Abstract:
A vehicle includes an engine, first clutch, transmission, and controller. The transmission includes a gearbox, position sensors, and a fluid circuit. The gearbox contains a second clutch. The fluid circuit includes a pump and a flow control solenoid valve. The controller opens the valve via flow control signals to allow fluid to pass into or from the particular clutch it feeds. The controller executes steps of a method to determine an actual flow rate through the valve as the clutch moves, and also calculates a compensation scale factor as a ratio of the commanded and actual flow rates. The controller modifies the flow control signals in a subsequent clutch actuation using the compensation scale factor, such as by multiplying a commanded flow rate corresponding to the flow control signals by the compensation scale factor. A system includes rotatable members connected by a clutch, the controller, valve, and position sensor.

Description:
TECHNICAL FIELD 
     The present disclosure relates to adaptive control of a flow control solenoid valve. 
     BACKGROUND 
     Hydraulic fluid circuits employ valves, pistons, and other various fluid powered components and flow control devices in order to perform useful work in a system. For example, a transmission typically employs hydraulic clutches having spaced friction plates. The friction plates are compressed via a clutch apply piston or a synchronizer fork by fluid pressure. When a fluid pump is running and/or a hydraulic accumulator is actively exhausting, fluid is delivered under pressure through any open branches of a fluid circuit. In a system having a clutch, for instance, a flow control valve may be selectively opened to allow the fluid to enter a clutch apply chamber for that particular clutch, thereby filling the clutch prior to clutch engagement. 
     SUMMARY 
     A vehicle is disclosed herein having a clutch and a flow control solenoid valve of the type noted above. The vehicle includes a controller that is programmed to learn the actual flow characteristics of the valve over time. Over time, the controller modifies flow control commands transmitted to the valve to thereby adapt to the changing performance of the valve, e.g., due to age, wear, temperature variations, and the like. 
     To accomplish these end goals, the controller may periodically update a series of lookup tables, each of which corresponds to a different measured temperature of the fluid used to actuate the clutch. The lookup tables are populated with commanded and actual flow rates. The commanded flow rate is a value that is available to the controller as part of its overall shift control logic, i.e., the particular flow rate needed for achieving a desired position of a clutch apply piston. The actual flow rate may be determined in various ways, including by calculating the actual flow rate as a function of measured clutch position and a surface area of a clutch apply piston of the clutch. 
     The controller may also calculate a compensation scale factor and include this factor in the lookup tables. The compensation scale factor may be derived by the controller by dividing the commanded flow rate by the actual flow rate at a given fluid temperature. The recorded compensation scale factor can be used in a future shift action involving the clutch, the valve for which the flow characteristics were learned, so as to yield an adapted commanded flow rate to be commanded at the next shift of the transmission using the same valve. The adapted commanded flow rate may be transmitted to the valve as a set of flow control signals, which in a solenoid valve embodiment are the electrical current commands required for energizing windings of the solenoid portion of the valve, as is well known in the art. 
     When used in this manner, the compensation scale factor helps to account for any differences that might be present between a generic flow vs. current (Q vs. i) characteristic table, typically provided by a valve supplier, and the actual performance of that particular valve. The use of multiple lookup tables to cover a number of different fluid temperatures may help to account for changes in oil viscosity and other temperature-dependent factors. Beneficial results of the adaptive methodology disclosed herein may include an improvement in overall shift feel and clutch durability. 
     In an example embodiment, the vehicle includes an engine, a first clutch, a transmission, and a controller. The transmission includes an input member that is selectively connectable to the output shaft of the engine via the first clutch. The transmission also includes a gearbox, first and second position sensors, and a fluid circuit. The gearbox may contain a second clutch or multiple such clutches. Each position sensor measures a corresponding position of a respective one of the first and second clutches. The fluid circuit includes a fluid pump and a solenoid valve, e.g., a flow control variable force solenoid (QVFS) valve, with the fluid pump circulating fluid under pressure to the valve for use by the particular clutch controlled via the valve whose flow performance is being evaluated. 
     In this example embodiment, the controller may selectively open the valve via flow control signals in the form of electrical current commands to thereby allow the fluid to flow into or out of the clutch. Execution of instructions by the controller causes the controller to receive the measured position signals from a selected one of the position sensors, e.g., in response to a requested shift. The controller then determines, from the received position signals, an actual flow rate through the valve as the clutch moves from a first calibrated position to a second calibrated position, and also calculates the compensation scale factor noted above as a ratio of the commanded flow rate to the actual flow rate. The controller then modifies the flow control signals for a subsequent actuation of the selected clutch using the calculated compensation scale factor, such as by multiplying a commanded flow rate corresponding to the flow control signals by the compensation scale factor to determine updated flow control signals. 
     The transmission may be embodied as a dual-clutch transmission (DCT) having a pair of input clutches as the first clutch. As is well known in the art of DCTs, one of the input clutches is applied to select oddly-numbered gears of the gearbox during a shift to an odd gear state, e.g., 1 st , 3 th , or 5 th  gear, and the other of the pair of input clutches is applied to select evenly-numbered gears of the gearbox during a shift to an even gear state such as 2 nd  or 4 th  gear. 
     The first and the second clutches may each have a respective clutch apply piston with a predetermined surface area. In such a design, the controller may calculate the actual flow rate through the valve as a function of the predetermined surface area of the clutch apply piston. Such information could be determined beforehand and recorded in memory of the controller as a calibration value. 
     The fluid circuit for the vehicle may also include a temperature sensor that measures a temperature of the fluid, for instance from a location within a fluid sump. The controller can record the compensation scale factor at different fluid temperatures for optimal performance. The controller may record the commanded flow rate, the actual flow rate, and the compensation scale factor at each temperature in a corresponding lookup table that is accessible by the controller. 
     A system is also disclosed herein. The system may include first and second rotatable members, for instance different shafts or axles of a powertrain, as well as a clutch that is operable for connecting the rotatable members together when the clutch is applied, and for disconnecting the rotatable members from each other when the clutch is released. The clutch in such a system may include a clutch apply piston. A position sensor measures a changing position of the clutch apply piston and outputs a set of measured position signals. The system includes a flow control solenoid valve that opens in response to flow control signals so as to allow fluid to pass into or from the clutch that is being fed by the valve, with the direction of flow depending on the flow control signals. 
     Additionally, a controller of the system adapts the flow control signals over time. By executing instructions embodying a method, for instance, the controller receives the set of measured position signals from the position sensor and determines, from the received set of measured position signals, an actual flow rate through the valve as the clutch apply piston moves from a first position to a second position. The controller also calculates a compensation scale factor as a ratio of the commanded flow rate and the actual flow rate, and then modifies the flow control signals in a subsequent actuation of the clutch using the compensation scale factor. 
     A method is also disclosed for use with a vehicle having an engine, a transmission, a clutch, a flow control solenoid valve, and a fluid pump operable for circulating fluid to the clutch via the flow control solenoid valve. The method includes receiving a set of position signals, via a controller, from a position sensor in response to a requested shift of the transmission, with the set of position signals describing a position of the clutch. The method includes determining, from the received set of position signals, an actual flow rate through the valve as the clutch moves from a first calibrated position to a second calibrated position. The controller then calculates a compensation scale factor as part of the method, doing so as a ratio of the commanded flow rate and the actual flow rate. The method further includes modifying the flow control signals for a subsequent actuation of the clutch using the calculated compensation scale factor. 
     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 example vehicle having a flow control valve and a controller that provides adaptive learning of actual flow performance characteristics of the flow control valve. 
         FIG. 2  is an example set of lookup tables usable by the controller of  FIG. 1 . 
         FIG. 3  is a flow chart describing an example method for learning the flow performance of the flow control valve shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawings, wherein like reference numbers correspond to like or similar components throughout the several figures, a vehicle  10  is shown in  FIG. 1 . The vehicle  10  includes a fluid circuit  20  having a fluid pump  22  and one or more flow (Q) control solenoid valves  24 . The valves  24  may be embodied as variable force flow control solenoid (QVFS) valves, which as is known in the art are responsive to electrical current signals so as to open and thereby pass pressurized fluid (arrow  29 ) at a desired flow rate. Such signals are indicated in  FIG. 1  as arrow I QC  from a controller (C)  50 , the structure and function of which is discussed in detail below. The flow control valves  24  are connected to the fluid pump  22  by a suitable conduit  23 , such as hoses, clamps, fittings, and the like. 
     The vehicle  10  includes various fluid powered components and control devices as explained below. The vehicle  10  therefore serves as a non-limiting example system suitable for use with a method  100  for adaptively learning the flow characteristics of the valve(s)  24 . However, those of ordinary skill in the art will appreciate that the present invention is not limited to vehicular applications. Other possible systems may include, by way of example, hydraulic presses, conveyors, and lifts typically used on a plant floor, provided any of these systems includes a hydraulic device that is actuated via a piston or other movable actuator whose position can be measured and controlled. 
     In all embodiments, the controller  50  of  FIG. 1  is in communication with the fluid circuit  20 . Position sensors (S P ) are disposed within the vehicle  10  as shown. Measured position signals (P X ) are output from the position sensors S P  and transmitted to the controller  50 , for instance over a controller area network bus, serial bus, or other suitable connection, and used in the execution of the method  100 . An example embodiment of the method  100  is described below with reference to  FIG. 3 . 
     As part of the method  100 , the controller  50 , using a processor (P) and memory (M), periodically updates a set of lookup tables (LUT)  52 , an example of which is shown in  FIG. 2 . Using the information recorded in the lookup tables  52 , the controller  50  ultimately derives and adapts the underlying commanded flow rate corresponding to flow control signals (I QC ), with the term “adapts” indicating the changing nature of the flow control signals (I QC ) over time to match the actual performance of the valve  24 . 
     In a possible configuration, the vehicle  10  of  FIG. 1  may include an internal combustion engine (E)  12  and a transmission (T)  14 , for instance a dual-clutch transmission (DCT) as shown having a first and a second input clutch CI 1  and CI 2 , respectively. Only one input clutch may be used in an alternative automated manual transmission (AMT). The transmission  14  includes an output member  17  that delivers output torque from the transmission  14  to the drive wheels  21  of the vehicle  10 , e.g., via a differential  19 . 
     Additionally, a temperature sensor S T  may be positioned in a fluid sump  26  of the fluid circuit  20 , with a fluid sump  26  containing a volume of fluid  27 , e.g., oil or transmission fluid. A portion of this fluid  27 , once circulated under pressure via the pump  22 , is ultimately discharged via the valves  24  as the pressurized fluid (arrows  29 ). The temperature sensor S T , which is in electrical communication with the controller  50 , may periodically or continuously transmit a measured fluid temperature (T F ) to the controller  50  for use in control of the valves  24 . The controller  50  may receive other signals as part of its overall control function within the vehicle  10 . 
     The controller  50  of  FIG. 1  may be embodied as a host computer device that includes elements such as the processor (P), the memory (M) including but not limited to read only memory (ROM), random access memory (RAM), electrically-programmable read-only memory (EPROM), flash memory, etc., and the required hardware devices  55 . Hardware devices  55  may include a high-speed clock (not shown), timers for use in the execution of certain steps of the method  100 , analog-to-digital (A/D) circuitry, digital-to-analog (D/A) circuitry, a digital signal processor (DSP), and input/output (I/O) devices and/or other signal conditioning and/or buffer circuitry. 
     Within the transmission  14 , a gear box  16  may contain another clutch C 1 , for instance a friction clutch or a clutch synchronizer. For simplicity, only one additional clutch C 1  is shown in the schematic view of  FIG. 1 . However, in an actual embodiment, any number of clutches and/or synchronizers may be used. The description provided herein with respect to the clutch C 1  of transmission  14  therefore applies to any position-controlled clutch used in the transmission  14  or outside of the transmission  14 , e.g., the input clutches CI 1  and CI 2 . 
     The clutches CI 1 , CI 2 , and C 1  may each utilize a respective one of the position sensors S P , positioned with respect to a clutch apply piston  11 , with engagement of each input clutch CI 1 , CI 2  in a non-limiting DCT embodiment respectively selecting only the oddly-numbered or evenly-numbered gears of the transmission  14 . As is known in the art, such a configuration allows the connection of an output shaft  13  of the engine  12  to such selected gears. That is, the transmission  14  may have an input member  15 A,  15 B, with the input member  15 A being the oddly-numbered gear shaft and input member  15 B being the evenly-numbered gear shaft, as is known in the art of DCTs. The specific rotatable members that are selectively connected and disconnected via engagement of a given clutch, e.g., CI 1 , CI 2 , or C 1 , may vary from those shown in  FIG. 1 , i.e., the output shaft  13  or input members  15 A,  15 B, without departing from the intended inventive scope. 
     The example clutches CI 1 , CI 2 , and C 1  are in fluid communication with the fluid pump  22 , and therefore are supplied with fluid  27  under pressure as needed to actuate the clutches CI 1 , CI 2 , and/or C 1 . The fluid  27  flows through the valve(s)  24  whenever the valves  24  are opened in response to receipt by the valves  24  of the flow control signals (I QC ) from the controller  50 . Such commands may be, as noted above, embodied as electrical current control signals transmitted to the valve(s)  24 , e.g., a commanded current level needed for energizing the coil (not shown) of any solenoid portion of the valves  24  so as to open the valves  24  in a typical normally-off valve design. 
     As one of its intended functions, the controller  50  of  FIG. 1  executes the instructions embodying the method  100  to thereby reduce the effects of variation in output flow from the valves  24  over time with respect to expected values. This in turn may help to reduce variation in clutch position and clutch torque, and ultimately improve overall shift quality. As is known in the art, conventional QVFS valves are provided with a characteristic flow/current (Q v. i) characteristic curve that is valid at a given pressure and temperature, typically as seen during steady-state operating conditions. However, variation from the values in a calibrated Q v. i characteristic curve may result at other pressures, temperatures, and/or due to age or wear of the valves  24 . To address this problem, the controller  50  of the present invention periodically learns the true output flow performance characteristics of the valves  24  and then adjusts the values in the lookup tables  52  in the manner set forth below so as to compensate for such variation, thereby creating a closer match between expected and actual performance. 
     Referring to  FIG. 2 , in an example embodiment the lookup tables  52  may include first, second, and third rows R 1 - 3 . The first row R 1  captures a commanded flow rate (Q CC ) underlying the flow control signals (I QC ) of  FIG. 1 . This value is shown nominally in an example range of −3 to +3, for instance in liters per minute or another suitable flow rate. The actual values in the first row R 1  will vary depending on the design of the valve  24 . In this instance, a negative flow, for instance −3, refers to an outflow of fluid  27  from the clutches CI 1 , CI 2 , or C 1  of  FIG. 1 , such as might occur when exhausting fluid  27  from the clutch CI 1 , CI 2 , or C 1 . 
     The second row R 2  may be populated with corresponding actual flow rates Q A , which as noted above may vary over time from the commanded flow rate Q CC  of the first row R 1 . The values in the second row R 2  are shown as Q 1 , Q 2 , Q 3 , . . . , Q N  for illustrative simplicity. The actual values recorded in the second row R 2  may be calculated by the controller  50 , for instance using the following equation: 
               Q   A     =     (       (       (       P   ⁢           ⁢   1     -     P   ⁢           ⁢   2       )       t   1       )     ·     (     A   1000000     )     ·   60     )           
where P 1  and P 2  are the measured positions of the clutch CI 1 , CI 2 , or C 1 , or more precisely of the clutch apply piston  11  thereof, as determined via the position signals P X  for a corresponding position sensor S P  for that clutch, A is the predetermined surface area of the same clutch apply piston  11 , and t 1  is a timer value indicating the elapsed time between the transition between positions P 1  and P 2 . The actual flow rate Q A  in other embodiments may be determined differently, for example using a flow meter, without departing from the intended inventive scope.
 
     Multiple lookup tables  52  may be created for different temperatures in some embodiments, with the different temperatures indicated as T 1 , T 2 , T 3 , T 4 , and T 5  in the example five-table embodiment of  FIG. 2 . In such an embodiment, multiple lookup tables  52  may be recorded in memory M of the controller  50  shown in  FIG. 1 . More than five lookup tables  52  may be used in other embodiments, while fewer than five lookup tables  52  may also be used. This way, the lookup table closest in temperature to the actual temperature at the time of the analysis can be used to minimize error, or values from multiple lookup tables  52  can be used to extrapolate a final value for use in the method  100 . A sufficiently large number of lookup tables  52  should be used so as to cover a useful range of likely temperatures, such as the five tables for temperatures T 1  T 2 , T 3 , T 4  and T 5  as shown in  FIG. 2 . 
     The controller  50  of  FIG. 1  uses the data in the respective first and second rows R 1  and R 2  of the lookup tables  52  to create a compensation scale factor F, and records this value in the third row R 3 , as indicated by the nominal scale factors F 1 , F 2 , F 3 , . . . , F N . The compensation scale factor F as used herein is a calculated ratio of the commanded flow rate Q CC  to the actual flow rate, i.e., 
             F   =         Q   CC       Q   A       .           
Using the compensation scale factor F, the controller  50  can readily adjust the commanded flow rate Q CC  from a prior application of one of the clutches CI 1 , CI 2 , or C 1  via the recorded compensation scale factor F, with the result being to a new or adapted value for the flow control signals (I QC ) of  FIG. 1 . Because this process is iterative, the controller  50  periodically updates the lookup tables  52  based on the calculated or measured actual flow Q A , a value which may change over time, to thereby ensure control accuracy of the valves  24 .
 
     Referring to  FIG. 3 , an example embodiment of the method  100  begins at step  102 , wherein the controller  50  commands a positive flow rate from the valve  24  of  FIG. 1  via an initial set of flow control signals (I QC ). Step  102  may be executed in response to a request for such flow, for instance by a driver of the vehicle  10  of  FIG. 1  when the driver requests a shift of the transmission  14  via throttle and/or braking action requiring the application or release of any of the clutches CI 1 , CI 2 , or C 1 . The method  100  then proceeds to step  104 . 
     Step  104  entails receiving the measured position signals P X  from the position sensors S P  of the particular clutch, the valve  24  for which whose performance is being evaluated in the present control loop. This value may be temporarily stored in memory M. The method  100  then proceeds to step  106 . 
     At step  106 , the controller  50  of  FIG. 1  next determines, from the received measured position signals P X  of step  105 , whether the clutch whose valve  24  is being evaluated has reached a first calibrated position (P 1 ). If the first calibrated position (P 1 ) has been attained, the method  100  proceeds to step  108 . The method  100  otherwise repeats step  106 . 
     At step  108 , the controller  50  starts a timer (K+), which may be included as part of the hardware  55  of the controller  50  as shown in  FIG. 1 . As noted above with reference to  FIG. 2 , the time of transition between calibrated positions may be used to calculate the actual flow rate Q A  for recording in each of the lookup tables  52 , and therefore the timer steps of  FIG. 3  are important to this calculation. The method  100  proceeds to step  110  when the timer has started. 
     Step  110  may entail determining, from the received measured position signals P X , whether the clutch whose valve  24  is being evaluated has reached a second calibrated position (P 2 ). If so, the method  100  proceeds to step  111 . The method  100  otherwise repeats step  110  while the timer continues counting. 
     At step  111 , the controller  50  stops the timer that was previously initiated at step  108  before proceeding to step  112 . The value of the counter in the transition between points P 1  and P 2  may be recorded in memory M for use in calculating the actual flow rate Q A  for lookup tables  52  of  FIGS. 1 and 2 . 
     Step  112  entails determining, from the received measured position P X , whether the clutch being evaluated has attained a third calibrated position (P 3 ). If not, the method  100  repeats step  112 . Otherwise, the method  100  proceeds to step  114 . 
     At step  114 , the controller  50  commands a negative flow rate from the valve  24  of the clutch being evaluated, doing so in response to the determination at step  112  that the clutch CI 1 , CI 2 , or C 1  has reached the third calibrated position (P 3 ). That is, to arrive at step  114 , the controller  50  first determines at steps  106  and  110  that the clutch has passed the first and second position thresholds P 1  and P 2 , respectively, and is thus being fully applied. However, step  112  determines that the clutch has moved beyond the second calibrated position (P 2 ) to the third calibrated position (P 3 ). In response to this, the controller  50  may command an outflow of fluid  27  from the clutch so as to move the clutch, or rather its clutch apply piston, back in the direction of the second calibrated position (P 2 ). After commanding such an outflow, the method  100  proceeds to step  116 . 
     Step  116  may entail determining whether the clutch position, from the measured position signals P X , has changed and is now less than the third calibrated position P 3 , i.e., the clutch apply piston  11  for the clutch being evaluated is presently located between the second and the third calibrated positions P 2  and P 3 . If this is the case, the method  100  proceeds to step  117 , with the method  100  instead repeating step  116  if the position has not yet changed as expected. 
     Step  117  involves initiating the timer from zero anew before proceeding to step  118 . 
     At step  118 , the controller  50  of  FIG. 1  next determines if the position of the clutch CI 1 , CI 2 , or C 1 , which was previously commanded by the controller  50  to approach the second position P 2  by the negative flow (−Q) commanded at step  114 , has in fact reached the second calibrated position P 2 . If so, the method proceeds to step  119 . Otherwise, the controller  50  repeats step  118 . 
     At step  119 , the timer that was previously started again at step  117  is now stopped (K−). The elapsed time of the move from the third threshold position P 3  back to the second threshold position P 2  is recorded in memory M of the controller  50 . The method  100  thereafter proceeds to step  120 . 
     Step  120  may include determining if the clutch being evaluated has moved to below the first threshold position (P 1  ), i.e., to a position between fully exhausted and the first calibrated position (P 1 ). If not, the method  100  repeats step  120  and continues to exhaust pressure from the clutch. The method  100  proceeds to step  122  once the clutch being evaluated has moved past the first threshold position (P 1 ). 
     At step  122 , the controller  50  of  FIG. 1  next calculates the compensation scale factor F for the positive and negative flow rates that occurred in the execution of steps  102 - 120 . As explained above with reference to  FIG. 2 , calculation of the compensation scale factor F involves the use of the data recorded in the lookup tables  52 , i.e., commanded and actual flow rates Q CC  and Q A , respectively, through the filling or emptying of the clutch CI 1 , CI 2 , or C 1 . The method  100  proceeds to step  123  when this step is complete. 
     Step  123  may optionally include incrementing a test counter (K T   + ). Such a test counter, also available as part of the hardware  55  shown in  FIG. 1 , may be tied to how many pairs of test flow rates are commanded during testing. For example, in the lookup tables  52  shown in  FIG. 2  there are seven (7) pairs of commanded flow rates, i.e., [−3, 3], [−2, 2], [−1, 1], [−0.75, 0.75], [−0.50, 0.50], [−0.35, 0.35], and [−0.25, 0.25], which in this example represents commanded flow rates (Q CC ) in liters per minute. The actual number of test pairs may vary with the design depending on the level of granularity that is desired, and the flow rates may likewise vary depending on the design of the transmission  14 . The method  100  then proceeds to step  124 . 
     At step  124 , the controller  50  of  FIG. 1  next determines whether the count of the test counter incremented at step  123  indicates that a given pair of commanded flow rates has been tested, i.e., K T =VAL? The method  100  proceeds to step  126  if a given pair has been tested. Alternatively, the method  100  may proceed to step  126  only if all seven flow rates pairs have been tested, although once a pair has been tested, that particular flow rate may be adapted for the next shift of the transmission  14  requiring that flow rate. The method  100  proceeds instead to step  129  if a given pair of commanded flow rates has not been tested. 
     At step  126 , the controller  50  updates the lookup tables  52  of  FIGS. 1 and 2 , as indicated by  52 + in  FIG. 3 , using the data determined in the execution of steps  102 - 124 , and after resetting the test timer for the particular pair of commanded flow rates whose test has been completed, thereafter proceeds to step  128 . 
     Step  128  entails applying the compensation scale factor F to the commanded flow Q CC  from the last application of the clutch CI 1 , CI 2 , or C 1 , indicated as Q CCi  for an initial use of the method  100 , such Q CCi  for the initial or an immediately prior shift action involving a particular commanded flow rate is set equal to the adapted commanded flow rate, as abbreviated by Q CCi =Q CC  in  FIG. 3 . This step allows the flow control signals (I QC ) transmitted from the controller  50  to the valve  24 , upon the next application of a clutch controlled by the valve  24 , to be adjusted or adapted upward or downward as needed via the compensation scale factor F. 
     The compensation scale factor F may be further limited by calibration values, and set to 1 when the learning process enabled by method  100  is not complete for a given commanded flow rate pair or temperature region. Likewise, the commanded flow rate (Q CC ) may remain limited to levels allowed by any calibrated flow command limit tables of the controller  50 . The method  100  is then completed (**) for one control loop, repeating anew at step  102 . The method  100  therefore runs continuously, updating the lookup tables  52  over time in the background of any existing clutch control logic in a non-intrusive and computationally simple approach. 
     Step  129  may include using the prior value for the commanded flow rate (Q CCi ) in the next use of the evaluated clutch. This decision is made based on a determination at step  124  that the test counter did not indicate that an affected pair of commanded flow rates, e.g., [−3, 3] LPM, has been completely tested. The method  100  is then completed (**) for one control loop, repeating anew at step  102 . 
     Using the method  100  and controller  50  as described above, a vehicle such as the vehicle  10  of  FIG. 1  or any other fluidic system using a positioned-controlled clutch apply piston may enjoy certain performance improvements. Current technology is to characterize flow versus solenoid current of a flow control solenoid valve at a single pressure and temperature, with this information provided via a supplier of the valve as noted above. The present invention as described above instead provides a nonintrusive, computationally efficient way to periodically characterize the actual output flow rate and adjust a commanded flow rate via the lookup tables  52  of  FIG. 2  so as to compensate for flow variation over time. The result should be a more predictable output flow from a given valve  24 . In turn, the improved accuracy in output flow control should result in an improved quality or feel of the particular fluid powered action that is being controlled, whether that is a shift of the transmission  14  in the example vehicle  10  of  FIG. 1  as described above or any other action of a positioned-controlled fluidic device. 
     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.