Electric brake system for a motor vehicle

The invention relates to an electric brake system for a motor vehicle wherein each wheel of the vehicle includes a brake actuator (2) assigned thereto. The invention also relates to a brake actuator for an electric brake system. During a braking operation, the brake linings (20a and 20b) are pressed against the brake disc (30) with a pregiven braking force by a braking-force device (10) of the brake actuator (2). At least one of the brake actuators (2) includes a sensor with which an elastic deformation of the braking-force device (10) or the brake yoke (18) is measured. This elastic deformation occurs during a braking operation. The braking force is determined from the measured elastic deformation and this braking force is then available for the control of the brake system.

FIELD OF THE INVENTION
 The invention relates to an electric brake system for a motor vehicle such
 as a passenger car which, inter alia, is equipped with a wheel brake for
 each wheel of the vehicle. Each wheel brake includes a brake actuator
 having, inter alia, a braking-force device which applies a braking force
 to a brake lining during a braking operation whereby a braking force is
 generated at the wheel. The brake actuator includes an element which
 supports the applied braking force. the invention further related to a
 brake actuator for an electric brake system.
 BACKGROUND OF THE INVENTION
 In recent times, brake systems of the brake-by-wire type were developed for
 motor vehicles, especially for passenger cars. In a brake system of this
 kind, the brake command of the driver is made apparent by applying a foot
 force to a brake pedal and the effect of the foot force on the pedal is
 detected by a sensor and converted into an electric signal. The signal is
 then transmitted to brake actuators of which one is assigned to each wheel
 of the motor vehicle and each brake actuator exercises, inter alia, a
 braking force on the brake lining with the aid of a braking-force device.
 The braking-force device is driven by an electric motor. The brake linings
 are pressed against the brake discs of the wheel brakes under the
 influence of the braking force whereby a braking force is generated at the
 wheel of the motor vehicle.
 The braking force which is applied by the braking-force device must be
 controlled in dependence upon the brake command of the driver of the motor
 vehicle. A simpler possibility to do this comprises supplying a specific
 motor current to the electric motors of the brake actuators in dependence
 upon the brake command of the driver in consequence of which a specific
 braking force is applied by the braking-force device of the brake
 actuators.
 This possibility is however problematic in that the brake actuators always
 exhibit an internal friction which leads to a hysteresis in the brake
 actuators. For this reason, it is not possible to clearly assign a braking
 force to a pregiven motor current; instead, for a pregiven motor current,
 the braking force always occurs at an undetermined point within a
 braking-force interval. Accordingly, only via a measurement of the braking
 force in the brake actuators can it be clearly and precisely determined
 which braking force is generated for a pregiven motor current by a
 braking-force device of a brake actuator. The measured value can be used
 for the purpose of adapting the actual braking force to the desired
 braking force corresponding to the brake command of the driver of the
 motor vehicle. For the above reasons, it is desirable that the braking
 force be measurable in the brake actuators as easily as possible.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide an electric brake system
 wherein the braking force applied by the braking-force device can be
 easily measured in at least one of the brake actuators. It is still
 another object of the invention to provide a brake actuator suitable for a
 brake system of this kind.
 The electric brake system of the invention is for a motor vehicle including
 a passenger car having a wheel brake for each wheel thereof, each wheel
 brake including a brake lining to which a braking force is imparted during
 a braking operation. The electric brake system includes: brake actuators
 corresponding to respective ones of the wheel brakes; each one of the
 brake actuators including a braking-force device for applying a braking
 force to the brake lining during a braking operation whereby a braking
 force is generated at the wheel; the one brake actuator also including an
 element for supporting the braking force; at least one of the
 braking-force device and the element being elastically deformable during
 the braking operation; and, at least one of the brake actuators including
 at least one sensor for measuring the elastic deformation occurring at one
 of the braking-force device and the element and for providing a signal
 representing the elastic deformation from which the braking force can be
 determined.
 The advantages achieved by the invention are seen in that the braking
 force, which is generated by the braking-force device, is determined from
 the elastic deformation of a component of the brake actuator. This
 component is anyhow always present in the brake actuator. In this way, the
 sensor can be configured of a few components and be integrated into the
 brake actuator and be well protected against external mechanical loads.
 According to another feature of the invention, at least a portion of the
 sensor is within the braking-force device, that is, within the element
 which supports the braking force. The advantage of this embodiment is seen
 in that the sensor, or at least a portion of the sensor, is surrounded on
 all sides by the braking-force device or by the element which supports the
 brake lining. In this way, protection against external mechanical loads is
 especially good.
 According to still another feature of the invention, the stiffness of the
 braking-force device or the element which supports the brake lining, is
 reduced in the measuring range of the sensor. This can take place, for
 example, via a reduction in the cross section of the material or by
 inserting a resilient element or by inserting a material having a lower
 modulus of elasticity than the remaining material of the braking-force
 device or of the element which carries the brake lining. The advantage of
 this feature of the invention is that even a slight change of the braking
 force generates an additional elastic deformation of the braking-force
 device or of the element which supports the brake lining which can be
 measured by the sensor. Accordingly, the measurement is especially precise
 which makes possible a correspondingly good control of the electric brake
 system on the basis of the measured actual braking forces.
 According to still another feature of the invention, the sensor is built
 into the brake actuator in such a manner that a clear measurement signal
 is generated thereby in the force-free state of the braking-force device.
 This clearly distinguishes from the measurement signals generated when the
 braking-force device applies a braking force. The advantage of this
 feature of the invention is that the force-free state of the braking-force
 device is reliably recognized. Accordingly, the situation described below
 cannot occur.
 An elastic deformation and therefore a braking force (because of
 measurement inaccuracies) is detected by the sensor even though no braking
 force is applied by the braking-force device. In this case, the
 braking-force device is driven back further by the motor of the brake
 actuator until an elastic deformation identical to 0 is indicated by the
 sensor and therefore a force-free condition of the braking-force device.
 This can, in some instances, lead to damage to the brake actuator.
 With this further feature of the invention, the case is reliably avoided
 that no elastic deformation and therefore the force-free condition of the
 braking-force device is indicated by the sensor (again, because of
 measuring inaccuracies) even though the braking-force device still applies
 force to the brake linings. Because of the defective indication of the
 sensor, the braking-force device is not driven back further by the
 electric motor so that the brake lining rubs continuously on the brake
 disc of the brake. In summary, it can be said that a destruction of the
 brake actuator or excessive wear of the brake linings is reliably avoided
 by this further feature of the invention.
 A reliable indication of the force-free state of the braking-force device
 can be provided by the sensor. This is done in that the sensor is
 assembled from at least two parts. A measurement signal is only generated
 by the sensor when one part of the sensor is located in the operating
 region of the other part of the sensor and the sensor is built into the
 brake actuator in such a manner that the one part of the sensor in the
 force-free state of the braking-force device is not located in the
 operating region of the other part of the sensor. The first part of the
 sensor can be brought out of the operating region of the second part of
 the sensor in the force-free state of the braking-force device. This is
 achieved, for example, in that the first part of the sensor is operatively
 connected to a spring which is pressed together when a braking force is
 applied by the braking-force device and presses the first part of the
 sensor into the operating region of the second part of the sensor and, in
 the force-free state of the braking-force device, presses the first part
 of the sensor out of the operating region of the second part of the
 sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
 In the description of the embodiments which follows hereinafter, reference
 is always made to a disc brake and that means especially that the element
 thereof, which supports the braking forces, is configured as a brake yoke.
 However, it is emphasized that the invention is not limited to a disc
 brake but is also applicable to drum brakes.
 FIG. 1 shows a brake actuator 2 (not to scale) which operates together with
 a brake yoke 18 which, in turn, supports the brake linings 20a and 20b.
 The operation of brake actuators 2 of this kind is known per se so that
 only a short explanation thereof will follow.
 The brake actuator 2 includes an electric motor 4 comprising a stator 6 and
 a rotor 8. With the aid of the stator 6 and rotor 8, a spindle nut 14 is
 rotated about an axis defined by the spindle 12. The rotational movement
 of the spindle nut 14 effects an axial movement of the spindle 12 and an
 axial movement of the brake piston 16. In this way, during a braking
 operation, the brake linings 20a and 20b, which coact with the brake
 piston 16, are likewise set into axial movement so that the brake linings
 20a and 20b lie against the brake disc 30 after passing through the air
 gap and bring about a braking force. The braking force is dependent upon
 the braking force which is applied by the spindle 12 to the brake piston
 16. After a braking operation, the brake linings 20a and 20b, the brake
 piston 16 and the spindle 12 are all moved back to their starting
 positions by return forces, that is, by the reverse drive of the electric
 motor 4.
 The braking force is applied by the braking-force device 10 (especially by
 the spindle 12) via the brake piston 16 on the brake linings 20a and 20b.
 This braking force leads to a force in the spindle 12 or in the brake yoke
 18 which is, in magnitude, precisely as large as the braking force but in
 the opposite direction. This force leads to an elastic deformation of the
 braking-force device 10 and especially of the spindle 12 or to an elastic
 deformation of the brake yoke 18 which can be measured with the aid of a
 sensor 24.
 For this purpose, the sensor 24 is either mounted in a cavity of the
 spindle 12 provided therefor in such a manner that the spindle 12
 coaxially surrounds the sensor 24 or, the sensor is located in a cavity of
 the brake yoke 18 provided therefor. The braking force applied to the
 brake linings 20a and 20b can be directly measured from the elastic
 deformation of the braking-force device 10 (especially the spindle 12 or
 the brake yoke 18) which is measured by the sensor 24.
 The elastic deformation to be measured can be amplified in that the
 stiffness of the spindle 12 or of the brake yoke is reduced in a targeted
 manner by inserting a resilient element in the region 50 of the sensor 24.
 This can be done, for example, with a resilient disc or by inserting a
 material having a modulus of elasticity in the region 50 which is less
 than the modulus of elasticity of the surrounding material. A further
 possibility is that the cross section of the material can be reduced in
 the region 50 as shown in FIG. 1.
 The sensor 24, which is shown in FIG. 1, is configured as an LVDT sensor
 having an operation which is known per se so that only a short description
 is needed. The LVDT (linear variable differential transformer) sensor can,
 for example, be obtained from the Lucas Schaevitz Company and from the
 Micro-Epsilon Company. The ferromagnetic core 26 dips deeper into the coil
 of the LVDT sensor 24 because of the elastic deformation of the
 braking-force device 10 and especially because of the deformation of the
 spindle 12 or because of the elastic deformation of the brake yoke 18. As
 a consequence thereof, the measuring signal applied to the connecting line
 32 changes.
 FIG. 2 shows a brake actuator 2 and a brake yoke 18 and the brake actuator
 2 is configured precisely as shown in FIG. 1. The only difference is that
 the sensor 24, which is accommodated in the cavity of the spindle 12 of
 the braking-force device 10 or in the cavity of the brake yoke 18, has
 another configuration and another mode of operation which is explained
 below.
 The hollow cavity, which is contained in the spindle 12 or in the brake
 yoke 18, is filled with a fluid and contains a pressure sensor 24 which is
 preferably attached to a wall of the cavity space. The signal is
 transmitted via a connecting line 32 for evaluation.
 As a consequence of a braking operation, an elastic deformation of the
 spindle 12 or of the brake yoke 18 occurs so that the volume of the hollow
 space in the spindle 12 or in the brake yoke 18 is reduced and the
 pressure in the fluid increases. The increase in pressure is detected by
 the pressure sensor 24 and a corresponding signal is outputted onto the
 connecting line 32 for evaluation so that the elastic deformation and
 therefore the applied braking force can be determined from the signal.
 FIG. 3 shows a brake actuator 2 which is configured precisely as the brake
 actuator 2 shown in FIG. 1 and likewise coacts with a brake yoke 18. The
 single difference is that the sensor 24 exhibits another configuration and
 another mode of operation which will be explained below. Here again, a
 detailed explanation is unnecessary because the sensor is known and
 described in detail in the SAE Technical Paper Series 910856.
 The sensor 24 comprises a permanent magnet core which is concentrically
 surrounded by the spindle 12. The spindle 12 is made of ferromagnetic
 material in the region of the permanent-magnetic core 34. The magnetic
 zones of this ferromagnetic material are directed in a preferred direction
 under the influence of the permanent-magnetic core 34 so that a magnetic
 field is developed by the ferromagnetic material of the spindle 12 in the
 region of the permanent-magnetic core 34. This magnetic field is
 superposed on the magnetic field generated by the permanent-magnetic core
 34 in accordance with the superposition principle. When there is an
 elastic deformation of the braking-force device 10 (especially the spindle
 12) because of a braking operation, then the magnetic field in this region
 changes under the influence of the pressure because of the displacement of
 the magnetic zones. This pressure is applied to the ferromagnetic region
 of the spindle 12 with this elastic deformation. As a consequence of the
 above, a change of the overall magnetic field takes place and this change
 is detected by a Hall sensor 36. A corresponding signal is outputted to
 the connecting line 32 and the braking force, which is applied during the
 braking operation, is determined from the signal of the Hall sensor 36.
 The special advantage of the embodiment shown in FIG. 3 is that the Hall
 sensor 36 is located outside of the spindle 12 and therefore a connecting
 line like the connecting line 32 is unnecessary in the spindle 12.
 Nonetheless, the Hall sensor 36 is well protected against external
 mechanical loads because it is surrounded by the brake piston 16.
 FIG. 4 shows a brake actuator 2 which is configured precisely as that shown
 in FIG. 1 and coacts with a brake yoke 18. The sensor 24 is mounted in a
 hollow space of the spindle 12 of the braking-force device 10. The sensor
 24 is modified with respect to the sensor shown in FIG. 1 in that the
 force-free state of the spindle 12 and therefore the liftoff of the brake
 linings 20a and 20b from the brake disc 30 can be reliably determined with
 the aid of the sensor 24. For this purpose, the rod 38 is guided through a
 first bore in the end face 40 of the spindle 12 and through a second bore
 in the end face 42 of the brake piston 16. The soft-magnetic core 26 is
 guided in the coil 28 with the aid of the rod 38. The rod 38 is configured
 to be so long that the end of the rod 38 projects beyond the end face 42
 of the brake piston 16 in the force-free state of the braking-force device
 10 (especially of the spindle 12) and lies in the air gap between the
 brake-lining support 22b and the end face 42 of the brake piston 16. The
 end of the rod 38 faces away from the soft-magnetic core 26.
 What happens during a braking operation will now be explained.
 The spindle 12 of the braking-force device 10, and therefore the brake
 piston 16, is set into an axial movement so that the end face 42 lies
 against the rear end of the brake-lining support 22b after passing through
 the air gap between the brake-lining support 22b and the end face 42 of
 the brake piston 16. For the axial movement of the brake piston 16 in the
 direction of the brake-lining support 22b, the rod 38 is pushed into a
 recess 44 of the brake piston 16. In this way, the stop 48 on the rod 38
 is moved toward the electric motor 4 and compresses the spring 46. With
 the movement of the rod 38, the soft-magnetic core 26 of the sensor 24
 plunges into the operating region of the coil 28 and only then is a signal
 generated by the sensor 24. A further increase of the braking force leads
 to an elastic deformation of the braking-force device 10 (especially the
 spindle 12) which is detected by the sensor 24 as described above with
 reference to FIG. 1.
 At the end of the braking operation, the spindle 12 of the braking-force
 device 10, and therefore the brake piston 16, moves in the axial direction
 away from the brake-lining support 22b so that an air gap is again formed
 between the rear end of the brake-lining support 22b and the end face 42
 of the brake piston 16. At that moment at which the end face 42 lifts off
 of the rear end of the brake-lining support 22b, the compressed spring 46
 begins to expand and pushes the end of the rod 38, which faces away from
 the soft-magnetic core 26, again into the air gap so that, after the
 braking operation, the start position shown in FIG. 4 is again reached and
 the soft-magnetic core 26 is no longer located in the operating region of
 the coil 28. In this way, in the force-free state of the braking-force
 device 10 (especially of the spindle 12), no signal (or no significant
 signal) is generated by the sensor 24 and the released brake is reliably
 indicated. The released brake is characterized by a lifting of the brake
 linings 20a and 20b from the brake disc 30. The force-free state is only
 indicated by an air gap present between the brake-lining support 22b and
 the brake piston 16. However, this ensures that the brake linings 20a and
 20b are actually lifted off of the brake disc 30 (for example, because of
 the brake disc impact).
 According to still another embodiment of the invention, the sensor 24
 includes a reflecting membrane 60 onto which light is guided which is
 reflected at the membrane 60. The membrane 60 is likewise deformed or
 displaced out of its rest position as a consequence of the elastic
 deformation of the braking-force device 10 or of the brake yoke 18. In
 this way, the reflection of the light at the membrane is again changed
 (for example, the reflection angle of the light changes with a
 deformation). The change of the reflection has a clear relationship to the
 deformation of the braking-force device 10 or of the brake yoke 18.
 In the following, a description is provided as to how the sensor 24, which
 includes the reflecting membrane, can, for example, be configured.
 FIG. 5 shows a brake actuator 2 which is configured precisely in the same
 manner as the brake actuator shown in FIG. 1 and coacts with a brake yoke
 18. The spindle 12 of the brake actuator 2 includes a hollow space in
 which a coupling element 52 is located. Here, the coupling element 52 is
 in the form of a ball. The ball 52 is operatively connected to a
 reflecting membrane 60 of the sensor 24.
 In addition to the membrane 60, the sensor 24 also includes a first light
 conductor 54 and a second light conductor 56 which run in a cable 58. This
 cable is brought out of the brake actuator 2 via a channel in the spindle
 12. The first light conductor 54 is disposed outside of the brake actuator
 2. Light is coupled into the end of the first light conductor 54 with the
 aid of a light-emitting diode. The light propagates up to the other end of
 the first light conductor 54 which is disposed in the hollow space of the
 spindle 12 and there exits from the first light conductor. The exiting
 light is reflected by the membrane 60 of the sensor 24 and, after the
 reflection, is coupled into the second light conductor 56. The light
 propagates within the second light conductor up to the end of the second
 light conductor which is disposed outside of the brake actuator 2 and
 there again exits from the light conductor 56. The intensity of the light
 exiting from the second light conductor 56 is measured outside of the
 brake actuator 2, for example, with the aid of a photodiode.
 What happens during a braking operation will be explained below.
 The spindle 12 of the braking-force device 10 is elastically deformed and,
 as a consequence thereof, a force is applied to the membrane 60 by the
 ball 52. Because of this deformation, the portion of the light which is
 coupled into the second light conductor 56 changes after the reflection at
 the membrane 60. At the output of the second light conductor 56 outside of
 the brake actuator 2, a light intensity is measured with the aid of the
 photodiode and this light intensity is different from the light intensity
 of the brake actuator 2 which is not actuated. The deformation of the
 membrane 60 and therefore the change of the measured light intensity forms
 a clear relationship with the braking force which is applied with the aid
 of the spindle 12 to the brake linings 20a and 20b. In this way, the
 applied braking force is determined with the aid of the sensor described
 in connection with FIG. 5.
 FIG. 6 shows a brake actuator 2 which is configured in the same manner as
 the brake actuator shown in FIG. 1 and coacts with a brake yoke 18. The
 sensor 24 shown in FIG. 6 is, for the most part, configured as the sensor
 24 shown in FIG. 5. A difference is seen only in that the membrane 60 of
 the sensor 24 is not, as shown in FIG. 5, located in the hollow space 40
 of the spindle 12; instead, the hollow face 40 of the spindle 12 is
 configured as the membrane 60.
 In a braking operation with the brake actuator 2 shown in FIG. 6, a direct
 deformation of the membrane 60 occurs which, in turn, leads to a change of
 the light intensity at the output of the second light conductor 56. The
 method of measuring the braking force in the brake actuator shown in FIG.
 6 is not different from the measuring method associated with the brake
 actuator shown in FIG. 5. A sensor of the kind described with respect to
 FIGS. 5 and 6 can be obtained, for example, from the OPTRAND Company,
 46155 Five Mile Road, Plymouth, Mich. 48170.
 It is understood that the foregoing description is that of the preferred
 embodiments of the invention and that various changes and modifications
 may be made thereto without departing from the spirit and scope of the
 invention as defined in the appended claims.