EP4577422A1

EP4577422A1 - A propulsion system for a heavy-duty vehicle, a heavy-duty vehicle and a method for controlling a propulsion system

Info

Publication number: EP4577422A1
Application number: EP22769252.2A
Authority: EP
Inventors: Esteban GELSO; Sachin JANARDHANAN; Leo Laine
Original assignee: Volvo Truck Corp
Current assignee: Volvo Truck Corp
Priority date: 2022-08-26
Filing date: 2022-08-26
Publication date: 2025-07-02
Also published as: CN119654248A; WO2024041740A1

Abstract

The invention relates to a propulsion system (102, 202) for a heavy-duty vehicle (100, 200), such as a truck, a bus or a construction equipment vehicle. The propulsion system comprises electric machines (EMs), such as electric motors and generators, and a control arrangement (126, 226) comprising a processor (128, 228) and a memory (130, 230). Each of the EMs are either in an EM electrically connected state, in which power is transferred between a power source and the wheels, and an EM torque is applied to the vehicle's wheels, or in an EM electrically disconnected state, in which no power is transferred between the power source and the wheels. Further, each of the EMs is associated with EM power loss characteristics for the EM electrically connected state and the EM electrically disconnected state, respectively. The EM power loss characteristics for the EM electrically disconnected state is related to inertia and/or friction losses caused by the EM when no power is supplied thereto from the power source. The control arrangement is arranged to define a cost function which depends on EM states for the EMs, wherein each of the EM states comprises the EM electronically connected state or the EM electronically disconnected state, wherein the cost function further comprises a motion request associated with desired values of global forces, to minimize the cost function over a search space comprising the EM states such that minimizing EM states are determined, and to control the EMs in accordance with the minimizing EM states.

Description

A propulsion system for a heavy-duty vehicle, a heavy-duty vehicle and a method for controlling a propulsion system

TECHNICAL FIELD

The invention relates to a propulsion system for heavy-duty vehicles. More particularly, it is related to systems and methods for providing more efficient propulsion of heavy-duty vehicles.

BACKGROUND

During the last years, the use of software-based vehicle motion management control systems has become increasingly popular in the field of heavy vehicles, such as trucks, buses and construction equipment. For instance, by being able to continuously collect parameter values related to the electric motors and friction brakes, it is made possible to provide more energy-efficient motion control, but also more precise steering and an improved driving dynamics.

A typical vehicle motion management control system comprises a plurality of actuators, such as steering devices, brakes, drives and dampers, communicatively connected to a control arrangement. In addition to the actuators, sensors are provided for feeding sensor data to the control arrangement. After processing the sensor data by using software instructions held in the control arrangement, control data can be transmitted from the control arrangement to the actuators such that for instance more energy efficient propulsion can be achieved.

More detailed information can be found in the Master Thesis by Johnsson-Holm, Erik, “Predictive Energy Management of Long-Haul Hybrid Trucks: Using Quadratic Programming and Branch-and-Bound”, Linkbping University, 2021 , in which it is presented how mixed-integer quadratic program (MIQP) can be used for lowering fuel consumption for a hybrid electric vehicle (HEV) having an electric propulsion system and a conventional internal combustion engine (ICE). Further, in the Master Thesis by Yadav, Dhananjay “Optimal torque split strategy for BEV powertrain considering thermal effects”, it is disclosed how two or more electric machines of a battery electric vehicle (BEV) can be controlled efficiently. Even though different approaches for vehicle motion management control has been suggested, there is room for further improvement. In particular, being able to efficiently reduce power losses in a vehicle comprising one or several electric machines would be of great benefit.

SUMMARY

An object of the invention is to provide a propulsion system having a number of electric machines (EMs), such as electric motors, that can be controlled in a manner such that power losses are reduced.

Generally, it has been realized that by allowing the EMs to be mechanically connected to a drivetrain, but electrically disconnected, i.e. not powered, additional degrees of freedom for controlling the propulsion system are made available. Even though these states give rise to power losses in the form of inertia and friction losses, these states may nevertheless be an optimal choice for reducing power losses. One reason for this is that the cost of changing states, e.g. from mechanically connected to mechanically disconnected, also result in a power loss.

According to a first aspect of the invention, the object is achieved by a propulsion system according to claim 1.

According to one embodiment, the propulsion system may further comprise mechanical actuators (MAs) which may be mechanically connected or mechanically disconnected. By also including states of the MAs in the search space of the cost function used, as well as power loss characteristics for different MA states, it is made possible to find a combination of power loss minimizing states of both the EMs and MAs. An advantage of assessing both the EMs and the MAs at the same time is that combinations of states can be found that would not be found if considering the two types in isolation.

According to a further embodiment, the control arrangement may be arranged to update minimizing MA states and minimizing EM states at a frequency of one time per second or more, such as every 10 milliseconds. Switching from electronically disconnected to electronically connected, and vice versa, can be made at a fraction of a second. As an effect, allowing state changes not involving going from mechanically connected to mechanically disconnected, or vice versa, is of particular benefit for sub-second power loss optimization. According to a second aspect of the invention, the object is achieved by a heavy-duty vehicle according to claim 10.

According to a third aspect of the invention, the object is achieved by a method according to claim 11.

According to a fourth aspect of the invention, the object is achieved by a computer program according to claim 13.

According to a fifth aspect of the invention, the object is achieved by a computer readable medium according to claim 14.

According to a sixth aspect of the invention, the object is achieved by a control arrangement according to claim 15.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples.

In the drawings:

Fig. 1 illustrates a hybrid vehicle with two axles by way of example,

Fig. 2 illustrates another hybrid vehicle with three axles by way of example,

Fig. 3 illustrates power losses for two electric motors and brakes,

Fig. 4 generally illustrates input and output for a motion coordination approach,

Fig. 5 illustrates, by way of example, the number of electric machines (EMs) used as a function of total longitudinal force request for a hybrid vehicle as illustrated in fig. 2, Fig. 6 illustrates, by way of example, torque distribution between the two EMs of the vehicle illustrated in fig. 2,

Fig. 7 illustrates, by way of example, the number of EMs used a function of total longitudinal force request for a vehicle with four EMs,

Fig. 8 illustrates, by way of example, torque distribution between the four EMs, Fig. 9 illustrates, by way of example, how states can be selected when using the motion control approach, and

Fig. 10 is a flowchart illustrating a method for controlling the propulsion system.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Fig. 1 generally illustrates, by way of example, a propulsion system 102 of a vehicle 100, such as a truck, a bus or a construction equipment vehicle. The propulsion system 102 should herein be understood to comprise any type of engine, such as an electric engine and/or an internal combustion (IC) engine, transmission components, such as a gear box, and other components arranged for providing a positive momentum on wheels 104a-d of the vehicle. In addition to these components, the propulsion system 102 can comprise components arranged for providing a negative momentum on the wheels 104a-d. For instance, the propulsion system 102 may comprise mechanical brakes as well as electric machines arranged to reduce a speed of the vehicle at the same time as generating electricity.

In the example illustrated in fig. 1 , the vehicle is a hybrid vehicle having an IC engine 106, herein generally referred to as a mechanical actuator (MA), connected via an MA clutch 108 to a transmission 110 in turn connected to a final drive 112. As illustrated in fig. 1 , the final drive 112 is connected to two of the wheels 104a, 104c such that power output from the IC engine 106 can provide rotation of these two wheels 104a, 104c. The combination of components used for propelling the vehicle 100, herein exemplified by the transmission 110 and the final drive 112, can generally be referred to as a drivetrain 113.

In addition to the IC engine 106, in the illustrated example, an electric motor 114, herein generally referred to as an electric machine (EM), is provided. The electric motor 114 is connected to a battery 116, or other power source. A switch 118 can be placed between the battery 116 and the electric motor 114 such that the electric motor 114 can be electrically connected to or electrically disconnected from the battery 116. Further, an EM clutch 120 can be provided between the electric motor 114 and the transmission 110. As an effect of having the EM clutch 120, the electric motor 114 can be mechanically connected to or mechanically disconnected from the transmission 110.

A steering system 122a, 122b may form part of the vehicle 100. In the illustrated example, the steering system 122a, 122b is coupled to a front axle, while the IC engine 106 and the electric motor 114 are coupled to a rear axle. Even though illustrated in this way, this is only one out of several possible embodiments.

Mechanical brakes 124a-d may be provided for each of the wheels 104a-d. The mechanical brakes 124a-d may be embodied in different ways, but all with the overall purpose of reducing speed or completely halt the vehicle. As a complement to the mechanical brakes 124a-d, which is an example of a mechanical actuator (MA), a generator can be used, even though not illustrated. The electricity formed by the generator can be used for charging the battery 116 while providing a braking effect.

In the context of this patent application, the IC engine 106 and the mechanical brakes 124a- d are generally referred to as MAs. The electric motor 114 is on the other hand referred to as an EM. In case a generator is added for charging the battery 116 while providing the breaking effect, this would also be considered an electric machine (EM). Put differently, any non-electric machine is in the context of this patent application to be considered a MA. The difference of the two can also be described as that the MA can be in two states; mechanically connected or mechanically disconnected, while the EMs can be provided in the mechanically connected or disconnected state as the MAs, but also in an electrically connected or disconnected state. Having this additional possibility allows for improved energy-efficiency. For instance, even though a mechanically connected and electronically disconnected electric machine, such as the electric motor 114, may negatively affect propulsion of the vehicle 100 by inertia and friction losses, having the electric motor 114 in this state may nevertheless be the most energy efficient option at a particular point of time due to that state changes may themselves come with an energy cost. In particular, this holds true when state changes are set to occur every second or even more frequent.

A control arrangement 126, comprising a processor 128 and a memory 130, can be provided for controlling states of the EMs and the MAs. In addition to adjusting the EMs and the MAs in accordance with minimizing MA states and minimizing EM states, that is, the states of the MAs and EMs resulting in a most energy-efficient propulsion of the vehicle 100, the control arrangement 126 may be arranged to set the force or torque exerted by the EMs and/or MAs.

Fig. 2 illustrates another example of a hybrid vehicle 200. In line with the example illustrated in fig. 1 , the vehicle 200 comprises a propulsion system 200 and wheels 204a-f. However, unlike the example illustrated in fig. 1 , the vehicle 200 illustrated in fig. 2 is provided with three axles and six wheels. In line with the example illustrated in fig. 1 , the vehicle 200 comprises an IC engine 206, an MA clutch 208, and a transmission 210. Since there is two axles coupled to the transmission 210, a first and a second final drive 112a-b can be used, one for each axle. The transmission 110 and the final drives 112a-b may together be referred to as a drivetrain 213.

A rearmost axle may be used as a start axle (S-A) and a mid-axle may be used as a cruise axle (C-A). A first electric motor 214a, a first battery 216a, a first switch 218a and a first EM clutch 220a may be coupled to the cruise axle (C-A) and a second electric motor 214b, a second battery 216b, a second switch 218b and a second EM clutch 220b may be coupled to the start axle (S-A). Even though illustrated that the first and second electric motor 214a, b have separate batteries, it is also possible to have one and the same battery, or other power source, shared among the two.

Similar to the example illustrated in fig. 1 , a steering system 222a-b can be provided on a front axle. In addition, mechanical brakes 224a-f can be coupled to each of the wheels 204a-f. A control arrangement 226, comprising a processor 228 and a memory 230, can be communicatively connected to the switches 218a-b, the EM clutches 220a-b and the mechanical brakes 224a-f.

In addition to the examples illustrated in fig. 1 and 2, the vehicle may also be an electric vehicle (EV), i.e. a vehicle having a propulsion system only including electric motors and no IC engines.

Further, even though only illustrated that each electric motor is coupled to one axle, it is also possible to have the electric motors, or more generally electric machines, connected to several axles. In addition, it is also possible to have several electric motors, or electric machines, coupled to one and the same axle.

By way of example, for hybrid vehicles as illustrated in fig. 1 and 2, to improve the energy efficiency of the propulsion systems 102, 202, forces of the MAs, such as service brakes, and the EMs can be chosen such that power losses are minimized. For instance, an optimal control problem design of the motion coordination, which minimizes the power losses, could be written as: where the u vector incudes the forces of the devices to be controlled, in this case n electric machines and m service brakes, wherein the service brakes are mechanical brakes, but u could also include steering devices.

B is called the control effectiveness matrix, V_des is the virtual control input with the desired values of global forces, and the power losses of n electric machines are approximated by nonlinear functions of the output torque, or forces, and in this example the nonlinear functions are quadratic functions that sum the copper losses, iron losses, windage losses and mechanical (friction/inertial) losses:

Where as is the rotational inertia losses due to rotor mechanical properties when current is switched off, i.e. electrically disconnected, and ao is the magnetic losses when current is supplied to the electric machine and inertia losses, a₃ « a₀ .

The terms ao, ai, a2 and as are time variant, and depend on working operating conditions like the electric machines rotational speed a)_em Even though the examples provided in fig. 1 and 2 are hybrid vehicles, the approach presented above can also be used for electric vehicle (EVs), i.e. vehicles not having any ICE.

Further, in the example provided above, power losses for the ICE are not taken into account. Even though not part of the example provided above, for hybrid vehicles the power losses for the ICE may also be taken into account.

In fig. 3, the power losses as a function of the torque on the wheels for an electric vehicle at 80 km/h is illustrated by way of example. The electric vehicle in this particular example is provided with two EMs of different type: one EM located in a cruise axle, and one located in a start axle, both with different designs. Moreover, the losses of the service brakes have been lumped in one term for the sake of clarity.

If mechanically disconnecting the electric motors 214a-b, or more generally the EMs, the losses can be assumed to be zero. However, by instead allowing the EMs to be mechanically connected but electronically disconnected, additional degrees of freedom can be allowed, resulting in increased possibilities to improve energy efficiency. However, the approach comes with the consequence that the losses related to the inertia and the friction losses of the EMs are to be taken into account. Thus, the terms ao of the EMs are now, when allowing these extra degrees of freedom, depending on the torque.

As an effect, the optimization problem can be reformulated as below. Unlike the problem presented above, this new approach includes new outputs of the control algorithm that are binary variables, called b_em, that take into account whether or not the different EMs should be electrically connected or electrically disconnected to reduce the power losses.

The power losses of the EMs are equal to: and the vector u of optimization variables is extended by including the new binary variables:

The factors “Cj” in the cost function above are parameters that represent costs of the power losses. The optimization problem can be written as a standard mixed-integer quadratic programming (MIQP) problem and solved using existing solvers in the literature.

The motion coordination approach described above is generally illustrated in fig. 4. The control effectiveness matrix B and the virtual control input Vdes with desired values of global forces can be input. In addition, an upper and a lower limit of u can be input. Pi_OSs parameters can be input, and also the costs c.

As illustrated, by using the motion coordination approach, it can be output force/torque of the EMs, the Connect/Disconnect states of the EMs, i.e. whether the different EMs should be electrically disconnected or electrically connected, and force/torque of the service brakes, or more generally the MAs.

In line with the examples provided above, having the EMs electrically disconnected, but mechanically connected, power losses will occur in the form of inertia and/or friction losses. When having the EM electrically connected, but mechanically disconnected, power losses will occur in the form of inverter losses. When having the EMs electrically disconnected and mechanically connected, the power losses are close to zero.

By using the approach suggested above, a torque split between the first and second motor 214a-b for the hybrid vehicle 200 illustrated in fig. 2 can be provided. By way of example, in fig. 5, for one vehicle speed point, a number of electric motors as a function of the total longitudinal force request is illustrated. In fig. 6, torque distribution between the first and second electric motor 214a-b is illustrated (in fig 6 referred to as EMi and EM2). As operating points for the vehicle change during operation, as well as motion global forces targets and capabilities of devices, such as electric motors, the motion control may recalculated to meet these new conditions.

In the example illustrated in fig. 7 and 8, four identical EMs are used. The four EMs are of the same type as the second electric motor 214b forming part of the example presented above. For the four EMs the maximum torque is 10 500 Nm. When having a set up with four EMs and using the motion control approach described above may result in the number of EMs in on state (i.e. electrically connected) as a function of the total longitudinal force request as illustrated in fig. 7. The torque distribution per machine may be as illustrated in fig. 8.

Since the four machines are identical in this example, it is possible to reduce the combinatorial problem of the MIQP. Instead of only solving one time the four cases where only one EM is on (electrically connected), and only one time the six cases where two EMs are on (electrically connected), and only once the four cases where three EMs are on (electrically connected), it is possible to in total solve five cases (equivalent to solve 5 QPs): {0,1 , 2, 3, 4} electric machines on. This also applies when we include knowledge of the preferred EMs to be used.

As described above, by using the approach described herein, the number of available states are increased and it is hence made possible to make more detailed settings, and thereby provide improved energy efficiency. Fig. 9 illustrates a state selection that can be chosen as the most energy efficient combination of states, or expressed differently, the combination of states in which power losses are minimized. Selected states are marked in bold in fig. 9.

The particular example illustrated in fig. 9 is related to the vehicle 200 illustrated in fig. 2. In this example, the MA #1 - #4, being the mechanical brakes, are mechanically disconnected. The MA #5, the IC engine, is mechanically connected. The first electric motor, EM #1, coupled to the start axle is mechanically connected and electrically connected, while the second electric motor EM #2, coupled to the cruise axle, is mechanically connected but electrically disconnected, i.e. generating inertia and friction losses. In fig. 10, it is provided a flowchart illustrating a method 1000 for controlling the propulsion system 102, 202 of the vehicle 100, 200. In a first step 1002, the cost function depending on the MA states and the EM states can be defined. In a second step 1004, the cost function can be minimized with the effect that minimizing MA states and minimizing EM states are determined, or put differently optimal MA states and optimal EM states. In a third step, the MAs and the EMs of the vehicle can be controlled by using the minimizing MA states and the minimizing EM states.

It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.

Claims

1. A propulsion system (102, 202) for a heavy-duty vehicle (100, 200), such as a truck, a bus or a construction equipment vehicle, said propulsion system comprising electric machines (EMs), such as electric motors and generators, and a control arrangement (126, 226) comprising a processor (128, 228) and a memory (130, 230), wherein each of the EMs are either in an EM electrically connected state, in which power is transferred between a power source and the wheels, and an EM torque is applied to the vehicle’s wheels, or in an EM electrically disconnected state, in which no power is transferred between the power source and the wheels, wherein each of the EMs is associated with EM power loss characteristics for the EM electrically connected state and the EM electrically disconnected state, respectively, wherein the EM power loss characteristics for the EM electrically disconnected state is related to inertia and/or friction losses caused by the EM when no power is supplied thereto from the power source, wherein the control arrangement is arranged to define a cost function which depends on EM states for the EMs, wherein each of the EM states comprises the EM electronically connected state or the EM electronically disconnected state, wherein the cost function further comprises a motion request associated with desired values of global forces, to minimize the cost function over a search space comprising the EM states such that minimizing EM states are determined, and to control the EMs in accordance with the minimizing EM states.

2. The propulsion system according to claim 1 , wherein each of the EMs being in the EM electrically disconnected state is mechanically connected to a drivetrain (113, 213) of the vehicle.

3. The propulsion system according to any one of claim 1 or 2, further comprising mechanical actuators (MAs), such as mechanical brakes and internal combustion engines, wherein each of the MAs are either in a MA mechanically connected state, in which an MA torque is applied to the vehicle’s wheels, or in an MA mechanically disconnected state, in which no MA torque is applied, wherein each of the MAs is associated with MA power loss characteristics for the MA mechanically connected state and the MA mechanically disconnected state, respectively, and wherein the cost function further depends on MA states for the MAs, wherein each of the MA states comprises the MA mechanically connected state or the MA mechanically disconnected state, wherein the search space further comprises the MA states such that minimizing MA states are determined as well, wherein the control arrangement (126, 226) is further configured to control the MAs in accordance with the minimizing MA states.

4. The propulsion system according to any one of the preceding claims, further comprising EM clutches (120, 220a-b) provided between a subset of the EMs and the drivetrain (113, 213), wherein, for each of the EMs in the subset, when being in an EM mechanically connected state, the EM clutches (120, 220a-b) are engaged, wherein, for each of the EMs in the subset, when being in an EM mechanically disconnected state, the EM clutches (120, 220a-b) are disengaged, wherein, for the subset, the EM states used in the search space further comprise the EM mechanically disconnected state, wherein, for the subset, the EM electronically connected state and the EM electronically disconnected state coincide with the EM mechanically connected state.

5. The propulsion system according to any one of the preceding claims, wherein the control arrangement is arranged to update the minimizing MA states and the minimizing EM states at a frequency of one time per second or more, such as every 10 milliseconds.

6. The propulsion system according to any one of the preceding claims, wherein the control arrangement is configured to provide control data to the drivetrain (113, 213) resulting in a change of at least one of longitudinal motion, lateral motion, yaw moment, roll moment and pitch moment of the vehicle (100, 200).

7. The propulsion system according to any one of the preceding claims, wherein a first electric machine (EM1) of the EMs is arranged to propel a start axle (S-A) of the vehicle (200) and a second electric machine (EM2) of the EMs is arranged to propel a cruise axle (C-A) of the vehicle.

8. The propulsion system according to any one of the preceding claims, wherein the cost function comprises power losses of the EMs, wherein a quadratic approximation of the power losses of the electric motors is set as a function of the electric motor torques for the EMs being in the EM electrically connected state.

9. The propulsion system according to any one of the preceding claims, wherein the cost function comprises a cost term adding an extra cost for switching between the EM electrically connected state and the EM electrically disconnected state for the EMs.

10. A heavy-duty vehicle, such as such as a truck, a bus or a construction equipment vehicle, comprising the propulsion system according to any one of the claims 1 to 9.

11. A method (1000) for controlling a propulsion system of a heavy-duty vehicle, such as a truck, a bus or a construction equipment vehicle, wherein the propulsion system comprises mechanical actuators (MAs), electric machines (EMs) and a control arrangement (126, 226) comprising a processor (128, 228) and a memory (130, 230), wherein each of the MAs are either in a MA mechanically connected state, in which an MA torque is applied to the vehicle’s wheels, or in an MA mechanically disconnected state, in which no negative torque is applied, wherein each of the electric motors are either in an electrically connected state, in which power is transferred between a power source and the wheels, and an EM torque is applied to the vehicle’s wheels, or in an electrically disconnected state, in which no power is transferred between the power source and the wheels, wherein each of the MAs is associated with MA power loss characteristics for the MA mechanically connected state and the MA mechanically disconnected state, respectively, and each of the EMs is associated with EM power loss characteristics for the EM electrically connected state and the EM electrically disconnected state, respectively, wherein the EM power loss characteristics for the EM electrically disconnected state is related to inertia and/or friction losses caused by the EM when no power is supplied to thereto, said method comprising defining (1002), by using the control arrangement, a cost function which depends on MA states for the MAs and EM states for the EMs, wherein each of the MA states comprises the MA mechanically connected state or the MA mechanically disconnected state, wherein each of the EM states comprises the EM electronically connected state or the EM electronically disconnected state, wherein the cost function further comprises a motion request associated with desired values of global forces, minimizing (1004), by using the control arrangement, the cost function over a search space comprising the MA states and the EM states such that minimizing MA states and minimizing EM states are determined, and controlling (1006), by using the control arrangement, the MAs and the EMs in accordance with the minimizing MA states and the minimizing EM states, respectively.

12. The method according to claim 11 , wherein each of the EMs being in the EM electrically disconnected state is mechanically connected to a drivetrain (113, 213) of the vehicle.

13. A computer program comprising program code means for performing the steps of any of claims 11 and 12 when said program is run on a computer.

14. A computer readable medium carrying a computer program comprising program code means for performing the steps of any of claims 11-12 when said program product is run on a computer.

15. A control arrangement for controlling a propulsion system of a vehicle, the control arrangement is configured to perform the steps of the method according to any of claims 11-12.