WO2017099655A1

WO2017099655A1 - A method and a system for controlling an output torque of an electric machine in a vehicle

Info

Publication number: WO2017099655A1
Application number: PCT/SE2016/051223
Authority: WO
Inventors: Niklas Pettersson
Original assignee: Scania Cv Ab
Priority date: 2015-12-08
Filing date: 2016-12-06
Publication date: 2017-06-15
Also published as: SE540416C2; DE112016005148T5; SE1651601A1

Abstract

The present disclosure relates to a method for controlling an output torque of an electric machine in a vehicle. The method comprises providing a first output torque of the electric machine, and determining a rotational speed of an output shaft of the electric machine. The method further comprises determining a temperature in an electronic component of the electric machine based on a model for the temperature in the electronic component of the electric machine and based on the first provided output torque and the determined rotational speed of the output shaft. The method even further comprises determining a maximum allowable output torque of the electric machine based on the determined temperature in the electronic component, and providing an updated output torque based on the determined maximum allowable output torque. The present disclosure further relates to a system for controlling an output torque of an electric machine in a vehicle, to a vehicle, to a computer program for controlling an output torque of an electric machine in a vehicle, and to a computer program product.

Description

A method and a system for controlling an output torque of an electric machine in a vehicle TECHNICAL FIELD

The present disclosure relates to a method and a system for controlling an output torque of an electric machine in a vehicle. The present disclosure further relates to a vehicle, a computer program, and a computer program product.

BACKGROUND ART

When using electric machines as propulsion means in vehicles one has to take special attention when driving off with the vehicle. If the electric machine is not rotating, or is rotating with a very low rotational speed, most of the current may be directed and pass through a specific electronic component, such as a specific transistor in the electric machine. Such a specific transistor may be an insulated-gate bipolar transistor, IGBT, of an inverter for the electric machine. This is in contrast to an electric machine with a reasonable high rotational speed, where the current will alternatingly pass through a number of different electronic components in the power controller of the electric machine, such as different transistors of the inverter.

If the current passes basically only through one specific electronic component in the power controller of the electric machine , this will cause the component to heat up due to losses in it. This heating up might lead to a temperature in the component which is so high that it can degrade the life-time of the component substantially. When the electric machine is rotating with a reasonable high rotational speed, the current will distribute over several electronic components, thus limiting the heating up of the single component. For preventing a too high temperature in the electronic component even for very low rotational speeds, or for a standing still of the electric machine, it is known in the art to limit the allowable output torque which the electric machine can provide at these rotational speeds. By limiting the allowable torque, the amount of current passing an electronic component will be limited and thus the temperature of the electronic component. For some specific inverters for electric machines with three phases the allowed torque is often limited to half of the possible output torque and this allowable output torque is then raised by raising rotational speed until it reaches it possible maximum at a rotational speed of around 100 revolutions per minute, rpm.

Limiting the allowable torque has, however, some drawbacks. One drawback is that the allowable torque might not be enough for moving the vehicle after a standing still. This might, for example, be the case when trying to move the vehicle after a stand-still on an uphill slope, or after the vehicle has been loaded. In case the vehicle has no extra combustion engine which can be used, the only solution then might be towing the vehicle or unloading it. Especially for trucks this might cause big costs and time losses.

It is further observed that such a limiting of the allowable torque might cause oscillations in the powertrain of the vehicle which can be experienced as very uncomfortable for a driver of the vehicle.

It has been tested to use temperature sensors close to the inverters or to other parts of the electric machine to prevent that the temperature in the electronic components are raising too much. It has, however, turned out that the reaction time of these temperature sensors is too slow. They indicate a too high temperature in the components only after the temperature has been too high for some time.

There is thus a need for lowering or removing the aforementioned drawbacks. Especially, there is a need for preventing a heating up of the electronic components to too high temperatures, whereas at the same time providing enough output torque to allow a moving of the vehicle and/or, preferably to prevent, or at least lowering oscillations in the powertrain.

Document DE102011090088A1 discloses a method for operating an electric machine in a vehicle. The electric machine is operated in an operating range which is limited to an operating range that is optimized for the electric machine. However, the electric machine may be operated at operating points outside of the optimized operating range only for a limited period of time which depends on the temperature of power electronics of the electrical machine. However, the aforementioned drawbacks are not removed by the method mentioned in this document.

SUMMARY OF THE INVENTION One object of the present invention is to provide a system, a method, a vehicle, a computer program, and a computer program product which lowers or removes the aforementioned drawbacks.

One object of the present invention is to provide a system, a method, a vehicle, a computer program, and a computer program product which prevents a heating up of electronic components of an electric machine to too high temperatures, whereas at the same time providing enough output torque to allow a moving of the vehicle, especially when moving from a stand still.

One object of the present invention is to provide a system, a method, a vehicle, a computer program, and a computer program product which prevents a heating up of electronic components of an electric machine to too high temperatures, whereas at the same time preventing, or at least lowering oscillations in a powertrain of the vehicle.

One object of the present invention is to provide an alternative system and an alternative method for controlling an output torque of an electric machine in a vehicle, as well as an alternative vehicle, an alternative computer program, and an alternative computer program.

At least some of the objects are achieved by a method for controlling an output torque of an electric machine in a vehicle. The method comprises a) providing a first output torque of the electric machine and b) determining a rotational speed of an output shaft of the electric machine. The method further comprises c) determining a temperature in an electronic component of the electric machine based on a model for said temperature in said electronic component of the electric machine and based on said first provided output torque and said determined rotational speed of the output shaft. The method even further comprises d) determining a maximum allowable output torque of the electric machine based on said determined temperature in said electronic component and e) providing an updated output torque based on said determined maximum allowable output torque.

This has the advantage that the output torque can be adapted to the temperature of the electronic components. As long as there is no risk of heating the electronic component too much, a maximum output torque can be provided, thus facilitating moving of the vehicle. By using the model it is also assured that the lifetime of the electronic components are not affected in an unaccepted negative way. The method further reduces oscillations in the powertrain as the cause for these oscillations is eliminated, or at least reduced.

In one example the method further comprises the step f) of determining a desired output torque of the electric machine. The first provided output torque and/or the updated provided output torque is then based on the desired output torque.

In one example of the method the steps b)-e), or b)-e) and f), respectively, are repeated. The temperature in the electronic component in step c) in a repeated run of said steps b)-e), or b)- e) and f), respectively, is determined based on the updated provided output torque of the electric machine. This is instead of said first provided output torque. This assures that the advantages of the method can be kept for any time period.

In one example, the only time-dependent input quantities in said model are said rotational speed of the output shaft and said provided first and/or updated output torque of the electric machine. This eliminates the need for a temperature sensor in or at the electric machine.

In one example, the method is started when the electric machine is not rotating, or is rotating below a pre-determined rotational speed. Especially under these conditions the method can provide its advantages.

In one example, the first provided output torque is allowed to take a value of above eighty per cent, preferably above ninety per cent of the maximum possible output torque of the electric machine. This especially assures facilitating a moving away of the vehicle from a standstill position.

In one example, the electronic component is at least one transistor. The electric machine may comprising six transistors, or three transistor pairs. Said transistors are preferably comprised in an inverter of the electric machine. Said transistors may be insulated-gate bipolar transistors, IGBT. At least some of the objects are achieved by a system for controlling an output torque of an electric machine in a vehicle. The system comprises a detector arrangement, being arranged for determining a rotational speed of an output shaft of the electric machine. The system further comprises a control unit. The control unit is arranged to control the electric machine so that a first output torque is provided by the electric machine. The control unit is further arranged to determine a temperature in an electronic component of the electric machine based on a model of the temperature, based on said first provided output torque of the electric machine, and based on the determined rotational speed of the output shaft. The control unit is even further arranged to determine a maximum allowable output torque of the electric machine based on the determined temperature in the electronic component. The control unit is yet even further arranged to control the electric machine so that an updated output torque is provided by the electric machine, wherein the updated output torque is based on said determined maximum allowable output torque.

In one embodiment, the system further comprises means for determining a desired output torque of the electric machine. The control unit is further arranged to provide said the and/or the updated output torque of the electric machine based on the desired output torque.

In one embodiment, the detector arrangement is arranged to determine the rotational speed of the output shaft of the electric machine repeatedly. The control unit is arranged to repeatedly determine the temperature in the electronic component of the electric machine based on the model of the temperature, based on the provided updated output torque of the electric machine, and based on the determined rotational speed of the output shaft. The control unit is further arranged to repeatedly determine the maximum allowable output torque of the electric machine based on the determined temperature in the electronic component. The control unit is even further arranged to repeatedly determine the updated output torque which is to be provided by the electric machine based on the maximum allowable output torque of the electric machine.

In one embodiment of the system, the control unit is arranged to determine the temperature in the electronic component based on the determined rotational speed of the output shaft and the provided first and/or updated output torque of the electric machine as the only time- dependent quantities in the model of the temperature.

In one embodiment of the system, the electronic component is at least one transistor. The electric machine may comprising six transistors, or three transistor pairs. Said transistors are preferably comprised in an inverter of the electric machine. Said transistors may be insulated- gate bipolar transistors, IGBT. At least some of the objects are achieved by a vehicle. The vehicle comprises the system according to the present disclosure.

At least some of the objects are achieved by a computer program for controlling an output torque of an electric machine in a vehicle. The computer program comprises program code for causing an electronic control unit or a computer connected to the electronic control unit to perform the steps according to the method of the present disclosure.

At least some of the objects are achieved by a computer program product containing a program code stored on a computer-readable medium for performing method steps according to the method of the present disclosure, when the computer program is run on an electronic control unit or a computer connected to the electronic control unit.

The system, the vehicle, the computer program and the computer program product have corresponding advantages as have been described in connection with the corresponding examples of the method according to this disclosure.

Further advantages of the present invention are described in the following detailed description and/or will arise to a person skilled in the art when performing the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows, in a schematic way, an embodiment of a vehicle according to the present disclosure. Fig. 2 shows, in a schematic way, an embodiment of a system according to the present disclosure.

Fig. 3a shows, in a schematic way, an example of a curve of a prior art system or method for controlling an output torque of an electric machine in a vehicle.

Fig. 3b shows, in a schematic way, an example of a curve of a system or method according to the present disclosure for controlling an output torque of an electric machine in a vehicle.

Fig. 4 shows, in a schematic way, an example of a flowchart of a method according the present disclosure. Fig. 5 shows, in a schematic way, an example of a device which can be used in connection with the present disclosure.

DETAILED DESCRIPTION Throughout this description the terms requested and desired are used interchangeably in relation to output torques. No different meaning is intended.

Fig. 1 shows a side view of a vehicle 100. In the shown example, the vehicle comprises a tractor unit 110 and a trailer unit 112. The vehicle 100 can be a heavy vehicle such as a truck. In one example, no trailer unit is connected to the vehicle 100. The vehicle 100 comprises an electric machine. The vehicle 100 comprises a system 299, se Fig. 2. The system 299 can be arranged in the tractor unit 110.

In one example, the vehicle 100 is a bus. The vehicle 100 can be any kind of vehicle comprising an electric machine as propulsion means. Other examples of vehicles comprising an electric machine are boats, passenger cars, construction vehicles, and locomotives. The innovative method and the innovative system according to one aspect of the invention are also well suited to, for example, systems which comprise industrial engines and/or engine- powered industrial robots.

The term "link" refers herein to a communication link which may be a physical connection such as an opto-electronic communication line, or a non-physical connection such as a wireless connection, e.g. a radio link or microwave link.

Fig. 2 shows, in a schematic way, an embodiment of a system 299 according to the present disclosure. The system 299 comprises an electric machine 210. In one example, the system 299 is part of a hybrid system. In one example, the system 299 works without any connection to a combustion engine, i.e. purely electrical. The electric machine 210 is arranged to propel the vehicle 100. The electric machine 210 can be a synchronous or an asynchronous motor.

The system 299 comprises an output shaft 240 of the electric machine 210. The output shaft 240 is arranged to transmit an output torque from the electric machine 210. Said output torque is in one example transmitted to ground contacting members of the vehicle (not shown in the figure). Examples of ground contacting members are wheels. Said transmission of the output torque from the electric machine 210 to ground contacting members can comprise further components, such as differentials, further shafts and/or axes, or the like. In one example the output shaft 240 is directly coupled to the electric machine 210. In one example the output shaft 240 is coupled to the electric machine 210 via intermediate components, such as differential(s), further shaft(s), axis/axes or the like.

The system 299 comprises a detector arrangement 220. Said detector arrangement 220 is arranged for determining a rotational speed of the output shaft 240 of the electric machine 210. In one example, said detector arrangement 220 comprises a rotational speed sensor. Since it is known in the art how to detect rotational speeds of a shaft, this is not discussed here any further.

The electric machine 210 comprises at least two electronic components 231, 232. Such electronic components may be transistors. In one example, the electric machine comprises six transistors, or three transistor pairs. Said transistors are preferably comprised in an inverter of the electric machine. In one example, said transistors are insulated-gate bipolar transistors, IGBT. A critical temperature is assigned to the electronic components. Above said critical temperature it is assumed that the lifetime and/or the performance of the electronic components is affected in an unacceptable way. Said critical temperature is preferably a predetermined temperature. The system 299 further comprises a first control unit 200. Said first control unit 200 is arranged for communication with said electric machine 210 via a link L210. Said first control unit 200 is arranged to receive information from said electric machine 210. Said first control unit 200 is arranged to control said electric machine 210. In one example, said control unit 200 is arranged to determine an output torque which should be provided by said electric machine. In one example, said control unit 200 is arranged to control the electric machine 210 so that said output torque is provided. In one example, said control comprises determining controlling a current supply to the electric machine. Said current supply originates in one example from a battery or another kind of energy storage (not shown in the figure). Said control comprises in one examples opening and closing of said at least two transistors 231, 232. Said first control unit 200 is arranged for communication with said detector arrangement 220 via a link L220. Said first control unit 200 is arranged to receive information from said detector arrangement 220. In one example, said received information relates to the rotational speed of the output shaft 240. The system 299 further comprises means 250 for determining a desired output torque of the electric machine. Said means for determining a desired output torque of the electric machine can comprise a gas pedal. Said means 250 can comprise a speed control apparatus. Said means 250 can comprise an interval determining system of the vehicle, such as a system determining interval(s) to objects in the driving direction of the vehicle. In one example, the control unit 200 is arranged to determine an output torque which is to be provided by the electric machine based on said desired output torque.

The first control unit 200 is arranged to determine a temperature in an electronic component of the electric machine based on a model of the temperature. The first control unit 200 is arranged to determine a temperature in an electronic component of the electric machine based on a provided output torque of the electric machine. The first control unit 200 is arranged to determine a temperature in an electronic component of the electric machine based on the determined rotational speed of the output shaft. The first control unit 200 is further arranged to determine a maximum allowable output torque of the electric machine based on the determined temperature in the electronic component. The first control unit 200 is even further arranged to control the electric machine so that an updated output torque is provided by the electric machine, wherein the updated output torque is based on said determined maximum allowable output torque. In one example, said first control unit 200 is arranged to perform any of the steps of the method described in relation to Fig. 4. This can be done either alone, or on combination with other elements, such as said detector arrangement 220. A second control unit 205 is arranged for communication with the first control unit 200 via a link L205 and may be detachably connected to it. It may be a control unit external to the vehicle 100. It may be adapted to conducting the innovative method steps according to the invention. The second control unit 205 may be arranged to perform the inventive method steps according to the invention. It may be used to cross-load software to the first control unit 200, particularly software for conducting the innovative method. It may alternatively be arranged for communication with the first control unit 200 via an internal network on board the vehicle. It may be adapted to performing substantially the same functions as the first control unit 200. The innovative method may be conducted by the first control unit 200 or the second control unit 205, or by both of them. In one example, said system 299 comprises a temperature sensor for determining a temperature of the environment (not shown in the figure). Said temperature sensor is preferably at a distance to the electric machine so that the temperature of the electric machine does not affect the temperature sensor. The control unit 200 is in one example arranged to receive data relating to the temperature of the environment from said temperature sensor. Fig. 3a shows, in a schematic way, an example of a curve of a prior art system or method for controlling an output torque of an electric machine in a vehicle. The curve shows the provided output torque τ of an electric machine as a function of the rotational speed n of an output shaft of the electric machine. For simplicity, Fig. 3a assumes that no clutch or the like is present and that the rotational speed of the output shaft is direct proportional to the rotational speed of the motor and to the speed of the vehicle. Thus, when the rotational speed of the output shaft is zero, the motor and the vehicle is at a standstill, and if the output shaft is rotating, the motor rotates as well and the vehicle moves. Adding clutches or the like would make the situation more complex, but would not change the underlying principle discussed here. At a standstill of the vehicle, i.e. at n=0, an initial output torque το is allowed. In this example, said initial output torque το is half the maximum output torque τι of the electric machine. The initial output torque το is chosen so that the temperature in the electronic components of the electric machine will under basically no circumstances experience a temperature which is so high that it impairs on the lifetime of the electronic components in an unaccepted way. This temperature is denoted critical temperature. A lower allowed output torque το requires less current through the electronic components, thus limiting the temperature arising from heat losses due to current resistance at the electronic component. In case the electric machine is at a standstill, all current might go through the same electronic component. Therefore, and since it is unknown how long the vehicle will remain at a standstill, this initial torque το is usually considerably lower than the maximum output torque τι, such as being half of the maximum output torque as in this case. As the vehicle starts moving, and thus the rotational speed of the output shaft and the electric machine are increasing, the provided current for the electric machine will distribute over several of the electronic components. Thus, not all heat losses in the electric machine will higher the temperature at the same electronic component. As a consequence, the allowed torque can increase. This continues until a first rotational speed ni of the output shaft, where the maximum torque το is allowed. At said first rotational speed of the output shaft, the motor will rotate fast enough, to distribute any heat losses due to current among the electronic components in such a way, that none of these electronic components receives a too high temperature, although the maximum current is provided. In the shown example, the increase of the torque between the initial torque το and the maximum torque τι is linear in the rotational speed of the output shaft. Other relations are, however, also possible.

The maximum output torque τι is then allowed above said first rotational speed ni and below a second rotational speed n₂. Above said second rotational speed n₂ of the output shaft, the output torque will be lowered again. This is indicated by the dashed line in Fig. 3a. The exact form of the dashed line and the reasons for the lowering above the second rotational speed is of no importance in connection to the present disclosure.

What has been discussed so far in relation to the torque relates to a maximum allowed torque as a function of the rotational speed. The torque which is provided in reality can be lower than the maximum allowed torque, for example if a lower torque is requested. It can, however, not be higher, although a higher torque might be requested.

The prior art solution according to Fig. 3a has at least two disadvantages. One disadvantage is that the initial output torque το might be too low to allow a moving of the vehicle from a standstill position. This might especially be problematic when trying to move the vehicle uphill from a standstill position and/or when the vehicle is loaded with heavy weight.

Another disadvantage has been turned to be that the vehicle can experience oscillations in the powertrain which can be uncomfortable to the driver. This is due to the fact that the curve between n=0 and said first rotational speed of the output shaft gives a positive feedback, i.e. an increased output torque will increase the rotational speed, which in itself will increase the output torque again. On the other hand, a decreased output torque will decrease the rotational speed, which in itself will decrease the output torque again. This behaviour has to be counteracted by the driver of the vehicle or a control unit of the vehicle. This

counteracting introduces said oscillations, which, besides being uncomfortable, can, in an extreme case, prevent a driving of the vehicle. However, the oscillations will be present even without the counteracting. This disadvantage is especially announced in heavy vehicles without a connected combustion engine, since a combustion engine due to friction and/or due to its inertia usually counteracts the effects of the positive feedback.

Fig. 3b shows, in a schematic way, an example of a curve of a system or method according to the present disclosure for controlling an output torque of an electric machine in a vehicle. Fig. 3b is not in the same scale as Fig. 3a. The behaviour of the prior art solution according to Fig. 3a is depicted with continuous lines in Fig. 3b. This facilitates a comparison between Fig. 3a and Fig. 3b despite the different scales. The values τ¾ τι and ni are the same in Fig. 3a and Fig. 3b. In the example of Fig. 3b the maximum allowable output torque for rotational speeds above ni coincides with the prior art solution of Fig. 3a. For values of the rotation speed between no and ni, the maximum allowable output torque is depicted by the dashed line. The dash-dotted lines 300, 310, 320, 330, and 340 depict alternative versions of the maximum allowable output torque. This will be explained in more detail further below.

For n=0, the maximum output torque τι is available. This has the advantage that it will be easier to move the vehicle from a standstill, especially when driving in an uphill direction, or when driving with heavy load. In some situations, this might allow a moving away, whereas in the prior art a moving away might have been impossible due to the available output torque only being half of the maximum output torque. In one example, said maximum output torque τι is available in the whole range between n=0 and ni. This is indicated by the dashed line. This has the further advantage that no positive feedback is present between the allowed torque and the rotational speed. Thus no oscillations are introduced in the powertrain due to positive feedback. The maximum output torque τι can be available in the whole range between n=0 and ni especially when the rotational speed of the output shaft is increasing to a value above ni during such a time period that the temperature in the electronic components will not rise above a critical temperature during that time period. This will be described in more detail later. Another example where the maximum output torque τι can be available in the whole range between n=0 and ni is when the desired output torque is below the allowed output torque according to the prior art solution. In this case it is assured that the temperature in the electronic components will not rise above a critical temperature, even if the rotational speed will be below ni for a long time period.

In case it is determined that the temperature in the electronic components might rise above the critical temperature, the allowable torque is lowered. Four such examples are depicted in Fig. 3b. If it is determined at a rotational speed n_a, n , n_c, or nd, that the temperature in the electronic components might rise above the critical temperature, the allowed output torque is lowered according to the curves 300, 310, 320, or 330, respectively. Once the output torque is in line with the prior art solution, it will follow the prior art solution until the rotational speed ni is reached. This is indicated by line 340. The further description will focus on an embodiment where the maximum allowable output torque will follow the prior art solution. In an alternative solution, however, the allowed output torque is allowed to stay above the prior art solution. After determining the temperature in an electronic component, it might be concluded that a value for the allowed maximum torque which is higher than the prior art solution still might be allowable, although being not the maximum possible output torque. Likewise, it might be concluded that only a value lower than the prior art solution might be the maximum allowable output torque. These two situations are not shown in the figure. It should, however, be understood that the present invention by no means is limited to follow the prior art curve. Obtaining values for the maximum allowable output torque above or below the prior art solution are alternative solutions throughout the whole description.

The exact shape of the lines 300-330 can differ. In one example, a line is linear, as with line 330. In one example, a line is not linear, as with line 300-320. In one example it is determined at a rotational speed n_a, n , n_c, or nd that the temperature in the electronic components can rise above said critical temperature when the rotational speed of the output shaft was below ni for a long enough time period. In one example it is determined at a rotational speed n_a, nb, n_c, or nd that the temperature in the electronic components can rise above said critical temperature when the rotational speed of the output shaft was below ni for a long enough time period and the desired output torque was above the allowable output torque according to the prior art solution. In the depicted examples of the lines 300-330, the output torque is lowered from the maximum output torque to an output torque according to the prior art solution once it is determined that the temperature in the electronic components can rise above the critical temperature. The rotational speeds n_a, n , n_c, or nd are only examples. In principle, the determination that the temperature in the electronic components can rise above said critical temperature can be done at any rotational speed below ni. In other words, when determining the maximum allowable torque, this is independent of the rotational speed. The situations depicted by the lines 300-340 have the advantage that the maximum output torque is available when the vehicle is moving from a standstill. The output torque is only lowered when the vehicle is in a move. Since starting moving a vehicle from a standstill might require a higher output torque than keeping a vehicle moving and/or since the slope of the road can change, the situations depicted by lines 300-340 might ease moving of the vehicle from a standstill as well. Oscillations in the powertrain are lowered in the situations depicted by lines 300-340. This is due to the fact that the area of the rotational speed where a positive feedback occurs, corresponding to line 340, is reduced as compared to the prior art solution.

Fig. 4 shows, in a schematic way, an example of a flowchart of a method according the present disclosure. In one example the method starts with step a). In step a) a first output torque of the electric machine is provided. Said first output torque can be determined by a control unit, such as the first control unit 200. Said output torque is preferably provided to an output shaft. This can be either directly, or indirectly via other component(s). In one example, step f) is performed before step a). Step f) will be described in more detail further down. Said first provided output can be based on a determined desired output torque of the electric machine. In one example, said first provided output torque is allowed to take a value of above eighty per cent, preferably above ninety per cent of the maximum possible output torque of the electric machine. In one example, said first provided output torque is allowed to take a value corresponding to the maximum possible output torque of the electric machine. The method continues with step b). In step b) a rotational speed of an output shaft of the electric machine is determined. In one example this is done with the help of a detector arrangement, such as detector arrangement 220. In one example, said determination is done by said detector arrangement. In one example, said determination is done by a control unit based on data from a detector arrangement. The method continues with step c). In step c) a temperature in an electronic component of the electric machine is determined based on a model for said temperature in said electronic component of the electric machine. Said determination is also based on said first provided output torque and said determined rotational speed of the output shaft. In one example, said model for the temperature is provided by the expression

C_pT = c_fr(n)T² + Q(T_b - T).

In this model, T denotes the temperature in the electronic component, t denotes the temperature change over time. C_p, C and Q are pre-determined constants, τ denotes the provided output torque, and n the rotational speed of the output shaft. r(n) is a function of n. T_b denotes the temperature of the environment.

In one example, the electronic component is a transistor and C_p denotes the heat capacity of a transistor. In one example, c denotes a constant for the heat losses in a transistor. In one example, Q denotes the heat transfer from a transistor to the environment. In one example, the temperature of the environment is determined by a temperature sensor. The temperature sensor is in one example at the vehicle. In one example, the temperature of the environment is determined by a weather data provider and transmitted to the vehicle. In one example, the temperature of the environment is assumed being a pre-determined constant.

In one example, the electronic component is a transistor and the function r(n) equals 1 at n=0. This is due to the fact that the current in the electric machine will only heat up one transistor in that case. In one example, the function r(n) equals 1/3 at the lowest value of the rotational speed where the maximum output torque can always be allowed without risking a rise of the temperature in a transistor above the critical temperature. In the example of Fig. 3a and Fig. 3b this corresponds to n=ni. The value 1/3 applies in case six transistors are present in the inverter of the electric machine. In case a number x of transistor pairs will be present, the function r(n) will equal 1/x at the lowest value of the rotational speed where the maximum output torque can always be allowed without risking a rise of the temperature in a transistor above the critical temperature. In one example, the value of the function r(n) falls linearly between n=0 and said the lowest value of the rotational speed where the maximum output torque can always be allowed without risking a rise of the temperature in a transistor above the critical temperature. However, the model above is also applicable for electronic components other than transistors. Such electronic components may be thyristors, diodes, capcitors, and the like.

In a preferred example, the electronic component is a transistor and the rotational speed of the output shaft and the provided output torque are the only time-dependent input quantities to the model. The determined temperature in the transistor is an output value of the value. The method continues with step d).

In step d) a maximum allowable output torque of the electric machine is determined based on said determined temperature in the electronic component, such as said transistor. The determined maximum allowable output torque is preferably determined so that there is no risk that the temperature in the transistor will rise above the critical temperature. In one example, if it is determined that the temperature in the transistor is close to the critical temperature, the allowed output torque will be limited to the curve according to the prior art solution. In one example, if it is determined that the temperature in the transistor is close to the critical temperature, the allowed output torque will be limited to a value lower than the prior art solution. In one example, if it is determined that the temperature in the transistor is substantially lower than the critical temperature, the allowed output torque will be the maximum possible output torque. The allowed output torque can also take any other value between these two cases. This is, for example, the case if the determined temperature is lower than the critical temperature, but risks to come close to it. The method continues with step e). In step e) an updated output torque is provided based on said determined maximum allowable output torque. Said updated output torque can differ from the first provided output torque of the electric machine. In one example, the provided updated output torque is based on a determined desired output torque of the electric machine. Said determined desired output torque can be determined as described in relation to step f). In one example, if the desired output torque is lower than the maximum allowable output torque, the desired output torque will be provided. In one example, if the desired output torque is greater than the maximum allowable output torque, the maximum allowable output torque will be provided.

In one example, the method ends after step e). In an optional step f) a desired output torque of the electric machine is determined. This is in one example done via analysing an input from the driver of the vehicle. This input could be an action on an input means, such as an action of a foot or a hand on a gas pedal, a speed and/or acceleration regulator, for example a lever, or the like. The desired output torque can also be determined based on information from a speed control system, or any other control system of the vehicle.

In one example, the method is repeated after step e), wherein the repetition starts at step b). During the repetition, the temperature in the electronic component, such as the transistor in step c) is determined based on said updated provided output torque of the electric machine. In one example, the method is started when the electric machine is not rotating, or is rotating below a pre-determined rotational speed.

In one example of the method, the term providing an output or determining an output relates to providing a current for the electronic components or determining a current for the electronic components, respectively. In one example, said current is proportional to the output torque. In one example of the method, said temperature in the electronic component does not necessarily correspond to the real temperature in the electronic component, but can be a virtual temperature of the electronic component. It is in principal enough that said virtual temperature changes equally fast as the real temperature. Said critical temperature value can then be defined for the virtual temperature. Using virtual temperatures has the effect that the pre- determined constants in step c) do not need to correspond to real physical quantities, but can be virtual as well. This facilitates an easy generation of the model since, as an example, said predetermined constants can be determined on empirical data or through mathematical modelling and do not necessarily have to be determined based on physical measurements.

The above described method does not necessarily have to be performed in the presented order. The steps a)-f) can be performed in a different order, or in parallel.

Figure 5 is a diagram of one version of a device 500. The control units 200 and 205 described with reference to Figure 2 may in one version comprise the device 500. The device 500 comprises a non-volatile memory 520, a data processing unit 510 and a read/write memory 550. The non-volatile memory 520 has a first memory element 530 in which a computer program, e.g. an operating system, is stored for controlling the function of the device 500. The device 500 further comprises a bus controller, a serial communication port, I/O means, an A/D converter, a time and date input and transfer unit, an event counter and an interruption controller (not depicted). The non-volatile memory 520 has also a second memory element 540. The computer program comprises routines for controlling an output torque of an electric machine in a vehicle.

The computer program P may comprise routines for providing a first output torque of the electric machine. This may at least partly be performed by means of said first control unit 200.

The computer program P may comprise routines for determining a rotational speed of an output shaft of the electric machine. This may at least partly be performed by means of said first control unit 200 and/or said detector arrangement 220. Said determined rotational speed of the electric machine may be stored in said non-volatile memory 520.

The computer program P may comprise routines for determining a temperature in an electronic component of the electric machine based on a model for said temperature in said electronic component of the electric machine and based on said first provided output torque and said determined rotational speed of the output shaft. This may at least partly be performed by means of said first control unit 200. Said determined temperature may be stored in said nonvolatile memory 520.

The computer program P may comprise routines for determining a maximum allowable output torque of the electric machine based on said determined temperature in said electronic component. This may at least partly be performed by means of said first control unit 200. Said determined maximum allowable output torque may be stored in said non-volatile memory 520.

The computer program P may comprise routines for providing an updated output torque based on said determined maximum allowable output torque. This may at least partly be performed by means of said first control unit 200.

The computer program P may comprise routines for determining a desired output torque of the electric machine. This may at least partly be performed by means of said first control unit 200. The program P may be stored in an executable form or in compressed form in a memory 560 and/or in a read/write memory 550.

Where it is stated that the data processing unit 510 performs a certain function, it means that it conducts a certain part of the program which is stored in the memory 560 or a certain part of the program which is stored in the read/write memory 550.

The data processing device 510 can communicate with a data port 599 via a data bus 515. The non-volatile memory 520 is intended for communication with the data processing unit 510 via a data bus 512. The separate memory 560 is intended to communicate with the data processing unit via a data bus 511. The read/write memory 550 is arranged to communicate with the data processing unit 510 via a data bus 514. The links L205, L210, L250-255, and L270, for example, may be connected to the data port 599 (see Figure 2).

When data are received on the data port 599, they can be stored temporarily in the second memory element 540. When input data received have been temporarily stored, the data processing unit 510 can be prepared to conduct code execution as described above. Parts of the methods herein described may be conducted by the device 500 by means of the data processing unit 510 which runs the program stored in the memory 560 or the read/write memory 550. When the device 500 runs the program, methods herein described are executed.

The foregoing description of the preferred embodiments of the present invention is provided for illustrative and descriptive purposes. It is neither intended to be exhaustive, nor to limit the invention to the variants described. Many modifications and variations will obviously suggest themselves to one skilled in the art. The embodiments have been chosen and described in order to best explain the principles of the invention and their practical applications and thereby make it possible for one skilled in the art to understand the invention for different embodiments and with the various modifications appropriate to the intended use.

Claims

1. A method for controlling an output torque of an electric machine in a vehicle, the method comprising the steps of:

a) provide a first output torque of the electric machine;

b) determine a rotational speed of an output shaft of the electric machine;

c) determine a temperature in an electronic component of the electric machine based on a model for said temperature in said electronic component of the electric machine and based on said first provided output torque and said determined rotational speed of the output shaft;

d) determine a maximum allowable output torque of the electric machine based on said determined temperature in said electronic component; and e) provide an updated output torque based on said determined maximum allowable output torque.

2. The method according to claim 1, further comprising the step of:

f) determine a desired output torque of the electric machine,

wherein said first provided output torque and/or said updated provided output torque is based on said desired output torque.

3. The method according to anyone of the previous claims, wherein the steps b)-e), or b)- e) and f), respectively, are repeated, and wherein the temperature in the electronic component in step c) in a repeated run of said steps b)-e), or b)-e) and f), respectively, is determined based on said updated provided output torque of the electric machine instead of said first provided output torque.

4. The method according to anyone of the previous claims, wherein the only time- dependent input quantities in said model are said rotational speed of the output shaft and said provided first and/or updated output torque of the electric machine.

5. The method according to anyone of the previous claims, wherein the method is started when the output shaft of the electric machine is not rotating, or is rotating below a predetermined rotational speed.

6. The method according to anyone of the previous claims, wherein said first provided output torque is allowed to take a value of above eighty per cent, preferably above ninety per cent of the maximum possible output torque of the electric machine.

7. The method according to anyone of the previous claims, wherein the electronic component is at least one transistor.

8. A system for controlling an output torque of an electric machine in a vehicle, the system comprising:

- a detector arrangement, being arranged for determining a rotational speed of an output shaft of the electric machine;

- a control unit, being arranged to control the electric machine so that a first output torque is provided by the electric machine, the control unit further being arranged to determine a temperature in an electronic component of the electric machine based on a model of the temperature, based on said first provided output torque of the electric machine, and based on the determined rotational speed of the output shaft, the control unit further being arranged to determine a maximum allowable output torque of the electric machine based on the determined temperature in the electronic component, the control unit even further being arranged to control the electric machine so that an updated output torque is provided by the electric machine, wherein the updated output torque is based on said determined maximum allowable output torque.

9. The system according to claim 8, further comprising means for determining a desired output torque of the electric machine, wherein the control unit is further arranged to provide said first and/or said updated output torque of the electric machine based on said desired output torque.

10. The system according to anyone of claim 8 or 9, wherein the detector arrangement is arranged to determine the rotational speed of the output shaft of the electric machine repeatedly, and wherein the control unit is arranged to repeatedly determine the temperature in the electronic component of the electric machine based on the model of the temperature, based on the provided updated output torque of the electric machine, and based on the determined rotational speed of the output shaft, the control unit further being arranged to repeatedly determine the maximum allowable output torque of the electric machine based on the determined temperature in the electronic component, and wherein the control unit further is arranged to repeatedly determine the updated output torque which is to be provided by the electric machine based on said maximum allowable output torque of the electric machine.

11. The system according to any of claim 8-10, wherein the control unit is arranged to determine said temperature in the electronic component based on the determined rotational speed of the output shaft and the provided first and/or updated output torque of the electric machine as the only time-dependent quantities in said model of the temperature.

12. The system according to any of claim 8-11, wherein the electronic component is at least one transistor.

13. A vehicle, comprising the system according to any of claim 8-11.

14. A computer program (P) for controlling an output torque of an electric machine in a vehicle, wherein said computer program (P) comprises program code for causing an electronic control unit (200; 500) or a computer (205; 500) connected to the electronic control unit (200; 500) to perform the steps according to anyone of the claims 1-7.

15. A computer program product containing a program code stored on a computer-readable medium for performing method steps according to anyone of claims 1-7, when said computer program is run on an electronic control unit (200; 500) or a computer (205; 500) connected to the electronic control unit (200; 500).