DE102011101728A1 - Method for providing fail-safe operation for electro-mechanical steering system of motor car, involves actuating power semiconductor to energize windings in residual phase curve, so that braking torque generates support moment - Google Patents

Abstract

The method involves detecting whether a power semiconductor (T1-T6) e.g. MOSFET transistors and half bridge circuit, is provided with low impedance defect. A portion of a phase curve in which braking torque due to the low impedance defect is determined. The power semiconductor is actuated in a manner that windings (L1-L3) are energized in a residual phase curve, so that a braking torque generates a support moment. A maximum support moment is generated with the low impedance defect during a guidance maneuver. An independent claim is also included for an electromotor comprising a half bridge circuit.

Description

The invention relates to a method for emergency operation of an electromechanical steering system and an electronically commutated electric motor for an electromechanical steering.

Such an electronically commutated electric motor is for example from the DE 102 23 139 A1 known. Usually, these electric motors have a three-phase excitation winding in the stator, wherein permanent magnets are arranged on the rotor. The power supply is a DC voltage source, for example a vehicle battery. The windings are connected in a star or in delta connection and are supplied with current via a half-bridge circuit with inverse diodes or free-wheeling diodes. The switching elements in the half-bridge circuit are usually power semiconductors, preferably MOSFET transistors. The windings are connected to each other via a further MOSFET transistor, via which the star point or in a delta connection, the phases can be separated. It is also described that errors in the operation of the motor, in particular errors in one of the semiconductor switches of the commutation arrangement, or by a short circuit in one of the winding strands can be. When passing a MOSFET transistor (low-resistance defect) flows due to the induced by the rotation of the motor voltages in the winding strands, a current through the two inverse diodes of the two remaining half-bridge transistors and generates a braking torque, which in the use of the drive for an electronic power steering in a motor vehicle practically leads to the inflexibility of the vehicle. The document proposes as a remedy the separation of the star point. The disturbance is thereby switched off and the drive remains functional, albeit with reduced power. Alternatively, a full bridge circuit is proposed, which allows a compensation of the braking torque without switching off the disturbance.

From the DE 10 2009 023 372 A1 For example, there is known a control apparatus for an electric power steering system which includes a multi-phase rotating electric machine and a power conversion circuit connected to the rotary electric machine, which includes a power control device configured to perform the power conversion circuit in a predetermined operating state a predetermined supply of the rotating electrical machine controls. Further, the control means includes an abnormality detecting means configured to detect an abnormality of the rotary electric machine and / or the power conversion circuit based on detection of the rotation angle of the rotary electric machine due to the predetermined feed. Further, the document teaches that the feed control means is arranged to perform a steering angle control assisting routine by controlling, when one phase is broken, the switching elements in the bridge circuit connected to the other two phases. By energizing the two remaining phases, a substantially uniform torque is generated, regardless of the rotation angle of the motor. Therefore, by controlling the absolute value of the currents in accordance with the required torque, the torque of the motor can be controlled so that the desired torque is obtained, whereby the assisting routine is performed.

It is assumed, however, that the power conversion circuit is fully functional and even has no defect and a low-impedance defect occurs on the connection path between the power conversion circuit and the engine.

The invention is based on the technical problem of providing a method for emergency operation of an electromechanical steering system, which can compensate for low-resistance defects of a power semiconductor in a half-bridge circuit with a simple structure of the steering system, as well as to create a suitable electronically commutated electric motor.

The solution of the technical problem results from the objects with the features of claims 1 and 7. Further advantageous embodiments of the invention will become apparent from the dependent claims.

• – Erfassen, ob und welcher Leistungshalbleiter der Halbbrückenschaltung einen niederohmigen Defekt aufweist,
• – Ermitteln des Teils des Phasenverlaufs, indem ein Bremsmoment aufgrund des niederohmigen Defektes erzeugt wird und
• – Ansteuern der Leistungshalbleiter derart, dass im restlichen Phasenverlauf die Wicklungen derart bestromt werden, dass ein dem Bremsmoment entgegenwirkendes Unterstützungsmoment generiert wird.

For this purpose, the method for emergency operation of an electromechanical steering system, by means of an electronically commutated electric motor with windings in a star or delta circuit, a half-bridge circuit of power semiconductors and freewheeling diodes and a control device for driving the power semiconductors, comprises the following method steps:

• Detecting whether and which power semiconductor of the half-bridge circuit has a low-impedance defect,
• - Determining the part of the phase curve by a braking torque is generated due to the low-impedance defect, and
• - Driving the power semiconductor such that in the remaining phase curve, the windings are energized so that a brake torque counteracting support torque is generated.

It is dispensed with a star point separating star point relay or such a star point relay forming MOSFETs and compensated exclusively by suitable control of the power semiconductor braking torque. This eliminates components (star point relay or MOSFET transistors) and only the software of the control device must be substantially adapted.

The windings of the electronically commutated electric motor are therefore connected directly to each other while avoiding switching elements. The electric motor is preferably a permanent-magnet synchronous motor.

Preferably, in the case of a low-impedance defect, one of the power semiconductors generates a maximum assist torque during a steering maneuver. This ensures that during a steering maneuver the abrupt braking torque is compensated as far as possible by the assist torque. Although this causes sudden haptic feedback (on the one hand from the braking torque and the other from the support torque) on the steering wheel, but it helps the driver to carry out the steering maneuver to the end. The current through the windings is preferably almost rectangular. It should be noted that the size of the support torque can also be selected depending on the type of steering maneuver or the steering movement. For example, the support torque can be chosen to be greater when cornering than smaller steering movements, such as occur only in very elongated curves, for example.

In a further embodiment, the assist torque is varied in different phases, preferably reduced in successive phases. Although this causes a lower moment of support in the later phases and therefore a greater resulting braking torque. On the other hand, the motor vehicle driver had enough time to adjust to the braking torque, so that the withdrawal of the assist torque does not affect the controllability. Furthermore, the assisting moment can be generated haptically more pleasantly on the steering wheel with retroactive effect. The withdrawal of the support torque is carried out, for example stepped, with the respective levels can be chosen differently (both in terms of height and length).

In a further embodiment, the assistance torque is selected as a function of the driving situation, for example, whether the motor vehicle is on a motorway journey, overland travel, city driving, parking or a winding route (eg serpentines). For this purpose, it is advantageous to use the hand torque applied by the motor vehicle driver and the steering angle as input variables for emergency operation. Thus, a steering request of the motor vehicle driver is initially visible in an increase of the manual torque, before the supporting electric motor is accelerated.

In a further embodiment, a reduced assist torque is generated after switching off the ignition or shutdown of the electric machine in an electric vehicle and reconnection of the prime mover. This should cause the user to have the defect repaired in a workshop, whereby only so much support torque is generated to allow such a startup of a workshop.

The electronically commutated electric motor with windings for electromechanical steering comprises a half-bridge circuit comprising power semiconductors and free-wheeling diodes, and a control device for driving the power semiconductors, wherein a low-impedance defect (in particular a superalloy) of a power semiconductor can be detected by means of a diagnostic unit and part of the phase characteristic can be determined a braking torque due to the low-impedance defect is generated, wherein the power semiconductors are so antsteuerbar that in the remaining phase curve, the windings are energized so that a braking torque counteracting support torque is generated.

In a further embodiment, a device detects a driving situation and / or steering situation, the assistance torque being set by the control device as a function of the driving situation and / or steering situation. Preferably, the driving situation and / or steering situation is determined by a control unit of the electromechanical steering and the control device provided. For this purpose, preferably applied by the motor vehicle driver manual torque and the steering angle are used as input variables for emergency operation.

In a further embodiment, the assist torque is varied in different phases.

In a further embodiment, the assist torque is reduced in successive phases.

The invention will be explained in more detail below with reference to a preferred embodiment. The figures show:

1 1 is a schematic representation of a half-bridge circuit for the windings of an electronically commutatable electric motor for the example of a star circuit,

2 an exemplary representation of the braking torque over the motor angle at a low-impedance defect of one of the power semiconductor of the half-bridge circuit,

3 an exemplary representation of the braking torque, an exemplary assist torque and a resulting mean brake torque over the engine angle,

4 an exemplary representation of various current waveforms in the windings to generate a support torque,

5 a representation of other possible current waveforms in the windings to generate a support torque and

6 a schematic flow diagram of a method for an emergency operation.

In the 1 schematically a half-bridge circuit for an electronically commutated electric motor with three windings L1, L2 and L3 is shown. In each case two power semiconductors T1, T2; T3, T4 and T5, T6 connected in series, wherein the series circuits are each connected between a battery voltage U _B and ground. The center taps of each series connection are each connected to one terminal of an associated winding L1-L3, the respective other terminals of the three windings L1-L3 being directly connected to one another. It should be noted that the number of windings can also be greater than three. Parallel to each power semiconductor T1-T6, an antiparallel-connected freewheeling diode D1-D6 is arranged. The power semiconductors T1-T6 are preferably designed as MOSFET transistors, IGBTs or similar switchable power semiconductors. The power semiconductors T1-T6 are controlled by a controller 10 controlled by control signals S1-S6 to control a directional excitation current in the windings L1-L3. The control device 10 is next with a diagnostic unit 11 connected by data technology. The control device 10 and the diagnostic unit 11 can also be integrated together in a control unit. Finally, parallel to the series circuits of the power semiconductors T1, T2; T3, T4; T5, T6 optionally a smoothing capacitor C arranged.

If a low-resistance defect occurs, for example because of a breakdown of a power semiconductor T1-T6, then a regenerative braking torque occurs.

Now suppose that the power semiconductor T5 is alloyed, the battery voltage U _{B is} permanently applied to its center tap. Depending on the voltage induced in the windings L1-L3, a current flow can take place via the freewheeling diodes D1, D3. If, for example, the induced voltage level at the winding L1 is above the battery voltage U _B , then the freewheeling diode D1 is switched through and the current can flow via the freewheeling diode D1, the power semiconductor T5, the winding L3 and the winding L1. Similar explanations apply if the voltage level at the winding L2 can switch through the freewheeling diode D3.

This results in a speed-dependent braking torque that can extend over approximately 120 ° -240 ° of the motor or rotor position of the electric motor. This leads, for example, to a braking torque curve M _B , as shown in FIG 2 is shown. In this case, the torque is plotted against the motor angle φ, the negative moments representing braking torques. This corresponds to the effect of a two-phase short circuit in the windings L1-L3, which is hardly manageable without countermeasure by the motor vehicle driver.

This is where the invention begins. By means of the diagnostic unit 11 In this case, it is first determined whether and which power semiconductor T1-T6 has a low impedance defect. This can be done for example by an internal current monitoring of the power semiconductors T1-T6.

Now suppose that the power semiconductor T5 was detected as alloyed, the power semiconductor T6 must not be turned on, as this would then cause a short circuit. But how the 2 can be seen, there is a part of the phase curve, where no braking torque M _{B is} generated. In these areas, a support torque M _M can be generated by, for example, by switching through the power semiconductors T1 and T4, a current flow through the windings L1 and L2 to ground. Analogously, a current flow can be generated by switching through the power semiconductors T3 and T2. By suitable control of the remaining power semiconductors T1-T4, the windings L1-L2 can thus be energized in such a way that a support torque M _M counteracting the braking torque is generated. This is in 3 shown. By the assist torque M _M M _R a resultant moment is generated in the middle over a full phase trajectory of 360 °, which may still cause a braking effect, but the amount of the motor vehicle driver is manageable. The braking torque is preferably compensated as much as possible or as far as possible, even a resulting positive assist torque M _R generated.

The profile of the assist torque M _M is dependent on the course of the current flow i through the intact power semiconductors T1-T4. Possible current curves i are in the 4 it is assumed that within 1/3 T, the assist torque M _M can be generated, whereas in 2/3 T, the regenerative braking torque M _{B is} generated. T represents the period of a rotor rotation of the electric motor. In this case, a first curve 1) is shown, in which the current is rectangular, which generates a maximum assisting moment _{M M.} The alternations 2) and 3) shown alternatively have lower slopes and produce smaller moments of support M _M. However, the course 1) leads to strong moment pulsations, ie, there are almost sudden changes in the moments that can be felt haptically on the steering wheel. Accordingly, while the support torque M _{M is} somewhat lower in the courses 2) and 3), but also the moment pulsations. It can also be provided that the generated support moments M _M in successive phases vary, so for example in a first phase of the course 1) is impressed as a stream, in a second phase of the course 2) and in a third phase of the course 3) impressed become. This ensures that at the beginning of the resulting braking torque M _{R is} the lowest and while increasing in the course of further phases, but at the same time the momentary pulsations are lower.

In the 5 are alternative courses of the current i shown, which also with the other courses in 4 can be combined, in contrast to the courses 1) -3) the curves 4) and 3) are asymmetrical in the part of the phase curve.

In the 6 is a possible flowchart of a method for emergency operation shown. The diagnostic unit checks in a step S1 whether a power semiconductor T1-T6 is alloyed through. If this is negated, the normal operation NB takes place. This is usually a 3-phase operation with full steering assistance. If, on the other hand, an alloyed power semiconductor T1-T6 is detected, a driver warning FW (acoustic and / or optical and / or haptic) and an error entry FE into an error memory take place.

In parallel, it is determined in a step S2 which power semiconductors T1-T6 are still available in which part of the phase curve in order to generate an assist torque _{M M.} In a step S3, the current driving situation is determined, for. B. whether it is a highway ride, a cross-country drive, a city trip, a parking or a winding route. For this purpose, it is advantageous to use the hand torque applied by the motor vehicle driver and the steering angle as input variables for the emergency operation, so that the steering maneuvers can be reliably detected and treated in accordance with the situation. Thus, a steering request of the motor vehicle driver is initially visible in an increase of the manual torque, before the supporting electric motor is accelerated. In the case of very small hand moments, there is no steering requirement of the motor vehicle driver, but nevertheless movements of the electric motor can occur which excite a purely rotationally controlled commutation to unwanted steering movements. While embracing the steering wheel during a steering movement while maintaining the steering assistance is wanted, but not possible with a pure speed-based control.

Depending on the result of step S3, the assist torque is varied in a step S4 in order to obtain a sufficient support torque M _M at a low torque pulsation in accordance with the driving situation. Accordingly, the electronically commutatable electric motor is energized via the remaining power semiconductors T1-T6. In a step S5, it is checked whether the engine of the motor vehicle has been switched off and switched on again (in the case of an internal combustion engine KL15, from → on).

If this is the case, the diagnosis begins again with step S1, since the defective parts have been replaced if necessary. It can also be provided that the assist torque after reclosing is lower.

LIST OF REFERENCE NUMBERS

10

control device

11

diagnostic unit

C

smoothing capacitor

D1-D6

Freewheeling diodes

L1-L3

windings

S1-S5

control signals

T1-T6

Power semiconductor

U _B

battery voltage

M _B

Braking torque path

M _M

assist torque

M _R

resulting moment

1)

first course

2)

second course

3)

third course

4) -5)

further sales

NB

normal operation

FW

driver warning

FE

error entry

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 10223139 A1 [0002]
• DE 102009023372 A1 [0003]

Claims (10)

Method for emergency operation of an electromechanical steering system, by means of an electronically commutated electric motor with windings (L1-L3), which in a star or delta connection, a half-bridge circuit of power semiconductors (T1-T6) and freewheeling diodes (D1-D6) and a control device ( 10 ) for driving the power semiconductors (T1-T6), comprising the following method steps: a) detecting whether and which power semiconductors (T1-T6) or which half-bridge circuit has a low-resistance defect, b) determining the part of the phase characteristic in which a braking torque (M _B ) is generated due to the low-impedance defect and c) driving the power semiconductors (T1-T6) such that in the remaining phase curve, the windings (L1-L3) are energized such that the brake torque (M _B ) counteracting support torque (M _M ) is generated. A method according to claim 1, characterized in that in a low-impedance defect during a steering maneuver a maximum assisting moment (M _M ) is generated. A method according to claim 1 or 2, characterized in that the assisting moment (M _M ) is varied in different phases. Method according to Claim 3, characterized in that the assisting momentum (M _M ) is reduced in successive phases. Method according to one of the preceding claims, characterized in that the assistance torque (M _M ) is selected as a function of a detected driving situation. Method according to one of the preceding claims, characterized in that after switching off and reconnection of a drive machine, a reduced assist torque (M _M ) is generated. Electronically commutated electric motor with windings (L1-L3) for electromechanical steering, comprising a half-bridge circuit comprising power semiconductors (T1-T6) and freewheeling diodes (D1-D6) and a control device ( 10 ) for driving the power semiconductors (T1-T6), characterized in that by means of a diagnostic unit ( 11 ) a low-impedance defect of a power semiconductor (T1-T6) can be detected and a portion of the phase profile can be determined in which a braking torque (M _B ) is generated due to the low-impedance defect, wherein the power semiconductors (T1-T6) are controlled such that in remaining phase curve, the windings (L1-L3) are energized such that the braking torque (M _B ) counteracting support torque (M _M ) is generated. Electric motor according to claim 8, characterized in that a device detects or determines a driving situation and / or steering situation, wherein the assisting moment (M _M ) is set as a function of the driving situation and / or steering situation. Electric motor according to claim 7 or 8, characterized in that the assisting moment (M _M ) is varied in different phases. Electric motor according to one of claims 7 to 9, characterized in that the assisting moment (M _M ) is reduced in successive phases.

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