Method for controlling a brake system

Abstract

A brake system comprises a cylinder-piston unit movable by an electromechanical actuator. Wheel brakes associated with at least one axle can be supplied with braking pressure via the hydraulic pressure chamber. The electromechanical actuator comprises a rotation-translation transmission and an electronically commutated synchronous machine having a stator with at least two phase windings, a rotor comprising at least one permanent magnet and at least one rotor position sensor. A torque-forming current and/or a magnetic field attenuating current are adjusted in a co-ordinate system which is fixed relative to the rotor. Voltages in the co-ordinate system are detected to serve as control variables and are transformed into a voltage phasor, which indicates for each phase winding of the stator, a voltage to be applied, and a set value for the magnetic field attenuating current is limited to a maximum value being determined from a predetermined characteristic map.

Claims

1. A method of operating a brake system for motor vehicles with an electrically controllable pressure supply device comprising: supplying brake pressure to a plurality of hydraulic wheel brakes from a hydraulic pressure chamber; displacing a piston within the hydraulic pressure chamber with an electromechanical actuator, wherein the electromechanical actuator comprises a rotation-translation gearbox and an electrical machine, wherein the electrical machine is an electronically commutated synchronous machine having a stator with at least two phase windings and a rotor comprising at least one permanent magnet as well as at least one rotor position sensor; regulating at least one of a torque-forming current and a magnetic field attenuation current in a coordinate system that is fixed relative to the rotor; transforming voltages in the coordinate system into control variables using a measured rotor position, wherein the control variables are a voltage vector that gives a voltage to be applied for each phase winding of the stator; determining a measured revolution rate of the rotor from a first predetermined characteristic field; and limiting a target for the magnetic field attenuation current to a maximum value according to the measured revolution rate and to not fall below a minimum value that is predetermined according to the measured revolution rate; and determining a target value for the magnetic field attenuation current according to the measured revolution rate of the rotor from the first predetermined characteristic field.

2. The method of claim 1, wherein there are three phase windings.

3. The method of claim 1, further comprising adapting the target value for the magnetic field attenuation current using a ratio between a reference voltage and a current supply voltage.

4. The method of claim 1, further comprising limiting a target value for the torque-forming current to a maximum value of the torque-forming current according to the measured revolution rate of the rotor.

5. The method of claim 4, wherein the target value for the magnetic field attenuation current and the maximum value of the torque-forming current are predetermined so that a predetermined limit value for a total current of the magnetic field attenuation current and the torque-forming current is not exceeded and that the torque of the electronically commutated synchronous machine is at a maximum while complying with the limit value for the total current.

6. The method of claim 1, wherein the limiting is carried out after the target value for the magnetic field attenuation current has been determined according to a difference between the magnitude of a predetermined maximum voltage, which corresponds to a current supply voltage minus a predetermined voltage interval, and the magnitude of a voltage vector formed by the torque-forming voltage and the magnetic field attenuation voltage, wherein the regulation of said difference is carried out.

7. The method of claim 6, wherein the regulation of the difference between the magnitude of a predetermined maximum voltage and the magnitude of a voltage vector formed from the torque-forming voltage and the magnetic field attenuation voltage is carried out when the magnitude of the voltage vector is less than a predetermined minimum value, wherein pre-control of the target value for the magnetic field attenuation current is carried out by predetermining the maximum permissible field attenuation current.

8. The method of claim 1, further comprising determining a target value for the torque-forming current according to a deviation between a target revolution Rate and the measured revolution rate of the rotor, and limiting the target value for the torque-forming current to a maximum value according to a target value for at least one of the magnetic field attenuation current, a maximum permissible total current, and a maximum permissible motor torque.

9. The method of claim 1, wherein a torque-forming voltage in the coordinate system that is fixed relative to the rotor is determined according to the difference between a target value for the torque-forming current and a measured torque-forming current, and limiting the torque-forming voltage according to a difference between the magnitude of a predetermined maximum voltage and the magnitude of a magnetic field attenuation voltage.

10. The method of claim 1, wherein the predetermined maximum voltage is a supply voltage.

11. The method of claim 1, wherein a magnetic field attenuation voltage in the coordinate system that is fixed relative to the rotor is determined according to the difference between a target value for the magnetic field attenuation current and a measured magnetic field attenuation current.

12. The method of claim 1, further comprising determining a measured torque-forming current and a measured magnetic field attenuation current in the coordinate system that is fixed relative to the rotor from measured currents through the phase windings of the rotor using the measured rotor position.

13. The method of claim 1, further comprising adapting the target value for the magnetic field attenuation current using at least one of a measured rotor temperature, a measured stator temperature, and a current supply voltage.

14. The method of claim 1, supplying the phase windings of the stator with current from a pulse width modulation circuit, wherein a voltage to be applied to a phase winding of the stator is converted into a degree of actuation of the corresponding pulse width modulation circuit.

15. The method of claim 14, further comprising adapting the degree of actuation according to a ratio of a reference voltage and a current supply voltage.

16. The method of claim 15, wherein the current supply voltage is a measured intermediate circuit voltage of the pulse width modulation circuit.

17. An electronic control unit for a brake system, comprising: an actuation circuit for an electronically commutated synchronous machine; a computation unit and semiconducting switch elements disposed in at least one pulse width circuit associated with each phase of a stator for the machine; wherein a maximum value for a magnetic field attenuation current is determined using pairs of values of measured revolution rates and associated maximum values read out from a non-volatile memory; wherein a measured revolution rate of the rotor is from a first predetermined characteristic field; wherein a target value for the magnetic field attenuation current is limited to a maximum value according to the measured revolution rate and is limited to not fall below a minimum value that is predetermined according to the measured revolution rate; and wherein the target value for the magnetic field attenuation current according to the measured revolution rate of the rotor from the first predetermined characteristic field.

18. The electronic control unit of claim 17, wherein the at least one bridge circuit is a pulse width modulation circuit associated with each phase of the stator.

19. A brake system comprising: an electrically controllable pressure supply circuit; a cylinder-piston arrangement with a hydraulic pressure chamber and a piston displaceable by an electromechanical actuator; a plurality of hydraulic wheel brakes that are associated with at least one axle of the vehicle and that can be supplied with brake pressure by the hydraulic pressure chamber; a sensor for detecting braking intention; wherein the electromechanical actuator comprises a rotation-translation gearbox and an electronically commutated synchronous machine comprising: a stator with at least two phase windings; a rotor comprising at least one permanent magnet; at least one rotor position sensor; and an electronic control unit comprising: an actuation circuit for the electronically commutated synchronous machine; a computation unit and semiconducting switch elements disposed in at least one pulse width modulation circuit associated with each phase of the stator; and wherein a maximum value for a magnetic field attenuation current is determined using pairs of values of measured revolution rates and associated maximum values read out from a non-volatile memory; and wherein a measured revolution rate of the rotor is from a first predetermined characteristic field; wherein a target value for the magnetic field attenuation current is limited to a maximum value according to the measured revolution rate and is limited to not fall below a minimum value that is predetermined according to the measured revolution rate; and wherein the target value for the magnetic field attenuation current according to the measured revolution rate of the rotor from the first predetermined characteristic field.

20. The brake system as of claim 19, wherein an electrical drive connected to at least one wheel of the motor vehicle, is actuated so that said drive produces a braking deceleration of the vehicle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:

(2) FIG. 1 shows a schematic representation of an actuation method or an actuation circuit according to a first exemplary embodiment of the invention;

(3) FIG. 2 shows a schematic representation of an actuation method or an actuation circuit according to a second exemplary embodiment of the invention;

(4) FIG. 3 shows a schematic representation of an actuation method or an actuation circuit according to a third exemplary embodiment of the invention; and

(5) FIG. 4 shows the basic design an arrangement for operating a permanently energized synchronous machine.

DETAILED DESCRIPTION

(6) Regarding the hydraulic design of the brake system, refer to DE 10 2010 040 097 Al, which is incorporated herein by reference.

(7) The actuation according is particularly suitable for actuating permanently energized synchronous machines with limited inductance that are to be operated in a wide range of revolution rates with high output power. The actuation of brushless motors with equal inductances in the directions of the d-axis and the q-axis is described below; in principle, however, motors with a reluctance torque can also be actuated according to a method according to the invention.

(8) According to a particularly preferred embodiment, the imposed magnetic field attenuation current id is not set by a regulator, but is predetermined so that the maximum possible motor torque can be output at a determined revolution rate. Said type of actuation makes use of the fact that different driven actuators, in particular in brake systems, primarily operate at the voltage and current limits of the drive. If the actuator is within the target range of the higher level system, i.e. for example almost at the target position, then the requested revolution rate is low and the working points of the drive lie at the current limit, so that no magnetic field attenuation current is necessary there. If the higher level system requests a changed rotor position (not necessarily because of position regulation), then said changed rotor position should often be approached in the shortest possible time, whereby the working points that are passed through lie at the voltage limit.

(9) The actuation method according to a first exemplary embodiment that is shown schematically in FIG. 1 therefore provides that target values id* for the magnetic field attenuation current are predetermined according to the measured revolution rate. This can be carried out advantageously by analyzing a characteristic field that describes the profile of the optimum magnetic field attenuation current at which the maximum power and/or the maximum torque is/are output against the revolution rate. The determination of the characteristic field can be carried out based on simulations and calculations using the motor parameters. Alternatively or additionally, the characteristic field can also be determined experimentally by increasing the component of the magnetic field attenuation current stepwise at different fixed revolution rates for the requested maximum phase current until the maximum output motor torque is reached. It is particularly advantageous if a characteristic field is calculated first and then confirmed by measurements.

(10) A target revolution rate n* is predetermined by a control unit of the overall system and is compared in the revolution rate regulator 1 with a measured motor revolution rate or a rotor revolution rate n in order to generate a control variable that corresponds to the requested torque-forming current iq*. The revolution rate regulator can in particular be implemented as a PI regulator, i.e. comprising a proportional component and an integral component.

(11) The iq current limiter 5 that is connected downstream limits the target current value iq* so that, depending on the current target value id* for the magnetic field attenuation current, the magnitude of the total current vector i.sub.total does not exceed a predetermined maximum value, wherein said magnitude can be calculated according to the following relationship:
i.sub.total={square root over (iq.sup.2+id.sup.2)}

(12) The respective maximum value iq.sub.max for the torque-forming current is determined by module 17, for example using the relationship:
iq.sub.max={square root over (i.sub.max.sup.2i.sub.d*.sup.2)}

(13) In addition, further limiting of the target current value iq* is carried out so that the maximum permissible motor torque is not exceeded:

(14) ${\mathrm{iq}}_{\max }=\frac{M_{\max }}{k_r}$

(15) Alternatively, it can also be provided that the maximum value iq.sub.max for the torque-forming current is determined by reading out a characteristic field according to the revolution rate.

(16) This is particularly advantageous because in module 18 the target value id* for the magnetic field attenuation current in said exemplary embodiment is determined from a characteristic field according to the revolution rate, and thus only a small amount of computing power is necessary for carrying out the method.

(17) The on-board electrical wiring of a vehicle can have varying supply voltages depending on the condition of the vehicle and the state of charge of the battery. If the actuated motor exhibits uniform behavior regardless of the current supply voltage, then it is advantageous to prescribe the characteristic curve for a predetermined reference voltage, in particular a minimum permissible value for the supply voltage, and when determining a target value id* for the magnetic field attenuation current to carry out scaling according to the ratio of the reference voltage and the current or measured supply voltage.

(18) If both the target value id* for the magnetic field attenuation current and the maximum value iq.sub.max for the torque-forming current are determined according to the revolution rate using a characteristic field, then the characteristic of the motor can be adapted according to the envisaged application by predetermining suitable characteristic fields.

(19) If an application requests that a certain input direct current is not exceeded in the motor mode, the values of id* and iq.sub.max can be predetermined according to the revolution rate so that the output motor torque is at a maximum while complying with the limit for the input direct current.

(20) If an application requests that a certain feedback current is not exceeded in the generator mode, the values of id* and iq.sub.max can be predetermined according to the revolution rate of so that the output motor braking torque is at a maximum while complying with the limit for the feedback current.

(21) If an application requests that the received maximum input direct current is reduced at supply voltages that are less than the rated voltage range or the reference voltage, then this can be achieved by reducing the maximum value iq.sub.max as a function of the measured or available supply voltage.

(22) Depending on the application, in addition the magnet temperature and the winding temperature can be taken into account when determining the values of id* and iq.sub.max.

(23) The determined target value id* is fed to the current regulator 3, which determines a target magnetic field attenuation voltage ud in the direction of the d-axis using the comparison of id* with the measured magnet field attenuation current id. It is advantageous if the current regulator 2 is implemented as a PI regulator, i.e. has a proportional component and an integral component.

(24) The target magnetic field attenuation voltage du may correspond to the maximum of the available voltage Umax in the intermediate circuit or to the supply voltage and is therefore limited to a corresponding value in the limiter 7.

(25) The current regulator 2 for the torque-forming current compares the target current value iq* with a current measured torque-forming current iq and generates a control variable that corresponds to the target voltage uq in the direction of the q-axis. Advantageously, the current regulator 2 can be implemented as a PI regulator, i.e. with a proportional component and an integral component.

(26) The target torque-forming voltage is fed to a limiter 8 that prevents the magnitude utotal of the total voltage vector of the torque-forming voltage and the magnetic field attenuation voltage from exceeding the maximum available voltage Umax:
u.sub.total{square root over (uq.sup.2+ud.sup.2)}

(27) The maximum value uq.sub.max for the torque-forming voltage is advantageously determined in module 15 using the following relationship:
uq.sub.max={square root over (U.sub.max.sup.2u.sub.d.sup.2)}

(28) Alternatively, it can also be provided to read out the maximum value for the limiter 8 from a characteristic field.

(29) The target values for the torque-forming voltage uq and for the magnetic field attenuation voltage ud, i.e. the voltage vector in the coordinate system that is fixed relative to the rotor, are transformed into the coordinate system that is fixed relative to the stator in module 10 using the measured rotor position and are converted in module 11 into a voltage vector that gives the voltages uu, uv, uw that are to be applied to the individual phase windings. This can be carried out with a suitable transformation, such as the inverse Clarke and Park transformation; methods for such a coordinate transformation are known.

(30) The phase windings of the stator are energized by means of a bridge circuit consisting of power semiconductors, wherein advantageously a pulse width modulation takes place. The semiconducting switches can for example be in the form of sense FETs in order to enable the measurement of the current flowing through the phase windings. Alternatively, direct measurement by means of a shunt or an inductive current sensor is also possible. The currents obtained iu, iv, iw are converted in module 13 in the coordinate system that is fixed relative to the stator and are transformed into the coordinate system that is fixed relative to the rotor in module 12 using the measured rotor position (or alternatively are transformed in one step).

(31) Advantageously, for measurement of the rotor position a resolver is used, from the signals of which an (electrical) rotor angle can be determined. Said rotor angle is fed to module 14, which determines a motor revolution rate or a rotor revolution rate (in particular from a change of the signals).

(32) Owing to the fact that a target value for the magnetic field attenuation current is determined using a characteristic curve, the actuation according to this implementation of the invention comprises a quite particularly simple structure.

(33) According to an alternative exemplary embodiment, which is represented in FIG. 2, the magnetic field attenuation current is also regulated, wherein the target value id* for the magnetic field attenuation current is limited according to the measured revolution rate. Modules that provide an identical function in this embodiment as in the first exemplary embodiment are provided with the same reference characters, and for a detailed description we refer to the above implementations.

(34) Revolution rate regulator 1 compares the target revolution rate n* and the current rotor revolution rate n and generates a target value iq* for the torque-forming current as a control variable. The limiter 5 that is connected downstream limits the target current value iq* so that, depending on the current target value of the current id*, the permissible magnitude of the total current vector is not exceeded. For this purpose, a maximum value iq.sub.max is determined in module 17 by calculation or by reading out from a characteristic field and is provided to the limiter. Current regulator 2 compares the target current value iq* and the currently existing torque-forming current iq and specifies a target voltage uq, which is limited in the limiter 8 that is connected downstream using the supply voltage and the target magnetic field-forming voltage du. In this case, in module 15 the maximum value is predetermined so that the magnitude of the voltage vector does not exceed the available supply voltage.

(35) The voltage vector of the torque-forming voltage and the magnetic field attenuation voltage that is determined in the coordinate system that is fixed relative to the rotor is subjected in modules 10 and 11 (or a combined module) to a suitable transformation, such as an inverse Clarke and Park transformation, in order to obtain the voltage vector of the voltages to be applied to the individual phase windings.

(36) Furthermore, the torque-forming voltage uq and the magnetic field attenuation voltage ud are fed to a module 16 for voltage monitoring, which determines the separation of the voltage vector from a voltage limit Ures, or subtracts the square of the voltage vector from the square of the voltage limit (the square root may then be formed). The voltage limit is advantageously smaller than the available voltage Umax by a predetermined voltage difference, so that a voltage reserve is reserved for imposing new target current values.
=Ures.sup.2ud.sup.2uq.sup.2

(37) The determined difference can be limited in limiter 9 to values lying in a predetermined interval before it is fed as a control difference to a field attenuation regulator 4. Said field attenuation regulator 4 can advantageously be implemented as an I or PI regulator, i.e. comprising integral and possibly proportional terms. The field attenuation regulator produces a target value id* for the magnetic field attenuation current as a control variable using the control difference.

(38) Said target value is limited in the negative direction to a predetermined maximum value id.sub.max in the limiter 6. The determination of said maximum value for id* is carried out according to the revolution rate in module 18 using a characteristic field. In contrast to the first exemplary embodiment, a target value is not determined, but only a maximum value idmax is determined using a characteristic field. The predetermined characteristic field can be calculated using simulations and/or measured or verified in experiments. In the limiter 6, furthermore, the target value id* is limited in the positive direction to zero. It is thereby ensured that the current applied in the direction of the d-axis has a magnetic field attenuating effect.

(39) The target value id*, which may be limited, is compared with the measured magnetic field attenuation current in the current regulator 3, wherein a target value ud for the voltage is generated in the direction of the d-axis as a control variable. The further modules, such as the limiter 7, the module 14 for determining the revolution rate and the modules 12 and for the Clarke and Park transformation of the measured currents through the phase windings operate as already described.

(40) According to a preferred embodiment, the magnetic field attenuation current is limited so that a minimal current in the negative d-axis direction is also predetermined and is switched to the output of the field attenuation regulator 4. Said minimal current can advantageously also be determined according to the revolution rate using a characteristic field.

(41) FIG. 3 shows such an exemplary embodiment of the method according to the invention, which builds on the implementation according to FIG. 2. Modules that provide an identical function in this embodiment as in the first exemplary embodiment are provided with the same reference characters and we refer to the above implementations for a detailed description.

(42) In order to increase the achievable system dynamics, advantageously the maximum permissible field attenuation current idmax is switched as a pre-control variable to the regulator output of the field attenuation regulator 4, which reduces the magnetic field attenuation current to the required value. It is advantageous if the pre-control variable is continuously reduced to zero after a predetermined time. When operating the motor in the field attenuation region, i.e. at high revolution rates with reduced torque, it can thereby be prevented that an unnecessarily large magnetic field attenuation current is imposed for a long time and that unnecessary current heat losses occur.

(43) The output of said regulator can be limited depending on the revolution rate so that it is never less than zero and never greater than the difference between the maximum permissible magnetic field attenuation current id and the magnetic field attenuation current id that is at least necessary at a revolution rate. For this purpose, module 19 can provide values determined according to the revolution rate using a characteristic field to the limiter 6.

(44) In the case of a dynamic target value request with the regulator having reached a steady state, the output value and the I-component of the regulator can be manipulated in a suitable manner. It can be provided to set the I-component of the regulator to a predetermined starting value according to the target value request.

(45) By suitably adapted characteristic fields, particularly dynamic operation of the electronically commutated synchronous machine is thereby guaranteed, wherein control requests are thus implemented in a minimal time. Especially in hazardous situations, a rapid build-up of pressure can be ensured thereby.

(46) FIG. 4 shows the schematic design of an arrangement for operating a permanently energized synchronous machine that can carry out a method according to the invention.

(47) The actuation circuit 41 comprises a computation unit 40, a memory 48 and a bridge circuit or a power end stage 42. In particular, the computation unit 40 can be implemented as a microcontroller comprising an integrated working memory and a non-volatile program memory 47. A suitable microcontroller can also comprise a memory 48, which for example can be implemented as a flash memory. It is advantageous if the microcontroller comprises one or more analog-to-digital converters that are connected to measurement devices on power end stages or rotors. The power end stage can for example comprise sense FETs in order to enable a current measurement. It is also advantageous if means for voltage measurement are provided. In principle, external sensors can also be connected by means of a data bus. The permanently energized synchronous machine 44 comprises a stator with phase windings that are energized by the power end stage 42 and a rotor that is mechanically connected to the load, which is not shown. The position of the rotor is determined by a sensor 46, which for example is implemented as a resolver or an optical rotary encoder. It is further advantageous if sensors for the measurement of the temperature of phase windings, magnets or generally the surroundings are provided. In principle, the method according to the invention can also be implemented by a customer-specific circuit comprising specifically adapted components and in particular being integrated on a semiconducting substrate.

(48) The foregoing preferred embodiments have been shown and described for the purposes of illustrating the structural and functional principles of the present invention, as well as illustrating the methods of employing the preferred embodiments and are subject to change without departing from such principles. Therefore, this invention includes all modifications encompassed within the scope of the following claims.