A Method for regulating a power-splitting transmission

Technical Field

The present invention relates to a method of controlling a powr-splitting transmission. At least certain embodiments of the invention relate to transmissions which regulate the transmission ratio between the input and output of a transmission in a continuously variable manner by means of a combination of mechanical transmission elements and the at least partially electrical transmission of the power flow .

Background

One example of a transmission of this type is described in document DE 103 14 234 B3. This transmission is a 4- shaft power-split transmission which has an internal combustion engine on the drive side and whose output is coupled to the drive side via a planetary gear. In this case, the internal combustion engine drives a planet carrier on which a three-stage stepped planet gear is arranged in a rotatable manner. A first stage drives the sun gear of the transmission output, a second stage of the planet carrier engages directly in a sun gear which is coupled to a first electrical machine, and a third stage of the stepped planet gear engages, via intermediate gears, in the sun gear which is connected to a second electrical machine. The transmission ratio of the transmission is defined by controlling the electrical machines. A transmission ratio between the internal combustion engine and the output is given by defining the rotational speed of an electrical machine and the fixed rotational speed of the output. This transmission ratio can be adjusted in a continuously variable manner by selecting the electrical machine rotational speed. If neither of the electrical machines is driven or braked, no supporting torque is generated

by the electrical machines. The single moment of resistance on the stepped planet gear therefore originates from the output, and this corresponds to an idling function without power transmission from the internal combustion engine. When a respective one of the electrical machines is at a standstill, a static transmission ratio is established in each case. As a result, no power is transmitted in an electrical manner. At all other operating points of the transmission, one of the electrical machines acts as a generator and one acts as a motor in each case. Therefore, no power flows in an electrical manner. The transmission is also suitable for a hybrid vehicle if an energy storage means is arranged in the electrical power flow. Without an energy storage means, the transmission is operated in an electrically compensated manner, with the emitted power of an electrical machine being equal to the consumed power of the respectively other electrical machine. In terms of its transmission ratio, the transmission can be electrically adjusted in a continuously variable manner and on account of an energy storage means in the electrical path can operate with energy gain or so as to emit electrical energy from the storage means, so that the transmission can fulfil the functions of recuperation and boosting during acceleration for use in a hybrid vehicle. Purely electrical driving is likewise possible when the electrical machines are fed from the energy storage means.

Control of a transmission of this type is complicated on account of the complex relationships. Separate regulation of the internal combustion engine and the electrical machines which control the transmission ratio is possible. Separate regulation of the two electrical machines and the internal combustion engine, which is controlled by means of the prespecified setpoint values of a controller, is described in DE 103

33 931 Al. In the illustrated power-split transmission with a manual transmission connected downstream, a controller outputs a prespecified setpoint value for the torque and rotational speed of the internal combustion engine for both electrical machines. The setpoint value for the respective drive is converted using separate control loops. In this case, the electrical machines are separately actuated by a single regulator in each case, with the rotational speed being regulated by forming a regulation difference in relation to the setpoint value by recording the actual rotational speed value of the electrical machine for each of the electrical machines individually. In this case, control of the electrical machines is based on separate control loops, with the control being coordinated in the controller by means of the prespecified setpoint value. The high number of existing degrees of freedom of the transmission means a very complex regulation system is required.

An object of the invention is to address such drawbacks. Accordingly, at least certain embodiments of the invention provide a simple regulation algorithm which permits the transmission ratio of a power- splitting transmission to be flexibly controlled by means of actuating at least two electrical machines.

Summary

According to one aspect of the present invention, there is provided a method as defined in claim 1 of the appended claims.

In embodiments, a power-based approach may be used, with the actuating variables for the electrical machines being calculated in a coupled manner in such a way that the rotational speed and/or the torque of the two electrical machines are calculated together on the

- A - basis of an electrical machine which is defined as the leading electrical machine.

Electrical and mechanical actuating variable restrictions for calculating the setpoint values are taken into account in a simple manner.

In embodiments, the method for controlling a power- splitting transmission is based on a design in which an internal combustion engine and at least two electrical machines which are mechanically coupled by means of transmission stages are present, with the electrical machine being arranged in the power flow of the transmission in such a way that the transmission ratio of the transmission is determined by the torque which is applied by the respective electrical machine and/or its rotational speed, and the electrical machines are coupled to one another by means of the transmission elements. The transmission ratio of the transmission is determined by defining the rotational speed of an electrical machine and the rotational speed of the output. In this case, the transmission may advantageously be a four-shaft power-split transmission with a technical design similar to the transmission described in DE 103 14 234 B3. The disclosure content of DE 103 14 234 B3 is explicitly incorporated here. However, the control method can also be applied to other power-split transmissions with an electrical and a mechanical transmission path of said design. The described transmission is controlled in such a way that one of the electrical machines is selected as the leading electrical machine for calculating the setpoint values for controlling the electrical machines on the basis of a current operating parameter of the electrical machines, for example the rotational speed. The rotational speed signals of the electrical machines provide information, which can be easily measured, about the operating state of the machines. In ranges of low rotational speeds, electrical machines experience a

severe change in efficiency, as a result of which regulation systems incorporating electrical machines operating at low rotational speeds are highly sensitive. The machines can change their direction of rotation during operation of the system according to embodiments of the invention. If such a zero crossing occurs in one electrical machine, the other electrical machine has, as a matter of principle, a higher rotational speed. At such operating points, it has proven expedient to use the machine with the higher rotational speed as the leading electrical machine, since the system therefore reacts less sensitively to changes in the actuating variables. The rotational speed signal is accordingly particularly suitable for selecting the leading electrical machine. The driver's request is the determining factor for transmission control. The rotational speed of the internal combustion engine is ascertained on the basis of the torque requested by the driver .Torque is for example determined from the position of the pedal value transmitter, using fuel consumption and/or power criteria. A best operating point can be selected, for example from the fuel consumption characteristic map for the engine, and the internal combustion engine is then actuated at the rotational speed which is optimum for fuel consumption. A setpoint torque value and/or a setpoint rotational speed value for the electrical machine which is defined as the leading electrical machine are/is formed using the predefined rotational speed of the internal combustion engine, with the setpoint value for the following electrical machine being ascertained by means of a power analysis, taking into account the torque which can be supplied by the electrical machines at the current rotational speed operating point.

In the regulation method, starting from the leading electrical machine, the power which is emitted or consumed by said leading electrical machine is set

equal to the power which is consumed or emitted by the following electrical machine for electrically compensated operation. In this case, the magnitudes of the power values are identical, but have been calculated with different algebraic signs.

In embodiments, the boosting, assistance, recuperation and charging operating modes are advantageously realized by adjusting the power equilibrium of the two electrical machines. In the operating modes boosting (assisting the internal combustion engine at full load) and assistance (assisting the internal combustion engine in operating ranges outside full load) , energy is drawn from the energy storage means, whereas energy is supplied to the energy storage means during recuperation (recovery of braking energy) and during charging of the energy storage means . Setpoint value control for these operating ranges can be realized in a simple manner by adjusting the power equilibrium. If the energy storage means is to be charged, the power equilibrium of the electrical machines is adjusted such that a positive power excess is generated. In this case, the electrical machine which is operated as a generator generates more power than is consumed by the electrical machine which is operated as a motor. This produces a generator torque which leads to a change in the transmission ratio. The rotational speed regulator for the internal combustion engine attempts to reduce this regulation difference and, depending on the application, increases the throttle valve angle or the injection quantity. This is equivalent to increasing the load point of the internal combustion engine. The excess energy is then supplied to the energy storage means . In the reverse case in which an additional drive torque is to be generated, the power consumption of the following electrical machine is set higher than the power which is generated by the leading electrical

machine. In this case, the additional energy is drawn from the energy storage means.

In a simple embodiment of the selection algorithm of the leading electrical machine, this electrical machine is determined on the basis of the rotational speed signal, with the electrical machine with the higher rotational speed always being selected as the leading electrical machine. This therefore prevents an electrical machine forming the basis for the calculation of the setpoint values in the zero crossing of the rotational speed in the event of the direction of rotation being reversed. In a development of the method, a changeover can be made between the leading electrical machine and the following electrical machine when the leading electrical machine reaches a lower rotational speed threshold and the rotational speed of the following electrical machine is above this threshold. The changeover is therefore made only when the leading electrical machine approaches the rotational speed zero crossing.

The method according to embodiments of the invention advantageously includes taking into account the electrical and mechanical limits of the electrical machines. The mechanical power, which is to be emitted or consumed, of the leading electrical machine is calculated on the basis of the actual rotational speed value and the setpoint torque value of said leading electrical machine and converted into an electrical power, which is to be emitted or consumed, at the efficiency of the electrical machine. The electrical power is limited to its maximum value on the basis of a threshold value of the maximum electrical power at the momentarily applicable rotational speed when the threshold value is exceeded. The electrical power, which is to be consumed or emitted, of the following electrical machine is then set equal to the electrical

power, which is to be emitted or consumed, of the leading electrical machine for the operating mode of electrically compensated operation without energy- storage These electrical powers are identical in magnitude but have a different algebraic sign, and the torque which is to be emitted is calculated from the thus ascertained setpoint value of the electrical power and the actual rotational speed value of the following electrical machine, with a check being made, on the basis of the electrical machine-specifically defined maximum mechanical torque, taking into account the actual rotational speed of the electrical machine, as to whether the following electrical machine can convert the required torque, and the torque is limited to the maximum torque when the limit value is exceeded. The maximum torque is then converted with the actual rotational speed value of the following electrical machine into the required electrical power of the following electrical machine. The thus calculated actuating variable for the electrical power is, with an inverse algebraic sign (identical in magnitude) , also predefined as a setpoint value of the electrical power of the leading electrical machine. Therefore, the invention has the advantage that the actuating variable restrictions of the electrical machines are taken into account when prespecifying the setpoint value. The calculation path over the two electrical machines, which regards the leading electrical machine as the starting point and calculates back to the leading electrical machine again with the actuating variable restriction of this and the following electrical machine, ensures that the two electrical machines can convert the required power which is to be emitted or consumed. Electrically compensated operation is realized at the same time.

In one development, the torque, which is to be emitted or consumed, of the respective electrical machine is

calculated on the basis of the electrical powers of the electrical machines together with the respective actual value of the rotational speed, and adjusted by a suitable subordinate regulation means.

According to another aspect of the invention, there is provided processing means programmed and operable to control the power-splitting transmission in accordance with the method defined in claim 1.

According to another aspect of the invention, there is provided a record carrier having thereon or threin a record indicative of instructions executable by processing means to control the power-splitting transmission in accordance with the method defined in claim 1.

The record carrier may be physical storage means. The record carrier may be a signal, such as an electrical or electromagnetic signal. The physical storage means may include solid state storage means, such as nonvolatile solid-state storage means, for example, flash memory, EPROM, EEPROM and/or similar means. The physical storage means may include, for example, a magnetic or optical storage disk, such as a harddrive, CD-ROM or DVD-ROM.

Brief Description of the Drawings

Emodiments of the invention are described below by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is an overview of the basic structure of the control method; and

Figure 2 is a detailed illustration of the setpoint value prespecification for the electrical machines.

Specific Description of Certain Exemplary Embodiments

Figure 1 shows the basic structure of a regulation method. On the basis of the driver's request, which is measured by means of the signal of a pedal value transmitter PWG, and the current vehicle speed v_veh, a desired wheel torque Dm_wheel_set is ascertained in a power interpreter 1. At the same time, the rotational speed Dz_Vm_opt, which is ascertained as being optimum from the point of view of fuel consumption, for the internal combustion engine is prespecified by means of a consumption characteristic map. A regulating device, in this case for example a throttle valve regulator 2, is used, on the basis of the respective optimum rotational speed Dz_Vm_opt and the measurement of the actual rotational speed of the internal combustion engine Dz_Vm_act, for adjusting the desired motor rotational speed Dz_Vm__opt which is optimum for consumption, with the position of a throttle valve Pos_Throttle being regulated for this purpose. As an alternative, for example for a diesel engine, the injection quantity could be calculated and correspondingly adjusted here. The pilot control setpoint values for the electrical machines El, E2 are calculated in a pilot control means 4, in parallel to the setpoint value prespecification for the internal combustion engine, on the basis of the transmission-specific constraints, from the desired wheel torque Dm_wheel_set and the rotational speeds of all transmission shafts. Said pilot control setpoint values are the torques Dm_El_v and Dm_E2_v of the electrical machines El, E2 which are to be realized by the pilot control means. Input variables of the pilot control means are the desired wheel torque Dm_wheel_set , the actual rotational speed value of the wheel Dz_wheel_act and of the internal combustion engine Dz_Vm_act and of the electrical machines

Dz_El_act, Dz_E2_act. The setpoint torques of the electrical machines Dm_El_Dr and Dm_E2_Dr are ascertained parallel to the pilot control means in a rotational speed regulator 3 from the actual rotational speed of the internal combustion engine Dz_Vm_act and the engine rotational speed Dz_Vm_opt which is optimum for consumption. In this case, the pilot control means converts the desired wheel torque, whereas the rotational speed regulator compensates for the difference between the setpoint engine rotational speed and the actual engine rotational speed. The torques ascertained by the rotational speed regulator 3 and the pilot control means 4 are added in each case. The setpoint torque values of the electrical machines Dm_El_set, Dm_E2_set are given by adding the torque values from the pilot control means and the rotational speed regulator. (Dm_El_set = Dm_El_Dr + Dm_El_v; Dm_E2_set = Dm_E2_Dr + Dm_E2_v) . The setpoint value of the torque for the respective electrical machine Dm_El_set and Dm_E2_set and the actual rotational speed values of the electrical machines Dz_El_act and Dz_E2_act are the input variables of the electrical machine control system 5 which outputs the required electrical power P_El_req and P_E2_req for electrical machine regulation of a subordinate regulation system for forming the actuating variables. This may be, for example, the frequency of a converter for actuating the electrical machines El, E2. The electrical machine control system 5 is described in detail in Figure 2.

As described in relation to Fig. 1, input variables of the electrical machine control system 5 are the setpoint torque Dm_El_set and Dm_E2_set and the actual rotational speed values of the electrical machines Dz_El_act Dz_E2_act. At its output, the electrical machine control system 5 generates the required electrical powers P_El_req and P_E2_req or, from these, the required torques of the electrical machines

Dm_El_req and Dm_E2_req. Both compensated electrical operation, and also operation in the recuperation, charging, assistance and boosting operating modes, are realized with the aid of the electrical machine control system 5. The starting point for control is a power analysis for the electrical machines in the context of the power-split transmission. One electrical machine El or E2 is selected as the leading electrical machine with the aid of the actual rotational speed signal of the electrical machines Dz_El_act and Dz_E2_act. In a simple embodiment, the leading electrical machine is the electrical machine with the highest rotational speed in each case. Calculation of the setpoint values starts on the basis of the desired torque of the leading electrical machine. If the leading electrical machine is El, the calculation starts from Dm_El_set and the rotational speed Dz_El_act of the leading electrical machine El . The desired electrical power P_El_w is calculated with knowledge about the efficiency phi. The desired power P_El_w is limited on the basis of the rotational speed Dz_El_act and a limit value, which is known specifically for the electrical machine at this rotational speed. On the basis of electrically compensated operation, the electrical power of the following electrical machine has to be of equal magnitude but be provided with a different algebraic sign. (P_E2_w = (-1) x (P_El_w) ) . The desired power of the following electrical machine is therefore determined from this value, which is limited in terms of its possible actuating variable, of the leading electrical machine. The internal torque DM__E2_int of the following electrical machine is calculated from the thus ascertained desired power of the following electrical machine ((-1) x (P_El_w) ) together with its actual rotational speed Dz_E2_act. The torque is limited to the prespecified maximum value at the actual rotational speed Dz_E2_act of the following electrical machine E2 on the basis of an electrical machine-

specific limit value when said limit value is exceeded. The capacity of the second electrical machine E2 at its actual rotational speed is therefore introduced into the calculation of the setpoint values. The actual rotational speed of the electrical machine Dz_E2_act and the efficiency phi are used to calculate the required electrical power consumption P_E2_req which is prespecified as the setpoint value with an inverse algebraic sign compared to the required electrical power P_El_req of the leading electrical machine. The consumed and emitted powers P_E2_req and P_El_req of the electrical machines can be converted by the electrical machines by taking into account the limit values of the respective electrical machines El, E2 at their respective current rotational speeds. The limit values of the two electrical machines are analysed during prespecification of the setpoint values. As an alternative, the required torques Dm_El_req and Dm_E2_req of the electrical machines are ascertained, for torque-based control of the electrical machines, from the required electrical power P_El_req and P_E2_req at the respective actual rotational speeds of the electrical machines, and passed to a subordinate torque regulator for regulating the electrical machines.

If the electrical machine E2 is selected as the leading electrical machine, the setpoint values are calculated in an analogous manner. The calculation differs due to the position of the switches S1-S4 which are switched so that the calculation is carried out analogously to the above-described method for the then leading electrical machine.

In an extension for operation with an energy storage means, the ratio of the required electrical powers P_El_req and P_E2_req, which are identical in magnitude during electrically compensated operation, is adjusted

in a deliberate manner. Discharge of electrical energy to the storage means or supply of energy from the storage means is realized by the then different setpoint value prespecification of P_El_req and P_E2_req. In this case, an energy management system 6 calculates the desired additional electrical power, which is superposed as power contribution P_Add_l/2 on top of the power P_El/2_req which is required for compensated operation, from at least the input variables Dz_El/2_act and the battery voltage U_Batt. In this case, the magnitude of the required additional power is calculated by a superordinate energy management system.

The method may be performed under the control of processing means such a microprocessor in an electronic control unit (ECU) executing an appropriate computer program stored on the ECU or in storage means, such as non-volatile solid state memory, to which the ECU has access.

List of reference symbols

1 Power interpreter

2 Throttle valve regulator

3 Rotational speed regulator

4 Pilot control means

5 Electrical machine control system

6 Energy management system

S_l ...4 Branch points of the calculation

PWG Pedal value transmitter signal v_veh Vehicle speed

Dm_wheel_set Desired wheel torque

Dm_wheel_act Actual value of the wheel torque

Dz_Vm_opt Optimum rotational speed of the internal combustion engine

Dz_Vm_act Actual rotational speed of the internal combustion engine

El, E2 Electrical machines Dz_El/2_act Actual value of the rotational speed of the electrical machine El or E2

Dz_El/2_set Setpoint value of the rotational speed of the electrical machine El or E2 Dm_El/2_v Setpoint torque value of the electrical machines from the pilot control means Dm_El/2_set Setpoint value of the torque of the electrical machine El or E2 at the input of the electrical machine control system

5

P_El/2_w Desired electrical power consumption/emission limited on the basis of the actual rotational speed and the electrical machine-specific capacity

Dm_El/2_int Calculated internal torque limited on the basis of the actual rotational speed and the electrical machine-specific capacity

P_El/2_req Required emitted/consumed electrical power of the electrical machine at the

output of the electrical machine control system

Dm_El/2__req Required torque of the electrical machines at the output of the electrical machine control system

U_Batt Battery voltage

P_Add_l/2 Power contribution

Pos_Throttle Throttle valve position

Dz_wheel_act Actual rotational speed of the wheel phi Efficiency