, phases (a) and (Jb) are not the same, a constant (R) can represent the ratio where

R can be used to simplify Eq. (7a).

Further, if the value of mean effective friction force Ff does not remain the same over two measurement phases for example phases (a) and (b) but is known to change in a predictable manner this can be allowed for by a calibration factor (r) determined by experiment or theory where:

and where Ff_a and Ff_b are the mean effective values of Ff over phases (a) and (b) . Substituting (R) and (r), equation (7) can be simplified to give:

A two stage two phase measurement is advantageously carried out with different average force/torque and velocities in each of the stages as shown in Figure (3a). The two stage measurement is carried out in order to eliminate the effect of changes in geographical gravitational elevation which causes work quantities WG_a and WG_b to be expended against gravity. The two phase measurement is carried out in order to eliminate resistance forces. One of the stages requires the use of a large amount of work to generate an increase in kinetic energy in order to reveal the mass of the vehicle and this need can lead to a relatively large terminal velocity. The other stage benefits from avoiding a large change in kinetic energy as the accurate measurement of velocity changes at low speed under gravity forces can offer an accurate measure of elevation work. It is therefore possible to increase accuracy if resistance forces are cancelled out between the phases for each stage at similar average velocities.

If on completion of the two phases (a) and (b) of the first stage the vehicle commences a new measurement period of work denoted phase (σ) but over the same distance as phase (a) followed by another successive measurement period denoted phase (d) over the same distance as phase (b) a modified mass equation can be written without the unknown terms WG_a and WG_b which relate to elevation work:

As illustrated in Figure 3A one pair of phases e.g. (a) and (b) may be devoted mainly to generating a large positive value for the measurement of work quantity W_α (through large acceleration torque) and if possible as large a negative value as possible for the measurement of work quantity W_b (through large deceleration torque). This is provided, of course, that the work measurements W_a and W_b account for all the driving and braking forces or torques causing the change in velocity but excluding forces which cancel out, namely Ff and gravity forces. For instance, torques applied by the brakes to the wheels of a vehicle must either be measured and allowed for as part of the measured work (W) or if measurement is impracticable, wheel braking must be avoided altogether during phases (a), (b), (c), and (d), as appropriate.

The second stage of a two stage measurement may be devoted mainly to the elimination of the effect of work against gravitational elevations encountered along the previous journey undertaken in the first stage. This can therefore be effectively done with little acceleration or deceleration and little applied force or torque as shown in Figure 3A. Once motion is established, gravitational work can be measured by the effect on measured velocity changes even when the vehicle coasts or free-wheels. Friction forces are substantially eliminated by assuming that Ff_c = Ff_d.

Gravity work cancels out since WG_a = WG_C and WG_b = WG_d and since movement in both phases (a) and (c) is over the same distance (X_2a-X_1a) (similarly for phases (b) and (d) over the same distance (X_2b-X_1b) .

If the distance over which a two phase measurement is carried out is known to be either truly horizontal, or to have a constant slope throughout, that is to satisfy the conditions:

Z_2a = Z_1a or ΔZ_a = 0 and ΔZ_b = 0

or the condition:

(Z_2a-Z_1a ) = (Z_2a-Z_1a)

so that ΔZ_a = ΔZ_b then no correction is needed for gravitational work since WG_b = WG_a or WG_b=WG_a=0. In such a simple case a second stage of measurements (c) and (d) is not necessary and a single stage, two phase measurement using Equation (11) simplifies into:

Where gravitational work exists and friction forces are easily cancelled out a two stage single phase measurement can be carried out. In the first stage relatively large acceleration is applied to yield an appreciably large W_a value and in the second stage low acceleration is applied followed by a 'coasting' movement to yield W_c of smaller value than W_a for use in Equation (8). If such a two stage single phase measurement is done over a short distance starting from rest, a vehicle may simply reverse back to the starting point before commencing the second stage.

If a vehicle is known to have a large aerodynamic or hydrodynamic resistance which is very speed sensitive the measurement procedure can be chosen to select distances so that V_2a = V_1b and V_1a - V_2b. The mean velocity over both phases (a) and (Jb) may be made similar so justifying the assumption that friction force Ff_a = Ff_b. Such procedure can reduce the calibration factor (r) in Eq.(10) to unity but may require a value for (R) , Eq. (9), other than unity. Such a case which requires phases (a) and (Jb) effectively to cancel out friction forces, may be best suitable for mass measurement of low mass vehicles subject to high aerodynamic or hydrodynamic drag.

If measurements are to be carried out on a pre-selected road, a pre-calibration of the gravitational elevation of that road can be made using the described measurement technique with vehicle of known mass Mi^* and MG^* . As a simplified illustration, Using Eq. (11) with R=r=1 and V_1a = 0, V_1b = V_2a , for simplicity, and with the aid of Eq. (15),

Once a road section has a known value of (ΔZb-ΔZa ) mass measurement can be repeatedly carried out on this road with one stage two phase measurements. For example, with R.r = 1 the substitution:

WG_a - WG_b = (Mi - MG) . g. (ΔZ_b - ΔZ_a) ........ (15) may be combined with Equation (11) to give:

and to allow the calculation of Mi .

The value of MG for a vehicle could be known from a previous measurement taken with a known value of Mw. Alternatively it could be neglected if MG is very small compared with Mw, in which case equation (16) is simplified with MG =0 and Mi-Mw.

In summary the following measurement techniques can be used according to choice and circumstances:

A) If the measurement distance is pre-calibrated for elevation changes, a single stage two phase is possible using Eq. (16).

B) If a distance is not pre-calibrated a vehicle may use a two stage single phase measurement, using Equation (8). Such a measurement ensures the exact re-positioning of the centre of gravity of the vehicle at the start and end of the measurement distance and can therefore be accurate as well as simple. The distance (X₂-X₁) may be chosen sufficiently short to allow the vehicle to reverse so as to return to the starting point without a need for a special roundabout journey. Since the terminal velocities are different in each of the two measurement stages there is some error due to the incomplete cancellation of aerodynamic drag but if the vehicle has a low drag coefficient and the terminal velocities reached are sufficiently small to keep aerodynamic forces low, the error in the measurement of mass may be kept to such a low value as to be negligible.

C) If greater accuracy is required with vehicles subject to large aerodynamic and rolling drag forces, a two stage two phase measurement may be used using Eq. (12).

It will be appreciated that the measurement distance chosen, (X₂-X₁) can be short e.g. ten metres or a few tens of metres. Also, measurement periods can start with the vehicle accelerating from rest. This facility helps in defining the starting position X₁ and allows a measurement with a low mean velocity and this has the beneficial effect of minimising wind resistance errors due to incomplete cancellation.

Figure 3B illustrates a measurement technique where a computer selects a distance from within the vehicle journey for the purpose of calculating the vehicle's mass. In this example of a two stage single phase measurement the processor selects the starting point for the measurement of each stage at distance X_la after the vehicle has began its journey at starting point X= 0. The processor also selects the same distance (X_2a-X_1a) for both phase (a) in stage 1 and phase (c) in stage 2 to ensure the cancellation of gravitational work. The processor stores the values of force or torque, distance and velocity for every sampling point along the journey and can therefore identify the exact location of points X_la and X_2a . This allows thee processor to discard parts of the journey which contain widely fluctuating torque or force which may reduce accuracy. Selecting only part of a journey for the distance may also assist in avoiding velocity measurement errors. Such a selective procedure can also be applied to the two stage two phase measurement shown in Figure 3A.

Brief Description of the Drawings

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic illustration of the forces acting on a moving vehicle;

Figure 2 illustrates the various measurements taken to measure the mass of a moving vehicle according to one preferred method of the invention;

Figures 3A and 3B are graphs of the variation of torque or force with distance travelled;

Figure 4 is a block circuit diagram of a preferred embodiment of apparatus according to the present invention for measuring the mass of a moving object such as a vehicle;

Figure 5 to 9 are diagrammatic illustrations of several examples of the application of the apparatus of Figure 4;

Figure 10 is an illustration of the application of the apparatus of Figure 4 to the measurement of the mass of the wagons of a train; and

Figure 11 is a block diagram of a further embodiment of apparatus according to the present invention for measuring the mass of a moving object.

Figure 4 shows a preferred form of apparatus for measuring the mass of a moving object, such as a vehicle in which the apparatus is installed. The apparatus uses the measurement of work energy and is first described with reference to work energy done by a moving force over a selected time period.

The apparatus has a force or torque transducer 1 which may be, for example, a strain gauge based load cell for force measurement or a strain gauge torque measuring tube for torque measurement. The electrical output from transducer 1 may be an analogue signal e.g. a voltage proportional to either force or torque, or a digital signal. Where it is an analogue signal it is fed into an A/D (analogue to digital) converter 2 where the measurements are converted to a digital signal. The output from the A/D converter is sampled through a gate 4 which is actuated by a trigger 3 which operates in a cyclic manner. The trigger 3 may be constructed, for example, as a rotating disc equipped with devices, equi-spaced around its rim, capable of energising a stationary pick-up transducer in proximity to generate trigger pulses. For force measurement, the angle through which the disc rotates between successive trigger events (trigger pulses) is directly proportional to the linear distance covered by the moving force between the said successive triggering events. The disc may conveniently be formed or driven by a wheel rolling along the surface on which the force is engaging for propulsion.

For torque measurement, the angle through which the disc rotates between successive triggering events is directly proportional to the angle of rotation of the torque- transmitting shaft between the successive triggering events. Here the rotating disc may conveniently be mechanically coupled to the rotating shaft transmitting the measured torque.

The number of trigger events during one revolution of the trigger 3 is chosen according to the extent of the fluctuation in the values of the force or torque being measured and the accuracy required for the work energy and/or mass measurements. The signals from the trigger 3 control the opening and closing of the gate 4 which allows, when open, the digitised signal representing measurement of either force or torque to be transmitted to a summing circuit 5 where it is added to the grand total value of all the preceding measurements. There is no need for preceding readings to be stored separately, only their sum total at any instant.

The number of trigger signals is counted in a counter 11 which effectively therefore measures the distance moved. Counter 11 may be used to stop or start a measurement period after a predetermined number of counts (hence a predetermined distance travelled) by closing gate 4. Counter 11 may be reset to zero to start a measurement period by a reset signal from reset 8.

Clock 12 counts the time elapsed between successive trigger events to provide a measurement of vehicle velocity in Velocity Calculator 7. The value of velocity can be recorded in memory in a processor or computer 6 at the beginning and at the end of a measurement period or at any other time. The total distance travelled during a measurement period, recorded in counter 11, can also be fed into processor 6. In a preferred arrangement such distance can be selected by the operator, by programming counter 11 to control the duration of the measurement period.

After a measurement period is completed, the grand total accumulated in 5, representing work energy quantity, is processed in processor 6, together with the values of vehicle velocity at the beginning and end of the measurement period, to measure M_i. The processor 6 may also have digital display facility 13 or a printer for recording measurements of total inertial mass.

Equations required in the calculations and data such as calibration factors, apparatus constants and physical constants are programmed into the calculator through a data input facility 9.

After a reading is taken at the end of a measurement period, it can be stored in a memory if desired. To prepare for a subsequent measurement period, the summing circuit 5 and clock 12 are reset to zero by a reset signal from a reset 8 which is activated either manually or automatically.

Alternatively, the processor may memorise the values of all or some of the sampled and calculated parameters involved during a measurement period, such as force, pulse number, cumulative distance, time interval between pulses, total elapsed time, velocity and cumulative work. If so programmed, the processor may select the measurement distance from within the stage (journey) undertaken, as shown in Figure 3B so as to avoid the beginning and end of stages, which may contain large fluctuations of torque or force which in turn can cause errors in the mass measurement. If so, the same distance must also be selected from the second stage measurement journey which ideally should be started at precisely the same starting point, preferably from a standing start.

Alternatively, for some applications the processor may be programmed to memorise just the minimum information of cumulative work, terminal velocities and cumulative distances so as to provide a simpler apparatus.

The two phase measurement which is used to cancel resistance forces can be conducted in a manner similar to a lap mode of a stop-watch. At the end of phase (a), after receiving a first distance signal from counter 11 (or a velocity signal from meter 7 depending on whether the measurement duration is controlled by distance or by velocity) the values in the summing circuit 5, counter 11 and velocity meter 7 are stored in processor 6 whilst phase (b) measurement continues awaiting a second distance signal from counter 11 (or velocity meter 7). At the end of measurement phase (b) after receiving the second distance signal, the values in the summing circuit 5, counter 11 and meter 7 are again stored and the calculation of mass, using the appropriate equation, can proceed whilst the measurement of accumulated work may continue, if desired, over the remaining journey.

The instrument for the measurement of mass can have an additional function for measuring the total work consumed by a vehicle over a journey. The measurement of work is useful since, when divided by total journey distance, it gives the average force used to move the vehicle over the journey. This information can identify unusually large drag forces caused by faults such as wheel drag due, for instance, to binding brakes. The measurement of work is also useful as it quantifies the average elapsed thermal efficiency of the engine of a vehicle over the journey, if compared with the energy, input associated with the quantity of fuel consumed, measured over the same journey period.

The ability of one instrument, on board a vehicle, to measure mass and average traction force as well as average engine thermal efficiency makes it an attractive instrument for a user of the vehicle.

Another way to process measured information for a mass measurement is to memorise in the processor 6 some or all of the values of velocity, sampled force or torque, accumulated work, trigger pulse number, distance from start, time from start, time between pulse intervals, when each of the samplings are taken as dictated by trigger 3. In this way the processor may select an arbitrary distance for the measurement phases from within the actual journey length travelled by the vehicle during the measurement period. The processor calculates the appropriate values of work and terminal velocities over such selected distances for use in the mass equations. For a second stage measurement where elevation work is cancelled out, the journey starts from the same geographical point, preferably from rest and the processor selects exactly the same distance from the journey starting point as it did for the first stage journey, in order to cancel out elevation changes over the same distance.

The following is offered as an example, together with Figure 3, to illustrate this technique. A journey may start from rest over a pre-selected distance. The driver is alerted by the processor, after a certain number of distance pulses are counted, when to stop accelerating and to start retardation on the overrun, using engine braking. The torque-distance history of such a journey is illustrated in Figure 3A during phases (a), acceleration and (b), overrun. Some complications are encountered during the start of the journey and the start of the overrun, as can be seen from the severe torque fluctuations. These may be caused by, for instance, vibrations of propeller shaft and energy absorbed into suspension springs.

The driver then returns to the same starting point for the second stage of the measurement needed to cancel out elevation work. He accelerates much less in phase (c) than he did in phase (a) and follows this by decoupling the engine from the driving wheels and coasting for the rest of phase (c) and for the whole of phase (d).

Figure 3B shows how the processor may select an arbitrary phase (a) for the calculation from within the length of phase (a) shown in Figure 3A, in order to exclude the large torque fluctuations shown at the beginning and end of the graph. The processor must be programmed to select exactly the same distance, in other words to start and end phase (c) at the same pulse counts value as it did for phase (a), during the second stage shown in Figure 3B. Figure 5 shows an illustration of a possible use of the apparatus of Figure 4 to measure the inertia mass of a wagon 22 pulled by a force provided by locomotive 20, as well as the work done during the journey. The force transducer is shown at 1 and the trigger which generates signals proportional to distance is activated by the rotation of the wagon wheel at 3. Alternatively the distance trigger 3 may be attached to one of the locomotive wheels or even to a special wheel which can be clamped on to either vehicle for the purpose of the measurement.

The force measured by transducer 1 must be in the direction of motion, otherwise an error will result.

Figure 6 shows a similar application to the one shown in Figure 5 for an articulated lorry arrangement where trailer 22 can be detached from the tractor 20 in an arrangement often used in practice. In this arrangement the force transducer 1 is located on the pin which transmits the force from the tractor to the trailer.

Figure 7 shows an application to a lorry but in this case an engine and cab form an integral part of a lorry whose total inertia mass Mi is being measured on the move. A torque transducer 1, is shown placed on the propeller shaft 50, connecting the engine and gear-box E, to the rear wheels which are driven by the engine. In this case the force transmitted by the tyre to the road is always in the direction of movement if the vehicle is moving in a straight line. A brake 100 is also provided on propeller shaft 50 to provide a larger retardation torque, say in the second phase following the acceleration torque in the first phase, measured by the torque transducer 1.

Figure 9 shows an application where a vehicle is towed by a cable 201 attached to a drum 200. The force measurement can be done on the cable by transducer 1 and the distance moved can be measured by distance trigger 3 attached to an idler pulley 202 engaged with the cable or to the drum 200 itself or to another device which can monitor the movement of the cable. The drum may be driven by an electric motor to pull the vehicle during the first phase of a measurement and then to disengage for the second phase. The vehicle finally comes to rest by application of the brakes at the end of the second phase which may be a marked position along the distance.

Alternatively, the drum may be attached to a heavy weight through another cable wound on it, not shown, and the vehicle can use its own traction in reverse gear and lift the weight by turning the drum through cable 201. The vehicle then stops, and after releasing the brakes is propelled forward for measurement, pulled by the descending weight. The weight may reach the ground near the end of the first phase of the measurement and the vehicle continues by inertia for the second phase and then stops using its brakes. If it is necessary to measure elevation work, a second stage may take place with much less force applied by the drum.

If such an arrangement is constructed as a vehicle weighing facility in a fixed location, for instance instead of a weighbridge, the elevation change over the measurement distance can be pre-measured to allow a measurement with one stage and two phases.

The following practices are recommended in order to obtain high accuracy in mass measurement:

1) The force measured should be in the direction of motion.

2) The vehicle should move in a straight line during the measurement to avoid lateral forces.

3) One measurement stage should result in appreciably large acceleration or deceleration values brought about by deliberate force application which is compatible with the available driving forces.

4) A second measurement stage, to eliminate elevation work, should minimise large force applications. The work consumed by elevation can be measured mainly by its effect on velocity changes.

5) Large velocities should be avoided hence short measurement distances are advantageous.

6) The surface on which the vehicle moves during measurements should be uniform in texture and without potholes or bumps or large stones.

7) If velocity measurement depends on it, the relationship between the distance trigger and vehicle distance movement should be accurately established and calibrated before a measurement, particularly if the rotation of load bearing pneumatic wheels is used to drive the distance trigger, since the radius of such a wheel may change with load and air pressure. A direct pre- calibration with the aid of a fifth wheel temporarily attached to the vehicle and plugged into the processor is both straightforward and quick and so is a measurement of the number of trigger pulses recorded over a known distance over which the vehicle moves.

8) Forces affecting vehicle velocity which are not cancelled out by subtraction and are not measured should be avoided during the measurement. Brakes applied to wheels is an example of forces to avoid. Brake 100, in Figure 7, is applied to the propeller shaft S in order to promote deceleration, whose effect is measured in the torque measurement at 1, and is acceptable.

9) Vehicles should return to the same starting point for a two-stage measurement and can start each journey from rest.

10) A measurement of the inertia mass of an unladen vehicle should take place and be memorised to allow direct subsequent measurements to establish cargo weight, when laden. The knowledge of and automatic subtraction of MG, the gyration mass of its rotating parts, is also possible.

The following methods of applying this mass measurement technique are suitable for towing apparatuss: a) The towing of a vehicle whose mass is being measured, over a short measurement distance, with a special powerful tractor (possibly with unsprung driving wheels) equipped with the apparatus of Figure 4 and a distance trigger wheel, can easily meet the measurement requirements mentioned above at.arbitrary locations. For example, an aircraft can be weighed in such a way when being towed on a runway. b) The towing of a vehicle over a short distance with a cable and winch arrangement (which can be provided on the vehicle's frame) by attaching the cable to an anchor point and fitting the vehicle with a distance trigger wheel for the purpose of the measurement.

Figure 10 shows an application similar to the one shown in Figure 5 but in this case the vehicle is a train with a number of wagons here denoted as Q, S and T pulled by locomotive P. The mass being measured Mi is the mass of each of the wagons being pulled by the locomotive but excluding the mass of the locomotive itself. In this illustration a force transducer is placed at each coupling between adjacent wagons and a distance trigger 3 may be attached to a single wheel rolling on the track, positioned anywhere but preferably at the locomotive end P or at the last wagon T. The output from each force transducer is transmitted through an A/D converter 2, gate 4 and summing circuit 5 to processor 6, together with the signals from the distance trigger. The transmission of signals from the transducers and trigger may be done through an electric cable or by radio transmission.

The distance between the centres of adjacent wagons should be known. For instance L_S-τ is the distance between.the centres of wagon T and wagon S and L_Q-S the distance between the centres of wagons S and Q. This can be measured automatically, for instance by an interrupted light beam, as the train moves slowly past a transmitter/receiver device which may communicate with the apparatus by radio signal. In such as a train, complications due to elevation changes can be allowed for, wagon by wagon, in the following manner:

The mass measurement is taken over a distance (X₂-X_ι) , starts at time = t₁ and finishes at time = t₂. The inertial mass of the last wagon T should be determined in advance of the measurement and the cumulative work done (W_τ) in moving this wagon is logged by the processor 6 at successive points along the journey. For instance, at time (t₁+δt) when the centre of wagon T arrives at the position where the centre of wagon S was at time t₁ , the cumulative work done is W_TS1. Again, at time (t_z+Δt) when the centre of wagon T reaches the position where the centre of wagon Q was at time t₂ the cumulative work recorded is W_TQ1 and so on. As the journey continues after the end of the measurement (time t₂) the centre of wagon T, at time (t₂+δt' ) passes the point where the centre of wagon S was at time t₂ to give a cumulative work value to that point of W_τS2 and finally at time (t₂+Δt ' ) and a distance of (L,_S-T+L_Q-S) from its position at the start of the measurement, wagon T reaches the point where the centre of wagon Q was at time t₂, and the cumulative work done has a value of W_TQ2.

The mass measurement for a train is based on a single stage single phase measurement for each wagon whilst the last wagon provides the necessary second stage for all other wagons. Work recorded at the forward and rear coupling of each wagon, are subtracted. For example, in the notation of Figure 10, W_S = (W_τ+W_S) -W_τ. A single phase measurement is possible since the resultant average friction resistance force Ff in Equations (6) may be taken to be nearly constant and can be measured afterwards at near constant speed and allowed for in the calculations. Ff is assumed nearly constant since aerodynamic resistance for wagons moving at low speed is small compared with acceleration traction forces and rolling resistance on a steel rail is also small and will change little during the measurement.

A single stage measurement is possible for each wagon by a use of the last wagon to measure elevation changes during the journey without a need to return to the starting point and repeat a measurement journey. Hence a train need only accelerate, from rest, over a straight stretch of track, over a short distance of say a few tens of metres, for a measurement of the mass of each of its carriages to be made on the move.

When the train accelerates over a distance (X₂-X₁) for a mass measurement, the work measurements 'across' wagon S, after subtraction, gives the net work energy operating on wagon S.

This can be substituted into Equation (6) as follows: ( WG_S2-WG_S1) = ( W_S2-W_S1) -Ff_S (X₂-X₁)-0.5Mi_S ( V² ₂- V² ₁) .. . .. . .. (6c) where W_S2 is the cumulative value of net work applied to wagon S at the end of the measurement, or at time t=t₂. Ff_S is the yet unknown friction force operating on wagon S, Mi_S is the unknown inertia mass of wagon S and (WG_S2-WG_S1) represent the unknown work expended against gravity by wagon S over the measurement distance. The friction force Ff_S resisting wagon S can be measured later when the train moves at nearly constant velocity. This can be done, for instance, soon after the mass measurement is finished, again using a single stage single phase measurement. The friction force Ff for any wagon can be calculated from Equation (6) at constant speed, for example, for wagon S:

where W_S= (W_S2-W_S1) and WG_S= (WG_S2-WG_S1) being net work quantities over the measurement distance.

The last wagon plays an important part in the measurement of elevation changes. The operator has to know its inertia revealed mass Mi^* _τ as well as its gyration mass MG^* _τ. These can be measured on a weigh bridge, which measures Mw and with a two stage single phase measurement of Mi , to give the value of MG. Alternatively, instead of a weigh bridge measurement the value of MG can be calculated from theory. The operator also has to know the friction force Ff^* _τ for the last wagon and this can be measured occasionally using a two stage single phase measurement over a given distance (X₂-X₁) but in the second stage the train has to move in the opposite direction to the first stage. The train may start from rest and move slowly forward over a short distance and then stop and start a slow return movement over the same distance. The distance (X₂-X₁) may be selected by the processor from within the journey distance, in both directions. This procedure eliminates the effect of elevation work WG to reveal the magnitude of the friction force Ff_τ of the last wagon. Applying Equations (6a) and (6b) gives:

where subscript (_R) denotes the reverse journey and subscript (_τ) denotes the last wagon.

The prior knowledge of Ff_τ ^* enables elevation effects to be calculated for all wagons. For instance, the value of Ff to be found for any other wagon using Equation (6d) above provided WG is known. When the train moves at constant speed the procedure shown in Figure 10 (and described above) is repeated to measure W for each wagon and WG from the measurement of the work consumed by the last wagon when it traverses the distance (X₂-X₁) over the geographical location occupied by each of the other wagons during the Ff measurement period.

We now refer to Figure 10 for the measurement of friction resistance Ff for each wagon undertaken at constant velocity over a distance (X₂-X₂) . Between time ( t₁+δ_t.) and ( t₂+δ_t ' ) the last wagon T moves over the measurement distance occupied by wagon S during the measurement. The work measured, of the last wagon T, over this distance is (W_τS2-W_τSl ) and this reveals the work done against elevation change by wagon T over this distance, namely (WG_τS2-WG_τεl ) through the relationship:

( WG_TS2-WG_TS1) = ( W_TS2-W_TS1) -Ff_τ ^* (X₂-X₁) ......(6f) where Ff_*τ is the now known value for wagon T.

If for any wagon the train changes its velocity during the elevation measurement the equation becomes:

( WG_TS2-WG_TS1) = ( W_TS2-W_TS1) -Ff_T(X₂-X₁) -0.5 (V² ₂- V² ₁) ... . (6g) from which (WG_TS2-WG_TS1 ) can be calculated.

The elevation work, in turn, reveals the elevation change itself ΔZ from: L

χ

where Mi^* _τ and Mg^* _τ above refer to known quantities for the last wagon.

Returning to Equation (6d) which calculates the friction resistance of a given wagon from the measurement of WG and the evaluation of ΔZ over the distance of this measurement, using the substitution for the elevation work WG in Equation (6d) where:

WG = ΔZ.(Mi-MG) . g ........(3b) where Mi and MG now refer to any wagon. MG can be substituted from a calculated value knowing the number of rotating axles or it may be justifiably neglected in many applications where it is very much smaller than Mw. If we neglect it, for the sake of simplicity we now have an expression for Ff for each wagon in turn, which contains the unknown value of mass Mi , for the same wagon, namely from Equation (6d):

Ff(X₂-X₁) = W - ΔZ.Mi .g ........ (6g)

This method now gives the value of friction force for each of the wagons in the train, which can be substituted into Equation (6c) in order to calculate mass Mi . In this instance it is denoted as Ff_S for wagon S.

There now remains the need to measure the elevation change experienced by each wagon during the original mass measurement distance, so as to substitute the last remaining unknown parameter WG into Equation (6c) for each wagon.

As seen in Figure 10 the mass measurement is taken over a distance (X₂-X₁) and started at time = t₂ and finished at time = t₂. Knowing the inertial mass of the last wagon T, the velocity measurement and the cumulative work recorded (W_τ) is logged by the processor and noted at successive points along the journey. For instance, at time (t_t+δ_t) when the centre of wagon T reaches the position where the centre of wagon S was at time t₁ the cumulative work W recorded is W_TS1 and again at time (t₁+Δt) when the centre of wagon T reaches the position where the centre of wagon Q was at time t₂ the cumulative work recorded is W_TQ1 and so on. As the journey continues after the end of the measurement (time t₂) the centre of wagon T, at time (t₂+<5_t') passes the point where the centre of wagon S was at time t₂ to give a cumulative work value to that point of W_TS2. Finally, at time ( t₂+Δt ') and a distance of (X₂+X₁+L_S+LQ) from its position at the start of the measurement, the centre of wagon T reaches the point where the centre of wagon Q was at time t₂ and records a cumulative work value of W_TQ2.

The value of ΔZ (elevation change) applicable to the distance over which each wagon moves during the mass measurement for that wagon is computed from equation (6c):

Where Mi_τ ^* and MG_τ ^* are the known masses of the last wagon, Ff_τ ^* the known resistance force for the last wagon and W, V₂ and V₂ are the work quantity and velocities measured over the distance (X₂-X₁) covered by any wagon over the measurement period. ΔZ can now be substituted in Equation (3b) to give a value for (WG₂-WG₁ ) , which in turn is used in Equation (6c), to calculate mass Mi for each wagon.

The measurement procedure for a train is as follows:

1) The train accelerates over a short distance (X₂-X₁ ) (for instance from rest) and the value of work W used over the measurement distance for each wagon is measured, being the difference between the work recorded at the front and rear couplings. The trigger wheel records distance moved an velocities. All information sampled at every trigger signal is stored in the processor 6.

2) The cumulative work for the measurement journey an beyond of the last wagon is recorded as shown in Figure 10. The processor identifies the values of work and velocity for every (X₂-X₁) stretch occupied by each of the wagons during the mass measurement.

3) A second measurement at constant or near constant speed, preferably at low speed, over a short distance, allows the calculation of friction force Ff for each wagon using Equations (4a), (3b) and (6g) in series as described above, again after the last wagon provides measurement of the elevation changes experienced by each wagon during the friction force measurement journey.

4) Using Equation (17) the processor evaluates the elevation change ΔZ experienced by each of the wagons during the mass measurements.

5) The processor now has all the information needed to calculate the inertia mass Mi of each wagon using Equation (6c).

An alternative form of apparatus for measuring the mass of an object through its inertial resistance to a change of its momentum samples the force or torque measurement at preselected time intervals to yield an aggregated impulse quantity. This technique of impulse mass measurement is a variation suitable for applications where there is no change of gravitational elevation during the measurement period, for instance a ship moving on calm sea.

When applying the physical relationships of force or torque and time to the measurement of mass using the measurement of force-time or torque-time impulse quantities the following equations are used with the additional terms:

F = Total force in direction of motion, measured.

Fk = Force used to change kinetic energy in direction of motion.

FG = Force exerted on mass by gravity in direction

against motion.

Ff = Mean effective friction force causing resistance to motion in the direction against motion averaged over the measurement period,

t = Time

δ - An incremental short time interval

δV - An incremental small change of velocity.

I = Accumulated impulse (force x time increment).

Using the other terms and subscripts used in previous equations to describe further equations governing the relationships used in the measurements:

F = Fk + Ff + FG . (18)

Fk. dt = Mi . dV . (19)

FG = Mw. g. (sine of angle between road and horizon) (20)

Ia = Cumulative measured quantity of successive values of F. dt , sampled during time interval ( t₂-t₁ ) , during measurement phase (a) (or cumulative measured Impulse).

Integrating equation (19) gives:

Equation (18) gives:

I = Mi . ( V₂-V₁) + Ff. (t₂-t₁) + FG. ( t₂-t₁) ........ (23) assuming forces Ff and FG remain constant during the measurement time interval (t₂-t₁ ) .

If a measurement is carried out on a road and the slope of the road is constant so that FG is a constant, and if the velocity change during the measurement interval is small enough to justify the assumption that Ff remains substantially constant during the measurement time interval, the values of I_a and I_b become:

I_a = Mi . (V_2a-V_1a) + Ff. (t_2a-t_1a) + FG_a . ( t_2a-t_1a) ........(24a)

I_b = Mi . ( V_2b-V_{1 b}) + Ff. ( t_2b-t_1b) + FG_b. (t_2b-t_1b) ........ (24b)

Eliminating Ff gives an expression for Mi as an alternative to equation (7):

On a level road FG=0, simplifying Equation (25).

If the apparatus is designed to carry out a pair of successive measurements periods, with or without a time interval in between, but triggering the start and finish of each measurement period (a) and (Jb) so that the time duration of measurement period (a) is either equal to or a multiple of the time duration of measurement period (Jb) the terms (t₂-t₁) can be eliminated.

If for example ( t_2b-t_1b) - 2.(t_2a-t_2a), on a level road:

If the duration of the second measurement period is related to the duration of the first measurement period by a calibration factor (R) so that:

and further if the value of friction force Ff is known to change slightly over the measurement periods this can be allowed for by a calibration factor (r) determined by experiment or theory:

where Ff_a and Ff_b are the time-mean effective values of Ff over measurements period (a) and (b) , then equation (25) can be adjusted to give:

The mass measurement based on accumulated impulse measurement using Equations 26 is less attractive than the mass measurement based on work measurement when applied to road vehicles since absolutely flat, horizontal roads are not easily available. The . latter method may, however, be very suitable for use in measuring the mass of vessels floating on water where change of elevation can be easily avoided.

Figure 8 shows such an example where a ship 300 is towed by a tug 310 with a cable. Provided the water is calm the force measurement at 1 can be a basis for mass measurement using the impulse method after making due allowance for the angle of the cable from the horizontal. It is important that an accurate method is available for measuring the velocity of the ship during the measurement. Instead of being accelerated by a tug a ship may trail a cable attached to a drag inducing object, such as a water parachute 330 and the deceleration force is measured and accumulated as an aggregated impulse. A third possibility is to measure the torque or reaction force on the propeller 320 and sample it against a clock to yield an accumulated; impulse quantity for mass measurement. In this last example experimental calibrations of the instrument are needed to reveal calibration factors for Equation (26b). As the relationship between hydrodynamic resistance and ship speed are theoretically understood, the value of (r) in Equation (28) can be allowed for.

Figure 11 is a block diagram of an apparatus for the measurement of mass based on the prior measurement of accumulated force-time or torque-time impulse quantities. The apparatus is similar to that of Figure 4 with like parts having the same reference numbers. In general, only the differences are described here.

Signals from the transducer 1 are fed through the A/D converter 2 to gate 4. The output from the converter 2 is sampled through gate 4 which is actuated by a trigger 31. The latter provides trigger signals separated by short, preferably equal time intervals. Trigger 31 can be an electronic clock or, for instance, a synchronous motor driving a rotating disc which activates a proximity transducer.

The time interval between trigger events is chosen according to the frequency and amplitude of fluctuation in the values of the force or torque measured and the accuracy required for the measurement.

The number of trigger signals is counted in counter 11 which effectively measures the time elapsed. Counter 11 may be used to stop a measurement period after a predetermined time period, by closing gate 4. Counter 11 may be reset to zero to start a measurement by reset signal from reset 8. All or some of these operations may be controlled automatically by a controller, not shown. Alternatively a measurement may be stopped after a chosen distance is covered, or after a chosen velocity is reached as measured by velocity meter 121. Meter 122 counts the distance moved by the mass to provide a measurement of its velocity. For example the velocity may be measured directly by an analogue velocity meter producing a signal proportional to speed. The value of velocity can be stored in memory in processor 6 at the beginning and at the end of a measurement period. The total time elapsed during a measurement period, recordable by using counter 11, can also be fed into processor 6 at the end of an experiment, if calculations are made on the basis of equation (25) and time duration of test periods can be made constant by programming counter 11 to control the duration of the measurement period as described below.

The period over which the total force-time impulse quantity is to be measured is controlled by gate 4 which can either be opened and closed manually or controlled by an automatic controller, not shown. The gate can open at the same time as the time measurement period starts and closes when the time measurement period finishes.

After a measurement is completed, the grand total accumulated in 5, representing force-time impulse or torque-time impulse quantity, is processed in processor 6, together with the values of vehicle velocity at the beginning and end of the measurement period, according to Equation (25) or (26). The processor may also have digital display facility for the resulting measurements of total inertial mass.

The reading memorised and or displayed in 9 can, for example, be in the mass unit kilogram or in a weight unit Newton.

The two successive measurements needed to measure mass can be conducted in a manner similar to a lap mode of a stop-watch. After receiving a first time signal from counter 11, the values in impulse summing circuit 5 and velocity meter 121 can be stored in processor 6 whilst measurement continues awaiting a second time signal from counter 11. The process is repeated after the second time signal, controlled by the controller (not shown) and the calculation of mass, using an equation in equation group (26), for instance, can proceed.

The duration of measurement periods (a) and (b) may alternatively be determined and controlled by a controller on the basis of either velocity values at the end of a measurement period or on the basis of distance travelled during a measurement period. The velocity controlled method may minimise friction drag errors whereas the distance controlled method can be used to ensure that a test remains within the bounds of a previously calibrated horizontal road section or a constant slope road section.

Claims

Claims

1. A method of measuring the mass of an object comprising: causing the object to move over a first measurement period (a) whereby said object moves over a first distance; measuring the accumulated first work energy or impulse done during movement of the object over said first distance to provide a first work energy or impulse value W_a,I_a; causing the object to move over a second measurement period (b, c) whereby said object moves over a second distance; measuring the accumulated second work energy or impulse done during movement of the object over said second distance to provide a second work energy or impulse value (W_b, W_c;I_b,I_c) ; measuring the velocity ( V_1a,V_1b,V_2a,V_2b;V_1a,V_1b,V_1c, V_2c) of the object at the beginning and end of each of said first and second distances; deriving a value for the mass (Mw,Mi ) of the object as a function of the measurements of said velocities and said first and second work energy or impulse values; wherein during movement of the object over at least one of said first and second distances the object undergoes a change of momentum.

2. A method as claimed in claim 1 wherein said object undergoes a smaller change in momentum in moving over said second distance than in moving over said first distance.

3. A method as claimed in claim 1 or 2 comprising applying a first force or torque to said object to cause said object to move over said first distance (a).

4. A method as claimed in claim 3 wherein said first force or torque is removed during at least a portion of at least one of said measurement periods.

5. A method as claimed in any preceding claim wherein: the accumulated first and second work energy is measured (W_a,W_c) ; said first and second measurement periods (a,c) are over the same distance; and comprising deriving a value for the mass Mi of the object as a function of the measurements of said velocities and said first and second work energy values by solving the following equations for Mi i

W_a = 0 . 5Mi . ( V² _2a- V² _1a) + Ff. (X_2a-X_1a) + WG_a W_c = 0 . 5Mi . ( V² _2c- V² _1c) + Ff. (X_2c-X_1c) + WG_C

Wherein: WG_a=WG_c for the same path of travel,

and Ff is assumed substantially constant.

6. A method as claimed in any of claims 1 to 4 wherein: the accumulated first and second work energy is measured (W_a ,W_b) ; said second distance (b) follows said first distance (a); and said first and second distances are over different paths of travel; and further comprising deriving a value for the mass Mw of the object as a function of the measurements of said velocities (v_1a,v_1b,V_2a,V_2b) and said first and second work energy values (W_a/W_b) ky solving the following equations for Mw:

W_a = 0 . 5 (Mw-MG) ( V² _2a- V² _1a) + Ff. (X_2a-X_1a) + Mw. g(Z_2a-Z_1a)

W_b = 0 .5 (Mw-MG) ( V² _2b- V² _1b) + Ff. (X_2b-X_1b) + Mw. g(Z_2b-Z_1b) wherein: MG is known or negligible, and

ΔZ =( Z₂-Z₁ ) is known.

7. A method as claimed in any of claims 1 to 4 or 6 wherein: accumulated first and second work energy is measured; further comprising: measuring the accumulated third work energy done during movement of the object over a third measurement period (c) whereby said object moves over a third distance to provide a third work energy value (W_c) ; wherein said third distance follows said second distance and said second distance follows said first distance, said distances being over different paths of travel; and further comprising deriving a value for the mass Mw of the object as a function of the measurements of said velocities and said work energy values by solving the following equations for Mw:

W_a = 0.5 (MW-MG) ( V² _2a- V² _1a) + Ff. (X_2a-X_1a) + Mw. g(Z_2a-Z_1a)

W_b = 0 .5 (MW-MG) ( V² _2b- V² _1b) + Ff. (X_2b-X_1b) + Mw. g(Z_2b-Z_1b)

W_c = 0.5 (MW-MG) ( V² _2c- V² _1c) + Ff. (X_2c-X_1c) ⁺ Mw. g(Z_2c-Z_1c)

wherein ΔZ = ( Z₂-Z₁ ) is known.

8. A method as claimed in any preceding claim further comprising measuring the change in elevation over a said distance by: causing a second object of known mass and known friction resistance force (Ff) to move over said distance (a,b, c,d) ; measuring the accumulated work energy done during movement of said second object over said distance to provide a work energy value (W_a,W_b,W_c,W_d) for said distance; measuring the velocities (V₁,V₂) of said second object at the beginning and end of said distance; and deriving a value for the change in elevation (ΔZ) over said distance using the equation:

W_a = 0 .5Mi* ( V² _2a- V² _1a) + Ff*. (X_2a-X_1a) + (Mi*-MG*) . g. (Z_2a-Z_1a)

9. A method as claimed in any of claims 1 to 6 or 8 wherein the accumulated first and second work energy is measured and further comprising: measuring the accumulated third work energy done during movement of the object over a third measurement period whereby said object moves over a third distance following said first distance, to provide a third work energy value (W_b) ; measuring the accumulated fourth work energy done during movement of the object over a fourth measurement period whereby said object moves over a fourth distance following said second distance, to provide a fourth work energy value

(W_a) ; wherein said first and second measurement periods (a,c) are over the same distance and said third and fourth measurement periods (b,d) are over the same distance; and further comprising: measuring the velocities (V_a,V_b, V_c, V_d) of the object at the beginning and end of said first, second, third and fourth distances; and deriving a value for the mass Mi of the object as a function of the measurements of said velocities and said first, second, third and fourth work energy values by solving the following equations for Mi :

W_a = 0.5Mi. ( V² _2a- V² _1a) + Ff. (X_2a-X_1a) + WG_a

W_b = 0.5Mi. ( V² _2b- V² _1b) + Ff. (X_2b-X_1b) + WG_b

W_c = 0.5.Mi. ( V² _2c- V² _1c) + Ff. (X_2c-X_1c) + WG_c

W_d = 0 . 5Mi . ( V² _2d- V² _1d) + Ff. (X_2d-X_1d) + WG_d

wherein: WG_a=WG_c

WG_b=WG_d

and Ff is assumed substantially constant.

10. A method as claimed in any of claims 1 to 9 wherein the step of measuring the work energy or impulse done and velocities over a said distance ( a,b, c,d) comprises: causing the object to move over a journey whose length of travel is greater than that of said distance; continually measuring said accumulated work energy value (W, I) , said velocity (V) and length of travel of said object over said journey; and selecting a length of said travel as said distance (a,b,c,.d).

11. A method as claimed in claim 10 wherein said distance is said first distance.

12. A method as claimed in any of claims 1 to 11 wherein the step of measuring accumulated work energy comprises measuring the accumulated energy in the form of mechanical work done by a moving force or rotating torque during movement of said object over a measurement period by the steps of: measuring said force or torque and providing a first signal representative of said measured force or torque; generating trigger signals at preselected increments of distance travelled by said force or torque; sampling said first signal in response to generation of said trigger signals; summing said sampled signals; and providing a work signal (W) which is a function of said summed sampled signals and is representative of the accumulated total amount of work energy done by said force or torque over said measurement period.

13. A method as claimed in any of claims 1 to 4 wherein: the accumulated first and second impulse is measured(I_a,I_b); said second distance (b) follows said first distance (a); and said first and second distances are over different paths of travel; and further comprising deriving a value for the mass Mi of the object as a function of the measurements of said velocities (V_1a,V_1b,V_2a,V_2b) and said first and second impulse values (I_a,I_b) by solving the following equations for Mi :

I_a = Mi . ( V_2a-V_1a) + Ff. ( t_2a-t_1a) + FG_a . ( t_2a-t_1a)

I_b = Mi . (V_2b-V_1b) + Ff. (t_2b-t_1b) + FG_b. (t_2b-t_1b)

wherein: FG_a=FG_b =0 or FG is constant and is known, and

Ff is assumed constant.

14. A method as claimed in any of claims 1 to 4 or 13 wherein the step of measuring accumulated impulse done comprises measuring the accumulated impulse done by a moving force or rotating torque during movement of said object over a measurement period by the steps of: measuring said force or torque and providing a first signal representative of said measured force or torque; generating trigger signals at preset time intervals; sampling said first signal in response to generation of said trigger signals; summing said sampled signals; and providing an impulse signal which is a function of said summed sampled signals and is representative of the accumulated total amount of impulse done (I) by said force or torque over said preset measurement period.

15. A method as claimed in any preceding claim further comprising clearing said summed signals after said preset measurement period.

16. A method as claimed in any preceding claim further comprising counting the number of trigger signals generated over the measurement period, said count being representative of the distance (a,b,c,d) travelled.

17. An apparatus for measuring the mass of a moving object comprising: first means (1-5; 1,2,4,5,31) for measuring the accumulated work energy or impulse done during movement of the object over a measurement period and providing an accumulated first work/impulse signal (W,I) representative thereof; velocity means (7,11,12; 11,121,122) for measuring the velocity of the object and providing first and second velocity signals (V) representative of the velocity of the object at the beginning and end of the measurement period respectively; store means (6) for storing said first work/impulse signal and velocity signals; and processor means (6) for deriving a mass signal representative of the mass of the object, as a function of said accumulated first work/impulse signal and velocity signals measured over at least two measurement periods

(a,b,c,d) .

18. An apparatus as claimed in claim 17 wherein said first means comprises: measuring means (1) for measuring force or torque associated with the movement of said object during said measurement period and providing a first signal representative of said measured force or torque; sampling means (4) for sampling said first signal; trigger means (3) for triggering sampling of said first signal at selected intervals of time; and summing means (5) for receiving and summing said sample signals to provide said accumulated first work/impulse signal.

19. An apparatus as claimed in claim 18 wherein said sampling means comprises gate means (4) which opens for a preset period of time in response to generation of a trigger signal by said trigger means (3,31), to sample said first signal.

20. An apparatus as claimed in claim 18 or 19 further comprising reset means (8) for clearing said summing means (5) after said measurement period.

21. An apparatus as claimed in claim 18, 19 or 20 wherein said trigger means (3) is operable to trigger sampling of said first signal at preselected increments of distance travelled by said force or torque, said preselected increments determining said selected time intervals.

22. An apparatus as claimed in claim 21 wherein said velocity means comprises: means (11,12) for measuring the length of each said selected interval of time between trigger signals and providing a time signal representative thereof; and velocity meter means (7) for providing a velocity signal representative of the velocity of the object as a function of said time signal.

23. An apparatus as claimed in claim 21 or 22 wherein said trigger means comprises a trigger transducer (3) which is actuable by a rotating device to produce a trigger signal at selected angular intervals during rotation, the angle turned during each said interval being proportional to the increment of distance travelled by the measured force or torque during said same interval.

24. An apparatus as claimed in claim 23 for measuring the work done by a moving force wherein the rotating device comprises a wheel engaged with a surface on which the force is moving.

25. An apparatus as claimed in claim 23 for measuring the work done by a rotating torque wherein the rotating device is coupled with a shaft which transmits said torque.

26. An apparatus as claimed in claim 18, 19 or 20 wherein said trigger means (31) is operable to trigger sampling of said first signal at preselected intervals of time.

27. An apparatus as claimed in claim 26 wherein said velocity means comprises: means (122) for measuring elapsed distance travelled between trigger signals from said trigger means (31) and providing a distance signal representative thereof; and velocity meter means (121) for providing a velocity signal representative of the velocity of the object as a function of said distance signal.