Patent Publication Number: US-2023151829-A1

Title: Hydraulic force fight mitigation

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the benefit of priority to U.S. Provisional Pat. Application No. 63/264,002, filed Nov. 12, 2021, the contents of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     This instant specification relates to fluid actuators, more specifically dual redundant hydraulic actuators for aircraft. 
     BACKGROUND 
     In some applications, such as aircraft control surface positioning or systems in which control redundancy is useful or required, a shared mechanical output is driven by two or more redundant fluid actuators. Force fight occurs when more than one actuator is operating on the same rigid mechanical output and there is a difference between them. No two actuators and/or control loops are perfectly identical and, as a result, the displacements of the two actuators is rarely, if ever, equal. When two (or more) actuators apply different levels of actuation to a shared, stiff or rigid mechanical output, a force fight is generated. Such force fights can place significant and potentially damaging stresses and/or torques across the shared mechanical output. 
     In an effort to mitigate the effects of force fight, some actuating systems are overbuilt in order to withstand not only external forces but also the internal force fighting loads. Some other force fight mitigation strategies implement differential pressure transducers in complex closed feedback control loops in an effort to control differential forces through the use of electronic control techniques. 
     SUMMARY 
     In general, this document describes fluid actuators, more specifically dual redundant hydraulic actuators for aircraft. 
     In an example embodiment, a fluid actuator system includes a first fluid actuator configured to actuate an output, a second fluid actuator configured to redundantly actuate the output, a first fluid valve configured to control a first fluid flow to the first fluid actuator, and a second fluid valve configured to control a second fluid flow to the second fluid actuator. 
     Various embodiments can include some, all, or none of the following features. The first fluid actuator can include a first piston, a first fluid chamber configured to receive fluid to urge movement of the first piston in a first direction, and a second fluid chamber configured receive fluid to urge movement of the first piston in a second direction opposite the first direction. The first fluid valve can include a first valve body configured to permit the first fluid flow to the first fluid chamber in a first configuration, permit the first fluid flow to the second fluid chamber in a second configuration, and block the first fluid flow to the first fluid chamber and the second fluid chamber in a third configuration. The second fluid actuator can include a second piston, a third fluid chamber configured to receive fluid to urge movement of the second piston in a first direction, and a fourth fluid chamber configured to receive fluid to urge movement of the second piston in a second direction opposite the first direction. The second fluid valve can include a second valve body configured to permit the second fluid flow to the third fluid chamber in a first configuration, permit the second fluid flow to the fourth fluid chamber in a second configuration, and permit the second fluid flow to the third fluid chamber and the fourth fluid chamber in a third configuration. The second fluid valve can include at least one fluid inlet, a first fluid outlet, a second fluid outlet, a valve body is configured to (1) fluidically connect the fluid inlet to the first fluid outlet in a first configuration, (2) fluidically connect the fluid inlet to the second fluid outlet in a second configuration, and (3) fluidically connect the fluid inlet to the first fluid outlet and the second fluid outlet in a third configuration. 
     In an example implementation a method of fluid actuation includes controlling, by a first fluid valve, a first fluid flow to a first fluid actuator, actuating, by the first fluid actuator, an output, controlling, by a second fluid valve, a second fluid flow to a second fluid actuator, and actuating, by the second fluid actuator, the output. 
     Various implementations can include some, all, or none of the following features. The method can include receiving, by a first fluid chamber of the first fluid actuator, the first fluid flow, urging, by the first fluid flow received by the first fluid chamber, a first piston of the first fluid actuator in a first direction, receiving, by a second fluid chamber of the first fluid actuator, the first fluid flow, and urging, by the first fluid flow received by the second fluid chamber, the first piston in a second direction opposite the first direction. The method can include configuring a first valve body of the first fluid valve to a first configuration, permitting, by the first valve body in the first configuration; the first fluid flow to the first fluid chamber, configuring the first valve body to a second configuration, and permitting, by the first valve body in the second configuration, the first fluid flow to the second fluid chamber. The method can include receiving, by a third fluid chamber of the second fluid actuator, the second fluid flow, urging, by the second fluid flow received by the third fluid chamber, a second piston of the second fluid actuator in a first direction, receiving, by a fourth fluid chamber of the second fluid actuator, the second fluid flow, and urging, by the second fluid flow received by the fourth fluid chamber, the second piston in a second direction opposite the first direction. The method can include configuring a second valve body of the second fluid valve to a first configuration, permitting, by the second valve body in the first configuration; the second fluid flow to the third fluid chamber, configuring the second valve body to a second configuration, and permitting, by the second valve body in the second configuration, the second fluid flow to the fourth fluid chamber. The method can include fluidically connecting a fluid inlet of the second fluid valve to a first fluid outlet of the second fluid valve in a first configuration, fluidically connecting the fluid inlet to a second fluid outlet of the second fluid valve in a second configuration, and fluidically connecting the fluid inlet to the first fluid outlet and the second fluid outlet in a third configuration. 
     In another example embodiment, a fluid actuator includes a housing, a first fluid actuator arranged within the housing and configured to actuate an output, and a second fluid actuator arranged within the housing and configured to redundantly actuate the output. 
     Various embodiments can include some, all, or none of the following features. The fluid actuator can include a first fluid valve configured to control a first fluid flow to the first fluid actuator, and a second fluid valve configured to control a second fluid flow to the second fluid actuator. The first fluid valve can include a first valve body configured to permit the first fluid flow to a first fluid chamber in a first configuration, permit the first fluid flow to a second fluid chamber in a second configuration, and block the first fluid flow to the first fluid chamber and the second fluid chamber in a third configuration. The second fluid valve can include a second valve body configured to permit the second fluid flow to a third fluid chamber in a first configuration, permit the second fluid flow to a fourth fluid chamber in a second configuration, and permit the second fluid flow to the third fluid chamber and the fourth fluid chamber in a third configuration. The second fluid valve can include, at least one fluid inlet, a first fluid outlet, a second fluid outlet, a valve body is configured to (1) fluidically connect the fluid inlet to the first fluid outlet in a first configuration, (2) fluidically connect the fluid inlet to the second fluid outlet in a second configuration, and (3) fluidically connect the fluid inlet to the first fluid outlet and the second fluid outlet in a third configuration. The first fluid actuator can include a first piston, a first fluid chamber configured to receive fluid to urge movement of the first piston in a first direction, and a second fluid chamber configured receive fluid to urge movement of the first piston in a second direction opposite the first direction. The second fluid actuator can include a second piston, a third fluid chamber configured to receive fluid to urge movement of the second piston in a first direction, and a fourth fluid chamber configured to receive fluid to urge movement of the second piston in a second direction opposite the first direction. The first fluid actuator can include a first piston rod affixed to the output, and the second fluid actuator comprises a second piston rod affixed to the output. 
     The systems and techniques described here may provide one or more of the following advantages. First, a system can provide redundant fluid actuation of an output with reduced force fight. Second, the system can reduce force fight without the use of a complex combination of sensors and control loops. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a chart of example prior art differential pressures in a prior art redundant fluid actuator. 
         FIG.  2    is a block diagram of an example redundant fluid actuator system 
         FIGS.  3 A- 3 C  are block diagrams of an example system  300  of example redundant fluid valves under various conditions. 
         FIGS.  4 A and  4 B  are charts of example fluid flows in an example redundant fluid actuator. 
         FIG.  5    is a chart of example differential pressures in an example redundant fluid actuator. 
         FIG.  6    is a flow diagram of an example process of redundant fluid actuation. 
     
    
    
     DETAILED DESCRIPTION 
     This document describes systems and techniques in which force fight between redundant fluid actuators is mitigated by a unique mechanical (e.g., fluidic control) design. In general, since many fluid actuators spend most of their operational lifetimes in a fixed position, most of the force fight that occurs during the operational lifetime will happen when the control valves are at null. This document describes examples of mechanical (e.g., fluid control) designs that mitigate force fight when the redundant fluid valves are at or near their null configurations. 
     In general, and as will be described in more detail throughout this document, a shared mechanical output is actuated by two (e.g., or more) redundant fluid actuators. Each of the redundant fluid actuators is controlled by a fluid (e.g., servo) valve, with one of the valves acting as the primary (e.g., leader) valve and the other acting as a secondary (e.g., follower) valve. The primary valve is configured to provide a conventional collection of three configurations in which fluid flow is directed to a first output (e.g., “extend”), to a second output (e.g., “retract”), or is blocked (e.g., null). The secondary valve is also configured to provide three configurations, including one in which fluid flow is directed to a first output (e.g., “extend”), another in which fluid flow is directed to a second output (e.g., “retract”), and another that permits at least a limited amount of fluid to flow between the fluid source and the first output and the second output (e.g., modified null). In general, by allowing the second valve to permit flow between opposing outputs of the secondary fluid actuator near null, the secondary actuator will be substantially unable to generate mechanical forces that could lead to force fight with the primary actuator when at or near null. 
       FIG.  1    is a chart  100  of example prior art differential pressures in a prior art redundant fluid actuator. In the illustrated example, two substantially similar, redundant fluid control systems are used. A line  110  represents pressure versus input (e.g., control) power for a first fluid servo valve (e.g., a primary valve), and a line  120  represents pressure versus input (e.g., control) power for a second fluid servo valve (e.g., a redundant valve). The pressure represented by the line  120  is substantially the inverse of the pressure represented by the line  110 . In the illustrated example, this inversion is used to illustrate a “worst case” scenario in which two valves connected to two redundant actuators are acting in opposition (e.g., when two control loops are both independently oscillating about a set point while independently trying to maintain a common target position). 
     When near the target position where only fine control movements are needed, the two control currents oscillate, sometimes in opposition, about the zero milliamp point. The difference between the pressures represented by the lines  110  and  120  is represented by a line  130  (e.g., differential pressure). As shown in the illustrated example, there is a relatively large swing  140  in differential pressures (e.g., +/- ~75% of supply pressure) across a relatively small range  150  of control current values (e.g., +/~1% of rated input current). In general, the steepness of the slope of the line  130  around zero pressure and zero current, represented by point  160 , provides a visual representation of the severity of force fight that can occur around null in previous systems. Typically, such force fight would result in stresses or torques across a redundantly actuated mechanical output, and can result in system inefficiency, power loss, excessive wear, damage, and/or malfunction. 
     Prior techniques for addressing such force fights have included oversizing of the mechanical components to make them strong enough to better tolerate force fight. However, such solutions can increase costs, size, and weight of the overall system. Other prior techniques have included the use of pressure sensors to measure differential pressures and closed control loops to control the positions of the redundant valves in order to mitigate force fight. However, such solutions increase system complexity, can require skilled tuning, and include components that have their own unavoidable differences that can actually contribute to force fight and at least partly offset the benefits they were intended to provide. 
       FIG.  2    is a block diagram of an example redundant fluid actuator system  200 . The system  200  includes an actuator  210  having a housing  211  configured to actuate a mechanical output  212  relative to a mounting point  214 . 
     The actuator  210  is a dual-redundant fluid actuator that includes a primary fluid actuator  220  and a secondary fluid actuator  240  within the housing  211 . The primary fluid actuator  220  and the secondary fluid actuator  240  are both configured to actuate the mechanical output  212  relative to the mounting point  214 , cooperatively or independently. 
     The primary fluid actuator  220  includes a piston  222  having a piston head  223  and a piston rod  225  and configured to receive fluid to urge movement of the piston  222  in a first direction (e.g., extend) based on fluid pressure in a fluid chamber  224 , and retract based on fluid pressure in a fluid chamber  226 . The piston rod  225  of the primary fluid actuator  220  is mechanically affixed to the mechanical output  212  at an end  228 . 
     The secondary fluid actuator  240  includes a piston  242  having a piston head  243  and a piston rod  245  and configured to receive fluid to urge movement of the piston  242  in a first direction (e.g., extend) based on fluid pressure in a fluid chamber  244 , and retract based on fluid pressure in a fluid chamber  246 . The piston rod  245  of the secondary fluid actuator  240  is mechanically affixed to the mechanical output  212  at an end  248 . 
     Without the force fight mitigation solutions that will be described in more detail below, since the end  228  and the end  248  are both affixed to the mechanical output  212 , any differences in the positions of the pistons  222  and  242  and/or differences in the amounts of force being provided by the pistons  222  and  242  can cause stress within the mechanical output  212 . In some examples, such stresses can cause the mechanical output  212  to develop stress fractures that can lead to eventual malfunction or failure. In some examples, such differences in outputs can cause the actuator  210  to warp or bend, causing bending stresses along the pistons  222  and/or  224 , causing binding and reduced output power, and/or causing excessing wear of internal fluid seals that can lead to eventual malfunction of failure. However, the actuator  210  includes force fight mitigation solutions that reduce or eliminate such problems when the actuator  210  is substantially motionless (e.g., aircraft flaps are either deployed or stowed for hours, with a few seconds of deployment and retraction in-between). 
     A primary valve  260  (e.g., leader valve) controls flow of a pressurized fluid from a pressurized fluid source  201  to the fluid chambers  224  and  226  of the primary fluid actuator  220 . In some embodiments, the primary valve  260  can be an electrohydraulic servo valve (EHSV). The primary valve  260  is configured to be controlled by a controller  202  (e.g., the controller  202  can provide control currents that actuate the primary valve  260 ). The primary valve  260  is configured to direct fluid flow from the fluid source  201  to the fluid chamber  224  in a first valve configuration (e.g., to extend the piston  222  and the mechanical output  212 ), to direct fluid flow from the fluid source  201  to the fluid chamber  226  in a second valve configuration (e.g., to retract the piston  222  and the mechanical output  212 ), and to block fluid flow among the fluid source  201 , the fluid chamber  224 , and the fluid chamber  226  in a third configuration (e.g., null, hydraulically blocking the primary fluid actuator  2   2   0 ). Excess fluid is directed to a drain  290 . An example of the primary valve  260  will be discussed further in the description of  FIGS.  3 A- 3 C . 
     A secondary valve  280  (e.g., follower valve, redundant backup valve) controls flow of the pressurized fluid from the pressurized fluid source  201  to the fluid chambers  244  and  246  of the secondary fluid actuator  240 . In some embodiments, the secondary valve  280  can be an electrohydraulic servo valve (EHSV). The secondary valve  280  is configured to be controlled by the controller  202  (e.g., the controller  202  can provide control currents that actuate the secondary valve  280 ). The secondary valve  280  is configured to direct fluid flow from the fluid source  201  to the fluid chamber  244  in a first valve configuration (e.g., to extend the piston  242  and the mechanical output  212 ), and to direct fluid flow from the fluid source  201  to the fluid chamber  246  in a second valve configuration (e.g., to retract the piston  242  and the mechanical output  212 ). The secondary valve  280  also has a third configuration that differs from the third configuration of the primary valve  260 . 
     In the third configuration of the secondary valve  280 , pressurized fluid flow from the fluid source  201  is blocked, but a (e.g., slight) fluid flow is permitted between the fluid chamber  244  and the fluid chamber  246 . In this third configuration, fluid pressure from the fluid source  201  is blocked, but the secondary fluid actuator is not hydraulically blocked in a small region around the null point of the secondary valve  280 . As such, when the secondary valve  280  is commanded to be at or near null, the secondary fluid actuator  240  exerts no fluid force (e.g., because any pressure in the fluid chambers  244  and  246  would leak down and/or equalize). With no fluid force being exerrted proximal the null position of the secondary valve, the secondary fluid actuator  240  will mechanically follow the movement of the primary fluid actuator  220  substantially without force fight. Excess fluid is directed to a drain  290 . An example of the secondary valve  280  will be discussed further in the description of  FIGS.  3 A- 3 C . 
       FIG.  3 A  is a block diagram of an example system  300  of example redundant fluid valves in a null configuration. The system includes a primary fluid valve  301  and a secondary fluid valve  351 . In some embodiments, the primary fluid valve  301  can be the example primary valve  260  of  FIG.  2   . In some embodiments, the primary fluid valve  351  can be the example secondary valve  280 . In some embodiments, one or both of the primary fluid valve  301  and the secondary fluid valve  351  can be electrohydraulic servo valves (EHSVs). 
     In use, the primary fluid valve  301  is configured to control a fluid flow from a pressurized fluid source (e.g., the example fluid source  201 ) to a first fluid actuator (e.g., the example primary fluid actuator  2   2   0 ). In use, the secondary fluid valve  351  is configured to control a fluid flow from a pressurized fluid source (e.g., the example fluid source  201 ) to a second fluid actuator (e.g., the example secondary fluid actuator  240 ). 
     The primary fluid valve  301  includes a fluid port  310  (e.g., a first fluid inlet) and a fluid port  312  (e.g., a second fluid inlet) that are configured to be fluidically connected to a first fluid pressure source (e.g., the example fluid source  201 ). A fluid port  314  is configured to be fluidically connected to a fluid drain (e.g., the example fluid drain  2   90 ). A fluid port  320  (e.g., a first fluid outlet) is configured to be fluidically connected to a first fluid chamber of a primary fluid actuator (e.g., the example fluid chamber  224  of the example primary fluid actuator  2   2   0 ). A fluid port  322  (e.g., a second fluid outlet) is configured to be fluidically connected to a second fluid chamber of a primary fluid actuator (e.g., the example fluid chamber  226  of the example primary fluid actuator  2   2   0 ). 
     The primary fluid valve  301  includes a valve body  330  (e.g., a valve spool) configured to direct fluid flows among the fluid ports  310 - 322 . The valve body  330  can be positioned in three configurations relative to the fluid ports  310 - 322 .  FIG.  3 A  shows the primary fluid valve  301  in a null configuration in which fluid flow through all of the fluid ports  310 - 322  is blocked. Additional configurations are discussed in the descriptions of  FIGS.  3 B and  3 C . 
     The secondary fluid valve  351  includes a fluid port  360  (e.g., a first fluid inlet) and a fluid port  362  (e.g., a second fluid inlet) that are configured to be fluidically connected to a second fluid pressure source (e.g., the example fluid source  201  or a redundant backup fluid source). A fluid port  364  is configured to be fluidically connected to a fluid drain (e.g., the example fluid drain  2   90 ). A fluid port  370  (e.g., a first fluid outlet) is configured to be fluidically connected to a first fluid chamber of a primary fluid actuator (e.g., the example fluid chamber  244  of the example secondary fluid actuator  240 ). A fluid port  372  (e.g., a second fluid outlet) is configured to be fluidically connected to a second fluid chamber of the secondary fluid actuator (e.g., the example fluid chamber  246  of the example secondary fluid actuator  240 ). 
     The secondary fluid valve  351  includes a valve body  380  (e.g., a valve spool) configured to direct fluid flows among the fluid ports  360 - 372 . The valve body  380  can be positioned in three configurations relative to the fluid ports  360 - 372 .  FIG.  3 A  shows the secondary fluid valve  351  in a null configuration in which fluid flow to the fluid port  364  is blocked (e.g., fluid drainage is blocked), but the valve body  380  is configured with a lapping  39   0   a  and a lapping  39   0   b  that permits an at least partial flow of fluid between the fluid ports  360  and  370  (represented by arrow  353   a ), and an at least partial flow of fluid between the fluid ports  362  and  372  (represented by arrow  353   b ). 
       FIG.  3 B  shows the system  300  in a configuration in which the fluid valves  301  and  351  have been urged into a configuration that could urge movement of a connected fluid actuator. In the illustrated example, the valve body  330  and the valve body  380  have been shifted slightly to the right (e.g., relative to the example shown in  FIG.  3 A ). The illustrated rightward shifting serves merely as an example, and slight leftward shifting would provide similarly opposite effects. 
     In the primary fluid valve  301 , the position of the valve body  330  blocks fluid flow though the fluid port  312 , permits fluid flow between the fluid port  310  (e.g., pressurized fluid) and the fluid port  320  (e.g., a fluid chamber of an actuator) (represented by arrow  303   a ) through an aperture  391  opened between the valve body  330  and the fluid port  310 , and permits fluid flow between the fluid port  322  (e.g., an opposing fluid chamber of the actuator) and the fluid port  324  (e.g., a fluid drain) (represented by arrow  303   b ). In use, this configuration would urge movement of a primary fluid actuator connected to the primary fluid valve  301 . 
     However, movement of a connected fluid actuator will urge movement of a corresponding secondary fluid actuator that is connected to the secondary fluid valve  351 . Such movement can cause fluid to be urged through the fluid ports  370  and  372 . Additionally, the valve body  380 , acting redundantly to the valve body  330 , is also moved slightly to the right (e.g., relative to the example shown in  FIG.  3 A ). 
     In the secondary fluid valve  351 , the lapping  39   0   a  of the valve body  380  permits fluid flow  353   a  between the fluid port  360  (e.g., pressurized fluid) and the fluid port  370  (e.g., a fluid chamber of an actuator), and the lapping  39   0   b  permits fluid flow  353   b  between the fluid port  372  (e.g., an opposing fluid chamber of the actuator) and the fluid port  364  (e.g., a fluid drain). As such, the fluid pressures provided to the opposing fluid chambers of the secondary actuator connected to the fluid ports  370  and  372  will be substantially equal and offsetting, and fluid movement cause by movement of the secondary fluid actuator (e.g., by movement of the primary fluid actuator) will be substantially unblocked and offer substantially no fluidic blocking action. Since the secondary actuator controlled by the secondary fluid valve  351  is essentially “floating” in this configuration, the secondary actuator provides substantially no force fight relative to the primary actuator being controlled by the primary fluid valve  301 . 
       FIG.  3 C  shows the system  300  in a configuration in which the fluid valves  301  and  351  have been urged into a configuration that could urge movement of a connected fluid actuator. In the illustrated example, the valve body  330  and the valve body  380  have been shifted even further to the right (e.g., relative to the examples shown in  FIGS.  3 A and  3 B ). The illustrated rightward shifting serves merely as an example, and further leftward shifting would provide similarly opposite effects. 
     In the primary fluid valve  301 , the position of the valve body  330  continues to block fluid flow though the fluid port  312 , continues to permit fluid flow between the fluid port  310  (e.g., pressurized fluid) and the fluid port  320  (e.g., a fluid chamber of an actuator) (represented by arrow  303   a ) through an aperture  391  opened between the valve body  330  and the fluid port  310 , and continues to permit fluid flow between the fluid port  322  (e.g., an opposing fluid chamber of the actuator) and the fluid port  324  (e.g., a fluid drain) (represented by arrow  303   b ). In use, this configuration would urge movement of a primary fluid actuator connected to the primary fluid valve  301 . 
     In the secondary fluid valve  351 , the position of the valve body  380  permits fluid flow  353   a  between the fluid port  360  (e.g., pressurized fluid) and the fluid port  370  (e.g., a fluid chamber of an actuator). However, the movement of the valve body  380  has exceeded the lapping  39   0   b  such that the lapping  39   0   b  no longer permits fluid flow  353   b  between the fluid port  372  and the fluid port  362 , and the position of the valve body  380  permits fluid flow between the fluid port  372  and the fluid port  364 . As such, the secondary fluid valve  351  provides a fluid flow having a direction that is substantially similar and redundant to the fluid flow that is provided by the primary fluid valve  301 . 
     Under normal operating conditions in which the primary fluid valve  301  and its respective primary fluid actuator are operating nominally, the forces caused by the secondary fluid valve and its respective secondary fluid actuator may be imbalanced. However, since both fluid valves  301 ,  351  are acting cooperatively, the force fight is generally much less than it might be near the null position where the fluid valves  301 ,  351  may be in opposition. Furthermore, in some embodiments, the fluid valves  301 ,  351  may only be configured to this potentially force-fight permitting condition for brief periods of time (e.g., only when periodically extending or retracting an redundant actuator that generally remains stationary most of the time, such as an aircraft flap actuator, a thrust reverser nacelle, or a landing gear actuator). 
     Under abnormal operating conditions in which the primary fluid valve  301  and/or its respective primary fluid actuator are not operating nominally, the secondary fluid valve  351  and its respective secondary fluid actuator can provide the control and fluid force used to control the mechanical output. The main behavioral difference that may be exhibited by the secondary fluid valve  351  compared to the primary fluid valve  301  is a slight lag in responsiveness due to the additional movement of the valve body  380  in order to overcome the lapping  39   0   a  and/or the lapping  39   0   b  before direction-controlling fluid flows begin. However, in some implementations the valve body  380  may be under closed-loop control that would compensate for the lapping  39   0   a  and the lapping  39   0   b . For example, if an associated mechanical output were to be extended or retracted, a control signal would be provided to move the configuration of the valve body  380 . If the movement of the valve body  380  were insufficient to cause movement of the controlled mechanical output, the control loop would likely increase the control current used to move the valve body  380  and eventually overcome the lapping  39   0   a  and/or the lapping  39   0   b . Once the lappings  39   0   a - 39   0   b  have been overcome, the response behavior of the secondary fluid valve  351  becomes substantially similar to the response behavior of the primary fluid valve  301  under similar control conditions. 
       FIGS.  4 A and  4 B  are charts of example fluid flows in an example redundant fluid actuator.  FIG.  4 A  shows a chart  400   a  of an example primary flow output  410   a  versus input (e.g., control) power for an example primary fluid valve. In some implementations, the primary flow output can represent the output of the example primary fluid valve  301  of  FIGS.  3 A- 3 C . As is shown in the illustrated example, small changes in control current about the null point (e.g., a between bout -0.25% to about +0.25% of rated input current) cause changes in output flow (e.g., between about -0.1% to about +0.1 % of maximum flow). In use, these flows can urge the movement of a primary fluid actuator of a dual-redundant fluid actuator such as the example primary fluid actuator  220  of the example actuator  210  of  FIG.  2   . 
       FIG.  4 B  shows a chart  400   b  of an example primary flow output  410   b  versus input (e.g., control) power for an example secondary fluid valve. In some implementations, the secondary flow output can represent the output of the example secondary fluid valve  351  of  FIGS.  3 A- 3 C . As is shown in the illustrated example, relatively larger changes in control current (e.g., compared to the chart  400   a ) about the null point (e.g., between about -1.0% to about +1.0% of rated input current) cause changes in output flow (e.g., between about -0.1 % to about +0.1 % of maximum flow). In use, these flows can urge the movement of a secondary fluid actuator of a dual-redundant fluid actuator such as the example secondary fluid actuator  240  of the example actuator  210  of  FIG.  2   . The relatively wider window of response around the zero input current point is due to the presence of lapping in the secondary fluid valve (e.g., the example lapping  39   0   a  and/or  39   0   b ). 
       FIG.  5    is a chart  500  of example differential pressures in an example redundant fluid actuator. In some implementations, the chart  500  can represent the pressures produced by the example systems  200  or  300  of  FIG.  2   -3C. In some implementations, the chart  500  can represent pressures that arise from the example flow outputs  410   a ,  410   b  of  FIGS.  4 A- 4 B . 
     A line  510  represents pressure versus input (e.g., control) power for a first fluid servo valve (e.g., a primary valve), and a line  520  represents pressure versus input (e.g., control) power for a second fluid servo valve (e.g., a redundant valve). The pressure represented by the line  520  is substantially the inverse of the pressure represented by the line  510 . In the illustrated example, this inversion is used to illustrate a “worst case” scenario in which two valves connected to two redundant actuators are acting in opposition (e.g., when two control loops are both independently oscillating about a set point while independently trying to maintain common target position). 
     When near the target position where only fine control movements are needed, the two control currents oscillate, sometimes in opposition, about the zero milliamp point. The difference between the pressures represented by the lines  510  and  520  is represented by a line  530  (e.g., differential pressure). As shown in the illustrated example, there is a relatively large window  550  of control currents about the null point (e.g., between about -1.0% to about +1.0% of rated input current) in which there is a relatively narrow range  560  of differential pressures (e.g., between about +10% to about -10% of supply pressure). In general, the flatness of the slope of the line  530  around zero pressure and zero input current within the window  550  provides a visual representation of the range in which force fight is mitigated (e.g., by the lappings  39   0   a - 39   0   b ). In some examples, mitigation of force fight around the null point can mitigate stresses or torques across a redundantly actuated mechanical output, which can improve the system efficiency, power, mechanical lifespan, and/or reliability of connected mechanical outputs. 
       FIG.  6    is a flow diagram of an example process  600  of redundant fluid actuation. In some implementations, the process  600  can be performed by the example redundant fluid actuator system  200  of  FIG.  2   . 
     At  610 , a first fluid valve is controlled to control a first fluid flow to a first fluid actuator. For example, the primary valve  260  can control flow of pressurized fluid from the pressurized fluid source  201  to the fluid chambers  224  and  226  of the primary fluid actuator  220 . 
     At  620 , the first fluid actuator actuates an output. For example, the primary fluid actuator  220  can receive fluid to urge movement of the piston  222  in a first direction (e.g., extend) based on fluid pressure in a fluid chamber  224 , and retract based on fluid pressure in a fluid chamber  226 . 
     At  630 , a second fluid valve is controlled to control a second fluid flow to a second fluid actuator. In some implementations, the process  600  can include receiving, by a third fluid chamber of the second fluid actuator, the second fluid flow, urging, by the second fluid flow received by the third fluid chamber, a second piston of the second fluid actuator in a first direction, receiving, by a fourth fluid chamber of the second fluid actuator, the second fluid flow, and urging, by the second fluid flow received by the fourth fluid chamber, the second piston in a second direction opposite the first direction. For example, the secondary valve can control flow of pressurized fluid from the pressurized fluid source  201  to the fluid chambers  244  and  246  of the secondary fluid actuator  240 . 
     At  640 , the second fluid actuator actuates the output. For example, the secondary fluid actuator  240  can receive fluid to urge movement of the piston  242  in a first direction (e.g., extend) based on fluid pressure in a fluid chamber  244 , and retract based on fluid pressure in a fluid chamber  246 . 
     In some implementations, the process  600  can include receiving, by a first fluid chamber of the first fluid actuator, the first fluid flow, urging, by the first fluid flow received by the first fluid chamber, a first piston of the first fluid actuator in a first direction, receiving, by a second fluid chamber of the first fluid actuator, the first fluid flow, and urging, by the first fluid flow received by the second fluid chamber, the first piston in a second direction opposite the first direction. For example, the primary fluid actuator  220  can receive fluid to urge movement of the piston  222  in a first direction (e.g., extend) based on fluid pressure in a fluid chamber  224 , and retract based on fluid pressure in a fluid chamber  226 . 
     In some implementations, the process  600  can include configuring a first valve body of the first fluid valve to a first configuration, permitting, by the first valve body in the first configuration, the first fluid flow to the first fluid chamber, configuring the first valve body to a second configuration, and permitting, by the first valve body in the second configuration, the first fluid flow to the second fluid chamber. For example, the primary valve  260  can be configured to direct fluid flow from the fluid source  201  to the fluid chamber  224  in a first valve configuration (e.g., to extend the piston  222  and the mechanical output  212 ), to direct fluid flow from the fluid source  201  to the fluid chamber  226  in a second valve configuration (e.g., to retract the piston  222  and the mechanical output  212 ), and to block fluid flow among the fluid source  201 , the fluid chamber  224 , and the fluid chamber  226  in a third configuration (e.g., null, hydraulically blocking the primary fluid actuator  2   2   0 ). 
     In some implementations, the process  600  can include configuring a second valve body of the second fluid valve to a first configuration, permitting, by the second valve body in the first configuration, the second fluid flow to the third fluid chamber, configuring the second valve body to a second configuration, and permitting, by the second valve body in the second configuration, the second fluid flow to the fourth fluid chamber. For example, the secondary valve  280  can be configured to direct fluid flow from the fluid source  201  to the fluid chamber  244  in a first valve configuration (e.g., to extend the piston  242  and the mechanical output  212 ), and to direct fluid flow from the fluid source  201  to the fluid chamber  246  in a second valve configuration (e.g., to retract the piston  242  and the mechanical output  212 ). 
     In some implementations, the process  600  can include fluidically connecting a fluid inlet of the second fluid valve to a first fluid outlet of the second fluid valve in a first configuration, fluidically connecting the fluid inlet to a second fluid outlet of the second fluid valve in a second configuration, and fluidically connecting the fluid inlet to the first fluid outlet and the second fluid outlet in a third configuration. For example, the example secondary fluid valve  351  can be configured to the configurations shown in  FIGS.  3 A- 3 C . 
     Although a few implementations have been described in detail above, other modifications are possible. For example, the logic flows depicted in the figures do not require the particular order shown, or sequential order, to achieve desirable results. In addition, other steps may be provided, or steps may be eliminated, from the described flows, and other components may be added to, or removed from, the described systems. Accordingly, other implementations are within the scope of the following claims.