Method for compensating for a brake torque in the event of a short-circuit failure in the power inverter of an assist motor

Abstract

A method for controlling a power steering system of a vehicle, having a steering wheel, and an inverter powering an assist motor, when a short-circuit failure is detected between a phase of the assist motor and an electric line of the inverter, wherein it includes:—a configuration step intended to determine, for a magnetic field of the assist motor, a controllable zone and a non-controllable zone, —a step of compensating for a space-average brake torque in the controllable zone.

Claims

1. A method for driving a power steering system of a vehicle, comprising a steering wheel, and an inverter electrically supplying an assist motor, when a short-circuit type failure is detected between a phase of the assist motor and an electric line of the inverter wherein it comprises: a configuration step intended to determine for a magnetic field of the assist motor a controllable zone and a non-controllable zone that are defined by an electrical angle and a direction of rotation and a rotational speed of the assist motor, a step of compensating a spatial average brake torque in the controllable zone, the compensation step comprising: a phase for detecting a relative electrical position of the assist motor (α.sub.r) with respect to the controllable zone, a phase for determining a spatial average motor torque (C.sub.mm) as a function of the measured steering wheel torque (C.sub.vm), a phase for converting the space average motor torque (C.sub.mm) into an instantaneous motor torque (C.sub.mi), a phase for controlling the phase currents of the assist motor.

2. The driving method according to claim 1, comprising an activation step comparing a direction of rotation of the steering wheel with the direction of rotation of the assist motor.

3. The driving method according to claim 2, wherein the activation step compares a steering wheel torque (C.sub.vm) and/or the rotational speed of the assist motor (V.sub.m) with a predetermined threshold.

4. The driving method according to claim 1, wherein the configuration step determines two phases (φ) of the assist motor not influenced by the failure.

5. The driving method according to claim 1, wherein the detection phase triggers an implementation of the phase for determining a spatial average motor torque (C.sub.mm).

6. The driving method according to claim 1, wherein the conversion phase implements a setpoint curve representing the instantaneous motor torque (C.sub.mi) as a function of the relative electrical position of the assist motor (αr) relative to the controllable zone.

7. The driving method according to claim 1, wherein the control phase successively uses a switching, called «deactivated inverter switching» (C0), a first switching group, and a second switching group for carrying out vector monitoring of the phase currents.

8. The driving method according to claim 1, comprising a step of estimating the spatial average brake torque induced by the failure in the non-controllable zone.

9. The driving method according to claim 8, wherein the determination phase uses the estimated spatial average brake torque (C.sub.fm) during the estimation step to determine the spatial average motor torque (C.sub.mm).

Description

(1) The invention will be better understood from the following description, which relates to an embodiment according to the present invention, given by way of non-limiting example and explained with reference to the appended schematic drawings, in which:

(2) FIG. 1 is a schematic representation of a method according to the invention;

(3) FIG. 2 is an equivalent diagram of an undriven inverter connected to an assist motor having a short-circuit type failure;

(4) FIG. 3 is a representation of the controllable zones and of the non-controllable zones of an assist motor as a function of the short-circuit type failure and of an assist torque direction;

(5) FIG. 4 is a representation of a setpoint curve representing an instantaneous motor torque as a function of a relative electrical position of the assist motor with respect to the controllable zone;

(6) FIG. 5 represents a switching table of an inverter.

(7) FIG. 2 illustrates an electrical diagram connecting an inverter 1 to an assist motor 2 in an electric power steering system of a vehicle.

(8) The inverter 1 is an electronic device electrically supplied by a direct current generator 11 comprising a ground portion 12 and a supply portion 13, making it possible to provide a three-phase alternating current.

(9) The inverter 1 contains three electric lines 14, 15, 16 disposed in parallel between the ground portion 12 of the generator 11 and the supply portion 13 of the generator 11. Each electric line 14, 15, 16 includes a «low side» switching cell 117, 118, 119, that is to say a switching cell linked to the ground portion 12 of the generator 11, and a «high side» switching cell 17, 18, 19, that is to say a switching cell linked to the supply portion 13 of the generator 11. The switching cells 17, 18, 19, 117, 118, 119 are of the MOSFET type. The inverter 1 therefore comprises three «low side» switching cells 117, 118, 119 and three «high side» switching cells 17, 18, 19.

(10) Each electric line 14, 15, 16 comprises between the «low side» switching cell 117, 118, 119 and the «high side» switching cell 17, 18, 19, a phase line U, V, W. There are therefore three phase lines U, V, W.

(11) Each phase line U, V, W supplies a coil 28, 27, 29 of the assist motor 2.

(12) In normal operation, electric currents flowing in the phase lines U, V, W create a rotating magnetic field determining a direction of rotation, a speed of rotation and a motor torque of the rotor 200 of the assist motor 2.

(13) A positive direction and a negative direction are arbitrarily defined. The positive direction corresponds in the remainder of the description to the trigonometric direction.

(14) In the diagram in FIG. 2, the «low side» switching cell 119 connected to the phase line U is defective, causing a failure of the inverter 1 of the short-circuit type between the phase line U and the ground portion 12 of the generator 11. The short-circuit switching cell is in a closed position. The phase lines V and W are still functional, but the phase line U is not.

(15) In the presence of a short-circuit type failure, electromotive forces are generated by the rotation of the rotor 200, creating a brake torque at the level of the assist motor 2.

(16) Depending on the defective phase line U, the speed and the direction of rotation of the assist motor 2 and the ground 12 or supply 13 portion of the inverter 11 in short-circuit, it is possible to determine on an electric revolution of the assist motor 2, a controllable zone ZC and a non-controllable zone ZNC.

(17) The non-controllable zone ZNC corresponds to an electrical angle over which the electromotive forces induce a brake torque.

(18) The controllable zone ZC corresponds to 1 electric revolution minus the electrical angle of the uncontrollable zone ZNC. An input angular position and an exit angular position Z.sub.c of the controllable zone ZC corresponding to the angular position Z.sub.c of the controllable zone ZC are defined relative to a determined coil 27, 28, 29. In FIG. 3, the angular position Z.sub.c is determined relative to the coil 27 electrically supplied by the phase line U. It is preferable to determine the angular position relative to the coil electrically supplied by the defective phase line.

(19) As can be seen in FIG. 3, there is a first non-controllable zone 25 corresponding to the short-circuiting of the phase line U with the ground portion 12 when the assist motor 2 assists in the positive direction and corresponding to the short-circuiting of the phase line U with the supply portion 13 when the assist motor 2 assists in the negative direction.

(20) There is a second non-controllable zone 22 corresponding to the short-circuiting of the phase line U with the ground portion 12 when the assist motor 2 assists in the negative direction and corresponding to the short-circuiting of the phase line U with the supply portion 13 when the assist motor 2 assists in the positive direction.

(21) There is a third non-controllable zone 24 corresponding to the short-circuiting of the phase line V with the ground portion 12 when the assist motor 2 assists in the positive direction and corresponding to the short-circuiting of the phase line V with the supply portion 13 when the assist motor 2 assists in the negative direction.

(22) There is a fourth non-controllable zone 21 corresponding to the short-circuiting of the phase line V with the ground portion 12 when the assist motor 2 assists in the negative direction and corresponding to the short-circuiting of the phase line V with the supply portion 13 when the assist motor 2 assists in the positive direction.

(23) There is a fifth non-controllable zone 26 corresponding to the short-circuiting of the phase line W with the ground portion 12 when the assist motor 2 assists in the positive direction and corresponding to the short-circuiting of the phase line W with the supply portion 13 when the assist motor 2 assists in the negative direction.

(24) Finally, there is a sixth non-controllable zone 23 corresponding to the short-circuiting of the phase line W with the ground portion 12 when the assist motor 2 assists in the negative direction and corresponding to the short-circuiting of the line phase W with the supply portion 13 when the assist motor 2 assists in the positive direction.

(25) When the speed of rotation of the assist motor 2 is zero, the angle of the non-controllable zone 21, 22, 23, 24, 25, 26 is equal to an electrical angle of 60°. Thus, the controllable zone ZC corresponds to 1 electric revolution of the assist motor 2 minus the angle corresponding to the non-controllable zone ZNC, that is to say 300°.

(26) As the rotational speed of the assist motor 2 increases, the angle of the non-controllable zone ZNC increases.

(27) The total value of the brake torque generated in the non-controllable zone ZNC and a distribution in the non-controllable zone ZNC depends on the speed of rotation of the assist motor 2.

(28) Thus, when the assist motor 2 is in a situation in which the phase U is in short circuit with the ground portion 12, and in the case of a closed loop monitoring, the method according to the invention as represented in FIG. 1 allows, during a configuration step, to define configuration parameters Z.sub.c, φ specific to the phase line U and to the portion 12 in short circuit. The configuration parameters Z.sub.c, φ are in particular the input and exit angular position Z.sub.c of the controllable zone ZC as well as the two functional phase lines φ.

(29) In closed loop monitoring, an estimation step receives as input the rotation speed V.sub.m of the assist motor 2, a measured electrical angle α.sub.m of the motor and information concerning the phase currents available i on each functional phase line V, W.

(30) The estimation step thus determines an estimated spatial average brake torque C.sub.fm exerted by the electromotive forces on the non-controllable zone ZNC. That is to say, the estimation step calculates the minimum value of torque to be provided in the following controllable zone ZC to counterbalance the brake torque exerted in the following non-controllable zone ZNC as a function of the speed of rotation.

(31) The method according to the invention implements an activation step receiving as input the speed of rotation V.sub.m of the assist motor 2 and the measured steering wheel torque C.sub.vm.

(32) The activation step allows activating a step of compensating the spatial average brake torque when the measured steering wheel torque C.sub.vm and the speed of rotation V.sub.m of the assist motor 2 are greater than a predetermined value, for example 5 N.Math.m for the measured steering wheel torque C.sub.vm and 50 rpm for the speed of rotation V.sub.m of the assist motor 2, and when they are in the same direction. For this, the activation step sends an activation signal on. When the conditions are not fulfilled, that is to say when the activation signal is not emitted, the activation step no longer drives the inverter 1 which is then in a deactivated state, called deactivated inverter switching. This activation step can also receive the phase currents available i on each functional phase line V, W.

(33) The compensation step includes a detection phase, a determination phase, a conversion phase and a control phase.

(34) The detection phase receives as input the measured electrical angle α.sub.m and a speed of rotation V.sub.m of the assist motor 2 as well as the input and exit angular position Z.sub.c of the controllable zone ZC determined during the configuration step.

(35) The detection phase determines a relative electrical position α.sub.r of the motor 2, unsigned, relative to the input of the controllable zone ZC and also defines a direction of the assist torque R of the functional phase lines V, W as a function of the direction of rotation of the assist motor 2.

(36) Finally, the detection phase activates, via an activation signal e, the realization of the determination phase at each electric revolution.

(37) The determination phase receives as input the activation signal e of the detection phase, the measured steering wheel torque C.sub.vm and the estimated spatial average brake torque C.sub.fm.

(38) The determination phase calculates the average motor torque C.sub.mm to be exerted on the controllable zone ZC so as to maintain an admissible steering torque for the driver. The determination phase takes into account the measured steering wheel torque C.sub.vm as in a failure-free situation and compensates the estimated spatial average brake torque C.sub.fm.

(39) The conversion phase transforms the average motor torque C.sub.mm to be exerted on the controllable zone ZC into an instantaneous motor torque C.sub.mi as a function of the relative electrical position α.sub.r of the assist motor 2. The average motor torque C.sub.mm is equal to the integral of the instantaneous motor torque C.sub.mi on the controllable zone ZC. The average motor torque C.sub.mm is distributed over the extent of the controllable zone ZC, in our case from 0° to 300°, according to a setpoint curve shown in FIG. 4. The non-controllable zone ZNC corresponds to the electrical angle ranging from 300° to 360°.

(40) The control phase receives the direction of the assist torque R of the assist motor, the two functional phases V, W and the instantaneous motor torque C.sub.mi so as to control the two functional phase lines V, W of the assist motor 2 via the inverter 1 according to a switching table as represented in FIG. 5.

(41) There are 21 possible switches of the inverter depending on the activation signal and the defective switching cell.

(42) A switching determines a position of each of the switching cells of the inverter.

(43) For example, the switching C31 defines that the «low side» switching cell of the phase line U is in the closed position, that the «high side» switching cell of the phase line U is in the open position, the «low side» switching cell of the phase line V is in the closed position, the «high side» switching cell of the phase line V is in the open position, the «low side» switching cell of the phase line W is in the closed position, the «high side» switching cell of the phase line W is in the open position.

(44) The deactivated switching inverter is passive switch C0 in which the 6 switching cells are in an open position. In the uncontrollable zone, the inverter is in the deactivated switching inverter.

(45) In the controllable zone, the inverter is successively in 2 possible active switches.

(46) For a failure as represented in FIG. 2, that is to say a failure by short-circuiting the phase line U with the ground portion 12, the «low side» switching cell of the line phase U is closed. Thus, the inverter can be controlled using one of the active switches C31, C34, C35, C36, C205, C206, C209, C210, that is to say the active switches whose «low side» switching cell 119 of the phase line U is in the closed position.

(47) Moreover, there are possible ‘active’ switches C32, C33, C37, C38, C207, C208, C211, C212 corresponding to the switches available when the phase line U is short-circuited with the supply portion 13.

(48) There are possible ‘active’ switches C31, C32, C36, C37, C201, C202, C210, C212 corresponding to the switches available when the phase line V is short-circuited with the ground portion 12.

(49) There are possible ‘active’ switches C33, C34, C35, C38, C203, C204, C209, C211 corresponding to the switches available when the phase line V is short-circuited with the supply portion 13.

(50) There are possible ‘active’ switches C31, C32, C33, C34, C201, C203, C205, C207 corresponding to the switches available when the phase line W is short-circuited with the ground portion 12.

(51) There are possible ‘active’ switches C35, C36, C37, C38, C202, C204, C206, C208 corresponding to the switches available when the phase line W is short-circuited with the supply portion 13.

(52) At each electric revolution when the activation signal is emitted, the inverter will successively go through a monitoring phase in the controllable zone using all or part of the 8 control switches available and in a passive phase in the non-controllable zone corresponding to the switch C0.

(53) Of course, the invention is not limited to the embodiments described and represented in the appended figures. Modifications remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without thereby departing from the scope of protection of the invention.