Abstract:
A method for calibrating a physical test cell includes the steps of: determining a set of inputs to be provided to the physical test cell based in part on a set of historical test data; providing the inputs to the physical test cell and receiving a set of outputs associated therewith, wherein the providing includes implementing a sequential space filling sampling procedure to substantially cover a region defined by the set of historical values; creating a virtual test cell comprising one or more response surfaces based on the set of outputs; and interrogating the virtual test cell to determine a calibration relationship between at least one of the inputs and at least one of the outputs. Smooth Kriging may be used to determine the virtual test cell.

Description:
TECHNICAL FIELD 
     The present invention generally relates to automated testing systems, and more particularly relates to testing systems incorporating a virtual test cell methodology for calibration of electromechanical components such as automotive powertrains and the like. 
     BACKGROUND 
     Automated testing procedures—particularly those used to characterize complicated electromechanical systems—are often costly and time-consuming. As technology levels increase, the number of variables and complexity associated with such components likewise dramatically increases. 
     In the case of powertrains and other automotive components, for example, there exist so many inputs and outputs associated with the system that the number of measurements required to properly calibrate even one aspect may take weeks to complete. A modern diesel engine, for example, may have as many as thirteen variables requiring calibration. Systematic and exhaustive measurements of such a system would be prohibitively time-consuming and expensive. 
     Prior art methods typically address this problem by performing individual tests for each calibration of interest. Design of Experiments (DOE) techniques are used to achieve test efficiency by limiting test conditions to a small set, with the assumption that certain inputs to not interact in their effect on the outputs, and/or that the test outputs follow a presumed mathematical relationship. These prior art testing methodologies may not properly take into account interaction between multiple variables, and therefore do not produce models amenable to optimization—e.g., optimization with respect to fuel economy, performance, quality, etc. 
     Accordingly, there is a need for improved methods and systems for calibration and testing of complicated electromechanical systems. 
     BRIEF SUMMARY 
     In general, the present invention provides methods and apparatus for automated testing of a physical test cell using a virtual test cell methodology. In general, and as described in further detail below, a limited set of test data is sequentially and strategically collected from the test cell, and a mathematical model is created based on the measured responses. The resulting model or set of models composes a virtual test cell which itself can be measured at points that were not included in the limited set of original test data. In this way, the time and expense associated with component calibration can be significantly reduced. 
     In accordance with one embodiment, a method for calibrating a physical test cell, includes the steps of: determining a set of inputs to be provided to the physical test cell based in part on a set of historical test data; providing the inputs to the physical test cell and receiving a set of outputs associated therewith, wherein the providing includes implementing a sequential space filling sampling procedure to substantially cover a region defined by the set of historical values; creating a virtual test cell comprising one or more response surfaces based on the set of outputs; and interrogating the virtual test cell to determine a calibration relationship between at least one of the inputs and at least one of the outputs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  is a general block diagram showing an overview of an exemplary test system; 
         FIG. 2  is a flowchart showing a virtual test cell test method in accordance with one embodiment; 
         FIG. 3  illustrates example conceptual plots relating to an exemplary sequential space-filling sampling plan; 
         FIG. 4  illustrates an exemplary test plan region in accordance with one embodiment; 
         FIG. 5  illustrates an exemplary space-filling in accordance with one embodiment; and 
         FIG. 6  further illustrates exemplary space-filling procedure. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is directed to automated testing of components (or “physical test cell”) using a virtual test cell methodology wherein a limited set of test data is collected from the physical test cell using a space-filling procedure, and a mathematical model is created based on the measured responses. The resulting model or set of models composes a virtual test cell which itself can be measured at points that were not included in the limited set of original test data. 
     As a preliminary matter, the following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Thus, while the present invention may be described in conjunction with engine powertrains and other automotive components, the present invention is not so limited, and may be employed to measure and calibrate any suitable electromechanical system. 
     Furthermore, embodiments of the invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more computers, microcontrollers, or other control devices. 
     For the sake of brevity, conventional techniques related to data processing, curve fitting, automotive components, and other well-known functional aspects of such systems may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. 
     The following description may refer to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature is directly joined to (or directly communicates with) another element/node/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically. The term “exemplary” is used in the sense of “example,” rather than “model.” Although the figures may depict example arrangements of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the invention. 
     Referring to  FIG. 1 , a simplified, conceptual test environment useful in describing one embodiment of the present invention generally includes two parts: a test system  102  and a physical test cell (e.g., a powertrain)  104  communicatively coupled to test system  102 . The term “powertrain” is used as an example physical test cell herein without loss of generality. Test system  102 , which will typically include one or more databases  103 , provides one or more inputs  110  to powertrain  104  in any suitable fashion, and simultaneously measures one or more outputs  120 . Suitable sensors (not shown) and actuators are typically used to measure outputs  120  and provide inputs  110  to powertrain  104 . 
       FIG. 2  provides a general method  200  in accordance with the present invention. First, in step  202 , a powertrain characterization test  202  is performed. This involves, initially, determining all measurements that can be made from the physical test cell calibration of powertrain  104 . Only the variable ranges for step  202  need to be specified (e.g., 600 rpm-6000 rpm). In contrast, traditional testing requires discrete variable levels to be specified (e.g., 600, 1000, 2000, 3000 rpm etc.) need not be specified. Accuracy required from the calibration drives the quantity of physical test rather than following a fixed test plan in the traditional process. 
     A virtual test cell  204  is then created, preferably using a sequential space-filling sampling method. That is, the system calibrates the virtual test cell to produce desired level of accuracy of output using: (a) efficient test plans with space-filling sampling, (b) historical boundaries for similar powertrains to create safe and stable test regions, (c) conditions for each test point chosen in a space filling sequence that systematically samples the entire operating region, (d) physical test measurements conducted at predetermined monitoring points chosen for gauging prediction errors of the response surface model associated with the virtual test cell, (e) response surface models (for each output) employing Kriging interpolation, and (f) convergence checks on each model. 
     Finally, various calibrations  210  are created using the virtual test cell ( 204 ). This involves “running” the virtual test cell for all desired test points, including points where no measurements were made in conjunction with step  202 . The virtual test cell can then be “run” as many times as required to produce data at all the points required including points at which no measurements were made. This step may employ mathematical optimization on the outputs of the virtual test cell using objective functions and constraints. In the illustrated embodiment in the context of a powertrain, calibration steps  210  include a torque model calibration, and a cam phaser calibration. Any number of such calibration steps may be performed in this manner. 
     Finally mathematical optimization is used to produce calibrations that optimize fuel economy, performance, quality, regulatory metrics etc. As mentioned above, when there are many variables that can be changed simultaneously, as is increasingly the case with advanced powertrain technologies, manual calibrations are far from optimal. 
     The described method using the “sequential space filling” test sampling plans is substantially more efficient and has been found to require approximately 15%-80% less data than traditional test plans. Many separate tests that were needed in the traditional process can be eliminated. 
       FIG. 3  depicts a sequential response surfaces corresponding to a sequential space-filling method in accordance with a simplified embodiment. As mentioned above, a space filling sequence systematically samples all or nearly all of the operating region. That is, the measured output is a response surface that progressively changes (in plots  302 ,  304 ,  306 , and  308 ) as the number of test points increases. In this illustration, in the interest of clarity, there are only two inputs (“input  1 ” and “input  2 ”). Those skilled in the art will recognize that a typical system may include many more such inputs. 
     In the first plot,  302 , a set of 10 points  312  are illustrated, with eight corresponding to the boundary. As space filling increases, twenty more points are selected based on the previous data points, resulting in a set  314  of 30 data points. Next, as the number of points increases to 70 points, the response surface plot  306  takes on a more detailed form. Additional points, as shown in plot  308  corresponding to set of points  318 , are used to monitor how much the response surface changes as new sampled points are added in the space-filling sequence. Physical measurements are not taken at these points; instead, the system only makes predictions of the outputs at the monitoring points and then tracks the extent to which these predictions change as new sampled points are added. When the predictions change very little, it is determined that the response surface has “converged,” providing the system with a suitable model. As mentioned previously, the monitoring points themselves are not used as data to model the response surface, but are used only to check for convergence. 
       FIGS. 5 and 6  show a more detailed space-filling process, expanding on what was illustrated in  FIG. 3 . That is,  FIG. 5  shows the first ten points ( 320 - 329 ) distributed within a sequenced, strength-two orthogonal array. Similarly,  FIG. 6  shows the order in which the points are added (p 1 , p 2 , p 3 , . . . , p 16 ) for the first sixteen points. Points are added one-by-one, but the monitoring points are not physically tested. Rather, the monitoring points are used as input locations to predict the output of the Kriging response surface model. They are usually located in finer grid points of the original input domain. Convergence checks are done by calculating the root mean square (RMS) of prediction errors from two consecutive Kriging response models. The difference between RMS values determines whether convergence has occurred. Thus, if the RMS difference between two consecutive surfaces gets within a pre-specified tolerance, then no further physical testing is needed. 
     The method by which space filling is achieved may be selected in accordance with the desired efficiency and accuracy. In one embodiment, for example, the system maximizes the minimum distance between the new point and the surrounding points (using a penalty measure). 
       FIG. 4  shows the use of historical boundaries for the same or similar powertrains to determine a safe and stable testing region. More particularly, a set of historical test data set  402  is provided. The boundary  404  of this data is then determined. This boundary may be a convex hull shape, but in any event is preferably a closed polygon that substantially surrounds the majority of data points in data set  402 . A set of data points  403  associated with a design of experiments (DOE) test plan is also provided. This test plan may include a regular array (in n-dimensional space) of data points, as shown, or may have any other suitable configuration. The boundary  404  is then superimposed on the DOE test plan  403  to provide an intersection of the two sets, resulting in the desired stable test plan  406 . 
     Table 1 below shows exemplary benefits of this method for test point reduction, which goes up dramatically as the number of variables increase. For example, a Diesel engine may have as many as thirteen variables for calibration, while a modern gasoline engine may have three such variables. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example test-point reduction 
               
             
          
           
               
                   
                   
                 % Reduction in Number of 
               
               
                 Number of 
                   
                 Test Points compared to 
               
               
                 Variables 
                 Examples 
                 traditional test 
               
               
                   
               
               
                 2 
                 No Cam Phasers (eg. Load, 
                 15-20% 
               
               
                   
                 Speed) 
               
               
                 3 
                 Dual Equal Cam Phaser 
                 50% 
               
               
                   
                 Engine 
               
               
                 4 
                 Dual Independent Cam 
                 80% 
               
               
                   
                 Phaser Engine 
               
               
                 5 or more 
                 SIDI, Diesel 
                 &gt;95%   
               
               
                   
               
             
          
         
       
     
     The methods described herein may be applied regardless of whether evaluations are done through test or through Computer Aided Engineering (CAE). 
     In one example, the described method was used in connection with torque model calibration of a powertrain. Using traditional methods this torque model testing would typically take approximately forty days to complete using a set of dynamometers. By using the virtual test cell methodology disclosed herein, however, the number of data points needed was reduced from 2688 to 600 and the overall duration of the procedure was reduced from forty days to nine days with a significant cost savings. In addition, the virtual test cell constructed from the torque model test allows several other time-consuming independent physical tests for cam phaser calibration, volumetric efficiency calibration, and the like to be eliminated. 
     While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention, where the scope of the invention is defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.