Patent Publication Number: US-2022227482-A1

Title: Actuator control for force fight mitigation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European Patent Application No. 21305053.7 filed Jan. 18, 2021, the entire contents of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to the field of actuator control and, in particular, the field of mitigating force fight in actuator control systems such as employed in aircraft flight control systems. 
     BACKGROUND 
     Actuators are used in various fields for moving components or surfaces. In aircraft, for example, flight control surfaces such as wing flaps, ailerons, elevators, rudders, etc. are moved by means of actuators in response to a command from the flight control computer or from a manual control e.g. operation of a control device by the pilot or crew. Similar actuation systems may be used in other fields such as in other vehicles or industrial uses. Often, e.g. where the component or surface to be moved is large or heavy, or where it is necessary to build in redundancy in the event of failure of an actuator, two or more actuators may be coupled to a single component or surface to be moved. This is the case, for example, in safety-critical applications such as actuation systems for primary flight controls of aircraft. In some systems, only one actuator will be active at a time, and the other(s) is on standby until needed. In other systems, both/all actuators may be active at the same time. The actuators in such active/active systems work together to position the surface based on commands which are used to provide respective position references to the individual actuator controllers which move the actuator positioning element to achieve the desired position. A feedback system is provided that senses the actuator or component/surface position and compares this with the desired position and the control signal is adjusted accordingly. Active/active systems improve system redundancy. Whilst, in theory, control commands will be provided to the actuators so that they work simultaneously to provide a balanced movement to the component or surface, in practice, due to differences in sensor tolerances, actuator or surface mechanical tolerances, signal disturbances/dissynchrony, load imbalance etc. force fighting can occur between the actuators. Force fighting can lead to wear or damage of the actuation system parts. 
     Conventionally, systems have had to be designed to be oversized or overrated to better be able to handle the stresses of force fighting and mitigate the wear or damage. This results in the design of the actuators being bigger and heavier than actually needed for the movement required. 
     Other solutions, aimed at mitigating force fighting between actuators, involve using a force fighting control process. Such processes generally operate by measuring the relative force or pressure on the actuators and to use the measurement to generate an offset in the position control commands sent to the respective actuators so as to equalize the load share on the actuators. In aircraft, actuators have traditionally been hydraulic actuators. Force fighting mitigation processes have involved measuring the differences in pressure of the actuators and using the pressure differential as part of a regulation scheme to offset the position references or commands being provided to each actuator so as to equalize the load share. 
     In aircraft, there is a trend towards More Electric Aircraft (MEA) or All Electric Aircraft (AEA) in which hydraulic systems are being replaced by smaller, lighter electrical systems. As part of this development, aircraft designers are now using electromechanical actuators (EMAs) in place of hydraulic actuators. Similar force fighting reduction schemes have been developed for EMAs using a force measurement from each EMA and using this is a position control loop to equalize load share. Other systems employ electro-hydraulic actuators (EHAs) which use local electrical motor drives to manage hydraulic pressure in the EHA to move the EHA piston. 
     While such force fighting controls schemes have proven effective, the response of the control can be fairly slow. Further, with EMAs the actuators can jam due to failure of the mechanical power transmission components. There is, therefore, a need for an improved force fighting mitigation control. 
     SUMMARY 
     According to the disclosure, there is provided a system for controlling two or more actuators acting together to move a component or surface. The system includes two or more actuators engaged with the component or surface to be moved. Each actuator is displaceable to move the component or surface in response to a component or surface position command, and has an associated actuator controller to receive the position command and to output an actuator displacement command. The system also includes force fight control means configured to determine a force differential from forces, or a pressure differential from pressures measured at each actuator and to derive a speed and/or a current offset signal, and to provide the offset signal to the actuator controllers to modify the actuator displacement commands. 
     Where the actuators are EMAs, forces are measured. In the case of EHAs, pressure measurements are used to generate the speed/current offset signal. 
     According to another aspect, there is provided a method of controlling two or more actuators acting together to move a component or surface, the method comprising: receiving a position command and outputting an actuator displacement command responsive to the position command; and determining a force differential from forces, or a pressure differential from pressures measured at each actuator to derive a speed and/or a current offset signal, and using the offset signal to modify the actuator displacement command to mitigate force fighting between the actuators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments will now be described by way of example only, with reference to the drawings in which: 
         FIG. 1  shows the configuration of a known active-active actuator control system for an aircraft flight surface control; 
         FIG. 2  shows a known position-based force fighting mitigation system; 
         FIG. 3  shows a conventional cascaded position control system; and 
         FIG. 4  shows a control system according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     To simplify explanation of the system of this disclosure, known control schemes will first be briefly described with reference to  FIGS. 1 to 3 . 
     Referring first to  FIG. 1 , a dual actuator position control system is shown. In this example, the surface  1  to be moved by the actuators  10 ,  20  is a flight control of an aircraft, or other movable surface of an aircraft, such as an aileron. This is one example, only, of the type of application of the system of this disclosure, and the system of the disclosure may find application in moving other components or surface in aircraft or in other fields, requiring active-active actuation of a component. 
     The surface  1  is controlled by two actuators  10 ,  20 , in active mode, acting in parallel. Of course, other numbers of actuators may be used and the disclosure is not limited only to control of two actuators. Also, the actuators shown are hydraulic actuators, but the disclosure may also apply to other types of actuator e.g. EMAs or electro-hydraulic actuators. Each actuator  10 ,  20  has an associated actuator controller  11 ,  21  which is responsible to moving the actuator rod or jack  12 ,  22  to cause resultant movement of the surface  1 , to which the jacks are attached. The actuator controls  11 ,  21  each receive a respective position command or position reference X 1 SP, X 2 SP defining the desired position of the respective jack  12 ,  22  to result in the desired position of the surface  1 . The position references are generated by the flight surface computer(s)  13 ,  23  (here Elevator and Aileron Computers ELAC). In the example shown, two computers  13 ,  23  are provided, one receiving commands from an autopilot, the other receiving manual commands from the pilot or crew e.g. by operation of a sidestick or lever. In other examples, the position references may be provided by a single computer or processor responsive to an input control command indicative of the desired position of the component or surface to be moved. The actuator controls  11 ,  21  may have dedicated or common/shared power supplies  100 ,  200 . In response to input commands e.g. autopilot and/or sidestick commands, the computer(s)  13 ,  23  will generate position references X 1 SP, X 2 SP 
     For the actuators, the position references provided to the actuator controls  11 ,  21  which cause the actuator jacks  12 ,  22  to move to position x 1 , x 2 . This causes corresponding movement (here rotation θSURF) of the surface. The actuator controllers  11 ,  21  will mitigate any disturbances resulting from e.g. the aerodynamic load Fload. 
     As mentioned in the Background, duel or multiple actuator control systems can result in force fighting i.e. unequal loads on the actuators.  FIG. 2  shows a conventional approach to force fighting mitigation as described above. 
     The force F 1 , F 2  on each actuator  10 ′  20 ′ is provided to a summer  30  which determines the force differential ΔF. This is fed to a PID controller  40 . The output of the PID controller provides an adjustment to the fed-back position signal X 1 , X 2  at summers  51 ,  52  to provide respective offsets to the actuators controls  11 ′,  21 ′ which then produce correspondingly adjusted position references for the actuators  10 ′,  20 ′. 
     As mentioned above, such force fighting mitigation schemes are effective but do not completely eliminate force fighting and have a delayed response. 
       FIG. 3  shows a control system and dynamic force equalization control for a dual EMA position control system. The EMAs  10 ″,  20 ″ each have an actuator position control  11 ″,  21 ″ for regulating the position X 1 , X 2  of the EMA based on the input position reference and the fed-back position Xsurf as described briefly above. The actuator position controls can be separate units for each actuator or can be a single unit configured to provide separate control signals to the respective actuators. For EMAs, the actuator controller  11 ″,  21 ″ is a servo-position control having a position control  111 , speed control  112  and current control  113 . The controller controls operation of the EMA, in response to the position reference, adjusted based on position, speed and current feedback, outputting a control voltage V 1 , V 2  for each EMA. In more detail, the position signal is fed back to the position control  111  which generates a position control signal ω 1  to the speed control  112 , this generates a current control signal i 1  to the current control  113  which outputs the voltage command reference V 1  for the EMA motor drive (not shown here as a separate component). 
     The present disclosure modifies the known force fighting mitigation scheme described above with particular reference to  FIG. 2 , but also such that it can be incorporated into the control of an EMA system such as described above with reference to  FIG. 3 . 
     The modified force fighting mitigation control will be described now specifically for EMA systems such as shown in  FIG. 3 . It should be noted, however, that the principles of the modified control can also be applied to hydraulic or other actuator systems such as shown in  FIG. 1 . 
     As discussed above, conventional force fighting mitigation control schemes are based on measuring the difference in actuator linear position and generating a position offset reference to offset the position to alleviate load imbalance. This is conventionally done by a centralised PID control as described with reference to  FIG. 3 . 
     In particular with servo-position controllers as described above, for use with EMAs, using the position control means that the system might not react quickly to force fighting. As can be seen from  FIG. 3 , the position control is the outermost loop of the control scheme and will have the lowest bandwidth of the three controls ( 111 ,  112 ,  113 ) in the actuator controller. This limits the speed with which the loop could react to a force fighting mitigation command. Whilst the impact of this is greater for EMA systems, using position control for force fighting mitigation also has disadvantages in conventional hydraulic systems such as described in relation to  FIGS. 1 and 2 . 
     The system of the present disclosure improves on conventional force fighting mitigation control by using the force or pressure differential, ΔF, between the actuators, to generate a speed offset and/or a current offset to generate an offset speed and/or current reference for each actuator rather than using the force differential to generate a position offset and provide an offset position reference. 
     In other words, the system controls the motor drive to facilitate position control of the actuator by regulating the motor speed (and/or current), thus controlling the motor angle and actuator position via gearing of the mechanical system, via control of the output voltage of the power converter. 
     An example of this modified control will be described with reference to  FIG. 4  for EMA actuators. 
     As described above, two EMA actuators  10 ′″,  20 ′″ are attached to the surface  1  to be moved. A position command (not shown in  FIG. 4 ) from the computer/user input is provided to each actuator controller  11 ′″,  21 ′″ to provide a respective control command to set the respective position X 1 , X 2  of the respective EMA jack  12 ′″,  22 ′″. As is conventional, actuator position, speed and current are fed back to the controller  11 ′″,  21 ′″, to the position controller  111 , speed controller  112  and current controller  113 , to provide positioning according to the position command. 
     Force fighting mitigation, according to the disclosure, as provided as follows. The load force of each actuator F 1 , F 2  is measured and a force differential ΔF is determined and provided to a feedback controller which may be, for example, a PID controller  400 . The feedback controller  400  generates, from the force differential, ΔF, a speed setpoint offset signal ΔωFF which is used by the speed controller  112  to offset the speed reference ω 1 , ω 2  to generate the current reference i 1 , i 2  which is used by the current controller  113  to generate the command voltage V 1 , V 2 . 
     The cascaded approach of position control, speed control, current control is just one example of a motor drive control scheme that can be used to modify a reference as generated by the force fight control system. Other techniques could be used e.g. model predictive schemes. 
     In an alternative example, instead of the PID  400  using the force differential to generate a speed setpoint offset, it generates a current setpoint offset (not shown). In other embodiments, the PID can generate both a speed and a current setpoint offset to adjust the command voltage. 
     In examples using hydraulic actuators or EHAs, the output force of actuator pressure is measured and used to generate a speed offset which is used to adjust the actuator controller outputs to balance the loads, rather than generating a position offset. 
     Using speed and/or current rather than position offsets in an inner control loop improves the rate of response of the system to force fighting. The current loop is tuned for the highest bandwidth due to the current time constants being in the millisecond range. The speed time constant is in the range of 10s or 100s of milliseconds. These inner control loops therefore respond faster than the outer position control loop. Also, since the force fighting performance is improved, the need to oversize or overrate the actuators is reduced, meaning the control systems can be smaller and lighter. 
     The control scheme can be integrated as part of a prognostics and health management strategy to monitor system loads and to flag up potential issues based on maximum loading.