Drum brake, making it possible to estimate a braking force

Abstract

The invention relates to a drum brake (1) which comprises a force sensor at a fixed point (13) where the friction segments (3, 4) are mounted on the structure of the brake, and this sensor is designed to make it possible to measure the reaction force of one of the segments (3) on the fixed point (13) while braking is applied. The value of this reaction is easily correlated to the braking torque. An ECU measuring the reaction can adjust this braking torque in real time via the actuator of the brake (1) in order to provide sufficient braking without locking the brake.

Description

DESCRIPTION TITLE: DRUM BRAKE, ALLOWING AN ESTIMATION OF BRAKING EFFORT TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to a drum brake, allowing an estimation of the braking force applied in service. STATE OF PRIOR ART

[0002] Drum brakes, commonly used in vehicles, are however hampered by the difficulty of controlling their braking forces, and therefore limiting the risk of locking the wheels they are fitted with. This difficulty remains for electronically controlled drum brakes, in which the movements of the components responsible for friction depend on electric motor actuators, including an ECU (electronic control unit) which controls the movements in certain specific situations, in particular (but not exclusively) to ensure emergency braking, in addition to other brakes that the driver conventionally controls by pressing the brake pedal.

[0003] It is known to have the ECU measure the movements of such actuators, and therefore those of the friction members, as well as the intensity of the current necessary to control the electric motor; by controlling determined quantities of these movements, we seek to apply braking moments that are also determined; but in reality there is no usable correlation between the movements of the actuator and the braking moment, which depends not only on the pressure exerted by the actuator on the rotor to be slowed down by means of the friction members, but on the friction coefficients of these, which are variable according to experience. STATEMENT OF THE INVENTION

[0004] The object of the invention is to remedy these shortcomings of the state of the art by proposing in particular a drum brake equipped with means allowing precise evaluation of the braking moment applied by the brake during its service, and thus capable of applying sufficient braking moments without being excessive.

[0005] To do this, according to a first aspect of the invention, a drum brake is proposed, comprising a support structure, two friction segments mounted on the support structure and intended, when braking is carried out, to exert a braking moment on a rotor, the segments each having a bearing end, anchored on a part called a fixed point which is integral with the support structure and exerting a reaction force on the bearing end via the fixed point when braking is carried out, the fixed point containing a force sensor, intended to measure said reaction force, characterized in that the drum brake comprises a preloaded elastic member configured to elastically stress at least one of the segments bearing on said force sensor in the absence of braking.

[0006] A teaching of the invention is that, if the braking moment is difficult to correlate well with the current passing through the electric motor of the actuator, or with the movements undergone by the actuator and transmitted by it to the segments, it can on the contrary be easily correlated with the reaction forces exerted by the support ends of the segments on the fixed point. The preloaded elastic member makes it possible to give the force sensor correct estimates of the reaction force while ensuring sufficient preload to avoid the consequences of service vibrations.

[0007] In particular, the preload exerted by the preloaded elastic member improves the measurement accuracy on the sensor by allowing the sensor to work in a range of force values where the forces are sufficiently high for the sensor to provide measurements with good accuracy. Furthermore, the preload of the preloaded elastic member prevents in particular vibrations of the segment against the sensor, for example hammering on the sensor, the shocks of which could damage the sensor.

[0008] Another lesson is that these reaction forces can be estimated globally using a single force sensor, even if two segments participate in the braking.

[0009] This makes it possible to obtain at any time sufficiently precise evaluations of the braking moment that the drum brake applies, without complicating or excessively costing its structure.

[0010] According to a particularly preferred embodiment, the brake comprises a force transfer part, guided movably in the fixed point, the force sensor is compressed between the fixed point and the force transfer part, and the force transfer part is compressed between the force sensor and the support end of a first of the segments.

[0011] An advantage of this construction is that the reaction force that the force sensor must measure is transmitted to it in an invariable and known direction, thus without introducing significant uncertainty that would be due to a deviation in position, or to movements of the segments. Damage to the force sensor, which could be produced by oblique or eccentric forces, is also avoided.

[0012] More particularly, it is recommended that the fixed point comprises a cavity in which the force sensor is housed, and the force transfer part slides in an opening of the cavity. The direction of transmission of the reaction force to the force sensor is then perfectly known; and the force sensor is conveniently mounted on the brake.

[0013] In addition, a seal can then be advantageously placed in the opening of the cavity, in order to make it watertight, and thus protect the force sensor simply and without any significant increase in weight.

[0014] According to a preferred embodiment, said first of the segments is located immediately upstream of the fixed point, according to a direction of rotation of the rotor. The direction of rotation mentioned here is the usual, predominant or exclusive direction of rotation of the rotor, which corresponds to forward motion in the case of a vehicle. This segment immediately upstream in fact normally undergoes greater forces than the other during braking: it therefore exerts a higher reaction force, capable of being measured with less uncertainty by the force sensor.

[0015] According to a preferred embodiment, the preloaded elastic member comprises a pair of springs, opposite ends of each of which are articulated respectively to the bearing end of the first segment on the fixed point and to the fixed point, the springs being loaded in tension, and the force sensor being located between the springs.

[0016] According to a further preferred embodiment, the preloaded elastic member comprises a pair of springs, opposite ends of each of which are articulated respectively to the bearing end of the first segment on the fixed point and to the bearing end of the second segment on the fixed point, the springs being loaded in tension, and the force sensor being located between the springs.

[0017] The joint springs, thus placed on either side of the force sensor, make it possible to better position the support end and the reaction force in front of it, and therefore to further avoid introducing uncertainties into the measurements it provides.

[0018] Additionally, using two springs reduces the risk of residual braking moment remaining after braking.

[0019] An improvement of this two-spring construction is obtained if the springs are arranged so as to be made parallel to a direction of mobility of the force transfer part at a mid-wear state of friction linings equipping the segments. This arrangement makes it possible to reduce the variation in amplitude of the preload that the springs impose on the force sensor, and therefore to limit the uncertainty in the measurement of the reaction force.

[0020] According to another aspect of the invention, it relates to a braking device, characterized in that it comprises the drum brake according to the above, an actuator of the drum brake, and an electronic control unit arranged to measure forces undergone by the force sensor, to deduce therefrom a reaction force of the segments on the fixed point, and to control the actuator as a function of said reaction force to impose a determined value of said reaction force.

[0021] Such a device allows the braking moment applied by the drum brake to be adjusted in real time, which contributes to braking efficiency while reducing the risk of rotor lock.

[0022] The improvement proposed here for the drum brake is very interesting if the actuator is electromechanical, because such actuators are particularly subject to the correlation defects, noted above, between the braking moment applied and the movements of the actuator.

[0023] Another aspect of the invention is a motor vehicle equipped with at least one braking device according to the above, in order to more easily use inexpensive drum brakes as service brakes of the vehicle in certain driving circumstances, such as emergency braking. BRIEF DESCRIPTION OF THE FIGURES

[0024] Other characteristics and advantages of the invention will emerge from reading the description which follows, with reference to the appended figures, which illustrate: -[Fig. 1]: a front view of a drum brake equipped with the invention; -[Fig. 2]: a detailed view of figure 1, illustrating the essential constituents of the invention; -[Fig. 3]: an exploded view of two parts of figure 2; -[Fig. 4]: a diagram of the brake control device; -[Fig. 5]: a flowchart of the brake control; -[Fig. 6]: in two parts A and B, the consequence of brake wear on the portion of figure 2; -[Fig. 7]: in three parts A, B and C, an evolution of a preload of the device of the invention, according to a first embodiment; -[Fig. 8]: in three parts A, B and C, an evolution of a preload of the device of the invention, according to a second embodiment; -[Fig. 9]: an explanatory diagram of the forces undergone by the brake during braking; -[Fig. 10]: another embodiment of a fixed point, equipped with the invention. DETAILED DESCRIPTION OF AN EMBODIMENT

[0025] Figure 1 illustrates a drum brake 1 equipped with the invention. The brake 1 is shown alone, without the rotor or drum whose rotation it is to brake, nor its control device. It comprises a plate 2 which is approximately flat and circular, on which are arranged a first segment 3 and a second segment 4 which are the friction members. The segments 3 and 4 are connecting rods in the shape of an arc of a circle, each extending to the periphery of the plate 2, over nearly a semicircle each, and the outer faces of which carry a lining 5. The linings 5 extend in front of a peripheral region 6 of the plate 2 in front of which the rotor to be braked extends, when the brake 1 is mounted on the device which it is to equip. By moving the segments 3 and 4 away from each other towards the periphery of the plate 2, the linings 5 will rub on the rotor and brake it.

[0026] This separation is accomplished by an actuator 7 located also near the periphery of the plate 2, between the segments 3 and 4. The actuator 7 comprises a mechanism 8 ending on two pistons 9 and 10, aligned and pointing in opposite directions, towards respective movable ends 11 of the segments 3 and 4. The opposite ends of the segments 3 and 4 are bearing ends 12 which remain essentially stationary. When the pistons 9 and 10 are deployed, they separate the movable ends 11 from each other, the segments 3 and 4 from each other by tilting them about their bearing ends 12, and braking becomes possible.

[0027] The situation of Figure 9 is obtained. The first segment 3 is, with this direction of rotation of the rotor (the counterclockwise direction, indicated by the arrow), subjected to general compression forces, and the second segment 4 to general tension forces. For each of the segments 3 and 4, the friction pressure p3 or p4 on the rotor is strongly increasing in said direction of rotation, and much greater normally on the first segment 3. It is difficult to properly correlate the braking moment T, which depends not only on the friction pressures p3 and p4 exerted by the linings 5 but of their coefficients of friction, with the deployments exerted by the pistons 9 and 10, even using maps based on preliminary tests.

[0028] However, the braking moment T can be correlated to the forces present in the segments 3 and 4, and transmitted to the other parts of the brake 1. In particular, it can be correlated to the reaction forces exerted by their support ends 12. If the (unknown) forces exerted by the pistons 9 and 10 each have a value F, the reaction forces can be evaluated at aF and b. F respectively for the segments 3 and 4, where "a" and "b" are known weighting coefficients, which can be respectively equal to 3 and 0.4 (for illustrative purposes). The invention is based on the idea of exploiting, by means of simple and precise equipment, one of these reaction forces to deduce the braking moment.

[0029] We now refer to Figures 2 and 3. The support ends 12 of the segments 3 and 4 are arranged on a part called a fixed point 13, screwed onto the plate 2 or, in the case of a vehicle, onto the axle stub carrier. The fixed point 13 comprises a cavity 14 opening towards one of the segments 3 and 4, here the first segment 3. The cavity 14 is occupied by a force transfer part 15, movable and more precisely sliding in a cylindrical opening of the cavity 14, and by a compression force sensor 16. The force sensor 16 is placed at the bottom of the cavity 14, aligned with the force transfer part 15 in its direction of mobility. The bearing end 12 of the first segment 3 presses on a bearing part 17, in the form of a ramp, constituting the outer end of the force transfer part 15, and the inner end of the force transfer part 15 presses the force sensor 16 against the bottom of the cavity 14.The force sensor 16 can thus measure the reaction force exerted by the first segment 3 on the fixed point 13 in the direction of movement of the force transfer part 15, which here corresponds to the direction of deployment of the pistons 9 and 10 (horizontal in FIGS. 1 and 2).

[0030] The device further comprises a so-called lower spring 18 and a so-called upper spring 19, superimposed in the vertical direction of Figures 1 and 2 and extending parallel to each other on either side of the force transfer part 15 and the force sensor 16. The upper spring 19 and the lower spring 18 each have a end articulated to the support end 12 of the first segment 3, and the opposite end articulated to the support end 12 of the second segment 4. They are tensioned, therefore holding the force transfer part 15 in the cavity 14, and causing it to exert a preload on the force sensor 16.

[0031] The bearing ends 12 of the segments 3 and 4 are each provided with a rounded edge 20 in an arc of a circle, which press respectively on an oblique face 21 at the end of the bearing piece 17, and on an oblique face 22 established on the fixed point 13. The oblique faces 21 and 22 have identical inclinations and in opposite directions which make them converge towards the top of figures 1 and 2 and towards the actuator 7. They define a so-called trapezoidal mounting of the bearing ends 12, known in itself and intended to promote the correct position of the segments 3 and 4, but which is not necessary for a suitable implementation of the invention.

[0032] The cavity 14 is sealed against dust and other impurities by an O-ring seal 23, established around the force transfer part 15 and rubbing against it without preventing its movement.

[0033] The support piece 17 is housed in a groove 24 near the opening of the cavity 14, in order to prevent the force transfer piece 15 from rotating. Finally, the reference 25 refers to a part for fixing the fixed point 13 to the plate 2 or to the stub axle of a vehicle by screws.

[0034] Figure 4 illustrates the complete braking device. The actuator 7 is electromechanical. It comprises an electric motor 26, located outside the brake 1, which drives the mechanism 8; the latter converts the rotations of the electric motor 26 into the deployments or retractions of the pistons 9 and 10. Electrical lines 28 connect an electronic control unit (ECU) T1 of the vehicle to the electric motor 26 on the one hand, and to the force sensor 16 on the other hand.

[0035] Figure 5 illustrates the control method. When a service of the brake 1 is requested by the ECU T1 (step E29), a rotation quantity of the electric motor 26 is estimated by using a mapping function of the brake 1 (step E30). This function is supposed to indicate, depending on the physical characteristics of the brake 1, of its wear or other parameters, the amount of rotation that would correspond to a desired value of the braking moment, thanks to an appropriate tilting of the segments 3 and 4. The electric motor 26 is then started to carry out this estimated amount of rotation (step E31). Then, and in accordance with the invention, the value of the load of the force sensor 16 is measured by the ECU 27 (step E32). If this value is equivalent, taking into account the preload exerted by the springs 18 and 19, to that which corresponds to the desired braking moment according to another map of the brake 1, the process of applying the brake 1 stops (step E34). Otherwise, said other map makes it possible to estimate a corrective rotation of the electric motor 26 to put the load of the force sensor 16 and, indirectly, the braking moment, at the desired value (step E33), and the process returns to step E32 to carry out this corrective rotation.This adjustment of the braking moment is accomplished in real time, while braking is being produced.

[0036] Figure 6 illustrates the effect of wear of the brake 1, more precisely of the wear of the linings 5, which is irregular due to the irregular pressure they exert on the rotor to be braked. Reference 35 refers to a plate for retaining the bearing ends 12, which helps to hold them against the fixed point 13. Part A illustrates the bearing ends 12 of the segments 3 and 4, and their positions relative to the fixed point 13, when the brake 1 is new. But when the brake 1 is considered to be very worn, part B shows a relatively large displacement d36 of sagging (towards the bottom of the figure) of the first segment 3, but a displacement d37 of elevation (towards the top of the figure), although less large, of the second segment 4 normally less subject to forces.

[0037] The consequences of these very different displacements d36 and d37 due to wear on segments 3 and 4 are explained by means of figure 7. Lines parallel to the direction h38 of movement of the force transfer part 15 (horizontal in this embodiment) have been drawn there. If, as shown in part A, the springs 18 and 19 are parallel to this direction when the brake 1 is new, they become oblique to it when the brake 1 begins to wear (part B), and the obliqueness becomes increasingly significant until the brake 1 is completely worn (part C), when the displacements d36 and d37 have been undergone. It can be seen that the springs 18 and 19 undergo a variable tension, and the force sensor 16 a variable preload, and increasingly important, as the brake 1 wears.

[0038] The variation in this preload must be taken into account when carrying out the process in Figure 5. This is possible if the mapping in step E33 is adapted, but uncertainties may still affect the final quality of the braking moment adjustment undertaken by this process.

[0039] This is why a possible (although by no means necessary) improvement can be proposed according to the teaching of Figure 8, parts A, B and C of which correspond to those of Figure 7. In a half-worn condition (part B), springs 18 and 19 are arranged to extend in the direction h38 corresponding to a nominal preload. This implies that they extend at a certain inclination relative to this direction h38 in the new condition of brake 1 (part A), and at another inclination, in the opposite direction but of approximately the same value, when brake 1 is very worn (part C).The difference between the nominal preload, having the lowest value (in part B of figure 8, and in part A of figure 7), and the preload having the highest value (in part A or C of figure 8, and in part C of figure 7) is approximately half as large in the configuration of figure 8, and the uncertainties in estimating the load of the force sensor 16, due to the uncertainties in the variation of the preload, will also be lower.

[0040] Therefore, a preferred embodiment of the invention consists in placing the springs 18 and 19 in an orientation such that they are parallel to the direction h38 of movement of the force transfer part 15 in a mid-wear state of the brake 1.

[0041] For example, a total preload applied by springs 18 and 19 might be 180 newtons for a brake 1 of 28 centimeters (11 inches) in diameter.

[0042] Figure 10 shows that the fixed point, here 39, is not limited to the embodiment of Figure 3. Here it has the shape of a roughly parallelepiped block directly fixed to the plate 2 of the brake 1. Figure 10 shows certain details of the embodiment which could also be present in that of Figure 3, such as an electrical connector 40, screwed onto the fixed point 39 by a fixing flange 43, and which is assembled with output pins 41 of the force sensor 16. It is thus possible to extract the measurements of the force sensor 16 from the brake 1 through the plate 2, perpendicular to the cavity 14, by a tubular connection 43 for the passage of one of the electrical lines 28, rigidly fixed to the fixed point 39 and leading to the outside of the brake 1. Figure 10 also illustrates a groove 44 in which the oblique face 22 is formed, and which receives the support end 12 of the second segment 4. Like the force sensor 16, the cavity 14 and the force transfer part 15 remain present and are not modified. The representation of figure 10 is a perspective section for reasons of clarity, like figure 3: the force transfer part 15 is still, in reality, sliding in the cavity 1 and resting on the force sensor 16, just as the connector 40 is electrically joined to the force sensor 16, and mechanically to the fixed point 39.

[0043] Naturally, the invention is described in the foregoing by way of example. It is understood that those skilled in the art are able to carry out different variant embodiments of the invention without departing from the scope of the invention.

[0044] For example, the measurement of the reaction force could concern the second segment 4: it would then be sufficient to reverse the orientation of the cavity 14, and to place its opening and the force transfer part 15 towards the support end 12 of this second segment 4. The force undergone by the force sensor 16 would then be less significant, but still suitable for estimating the braking moment. The estimation uncertainty would perhaps be greater, but the force sensor 16 would also be less stressed. This construction variant is therefore in no way excluded.

[0045] A single spring for returning the bearing ends 12 toward each other, such as the lower spring 18, could be sufficient to give the force sensor 16 correct estimates of the reaction force while ensuring sufficient preload to avoid the consequences of service vibrations, such as hammering of the force sensor 16.

[0046] Alternatively, the brake comprises a pair of springs 18, 19, opposite ends of each of which are hinged respectively to the bearing end 12 of the first segment 3 on the fixed point 13, 39 and to the fixed point 13, 39, the springs 18, 19 being loaded in tension, and the force sensor 16 being located between the springs.

[0047] The brake according to the invention can be fitted to a motor vehicle wheel and function as a service brake when the vehicle is in motion, but it can also be used as a simple parking brake.

[0048] The invention can be fitted to different models of drum brakes, such as the so-called "simplex" model illustrated here, the "duo servo" model, etc.

[0049] NOMENCLATURE

Claims

CLAIMS 1. Drum brake (1), comprising: a support structure (2, 13, 39), two friction segments (3, 4) mounted on the support structure and intended, when braking is carried out, to exert a braking moment (T) on a rotor, the segments each having a bearing end (12), anchored on a part called a fixed point (13, 39) which is integral with the support structure and exerts a reaction force (a. F, b. F) on said bearing end via the fixed point when braking is carried out, the fixed point (13, 39) containing a force sensor (16), intended to measure said reaction force, characterized in that the drum brake (1) comprises a preloaded elastic member (18, 19) configured to elastically stress at least one of the segments (3, 4) bearing on said force sensor in the absence of braking.

2. Drum brake according to claim 1, characterized in that it comprises a force transfer part (15), guided movably in the fixed point (13, 39), in that the force sensor (16) is compressed between the fixed point and the force transfer part, and the force transfer part (15) is compressed between the force sensor (16) and the support end (12) of a first of the segments (3).

3. Drum brake according to claim 2, characterized in that the fixed point (13, 39) comprises a cavity (14) in which the force sensor (16) is housed, and the force transfer part slides in an opening of the cavity.

4. Drum brake according to claim 3, characterized in that a seal (23) is arranged in the opening of the cavity (14), to make it watertight.

5. Drum brake according to any one of claims 2 to 4, characterized in that said first of the segments (3) is located immediately upstream of the fixed point (13, 39), according to a direction of rotation, either predominant or exclusive, of the rotor.

6. Drum brake according to any one of claims 1 to 5, characterized in that the preloaded elastic member comprises a pair of springs (18, 19), opposite ends of each of which are articulated respectively to the bearing end (12) of the first segment (3) on the fixed point (13, 39) and to the fixed point (13, 39), the springs (18, 19) being loaded in tension, and the force sensor (16) being located between the springs.

7. Drum brake according to any one of claims 1 to 5, characterized in that the preloaded elastic member comprises a pair of springs (18, 19), opposite ends of each of which are articulated respectively to the bearing end (12) of the first segment (3) on the fixed point (13, 39) and to the bearing end (12) of the second segment (4) on the fixed point (13, 39), the springs (18, 19) being loaded in tension, and the force sensor (16) being located between the springs.

8. Drum brake according to claim 7, characterized in that the springs (18, 19) are arranged so as to be made parallel to a direction of mobility (h38) of the force transfer part (15) in a state of mid-wear of friction linings (5) equipping the segments (3, 4).

9. Braking device, characterized in that it comprises the drum brake according to any one of claims 1 to 8, an actuator (7) of the drum brake, and an electronic control unit (27) arranged to measure forces experienced by the force sensor (16), to deduce therefrom a reaction force (a. F) of the segments on the fixed point (13, 39), and to control the actuator (7) as a function of said reaction force to impose a determined value of said reaction force.

10. Braking device according to claim 9, characterized in that the actuator (7) is electromechanical.

11. Motor vehicle, characterized in that it comprises the braking device according to any one of the preceding claims to ensure service braking of the vehicle.

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