TECHNICAL FIELD 
     The present invention relates generally to the control of an electro-hydraulic system, and in particular to an apparatus and method for maintaining control and operation of an electro-hydraulic system or fluid circuit having a failed pressure or position sensor. 
     BACKGROUND OF THE INVENTION 
     Electro-hydraulic systems or fluid circuits utilize various electrically-actuated and hydraulically-actuated devices, alone or in combination, to provide open-loop or closed loop feedback control. In a closed-loop system in particular, feedback mechanisms or sensors can be used to monitor circuit output values. Each sensor can generate a signal that is proportional to the measured output, and using a suitable control logic device or controller the output can be compared to a particular input or command signal to determine if any adjustments or control steps are required. Sensors for use in an electro-hydraulic fluid circuit ordinarily include pressure transducers, temperature sensors, position sensors, and the like. 
     In a conventional fluid circuit, the precise control of the operation of the fluid circuit can be maintained by continuously processing the various measured or sensed output values. Supply and tank pressures, as well as pressures operating on particular ports or chambers of a control valve, cylinder, or fluid motor used within the circuit, can be continuously fed to a control unit or controller. However, system control can be lost or severely degraded in a conventional fluid circuit if any of the required pressure or position sensors fails or ceases to function properly for whatever reason. While certain code-based methods exist for detecting out-of-range sensor operation, or for determining shorted or open circuits, such methods usually result in a temporary shutdown of the process utilizing the fluid circuit, and therefore can be less than optimal when continuous fluid circuit operation is required. 
     SUMMARY OF THE INVENTION 
     Accordingly, an electro-hydraulic system or fluid circuit includes a sump or a tank configured for holding a supply of fluid, a hydraulic device having a predetermined load configuration, and a pump for drawing fluid from the tank and delivering it under pressure to the hydraulic device. Sensors are adapted for measuring a supply pressure, a tank pressure, and a position of a moveable spool portion or other moveable portion of the hydraulic device, as well as one or more additional valves, such as a fluid conditioning valve positioned in fluid parallel with the hydraulic device. A controller has an algorithm suitable for estimating or reconstructing an output value of a failed one of any of the plurality of sensors in the fluid circuit using the predetermined load configuration, thereby ensuring the continued operation of the hydraulic device and the fluid circuit. 
     Using the method of the invention, which can be embodied by the computer-executable algorithm mentioned above, at least some level of control can be maintained over the fluid circuit despite the presence of the failed sensor. A quasi-steady analysis of the fluid circuit can capture the fundamentals of the fluid circuit. In a fluid circuit having a pump, a reservoir or tank, a plurality of check valves and/or fluid conditioning valves, and a cylinder, fluid motor, or other device having a first and a second work chamber or port, unknown variables Q a , Q b , and Q ƒcv  are present, wherein Q a  describes the flow into and out of a first work chamber of the cylinder, Q b  is the flow into and out of a second work chamber of the cylinder, and Q ƒcv  is the flow through an orifice of a fluid conditioning valve positioned or connected in fluid parallel with the cylinder and pump. In accordance with the invention, a fluid circuit configured in this manner can be modeled via a predetermined set of non-linear equations that differ depending on the failed state of the fluid circuit, i.e., a failure of a sensor occurring when the fluid circuit is active, that is, when fluid is flowing from the work chamber a to the work chamber b, or from work port b to a, as described below. 
     The method therefore allows for the estimating or reconstructing of an otherwise lost or unavailable sensor signal using a calibrated, known, or predetermined load configuration, e.g., in a two-port device such as a cylinder or fluid motor, the relationship between the flow rates through the respective work chambers or ports. A fluid circuit adapted for executing the method can include a controller having an algorithm suitable for processing the output values from a plurality of pressure and position sensors, calculating any required flow information using calibrated volumetric and measured pressure and/or other required data in conjunction with the pressure and position measurements, and estimating the missing sensor value using a set of non-linear equations. The controller then automatically controls the fluid circuit using the estimated value until such time as the sensor can be diagnosed, repaired, or replaced. 
     More particularly, the method allows for the estimation or reconstruction of an output value of any one sensor of a plurality of sensors in a fluid circuit having a controller, a pump, a tank, a hydraulic device, and a fluid conditioning valve. The conditioning valve is in fluid parallel with the hydraulic device. The method includes sensing a set of output values from the plurality of sensors, processing the output values using the controller to determine the presence of a failed sensor, and using the controller to calculate an estimated output value of the failed sensor using a predetermined load configuration of the hydraulic device. The hydraulic device can be controlled using the estimated output value until the failed sensor can be repaired or replaced, thereby ensuring continuous operation of the fluid circuit. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary fluid circuit in a first sensory failure state having a controller in accordance with the invention; 
         FIG. 2  is a schematic illustration of the exemplary fluid circuit of  FIG. 1  in a second sensory failure state; and 
         FIG. 3  is a flow chart describing a control method usable with the fluid circuit of  FIGS. 1-2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings wherein like reference numbers correspond to like or similar components throughout the several figures, and beginning with  FIG. 1 , a fluid circuit  10  is shown in a first possible sensory failure state, as will be described below. The fluid circuit  10  includes a pump (P)  12  and a low-pressure reservoir, sump, or tank  14 . The tank  14  holds or contains a supply of fluid  15 , which is drawn by the pump  12  and delivered under pressure (P s ) via a supply line  11  to a hydraulic device  24 . In the exemplary embodiment of  FIG. 1 , the hydraulic device  24  is configured as a dual-chamber cylinder  27  containing a spool or piston  26 , with the cylinder  27  having a first and a second work port,  31  and  33 , respectively, in communication with the work chambers a and b defined by and within the cylinder  27  and piston  26 . 
     Control logic or an algorithm  100  for executing the method of the invention can be programmed or recorded within a controller (C)  30  and implemented to selectively control the various fluid control devices within the fluid circuit  10  as needed to power a downstream fluid circuit (FC)  28 , including items such as but not limited to hydraulic machinery, valves, pistons, accumulators, etc. The FC  28  in turn is in fluid communication with the tank  14  via a return line  13 . 
     The controller  30 , which can be directly wired to or in wireless communication with the various components of the fluid circuit  10 , receives a set of pressure and position input signals (arrow  25 ) from sensors  18 A-D and  19 A-C, as explained below. The fluid circuit  10  can be configured as a digital computer generally including a CPU, and sufficient memory such as read only memory (ROM), random access memory (RAM), electrically-programmable read only memory (EPROM), etc. The controller  30  can include a high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O), as well as appropriate signal conditioning and buffer circuitry. Any algorithms resident in the controller  30  or accessible thereby, including the algorithm  100  described below with reference to  FIG. 3 , or any other required algorithms, can be stored in ROM and automatically executed by the controller  30  to provide the required circuit control functionality. 
     The fluid  15  is selectively admitted into the fluid circuit  10  via the supply line  11  at the supply pressure (P s ). A fluid conditioning valve  16  is positioned in fluid parallel with the hydraulic device  24  between a pair of pressure sensors  18 A and  18 B, e.g., pressure transducers or other suitable pressure sensing devices. The sensor  18 A is positioned and adapted for measuring the supply pressure (P s ), while the sensor  18 B is positioned and adapted for measuring the return line or tank pressure (P t ). As needed, some or all of the fluid  15  flowing from the pump  12  can be diverted from the hydraulic device  24  through the conditioning valve  16  and back to the tank  14 . 
     The fluid circuit  10  includes position sensors  19 A,  19 B, and  19 C adapted for measuring the position of respective spools in the conditioning valve  16 , the valve  20 , and the valve  22 , respectively. Additional pressure sensors  18 C,  18 D are positioned in fluid series with the hydraulic device  24 . The sensor  18 C is positioned and adapted for measuring the fluid pressure (P a ) operating on work chamber a or the first work port  31  of the hydraulic device  24 , and is positioned downstream of a first valve  20 . The first valve  20  can be configured as any suitable fluid control valve suitable for directing fluid  15  from the pump  12  in the direction of arrow C, and into the first work port  31  of the hydraulic device  24  in order to move the piston  26  in the direction of arrow C. A second valve  22  prevents a flow of fluid  15  into the work port  33 . The sensor  18 D is positioned and adapted for measuring the fluid pressure (P b ) operating on work chamber b or the second work port  33  of the hydraulic device  24 . 
     Under normal operating conditions, the variables P s , P t , P a , and P b  are known, being sensed or measured by the respective pressure sensors  18 A- 18 D. The position variables x a , x b , and x ƒcv  are also known, being sensed by the position sensors  19 A-C. The variables x a  and x b  describe the position of the piston  26  in work chambers a and b, respectively, while x ƒcv  describes the position of a spool portion of the fluid conditioning valve  16 . Three unknown variables include Q a , Q b , and Q ƒcv , as noted above, i.e., the flow into the first work port  31 , the second work port  33 , and the conditioning valve  16 , respectively. A unique solution is thus provided for these values using the following three-function equation set:
 
ƒ1( Q   a   , P   s   , P   a   , x   a )=0;
 
ƒ2( Q   b   , P   t   , P   b   , x   b )=0; and
 
ƒ3( Q   ƒcv   , P   s   , P   t   , x   ƒcv )=0
 
For example, ƒ1(Q a , P s , P a , x a )=Qa−c d A(x a )sgn(P s −P a )√{square root over (2/ρ|Ps−Pa|)}, where c d  is the discharge coefficient, ρ is the density of the fluid, and A is the orifice area as a function of spool position.
 
     However, in a sensory failure state in which one of the sensors  18 A-D or  19 A-C fails, the set of equations above cannot be uniquely solved without resorting to additional information. For example, if the pressure at work port  31  or P a  is unavailable due to a failure of sensor  18 C, the remaining known variables are P s , P t , P b , x a , x b , and x ƒcv . We now have four unknown variables, i.e., Q a , Q b , and Q ƒcv  as before, as well as the unknown value of P a . 
     In an observer-based model, state variables can be estimated by comparing the model outputs to actual measurements. A signal can be easily reconstructed only if the system itself is fully observable. However, observer-based models are severely challenged in the face of unknown load conditions, such as the velocity of a piston positioned within a fluid cylinder, a portion of a fluid motor, or any moveable portion of a typical two-port fluid device. 
     For example, a fluid circuit can be modeled via the following equation:
 
 {dot over (P)}   a =(β/ V )( Q   a ( P   s   ,P   a   ,x   a )− A{dot over (x)}   cyl )
 
wherein {dot over (P)} a  refers to the change in fluid pressure at a first port or “work port a” of a 2-port device, β is the bulk modulus of the fluid used in the circuit, V is the volume of the cylinder, Q a  is the flow rate through work port a, P s  is the supply pressure, P a  is the pressure at chamber a or work port  31 , and x a  is the spool position of a spool or piston at chamber a or work port  31 . Additionally, A is the cross-sectional area of the cylinder, and {dot over (x)} cyl  is the rate of change in position of the cylinder, i.e., the velocity thereof. The value A{dot over (x)} cyl  is an unknown load condition in such an exemplary cylinder.
 
     Using the algorithm  100 , the load configuration of the hydraulic device  24  can provide further constraints as determined using the unknown variables. For example, Q a =−Q b  for a cylinder/motor connection as shown in  FIGS. 1 and 2 , if the work chambers on either side of the cylinder  27  are equally sized, or Q a =−(A a /A b )(Q b ) where A a  is piston area in work chamber a and A b  is position area in work chamber b, if the work chambers a and b are differently sized. Therefore, the algorithm  100  can use non-linear equations to determine the unknown three variables in a first sensory failure mode. Accordingly, any one of the sensor signals P s , P t , P a , P b , x a , and x b  can be estimated using the above equations. 
     Referring to  FIG. 2 , the fluid circuit  10  of  FIG. 1  is shown in a second failure sensory state, i.e., when fluid is being applied at work port  33  to move the piston  26  in the direction of arrow D. As above, any one of the missing sensor signals P s , P t , P a , P b , x a , and x b  can be estimated or reconstructed using the known load configuration for the hydraulic device  24 . 
     Referring to  FIG. 3  in conjunction with the fluid circuit  10  of  FIGS. 1 and 2 , the method of the invention can be executed via the algorithm  100 . Beginning at step  102 , the controller  30  continuously or in accordance with a specified periodic cycle time reads the output values from each of the sensors  18 A-D and  19 A-C. In normal operation, the controller  30  processes these values using control logic, and selectively actuates the hydraulic device  24  and, if used, any additional downstream devices in the downstream fluid circuit  28  according to such control logic. The algorithm  100  then proceeds to step  104 . 
     At step  104 , the controller  30  determines whether any of the sensors  18 A-D and  19 A-C has failed. If not, the algorithm  100  is finished, effectively resuming with step  102  and repeating steps  102  and  104  until such a sensor failure is determined to be present. If a sensor has failed, the algorithm  100  proceeds to step  106 . 
     At step  106 , the algorithm  100  estimates or reconstructs the value for the failed sensor. This estimated value is represented in  FIG. 3  as the value (e). For example, if the sensor  18 C has failed the output value P a  would be unavailable as a result. Continuing with the example of sensor  18 C, the unknown variables would be Q a , Q b , Q ƒcv , and P a . However, given a known load configuration such as Q a =−Q b  for the cylinder or motor connection shown in  FIGS. 2 and 3 , the four unknowns reduce to three: Q a  (or Q b ), Q ƒcv , and P a . The algorithm  100  then uses the non-linear equations as set forth above, i.e., ƒ1(Q a , P s , P a , x a )=0; ƒ2(Q b , P t , P b , x b )=0; and ƒ3(Q ƒcv , P s , P t , x ƒcv )=0, to estimate the value (e). 
     Once the estimated value (e) has been determined or calculated at step  106 , the algorithm  100  proceeds to step  108 , wherein the controller  30  executes control of the fluid circuit  10  of  FIGS. 1 and 2  using the estimated value (e). Continued control of the fluid circuit  10  can therefore be maintained. The algorithm  100  can then be finished, or can optionally proceed to step  110 . 
     At step  110 , an alarm can be activated, or another suitable control action can be taken, to ensure that attention is drawn to the presence of the failed sensor. In this manner, the sensor failure can be properly diagnosed, repaired, or replaced as needed. 
     Accordingly, using the control algorithm  100  as set forth above as part of the fluid circuit  10  of  FIGS. 1 and 2 , single sensor fault operation of the fluid circuit  10  can be achieved. Given the load configuration, it is possible to reconstruct most of a single failed sensor signal if service is running at the time of the sensor failure. If service stops, i.e., if both work ports  31  and  33  of the hydraulic device  24  close, it can be difficult to accurately estimate the failed sensor signal. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims. Likewise, while the invention has been described with reference to a preferred embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
A fluid circuit includes a tank for holding fluid, a hydraulic device having a predetermined load configuration, and a pump for delivering the fluid under pressure to the hydraulic device. Sensors measure at least one of a supply pressure, a tank pressure, and a position of a portion of the hydraulic device. A controller estimates or reconstructs an output value of any one sensor using the predetermined load configuration in the event of a predetermined failure of that sensor, ensuring continued operation of the hydraulic device. A method for estimating the output value includes sensing output values using the sensors, processing the output values using the controller to determine the presence of a failed sensor, and calculating an estimated output value of the failed sensor using the predetermined load configuration. Operation of the hydraulic device is maintained using the estimated output value until the failed sensor can be repaired.