Abstract:
The disclosure relates to a method for charging an intermediate circuit capacitor in an electric drive unit comprising an electric motor. The intermediate circuit capacitor is charged by an intermediate circuit current that is supplied by a battery. The output voltage of the battery is settable to one or more voltage values. A target value of the intermediate circuit current is determined, and an actual value of the intermediate circuit current is ascertained. The actual value of the intermediate circuit current is then compared with the target value of the intermediate circuit current. An optimal output voltage of the battery is determined on the basis of the comparison of the actual value of the intermediate circuit current with the target value of the intermediate circuit current. Then, the optimal output voltage of the battery is set.

Description:
This application is a 35 U.S.C. § 371 National Stage Application of PCT/EP2011/070350, filed on Nov. 17, 2011, which claims the benefit of priority to Serial No. DE 10 2011 002 608.8, filed on Jan. 13, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a method for charging an intermediate circuit capacitor in an electric drive unit with an electric motor and a control device for implementing the method according to the disclosure. 
     BACKGROUND 
     It would appear that in the future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet requirements set for a respective application in respect of voltage and available power, a high number of battery cells are connected in series. Since the current provided by such a battery needs to flow through all of the battery cells and a battery cell can only conduct a limited current, additional battery cells are often connected in parallel in order to increase the maximum current. This can be provided either by providing a plurality of cell coils within a battery cell housing or by external interconnection of battery cells. However, it is problematic that compensation currents between the battery cells connected in parallel may arise owing to cell capacitances and voltages which are not exactly identical. 
     The basic circuit diagram of a conventional electric drive unit, as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the rotor blade adjustment of wind energy installations, is illustrated in  FIG. 1 . A battery  10  is connected to a DC voltage intermediate circuit, which is buffered by an intermediate circuit capacitor  11 . A pulse-operated inverter  12 , which provides sinusoidal voltages which are phase-shifted with respect to one another for the operation of an electric drive motor  13  via in each case two switchable semiconductor valves and two diodes at three outputs, is connected to the DC voltage intermediate circuit. The capacitance of the intermediate circuit capacitor  11  needs to be high enough for the voltage in the DC voltage intermediate circuit to be stabilized for a period of time in which one of the switchable semiconductor valves is switched on. In a practical application such as an electric vehicle, a high capacitance in the mF range results. 
       FIG. 2  shows the battery  10  shown in  FIG. 1  in a more detailed block circuit diagram. A large number of battery cells are connected in series and optionally additionally in parallel in order to achieve a high output voltage and battery capacity desired for a respective application. A charging and isolating device is connected between the positive pole of the battery cells and a positive battery terminal  14 . Optionally, in addition an isolating device  17  can be connected between the negative pole of the battery cells and a negative battery terminal  15 . The isolating and charging device  16  and the isolating device  17  each comprise a contactor  18  and  19 , respectively, which are provided for isolating the battery cells from the battery terminals  14 ,  15  in order to switch said battery terminals to be voltage-free. Owing to the high DC voltage of the series-connected battery cells, there is otherwise a considerable potential risk for maintenance personnel or the like. In addition, a charging contactor  20  with a charging resistor  21  connected in series with the charging contactor  20  is provided in the charging and isolating device  16 . 
     The charging resistor  21  limits a charging current for the intermediate circuit capacitor  11  if the battery is connected to the DC voltage intermediate circuit. For this purpose, first the contactor  18  is left open and only the charging contactor  20  is closed. If the voltage at the positive battery terminal  14  reaches the voltage of the battery cells, the contactor  18  can be closed and possibly the charging contactor  20  can be opened. 
     The charging contactor  20  and the charging resistor  21  represent significant extra complexity in applications which have a power in the region of a few 10 kW, with this extra complexity being required only for the charging operation of the DC voltage intermediate circuit which lasts a few hundred milliseconds. Said components are not only expensive but are also large and heavy, which is particularly disruptive for the use in mobile applications such as electric motor vehicles. 
     SUMMARY 
     According to the disclosure, a method for charging an intermediate circuit capacitor in an electric drive unit with an electric motor is provided. The intermediate circuit capacitor is charged by an intermediate circuit current, which is provided by a battery, whose output voltage can be adjusted to one of a plurality of voltage values. The available voltage values may also be temporally averaged voltage values which are generated by the use of known modulation methods, for example pulse width modulation. The method is characterized by the fact that, first, a setpoint value of the intermediate circuit current is fixed and an actual value for the intermediate circuit current is determined. Then, the actual value for the intermediate circuit current is compared with the setpoint value for the intermediate circuit current. On the basis of this comparison, an optimum output voltage for the battery is determined, which output voltage is suitable for favorably influencing the development of the intermediate circuit current over time. This optimum output voltage of the battery is finally adjusted. The provided method has the advantage that it manages without any charging switches and charging resistor, which are expensive, large and heavy. 
     It is preferred that the method is implemented using a first controller. A controller continuously compares a signal of a setpoint value with a measured actual value for the controlled variable within a control loop and determines, from the difference between the two variables, which is referred to as the control difference, a manipulated variable which influences a controlled system to the extent that the control difference is minimized. In the present case, the control difference of the first controller is provided by the difference between the actual value and the setpoint value for the intermediate circuit current, while the manipulated variable is provided by the optimum output voltage of the battery. In the specific configuration of the first controller, recourse can be made to the embodiments known from the prior art. For example, the first controller can contain a proportional component, an integrating component and/or a differentiating component of the amplification. The configuration in the form of a two-state controller is also possible. 
     The setpoint value for the intermediate circuit current can be fixed on the basis of a comparison of an actual value for an intermediate circuit voltage which is present at the intermediate circuit capacitor with a setpoint value for the intermediate circuit voltage. In this case, preferably a second controller is used, in which the control difference is provided by the difference between the actual value and the setpoint value for the intermediate circuit voltage and the manipulated variable is provided by the setpoint value for the intermediate circuit current. The second controller can also have any desired configuration in a similar way to the first. The use of the second controller makes it possible for the intermediate circuit voltage to also be freely selectable and for it to be adjusted to different values, for example in different driving situations in an electric vehicle. This can be adjusted continuously by the second controller. 
     It is also preferred that the intermediate circuit capacitor is charged via an inductance. As a result, a smoothing low-pass filter effect is achieved, with the result that the intermediate circuit current is not subjected to any abrupt changes. 
     In a preferred embodiment of the disclosure, the battery comprises at least one battery module string with a plurality of battery modules connected in series. Each battery module comprises at least one battery cell and a coupling unit. The at least one battery cell is connected between a first input and a second input of the coupling unit. The coupling unit is designed to connect the at least one battery cell between a first terminal of the battery module and a second terminal of the battery module in response to a first control signal and to connect the first terminal to the second terminal in response to a second control signal. As a result, the output voltage of the battery is adjustable stepwise. 
     By virtue of the use of the coupling device, battery cells of each battery module can either additively contribute to the output voltage of the battery or can be bridged, with the result that the battery cells of the battery module do not contribute to the output voltage of the battery. By varying the time interval in which a battery module is in one of the two states within a specific period duration, each battery module voltage can be adjusted between zero volt and the maximum module voltage when averaged over time. For this, known modulation methods, such as pulse width modulation, for example, can be used. The output voltage of the battery can thus be adjusted continuously from zero volt (if all of the coupling units are connected in such a way that the battery cells are bridged) up to a maximum output voltage (if all of the coupling units are connected in such a way that the cells of the battery modules additively contribute to the total voltage of the battery). 
     A further aspect of the disclosure relates to a control unit which is designed to determine an actual value for an intermediate circuit current, by means of which an intermediate circuit capacitor in a drive unit with an electric motor is charged. In addition, the control unit is designed to adjust an output voltage of a battery to one of a plurality of voltage values. The control unit is designed to implement the method according to the disclosure. The control unit can be part of a battery, whose output voltage can be adjusted to one of a plurality of voltage values. The battery is preferably a lithium-ion battery. It is preferred here that the battery comprises the described battery modules with coupling units, as a result of which the output voltage of the battery is adjustable stepwise. 
     The control unit can likewise be part of a drive unit with an electric motor. 
     A further aspect of the disclosure relates to a motor vehicle with a drive unit according to the disclosure for driving the motor vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure will be explained in more detail with reference to the drawings and the description below. In the drawings: 
         FIG. 1  shows an electric drive unit in accordance with the prior art, 
         FIG. 2  shows a block circuit diagram of a battery in accordance with the prior art, 
         FIG. 3  shows a coupling unit which can be used in a method according to the disclosure, 
         FIG. 4  shows a first embodiment of the coupling unit, 
         FIG. 5  shows a second embodiment of the coupling unit, 
         FIG. 6  shows the second embodiment of the coupling unit in a simple semiconductor circuit, 
         FIGS. 7 and 8  show two arrangements of the coupling unit in a battery module, 
         FIG. 9  shows the coupling unit illustrated in  FIG. 6  in the arrangement illustrated in  FIG. 7 , 
         FIG. 10  shows an electric drive unit in which the method according to the disclosure can be implemented, and 
         FIG. 11  shows a block circuit diagram of a system in which the method according to the disclosure is implemented. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  shows a coupling unit  30 , which can be used in a method according to the disclosure. The coupling unit  30  has two inputs  31  and  32  and an output  33  and is designed to connect one of the inputs  31  or  32  to the output  33  and to decouple the other. In specific embodiments of the coupling unit, said coupling unit can also be designed to disconnect both inputs  31 ,  32  from the output  33 . However, no provision is made for both the input  31  and the input  32  to be connected to the output  33 . 
       FIG. 4  shows a first embodiment of the coupling unit  30 , which has an inverter  34  which can in principle only connect one of the two inputs  31 ,  32  to the output  33 , while the respective other input  31 ,  32  is decoupled from the output  33 . The inverter  34  can have a particularly simple realization as an electromechanical switch. 
       FIG. 5  shows a second embodiment of the coupling unit  30 , in which a first and a second switch  35  and  36  are provided. Each of the switches is connected between one of the inputs  31  and  32  and the output  33 . In contrast to the embodiment shown in  FIG. 4 , this embodiment has the advantage that both inputs  31 ,  32  can also be decoupled from the output  33 , with the result that the output  33  will have a high resistance. In addition, the switches  35 ,  36  can be implemented simply as semiconductor switches such as metal-oxide semiconductor field-effect transistors (MOSFETs) or insulated gate bipolar transistors (IGBTs), for example. Semiconductor switches have the advantage of a favorable price and a high switching speed, with the result that the coupling unit  30  can respond to a control signal or a change in the control signal within a short period of time and high switchover rates can be achieved. 
       FIG. 6  shows the second embodiment of the coupling unit in a simple semiconductor circuit, in which each of the switches  35 ,  36  comprises in each case one semiconductor valve which can be switched on and off and one diode which is connected in parallel therewith. 
       FIGS. 7 and 8  show two arrangements of the coupling unit  30  in a battery module  40 . A plurality of battery cells  41  is connected in series between the inputs of a coupling unit  30 . However, the disclosure is not restricted to such a series circuit comprising battery cells; it is also possible for only one individual battery cell to be provided or else a parallel circuit or a mixed series and parallel circuit of battery cells. In the example shown in  FIG. 7 , the output of the coupling unit  30  is connected to a first terminal  42  and the negative pole of the battery cells  41  is connected to a second terminal  43 . However, a minor-image arrangement as in  FIG. 8  is possible, in which the positive pole of the battery cells  41  is connected to the first terminal  42  and the output of the coupling unit  30  is connected to the second terminal  43 . 
       FIG. 9  shows the coupling unit  30  illustrated in  FIG. 6  in the arrangement illustrated in  FIG. 7 . Actuation and diagnosis of the coupling units  30  takes place via a signal line  44 , which is connected to a control device (not illustrated). 
       FIG. 10  shows an electric drive unit according to the disclosure with an electric motor  13 , in which the method according to the disclosure can be implemented. As in  FIG. 1 , a battery  10  is connected to a DC voltage intermediate circuit, which is buffered by an intermediate circuit capacitor  11 . A pulse-operated inverter  12 , which supplies an electric drive motor  13 , is connected to the DC voltage intermediate circuit. The disclosure provides for the battery  10  to comprise a battery module string  50 , which comprises a plurality of series-connected battery modules  40 - 1 , . . . ,  40 - n , which each comprise a coupling unit  30  and are constructed as illustrated in  FIG. 7 or 8 . When combining battery modules  40 - 1 , . . . ,  40 - n  to form the battery module string  50 , in each case the first terminal  42  of a battery module  40 - 1 , . . . ,  40 - n  is connected to the second terminal  43  of an adjacent battery module  40 - 1 , . . . ,  40 - n . 
     A negative pole  51  and a positive pole  52  of the battery module string  50  represent the taps of the battery  10 . Owing to the fact that the battery modules  40 - 1 , . . . ,  40 - n  arranged between the taps each comprise coupling units  30 , the output voltages which can be adjusted at the taps are adjustable stepwise. 
     A control unit (not illustrated) is designed to output a first control signal to a variable number of battery modules  40 - 1 , . . . ,  40 - n , by means of which control signal the coupling units  30  of the battery modules  40 - 1 , . . . ,  40 - n  actuated in this way connect the battery cell (or the battery cells)  41  between the first terminal  42  and the second terminal  43  of the respective battery module  40 - 1 , . . . ,  40 - n . At the same time, the control unit outputs a second control signal to the rest of the battery modules  40 - 1 , . . . ,  40 - n , by means of which the coupling units  30  of these remaining battery modules  40 - 1 , . . . ,  40 - n  connect the first terminal  42  and the second terminal  43  of the respective battery module  40 - 1 , . . . ,  40 - n , as a result of which the battery cells  41  of this battery module  40 - 1 , . . . ,  40 - n  are bridged. 
     By suitably actuating the plurality of battery modules  40 - 1 , . . . ,  40 - n , different voltages can thus be output at the taps  51 ,  52  of the battery  10 . 
     By suitably selecting the switching states of the coupling units  30 , the voltage between the taps  51  and of the battery  10  can thus be adjusted stepwise between zero volt and a maximum value. The quantization steps in the adjustment of the output voltage correspond to the module voltages of the battery modules  40 - 1 , . . . . ,  40 - n  and are thus dependent on the number of battery cells  41  in the battery modules  40 - 1 , . . . ,  40 - n  and the state of charge of the battery cells  41 . 
     A coil  60  is connected between the intermediate circuit capacitor  11  and the battery  10 . The inductance of the coil  60  can be selected to be relatively low since, owing to the use of the battery modules  40 - 1 , . . . ,  40 - n , the voltage difference at the coil  60  and therefore also the current ripple are very low within a pulse period. Using pulse width modulation, the output voltage, present between the taps  51 ,  52 , of the battery is adjustable substantially continuously, which is assumed in the following description of the method according to the disclosure. 
       FIG. 11  shows a block circuit diagram of a system comprising the battery  10 , the coil  60  and the intermediate circuit capacitor  11 , in which the method according to the disclosure is implemented. Transformation functions are specified in the various blocks, which result from Laplace transformation in the complex spectral range with complex variable p. 
     The method according to the disclosure for charging the intermediate circuit capacitor  11  comprises the following method steps: In method step  101 , a setpoint value for the intermediate circuit current is fixed. In method step  102 , an actual value for the intermediate circuit current is determined. In method step  103 , a difference between the actual value and the setpoint value for the intermediate circuit current is formed, which forms the control difference for a current controller, which determines, in method step  104 , an optimum output voltage of the battery  10  and outputs this output voltage as manipulated variable. The transformation function of the current controller is provided by the transformation function F RI (p). In method step  105 , the optimum output voltage of the battery  10  is adjusted, wherein it is assumed that the output voltage is continuously adjustable, which can be achieved by suitable pulse width modulation. 
     If only the intermediate circuit current for charging the intermediate circuit capacitor  11  is intended to be controlled, the previously outlined current control loop is sufficient. The desired intermediate circuit current is input to said current control loop as setpoint value. If an intermediate circuit voltage at the intermediate circuit capacitor  11  has reached the output voltage of the battery  10 , the current controller is deactivated, and the battery  10  is connected directly to the intermediate circuit capacitor  11  via the coil  60 . 
     However, it is also possible to superimpose a further control of the intermediate circuit voltage which is present at the intermediate circuit capacitor  11  on the already outlined control of the intermediate circuit current. Thus, different intermediate circuit voltages can also be adjusted, for example for different driving situations in an electric vehicle. 
     This takes place by virtue of the fact that method steps  106  to  108  are introduced before method step  101 , in which the setpoint value for the intermediate circuit current is fixed. In method step  106 , an actual value for the intermediate circuit voltage is measured. In method step  107 , a difference between the actual value and a setpoint value of the intermediate circuit voltage is determined and, in method step  108 , this is converted into an optimum intermediate circuit current by means of a voltage controller with the transformation function F RU (p). 
     If a controlled voltage value is desired for the intermediate circuit voltage which is between the possible levels of the output voltage, the voltage controller used in method step  108  is continuously active. The voltage controller in this case continuously calculates a setpoint value for the subordinate current controller from the desired setpoint value for the intermediate circuit voltage. 
     The right-hand part of the block circuit diagram shown in  FIG. 11  (to the right of the dashed line  109 ) describes the physical response of an electromagnetic resonance circuit which comprises the components coil  60  (transformation function 1/pT L ), intermediate circuit capacitor  11  (transformation function 1/pT C ) and a system resistor R (not known in more detail) (transformation function K R ). The effect of multiplying by the respective transformation functions is in this case that of converting a voltage into a current and, vice versa, a current into a voltage. In this case, the actual value for the intermediate circuit current is generated at the node  110 . The actual value for the intermediate circuit voltage present at the intermediate circuit capacitor  11  is generated at the node  111 . Both values are measured in method steps  102 ,  106 . 
     The proposed method for charging an intermediate circuit capacitor requires only the coil  60  as additional hardware component. The actuation of the coupling units  30  arranged in the battery modules  40 - 1 , . . . ,  40 - n  can be realized via software functions in the control device (not illustrated). The actual values for the intermediate circuit current and the intermediate circuit voltage are typically detected for other reasons and are therefore available for the closed-loop control.