Abstract:
The invention relates to a method of controlling a vehicle brake that is adapted to exert a braking force in response to an actuation setpoint, the method comprising the following steps:
       from a braking setpoint, determining a nominal actuation setpoint for the brake actuator, taking account of all of the components of the braking setpoint;   from the same braking setpoint, and from a measurement of the torque developed by the brake, determining a correction for the nominal actuation setpoint, this correction taking account only of low-frequency variations in the braking setpoint; and   adding the correction to the nominal setpoint.

Description:
The invention relates to relates to a method of controlling a vehicle brake with torque correction. 
     BACKGROUND OF THE INVENTION 
     Vehicle braking systems include braking actuators (which may be hydraulic or electromechanical) for applying braking torque to the wheels of a vehicle, thereby tending to slow down the vehicle. 
     Most of the brake controls that are known for use in aviation make use of a setpoint that is converted either into a pressure if the brakes are hydraulic, or into a force that is to be applied, or into a displacement of the pusher, if the brakes are electromechanically actuated. 
     Controls making use of a torque setpoint and organizing a feedback loop based on measured torque have been proposed, as in document US 2005/0001474. Those controls present the advantage of taking account of the overall action of the brake by monitoring the torque that it generates, thereby making it possible to adapt to dispersions in the braking torque response for a given braking force. 
     Nevertheless, controls having a broad passband can interfere with protection for preventing the wheels from locking, particular if there is a phase offset between torque control signals and anti-locking control signals. Under certain grip conditions, torque control delivers a torque setpoint that is temporarily zero in order to prevent the wheels from locking. However, if a wheel locks in untimely manner, then the torque as measured becomes zero quite suddenly and the measured torque is then equal to a torque setpoint of zero. The wheel thus remains locked, and the brake is not controlled for the purpose of releasing the wheel. 
     OBJECT OF THE INVENTION 
     An object of the invention is to provide brake control that makes use of a force or position setpoint, while nevertheless taking account of measured torque. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In order to achieve the invention, there is provided a method of controlling a vehicle brake that is adapted to exert a braking force in response to an actuation setpoint, the method comprising the following steps:
         from a braking setpoint, determining a nominal actuation setpoint for the brake actuator, taking account of all of the components of the braking setpoint;   from the same braking setpoint, and from a measurement of the torque developed by the brake, determining a correction for the nominal actuation setpoint, this correction taking account only of low-frequency variations in the braking setpoint; and   applying the correction to the nominal actuation setpoint.       

     Thus, the brake is indeed controlled in accordance with the braking setpoint and not in accordance with torque. The torque measurement is used herein merely to produce a low frequency correction of the nominal actuation setpoint, which is itself calculated while taking account of high-frequency components in the braking setpoint. 
     Low-frequency correction as proposed in this way thus enables dispersions in braking torque to be reduced for a given braking setpoint, where such dispersions can be caused by dispersions in applied braking force, or to dispersions in the braking torque response to an applied braking force. 
     In addition, low-frequency correction as proposed remains compatible with anti-locking protection that provides high-frequency modulation of the braking setpoint in order to prevent the wheel from locking. 
     One of the advantages of the invention is that it enables the brake to operate in a degraded mode in the event of the torque sensor failing. The correction is then arbitrarily set to zero or maintained to its current value, and the brake is then controlled solely as a function of the braking setpoint. 
     In the special circumstance of using a position setpoint, the torque correction of the invention makes it possible to compensate for thermal expansion that can cause the force that is applied by the brake to vary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention can be better understood in the light of the following description with reference to the sole FIGURE that constitutes a block diagram of a particular implementation of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is described below in application to an aircraft brake of the type including electromechanical actuators that are controlled in displacement. A computer (not shown) generates a braking setpoint  F . This setpoint is corrected at high frequency by a system for providing protection against wheel lock, which system continuously verifies the slip rate of the wheel, detects any starting of wheel lock, and decreases the braking setpoint  F  accordingly in order to prevent the wheel from locking. 
     In known manner, a converter  1  transforms the braking setpoint  F  into a position setpoint  X  for the pusher of the actuator, in this example in application of a model  1  that is not linear. Calculation of the position setpoint  X  is performed at a calculation frequency that is high and compatible with the operating anti-locking protection, such that the position setpoint  X  takes account simultaneously of low-frequency components and of high-frequency components in the braking setpoint  F . 
     According to the invention, a low-frequency position correction x corr  is calculated and added by means of a summing circuit  20  to the position setpoint  X  in order to obtain a corrected position setpoint  X   corr =  X +x corr . This position setpoint x corr  takes account of the measured torque as follows. 
     Initially, an image of a mean torque C mean  is generated that corresponds to the braking setpoint  F . For this purpose, the braking setpoint  F  is delivered to a proportional stage  2  of gain K 1  in order to make it comparable to a torque, and it is then subjected to a first lowpass filter  3  in order to eliminate all high-frequency components, and in particular those that are due to implementing anti-lock protection. 
     Furthermore, use is made of a measurement of the torque C mes  actually exerted by the brake, which measurement is provided to a proportional stage  4  of gain K 2 , and is then subjected to a second lowpass filter  5  in order to eliminate all of the high-frequency components, together with measurement noise. This produces a calibrated measured torque {tilde over (C)} mes . 
     The mean torque C mean  and the calibrated measured torque {tilde over (C)} mes  are supplied as inputs to a comparator that generates an error ε. This error is subjected to processing, by being delivered to a controller that includes a proportional action  6  of gain K 3 , an integral action  7 , and finally a saturation stage  8  having the purpose of restricting the correction to values lying within the range [x min ,x max ]. This saturation prevents excessively large corrections that would disturb proper operation of the brake or that would lead to too great a force being applied, i.e. a force above a limit force that can be accepted by the brake. 
     Preferably, and in conventional manner, the integral action  7  includes anti-runaway protection that freezes the integral action in the event of the correction being saturated by the saturation stage  8 , so as to avoid incrementing the integral of the error ε so long as the correction is saturated. 
     The output from the saturation stage  8  is then delivered to a slope limiter  9  that has the function of ensuring that variations in the correction are progressive. This produces the desired position correction x corr . 
     When the aircraft is stationary, a braking force can nevertheless be applied, e.g. for presenting the aircraft from moving while parked. The force as applied in this way leads to a non-zero mean torque C mean , whereas the measured torque C mes  is zero, or very low. Under such circumstances, the torque error would be large and would lead to a large amount of correction, further increasing the travel of the actuator pushers, and thereby contributing to increasing the applied force. In order to avoid such a situation, the correction is neutralized. To perform this neutralization, provision is made for a switch  10  under the control of a member  11  for deactivating correction, thus making it possible to switch the input of the slope limiter  9  to a fixed value, which value is selected in this example to be equal to zero. This switching also serves to neutralize the correction when it is detected that the torque sensor that provides the torque measurement C mes  is faulty. The slope limiter  9  connected downstream from the switch  10  then serves to avoid jolty correction in the event of such switching, and when switching in the opposite direction. 
     The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims. 
     In particular, although the invention is illustrated with reference to brakes having electromechanical actuators that are controlled in position, the invention applies more generally to any other type of control. For example, it is possible to generate force actuation setpoints for brakes of the same type, or pressure actuation setpoints for hydraulic brakes, such actuation setpoints being corrected in accordance with the invention by measuring the torque generated by the brake. 
     Although it is stated above that the correction changes suddenly to the value zero when the correction is neutralized, provision can be made to deactivate correction in some other way, e.g. by maintaining the most recent correction value prior to neutralization, and then when correction is reactivated, by starting again from said most recent value. It is possible to neutralize the correction by other means, e.g. by using a conditional summing circuit  20  that stops summing the position correction x corr  to the position setpoint  X  in response to an instruction to neutralize correction. 
     Although it is stated that in order to form an image of the mean torque C mean  from the braking setpoint  F , the braking setpoint is multiplied by a constant gain K 1 , it is naturally possible to make use of a gain K 1  that is variable and that is determined in real time as a function of parameters p such as the speed of the aircraft, the temperature of the brake, or the operating point of the brake, by using an appropriate digital model. Advantageously, account should be taken not only of parameters of the brake that is being regulated, but also parameters that relate to other brakes, thus making it possible to ensure that brake wear and heating is made uniform. 
     Although the description refers to a controller of the proportional-integral type, it is also possible to use other types of controller, for example a proportional-integral-derivative or other controller. 
     Although the image of the mean torque C mean  and the torque measurement C mes  are filtered independently by two independent lowpass filters, it is also possible to omit those two filters and replace them with a single lowpass filter that is located downstream from the comparator so as to filter the error ε. 
     Although it is stated that a saturation stage  8  is used for saturating the correction x corr , it is possible to use a saturation stage  21  for saturating the corrected setpoint  X   corr  either in addition as shown in the FIGURE, or as a replacement for the saturation stage  8 , thereby making it possible to guarantee that the corrected setpoint remains within levels that are compatible with the structural integrity of the brake components. 
     Finally, although it is stated that in order to determine the correction x corr  for the position setpoint, use is made of the braking setpoint  F  that contains the anti-locking correction and from which low-frequency components are extracted by means of a lowpass filter, it is possible to determine this position correction in some other way, for example by using the low-frequency braking setpoint taken prior to it being subjected to high-frequency correction by the device for providing anti-locking protection (the low-frequency braking setpoint may for example come from pedals operated by the pilot, or from a deceleration setpoint when braking in so-called “autobrake” automatic mode). The nominal setpoint  X  is itself determined by an input that is the sum of the low-frequency braking setpoint plus the high-frequency anti-locking correction.