Abstract:
A method is disclosed for determining off-temperature accuracy information of a solenoid fluid control valve. The method comprises performing a current sweep at a first temperature, and for at least one command pressure, determining an error in a control pressure based on the current sweep. The error may be determined at the first temperature. A metric may then calculated based on the current sweep, and a pressure error offset may be determined for a second temperature based on the metric. The pressure error offset may describe a difference between the error in control pressure at the first temperature and an expected error in control pressure at the second temperature.

Description:
FIELD OF INVENTION 
     The present invention relates to determining off-temperature pressure accuracy of a solenoid fluid control valve. 
     BACKGROUND 
     Solenoid fluid control valves are often used to control fluid pressure in a variety of systems, including clutch mechanisms and other devices in an automobile. The environment in which these actuators must function may have a wide temperature range, leading to variation in the solenoid fluid control valve&#39;s response to a given command current. To monitor this variation, when a manufacturer supplies a solenoid fluid control valve to a customer, the customer may request pressure accuracy information for a variety of temperatures. While the pressure accuracy information may be quickly obtained at a single temperature, performing an accuracy audit at a variety of temperatures may be time-consuming and inefficient. 
     SUMMARY 
     A method is disclosed for determining off-temperature accuracy information of a solenoid fluid control valve. The method comprises performing a current sweep at a first temperature, and for at least one command pressure, determining an error in a control pressure based on the current sweep. The error may be determined at the first temperature. A metric may then calculated based on the current sweep, and a pressure error offset may be determined for a second temperature based on the metric. The pressure error offset may describe a difference between the error in control pressure at the first temperature and an expected error in control pressure at the second temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an overview of the inventive temperature accuracy method; 
         FIG. 2  shows control pressure and fluid flow versus command current for a solenoid fluid control valve operated at 70° C.; 
         FIG. 3  shows pressure error versus command pressure for a solenoid fluid control valve operated at 70° C.; 
         FIG. 4  illustrates the average slope, slope liftoff , in the liftoff region of a control pressure versus command current curve; 
         FIG. 5  illustrates the maximum slope, slope max , of a control pressure versus command current curve; 
         FIG. 6  shows a look-up table giving pressure error offsets as a function of a command pressure and a flow and slope temperature (FST) factor; 
         FIG. 7  shows a measured pressure error versus command pressure for a solenoid fluid control valve operated at 70° C. and an expected pressure error versus command pressure for a solenoid fluid control valve operated at 110° C.; 
         FIG. 8  shows a measured pressure error versus command pressure for 100 solenoid fluid control valves operated at 110° C.; 
         FIG. 9  shows an expected pressure error versus command pressure for 100 solenoid fluid control valves operated at 110° C.; 
         FIG. 10  shows an average of the measured data plotted in  FIG. 8 , and an average of the expected data plotted in  FIG. 9 ; and 
         FIG. 11  shows an application specific integrated circuit (ASIC) that may be configured to perform the methods disclosed herein. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The solenoid actuator manufacturing process introduces variations in solenoid performance and response. These variations may be particularly pronounced at extreme temperatures. When a solenoid actuator is used in an automobile transmission, error in the output pressure for a given command current may result in lower shift quality. Accordingly, when a product is provided to a customer, the customer may request an off-temperature audit to confirm that the solenoid performs satisfactorily at a variety of temperatures. 
     A typical solenoid fluid control valve may have a coil, and an armature that moves in response to a current through the coil. The motion of the armature may result in the motion of a spool that regulates fluid flow from a supply port to a control port. The fluid pressure at the control port, knows as the “control pressure,” may be measured as a function of the current through the solenoid&#39;s coil. 
     In the prior art, a temperature audit of a solenoid fluid control valve may entail testing the valve&#39;s performance at a variety of temperatures. The valve may be placed in an environment set at a certain temperature, and a current sweep may be performed. As the current through the coil of the solenoid fluid control valve is increased and then decreased, the resulting control pressure for the changing current may be recorded. This information, or transfer function, may be used immediately to perform the audit. The audit may focus on a discreet number of control pressures chosen by the customer. For each of the control pressures, the corresponding current as indicated by the transfer function may be applied to the solenoid fluid control valve. The difference between the expected control pressure and the measured control pressure may be required to fall within an acceptable error range. 
     The temperature audit process can be time consuming, often requiring a few days for the audit of the solenoid fluid control valve to be requested, performed, and analyzed. The audit may also require the use of cold and hot chambers for taking data at various temperatures. These chambers may not always be available, further slowing the auditing process. While the audit may provide accurate data for the valve being tested, the information only applies to a single solenoid fluid control valve at the particular temperature that was tested. Accordingly, the process is highly inefficient when applied to a large quantity of solenoid fluid control valves at a variety of temperatures. 
     The inventive temperature accuracy audit method uses control pressure and flow signature data for a solenoid fluid control valve at a characterization temperature to determine a pressure error offset at a variety of other temperatures. The method may be used by a manufacturer to efficiently provide accuracy information to a customer. The information may be provided in the form of software. The customer may enter information into a plurality of required fields and the software may calculate an accuracy offset. 
     Referring to  FIG. 1 , the temperature accuracy method may begin with a sweep of the command current for a particular solenoid fluid control valve, and the corresponding control pressure and fluid flow for a plurality of command currents may be recorded (step  100 ). A pressure accuracy test may then be run to test the error in the command pressure (step  102 ). This step may be performed as part of step  100 , or as a separate process. A flow and slope temperature (FST) factor may be calculated based on the data collected during the command current sweep (step  104 ). This step may be performed by software provided to the customer. A supply pressure and temperature may then be chosen for the accuracy results (step  106 ). The supply pressure and temperature may be chosen by the customer and input to the software provided by the manufacturer. A lookup table may be used to determine a pressure error offset for the chosen supply pressure and temperature (step  108 ). This step may also be performed by the software. Finally, the pressure error offset may be applied to the accuracy data collected in step  102  to determine an expected pressure error at the chosen temperature (step  110 ). The software may provide the pressure error offset or the expected pressure error to the user. The method is described in detail below. 
     Referring to  FIG. 2 , the method begins with a sweep of the current supplied to the solenoid fluid control valve&#39;s coil. This current is referred to herein as the “command current.” The sweep may be performed at a temperature that is close to a normal operating temperature of the solenoid fluid control valve&#39;s environment. For example, if the solenoid fluid control valve is to be used in an automobile transmission, the sweep may be performed at the normal operating temperature of a transmission, approximately 70° C.-80° C. The sweep may also be performed with a pressure at the valve&#39;s supply port, i.e., “supply pressure,” that is close to the valve&#39;s supply pressure under normal operating conditions. The data in  FIG. 2  corresponds to a supply pressure of 896 kPa, and a temperature of 70° C. The curve  200  shows control pressure versus command current for an increasing command current. As current is applied to the solenoid fluid control valve, the armature becomes displaced, allowing a fluid flow through the valve that results in a non-zero control pressure. The curve  202  shows control pressure versus command current for a decreasing command current. The curves  204 ,  206  show the fluid flow versus command current for the increasing and decreasing command current, respectively. 
     Next, an audit is performed at the characterization temperature to measure the accuracy of the command current versus control pressure data shown in  FIG. 2 . A set of control pressures is chosen, and for each control pressure, the command current expected to produce that control pressure is applied to the solenoid. Because the chosen control pressures will be “commanded” by applying the corresponding command currents, they are referred to as “command pressures.” The resulting control pressure for each of the command pressures is measured, and the difference between the command pressure and measured control pressure is calculated. This difference is referred to as the “pressure error.” An example of the pressure error versus command pressure data  300  is shown in  FIG. 3 . As shown in  FIG. 3 , not all of the data point shown in  FIG. 2  may be accuracy tested. 
     Once the accuracy test has been performed, the control pressure and flow data shown in  FIG. 2  may be analyzed to determine a flow and slope temperature (FST) factor. The FST factor is given by 
                       FST   ⁢           ⁢   factor   ⁢           ⁢     (     ml   ⁢     /     ⁢   min     )       =           slope   liftoff     ⁡     (     kPA   ⁢     /     ⁢   A     )           slope     ma   ⁢           ⁢   x       ⁡     (     kPA   ⁢     /     ⁢   A     )         *       flow   peak     ⁡     (     ml   ⁢     /     ⁢   min     )           ,           Equation   ⁢           ⁢     (   1   )                 
where slope liftoff  is the average slope of the control pressure versus command current curve in a liftoff region of the current sweep, and slope max  is the maximum value of the slope of the control pressure versus command current curve over the whole current sweep. flow peak  is the peak fluid flow, the maximum value of the fluid flow through the valve.
 
       FIG. 4  shows the liftoff region  400 . An average slope, slope liftoff , of the command current versus control pressure curve  402  for increasing control pressure is determined for the liftoff region  400 . The liftoff region  400  is generally bounded by the command current required to open the fluid control valve, and the command current corresponding to the peak flow. The dashed line  404  illustrates a line whose slope equals slope liftoff , the average slope of curve  402  in the liftoff region  400 .  FIG. 5  shows the full current sweep shown in  FIG. 2 , and the dashed line  500  has a slope equal to slope max , the maximum slope at any point in the control pressure versus command current curve  502  for an increasing command current. 
     The FST factor of Equation 1 takes into account the fact that the slope of the control pressure versus command current curve in the liftoff region, slope liftoff , may not be equal to the maximum slope of the curve over the whole current sweep, slope max . The FST factor also takes into account the fluid flow through the valve, depending directly on the peak flow. 
     Once the FST factor is calculated, a look-up table may be used to determine command pressure accuracy information at temperatures and supply pressures other than the characterization temperature and supply pressure. The look-up table may be based on an average of measurements recorded for a large number of solenoid fluid control valves. Each valve may undergo the current sweep and temperature audit at a variety of temperatures, not just the characterization temperature. An FST factor may also be calculated for each valve based on data taken at the characterization temperature. A table may then be created with information for a variety of off-temperatures, temperatures not equal to the characterization temperature. For each off-temperature, the FST factors and audit information for large quantity of valves may be combined to create a look-up table for that temperature. 
     The supply pressure may also be varied, and the look-up tables may be defined for a specific supply pressure as well as temperature. An example table is shown in  FIG. 6  for a temperature of 110° C. and a supply pressure of 896 kPa. A pressure error offset is given based on the FST factor (top row) and the command pressure (left-most column). 
     Referring to the table in  FIG. 6 , for a command pressure of 228 kPa in a solenoid with an FST factor of 1.3 ml/min, a pressure error offset of −2.067 kPa may be expected.  FIG. 6  shows the pressure error offset for ten additional command pressures. The pressure error offsets may be applied to the command pressure accuracy plot in  FIG. 3  to estimate a command pressure error at 110° C. This concept is illustrated in  FIG. 7 . 
       FIG. 7  shows the pressure error versus command pressure  700  for a solenoid at 70° C. This data, also shown in  FIG. 2 , is taken after the initial sweep of the solenoid is performed.  FIG. 7  also shows the expected pressure error versus command pressure  702  for solenoid at 110° C. This data has not been directly measured, but has been estimated using the method described above. The table in  FIG. 6  provides an expected difference, or offset, between each pair of data points in  FIG. 7 . For a command pressure of 228 kPa and an FST factor of 1.3, the table indicates a pressure error offset of −2.067. This means that at 110° C., the pressure error at 304 kPa may be expected to differ from the pressure error at 70° C. by −2.067 kPa. 
     The temperature accuracy data  300  in  FIG. 3  indicates that for the particular solenoid fluid control valve being tested, the pressure error for a control pressure of 228 kPa at 70° C. is 3.5 kPa. This valve has an FST factor Of approximately 1.3 ml/min. The pressure error offset table in  FIG. 6  indicates that at 110° C., the pressure error for the solenoid fluid control valve is 3.5 kPA−2.067 kPa=1.433 kPa. 
     While the table in  FIG. 6  only shows data for a discreet set of command pressures and FST factors, pressure error offsets for other values may be determined from the information provided. For example, for an FST factor of 1.4 ml/min, an average of the pressure error offsets for FST factors 1.3 and 1.5 may be used. Additionally, more or fewer command pressures and FST factors may be included in the table. 
       FIG. 8  shows pressure error versus command pressure data for 100 solenoid fluid control valves operated at 110° C. For each line, a solenoid fluid control valve was placed in a 110° C. environment, and a first current sweep was performed. The pressure error was then measured based on the current sweep for 10 command pressures, still in the 110° C. environment. 
       FIG. 9  shows analytical data for the same 100 solenoid fluid control valves. The data in  FIG. 9  has been calculated using the accuracy model described herein. For each line in  FIG. 9 , a solenoid fluid control valve was placed in a 70° C. environment, and a first current sweep was performed. The pressure error was then measured based on the current sweep for 10 command pressures, still in the 70° C. environment. The FST factor was calculated, and a pressure error offset was determined for each of the 10 command pressures based on a look-up table, such as the table shown in  FIG. 6 . This offset was added to the pressure error measured at 70° C. to estimate a pressure error at 110° C. The estimated pressure error is plotted as a function of command pressure in  FIG. 9 . 
     The average of the 100 measured and estimated pressure errors is plotted in  FIG. 10 . The average of the measured data  1000  differs only slightly from the average of the estimated data  1002 , indicating that the accuracy model provides a good off-temperature estimate of the pressure error. 
     The methods described herein may be performed in part or in whole by software implemented on an application specific integrated circuit (ASIC) or any other type of processor. Referring to  FIG. 11 , the software may be implemented on an ASIC  1100  configured to receive as input the data  1102  from the sweep of the command current, the data  1104  from the pressure accuracy test, and the temperature  1106  for which the error offsets will be estimated. The supply pressure  1108  for which the error offset will be estimated may also be input. The software implemented on the ASIC  1100  may use the input data to determine the FST factor, and may provide as output  1110  a series of pressure error offsets, such as a single column of the data shown in  FIG. 6 . The offsets may correspond to the calculated FST factor. Alternatively, the software my add the pressure error offsets to the pressure error data indicated in the data  1104  for the pressure accuracy test, and may provide as output  1110  the estimated pressure error as a function of command pressure, like the data  702  shown in  FIG. 7 . 
     A manufacturer of solenoid fluid control valves may provide the software to a customer. The data shown in  FIGS. 2 and 3  may also be provided to the customer for each solenoid fluid control valve. The customer may then use the software and the data to estimate a device&#39;s command pressure accuracy. This eliminates the need for the manufacturer and/or the customer to measure the device&#39;s accuracy at multiple temperatures. Instead, a single current sweep and accuracy analysis may be performed at a characteristic temperature, and accuracy information for other temperatures may be determined based on the current sweep and accuracy analysis.