Abstract:
A control method for a vehicle drive assembly includes applying a conduction angle set point value to a variable reluctance machine. The method also includes regulating the conduction angle set point value by comparing a table set point torque from an angle table and a torque estimation value to determine a control measurement of torque delivered by the drive assembly. The control measurement of torque is corrected to an additional conduction angle which is added to a set point angle from the angle table to determine the regulated conduction angle set point value.

Description:
BACKGROUND OF THE INVENTION 
   I. Field of the Invention 
   The invention relates to the drive assemblies of vehicles comprising an internal combustion engine, and control methods for such assemblies. 
   II. Description of Related Art 
     FIG. 1  represents one conventional exemplary embodiment of such an assembly. The assembly  2  forms a motor vehicle drive assembly. In a manner known per se, this assembly comprises an internal combustion engine  4 , such as a petrol motor or a diesel motor. It comprises an alternator-starter formed by a variable reluctance machine  6  of a type known per se. This machine  6  is mechanically linked to the engine  4  to provide the latter with a torque, particularly in order to start it when it is stopped. This mechanical link of a type known per se carries the reference  8  in  FIG. 1 . The assembly comprises a control unit  10  comprising conventional computer means enabling it to control the various units of the assembly as will be seen below. The assembly  2  also comprises an inverter  12  by means of which the control unit  10  drives the operation of the machine  6 . 
   The assembly  2  also comprises a position sensor and/or a speed sensor  14  enabling the unit  10  to know a measurement of the speed of the machine  6  at each instant. 
     FIG. 2  represents a conventional inverter topology for controlling the motor phase. In this  FIG. 2 , V DC  represents the DC voltage powering the inverter, I 1  and I 2  two electric switches that can be switched open and closed and D 1  and D 2  are two diodes. 
   A control method for such a drive assembly installed on a vehicle (comprising, as has been seen, a reluctance machine associated with an inverter) is normally used to control the torque delivered by this drive assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   However, in the context of large-scale industrial production, it is essential to guarantee the lowest possible variability of the torque supplied by two different assemblies, all other things being equal (control, power supply voltage, etc.). 
   Now the torque delivered by a switched reluctance machine depends strongly on the geometric dispersions of the machine, the magnetic dispersions of the plates, or even the electrical dispersions of the wires. 
     FIGS. 3 and 4  illustrate the shape of the phase currents and voltages as represented in  FIG. 2 , respectively at low speed and at high speed, according to the electrical angle of the rotor (this angle being zero when the tooth of the rotor is in conjunction with the tooth of the stator). 
   It is known that a variable reluctance machine such as the machine  6  in  FIG. 1  is essentially controlled by three variables:
         the start of magnetization ON and end of magnetization OFF angles;   and, at low speeds, the peak phase current which is regulated (I lim ).       

   The phase of the machine is magnetized in the time interval formed by the conduction angle Θp=OFF-ON. 
   In order to control the variable reluctance machine, the control parameters (ON, Θp, I lim ) at low speed and (ON, Θp) at high speed are used as input parameters for the control unit  10 ; these parameters being optimized for each torque-speed operating point. 
   As illustrated in  FIG. 1 , and in a manner known per se, the machine  6  is controlled by the control unit  10  by means of control laws taken from tables that give the ignition (ON), conduction (Θp table ) and current (I lim ) angles according to the speed of the alternator obtained from the sensor  14 . These quantities are also determined according to the torque to be delivered by the link  8  to the engine  4 . If necessary, it is also possible to take account of the DC power supply voltage V DC  of the inverter if the latter is likely to vary, and other parameters such as the temperature of the windings, for example. 
   In each electrical period, a pointer is defined according to these external parameters (speed, power supply voltage, temperature of the windings, etc.) and the torque set point. This pointer then addresses the tables which give (possibly after extrapolation) the control parameters (ON, Θp, I lim ) at low speed and (ON, Θp) at high speed. 
   However, this conventional open loop mode control law, according to which the control instructions applied depend only on the position, the speed, the torque set point and the current (only taken into account at low speed when it is set to I lim ), is unsatisfactory: the torque actually delivered remains totally dependent on the dispersions (mechanical, magnetic, electrical, etc.) of the machine. 
   Referring to  FIG. 5 , there are thus represented the effects of the variability of the air gap between rotor and stator on the electromagnetic torque delivered (i.e., the surface area of the energy cycle) when the conventional control is used. 
   It should be noted that the effects of the variation of other parameters (geometric dimensions, electrical or magnetic parameters) are relatively similar. 
   In  FIG. 5 , the air gap  1  is greater than the air gap  2  which generates a difference in the saturation curves when a rotor tooth is in conjunction with a stator tooth of the phase concerned. In opposition, the effect of the variation of the air gap is negligible. 
   It can be seen from this  FIG. 5 , for one and the same control, the electrical torque delivered is different according to the air gap of the machine. 
   In order to improve the quality level and provide a service that is identical from one assembly to another, a first solution consists in reducing the various tolerances of the series-produced machine (particularly, the mechanical, magnetic and electrical tolerances). This solution is, however, expensive, particularly in terms of manufacturing processes and raw materials. 
   One aim of the invention is to propose a solution that makes it possible to compensate for the inherent dispersions of the machine when the latter is operating, without in any way increasing the cost of such a machine. 
   To this end, the invention proposes, according to a first aspect, a control method for a vehicle drive assembly in which a conduction angle set point value is applied to a variable reluctance machine, characterized in that said conduction angle set point value is regulated using an additional conduction angle determined according to a control measurement of the torque delivered by the drive assembly to the engine. 
   Certain preferred, but non-limiting aspects of this method are as follows:
         the torque control measurement is carried out by calculating the difference between the set point torque and a torque estimation value;   the difference is processed by a corrector provided with a correction law suitable for determining the additional conduction angle intended to be applied to the alternator to regulate said conduction angle set point value and render said difference zero;   the torque control measurement is carried out using an estimation of the phase flux;   the flux is estimated by integrating the voltage applied to this phase in an electrical period according to Φ estimated =∫(U phase −R phase ·I phase )dt, where R phase  designates the resistance of the stator winding, U phase  the phase voltage and I phase  the phase current;   a direct measurement of the phase voltage is carried out;   the start and end of magnetization control instructions and the phase current are used to determine the phase voltage, the latter being equal to the power supply voltage V DC  during the magnetization time, to −V DC  during the demagnetization time, and to zero during the free-wheeling phases or when the phase current is zero;   the flux estimation is periodically updated using a measurement flux value;   the flux estimation is realigned in each electrical period, at the time of the transition to opposition, on the value L 0 ·I phase  where L 0  is the inductance in opposition and I phase  the current measured at the instant of the transition to opposition;   the torque is estimated, using the flux estimation, by calculating the difference between the energy transmitted in the magnetization phase and the energy returned in the demagnetization phase;   the correction law is of proportional-integral type;   there is applied to the machine, over an electrical period, a conduction angle set point value calculated from the estimation of the flux calculated in the preceding electrical period;   the conduction angle set point value is equal to the sum of the additional conduction angle and a conduction angle value supplied by an angle table addressed by the set point torque value;   said conduction angle set point value is equal to the sum of the additional conduction angle and a conduction angle value supplied by an angle table addressed by the set point torque value;   the variable reluctance machine operates in discontinuous mode or in continuous current mode, said conduction angle value supplied by the angle table being equal to 180° when operating in continuous current mode.       

   According to a second aspect, the invention relates to a drive assembly for motor vehicle comprising:
         an internal combustion engine;   a variable reluctance machine; and   a control unit suitable for applying a conduction angle set point value to the machine,
 
characterized in that the control unit is also suitable for regulating said conduction angle set point value using an additional conduction angle determined according to a control measurement of the torque delivered by the drive assembly.
       

   Some preferred, but non-limiting, aspects of this drive assembly are as follows:
         it comprises appropriate means for calculating an estimation of the torque and carrying out a control measurement of the torque by measuring the difference between the set point torque and said torque estimation, and means for adjusting the conduction angle set point value according to said torque control measurement;   it comprises means for calculating an estimation of the flux and means for calculating an estimation of the torque based on said flux estimation;   an angle table being addressed by the torque set point value to supply a conduction angle value, the control unit is suitable for applying to the machine a conduction angle set point value equal to the sum of the additional conduction angle and the conduction angle value supplied by the angle table.       

   Finally, the invention also provides for a vehicle comprising a drive assembly according to the second aspect of the invention. 
   Other aspects, aims and advantages of the invention will become more apparent from reading the detailed description below of preferred embodiments of the latter, given by way of nonlimiting example and described with reference to the appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  represents a conventional exemplary embodiment of a drive assembly for a vehicle; 
       FIG. 2  represents a conventional inverter topology for controlling a motor phase; 
       FIG. 3  illustrates the shape of the phase currents and voltages as represented in  FIG. 2 , at low speed, according to the electrical angle of the rotor; 
       FIG. 4  illustrates the shape of the phase currents and voltages as represented in  FIG. 2 , at high speed, according to the electrical angle of the rotor; 
       FIG. 5  represents the effects of a variation of an air gap between rotor and stator on the electromagnetic torque delivered; 
       FIG. 6  represents the energy transmitted in an electrical period at high speed; 
       FIGS. 7 and 8  respectively represent the energy transmitted in the magnetization phase and in the demagnetization phase; 
       FIG. 9  represents one possible embodiment of the drive system according to the invention; 
       FIG. 10  gives sequencing examples of the torque regulation calculation; 
       FIG. 11  represents the trend of the energy cycle over several electrical periods when the conduction angle set point value is regulated as is proposed in the context of the invention; 
       FIG. 12  represents one possible embodiment of a phase flux estimator. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention aims to compensate for the inherent dispersions of the machine and its component parts in order for the electromagnetic torque delivered to be affected as little as possible by these dispersions. 
   To this end, the invention proposes using an electro-magnetic torque observer. This observer is used more specifically to estimate the delivered torque. A regulation law is then defined which makes it possible to correct the differences between the set point torque and the estimated torque. This correction is translated by the calculation of an additional conduction angle which is used to regulate the conduction angle set point value Θp. 
   The estimation of the electromagnetic torque is carried out based on an estimation of the flux in a phase. 
   The estimation of the flux of a phase is obtained by integrating the voltage applied to this phase: Φ estimated =∫(U phase −R phase ·I phase )dt, where R phase  designates the resistance of the stator winding, U phase  and I phase  respectively designate the phase voltage and current (as is indicated in  FIGS. 2 ,  3  and  4 ). 
   The phase voltage U phase  corresponds to the voltage of the power supply V DC  during the magnetization time, the voltage −V DC  during the demagnetization phase, and 0 during the freewheeling phases or when the current is zero. 
   In order to calculate the phase voltage U phase , there is used either a direct measurement of the voltage of each stator winding, or even the phase current I phase , the DC power supply voltage V DC  and the ON and OFF control signals. In practice, the start and end of magnetization control instructions and the measurement of the phase current make it possible to know the sign of the voltage applied to the winding (positive, negative or zero), so replacing a direct measurement of the voltage of each stator winding. 
   The value of the phase current I phase  is obtained, in a manner known per se, as is represented in  FIG. 1 , by electrical measurement at the output of the inverter  12 . 
     FIG. 12  represents one possible embodiment of a phase flux estimator  17 . 
   The estimator  17  thus comprises means  22  for calculating the phase voltage U phase . These means  22  take as input parameters either a direct measurement of the voltage of each stator winding, or even the phase current I phase , the power supply voltage V DC  and the ON and OFF control signals. 
   The value of the phase current I phase  is obtained, in a manner known per se, as is represented in  FIG. 1 , by electrical measurement at the output of the inverter  12 . 
   An integrator  23  can be used to calculate the flux estimation according to the equation Φestimated=∫(U phase −R phase ·I phase )dt, presented above. 
   In practice, however, the measurements always present errors and, above all, the resistance of the winding of a phase R phase  is highly variable (particularly with temperature). According to one advantageous embodiment of the estimator  17 , the estimated flux at the output of the integrator  23  is realigned on each electrical period. 
   According to one possible embodiment of the invention, this realignment is done on the transition to opposition because, at this moment, the inductance of the phase is at its most constant (therefore independent of the phase current) which minimizes the errors and simplifies the flux estimation. The estimated flux on the transition to opposition Φestimated|opposition is therefore such that Φestimated|opposition=Lo·I phase , where Lo is the inductance in opposition (which can be measured with the rotor blocked in opposition, for example) and I phase  is the phase current measured at this instant. The output of the integrator  23  is thus forced in each electrical period, on the transition to opposition, by the updating of the flux value (Lo·I phase ). 
   The torque delivered by the machine at constant speed is proportional to the energy transmitted by a phase of the engine. It is therefore proportional to the surface area of the curve illustrated in  FIG. 6  which shows the trend of the flux associated with a phase of the alternator according to the phase current, for a machine operating at high speed. The energy obtained from the electromechanical conversion is thus characterized by the surface area (or energy cycle) which is delimited by the path traveled by the phase flux and the phase current during an electrical period. 
   Knowing the trend of the flux during a period, it then becomes possible to estimate the electromagnetic torque transmitted by calculating the energy transmitted as graphically represented in  FIG. 6 . 
   Knowing the flux, the torque can actually be estimated by the difference between the energy Emag transmitted in the magnetization phase and the energy Edemag returned in the demagnetization phase. 
   The energy transmitted on magnetization is: Emag=∫ ON   OFF I phase dφ, where Φ is the estimated phase flux. It should be noted that the flux is strictly increasing during this phase; it reaches its maximum at the end of magnetization (OFF). 
   On demagnetization, the returned energy is Edemag=∫ OFF   ON I phase dφ. In this phase, the flux is strictly decreasing. 
   Algebraically, Emag and Edemag have opposite signs. The sum of these two quantities (or the difference of their absolute values) represents the converted energy and is therefore an image of the electromechanical torque of the machine. 
   Over an electrical period, the average electromechanical torque delivered is proportional to the surface area of the energy cycle represented by  FIG. 6 . 
   Knowing the flux at each instant from flux estimator  17  described previously, and having a measurement of the phase current and an image of the voltage applied to the winding (which, as has already been mentioned, can be measured directly or estimated via the control and the value of the current), it is possible to have an image of the electromechanical torque by direct calculation of the energy represented by this surface area. 
   This energy can thus be calculated in two phases. 
   During a first phase, the energy exchanged in the magnetization phase Emag=∫ OFF   ON I phase dφ, as represented by the surface area of  FIG. 7 , is calculated. 
   During a second phase, the energy exchanged in the demagnetization phase Edemag=∫ OFF   ON I phase dφ, as represented by the surface area of  FIG. 8 , is calculated. 
   The converted energy corresponds to the difference of the two surface areas represented in  FIGS. 7 and 8 , or even to the algebraic sum of the values Emag and Edemag (these being of opposite signs). 
     FIG. 9  represents one possible embodiment of an assembly  20  according to the invention implementing a regulation aiming to compensate for the effect of the dispersions, in particular the mechanical dispersions (as discussed in relation to a variation of the air gap in light of  FIG. 5 ), magnetic dispersions or electrical dispersions. 
   In this  FIG. 9 , the elements of the same type as those of  FIG. 1  have the same numerical references. 
   Starting from the conventional regulation arrangement of  FIG. 1 , the assembly  20  of  FIG. 9  according to one possible embodiment of the invention comprises means  17  of estimating the flux followed by means  18  of estimating the torque 
   The torque C estimated  estimated using the means  18  is compared using a comparator  19  to the set point torque C setpoint  which is also supplied to the angle table  21 . At the output of the comparator  19 , there is a measurement of the difference between the estimated torque C estimated  and the set point torque C setpoint  in the form of an error ε. 
   The error ε is processed by a corrector  22  provided with a law (proportional-integral for example) suitable for determining an additional conduction angle ΔΘp making it possible to regulate the conduction angle set point value Θp applied to the alternator so that the error ε is rendered zero (that is, so that the estimated torque is then equal to the set point torque). 
   The conduction angle set point value Θp is thus equal to the sum of the set point value Θp table  obtained from the angle tables and said additional conduction angle ΔΘp:Θp=Θp table +ΔΘp. 
   According to the diagram of  FIG. 9 , the conduction angle Θp is controlled so as to enable the torque of the machine to be controlled directly. 
   The torque differences due to the various dispersions are thus compensated; for one and the same operating point, the conduction angle Θp applied to the alternator will thus be different between two machines of the same type. 
   The torque estimation requires an electrical period. Thus, the torque required is established (and the dispersions are then compensated) only after several electrical periods. In the context of the control discussed previously in light of  FIG. 9 , in the first electrical period, or after a change of set point torque, a conduction angle set point value Θp equal to the angle Θp table  supplied by the angle table  21  is applied (that is, the error ε is set to zero). 
   The additional conduction angle ΔΘp is then calculated and, if necessary, modifies the conduction angle applied to the machine during the second period (or even during the period following the one in which the change of set point torque occurred). 
   This procedure is repeated as long as the set point torque does not vary significantly, in particular regarding the resolution of the angle table. 
   During an electrical period, several tasks must therefore be sequenced. 
   The magnetization start instant ON (i.e. the ON angle) remains unchanged. During the magnetization period, the set point giving the conduction angle value Θp (having to be applied until the magnetization end instant OFF) is calculated. 
   According to a first option, the calculation of the control Θp can be carried out in parallel with the estimation of the torque (and therefore of the flux) during the start of magnetization. The bottom timing diagram of  FIG. 10  illustrates this first option. 
   According to a second option, the start of the magnetization period can be devoted to calculating the control Θp, the torque estimation commencing thereafter. A linear extrapolation is then done over this period so as to estimate the torque and the flux. The top timing diagram of  FIG. 10  illustrates this option. 
     FIG. 11  represents one example of the effect of the regulation of the torque in discontinuous mode. Considering a machine (provided with an air gap  1 ), the torque of which is less than (surface area of the bottom energy cycle), given equal control, a reference machine (that provided with the air gap  2 ) as illustrated in  FIG. 5 . 
   Regulating the set point value of the conduction angle Θp makes it possible to compensate for the dispersions (here, the thickness of the air gap) by obtaining the output reference torque. The conduction angle increases over several periods until the energy cycle has a surface area that is equivalent (i.e., an identical torque) to that of the alternator provided with the initial air gap  2 . It should be noted that the energy cycles are not equivalent; only their surface area (and therefore the torque) is identical. 
   In the context of the invention, the conduction angle Θp is regulated as indicated previously both in discontinuous mode and in continuous current mode. 
   In discontinuous mode, such a control makes it possible to compensate for the dispersions between machines of the same type. 
   In continuous current mode, such a control makes it possible to stabilize the torque delivered. This is of particular interest in as much as the continuous current mode, where it provides for a torque gain at high speed, is normally unstable. 
   In the context of the invention, the switch from the discontinuous mode to the continuous current mode is therefore done naturally, the torque being permanently regulated with the same control law. 
   Moreover, the various dispersions are permanently compensated for (whether in continuous or discontinuous mode). Thus, when the table gives a conduction angle Θp table  of 180°, two machines from the same lot can be, for one and the same output torque, and according to the dispersions, one in continuous current mode and the other in discontinuous mode. 
   It should be mentioned here that the angle table can be addressed over the entire torque range (discontinuous and continuous modes). In the area (high speed and high torque) that can be reached with the continuous current mode, the table supplies a set point Θp table =180°; the control law of the torque then regulating the conduction angle Θp applied to the alternator so as to stabilize the continuous current mode on the one hand, and to supply the set point torque on the other hand.