Patent ID: 12246687

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

With reference toFIG.1, there is represented a drum brake1, comprising a revolution plate2equipped with a first and a second brake shoe3and4in the form of an arc of a circle and radially movable so as to be able to be pressed against the rotating face of a drum to be braked, not represented.

The shoes3and4each have a flat sheet metal core3a,4ain the form of a circular ring portion which carries a friction lining3b,4b, and are mounted diametrically opposite each other with their ends bearing against both a hydraulic wheel cylinder5and an electromechanical actuator6carried by the plate2. These shoes3and4are also returned towards each other by two return springs7and8, and each pressed against the plate2by a so-called side spring9,10.

A wear adjustment rod11extends along the wheel cylinder5, having a first end bearing against the core3aof the first shoe3and a second end bearing against the core4aof the second shoe4when the brake is at rest.

The wheel cylinder5, which is a hydraulic actuator, is to be actuated when the drum brake1is used according to a first so-called “simplex” operating mode, which ensures gradual braking particularly adapted to brake the vehicle in service. It comprises a cylindrical hydraulic chamber closed at its ends by two pistons which move apart when the hydraulic pressure increases and pushes the associated ends of the shoes3and4.

The electromechanical actuator6in turn ensures parking braking by moving the associated ends of the shoes apart to ensure rapid and powerful locking of the vehicle wheels in a so-called “duo-servo” operating mode, as well as keeping them in the locked state, particularly when the wheel cylinder5is inactive. Further, the actuator6provides emergency braking.

As is visible inFIG.2, the electromechanical actuator6according to the invention comprises an electric motor12, a calculator13, and a screw-nut type motion converter14. The calculator13drives the electric current delivered to the motor12during its activation. Placed at the terminals of the motor12, this calculator13is able to detect fluctuations in the rated intensity In of the electric current drawn by the motor, and to interrupt providing this current to stop the motor.

The motion converter14comprises a nut16and a screw17rotatably driven by the motor12, directly or indirectly via a motor reducer18ensuring a reduction in the speed of rotation measured at the motor output. The screw17, with axis AX of longitudinal direction, comprises a male thread which cooperates with a female thread of the nut16, this nut being carried by the screw. In particular, the screw17is translationally fixed while the nut is rotatably locked and free to translate along the screw.

The nut16is intended to accompany one of the shoes3and4in the brake1equipped with such an electromechanical actuator6in its movement, in order to selectively apply it against the rotating drum surface marked by22, and to move it apart from this rotating surface22.

More precisely, the stroke of the nut16, marked by X, is bounded between a fixed portion of the actuator6forming a fixed stop23, representing the fully retracted position of the nut, denoted as Xbf, and a so-called movable stop representing the braking position, denoted as Xbm and shown in dotted line, that is for which the friction lining3bor4bis in contact with the rotating surface22. The fixed stop23is located as an extension of the distal end of the screw17relative to the rotating surface22. The position of the movable stop is in turn determined by the thickness of the friction lining3b, which wears down with each braking/brake releasing cycle, thus gradually moving this movable stop apart from the fixed stop23, towards the rotating surface22.

The idea underlying the invention is to equip the electromechanical actuator6with a means changing the resistance to progress of the nut at at least one predetermined position, located at a distance from the stops, in order to change the intensity of the current flowing through the motor when the nut passes through this predetermined position.

According to a first embodiment of the invention, the electromechanical actuator6comprises a spring, for example, a coil spring26extending over a portion of the screw17from a first end of the spring attached to the fixed stop23, to a second end extending at a distance from the fixed stop23, as is visible inFIG.2. With this solution, when the nut16, in translation towards the fixed stop23, comes to bear against the second end of the spring, the spring is sandwiched between the fixed stop and the nut, exerting a force opposing the progress of this nut.

With this arrangement, the intensity In measured is generally constant until the nut16contacts the second end of the spring, this level of progress of the nut being denoted as Xe. As the nut continues to move towards the fixed stop23, there is an increase in the current In drawn by the motor12to counteract the resistance to progress of the nut.

In the example ofFIG.2, this increase in intensity follows a ramp, in other words a linear increase, but it is understood that the invention is not limited to this feature, given that the time course of the intensity curve is a function of the nature of the spring, namely according to its spring constant and morphology.

It is understood here that the invention provides for the setting of an intensity threshold value, denoted as S, for which the calculator13causes stop of the power supply, taking into consideration the nature of the spring and the response time of the calculator so that the actual stopping of the nut is carried out before the nut is completely compressed, in other words, before it reaches its full length. This is because the fully compressed spring26would act in the same way as the fixed stop23and would cause the nut to be clamped against it.

Since the pitch of the screw, the spring constant of the spring26which induces a predetermined change in the intensity, and the response time of the calculator13are known data for a given application, the position of the nut being stopped can thus be determined in relation to the number of revolutions made by the screw since the increase in intensity In of the current. With this arrangement, the actuator6according to the invention thus makes it possible to know the position of the nut along the screw, while at the same time preventing the nut from being clamped against the fixed stop23, which would lead to an intensity peak, visible as a dotted line inFIG.2, and to damage.

An alternative in which the spring26is attached to the nut16at its first end and contacts the fixed stop23at the progress level Xe, or an alternative arrangement in which the spring is floatingly mounted to the screw17, can be adopted without departing from the scope of the invention. With a floating arrangement, the spring26is free to move between the nut16and the fixed stop23until the nut reaches the progress level Xe in its movement towards the fixed stop, in other words when the distance measured between the fixed stop and the nut corresponds to the length of the spring, marking the contact of the spring against both the fixed stop and the nut.

Also, the invention advantageously provides for pre-stressing the spring26so as to provide a greater resistance to progress of the nut. Such an arrangement makes it possible to tend towards a more rapid increase in current intensity and consequently to favour a more rapid stop of the nut, the intensity reaching the threshold value S for a lesser movement of the nut in comparison with the use of a non-prestressed spring.

According to a second embodiment, the change in the resistance to progress of the nut is achieved by means of an irregular thread of the screw.

In the example ofFIG.3, using the same electromechanical actuator architecture asFIG.2except for the spring26, the screw17comprises a thread, a portion of which marked27is oversized. This oversized portion27, bounded between two ends located at a distance from the movable and fixed stops, increases the local friction between the screw17and the nut16when the latter moves at the screw, and consequently increases the resistance to progress of the nut. The rated intensity In changes in the form of a square wave as the nut16passes through the oversized portion27. More precisely, the intensity In successively increases in the form of a ramp from a level of progress of the nut denoted as Xi, corresponding to its engagement in the oversized portion27. This increase is gradual in that the further the nut progresses along this portion27, the greater the friction, given that the nut meshes more with it. In a non-limiting manner, when the length of the oversized portion27along the longitudinal direction is greater than that of the nut, it follows that from a given progress, the nut completely meshes with this portion. At this point, the rated intensity In follows a constant change until the nut starts its disengagement from the oversized portion27, leading to a decrease in the form of a ramp of the rated intensity In. Once the nut is fully disengaged from this thread portion27, at a point of progress denoted as Xf, the current displays a constant value, corresponding to the value measured before the nut passed over this portion27.

In this embodiment, it is understood that the intensity threshold value S for which the calculator13causes the stop of the power supply is defined so as to have a value lower than that measured when the nut16evolves at the level of the oversized portion27, that is to say so that the calculator detects an overshoot. As in the first embodiment, the position of the nut16being stopped, at a distance from the fixed and movable stops, can be determined in relation to the number of revolutions made by the screw since the increase in the current intensity In.

In the example ofFIGS.2and3, the screw17is a driving element while the nut16is a driven element, with a rotational movement of the screw being transformed into a translational movement for the nut, but the addition of a spring26or the formation of an oversized portion27on the screw thread are solutions that can also be applied for a reverse arrangement. In other words, these solutions are applicable in the case where the nut16is the driving element and the screw17is the driven element. With this arrangement, illustrated inFIGS.4and5, a rotational movement of the nut16, rotatably driven by the electric motor12, is transformed into a translational movement of the screw17in order to directly move the shoe3provided at the end of the screw. It is understood that an arrangement in which the screw17indirectly pushes the shoe by means of a part interposed between this shoe and the screw end facing the shoe, does not depart from the scope of the invention.

Thus, in a third embodiment of the electromechanical actuator, visible inFIG.4, which uses the same components as in the case of the first mode but modulated differently, the spring26extends along a portion of the screw17by being attached to it at the end against which the shoe is held under the action of the return springs7and8. The nut16forms a fixed stop on which the spring26bears when the screw17moves away from the rotating surface22, that is along the retraction direction of the shoe. In this respect, it is understood that the rated intensity in the case of this third embodiment changes in the same way as in the case of the first embodiment. The intensity increases as the screw end fixedly carrying the spring approaches the nut once the spring bears against this nut. In the same way as in the first embodiment, this increase in intensity In, in response to the increase in the resistance to progress of the screw17under the effect of the elastic return of the spring26, is detected by the calculator13above a threshold value S.

In an alternative embodiment, and in the same way as for the first embodiment, the invention provides for the spring26to be attached to the nut16, at its first end, or else for it to be floatingly mounted to the screw17between the nut16and the shoe3or4, which is held bearing against the screw17under the effect of the return springs.

Finally, according to a fourth embodiment, visible inFIG.5, which uses the same components as in the case of the second embodiment but modulated differently, the screw17comprises an oversized thread portion27inducing in the same way an increase in the resistance to progress of the screw17when it passes through the nut16. This increase in force is reflected in an increase in intensity In corresponding to that illustrated in the case of the second embodiment.

The invention has been described for a particular location of the spring26or the oversized portion27, so as to lead to a change in the rated intensity (In) of the current passing through the motor in the retraction direction of the shoe3or4. With this arrangement, the position of the driven element can be determined and the stop of the power supply can also be ordered before the shoe reaches its fully retracted position. As a result, the service life of the actuator is increased.

It is understood that the invention is not limited to this purpose, as the location of the spring26or the oversized portion27may vary as required. In other words, these elements may be placed so that the change in intensity resulting from their bias against the movement of the driven element indicates a particular position other than that indicating proximity to the fully retracted position.

By way of example, their placement may be defined such that the variation in induced rated intensity In is indicative that the friction lining3bor4bof the shoe is almost completely consumed, prompting the user of the vehicle equipped with such an actuator to change the shoe. In the case of an architecture according to the first embodiment, such a result is obtained by placing the spring along the screw17between the nut16and the shoe.

In particular, the electromechanical actuator may in particular comprise a plurality of springs or have an irregular screw thread at different locations, so as to mean several particular positions.

Within the scope of the first and third embodiments, the invention has been explained for the case where the resistance to progress of the driven element is provided by the elastic return of a coil spring. However, any other form of spring or any other member ensuring compressive elastic return is also contemplatable as long as it is attached or bounded in its movement so as to exert a force opposing the progress of the driven element at a predetermined position. As an example, the coil spring26could be replaced by a leaf, one end of which is attached to a fixed portion of the actuator6and one end of which is located on the stroke of the driven element to achieve resistance to progress by elastic deformation. Also, the use of spring washers, also known as Belleville washers, cascaded to form a spring with significant spring constant, is an efficient substitute for the use of a conventional pre-stressed spring.

Within the scope of the second and fourth embodiments, the invention has been explained by the formation of an oversized portion27, that is formed with an oversize, however it is understood that the portion may be formed by a thin layer deposition of a material on the thread or may be limited to a point-like protrusion protruding from the screw17. Also, the invention could conversely provide for an undersized or recessed portion of the thread so as to cause a drop in intensity In. It is understood that in this alternative, the threshold value S is defined as a minimum value below which the calculator13is prompted to stop power supply. Specifically, the electromechanical actuator according to the second embodiment or the fourth embodiment has a singularity at the screw thread to induce a variation in the rated intensity In of the current drawn by the electric motor12.

Generally speaking, the electromechanical actuator6according to the invention comprises at least one means for varying, that is increasing or decreasing, the resistance to progress at a predetermined position of a driven element with respect to a driving element converting a rotational movement into a translational movement advantageously selected from a nut and a screw meshed together. This variation in resistance to progress induces an event on the curve of rated intensity In, measurable by means of a calculator or any other similar device, and thus makes it possible to evaluate the position of the driven element.

The electromechanical actuator6has been explained for the case where it equips a drum brake having shoes, but an application for a disc brake, with the driving element moving a brake pad relative to a brake disc, may be adopted without departing from the scope of the invention. Generally speaking, the electromechanical actuator6according to the invention allows a block, designating a shoe or a pad, to be selectively moved against a rotating surface to be braked.

NOMENCLATURE

1drum brake2plate3,4brake shoe3a,4acore3b,4bfriction lining5wheel cylinder6electromechanical actuator7,8return spring9,10side spring11wear adjustment rod12electric motor13calculator14motion converter16nut17screw18motor reducer22rotating drum surface23fixed stop26spring27oversized thread portionAX axis of the screw along a longitudinal directionS intensity thresholdX stroke of the nutXbf, Xbm, Xe, Xi, Xf particular positions of the nut