Method for electrically controlling a parking brake with estimation of cooling

Abstract

The invention relates to a method for electrically controlling a parking brake for a motor vehicle, the brake comprising at least one friction means connected to an electromechanical actuator capable of moving the friction means towards a braking member, wherein the following steps are carried out: - defining a clamping force for immobilising the vehicle; - estimating a temperature variation of the braking member from a convective heat transfer model, ignoring conductive and radiative heat transfer phenomena; - defining, from the estimation of the temperature variation, a correction of a clamping to be performed in order to preserve the defined clamping force; and - applying the clamping force with correction of the clamping via the actuator.

Description

Method of electrically controlling a parking brake with cooling estimation

[0001] The invention relates to a method for electrically controlling a parking brake, also called a parking brake, for a motor vehicle.

[0002] Disc or drum brakes are known from the prior art. A brake generally comprises friction means connected to an actuating member, also called an actuator, capable of moving the friction means towards a braking member fixed to a wheel of the vehicle. This is intended to bring the friction means, for example brake linings or pads, into contact with the braking member to brake the vehicle by friction or to move them away from the braking member in order to stop braking. When the braking system is a disc brake, the braking member is formed by a disc rotating integrally with the wheel. In the case of a drum brake, the braking member is formed by a drum rotating integrally with the wheel.

[0003] A single brake can include several actuators. For example, it can include a hydraulic actuator used for service braking and an electric actuator for parking and emergency braking. In the latter cases, we also speak of an electrically controlled parking brake or electric parking brake. The electric parking brake is increasingly used as a replacement for manual parking brakes. Using an electric parking brake is simpler for the vehicle user and is more compact. When a user wants to move the vehicle out of parking, they simply press a button to release the electric parking brake, wait, if necessary, for a signal to indicate that the electric parking brake has been released, and then accelerate.

[0004] However, using a brake causes the braking component to heat up, the disc when the brake is a disc brake, or the drum when the brake is a drum brake.

[0005] When defining a parking brake application strategy, it is necessary to take this heating into account, because once parked, the braking component will cool down, and this cooling will cause the brake to release, or to over-apply. For example, when the disc cools down, its expansion decreases, and therefore the contact with the friction means (pad or shoe) is impacted, to the point of reducing the application force. The reduction in application force can then cause the vehicle to move.

[0006] Thus, estimating the cooling of the braking device is essential to define an effective clamping force strategy.

[0007] It is known to estimate the cooling of the braking system using models taking into account three heat transfer phenomena: convection, conduction and radiation.

[0008] In order to estimate these three phenomena, many parameters are necessary. These parameters can only be determined by vehicle tests. The following steps must then be carried out: implement equations with generic parameters; run tests on different road conditions with a temperature sensor; run a program (Matlab® script for example) to find the best parameters to have the least error between the estimate and the actual temperature; implement new parameters in the software equations; run new tests on different road conditions to validate the temperature estimate.

[0009] This type of method therefore requires the management of abstract parameters, many parameters dependent on the type of vehicle, and many expensive tests. This leads to an estimation that is costly, particularly in terms of time, and inaccurate in practice, which can lead to an overestimation of the application that can reduce the service life of the brake, or to an underestimation that can lead to an unexpected start of the vehicle. The maximum error in the temperature estimation is taken into account in the application strategy: the application strategy corresponding to a temperature equal to an estimate of the temperature to which a maximum error in the temperature estimation is added is chosen.

[0010] The invention aims in particular to solve this problem, by proposing a method for electrically controlling a parking brake for a motor vehicle, from an estimation of the cooling of the braking member based on a cooling model in which the phenomena of conduction and radiation are neglected and only a convection heat transfer model is used.

[0011] To this end, the subject of the invention is a method for electrically controlling a parking brake for a motor vehicle, the brake comprising at least one friction means (pad / jaw lining) connected to an electromechanical actuator capable of moving the friction means towards a braking member (disc / drum) for braking, in which the following steps are carried out: a brake application force is defined for immobilization of the vehicle; a temperature variation of the braking member is estimated from a convection heat transfer model, neglecting conduction and radiation heat transfer phenomena; a correction of a clamping to be carried out to maintain the defined clamping force is defined from the estimation of the temperature variation; the clamping force is applied with correction of the clamping via the actuator.

[0012] Such a convection heat transfer model has few parameters, and these parameters depend essentially on the mechanical characteristics of the brake, and are therefore perfectly known. There is no need to carry out a calibration of the parameters. Such a model leads to a better accuracy in the estimation of cooling, and therefore of the actual temperature of the braking component.

[0013] Furthermore, because this cooling estimate is reliable, the vehicle's ECU can be switched off more quickly. The ECU sets an initial clamping force based on the slope, then adjusts the clamping / release action based on the temperature. In other words, the clamping strategy will be adapted to the actual temperature and therefore oversizing the brake can be avoided. It will also be possible to reduce the time the ECU remains switched on after clamping, as in this case, retightening is not necessary.

[0014] According to other optional characteristics of the process taken alone or in combination: the convection heat transfer model includes the following parameters: h: convection heat transfer coefficient of the air; m: mass of the braking device; c p : heat capacity of the braking component material;A s : contact surface between the air and the braking device; the heat transfer model is written:

[0015]

[0016] where dT conv is the temperature variation due to the phenomenon of heat transfer by convection, T disc is the temperature of the braking device, T ambis the air temperature at the contact surface between the air and the braking member, and v is the air velocity at the contact surface between the air and the braking member;the convection heat transfer coefficient h is determined using a model, such as the Jürges model;the mass variation of the braking member over time is taken into account in the convection heat transfer model;the brake application force is defined as a function of the slope on which the vehicle is resting and the mass of the vehicle;the application correction is carried out by reducing the application;the application correction is carried out by increasing the application;the brake is a drum brake, the braking member is a drum, the friction member is a shoe provided with lining;the brake is a disc brake, the braking member is a disc, the friction member is a brake pad.

[0017] The invention also relates to a motor vehicle comprising an electronic control unit configured to implement the method according to the invention. Brief description of the figures

[0018] The invention will be better understood on reading the following description, given solely by way of example and with reference to the appended drawings in which:

[0019] is a schematic representation of the steps of an example of an electrical control method according to the invention;

[0020] illustrates the actual temperature evolution of a disk over time, as well as two estimation curves with two different models. Detailed description

[0021] The diagram schematically represents the steps of an example of a method for electrically controlling a parking brake for a motor vehicle according to the invention.

[0022] The parking brake comprises at least one friction means connected to an electromechanical actuator capable of moving the friction means towards a braking member for braking.

[0023] In a first embodiment, the brake is a drum brake, the braking member being a drum, and the friction member being a shoe provided with lining.

[0024] In a second embodiment, the brake is also a disc brake, the braking member being a disc, and the friction member being a brake pad.

[0025] The method comprises the following steps: a brake application force FS is defined (step 0) for immobilization of the vehicle; a temperature variation of the braking member (heating and cooling) is estimated (step 1) from a convection heat transfer model, neglecting heat transfer phenomena by conduction and radiation; a correction of an application to be made to maintain the defined brake application force FS is defined (step 2); the brake application force FS is applied (step 3) with application correction via the actuator.

[0026] In particular, according to the method, a brake application force FS is defined as a function of the slope on which the vehicle is resting and the mass of the vehicle (step 0). This application force is determined by applying the following equation:

[0027]

[0028] With: m: the mass of the vehicle; g: the constant of gravity; µ: the heat loss; “slope”: slope of the road; “static wheel radius”: radius of the wheel; and “effective wheel radius”: point where the force is applied (center of the piston which is at the center of the wheel).

[0029] A temperature variation of the braking device is then estimated from a convection heat transfer model, neglecting heat transfer phenomena by conduction and radiation (step 1).

[0030] Estimating this variation in temperature of the braking device, being a result of heating and cooling over time (rather cooling during immobilization by using a parking brake), makes it possible to determine the evolution of the clamping force.

[0031] For example, when the brake is a disc brake, a decrease in the temperature of the braking component leads to a decrease in the expansion of the braking component and therefore to a distance between the pads and the disc. The clamping force will therefore decrease. The phenomenon is reversed for a drum brake, in which the decrease in expansion leads to a closer movement of the drum and the shoes, and therefore to an increase in the clamping force. In both cases, the real-time clamping force moves away from the clamping force FS previously defined.

[0032] Following the estimation of the variation in braking temperature, a correction of a tightening to be carried out to maintain the defined tightening force is defined from the latter (step 2). As explained above, the variation in temperature leads to a variation in the tightening force moving away from the tightening force FS. Quantifying the temperature variation makes it possible to quantify the variation in the tightening force, and therefore to estimate the tightening correction to be carried out to return to the tightening force FS.

[0033] For example, when the brake is a disc brake, the cooling of the disc is estimated. The tightening correction corresponds to an increase in the tightening (of the pads on the disc) to compensate for the distance (by reducing the expansion) of the disc from the pads and thus re-attain the initially defined tightening force.

[0034] When the brake is a drum brake, the cooling of the drum is estimated. The clamping correction corresponds to a reduction in the clamping (of the shoes on the drum) to compensate for the approach (by reducing the expansion) of the drum in relation to the shoes and thus re-attain the clamping force initially defined.

[0035] Finally, the clamping force is applied with correction of the clamping via the actuator (step 3). Optimal immobilization of the vehicle is therefore obtained by applying the clamping force FS, but the latter is obtained by applying a different clamping force than that which would have been applied without estimating the temperature variation, and which would not have made it possible to obtain the clamping force FS, but a clamping force more or less important than the latter, depending in particular on the type of braking device.

[0036] Concerning the convection heat transfer model used in step 1, the latter includes the following parameters: h: convection heat transfer coefficient of the air; m: mass of the braking device; c p : heat capacity of the braking component material;A s : contact surface between the air and the braking component.

[0037] According to a preferred embodiment, the heat transfer model is written:

[0038]

[0039] With :dT conv : the temperature variation due to the phenomenon of heat transfer by convection;T disc : the temperature of the braking device;T amb : the air temperature at the contact surface between the air and the braking member; andv: the air speed at the contact surface between the air and the braking member.

[0040] The parameters v and T ambcome from sensors, and their values ​​are therefore obtained immediately.

[0041] The parameter T disc corresponds to an estimate of the temperature of the braking component (sum of cooling and heating).

[0042] The parameters m, c p and A s are mechanical characteristics, and are therefore known.

[0043] As an example, the table below gives characteristics of a disc for three types of brake.

[0044] [Tab. 1]Type 1Type 2Type 3Diameter(m)0.2950.2950.33Thickness(m)0.0110.0120.022Surface (m 2 )0.06830.06830.0855Mass(kg)5.97.395.08MaterialCast ironCast ironAluminiumC p (J Kg -1 K -1 )450450900

[0045] Concerning the parameter h, the heat transfer coefficient by air convection, it must be estimated as a function of the speed v of the air at the level of the contact surface between the air and the braking member.

[0046] According to a particular embodiment, a Jürges model is used:

[0047] If v ≤ 5 m / s then: h = 4×v+5.6

[0048] If v > 5 m / s then: h = 7.1×v^(0.78)

[0049] Thus, the estimation of the temperature variation of the braking member is advantageously carried out in a simple, immediate and precise manner, as illustrated in the. The latter illustrates the evolution of the temperature (°C) of a disc over time (s). Cooling is therefore observed. The figure includes three curves: estimation by a heat transfer model taking into account heat transfers by convection, conduction and radiation; estimation by a heat transfer model according to the invention; the actual temperature, measured.

[0050] We note that: the average difference between the estimated curve and the actual curve is 21.39°C for curve a, and only 14.15°C for curve b; the maximum difference between the estimated curve and the actual curve is 46.65°C for curve a, and only 36.7°C for curve b.

[0051] According to a particular embodiment, the variation in mass of the braking member over time is taken into account in the convection heat transfer model. Indeed, the use of the brakes causes wear over time, particularly of the braking member: the latter, through friction and corrosion, loses material, and therefore its mass decreases.

[0052] The parameter m in the convection heat transfer model therefore becomes a parameter that is a function of time: m(t). This parameter can then be evaluated, as for the parameter h, by a predictive model of disc wear.

[0053] The invention also relates to a motor vehicle comprising an electronic control unit configured to implement the method according to the invention.

Claims

Method for electrically controlling a parking brake for a motor vehicle, the brake comprising at least one friction means connected to an electromechanical actuator capable of moving the friction means towards a braking member for braking, characterized in that the following steps are carried out: a clamping force is defined for immobilization of the vehicle without application of the clamping force; a temperature variation of the braking member is estimated from a convection heat transfer model, neglecting conduction and radiation heat transfer phenomena; a correction of a clamping to be carried out to maintain the defined clamping force is defined from the estimation of the temperature variation; a first clamping force is applied with correction of the clamping via the actuator. Method according to claim 1, in which the convection heat transfer model comprises the following parameters: h: convection heat transfer coefficient of the air; m: mass of the braking member; c p : heat capacity of the braking component material;A s : contact surface between the air and the braking component. Method according to the preceding claim, in which the heat transfer model is written: where dT conv is the temperature variation due to the phenomenon of heat transfer by convection, T disc is the temperature of the braking device, T amb is the air temperature at the contact surface between the air and the braking member, and v is the air speed at the contact surface between the air and the braking member. A method according to claim 2 or 3, wherein the convection heat transfer coefficient h is determined using a model, such as the Jürges model. Method according to one of claims 2 to 4, in which the variation in mass of the braking member over time is taken into account in the convection heat transfer model. A method according to any preceding claim, wherein a brake application force is defined as a function of the slope on which the vehicle is resting and the mass of the vehicle. A method according to any preceding claim, wherein the tightening correction is effected by reducing the tightening. A method according to any preceding claim, wherein the tightening correction is effected by increasing the tightening. Method according to any one of the preceding claims, in which the brake is a drum brake, the braking member is a drum, the friction member is a shoe provided with lining. Method according to any one of the preceding claims, in which the brake is a disc brake, the braking member is a disc, the friction member is a brake pad. Motor vehicle comprising an electronic control unit configured to implement the method according to any one of claims 1 to 10.

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