Patent Description:
This instant specification relates to servo valve based control of hydraulic actuators.

Hydraulic actuators are used to actuate mechanical outputs such as valves and articulated motion control outputs. In order to achieve various safety, reliability, and performance requirements, various forms of redundancy are utilized.

Some existing systems provide redundancy by including doubled coils on servo valves that control the flow of fluid to hydraulic actuators through shared hydraulic paths. Some other existing systems provide redundant pressure control. <CIT> describes a method which controls a hydraulic actuator drive that has an actuator and the position of the actuator is set by at least one electrohydraulic proportional valve that has at least one electrical input and at least one hydraulic output. First, the position of the actuator is detected and an actual position value is generated therefrom. Second, a target position value is specified in dependence on a default for a desired position of the actuator and a target/actual value difference is corrected by the at least one electrohydraulic proportional valve, so that the actuator assumes a position corresponding to the target position value. By the method the target position value is also varied if the default remains unchanged for a desired position of the actuator.

In general, this document describes systems and techniques for servo valve based control of hydraulic actuators.

An electrohydraulic positioning control system and a method for controlling an electrohydraulic positioning control system according to the present invention are set out in the independent claims. Further advantageous developments of the present invention are set out in the dependent claims.

In a general aspect, a method of operating a hydraulic actuator system, not according to the claimed invention, includes actuating a closure member of a valve assembly at a predetermined first velocity a predetermined first number of cycles between a first configuration in which a fluid flow path is flushed for a predetermined first drain period and a second configuration in which fluid flow is flushed for a predetermined first flushing period, actuating the closure member at a predetermined second velocity a predetermined second number of cycles between the first configuration for a predetermined second drain period and the second configuration for a predetermined second flushing period, actuating the closure member at a predetermined third velocity a predetermined third number of cycles between the first configuration for a predetermined third drain period and the second configuration for a predetermined third flushing period, actuating the closure member at a predetermined fourth velocity a predetermined fourth number of cycles between the first configuration for a predetermined fourth drain period and the second configuration for a predetermined fourth flushing period, and actuating the closure member at a predetermined fifth velocity a predetermined fifth number of cycles between the first configuration for a predetermined fifth drain period and the second configuration for a predetermined fifth flushing period.

Various implementations, not according to the claimed invention, can include some, all, or none of the following features. The closure member can be configured to flush air residuals trapped in the valve assembly with hydraulic fluid provided to the valve assembly while in the second configuration. The second drain period can be longer than the first drain period and the third drain period, the fourth drain period can be longer than the second drain period, and the fifth flushing period is longer than fourth flushing period. The second flushing period can be longer than the first flushing period and the third flushing period, and the fourth flushing period can be longer than the second flushing period. The fifth velocity can be less than the first velocity, the second velocity, the third velocity, and the fourth velocity. One or more of the first number of cycles, the second number of cycles, the third number of cycles, the fourth number of cycles, the first drain period, the second drain period, the third drain period, the fourth drain period, the fifth drain period, the first flushing period, the second flushing period, the third flushing period, the fourth flushing period, and the fifth flushing period can be based on a pressure of hydraulic fluid provided to the valve assembly. The first drain period can be less than <NUM> seconds, the second drain period can be less than <NUM> seconds, the third drain period can be less than <NUM> seconds, the fourth drain period can be less than <NUM> seconds, the fifth drain period is less than <NUM> seconds, the first flushing period can be less than <NUM> second, the second flushing period can be less than <NUM> seconds, the third flushing period can be less than <NUM> second, the fourth flushing period can be less than <NUM> seconds, and the fifth flushing period can be between <NUM> seconds and <NUM> seconds. The method can also include providing a hydraulic fluid at a pressure less than or equal to <NUM> MPa (<NUM> psig), wherein the first number of cycles can be between <NUM> and <NUM>, the second number of cycles can be between <NUM> and <NUM>, the third number of cycles can be between <NUM> and <NUM>, the fourth number of cycles can be between <NUM> and <NUM>, and the fifth number of cycles is between <NUM> and <NUM>. The method can also include providing a hydraulic fluid at a pressure greater than <NUM> MPa (<NUM> psig), wherein the first number of cycles can be between <NUM> and <NUM>, the second number of cycles can be between <NUM> and <NUM>, the third number of cycles can be between <NUM> and <NUM>, the fourth number of cycles can be between <NUM> and <NUM>, and the fifth number of cycles is between <NUM> and <NUM>. The first velocity can be between <NUM>%/sec and <NUM>%/sec of a travel of the closure member, the second velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, the third velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, the fourth velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, and the fifth velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel. The valve assembly can include a fluid supply port, a fluid drain port, and a fluid control port, and the closure member is configurable into a plurality of valve configurations including the first configuration in which the fluid control port is in fluid communication with the fluid drain port, and the fluid supply port is blocked, the second configuration in which the fluid control port is in fluid communication with the fluid supply port and is in fluid communication with the fluid drain port through a fluid restrictor, and the fluid flow comprises flow from the fluid control port to the fluid drain port through the fluid restrictor, a third configuration in which fluid communication between the fluid control port, the fluid supply port, and the fluid drain port is blocked, and a fourth configuration in which the fluid control port is in fluid communication with the fluid supply port, and the fluid drain port is blocked.

In another general aspect, a hydraulic actuator system, not according to the claimed invention, includes a valve assembly having a fluid supply port in fluid communication with the main fluid supply conduit, a fluid drain port, and a fluid control port in fluid communication with the main fluid control conduit, and a controller configured to control operation of the valve assembly, the operations including actuating a closure member of the valve assembly at a predetermined first velocity a predetermined first number of cycles between a first configuration in which fluid flow is drained for a predetermined first drain period and a second configuration in which a fluid flow path is flushed for a predetermined first flushing period, actuating the closure member at a predetermined second velocity a predetermined second number of cycles between the first configuration for a predetermined second drain period and the second configuration for a predetermined second flushing period, actuating the closure member at a predetermined third velocity a predetermined third number of cycles between the first configuration for a predetermined third drain period and the second configuration for a predetermined third flushing period, actuating the closure member at a predetermined fourth velocity a predetermined fourth number of cycles between the first configuration for a predetermined fourth drain period and the second configuration for a predetermined fourth flushing period, and actuating the closure member to at a predetermined fifth velocity a predetermined fifth number of cycles between the first configuration for a predetermined fifth drain period and the second configuration for a predetermined fifth flushing period.

Various embodiments, not according to the claimed invention, can include some, all, or none of the following features. Actuation of the closure member can mix and flush air residuals trapped in the valve assembly with hydraulic fluid provided to the valve assembly. The second drain period can be longer than the first drain period and the third drain period, and the fourth drain period can be longer than the second drain period. The second flushing period can be longer than the first flushing period and the third flushing period, the fourth flushing period can be longer than the second flushing period, and the fifth flushing period is longer than the fourth flushing period. The fifth velocity can be less than the first velocity, the second velocity, the third velocity, and the fourth velocity. One or more of the first number of cycles, the second number of cycles, the third number of cycles, the fourth number of cycles, the first drain period, the second drain period, the third drain period, the fourth drain period, the fifth drain period, the first flushing period, the second flushing period, the third flushing period, the fourth flushing period, and the fifth flushing period can be based on a pressure of hydraulic fluid provided to the valve assembly. The first drain period can be less than <NUM> seconds, the second drain period can be less than <NUM> seconds, the third drain period can be less than <NUM> seconds, the fourth drain period can be less than <NUM> seconds, the fifth drain period can be less than <NUM> seconds, the first flushing period can be less than <NUM> second, the second flushing period can be less than <NUM> seconds, the third flushing period can be less than <NUM> second, the fourth flushing period can be less than <NUM> seconds, and the fifth flushing period can be between <NUM> seconds and <NUM> seconds. The operations can also include providing a hydraulic fluid at a pressure less than or equal to <NUM> MPa (<NUM> psig), wherein the first number of cycles can be between <NUM> and <NUM>, the second number of cycles can be between <NUM> and <NUM>, the third number of cycles can be between <NUM> and <NUM>, the fourth number of cycles can be between <NUM> and <NUM>, and the fifth number of cycles is between <NUM> and <NUM>. The operations can also include providing a hydraulic fluid at a pressure greater than <NUM> MPa (<NUM> psig), wherein the first number of cycles can be between <NUM> and <NUM>, the second number of cycles can be between <NUM> and <NUM>, the third number of cycles can be between <NUM> and <NUM>, the fourth number of cycles can be between <NUM> and <NUM>, and the fifth number of cycles is between <NUM> and <NUM>. The first velocity can be between <NUM>%/sec and <NUM>%/sec of a travel of the closure member, the second velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, the third velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, the fourth velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel, and the fifth velocity can be between <NUM>%/sec and <NUM>%/sec of the closure member's travel. The valve assembly can include a fluid supply port, a fluid drain port, and a fluid control port, and the closure member is configurable into a plurality of valve configurations including the first configuration in which the fluid control port is in fluid communication with the fluid drain port, and the fluid supply port is blocked, the second configuration in which the fluid control port is in fluid communication with the fluid supply port and is in fluid communication with the fluid drain port through a fluid restrictor, and the fluid flow comprises flow from the fluid control port to the fluid drain port through the fluid restrictor, a third configuration in which fluid communication between the fluid control port, the fluid supply port, and the fluid drain port is blocked, and a fourth configuration in which the fluid control port is in fluid communication with the fluid supply port, and the fluid drain port is blocked.

According to claim <NUM>, an electrohydraulic positioning control system includes a shuttle valve configured to direct fluid flow between a selectable one of a first fluid port and a second fluid port, and a fluid outlet configured to be fluidically connected to a fluid actuator, a first servo valve controllable to selectably permit flow between the first fluid port and a fluid source, permit flow between the first fluid port and a fluid drain, and block fluid flow between the first fluid port, the fluid source, and the fluid drain, a second servo valve controllable to selectably permit flow between the second fluid port and the fluid source, permit flow between the second fluid port and the fluid drain, and block fluid flow between the second fluid port, the fluid source, and the fluid drain, a first servo controller configured to provide a first health signal and control the first servo valve based on a position demand signal, a position feedback signal, a first priority signal, and a second health signal, and a second servo controller configured to provide the second health signal and control the second servo valve based on the position demand signal, the position feedback signal, a second priority signal, and the first health signal, wherein either the first priority signal or the second priority signal comprises a representation of an operational condition comprising a high priority command provided to a selected one of the first servo controller or the second servo controller to act as a primary servo controller.

Various embodiments can include some, all, or none of the following features. The other of the first priority signal or the second priority signal comprising the high priority command, can comprise a representation of an operational condition comprising a low priority command provided to the other of the first servo controller or the second servo controller to act as a reserve servo controller. The first servo controller can be configured to perform operations that include receiving, by the first servo controller, the high priority command as the first priority signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. At least one of the first health signal and the second health signal can be configurable to comprise representations of one or more operational conditions including (a) an operable condition indicating an absence of failure, (b) a fail condition indicative of a failure that is addressable a shutdown of a corresponding one of the first servo valve or the second servo valve, and (c) a failure of the health signal that represents an inability to transmit any of above conditions. The first servo controller can be configured to perform operations that include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the fail condition in the second servo controller or the second servo valve, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. The first servo controller can be configured to perform operations including receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the operable condition in the second servo controller and the second servo valve, and controlling, by the first servo controller, the first servo valve to provide a fluidic connection from the first fluid port to the fluid drain and to block the fluid source. The first servo controller can be configured to perform operations including receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, failure of the second health signal, determining, by the first servo controller and based on the detecting, a modified position demand that is less than a position demand represented by the position demand signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator based on the modified position demand by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. The first servo controller can be configured to perform operations including receiving, by the first servo controller, the low priority command as the first priority signal, controlling, by the first servo controller and based on the receiving, the first servo valve to a standby position based on a standby demand, detecting, by the first servo controller and based on the second health signal, the operable condition in the second servo controller and the second servo valve, receiving, by the first servo controller, a command signal representative of a silt reduction operation, controlling, by the first servo controller and in response to the received first priority signal, the first servo valve to a first modified position that is below standby position based on the standby demand, and controlling, by the first servo controller and in response to the received first priority signal, the first servo valve to the standby position based on a standby demand.

According to claim <NUM>, a method for controlling an electrohydraulic positioning control system includes controlling, by a first servo controller configured to provide a first health signal, a first servo valve to selectably permit flow between a first fluid port and a fluid source, permit flow between the first fluid port and a fluid drain, and block fluid flow between the first fluid port, the fluid source, and the fluid drain, wherein the controlling is based on a position demand signal, a position feedback signal, a first priority signal, and a second health signal, providing, by the first servo controller, the first health signal, controlling, by a second servo controller, a second servo valve to selectably permit flow between a second fluid port and the fluid source, permit flow between the second fluid port and the fluid drain, and block fluid flow between the second fluid port, the fluid source, and the fluid drain, wherein the controlling is based on the position demand signal, the position feedback signal, a second priority signal, and the first health signal, providing, by the second servo controller, the second health signal, wherein either the first priority signal or the second priority signal comprises a representation of an operational condition comprising a high priority command provided to a selected one of the first servo controller or the second servo controller to act as a primary servo controller, and directing, by a shuttle valve, fluid flow between a selectable one of the first fluid port and the second fluid port, and a fluid outlet configured to be fluidically connected to a fluid actuator.

Various implementations can include some, all, or none of the following features. The other of the first priority signal and the second priority signal comprising the high priority command, can comprise a representation of an operational condition comprising a low priority command provided to the other of the first servo controller or the second servo controller to act as a reserve servo controller. The method can also include receiving, by the first servo controller, the high priority command as the first priority signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. At least one of the first health signal and the second health signal can be configurable to include representations of one or more operational conditions that include (a) an operable condition indicating an absence of failure, (b) a fail condition indicative of a failure that is addressable a shutdown of a corresponding one of the first servo valve or the second servo valve, and (c) a failure of the health signal that represents an inability to transmit any of above conditions. The method can include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the fail condition in the second servo controller or the second servo valve, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. The method can also include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the operable condition in the second servo controller and the second servo valve, and controlling, by the first servo controller, the first servo valve to provide a fluidic connection from the first fluid port to the fluid drain and to block the fluid source. The method can also include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, failure of the second health signal, determining, by the first servo controller and based on the detecting, a modified position demand that is less than a position demand represented by the position demand signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator based on the modified position demand by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. The method can also include receiving, by the first servo controller, the low priority command as the first priority signal, controlling, by the first servo controller and based on the receiving, the first servo valve to a standby position based on a standby demand, detecting, by the first servo controller and based on the second health signal, the operable condition in the second servo controller and the second servo valve, receiving, by the first servo controller, a command signal representative of a silt reduction operation, controlling, by the first servo controller and in response to the received command signal, the first servo valve to a first modified position that is below standby position, and controlling, by the first servo controller and in response to the received command signal, the first servo valve to the standby position.

The systems and techniques described here may provide one or more of the following advantages. First, the system can provide redundant control of a controlled process. Second, the system can improve system uptime. Third, the system can detect internal faults independently of a supervising controller. Fourth, the system can engage its redundant features independently of a supervising controller. Fifth, the system can bleed residual air without interrupting active control operations. Sixth, the system clear itself of contaminant buildup without interrupting active control operations.

This document describes systems and techniques for redundant hydraulic servo control. In general, system uptime and reliability are highly important factors in some processes that are controlled by hydraulic servo control systems. For example, some operations require a system to operate for <NUM>-<NUM> years without shutdown. In order to mitigate operational risks of critical components, the hydraulic control systems described in this document include features that provide redundancy (e.g., a primary hydraulic servo valve and controller, a backup hydraulic servo valve and controller that is kept online, and an automatic failover process for transferring control between the primary and the backup units) and online serviceability (e.g., one servo valve can be replaced and purged while the other maintains control) that can reduce or eliminate operational downtime.

<FIG> is a perspective view of an example hydraulic control system <NUM>. The system <NUM> includes an electro hydraulic servo valve (EHSV) module 120a and an EHSV module 120b connected into a single manifold <NUM>. An electrical junction box <NUM> houses power and control components for the system <NUM>. Each of the EHSV modules 120a-120b includes a controller and electromechanical components that can control the flow of hydraulic fluid to the manifold <NUM>. The manifold <NUM> includes isolation valves, needle valves, and a shuttle valve subassembly. A pressure gauge 115a is configured to show an output pressure of the EHSV module 120a, and a pressure gauge 115b is configured to show an output pressure of the EHSV module 120b. An isolation valve 140a provides an operator with a capability to fluidically isolate the EHSV module 120a from the rest of the system <NUM>, and an isolation valve 140b provides an operator with a capability to fluidically isolate the EHSV module 120b from the rest of the system <NUM> (e.g., to permit service or replacement of one EHSV module while the other remains in service).

In the illustrated example, the system <NUM> provides two substantially identical, redundant hydraulic-position controllers (servos), two substantially independent sensors, and substantially independent flow paths. In use, the system generally uses the EHSV module 120a as a primary valve controller, and keeps the EHSV module 120b in reserve as a redundant backup (although in some implementations, the valve roles may be reversed).

<FIG> is a schematic diagram of an example hydraulic control system <NUM>. In some embodiments, the system <NUM> can be the example system <NUM> of <FIG>. The system <NUM> includes a fluid control system <NUM> that is configured to control a flow of fluid (e.g., hydraulic fluid) from a fluid reservoir <NUM> or other fluid pressure source to a fluid actuator <NUM> (e.g., a hydraulic cylinder, a hydraulic actuator). The fluid reservoir <NUM> provides fluid to a main fluid supply conduit <NUM>. A main fluid control conduit <NUM> (e.g., a fluid outlet) is configured to provide fluid communication with a pressure chamber of the fluid actuator <NUM>. A position sensor <NUM> is configured to provide signals representative of the position or configuration of the fluid actuator <NUM>.

The fluid control system <NUM> includes an electro hydraulic servo valve (EHSV) 220a and an EHSV 220b. The configuration of the EHSVs 220a and 220b will be discussed in more detail in the description of <FIG>.

The EHSV 220a includes a fluid supply port 222a in fluid communication with the main fluid supply conduit <NUM>, a fluid drain port 224a in fluid communication with a drain 228a, and a fluid control port 226a in fluid communication with the main fluid control conduit <NUM>. The EHSV 220a is configured to actuate a closure member 229a to selectably provide several configurations that provide and/or block various fluid interconnections between the main fluid control conduit <NUM>, the main fluid supply conduit <NUM>, and the drain 228a.

The EHSV 220a also includes a valve controller 234a and a valve position sensor 232a configured to sense the configuration of the closure member 229a. The valve controller 234a is configured to control the operation of the EHSV 220a based on commands from a controller <NUM> (e.g., priority signals that identify which of the EHSVs is to act as the primary controller and which is to act as the secondary controller), position feedback from the valve position sensor 232a, position feedback from the position sensor <NUM>, and a health signal from the EHSV 220b. The health signal is communicated over a communication bus <NUM>.

The EHSV 220b includes a fluid supply port 222b in fluid communication with the main fluid supply conduit <NUM>, a fluid drain port 224b in fluid communication with a drain 228b, and a fluid control port 226b in fluid communication with the main fluid control conduit <NUM>. The EHSV 220b is configured to actuate a closure member 229b to selectably provide several configurations that provide and/or block various fluid interconnections between the main fluid control conduit <NUM>, the main fluid supply conduit <NUM>, and the drain 228b. In some embodiments, the drain 228a and the drain 228b may be fluidly interconnected (e.g., to provide a fluid return to the fluid reservoir <NUM>).

The EHSV 220b also includes a valve controller 234b and a valve position sensor 232b. The valve controller 234b is configured to control the operation of the EHSV 220b based on commands from the controller <NUM> (e.g., priority signals that identify which of the EHSVs is to act as the primary controller and which is to act as the secondary controller), position feedback from the valve position sensor 232b, position feedback from the position sensor <NUM>, and a health signal from the EHSV 220a. The health signal is communicated over the communication bus <NUM>.

The EHSVs 220a and 220b are in communication with, or otherwise controlled by, the controller <NUM>. The controller <NUM> is configured to provide control signals to the EHSVs 220a and 220b to EHSV command the position demand of the fluid actuator <NUM> and provide priority signals to them. The controller <NUM> is also configured to receive feedback signals from the EHSVs 220a and 220b to determine the actual conditions of the EHSVs 220a and 220b and the actual position of the fluid actuator <NUM>.

The fluid control port 226a and the fluid control port 226b are in fluid communication with the main fluid control conduit <NUM> through a shuttle valve <NUM>. The shuttle valve <NUM> is configured to selectively provide fluid communication between the main fluid control conduit <NUM> and a selected one of the fluid control port 226a and the fluid control port 226b, while blocking fluid communication to the other one of the fluid control port 226a and the fluid control port 226b. The shuttle valve <NUM> is configured to select the interconnection based on which of the fluid control port 226a and the fluid control port 226b is providing the relatively higher fluid pressure.

Under normal operations, the EHSV 220a is controlled in order to control actuation of the fluid actuator <NUM>, while the EHSV 220b is held in standby. The controller <NUM> is configured to detect a state of operation of both the EHSV 220a and the EHSV 220b. In the event of a failure of the primary EHSV or a failure of communications with the primary EHSV, the secondary EHSV is used in order to provide substantially uninterrupted control of the fluid actuator <NUM>.

In some embodiments, a secondary EHSV can be held at below null position, so as to not interfere with shuttle valve position. For example, the fluid control port 226b may be disconnected to the fluid supply port 222b, but the closure member 229b can be positioned close to a fluid communication position in case fast action is needed to allow the EHSV 220b take over control from EHSV 220a.

In some embodiments, the system <NUM> can provide demand offset when one of the EHSV's status is unknown (e.g., designated secondary EHSV but in control, healthy link failed). Offset demand on a designated secondary EHSV that is in operation (e.g., due to the other EHSV's condition being unknown) can reduce or avoid hydraulic pressure equalization on the inputs to the shuttle valve <NUM> and thus increase stable positioning and stable flow on the main fluid control conduit <NUM>.

In some embodiments, the system <NUM> can be configured to perform air bleeding procedures (e.g., to facilitate online replacement of one of the EHSVs 220a-220b). For example, one or both of the EHSVs 220a-220b can be actuated in a manner that permits or promotes a release of air from within a closed cavity (e.g., air trapped inside a new installed, dry EHSV), while not interfering with normal cylinder operation. Examples of air bleeding procedures will be discussed in more detail in the descriptions of <FIG>.

<FIG> are schematic diagrams of an example EHSV <NUM> in various operational configurations. In some embodiments, the EHSV <NUM> can be the example EHSV module 120a of <FIG>, the example EHSV module 120b, the example EHSV 220a of <FIG>, and/or the example EHSV 220b. The EHSV <NUM> includes a fluid supply port <NUM> configured to be in fluid communication with a supply conduit (e.g., the main fluid supply conduit <NUM>), a fluid drain port <NUM> configured to be in fluid communication with a drain, and a fluid control port <NUM> configured to be in fluid communication with a main fluid control conduit (e.g., the main fluid control conduit <NUM>) through the shuttle valve <NUM>. The EHSV <NUM> is configured to selectably provide several configurations that provide and/or block various fluid interconnections between the main fluid control conduit, the main fluid supply conduit, and the drain.

The EHSV <NUM> includes a housing <NUM> and a closure member <NUM>. The closure member <NUM> is positioned relative to the housing <NUM> by an actuator <NUM>. The actuator <NUM> is configured to be controlled by a controller, such as the example valve controller 234a or the example valve controller 234b of <FIG>. The EHSV <NUM> also includes a sensor <NUM> that is configured to provide a signal that represents the position of the closure member <NUM> relative to the housing <NUM>, or the configuration of the EHSV <NUM>. The sensor <NUM> is configured to provide the sensor signal as feedback to a controller, such as the example valve controller 234a or the example valve controller 234b.

The EHSV <NUM> is configured to provide four fluid interconnection configurations. In a configuration 390a, shown in <FIG>, the fluid control port <NUM> is fluidically connected to the fluid drain port <NUM> while the fluid supply port <NUM> is fluidically blocked. In a configuration 390b, shown in <FIG>, the fluid control port <NUM>, the fluid drain port <NUM>, and the fluid supply port <NUM> are all fluidically blocked (e.g., a null position). In a configuration 390c, shown in <FIG>, the fluid control port <NUM> is fluidically connected to the fluid supply port <NUM> while the fluid drain port <NUM> is fluidically blocked.

In a configuration 390d, shown in <FIG>, the fluid control port <NUM> is fluidically connected to both the fluid drain port <NUM> and the fluid supply port <NUM>. In the configuration 390d, a fluid connection <NUM> between the fluid control port <NUM> and the fluid drain port <NUM> is relatively (e.g., substantially) more restrictive to fluid flow than a fluid connection <NUM> between the fluid control port <NUM> and the fluid supply port <NUM>.

In various circumstances, air may become present in the fluid lines that pass through the EHSV <NUM>. For example, air may enter the fluid circuit during maintenance, or during rapid actuation of the example actuator <NUM> (e.g., air may leak past hydraulic seals that define the pressure chamber of the actuator). Such air is generally unwanted, as it can degrade the performance of the actuator being controlled (e.g., sponginess or springiness due to the relative compressibility of gaseous fluids compared to liquids).

In use, the EHSV <NUM> can be configured to the configuration 390d in order to purge (e.g., bleed) air from the fluid pathways inside and/or downstream from the EHSV <NUM>. In previous designs, trapped air would be purged from the fluid circuit manually. Such previous processes would typically require operational downtime and/or manual access to the fluid lines (e.g., ground maintenance). In the illustrated example, air trapped in the fluid is able to exit to the fluid drain port <NUM> through the fluid connection <NUM> more easily than can the surrounding fluid, thus allowing the air to be purged from the fluid circuit as a mechanical or automated function of the EHSV <NUM> instead of requiring manual access to the fluid circuit. In some implementations, the EHSV that is to be air-bled can be shifted out of process control, and such operations can be performed by its redundant companion EHSV while the EHSV in need of bleeding can be cleared of air.

The EHSV <NUM> also includes a bias member <NUM> configured to urge the closure member <NUM> into a predetermined (e.g., failsafe) configuration. In the illustrated example, the failsafe configuration is the configuration 390a, but in other embodiments the failsafe configuration can be any one of the configurations 390a-390d. In some embodiments, the bias member <NUM> can be configured to urge the closure member <NUM> away from a predetermined one of the configurations 390a-390d (e.g., to prevent accidental use of the configuration 390d).

<FIG> shows an example schematic view of an example hydraulic control system <NUM>. In some embodiments, the system <NUM> can be part of the system <NUM> of <FIG> or the system <NUM> of <FIG>.

The system <NUM> includes an EHSV module 401a and an EHSV module 401b. The EHSV module 401a includes a valve controller 434a and an EHSV 420a, and the EHSV module 401b includes a valve controller 434b and an EHSV 420b. In general, the EHSV modules 401a and 401b are configured to be redundant, substantially self-contained, replaceable modules within the system <NUM>.

The valve controller 434a includes a control current output 410a that actuates the EHSV 420a, and a position feedback input 412a that is configured to receive position feedback sensor signals from the EHSV 420a (e.g., from a variable displacement transformer linked to a moveable closure member of the valve). The valve controller 434a also includes a position feedback input 413a and a position feedback input 414a that are configured to receive position feedback sensor signals from the fluid actuator <NUM> (e.g., from a variable displacement transformer or other appropriate position sensor linked to a moveable component or to an output of the actuator).

In some embodiments, the fluid actuator <NUM> can be configured with redundant position sensors, and the position feedback input 413a and a position feedback input 414a can be configured to read the redundant signals provided by the redundant sensors. The valve controller 434a also includes an input/output module 406a that is configured to receive commands and demands from the controller <NUM>, and send and/or receive feedback and/or status signals to/from the controller <NUM>.

The valve controller 434b includes a control current output <NUM>0b, a position feedback input 412b, a position feedback input 413b, a position feedback input 414b, and an input/output module 406b that perform functions that are substantially similar to their counterparts in the valve controller 434a. In some embodiments, the position feedback inputs 413a and 413b can be configured to receive the same position feedback signal, and the position feedback inputs 414a and 414b can be configured to receive the same redundant position feedback signal.

The valve controller 434a includes a health status transmitter 416a and a health status receiver 418a, and valve controller 434b includes a health status transmitter 416b and a health status receiver 418b. The health status transmitter 416a is configured to transmit a health status signal 437a over a communication bus <NUM>, and the health status receiver 418b is configured to receive the health status signal 437a. The health status transmitter 416b is configured to transmit a health status signal 437b over the communication bus <NUM>, and the health status receiver 418a is configured to receive the health status signal 437b. Such a configuration allows the valve controllers 434a and 434b to monitor each other's status.

The valve controller 434a is configured to provide closed-loop control of the EHSV 420a and, by extension, the fluid actuator <NUM>, by providing control current at the control current output 410a based on a demand signal (e.g., received from the controller <NUM> at the I/O module 406a), position feedback signals received at the position feedback inputs 412a, 413a, and 414a, the health status of the EHSV module 401a, and the health status signal 437b. The valve controller 434b is configured to provide closed-loop control of the EHSV 420b and, by extension, the fluid actuator <NUM>, by providing control current at the control current output 410b based on the demand signal (e.g., received from the controller <NUM> at the I/O module 406a), position feedback signals received at the position feedback inputs 412b, 413b, and 414b, the health status of the EHSV module 401b, and the health status signal 437a.

The EHSV modules 401a and 401b are configured to receive commands (e.g., demand signals) from the controller <NUM> to control fluid flow from a fluid supply <NUM> to a fluid actuator <NUM> (e.g., a hydraulic actuator or cylinder) through a shuttle valve <NUM>. The shuttle valve <NUM> is configured to fluidically connect whichever of the EHSV 420a or the EHSV 420b is providing the highest output pressure. In use, one of the EHSVs 420a or 420b is operated as a primary EHSV providing operational flow and pressure, while the other EHSV is operated as a secondary (e.g., backup) unit. In some implementations, the secondary EHSV may be operated in parallel with the primary EHSV, but at a slightly lower position demand (e.g., enough to prevent switchover of the shuttle valve away from the primary EHSV). In the event of a sudden failure of the primary EHSV, the fluid pressure from the primary EHSV may drop abruptly. By keeping the secondary EHSV online but controlling slightly low (e.g. based on a modification of the demand signal), the shuttle valve <NUM> can switch over based on the still-present secondary pressure with little interference with the operation of the actuator, allowing the secondary EHSV to take control immediately and then identify its new status as the controlling EHSV. Once the secondary EHSV recognizes its new status (e.g., based on a response to a received health signal and/or a signal from the controller <NUM>), it can remove the modification to its own demand so it controls cylinder position to follow the demanded position without the slight reduction caused by the modification.

In some implementations, a "healthy" signal can be a signal that is transmitted when the transmitter identifies itself as operating normally (e.g., an operable condition absent of failure, without identified malfunction). Since in some implementations, notification by a valve controller (and subsequent detection by a companion valve controller) can be of highest priority, of which a change in that status needs to be communicated quickly. The healthy signal can be transmitted with the relatively fastest frequency that can be correctly recognized by a receiver, and any further modification detected on the receiver side can be detected as a failure of sender.

In some implementations, a "slow fail" signal can be a signal that is transmitted when the transmitter identifies itself as experiencing or predicting a malfunction, failure, or other condition that is addressable by a slow, controlled shutdown of a corresponding one of the EHSVs. In some implementations, a "fast fail" signal can be a signal that is transmitted when the transmitter identifies itself as experiencing or predicting a malfunction, failure, or other condition that is addressable by a rapid shutdown of the corresponding one of the EHSVs. In some implementations, the signals can be the health signals received by the example EHSV modules 120a and 120b, by the example valve controllers 234a and 234b, or by the example valve controllers 434a and 434b, from their corresponding redundancy devices. In general, health signals can be received and interpreted by the receiver to determine several different states of health of the sending device and/or the communication bus used to communicate the signal.

In some implementations, the operation of the example fluid control system <NUM> can be based, at least in part, on health signals. For example, the system <NUM> can operate in a normal operation mode based on identification of a healthy signal). In an example of normal operations, a selected valve controller takes control over the position of the fluid actuator <NUM> by modulating passages from main fluid supply conduit <NUM> to the fluid actuator <NUM> and from fluid actuator <NUM> to the drain ports 224a and 224b.

The unit that is not performing control operations while being in standby provides a continuously opened passage to drain at limited opening so its side of the shuttle valve <NUM> can have a low pressure equal to drain pressure. Servo positioning keeps the corresponding closure member 229a or 229b close to the null position (e.g., configuration 390b) in case fast action is needed to take over control. The unit that is not performing control operations opens to full drain in case the demanded position of the fluid actuator <NUM> is close to zero. In some implementations, this is to make the full flow drain from its side of shuttle valve <NUM> and allow the controlling EHSV to realize positioning of the fluid actuator <NUM> without interference (e.g., mostly during fast governor valve shutdown).

Both of the valve controllers 234a and 234b receive position demand from the controller <NUM>, and both valve controllers 234a and 234b are configured to receive two (e.g., redundant) position feedback signals from the fluid actuator <NUM> (e.g., both valve controllers get the same value of demand and position feedback all the time). The valve controllers 234a and 234b transmit health signals to each other over separate lines to inform each other that they are healthy (e.g., operable, not in failure), experiencing a slow fail (e.g., faulty but the failure is controlled so the shutdown of the unit is not severe), or experiencing a fast fail (e.g., faulty in critical way, shutdown of the unit needs to be performed with its maximum speed). Other states of signals are considered as line failures, however it is the receiver that identifies whether the line failure is a type of short circuit, a disconnection, or noise.

In some implementations, the system <NUM> can determine that the unit that is designated to be in control of the process has failed. Thanks to the exchange of status information, there is no need for action by an external system (e.g., the controller <NUM>) in case of failure in the controlling valve controller 234a or 234b or the controlling EHSV 220a or 220b. The failed valve controller that is in control can determine that it has a fault and is unable to continue controlling the fluid actuator <NUM>. The controlling valve controller communicates this status to the standby valve controller by altering its transmitted health signal. The designated standby valve controller takes control immediately upon identification of the changed health status. As some upset of positioning is expected, the standby unit adds boost into its servo valve position when taking control, to better fulfill a demanded position of the fluid actuator <NUM>. Once it has taken control, the designated standby valve controller communicates with the controller <NUM> to notify it that it is now operating as the primary controller for operations of the controlled process. The failed valve controller communicates with the controller <NUM> to notify it of the fault and that it is no longer in operation.

In some implementations, the system <NUM> can determine that the unit that is currently in standby has failed. In some implementations, the failed secondary valve controller can inform the other unit that it is faulty and thus unable to take over control if needed. The failed standby unit also notifies the controller <NUM> that it is faulty. The current primary valve controller that is in control is informed that the other unit is inoperable, and will keep its own control over the position of the fluid actuator <NUM> despite whatever mode is demanded from controller <NUM>. For example, even if the valve controller that is in control is commanded to transfer to standby operation, it will stay in control to maintain continuity of the controlled operation. Based on the internal exchange of health status information, there is no need for action by an external system (e.g., the controller <NUM>) in case of standby EHSV failure.

In some implementations, a valve controller can identify a communication link failure and respond. For example, the standby unit can respond by outputting an alarm signal (e.g., to the controller <NUM>) to indicate a fault of the communication bus <NUM>. When the standby unit senses that the health signal is not recognizable (e.g., short circuit, disconnection, noisy signal), it then attempts to take over control, and identifies itself as acting as the primary valve controller that is in charge of controlling the fluid actuator <NUM>. When the reason for the communication failure is unknown (e.g., cannot determine if the other valve controller has failed, or if it is only a wiring issue and the other unit is still functioning normally), the secondary valve controller can modify its demand by subtracting a small offset (e.g., about <NUM>% of fluid actuator full stroke). In some implementations, this demand modification can create a slightly lower pressure on its side of the shuttle valve <NUM>, so as to not interfere with the operation of the primary EHSV if the two units are attempting to control the fluid actuator <NUM> at the same time. Offset on the demand signal can reduce or avoid the hydraulic pressure equalization on the inputs of the shuttle valve <NUM> and can help maintain stable positioning and/or stable flow on the main fluid control conduit <NUM>.

In another example, the primary valve controller can determine a fault in health signal communications from the standby valve controller. In some implementations, the primary valve controller can respond by outputting an alarm signal (e.g., to the controller <NUM>) to indicate a fault of the communication bus <NUM>. The primary valve controller can keep operational control of the fluid actuator <NUM>. Since the reason for the fault may not be entirely known, the primary unit may assume that the other unit might not be operable, and will keep operation and control over the fluid actuator <NUM> even if the controller <NUM> commands it to transfer to secondary or backup operation. Since the reason for the communication failure is unknown the formerly primary valve controller can modify its demand. For example, the valve controller 234a or 234b can modify its demand by subtracting a small offset (e.g., about <NUM>% of fluid actuator full stroke).

In some implementations, the valve controllers 234a-234b can be commanded (e.g., by the controller <NUM>) to trade their operating roles. For example, an operator may access a control panel or other input to the controller <NUM> to command an immediate swap of the primary/secondary designations of the two units. In some implementations, if any overlap of signal is foreseen, both units may be set to act as primary units first, before setting one as secondary (e.g., it may be preferable to have both units designated as primary for a short while than to have both units designated as secondary). In such an example, both units are operable, healthy, and receive information that the other unit is healthy too. In such an example, both units can execute exactly what is given as designation from controller <NUM>. The secondary unit will transfer to primary operational mode based on a command from the controller <NUM>, and because some minimal upset to the positon of the fluid actuator <NUM> is expected during the control switch, the unit can apply additional boost on its position control of its corresponding EHSV to compensate for process upset. In response to the control transfer signal from the controller <NUM>, the former primary valve controller will switch into secondary control mode, and it can control its corresponding EHSV to a configuration having a slight drain. In some implementations, both units can indicate their current primary/secondary state through discrete communication outputs (e.g., to the controller <NUM>).

In some implementations, the valve controllers 234a-234b can perform operations that prevent or reduce build-up (e.g., dirt, silt) that may have accumulated in the EHSVs 220a-220b. Depending on the site condition and quality of the hydraulic oil, it can be desirable to perform a build-up reduction process. For example, periodically (e.g., daily, weekly, other period), the valve controllers 234a-234b can oscillate their corresponding closure members 229a-229b by a small amount (e.g., a single cycle) to allow accumulated contamination to release. In some implementations, this function may be useful where one or both of the EHSV's 220a-220b are held in one stable configuration for a long period of time. When decontamination is commanded, the primary valve controller can respond by moving its corresponding closure member in a short position step down and then by a similar step up above desired servo valve position (e.g., use of opposite, semi-symmetrical movements can reduce impact on actuator position). Since the secondary unit is continuously at drain and typically will stay at steady position for a long time, a similar operation may also be implemented. Since the secondary unit is configured to not interfere with operation by the primary unit, its output pressure needs to remain below the output pressure of the primary unit at the shuttle valve <NUM>. In some implementations, this can be taken into account by having the secondary valve controller respond to its own designation as a secondary unit, and perform the build-up reduction process by only short stepping down, and in some examples by also maintaining that position longer than a primary unit would do, and then return back to normal position. The positive pulse is not executed, to avoid upsetting the system operation.

In some implementations, parts or all of an EHSV module (e.g., the example EHSV modules 120a-120b, the example EHSVs 220a-220b, the valve controllers 234a-234b) can be replaced online (e.g., one redundant part of the system can be replaced while the other maintains operational control). Referring to <FIG>, an operator can use the isolation valves 140a-140b, the pressure gauges 115a-115b, and software tools to facilitate an online replacement of a redundant component. The mechanical design of the system <NUM> reduces the open cavity volume of the assembly and reduces space in which air can become trapped during online replacement. Parameterization of the unit can be copied from the disassembled servo or from an earlier-stored configuration file. Having the configuration file loaded to a newly installed servo, there is a reduced need to configure it manually and there is a reduced need to perform cylinder calibration on the installed servo. In some embodiments, monitoring software (e.g., a customer service tool) can be included to provide monitoring and to verify proper operation of newly installed EHSVs before they are hydraulically joined to the operational (e.g., live, running, pressurized) system by opening isolation valves.

Returning briefly again to <FIG>, the valve controllers 234a-234b are configured to be able to perform an automatic air bleeding procedure that can be performed after an online replacement. The procedure is configured to releasing the air from a closed cavity (e.g., air trapped in a newly installed, dry EHSV), while substantially not interfering with normal operation of the fluid actuator <NUM>.

Referring now to <FIG>, a cross-sectional view of an example hydraulic servo valve (EHSV) <NUM> is shown. In some embodiments, the EHSV <NUM> can be the example EHSV module 120a or 120b of <FIG>, the example EHSV 220a or 220b of <FIG>, the example EHSV <NUM> of <FIG>, or the example EHSV 420a or 420b of <FIG>. The air bleeding procedure described above utilizes additional holes <NUM> (not visible in <FIG>, see <FIG>) provided in a closure member <NUM> (e.g., valve spool) of the EHSV <NUM>. The holes <NUM> provide small oil paths for flushing out air that can be trapped or can accumulate within the EHSV <NUM>. The valve controllers 234a-234b are configured to move the closure member <NUM> with dynamic movements of different lengths to create pressure differences and flow that releases trapped air. Examples of such movements are described in more detail in the descriptions of <FIG>.

<FIG> are various views of the example closure member <NUM> of <FIG>. In some embodiments, the closure member <NUM> can be the example closure member 229a or 229b of <FIG>, or the example closure member <NUM> of <FIG>. <FIG> shows a perspective view of the closure member <NUM> and one of the holes <NUM>. A portion <NUM> of the closure member <NUM> is shown enlarged in <FIG>. <FIG> shows a side view of the closure member <NUM> and two of the holes <NUM>. A cross-sectional view of the closure member <NUM> taken through a section <NUM> is shown enlarged in <FIG>.

A collection of holes <NUM> are provided as a selectably controllable (e.g., by partly rotating the closure member <NUM> within the EHSV <NUM>) primary fluid flow path through the closure member <NUM> (e.g., between various combinations of the fluid source, drain, and/or control lines), while the holes <NUM> are configured to provide a restricted flow path (e.g., to allow air to purge to drain). In some embodiments, the holes <NUM> can provide the example fluid connection <NUM> of <FIG>, while the holes <NUM> can provide the example fluid connection <NUM>. The holes <NUM> provide limited passages that make it possible to create controlled bleeding flows from a fluid supply, through a control line, to a drain port. Such construction allows air residuals to be evacuated when a rapid flow (e.g., high volume flushing) process is not allowable. The example design incorporates three such bleeding holes to allow for the release of air trapped inside the closure member.

<FIG> are graphs of servo valve demands during an example air bleeding process. In use, a closure member such as the example closure member 229a or 229b of <FIG>, the example closure member <NUM> of <FIG>, or the example closure member <NUM> of <FIG> can be operated through one or more predetermined sequences of operations configured to purge air that is trapped within the closure member <NUM>. In some embodiments, the purging process can be predetermined for a specific application. In some embodiments, multiple purging processes can be determined for multiple specific applications.

In an example implementation in which control pressure is less than or equal to <NUM> MPa (289psig), the closure member can be operated in five phases.

Phase <NUM>: The closure member can be closed (e.g., spool position = <NUM>%, drain position, configuration 390a) for <NUM> seconds and then opened (e.g., spool position = <NUM>%, flush position, configuration 390d) for <NUM> seconds. During this phase, the closure member can be moved at a rate of <NUM>%/sec (e.g., full transition from <NUM>% to <NUM>% can take about <NUM>, where <NUM>% represent the travel between minimal and maximal position of the closure member). This movement can be repeated for <NUM> cycles. In some implementations, this process can be visualized as the graph 700a of <FIG>. In phase <NUM>, dynamic pressure changes cause residual air to mix with oil, and depending on supply pressure an oil-air foam may be created.

Phase <NUM>: The closure member can be closed (e.g., configuration 390a) for <NUM> and then opened (e.g., configuration 390d) for <NUM>. During this phase, the closure member can be moved at a rate of <NUM>%/sec. This movement can be repeated for <NUM> cycles. In some implementations, this process can be visualized as the graph 700b of <FIG>. In phase <NUM>, the air-oil mixture is stabilized, more air residuals are pushed out of the bleeding holes and internal unit leakage in the form of small bubbles in oil or in foam.

Phase <NUM>: The closure member can be closed (e.g., configuration 390a) for <NUM> and then opened (e.g., configuration 390d) for <NUM>. During this phase, the closure member can be moved at a rate of <NUM>%/sec. This movement can be repeated for <NUM> cycles. In some implementations, this process can be visualized as the graph 700c of <FIG>.

Phase <NUM>: The closure member can be closed (e.g., configuration 390a) for <NUM> and then opened (e.g., configuration 390d) for <NUM>. During this phase, the closure member can be moved at a rate of <NUM>%/sec. This movement can be repeated for <NUM> cycles. In some implementations, this process can be visualized as the graph 700d of <FIG>.

Phase <NUM>: The closure member can be closed (e.g., configuration 390d) for <NUM> and then opened (e.g., configuration 390d) for <NUM>. During this phase, the closure member can be moved at a rate of <NUM>%/sec. This movement can be performed one or more times (e.g., three, five, ten, or another other appropriate number of cycles). In some implementations, this process can be visualized as the graph 700e of <FIG>.

The five phases just described, when performed sequentially, can provide an air purging process that can be completed in about <NUM> minutes.

In another example implementation in which control pressure is greater than <NUM> MPa (289psig), the closure member can be operated in another example five phases:.

As mentioned above, these are just two examples of a large number of possible combinations having greater or fewer phases, longer or shorter open and close (e.g., flushing and drain) times, faster or slower actuation speeds, and/or greater or fewer cycles per phase.

One of the benefits of performing the on-line air bleeding is that it is possible to bleed the air from closed cavities without using openings such vent valves. For example, it can be dangerous to release pressurized oil with air residuals that is being provided to a running process.

Furthermore, the purging configuration may be selected, and the purging operation may be performed, during normal operations if necessary. For example, the configuration 390c can be a configuration that provides pressurized fluid to actuate an actuator. If it is determined (e.g., manually or automatically) that purging is needed, the valve <NUM> can be switched into the configuration 390d. The configuration 390d still provides the pressurized fluid to the actuator through the fluid connection <NUM>, but also provides the fluid connection <NUM> for trapped air to escape.

<FIG> is a flow diagram of an example air bleeding process <NUM>. In some implementations, the process <NUM> can be performed by the example hydraulic control system <NUM> of <FIG>, the example hydraulic control system <NUM> of <FIG>, or the example hydraulic control system <NUM> of <FIG>.

At <NUM>, a closure member of a valve assembly is actuated at a predetermined first velocity a predetermined first number of cycles between a first configuration (e.g., configuration 390a), in which fluid flow is permitted from control port to drain port for a predetermined first drain period (e.g., held in configuration 390a), and a second configuration (e.g., configuration 390d) in which fluid flow is permitted from supply to control port and from control port to drain for a predetermined first flushing period (e.g., held in configuration 390d). For example, the valve controller 234a can control the closure member 229a of the EHSV 220a in a pattern such as the example pattern shown in <FIG>.

In some implementations, the closure member can be configured to mix air residuals trapped in the valve assembly with hydraulic fluid provided to the valve assembly while in the second configuration. For example, the example closure member <NUM> includes the fluid connection <NUM>, which provides a fluid pathway for bleeding air from the fluid control port <NUM> to the fluid drain port <NUM>.

At <NUM>, the closure member is actuated at a predetermined second velocity a predetermined second number of cycles between the first configuration, for a predetermined second drain period, and the second configuration for a predetermined second flushing period. For example, the valve controller 234a can control the closure member 229a of the EHSV 220a in a pattern such as the example pattern shown in <FIG>.

At <NUM>, the closure member is actuated at a predetermined third velocity a predetermined third number of cycles between the first configuration, for a predetermined third drain period, and the second configuration for a predetermined third flushing period. For example, the valve controller 234a can control the closure member 229a of the EHSV 220a in a pattern such as the example pattern shown in <FIG>.

At <NUM>, the closure member is actuated at a predetermined fourth velocity a predetermined fourth number of cycles between the first configuration, for a predetermined fourth drain period, and the second configuration for a predetermined fourth flushing period. For example, the valve controller 234a can control the closure member 229a of the EHSV 220a in a pattern such as the example pattern shown in <FIG>.

At <NUM>, the closure member is actuated to the first configuration at predetermined fifth velocity for a predetermined fifth drain period, and to the second configuration at a predetermined fifth velocity for a predetermined fifth flushing period. For example, the valve controller 234a can control the closure member 229a of the EHSV 220a in a pattern such as the example pattern shown in <FIG>.

In some implementations, the second drain period can be longer than the first drain period and the third drain period, and the fourth drain period can be longer than the second drain period. For example, the drain period of the example phase <NUM> pattern illustrated by <FIG> is longer than the drain periods of phases <NUM> and <NUM> illustrated by <FIG> and <FIG>, and the drain period of the example phase <NUM> pattern illustrated by <FIG> is longer than the drain period of phase <NUM> illustrated by <FIG>.

In some implementations, the second flushing period can be longer than the first flushing period and the third flushing period, and the fourth flushing period can be longer than the second flushing period. For example, the flushing period of the example phase <NUM> pattern illustrated by <FIG> is longer than the flushing periods of phases <NUM> and <NUM> illustrated by <FIG> and <FIG>, and flushing period of the example phase <NUM> pattern illustrated by <FIG> is longer than the flushing period of phase <NUM> illustrated by <FIG>.

In some implementations, the fifth velocity can be less than the first velocity, the second velocity, the third velocity, and the fourth velocity. For example, the velocity of the closure member during the example phase <NUM> pattern illustrated by <FIG> is slower than the velocities used for phases <NUM>-<NUM>.

In some implementations, one or more of the first number of cycles, the second number of cycles, the third number of cycles, the fourth number of cycles, the first drain period, the second drain period, the third drain period, the fourth drain period, the first flushing period, the second flushing period, the third flushing period, the fourth flushing period, and the fifth flushing period can be based on a pressure of hydraulic fluid provided to the valve assembly. For example, in the descriptions of <FIG>, this document describes examples of two different configurations of five different air-bleeding phases for two different pressure ranges. Additional configurations may be used, as such configurations can be adapted for use with different application-specific pressures, flow rates, actuator fluid viscosities, nominal operating temperatures, and combinations of these and/or any other appropriate factor that can affect the amount of air that can be trapped in a system and/or the system's ability to be purged.

In some implementations, the first drain period can be less than <NUM> seconds, the second drain period can be less than <NUM> seconds, the third drain period can be less than <NUM> seconds, the fourth drain period can be less than <NUM> seconds, the fifth drain period can be less than <NUM> seconds, the first flushing period can be less than <NUM> second, the second flushing period can be less than <NUM> seconds, the third flushing period can be less than <NUM> second, the fourth flushing period can be less than <NUM> seconds, and the fifth flushing period can be between <NUM> seconds and <NUM> seconds. For example, the closure member <NUM> can be at drain for <NUM> seconds per oscillation during the example phase <NUM> illustrated by <FIG>, the closure member <NUM> can be at drain for <NUM> second per oscillation during the example phase <NUM> illustrated by <FIG>, the closure member <NUM> can be at drain for <NUM> seconds per oscillation during the example phase <NUM> illustrated by <FIG>, the closure member <NUM> can be at drain for <NUM> seconds per oscillation during the example phase <NUM> illustrated by <FIG>, and the closure member <NUM> can be at drain for <NUM> seconds during the example phase <NUM> illustrated by <FIG>.

In some implementations, the process can include providing a hydraulic fluid at a pressure less than or equal to <NUM> MPa (<NUM> psig), wherein the first number of cycles is between <NUM> and <NUM>, the second number of cycles is between <NUM> and <NUM>, the third number of cycles is between <NUM> and <NUM>, the fourth number of cycles is between <NUM> and <NUM>, and the fifth number of cycles is between <NUM> and <NUM>. For example, for a pressure of less than <NUM> MPa (289psig), the example phase <NUM> of <FIG> is described as having <NUM> cycles, the example phase <NUM> of <FIG> is described as having <NUM> cycles, the example phase <NUM> of <FIG> is described as having <NUM> cycles, and example phase <NUM> of <FIG> is described as having <NUM> cycles, and the example phase <NUM> of <FIG> is described as having one cycle (e.g., between <NUM> and <NUM> cycles).

In some implementations, the process <NUM> can include providing a hydraulic fluid at a pressure greater than <NUM> MPa (<NUM> psig), wherein the first number of cycles is between <NUM> and <NUM>, the second number of cycles is between <NUM> and <NUM>, the third number of cycles is between <NUM> and <NUM>, the fourth number of cycles is between <NUM> and <NUM>, and the fifth number of cycles can be between <NUM> and <NUM>. For example, for a pressure greater than <NUM> MPa (289psig), the example phase <NUM> of <FIG> is described as having <NUM> cycles, the example phase <NUM> of <FIG> is described as having <NUM> cycles, the example phase <NUM> of <FIG> is described as having <NUM> cycles, and the example phase <NUM> of <FIG> is described as having <NUM> cycles, and the example phase <NUM> of <FIG> is described as having one cycle (e.g., between <NUM> and <NUM> cycles).

In some implementations, the first velocity can be between <NUM>% and <NUM>% of the closure member's travel per second, the second velocity can be between <NUM>% and <NUM>% of the closure member's travel per second, the third velocity can be between <NUM>% and <NUM>% of the closure member's travel per second, the fourth velocity can be between <NUM>% and <NUM>% of the closure member's travel per second, and the fifth velocity can be between <NUM>% and <NUM>% of the closure member's travel per second. For example, the example phase <NUM> of <FIG> is described as being performed at a velocity of <NUM>%/sec, the example phase <NUM> of <FIG> is described as being performed at a velocity of <NUM>%/sec, the example phase <NUM> of <FIG> is described as being performed at a velocity of <NUM>%/sec, the example phase <NUM> of <FIG> is described as being performed at a velocity of <NUM>%/sec, and the example phase <NUM> of <FIG> is described as being performed at a velocity of <NUM>%/sec.

In some implementations, the valve assembly can include a fluid supply port, a fluid drain port, and a fluid control port, and the valve body is configurable into a collection of valve configurations including the first configuration in which the fluid control port is in fluid communication with the fluid drain port, and the fluid supply port is blocked, the second configuration in which the fluid control port is in fluid communication with the fluid supply port and is in fluid communication with the fluid drain port through a fluid restrictor, and the fluid flow comprises flow from the fluid control port to the fluid drain port through the fluid restrictor, a third configuration in which fluid communication between the fluid control port, the fluid supply port, and the fluid drain port is blocked, and a fourth configuration in which the fluid control port is in fluid communication with the fluid supply port, and the fluid drain port is blocked. For example, the process <NUM> can be performed using the EHSV <NUM> of <FIG>.

<FIG> is a flow diagram of an example process <NUM> for communicating servo valve health status. In some implementations, the process <NUM> can be performed by the example hydraulic control system <NUM> of <FIG>, the example hydraulic control system <NUM> of <FIG>, or the example hydraulic control system <NUM> of <FIG>.

At <NUM> a first servo valve is controlled by a first servo controller configured to provide a first health signal to selectably permit flow between a first fluid port and a fluid source, permit flow between the first fluid port and a fluid drain, and block fluid flow between the first fluid port, the fluid source, and the fluid drain, wherein the controlling is based on a position demand signal, a position feedback signal, a first priority signal, and a second health signal. For example, the EHSV 220a is controlled by the valve controller 234a.

At <NUM>, the first health signal is provided by the first servo controller. For example, the valve controller 434a can transmit the health status signal 437a over the communication bus <NUM>.

At <NUM> a second servo valve is controlled by a second servo controller to selectably permit flow between a second fluid port and the fluid source, permit flow between the second fluid port and the fluid drain, and block fluid flow between the second fluid port, the fluid source, and the fluid drain, wherein the controlling is based on the position demand signal, the position feedback signal, a second priority signal, and the first health signal. For example, the EHSV 220b is controlled by the valve controller 234b.

At <NUM>, the second servo controller provides the second health signal. For example, the valve controller 434b can transmit the health status signal 437b over the communication bus <NUM>.

At <NUM>, a shuttle valve directs fluid flow between a selectable one of the first fluid port and the second fluid port, and a fluid outlet configured to be fluidically connected to a fluid actuator. For example, the shuttle valve <NUM> can switch between connecting the main fluid control conduit <NUM> to the fluid control port 226a and connecting main fluid control conduit <NUM> to the fluid control port 226b.

Either the first priority signal or the second priority signal includes a representation of an operational condition including a high priority command provided to a selected one of the first servo controller or the second servo controller to act as a primary servo controller. The other of the first priority signal or second priority signal can include a representation of an operational condition including a low priority command provided to the other of the first servo controller or the second servo controller to act as a reserve servo controller. For example, the controller <NUM> can send a command to the valve controller 234a to operate as the primary controller for the fluid actuator <NUM>, and the controller <NUM> can send a command to the valve controller 234b to operate as a secondary (e.g., backup or standby) controller for the fluid actuator <NUM>.

In some implementations, the process <NUM> can also include receiving, by the first servo controller, the high priority command as the first priority signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. For example, when the valve controller 234a is commanded to act as the primary controller, the valve controller 234a can take control over the fluid actuator <NUM> by controlling the EHSV 220a.

In some implementations, at least one of the first health signal and the second health signal include representations of one or more operational conditions including an operable condition indicating an absence of failure, a fail condition indicative of a failure that is addressable by a shutdown of a corresponding one of the first servo valve or the second servo valve, and a failure of the health signal that represents an inability to transmit any of above conditions. For example, the health status receiver 418b can receive the health status signal 437a and determine if the valve controller 434a is in a normal operational state or if it has detected a malfunction and needs to be shut down.

In some implementations, the process <NUM> can also include controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by modulating fluid connectivity from the fluid source to the fluid actuator and from the fluid actuator to the fluid drain, and controlling, by the second servo controller, the second servo valve to provide a restricted fluidic connection from the second fluid port to the fluid drain when the position demand signal indicates a nonzero demanded position, and to provide an unrestricted fluidic connection from the second fluid port to the fluid drain when the position demand signal indicates a zero-proximal demanded position. For example, when the EHSV 220b is acting as the primary EHSV to control the fluid actuator <NUM>, the EHSV 220a can be 229b close to the null position (e.g., configuration 390b) in case fast action is needed to take over control.

In some implementations, the process <NUM> can also include receiving, by the first servo controller, the low priority command as the first priority signal detecting, by the first servo controller, the fail condition in the second servo controller or the second servo valve, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating fluid connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. For example, when the valve controller 234a is commanded to act as the secondary controller but detects that the servo controller 234b has a fault, the valve controller 234a can immediately take control over the fluid actuator <NUM> by adequate control of EHSV 220a.

In some implementations, the process <NUM> can also include controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by modulating fluid connectivity from the fluid source to the fluid actuator and from the fluid actuator to the fluid drain, detecting, by the first servo controller, a fault condition in the first servo controller or the first servo valve, transmitting a fault signal indicative of the detected fault condition as the first health signal, and controlling, by the second servo controller and in response to the fault signal, the second servo valve to selectably permit flow between the second fluid port and the fluid source, permit flow between the second fluid port and the fluid drain, and block fluid flow between the second fluid port, the fluid source, and the fluid drain. For example, the valve controller 234a can identify a fault within itself while functioning as the primary controller for the fluid actuator <NUM>, and respond by modifying its health signal to indicate the fault (e.g., a slow fail signal or a fast fail signal). The valve controller 234b can receive and interpret the health signal, and respond by taking over control of the fluid actuator <NUM> from the valve controller 234a.

In some implementations, the process <NUM> can also include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the operable condition in the second servo controller and the second servo valve, and controlling, by the first servo controller, the first servo valve to provide a fluidic connection from the first fluid port to the fluid drain and to block the fluid source. For example, the controller <NUM> can command the valve controller 234a to operate as the secondary, backup controller, and if it also detects that the valve controller 234b is indicating that it is fully operational, the valve controller 234a can transition into standby mode by controlling the EHSV 220a to the configuration 390a.

In some implementations, the process <NUM> can also include controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator by modulating fluid connectivity from the fluid source to the fluid actuator and from the fluid actuator to the fluid drain, detecting, by the second servo controller, a fault condition in the second servo controller or the second servo valve, transmitting a fault signal indicative of the detected fault condition as the second health signal, receiving, by the first servo controller, a command signal configured to transfer control of the fluid actuator from the first servo controller and the first servo valve to the second servo controller and the second servo valve, and ignoring, by the by the first servo controller and based on the fault signal, the command signal. For example, when the valve controller 234a is acting as the primary controller for the fluid actuator <NUM> and a fault signal is received from the valve controller 234b, the valve controller 234a may ignore a command from the controller <NUM> to transfer control to the valve controller 234b (e.g., to prevent switchover to a faulty EHSV module).

In some implementations, the process <NUM> can also include receiving, by the first servo controller, the low priority command as the first priority signal, detecting, by the first servo controller, the failure of the second health signal, determining, by the first servo controller and based on the detecting, a modified position demand that is less than a position demand represented by the position demand signal, controlling, by the first servo controller, the first servo valve to control a position of the fluid actuator based on the modified position demand by (a) modulating fluid connectivity from the fluid source to the first fluid port, (b) modulating connectivity from the first fluid port to the fluid drain, and (c) blocking fluid flow between the first fluid port, the fluid source, and the fluid drain. For example, when the valve controller 234b detects a failure of the health signal from the valve controller 234a (e.g., as opposed to a fault in the valve controller 234a itself), the exact status of the valve controller 234a can be unknown (e.g., cannot differentiate between a malfunction of the valve controller or a malfunction in the communications downstream from the valve controller). In circumstances such as this, the valve controller 234b can switch into a parallel primary controller mode, where the EHSV 220b is controlled based on a modification of the demanded position to place the EHSV 220b in a state of operation that closely follows the output that the EHSV 220a may or may not still be providing. In some examples, this type of operation can create a safe fallback position without causing the shuttle valve <NUM> to switch over from the EHSV 220a if it is still operating normally.

In some implementations, the process <NUM> can also include detecting, by the second servo controller, a failure of the first health signal, determining, by the second servo controller and based on the detecting, a modified position demand that is less than a position demand represented by the position demand signal, and controlling, by the second servo controller and based on the modified position demand, the second servo valve. For example, the valve controller 234b can detect a short to ground, a short to battery, or an undefined (e.g., noise) state on the health signal from the valve controller 234a. Persons of skill in the art utilize a number of existing communication techniques that can be used to convey operational status and/or control messages while also determining an operational status of the communication link itself. For example, 4mA to 20mA current loops are used, in which information is communicated on a digital signal that uses 20mA as a high or "<NUM>" signal and uses 4mA as a low or "<NUM>" signal, while currents closer to zero can represent a shorted or open communication circuit. In another example, digital communications can include checksums, in which communicated information (e.g., commands, statuses) is accompanied by mathematically hashed information that can be compared to received communications to determine if the information was received correctly or if the information had been corrupted by noise. Since these states may indicate a communication rather than a control fault (e.g., the valve controller 234a and the EHSV 220a may still be operating normally), the valve controller 234b may respond by controlling the EHSV 220b in a manner that causes it to provide slightly less than the pressure that is commanded by the controller <NUM>. As such, the provided pressure is nearly the same as the pressure that may or may not be getting provided by the EHSV 220a (e.g., to act as a close fallback for the commanded pressure level) but will not cause switchover of the shuttle valve <NUM> if the EHSV 220a is still operating normally.

In some implementations, the process <NUM> can include receiving, by the first servo controller, a command signal configured to transfer control of the fluid actuator from the first servo controller and the first servo valve to the second servo controller and the second servo valve, receiving, by the second servo controller, a command signal configured to transfer control of the fluid actuator from the first servo controller and the first servo valve to the second servo controller and the second servo valve, controlling, by the second servo controller and in response to the received command signal, the second servo valve to selectably permit flow between the second fluid port and the fluid source, permit flow between the second fluid port and the fluid drain, and block fluid flow between the second fluid port, the fluid source, and the fluid drain, and controlling, by the first servo controller and based on the received command signal, the first servo valve to at least permit fluid flow between the first fluid port and drain, and block the fluid supply. For example, if the valve controller 234a is in control of the process and the controller <NUM> requests a switchover of control, the valve controller 234b can respond by controlling the EHSV 220b to control the fluid actuator <NUM>, and the valve controller 234a can control the EHSV 220a to provide an output pressure that is slightly below the commanded pressure.

In some implementations, the process <NUM> can also include receiving, by the first servo controller, the low priority command as the first priority signal, controlling, by the first servo controller and based on the receiving, the first servo valve to a standby position based on a standby demand, detecting, by the first servo controller and based on the second health signal, the operable condition in the second servo controller and the second servo valve, receiving, by the first servo controller, a command signal representative of a silt reduction operation, controlling, by the first servo controller and in response to the received command signal, the first servo valve to a first modified position that is below standby position, and controlling, by the first servo controller and in response to the received command signal, the first servo valve to the standby position. For example, the controller <NUM> can request the valve controller 234a to switch over to standby (e.g., secondary, backup) mode, and if the valve controller 234a determines that it is safe to do so (e.g., receiving a healthy operation signal from the valve controller 234b), then the valve controller 234a can switch over to standby operation. The controller <NUM> can request the valve controller 234a to perform an operation that prevents or reduces build-up (e.g., dirt, silt) that may have accumulated in the EHSV 220a. In response, the valve controller 234a can causes the closure member 229a to oscillate slightly in a manner in which the closure member changes its position by the distance that accumulated dirt releases from the closure member and valve surfaces (e.g., to agitate and loosen internal buildup of contamination). The movement is directed only into the drain direction to avoid potential disturbances on the fluid actuator.

In some implementations, the process <NUM> can also include receiving, by the first servo controller, a command signal representative of a silt reduction operation, receiving, by the second servo controller, the command signal, wherein the second servo controller is operating at a standby demand, controlling, by the first servo controller and in response to the received command signal, the first servo valve to a first modified position that is below a position demand represented by the position demand signal, controlling, by the first servo controller and in response to the received command signal, the first servo valve to a second modified position that is above the position demand, controlling, by the first servo controller and in response to the received command signal, the first servo valve based on the position demand, controlling, by the second servo controller and in response to the received command signal, the second servo valve to a third modified position that is below the standby demand, and controlling, by the second servo controller and in response to the received command signal, the second servo valve based on the standby demand. For example, the controller <NUM> can request the valve controller 234a to perform an operation that prevents or reduces build-up (e.g., dirt, silt) that may have accumulated in the EHSV 220a. In response, the valve controller 234b can operate the EHSV 220b slightly below the demanded pressure (e.g., to act as a backup in case the EHSV 220a malfunctions during the cleaning process). The valve controller 234a remains in control of the fluid actuator <NUM>, and causes the closure member 229a to oscillate slightly in a manner that causes the output pressure to repeatedly vary slightly above and slightly below the demanded pressure (e.g., to agitate and loosen internal buildup of contamination).

In some implementations, the process <NUM> can also include moving a selected one of the first servo valve and the second servo valve between a first position to permit flow between drain and corresponding one of the first fluid port and the second fluid port, and moving a selectable one of the selected servo valve to a second position configured to provide an air bleeding fluid path between the fluid drain and the fluid source and the corresponding one of the first fluid port and the second fluid port. For example, one or both of the EHSVs 220a and 220b can be controlled to have the example configuration 390d of <FIG>. In another example, one or both of the EHSVs 220a and 220b can be controlled to perform the example air bleeding operations discussed in the descriptions of <FIG>.

<FIG> is a schematic diagram of an example of a generic computer system <NUM>. The system <NUM> can be used for the operations described in association with any or all of the example controller <NUM>, the example EHSV module 120a, the example EHSV module 120b, the example controller <NUM>, the example controller <NUM>, the example valve controller 234a, the example controller 324b, the example valve controller 434a, or the example controller 434b.

The system <NUM> includes a processor <NUM>, a memory <NUM>, a storage device <NUM>, and an input/output device <NUM>. Each of the components <NUM>, <NUM>, <NUM>, and <NUM> are interconnected using a system bus <NUM>. The processor <NUM> is capable of processing instructions for execution within the system <NUM>. In one implementation, the processor <NUM> is a single-threaded processor. In another implementation, the processor <NUM> is a multi-threaded processor. The processor <NUM> is capable of processing instructions stored in the memory <NUM> or on the storage device <NUM> to display graphical information for a user interface on the input/output device <NUM>.

The features described can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. The apparatus can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device for execution by a programmable processor; and method steps can be performed by a programmable processor executing a program of instructions to perform functions of the described implementations by operating on input data and generating output. The described features can be implemented advantageously in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. A computer program is a set of instructions that can be used, directly or indirectly, in a computer to perform a certain activity or bring about a certain result.

Claim 1:
An electrohydraulic positioning control system (<NUM>, <NUM>) comprising:
a shuttle valve (<NUM>, <NUM>) configured to direct fluid flow between a selectable one of a first fluid port (226a) and a second fluid port (226b), and a fluid outlet configured to be fluidically connected to a fluid actuator (<NUM>);
a first servo valve (220a, 420a) controllable to selectably permit flow between the first fluid port (226a) and a fluid source (<NUM>, <NUM>), permit flow between the first fluid port (226a) and a fluid drain (228a, 228b), and block fluid flow between the first fluid port (226a), the fluid source (<NUM>, <NUM>), and the fluid drain (228a, 228b);
a second servo valve (220b, 420b) controllable to selectably permit flow between the second fluid port (226b) and the fluid source (<NUM>, <NUM>), permit flow between the second fluid port (226b) and the fluid drain (228a, 228b), and block fluid flow between the second fluid port (226b), the fluid source (<NUM>, <NUM>), and the fluid drain (228a, 228b);
a first servo controller (234a, 434a) configured to provide a first health signal (437a) and control the first servo valve (220a, 420a) based on a position demand signal, a position feedback signal (412a, 412b), a first priority signal, and a second health signal (437b); and
a second servo controller (234b, 434b) configured to provide the second health signal (437b) and control the second servo valve (220b, 420b) based on the position demand signal, the position feedback signal (412a, 412b), a second priority signal, and the first health signal (437a)
wherein either the first priority signal or the second priority signal comprises a representation of an operational condition comprising a high priority command provided to a selected one of the first servo controller (234a, 434a) or the second servo controller (234b, 434b) to act as a primary servo controller.