Abstract:
An electrical fly-by-wire system for operating an aircraft rudder includes a low-pass filter, arranged between a rudder bar and an actuator of a rudder. The low-pass filter receives a control command from the rudder bar corresponding to the degree of travel the rudder bar has experienced from a neutral position. Based on the amplitude of the control command, the filter generates an operating command for the actuator. Additionally, the filter operates such that the higher the fraction of the rudder bar&#39;s travel away from the neutral position, with respect to its maximum value of travel, the higher the filter&#39;s time constant is set.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrical fly-by-wire system for operating an aircraft rudder. 
     BACKGROUND OF THE INVENTION 
     It is known that, at the present time, in most aircraft, a rudder is operated via a mechanical link positioned between the rudder bar, actuated by the pilot, and the rudder. However, electrical fly-by-wire operation of such a rudder has already been envisaged, in the image of what is already done with the other control surfaces, the flaps, ailerons, spoilers, etc. 
     Furthermore, it is known that such a rudder is engineered on the basis of calculated loadings applied to the aircraft during standardized maneuvers. In roll and yaw, these maneuvers consist in influencing the rudder by sharp actions on the rudder bar, up to the point where the rudder has reached its full travel. 
     SUMMARY OF THE INVENTION 
     A subject of the present invention is an electrical fly-by-wire system for operating a rudder, by virtue of which it is possible to limit the lateral loadings applied during maneuvers to the rudder and therefore reduce the size and mass thereof, without thereby reducing the flyability of the aircraft or flight safety. 
     To this end, according to the invention, the electrical fly-by-wire system for operating an aircraft rudder, the rudder being mounted so that it can rotate about an axis so that it can adopt any angular position whatsoever within a range of travel extending on each side of the neutral position of the rudder and limited on each side of this neutral position by a maximum travel value, and the system including: 
     a rudder bar actuated by the pilot and associated with a transducer that delivers an electrical control command that represents the action of the pilot on the rudder bar; and 
     an actuator receiving an operating command derived from the control command and moving the rudder about the axis. 
     The system is notable in that: 
     between the rudder bar and the actuator there are filters of the low-pass type receiving the control command from the transducer and generating the operating command for the actuator; and 
     the higher the fraction of the maximum travel value to which the amplitude of the control command corresponds, the higher the time constant of the filter. 
     Thus, by virtue of the present invention, non-linear filtering which depends on the travel available to the rudder is introduced into the control commands at the rudder bar, this filtering being all the greater the nearer the rudder gets to the end stops delimiting maximum travel, thus limiting the loadings applied to the rudder and therefore making it possible for its size and mass to be reduced. 
     Furthermore, it is known that it is customary for an operating system of the type recalled hereinabove to include, in addition, a yaw-stabilizer that generates a stabilizing command which is added to the control command at the rudder bar. In this case, the level of the maximum loadings on the rudder becomes particularly critical when these commands are of the same sign. 
     Hence, according to another particular feature of the present invention, the operating system additionally includes a yaw-stabilizer that stabilizes the aircraft in terms of yaw, generating a yaw-stabilizing command, and a first adder that sums the yaw-stabilizing command and the actuator operating command. Also, a sign identifier is provided, which is capable of determining whether the control command and the yaw-stabilizing command are of the same sign or of opposite signs. The sign identifier acts on the filters to increase their time constant when the control command and the stabilizing command are of the same sign. 
     Thus, the loadings applied to the rudder are reduced even more by further filtering of the control command at the rudder bar when the rudder is close to its position of maximum travel and when this command and the yaw-stabilizing command are of the same sign. 
     In a practical embodiment, the system according to the present invention includes: 
     a limiter receiving the control command and delivering an output signal which is: 
     either the control command, when the amplitude thereof corresponds to a travel value below a limit equal to a predetermined fraction of the maximum travel value; 
     or a limit value corresponding to the limit when the amplitude of the control command is greater than this limit value; 
     a first low-pass filter having a first time constant and receiving the output signal from the limiter; 
     a subtractor calculating the difference between the control command and the output signal from the limiter; 
     a second low-pass filter having a second time constant higher than the first time constant and receiving the difference calculated by the subtractor; and 
     a second adder summing the output signals from the first and second filters, so as to generate a filtered control command for the actuator. 
     When this system is provided with the aforementioned yaw-stabilizer, it may additionally include: 
     a third low-pass filter having a third time constant higher than the second time constant and receiving the difference calculated by the subtractor; 
     a controlled switch inserted between the second and third low-pass filters, on the one hand, and the second adder, on the other hand, so as to be able to send to the second adder, either the output signal from the second low-pass filter or the output signal from the third low-pass filter; and 
     a switch controller that: 
     connects the second low-pass filter to the second adder when the yaw-stabilizing command and the electrical control command are of opposite signs; or 
     connects the third low-pass filter to the second adder when the yaw-stabilizing command and the electrical control command are of the same sign. 
     Preferably, the first, second and third low-pass filters are of the first-order type, with a transfer function of the form          1     1   +     τ                 p         ,                          
     τ being the respective time constant τ 1 , τ 2  or τ 3  of the first, second and third filters and p being the LAPLACE variable. 
     The first (τ 1 ), second (τ 2 ) and third (τ 3 ) time constants may have respective values of between 100 ms and 500 ms; 500 ms and 1 second; and 1 second and 2 seconds. 
     Furthermore, the limit may correspond to roughly 70% of the maximum value of travel of the rudder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The figures of the appended drawings will make it easy to understand how the invention may be embodied. In these figures, identical references denote elements which are similar. 
     FIG. 1 shows the block diagram of one embodiment of the electrical fly-by-wire operating system according to the present invention; 
     FIG. 2 is a diagram illustrating, in a plan view, the movements of the aircraft rudder operated by the system of FIG. 1; and 
     FIGS. 3,  4  and  5  illustrate the filtering of the operating commands for the rudder, for three different command amplitudes, respectively. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The electrical fly-by-wire operating system according to the present invention and depicted in FIG. 1 is intended to operate an aircraft rudder  1  mounted to rotate in both directions about an axis Z—Z in the way symbolized by the double headed arrow  2 . As illustrated in the schematic plan view of FIG. 2, the rudder  1  can adopt any angular position whatsoever about axis Z—Z, within a range of travel  3  extending on each side of the aerodynamically neutral position  4  of rudder  1 . The range of travel  3  is limited on each side of the neutral position  4  by a position  5 D or  5 G, corresponding to the maximum travel value M (to the right and to the left respectively) and embodied by end stops  6  for rudder  1 . 
     The electrical fly-by-wire operating system comprises a rudder bar  7  available to the pilot (not depicted), associated with a transducer  8  delivering an electrical yaw-control command y, and an actuator  9  receiving, from the output of an adder  10 , an operating command c capable of moving rudder  1  about axis Z—Z. 
     The electrical fly-by-wire operating system of FIG. 1 additionally comprises yaw-stabilizing means  11  (flight computer), generating a yaw-stabilizing command s sent to one of the inputs of adder  10 . The other input of adder  10  receives a command yf, corresponding to yaw-control command y after filtering via an arrangement  12  arranged between transducer  8  and adder  10 . 
     The operating command c for actuator  9  is therefore the sum of the filtered command yf and of the yaw-stabilizing command s. 
     The filtering arrangement  12  comprises a limiter  13  receiving, at its input  13 E, the yaw-control command y and capable of limiting it in amplitude to a limit value l corresponding to a predetermined fraction L of the maximum travel value M. For example, the limit L is equal to 70% of the maximum value M (see FIG.  2 ). The limiter  13  operates as follows: 
     if the amplitude y 1  of the control command y is less than the limit value l, it is signal y which appears at the output  13 S of limiter  13 ; 
     by contrast, if the amplitude y 2  of the control command  y  is greater than the limit value l, it is this limit value l which is present at output  13 S. 
     Filtering arrangement  12  additionally comprises three first-order low-pass filters  14 ,  15  and  16 , a subtractor  17 , an adder  18 , a controlled switch  19 , an operating device  20  for the switch, and a multiplier  21 . 
     These various elements are connected as follows: 
     input  14 E and output  14 S of the filter  14  are connected respectively to output  13 S of limiter  13  and to one of the inputs  18 E 1  of adder  18 ; 
     the positive input  17 P and the negative input  17 N of subtractor  17  are connected respectively to the output of transducer  8  and to output  13 S of limiter  13 , so that subtractor  17  at its output  17 S delivers the difference between the electrical yaw-control command y and this same command limited by limiter  13 ; 
     inputs  15 E and  16 E of filters  15  and  16  are connected in common to output  17 S of subtractor  17 ; 
     outputs  15 S and  16 S of filters  15  and  16  are connected respectively to the two inputs  19 E 1  and  19 E 2  of the controlled switch  19 ; 
     output  19 S of the controlled switch  19  is connected to the other input  18 E 2  of adder  18 , so that the latter receives either the signal filtered by filter  15  or the signal filtered by filter  16 , depending on the position of switch  19 ; 
     the control device  20  operating switch  19  is itself controlled by multiplier  21 , which receives both the yaw-stabilizing command s and the yaw-control command y. 
     The way in which the system, according to the invention, works is described hereinafter with reference to the diagrams of FIGS. 3,  4  and  5 , which represent the yaw-control command y as a function of time t. The diagrams also show the limit values l and m corresponding respectively to the limit angular values L and M. 
     FIG. 3 depicts the scenario in which the given command y is in the form of a square pulse  22 , the amplitude yl of which is below the limit l. In this case, the limiter  13  allows the square pulse  22  to pass in its entirety, and this appears at its output  13 S. Thereafter: 
     subtractor  17  receives the same square pulse  22  on its two inputs  17 P and  17 N, which means that no signal is present on its output  17 S and neither of filters  15  and  16  is active; 
     filter  14  receives the square pulse  22  and filters it, rounding off the sharp rising  22 A and falling  22 R edges, in the way depicted in FIG.  3 . 
     The signal yf in this case therefore consists entirely of this square pulse with rounded rising and falling edges  22 A and  22 R. 
     If, now, the given command y is in the form of a square pulse  23 , the amplitude y 2  of which is above the limit value l (see FIGS.  4  and  5 ), the limiter  13  is active and at its output  13 S delivers a square pulse corresponding to the square pulse  23 , but limited to the amplitude l. Thereafter: 
     filter  14  receives the square pulse  23 , capped of its excess  24  above the amplitude l; and 
     subtractor  17  delivers on its output  17 S the excess  24  above the amplitude l, sent to the inputs  15 E and  16 E of the filters  15  and  16 . 
     The square pulse  23 , capped of the excess  24 , is filtered by filter  14  in the way similar to the one indicated above for the square pulse  22  (note the rising and falling edges  23 A and  23 R). 
     In addition, excess  24  is filtered either by the filter  15  or by filter  16 , depending on the signs of the commands y and s. 
     If these signs are opposite, something which is detected by multiplier  21 , the switch  19 , controlled by device  20 , connects the output  15 S of filter  15  to the input  18 E 2  of adder  18 , so that this excess  24  is filtered by filter  15 , more strongly than filter  14  filters the capped square pulse  23 , as indicated by the curved segment  25  in FIG.  4 . This figure also represents, in dashed line, by way of comparison, the continuation of the rounded rising edge  23 A that would have resulted from filtering by filter  14 . 
     By contrast, if the commands y and s are of the same sign, the device  20 , under the control of multiplier  21 , switches switch  19  so that the output  16 S of filter  16  is now connected to the input  18 E 2  of adder  18 . The excess  24  is therefore more strongly filtered by filter  16  than by filter  15 , as shown by the curved segment  26  in FIG.  5 . In this last figure, a dashed line has been used, for comparison purposes, to depict the continuations of the rounded rising edge  23 A which would have resulted from filtering by filters  14  and  15  respectively. 
     In both instances of FIGS. 4 and 5, the filtered command yf therefore consists of the sum of the capped square pulse  23 , filtered by filter  14 , and of the excess  24 , filtered either by filter  15  or by filter  16  (FIG. 4 or FIG.  5 ). 
     The low-pass filters  14 ,  15  and  16  have time constants which, for example, are respectively between 100 ms and 500 ms; 500 ms and 1 second; and 1 second and 2 seconds. Thus: 
     the filtering afforded by filter  14  corresponds to high flyability criteria; 
     filter  15  allows a significant reduction in the loadings applied to the rudder, when the action on the rudder bar and the action of the yaw stabilizer oppose one another; and 
     filter  16  allows a significant reduction in the loadings even when the action of the rudder bar and the action of the yaw stabilizer combine. 
     Such a reduction in the loadings applied to the rudder allows its size and therefore mass to be reduced.