TECHNICALFIELD The present invention relates to a method and system for estimating the weight of a vehicle, in particular large vehicles like lorries and trucks, even in the cases where the vehicle is connected to a trailer, semi-trailer or the like.

BACKGROUND OF THE INVENTION Today's vehicles contain highly sophisticated control systems using algorithms for controlling a number of functions like the engine, brakes, gearbox, cruise control etc., which require a number of parameters from e.g. sensors and the like.

Regarding control of the engine in order to obtain an optimal performance in view of output, fuel economy, exhaust condition, the behavior of the vehicle and the like, one parameter that may be an important factor is the weight of the vehicle. The control algorithms should then act somewhat differently depending on if the vehicle is unloaded or having full load, possibly together with a trailer.

A number of attempts have been made to estimate the weight of a vehicle with on-board systems, i.e. which do not require actual weighing of the vehicle on scales. One such system is described in the US patent No. 5,610,372 where the weight of a vehicle is estimated by using Newton's second law relating the acceleration that an object experiences through the application of a particular force to the mass of the object; F = m a where F is the force applied to the vehicle, α is the resulting acceleration and m is the mass of the vehicle.

In one embodiment of US 5,610,372 acceleration and force sensors are used in order to obtain values for calculating the weight of the vehicle. A similar approach is described in a Thesis for Degree of Master of Science at Linkδping University, Division of Vehicular Systems, "Adaptive Vehicle Weight Estimation" by Emil Ritzen; Reg. No: LiTH-ISY- EX- 1883.

In the thesis a method is developed for estimating the weight of a vehicle during driving by comparing the acceleration of the vehicle with the estimated torque from the engine. The important parameter regarding the torque is really the torque required to propel the vehicle. The energy loss from parameters that do not contribute to useful energy on the drive wheels thus has to be deducted from the energy delivered by the engine.

One of the energy losses that for large trucks can be quite important is the rotational acceleration of the wheels, i.e. the energy required to rotate the wheels of the vehicle. Regarding trucks there is a problem in estimating the weight because there may be different number of wheels on the vehicle depending on the situation, such as if the tag axle of a three axle tractor is lifted or not, if a trailer or a semi-trailer is connected to the tractor or not.

None of the above mentioned documents deals with how to obtain information regarding the number of wheels on the vehicle that are actually rolling. In US 5,610,372 the factors that cause energy loss are assumed to be constant over a very short time span whereby a plot of the force to move the vehicle against the acceleration will result in a curve, which, properly scaled, will have a slope that is the inverse of the current vehicle weight. This assumption is then used to create a vector whose slope represents the current estimate of the inverse of the weight of the vehicle and whose elements are the composite estimates of the acceleration and force. The vector is then corrected or updated with a vector that is formed between the acceleration and force measurements that are next to be integrated into the composite estimate and the previous acceleration and force measurements.

The thesis "Adaptive Vehicle Weight Estimation" does not mention how information regarding the number of wheels is obtained, but because this is a development project one can assume that the number of wheels are counted and entered into the algorithm manually.

However, even if the basic concept of utilizing Newton's second law for weight estimation of moving vehicles has proven its advantages, there is a need for improving and automating the concept so that it can be used as a useful control parameter in a modern vehicle during different driving and load situations.

BRIEF DESCRIPTION OF THE INVENTION The aim of the present invention is to provide a method for vehicle weight estimation that takes care of different driving and load conditions in a simple, reliable and useful way.

This aim is solved by the features of claim 1. Preferable embodiments of the invention are found in the dependent claims.

According to a main aspect of the invention, it comprises a method for estimating the weight of a vehicle, comprising the steps of obtaining information from first sensor means relating to the acceleration of the vehicle, and estimating the acceleration of the vehicle based on the information obtained by the first sensor means obtaining information from second sensor means relating to the force obtained from the engine of the vehicle, and estimating the force obtained from the engine based on the information obtained by the second sensor means, obtaining information from third sensor means and estimating the current number of wheels on the vehicle that are in contact with the ground based on the information obtained bv the third sensor means, estimating the force needed to move the vehicle taking into account losses due to the forces needed to rotate the wheels of the vehicle based on the estimation regarding the current number of wheels that are in contact with the ground, estimating the force that is moving the vehicle by subtracting the estimated force needed to move the vehicle from the estimated force obtained from the engine, and estimating the weight of the vehicle based on the mathematical correlation that the weight of the vehicle is the quotient of the force that is moving the vehicle and the estimated acceleration.

In this context, information relating to the acceleration can be obtained from first sensor means that e. g. could be sensor means detecting the velocity of the vehicle, via the rotational speed of the wheels, or sensors detecting the rotational speed of different parts of the drive line. Further, information relating to the force obtained from the engine may be obtained by second sensor means detecting the torque from the output shaft of the engine, sensors detecting the amount of injected fuel, or any other suitable sensors capable of measuring torque, output power directly or indirectly.

According to another aspect of the invention, the information for estimating the current number of wheels that are in contact with the ground is based at least partly on whether a trailer is comprised in the vehicle or not.

According to a further aspect of the invention, the information for estimating the current number of wheels that are in contact with the ground is based at least partly on the position of a liftable and lowerable axle of the vehicle. The third sensor means can thus be sensors arranged to the connecting point between the truck and a trailer as well as sensors arranged to the tag axle capable of detecting its position.

The estimated weight is then transmitted to the control system of the engine of the vehicle.

The benefit of the invention is that the weight estimation system and method presented utilizes information from different sensors in order to ascertain the number of "active" wheels that are in contact with the ground, i.e how many wheels that are currently running on the actual vehicle. This means if the tag axle, if the truck is equipped with such, is lifted or not, if there is a trailer connected to the truck and possibly the kind of trailer.

The information may be obtained from the control system that is present in modern vehicles such as heavy trucks. The weight estimation system is then automatically adapted to the number of wheels that are actually in contact with the ground. With this information and other information from the control system of the vehicle, such as the acceleration of the vehicle that can be obtained from the velocity and the torque of the engine, the weight of the vehicle can be estimated based on Newton's second law and transmitted to the control unit of the engine.

Should the information regarding the actual number of "active" wheels be insufficient, the method utilizes information regarding the total allowed weight of the vehicle, including trailer if the vehicle is equipped with trailer connection, and from information regarding the maximum permissible weight that each axle can carry according to road legislations. Based on this, the number of axles required can be estimated, and thus the number of wheels on the vehicle. It is then possible to estimate the actual weight of the moving vehicle. By obtaining an estimation of the actual weight of the vehicle the driving behavior of the vehicle, the performance, optimization of fuel consumption, exhaust emissions and the like control parameters of the engine may be adapted in an efficient way, where the estimation of the weight has been performed in a highly automated way.

These and other aspects of and advantages with the present invention will become apparent from the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed description of the invention reference will be made to the accompanying drawings, of which

Fig. 1 is a schematic view of the longitudinal forces acting on a vehicle, Fig. 2 is a physical model of a drive line of a vehicle,

Fig. 3 is a schematic representation of a weight estimation unit comprising the present invention, and

Fig. 4 shows a flow chart for obtaining information and estimating the number of wheels that are running on an actual vehicle, which is used for estimating the weight of the vehicle. •

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a method for weight estimation based on Newton's second law and treating the whole vehicle (truck and possible trailer) as one rigid body. The basis of the method is outlined in the above mentioned document "Adaptive Vehicle Weight Estimation", which basis is utilized by the present invention. The vehicle is treated as one mass and the power from the engine as a longitudinal force causing longitudinal acceleration, F = m a. An equivalent assumption is to treat the vehicle as a moment of inertia and the power from the engine as a torque, ώwJ = M.

The longitudinal forces acting on the vehicle are shown in Fig. 1 where rw is the wheel radius, m' t is the weight of the vehicle without wheels, Fw is the composite force of all external longitudinal forces acting on the vehicle, ωWf is the rotational speed of the front wheel.

The following equations may be formed at the rear wheel:

M^a = F1n - F1 (2)

where α is the acceleration, Jwr is the moment of inertia of the rear wheels, TTiwr is the weight of the rear wheels, FfΛ is the friction force on the rear wheels,

Fj is the force between the chassis of the vehicle and the rear wheels, is the acceleration of the rear wheels, At the front wheels, the following equations may be formed:

Ffr2rw = ώMfJκf = —Jκf (3)

F2 - Fn = Tn^a (4) where JwJ is the moment of inertia of the front wheels, Ffr2 is the friction force of the front wheels, πiwf is the weight of the rear wheels, is the force between the chassis of the vehicle and the front wheels, ω V. is the acceleration of the front wheels,

The longitudinal forces may be formed according to Newton's second law

By combining equations (1) to (5), the following expression can be derived

Mw = [Jmr +Jwf +{m't +mwr +TThf{)r2w] ώw =

The transmission 14 of the vehicle may, according to Fig. 2, be calculated as

ωe = Kgtωt (7)

where is the rotational speed of the engine, Kgt is the gear ratio of the transmission, ωt is the output rotational speed from the gear box, J9 is the mass moment of inertia in the transmission, Me is the output torque of the engine; Mt is the output torque of the transmission.

The final drive 16 may, according to Fig.2, be modelled as

where ωw is the rotational speed of the wheels, Kgf is the gear ratio of the final drive, Jf is the moment of inertia of the final drive, Mu; is the output torque to the wheels.

If eq. (8) is combined with eq. (10), then we obtain

ώwJf =(-ωeJg+Me)Kg-Mw (11) where Kg = KgtKgf. Newton's second law applied to the engine 12 according to Fig.2 gives

where Je is the mass moment of inertia of the engine, Mc is the torque delivered from the combustion compensated by internal engine friction, Me is the torque output from the engine when the mass moment of inertia Je is taken into consideration.

Combining eq. (11) and eq. (12) gives

fh J. =(-ω (J +J Λ + M ΛK -M fl3) Insertion of eq. (7) and eq. (9) into (13) gives

ώjf = -Kg2ώ(Jg+Je) + McKg -Mw (14)

Finally eq. (6) and (14) provides the desired model of estimating the weight

ώw[jf +Jw+™,r> +Kg2(Jg -Fwrw (15)

The external forces, comprising air resistance Fav, rolling resistance Frou and sloping resistance Fsiope, have to be modelled in a suitable way, such as

Fair = ±p-cwA(v + v0)2 (16)

where p is the air density, Cw is the drag coefficient, A is the frontal area of the vehicle, u is the speed, and υo is the headwind speed.

Froll=mt{crX+cr2v) (17)

where CΓJ, Cro. are parameters depending on e.g. type of tires and road pavement.

Fsloope=mtgύκχ (18) where χ is the slope of the road.

By combining eq. (15) - (18) the resulting model is obtained:

ά{Jf + Jw + mtrl + Ks2(Jg + Je] = 1 2 <19) McKg ~i:P - cwA(.v + vo) rw - mt(crl + cr2v)rw - mtg - rw smχ

From the model it is then possible to estimate the weight. Certain parameters in the model need to be measured and communicated to an electronic control unit 20 of a weight estimation system 21. One parameter is acceleration, ώw . The acceleration can be obtained by measuring and differentiating the velocity of the vehicle. The velocity may be obtained from e.g. first sensor means 23, Fig. 3, in the form of wheel speed sensors if an ABS system is fitted on the vehicle, from the speed of an alternator, a crankshaft, a flywheel or from the output shaft of the transmission. Signals from these first sensor means can be fed to the electronic control unit for calculating the acceleration of the vehicle. It is however to be understood that the acceleration may be calculated in other parts of the electrical system of the vehicle, such as by the sensors themselves or by other electronic units.

Another parameter is the torque of the engine, Mc. This parameter can usually not be measured directly and has thus to be approximated. A simple approximation that has proved adequate for the purpose is that the torque of a diesel engine can be said to be a linear function of the injected amount of fuel, which linear function in an improved embodiment is made even better if account is taken for the engine speed. Modern diesel vehicles are equipped with second sensor means 25 in the form of sensors connected to the fuel line 27 of the engine capable of detecting the fuel consumption and sensor means capable of detecting the engine speed via e. g. the flywheel 29 and thus signals from these second sensor means can be fed to the electronic control unit for calculating the torque of the engine; the force obtained from the engine. It is however to be understood that other means and sensors may be applicable for measuring or sensing the output power from the engine directly or indirectly. It is also to be understood that the force obtained from the engine may be calculated in other parts of the electrical system of the vehicle, such as by the sensors themselves or by other electronic units.

The gear ratio, K9, may be estimated by sensors measuring the speed on the input and output shaft of the transmission. The signals are then filtered in a suitable way and divided in order to obtain the gear ratio.

When all variables are measured, a straight line approximation in line with y = kx + I gives the weight of the vehicle and the gradient of the road, where, when looking at eq. (19)

y is MeKg - άlJf +Jw +Kg2(Jg + Je)] -±p -cwA(v + v0)2rw

x is ώw I is rwm,(gsinχ + crl)

As seen above the estimation of the weight, πit, is simply done by dividing the slope of the line with rl . Extracting the gradient, χ , from I can only be done with knowledge of the weight, but this problem is easily solved since the estimation of the weight is independent from χ and therefore can be done in advance. As χ is small, sin χ could be approximated to χ . Since it is not possible to measure the headwind speed, the estimation of the gradient has to be made with the assumption that the influence of headwind speed is negligible. The part of the rolling resistance that depends on the velocity, cr2rwrrkv is small compared to the other parts in the equation, so a good approximation is to ignore it.

As can be seen from the above model for estimating the weight, the total moment of inertia of the wheels of the vehicle, Jw, plays an important role. It is therefore important that the number of "active" wheels, i.e. that are in contact with the ground on the actual vehicle, are known or can be derived in a simple and reliable way.

When implementing a weight estimation system according to the present invention, information regarding the wheels of the truck or tractor itself can be entered and stored in the electronic control unit 20, because the number of wheels of the truck is known.

However, if the truck is equipped with a set of wheels that can be lifted or lowered depending on the load of the vehicle, i.e. if the vehicle is equipped with a tag axle 30, the electronic control unit 20 needs to obtain information regarding the number of "active" wheels. The weight estimation system 21 thus comprises third sensor means 32 capable of providing a signal from the system handling the tag axle 30, regarding the current position of the tag axle so that the electronic control unit can take into account the number of "active" wheels.

Because the number of wheels on the tag axle is known, their contribution to the moment of inertia of all the "active" wheels may also be stored in the memory means of the electronic control unit during implementation. Apart from the actual truck, the electronic control unit requires information regarding possible trailers or semi-trailers that are connected. The weight estimation system is capable of deriving information from the electrical connection between the truck and the trailer. In its simplest form, a third sensor means in the form of a current sensor 34 may be arranged to the electrical wires of the trailer connection socket 36 in order to detect that current is flowing to the trailer. Some of the modern truck/ trailer systems are provided with more advanced information transfer means enabling different functions of the trailer, such as ABS brake systems and air suspension, to communicate with the control systems of the truck. The electronic control unit can thus utilise this in order to obtain information regarding the presence of a trailer.

For most trucks that frequently pull trailers it is fairly simple to estimate the number of axles for example depending on if the truck has a fifth wheel for pulling semi- trailers. A cargo trailer usually has two axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. In Europe, most semi-trailer tractors have 2 axles, again with the front, steer, having two wheels, and rear, drive, having a pair of double wheels on each side. Thus, the most common configuration has 6 wheels. Conversely, the cargo trailer usually has three axles at the rear, each with dual wheels, or 12 wheels in total. One way or the other, the entire vehicle thus usually has 5 axles and 18 wheels in total, although the trailers can vary in number of wheels. The weight estimation system can thus be pre-programmed with the number of wheels that the total vehicle can have as "active" wheels, and depending on the input signals from the tag axle and trailer connections, the total moment of inertia from the "active" wheels is used in the algorithm for weight estimation. If it is not possible to obtain sufficient information on the number of axles, for example if one of the above mentioned sensor means for providing information regarding the position of the tag axle or if a trailer is connected or not, are not present, or if one of these are mal- functioning, an estimation regarding the total allowed weight of the vehicle, possibly including a trailer of the vehicle is equipped with a trailer connection, can be used as a starting point and the approximate load that one axle can carry according to vehicle regulations regarding maximum permissible axle loads. As an example, a 60 000 kg vehicle, where the maximum permissible load on each axle is 9 000 kg, requires at least 7 axles to carry that load. From this information it is then possible to roughly estimate the number of wheels arranged on this vehicle and thereby estimate the weight of the moving vehicle by the aforementioned method. This estimation is however not as accurate as when information is obtained from sensors, especially when no information can be obtained regarding a trailer, but provides an acceptable enough estimation of the weight that can be used for controlling the engine.

The estimated weight is then transmitted to the control system 40 of the engine of the vehicle, which is used as one parameter for controlling the performance and behaviour of the engine depending on the actual conditions of the vehicle.

Figure 4 shows a flow chart depending on different situations and sensors on the vehicle.

One case is where a tag axle is fitted on the vehicle. With the aid of the third sensor means 32 it is possible to determine, 110, if the tag axle is lowered or not. In any of those positions it is possible to estimate the current number of wheels of the truck that are in contact with the ground, 112, 114, because, as said above, this information is known and implemented into the unit when the system is setup. The other case is where no tag axle is fitted. Also in this case the information regarding number of wheels is directly implemented in the system.

Apart from the actual truck, there are three scenarios depending on the equipment of the vehicle. The first scenario, 116, is when no trailer connection is fitted. Based on the information stored in the electronic control unit regarding the wheels of the truck it is possible to calculate the moment of inertia of the wheels, 117.

The second and third scenarios comprise fitted trailer connections. In the second scenario, 118, it is possible to obtain information, 120, regarding if a trailer is connected or not, as outlined above using a third sensor means for detecting if a trailer is connected or not to the truck. If the system senses that there is no trailer connected, then it is possible to directly calculate the moment of inertia from the information stored in the electronic control unit.

If the system senses that a trailer is connected, the number of wheels of the trailer is estimated, 122, preferably depending on information stored in the electronic control unit regarding the type of trailer connected and thus the number of axles/number of wheels, as discussed above. The estimated number of wheels of the trailer is added to the number of wheels of the truck in order to calculate the moment of inertia of the total number of wheels of the complete vehicle, 117.

In the third scenario, 124, no information can be obtained as whether a trailer is connected or not. In this case the estimation starts with the knowledge of the total allowed weight of the vehicle and the permissible load of an axle, and from there estimate the minimum number of axles and in turn the minimum number of wheels, 126. This estimation is then added to the number of wheels of the truck in order to calculate the moment of inertia of the total number of wheels of the complete vehicle, 117.

For all the above mentioned cases and scenarios, the calculated moment if inertia, Jw, is then used, 128, as input for the weight estimation algorithm.

The algorithm for estimating the weight of the vehicle is preferably implemented in software stored in the electronic control unit that could be a hardware unit of its own 20 or integrated in one of the hardware units that are a part of a modern vehicle, for instance the engine control unit. A part of the program may be stored in a processor or a memory 22 that may be any of the following: ROM (read only memory), PROM, Programmable ROM) EPROM (Erasable PROM), Flash, EEPROM (Electrically EPORM) or any other type of memory media capable of storing data such as a magnetic disk, CD-ROM or DVD disk, hard disk, magneto-optical memory storage means, as firmware.

The electronic control unit may comprise filters for filtering the signals, A/D-converters for converting and sampling the signals, input/output circuitry 23 and one or more micro processors 24. The micro processor (or processors) comprises a central processing unit CPU performing the following functions: collection of measured values, processing of measured values, estimating the weight and output of result from calculation. The micro processor (or processors) further comprises a data memory and a program memory.

A computer program for carrying out the method according to the present invention is stored in the program memory. The software includes computer program code elements or software code portions that make the computer perform the said method using the equations, algorithms, data and calculations previously described.

The weight estimation unit is capable of communicating with other systems of the vehicle for input and output of signals via for example a data bus system 26 such as a CAN-bus (Controller Area Network) commonly used for heavy vehicles. It is however to be understood that many other types of communication system, such as e.g. Flexray, TTCan and Most may be feasible within the scope of the present invention.

Even though the present invention has been described in relation to one algorithm for utilising Newton's second law, it is to be understood that different algorithms or modifications may be utilised within the scope of protection.

Although specific embodiments have been shown and described herein for purposes of illustration and exemplification, it is understood by those of ordinary skill in the art that the specific embodiments shown and described may be substituted for a wide variety of alternative and/or equivalent implementations without departing from the scope of the present invention. Those of ordinary skill in the art will readily appreciate that the present invention could be implemented in a wide variety of embodiments, comprising hardware and software implementations, or combinations thereof. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Consequently, the present invention is defined by the wordings of the appended claims and equivalents thereof.