Patent Description:
In this regard, quality control techniques assume fundamental relevance from the supplier's viewpoint, as does the guarantee of product quality for the customer.

At present, therefore, quality management is aimed at the production of an extremely reduced number of defective parts.

Every production system is affected by errors.

In the case of quality control of brake pads for motor vehicles, this operation is normally carried out by withdrawing a sample pad, after a predefined number of pads produced, and then analysing it in a remote place.

If the pad is judged to be in conformity, i.e. free of defects, the entire product lot to which the sample pad belongs will likewise be judged to conform.

This type of control cannot be performed for a substantial number of pads, but only for a limited number of the same, and consequently cannot be applied on the pad production line.

Furthermore, quality control of a pad becomes impracticable with the current techniques if the pad is not of a traditional type, but it rather sensor-equipped, that is, fitted with a sensor capable of picking up information on the braking of a vehicle.

For example, in production lines for the manufacture of sensor-equipped pads, the main difference compared to a standard pad is the presence of a completely new manufacturing step called calibration.

The calibration process is a test applied at the end of the production line or during an intermediate phase thereof in order to evaluate the technical and functional characteristics of the sensor-equipped pad and it is also an actual calibration process designed to ensure that the sensor-equipped pads actually act as absolute sensors.

<CIT> teaches a method for the real time estimation of the applied pressure and noisiness in a brake element.

The task that the present invention sets itself is to provide a method for the automatic calibration of a brake pad that does not have the limitations of the prior art.

The task is achieved by a method according to Claim <NUM>.

With particular reference to the above-described figures, the device implementing the method according to the invention for automatic calibration. denoted in its entirety by the reference number <NUM>, comprises a supporting base <NUM> for stabilising the device during its operation.

The base <NUM> is preferably metallic and provides stability and support to the entire device implementing the method according to the invention during calibration and compression of the pads, relative to the braking device of a vehicle, due to the high level of pressures and forces applied.

Hereinafter pad will mean a pad equipped with a sensor <NUM> of the type comprising a metal support element, a block of friction material supported by the metal support element, and one or more sensors supported by the metal support element and interposed between the block of friction material and the metal support element.

The sensors are capable of detecting, during use on a vehicle, the forces that are exchanged during use between the block of friction material and the element to be braked, defined by the disc constrained to the wheel.

A sensor-equipped pad results in the possibility of detecting and/or foreseeing the generation of numerous problems, such as anomalous wear on the brake pads, for example because they "touch" the disc even when the brake is not activated, for example due to a poor adjustment of the brake callipers, rather than undesirable noise, vibrations and squealing during braking.

The automatic calibration device <NUM> implementing the method according to the invention comprises at least one control panel <NUM>, a pad retaining means, a mechanical stress means generating on the pad a normal force in combination with a shear force or a normal force in combination with a torque and configured so as to reproduce the typical forces acting upon said pad during real braking of a vehicle, and sensors for detecting the normal force and the shear force or sensors for detecting the normal force and the torque.

On the device <NUM> implementing the method according to the invention there is a control panel <NUM> for the management thereof, which comprises all the push buttons for blocking the calibration test for emergency reasons during the calibration operations.

The panel must further have service push buttons for starting and selecting the calibration tests to be performed and a screen <NUM> for displaying information related to the test underway, for example, information relative to the pressure and/or forces and/or torques applied.

The panel <NUM> must also be able to provide calibration test reports stating the values measured by the device and relating to a type of pad during the calibration procedures, as well as the statistical values thereof, such as mean maximum, minimum and standard deviations and, finally, the calibration parameters and an indication as to whether the test has been passed or not according to the initial predefined values of the parameters.

The panel <NUM> must take into consideration the control of the ECU in order to measure the pads and synchronise them with the measurements of the sensors installed in the device <NUM> implementing the method according to the invention and used as a reference for comparison with those of the pad.

We shall now make reference to the embodiments illustrated in <FIG>. In a first technical solution, the device <NUM> implementing the method according to the invention comprises at least one fluid-dynamic compressor <NUM> for generating pressure ramps and exerting an optimal pressure on a pad <NUM> to be calibrated, operating at a pressure comprised between approximately <NUM> and <NUM> bar.

The device <NUM> implementing the method according to the invention also has at least one supporting column <NUM> associated with the supporting base <NUM> and suitable for mechanically supporting at least the mechanical stress means, which in this case is a pressure means, generically indicated by <NUM>, for generating a pre-established pressure on the pad <NUM> and defined, in said first technical solution, by a first fluid-dynamic piston <NUM> suitable for delivering the necessary pressure to the pad <NUM> so as to obtain a shear force beyond a normal force as in real braking in a brake calliper of a vehicle.

As will be seen further below, in a second preferred technical solution, the pressure means <NUM> is defined by the first piston <NUM> and by a second piston <NUM>.

There is also an adjustment means, generically indicated by <NUM>, for adjusting the pressure means <NUM> so as to exert on the pad <NUM> a shear force, in addition to a normal force.

The pad retaining means <NUM> comprises a first lock-in means <NUM> for locking in the friction material <NUM> of the pad <NUM> with the pressure means <NUM> and a second lock-in means <NUM> for locking in the pad <NUM>, again on the base <NUM>, for the purpose of reproducing the typical forces that act on the pad during the braking of a vehicle.

In particular, the first lock-in means <NUM> comprises a first profiled edge <NUM> which reproduces the conformation of the friction material <NUM> of the pad <NUM> so as to lock it in optimally during the pressure exerted thereupon by the pressure means <NUM> in order to enable the transmission of the shear forces during the calibration of the pad in as uniform a manner as possible.

The second lock-in means <NUM> comprises a second profiled edge <NUM> which reproduces the conformation of the metal plate <NUM> or backplate of the pad <NUM> so as to lock it in during the pressure exerted thereupon in order to enable the transmission of shear forces during the calibration of the pad while avoiding the shifting thereof relative to the base <NUM>.

The metal plate <NUM> or backplate of the pad conveniently has pins 13a for maintaining electrical contact of the pad <NUM> with the calibration device <NUM> implementing the method according to the invention, for example via a connector <NUM>, by means of a contact device that can be manually or automatically actuated and must have a lever to enable contact to be made between the pins and connector in a very precise manner when the pad is positioned in the calibration device at the start of the calibration test.

<FIG> shows a contact device comprising, by way of example, a mechanism with shock absorbers <NUM> which enable the contacts of the connector <NUM> to be retracted if the positioning of the same is not optimal, in order not to damage the same by bending them during incorrect contacting, as well as a mechanised system <NUM> for automatic contacting which comprises, solely by way of example, a specially motorised translating shaft <NUM> connected by means of a system of connecting rods <NUM> to a plate <NUM> for supporting the mechanism with shock absorbers <NUM>.

As noted, the device <NUM> implementing the method according to the invention further comprises pressure sensors <NUM> and force sensors <NUM> for measuring the normal forces and shear forces; they are positioned in such a way as to maximise the reliability of their measurements relative to the values actually detected and transmitted to the pad <NUM> via the pressure means <NUM>.

More specifically, in the first technical solution the pressure means <NUM> comprises at least a first fluid-dynamic piston <NUM>, which is advantageously supported by at least one supporting column <NUM> and can be inclined via the adjustment means <NUM>.

The column <NUM> also provides mechanical support to the first piston and the compressor <NUM> for the fluid-dynamic system, which can use oil or air.

Furthermore, in this solution the adjustment means <NUM> is defined by the column <NUM> itself, which is used to modify the angle of incidence of the first piston in a range of <NUM>° to <NUM>° with an optimal value of <NUM> degrees (the optimal value obviously depends on the nominal friction coefficient of the brake pad).

In order to modify this angle, the column <NUM> must be capable of being inclined backward or moved upward relative to the base <NUM>.

Once the first piston has been correctly positioned with a desired angle, the device <NUM> implementing the method according to the invention has a locking device, not illustrated, for fixing this angle permanently in total safety during the ensuing calibration test.

The device also has a mechanism for lifting the first piston <NUM> before and after the calibration of said pad for the removal of a calibrated pad with a pad to be calibrated.

In particular the lifting mechanism can be obtained, for example, by means of at least one guide <NUM> for the longitudinal movement of the column <NUM>.

The guide <NUM> must be provided with nuts or other locking systems for permanently fixing the chosen angle.

It should also be specified that the first lock-in means <NUM> comprises a level of the pin <NUM> of the first piston <NUM>, a device <NUM> for enabling a precise reading, during the calibration, of the angle formed between the axis of the first piston and the plane of the base <NUM>, on which the pad <NUM> is positioned, in such a way as to have good control over said parameter, which is fundamental for a positive outcome of the calibration.

In a variant embodiment, the pressure means <NUM> can comprise an electric motor which activates an endless screw, not illustrated, located on the first piston in order to effect the movement thereof. It is further specified that the base <NUM> must have a mass and a shape such as to assure high mechanical strength in order to avoid any shifting or fall of the entire device <NUM> implementing the method according tto the invention during the applications of pressure.

Furthermore, the base <NUM> must not be subject to deformations.

The upper part of the base <NUM> must further have threaded holes for mounting and removing the second lock-in means <NUM>, which must be changed on each occasion to provide the flexibility of use necessary when the size of the pads <NUM> changes.

All of the mechanical components must be selected so as to ensure structural and mechanical stability and prevent wear after intensive calibration cycles.

This can be obtained through the structural dimensioning of the components of the device <NUM> implementing the method according to the invention and choosing resistant stainless steel materials with a high level of mechanical strength.

The base <NUM> must also have supporting uprights <NUM> to adequately support the column <NUM>.

As seen, the column <NUM> is connected to the guide <NUM> to enable the longitudinal movement thereof relative to the base <NUM>, so that disposed inside the latter there are all the data transmission cables and the power supply cable, as well as all the tubes for the oil or compressed gas serving to provide the required thrust to the first piston.

Furthermore, the column <NUM> must be dimensioned in such a way as not to undergo deformations that could either modify the way in which the forces are transmitted or even disable the entire device, for example by precluding the correct setting of the angle of incidence and the loading and unloading of the pads <NUM> during the calibration process.

The pressure sensor <NUM> is positioned in the column <NUM> to permit the measurement of the fluid-dynamic pressure of the fluid-dynamic or gas circuit suitable for cooperating with a safety valve <NUM>.

In the device there are also valves 22a for loading and unloading the oil or gas from the fluid-dynamic system.

Advantageously, as seen, the first piston <NUM> exerts a pressure on the pad <NUM>, and is connected to the column <NUM> by means of a connection hub <NUM> and connected to the fluid-dynamic circuit.

The first piston must have a length such as to end at the first lock-in means <NUM> so as to be easily mountable and removable, but simultaneously robust and reliable in order to ensure a correct positioning relative to the pad <NUM> and to enable a pressure as uniform as possible on the latter and an optimal transmission of the shear forces on the friction material <NUM> of the pad <NUM>.

Before any calibration cycle, the first piston <NUM> must be capable of being moved laterally or vertically to ensure facilitated access to the pads <NUM> so that they can be positioned or removed from the device <NUM> implementing the method according to the invention.

An automatic device <NUM> can also be provided to set the pressure of the oil or gas to zero to enable the positioning of the pad <NUM> in the second lock-in means in a correct position before the start of the calibration procedure and restore the pressure of the oil or gas after the correct positioning of a new pad <NUM> to be calibrated.

The pressure of the fluid-dynamic circuit which supplies pressure to the first piston <NUM> must be checked and measured during the calibration of the pad by a dedicated sensor <NUM> connected to the fluid-dynamic circuit.

The force actually transmitted to the piston, and consequently to the pad <NUM>, is measured with precision both in the normal direction and tangentially to the surface of the pad <NUM> by means of load cells <NUM> positioned conveniently, for example, on the pin <NUM> or in the hinge <NUM> of the first piston <NUM> or also on the base <NUM> or in any other equivalent position for such measurements.

All of the sensors must be connected to the control panel <NUM> to enable a constant, continuous measurement of the forces during calibration.

The fluid-dynamic pressure of the oil that pushes the first piston must have values comprised between <NUM> bar and <NUM> bar, and enable a precision of at least <NUM>% for reduced pressure values <<NUM> bar and <NUM>-<NUM>% for values above <NUM> bar.

In detail, the first lock-in means <NUM> for locking in the friction material <NUM> must be capable of being slightly inclined so as to be adapted to the surface profile of the pad <NUM> and this is achieved by means of the pin <NUM> in the first technical solution or, as we shall see, by means of a first and a second bearing <NUM> and <NUM> in a preferred technical solution.

The first lock-in means <NUM> has a coupling mechanism <NUM> for locking or releasing it relative to the first piston <NUM> in such a way that it can be changed according to the conformation of the pad to be calibrated or in the event of damage or wear.

The first lock-in means is defined by a first template <NUM>, which must have a shape and an edge <NUM> such as to precisely reproduce the profile of the friction material <NUM> of the pad <NUM>.

The height of the edge <NUM> must be about <NUM> depending on the pad shape and model.

In order to prevent damage to the friction material <NUM>, it will be advantageous to slightly round the margin of the edge <NUM> and possibly use soft material to reduce the mechanical stress on the corresponding edge of the friction material of the pad.

In a different embodiment, the first template <NUM> can be integrated into the first piston without having, in this case, the edge <NUM>.

In such a case it would be desirable to use stainless steel or a material similar to the one used in the vehicle brake disc so as to have the same friction coefficients on the friction material <NUM> during the calibration of the pad.

The second lock-in means <NUM> of the pad <NUM> acts on the backplate <NUM> thereof and comprises a housing plate <NUM>, which can be made of stainless steel, connected directly to the base <NUM> by screws and bolts suitable for the securing thereof in such a way as to ensure long-term operating stability despite the enormous forces acting on it.

The housing plate <NUM> has a second template <NUM> having an edge <NUM> that exactly reproduces the profile of the backplate or metal plate <NUM> of the pad <NUM>.

The housing plate <NUM> also has a connection system having the above-described pins 13a to permit an electrical connection to the pad <NUM> during the calibration thereof and send the signals required by the electronic system for the acquisition of the necessary data during the calibration procedure.

The pins 13a must be removably associated in the housing plate <NUM> and be positioned in a manner that is compatible with the position of the connector <NUM> of the pad <NUM>, which can vary on each occasion depending on the specific model of the pad to be calibrated.

At the outlet of the housing plate <NUM> of the backplate of the pad <NUM> there is a cable for transmitting the signal, not illustrated, having a number of wires necessary to receive the signals of all the sensors integrated into the brake pad, said wires being of a standard length, typically (but not necessarily) less than <NUM> metre and incorporated in a cable compatible with the pin 13a of the electronic control unit (ECU), which is built specifically to measure and receive the signals of the pad.

In a preferred variant embodiment, the pressure means <NUM> comprises, as said, both the first piston <NUM>, which is used, however, in order to be able to apply only a normal force on the pad <NUM>, and at least a second piston <NUM> for applying thereupon a normal force or shear force.

Advantageously, the first and second pistons <NUM> and <NUM> are disposed perpendicularly to each other on the base <NUM>.

In this solution the base <NUM> supports two main columns <NUM>, which maintain fixed a third crossbar <NUM>, which has the function of supporting the first piston <NUM> in a vertical position so as to provide vertical pressure to the pad <NUM>.

The crossbar <NUM> is not inclinable, but rather fixed, as the shear forces are provided by the second piston <NUM>, parallel to the plane of the base <NUM>, and, consequently, to the surface of the friction material <NUM>.

In particular, the second piston <NUM> is disposed so as to directly deliver the forces to the surface of the pad <NUM> on one side thereof, and in particular laterally to the friction material <NUM>.

The first and second pistons <NUM> and <NUM> are driven, respectively, by a first and a second electric motor <NUM> and <NUM>.

The first and second electric motors will provide the energy necessary to supply suitable forces to be applied on the pad <NUM> via the first and second pistons.

This preferred technical solution assures several advantages compared to the previous one.

Firstly, the two normal and horizontal forces can be exerted separately, making it possible to perform solely a compressibility test on the pad, which in this case can also not be of the sensor-equipped type, but rather be a traditional braking pad devoid of sensors capable of transmitting the values of the forces that are applied thereon.

In this manner, one can apply a compression force alone on the pad, with a direction normal to the surface thereof, without applying shear forces, and measure the variation in thickness as a function of the pressure applied on the pad.

This possibility is very important because it provides a parameter that is extremely important for characterising the production and process of manufacture both of sensor-equipped pads and traditional pads and enables virtually <NUM>% control of production.

The second advantage is an increase in flexibility for delivering different shear forces at a given pressure, it being understood that the optimal condition defined by the formula
<MAT>
can always be restored by setting the ratio between the two normal and horizontal forces applied on the pad, which in this case is sensor-equipped.

In particular, the first and second pistons apply the vertical force and the horizontal force on a first element that acts directly on the pad held fixed by a second element which houses both the pad and the pins 13a to permit an electrical connection between the pad and the external electronic circuit for receiving the signals of the pad.

The first take-up element <NUM> for taking up the force from the first piston <NUM> and from the second piston <NUM> has a flat clamping surface on the exposed surface of the friction material of the pad <NUM>.

The first and second pistons <NUM> and <NUM> have, in proximity to the base <NUM>, a first and a second bearing <NUM> and <NUM> to provide a minimum tilting capability to the first and second elements <NUM> and <NUM> to eliminate the friction forces between them and the first and second pistons and maintain solely the contribution of the friction force between the pad and the steel plate, in order to give a real representation of the process of rubbing between the disc and pad in the brake calliper.

The first and second bearings <NUM> and <NUM> have load cells <NUM> and <NUM> inside them which are capable of communicating data related to the pressure exerted on the pad.

In this manner, advantageously, on the surface of the pad <NUM>, in particular on the side of the friction material <NUM>, stability is obtained thanks to the friction forces that arise between the pad and the first element <NUM>.

In such a case, in order to avoid mechanical failures, the condition
<MAT>
must be satisfied, where µs is the static friction coefficient.

On the side of the backplate <NUM>, the second element <NUM> will be used; it has a base <NUM> having a recess <NUM> for reproducing the profile of the backplate <NUM>, which has a thickness ranging from <NUM> to <NUM> and shapes varying according to the pad model.

The recess <NUM> has a thickness of at least <NUM>-<NUM> to enable a lateral hold on the pad <NUM> during the calibration operation and limit all lateral movements thereof.

The second element <NUM> further has a connector <NUM> for electrically connecting the pad, which, as said, is sensor-equipped in this case, with the electrical part of the device <NUM> implementing the method according to the invention.

The second element <NUM> also has two handles <NUM> to permit a simplified removal thereof following pad changes or simply because of wear.

The second element will be maintained fixed to the base <NUM> by mechanical attachment points suited to the purpose and not illustrated.

In this case as well, the pressure means can have an endless screw <NUM> suitable for exerting pressure on the pad.

It should also be specified that, in the preferred variant embodiment, the first piston has threaded bars <NUM> for adjusting its distance from the base <NUM> and a spring-operated presser <NUM> for maintaining the end thereof that acts on the pad perfectly parallel to the surface of the same.

The verticality of the spring-operated presser is suitably adjusted by means of an alignment cam <NUM>.

In light of the foregoing, it is very important for the control panel <NUM> to be able to correctly run the calibration procedure according to the calibration protocol defined at the time of setting the calibration procedure itself.

In particular, the control panel is capable of ensuring the safety of the operators and technicians who perform the calibration by blocking any calibration run if the overall hardware or software controls are not positive, of managing the start and stop of the calibration, of permitting and managing the loading and unloading of the hydraulic or electrical systems and maintenance operations in general, of managing the loading and removal of the pads to be calibrated in the calibration system, of controlling and maintaining the level of forces and pressure according to the procedure and test protocols, of memorising the data of the sensors during the calibration procedure and the data on a central or locale database, of sending and receiving the data or calibration settings of the calibration system, of managing the step of printing on the brake pad after the calibration in order to print a permanent Data Matrix and memorise, accordingly, the values received to permit the traceability of the structural and functional characteristics of the pad.

Control of the calibration of the pad must be performed, insofar as the pressure is concerned, on the fluid-dynamic circuit or by means of the electric motor driving the piston, and insofar as the applied forces are concerned, by using the normal force measured by the load cells.

The number of pressure values detected during the calibration will preferably be a minimum of <NUM> and a maximum of <NUM>.

The pressure values must preferably correspond to any of the following values expressed in bars: <MAT>.

Where i is an index of the specific point of measurement and N is the total selected number of points to be measured during the calibration.

In this case a constant specific interval of between <NUM> and <NUM> bar is assumed.

The control panel must have power inputs, preferably <NUM>/<NUM> VAC, in order to power the entire device <NUM> implementing the method according to the invention.

The electrical insulation and earthing must be suitably set so as to avoid leakage currents and EMC emissions or problems of immunity and electrostatic discharges (ESD).

In particular, any leakage of current toward the base or toward the pads must be avoided in order to eliminate any disturbance in the measurement of data during the calibration.

The control panel must also have the possibility of interfacing with an external unit such as a PC to be configured to enable the downloading either of the calibration data or DataMatrix of the pad and it must also control a trigger preferably of the -<NUM>, <NUM> V type to manage the calibration steps and coordinate the various electronic components for acquiring the data of the pad sensors. During a calibration session, each value of pressure used to compress the pad must be maintained stable for a period of time, typically comprised between <NUM> and <NUM>, which should be configured by the software in the control panel during the calibration setting phase.

Finally, the panel should also have indicators, such as LEDs, to enable real-time readings and monitoring of the values observed via the pressure and force sensors installed in the device <NUM> implementing the method according to the invention.

This makes it always possible for the control and alarms to be activated in the event of an anomaly relative to the predetermined values fixed during the setting phase.

Thanks to the device <NUM> implementing the method according to the invention, the calibration procedure substantially corresponds to the real conditions in terms of pressure and shear forces, and to those of the brake calliper during braking of a vehicle.

In particular, the ratio between the normal forces, correlated to the pressure, and the shear forces, correlated to the friction forces, must be as close as possible to the real coefficient of friction between the brake pad and disc of the vehicle.

The operation of the device <NUM> implementing the method according according to the invention is as follows.

The value of the friction coefficient is known from the design stage and is adjusted at the start of the calibration procedure.

The angle of incidence of the piston is adjusted and the angle of inclination thereof is changed until reaching the desired value according to the following formula: <MAT>.

Where Ft is the tangential force, FN the normal force, and q the angle between the axis of the piston and the plane of the surface of the friction material of the brake pad.

This formula is very important because it takes account of the non-linear behaviour of the friction material under pressure, thanks to the fact that the elastic constants thereof depend on the pressure.

Consequently, failure to reach a condition close to the friction coefficient, in terms of the ratio of forces, will cause an incorrect calibration of the pad, since the forces will be transmitted in a different manner from the real braking conditions of the vehicle.

In a normal calibration of a pad <NUM>, after the calibration parameters have been set by means of suitable software, which is not the subject matter of the present invention, the device <NUM> implementing the method according to the invention will be set in a pad loading/unloading mode to facilitate the movement of the first piston <NUM> and enable correct positioning of the pad in the second lock-in means <NUM>.

The first piston is repositioned with the first lock-in means <NUM> for locking in the friction material <NUM> in such a way that the edge <NUM> thereof adheres perfectly to the edge of the friction material. Then the device <NUM> implementing the method according to the invention is set back into the calibration mode by restoring pressure to the fluid-dynamic circuit so as to lock the first piston in the pre-established position.

The safety cage <NUM> is closed again so that calibration of the pad can begin.

The test is performed automatically and in sequence according to the number of points established and defined via the software.

In the absence of such a definition, the calibration will be performed according to the default setting, that is, by carrying out <NUM> measurements at <NUM>, <NUM>, <NUM> and <NUM> bar, maintained for <NUM> seconds with <NUM> second intervals of pressure increases to the next value and stabilisation thereof. At the end of calibration, according to what was previously specified, the device <NUM> implementing the method according to the invention automatically comes to a stop and saves all the data in a predefined file via configuration software.

During the calibration of the pad the values of the sensors must be visible in real time also from the control panel <NUM> via dedicated LED indicators.

At the end of calibration, the operator must set the machine in the brake pad unloading mode, open the safety cage <NUM> by releasing the lock mechanism of block and move the first piston to access the pad and remove it.

The whole procedure will be repeated to calibrate a new pad.

In case of the device with a first and second piston disposed perpendicular to each other, the operation is the same as that of the device having only the first piston, since only the method of applying the forces changes, while the result does not change.

Another important additional feature of the machine is that of enabling an in-line measurement of compressibility in <NUM>% of the pads produced, thus assuring high standards of quality. This is possible thanks to the inclusion of an additional module, preferably applied to the piston <NUM> in <FIG>, consisting of a system for measuring the distance between two fixed points, one on the pusher and one on the base on which the pad to be calibrated rests. This measurement can be conveniently performed by means of laser measurement systems, which in any case assure precisions within about one micron. In this manner, if, for example, only a normal force is exerted, it will be possible to simultaneously measure both the pressure exerted and the consequent compression of the friction material and/or underlayer of the brake pad, which provide the values of compressibility of the pad itself.

We shall make reference, finally, to the embodiment of the invention illustrated in <FIG>.

In the previous cases, the forces required on the pad in order to reproduce the same vertical or pressure loads and the same shear loads experienced by the braking device are generated by a single piston or by two pistons.

In this case the stress means comprises a motorised disk <NUM> which applies torque to the pad <NUM> and a vertical piston <NUM> which applies the normal force to the pad <NUM>.

The pad retaining means <NUM> comprises a terminal pressing plate <NUM> of the vertical piston <NUM>.

The pressing plate <NUM> supports the pad <NUM> and presses it against the disk <NUM>.

More precisely, the supporting base for stabilising the device during the operation thereof comprises a structure with uprights <NUM> which supports the vertical piston <NUM>.

The vertical piston <NUM> is driven by a motor <NUM>, preferably an electric motor, in order to provide the normal force which the piston <NUM> exerts on the pad <NUM>.

The pressing plate <NUM> has a bearing for micrometric adjustment in the event of imperfect planarity, so as to provide a pressure as uniform as possible.

The piston <NUM> presses the pad against the disk <NUM>, as occurs in the real world.

The disk <NUM> can be an actual brake disk used in a vehicle equipped with the same braking device and the same brake pad undergoing calibration, or else it can be a disk made with the same material as a brake disk.

A tool <NUM> is used to install disks <NUM>, which may also differ in terms of size and material, on the calibration device.

The disk <NUM> and the position of the pad <NUM> must be selected in a manner that is suitable for simulating the same condition as in the braking device of the vehicle with the same radius provided for in the braking device of the vehicle.

The tool <NUM> is then integrated into the calibration device to support the disk <NUM> and enable the delivery of torsion force to the pad <NUM> so as to simulate the braking torque experienced by the pad during a braking of the braking device.

Because of the intense vertical forces due to the high pressure intervals required to replicate the conditions experienced by the braking device (<NUM> - <NUM> Bar), the tool <NUM> is mechanically robust and allows very small torsional movements at the pad <NUM> about the axis of the disk <NUM>, as actually occurs in a vehicle.

To enable these movements, there is provided a bearing fixed rigidly onto the base of the calibration device so as to guarantee mechanical robustness and at the same time the rotational movement about the axis of the disk <NUM>.

In this case as well, the calibration device also comprises a motor <NUM> for rotating the tool <NUM> and the disk <NUM> fixedly supported by it, and a coupling <NUM>, for example a differential coupling, for connecting the motor <NUM> to the shaft <NUM> that supports the tool <NUM> in rotation.

With the differential coupling <NUM>, torque values in the order of <NUM>-<NUM> are achieved without any need to use motors that are particularly large or powerful.

The mechanical stability of the system is guaranteed by the following condition: <MAT>.

Where µ is the nominal static friction coefficient of the specific pad <NUM>.

The aforesaid condition is necessary to avoid rotational movement and maintain a static measurement.

In this case, FN represents the vertical forces, whilst Ft derives from the braking torque τ= RFt, with R representing the actual radius of the braking device, which will be the same as used in the dynamometric measurements for that braking device.

The measurement should preferably be made not far from the aforesaid condition in order to reproduce the conditions experienced by the braking device.

A specific control of the torque value is provided for to maintain the aforesaid condition while the pressure increases during the test.

The particularity of this embodiment of the invention consists in the fact that a real torque is provided by the motor <NUM> so as to subject the pad to a torsional movement and produce a deformation by shear forces on the friction material precisely as occurs in the braking device. Therefore, the main difference is the provision of a real torque instead of shear forces to produce essentially the same effect as in the solutions previously illustrated and described.

In this case, therefore, the torque is directly controlled instead of the shear forces.

Finally, in this specific case a torque sensor (not shown) must be provided for direct measurement of the torque delivered to the pad.

This torque sensor must be positioned along the drive shaft <NUM>, preferably near the coupling <NUM> or the tool <NUM> where the disk <NUM> is housed.

Claim 1:
A method for automatic calibration of a brake pad (<NUM>) comprising a block of friction material (<NUM>), a support plate or backplate (<NUM>) and force sensors capable of detecting forces that are exchanged between the block of friction material (<NUM>) and an element to be braked, characterised in that of comprising following steps:
- providing an automatic calibration device (<NUM>) comprising a supporting fixed base (<NUM>), at least one control panel (<NUM>), brake pad (<NUM>) mechanical stress means generating on said pad a normal force in combination with a shear force, and brake pad (<NUM>) retaining means in turn comprising a first lock-in means (<NUM>) for locking in said friction material with said mechanical stress means and a second lock-in means (<NUM>) for locking said brake (<NUM>) pad on said fixed base (<NUM>), and sensors for detecting said normal force and said shear force;
- locking in said brake pad (<NUM>) on said supporting fixed base (<NUM>) by said a second lock-in means (<NUM>);
- locking in said friction material (<NUM>) with said mechanical stress means by said a first lock-in means (<NUM>);
- generating on said brake pad (<NUM>) locked in by said first and second lock-in means (<NUM>, <NUM>), a normal force in combination with a shear force adjusted to reach a condition, in terms of ratio of shear force to normal force, equal or close to a value known from a design stage of the friction coefficient between said block of friction material (<NUM>) and said retaining means;
- receiving from the force sensors in the brake pad (<NUM>) signals providing calibration data, wherein measurements of force from the force sensors in the brake pad (<NUM>) are synchronised and compared with the measurements of force from the sensors installed in the device (<NUM>);
- memorising said calibration data.