Patent Publication Number: US-2015073601-A1

Title: Test system having a compliant actuator assembly and iteratively obtained drive

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/875,645 filed Sep. 9, 2013. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a control of a system, machine or process. More particularly, the present invention relates to calculating a model to be used for generating drive signals as input to a vibration or other controlled system. 
     Vibration systems that are capable of simulating loads and/or motions applied to test specimens are generally known. Vibration systems are widely used for performance evaluation, durability tests, and various other purposes as they are highly effective in the development of products. For instance, it is quite common in the development of automobiles, motorcycles, or the like, to subject the vehicle or a substructure thereof to a laboratory environment that simulates operating conditions such as a road or test track. Physical simulation in the laboratory involves a well-known method of data acquisition and analysis in order to develop drive signals that can be applied to the vibration system to reproduce the operating environment. This method includes instrumenting the vehicle with transducers “remote” to the physical inputs of the operating environment. Common remote transducers include, but are not limited to, strain gauges, accelerometers, and displacement sensors, which implicitly define the operating environment of interest. The vehicle is then driven in the same operating environment, while remote transducer responses (internal loads and/or motions) are recorded. During simulation with the vehicle mounted to the vibration system, actuators of the vibration system are driven so as to reproduce the recorded remote transducer responses on the vehicle in the laboratory. 
     However, before simulated testing can occur, the relationship between the input drive signals to the vibration system and the responses of the remote transducers must be characterized in the laboratory. Typically, this “system identification” procedure involves obtaining a respective model or transfer function of the complete physical system (e.g. vibration system, test specimen, and remote transducers) hereinafter referred to as the “physical system”; calculating an inverse model or transfer function of the same; and using the inverse model or transfer function to iteratively obtain suitable drive signals for the vibration system to obtain substantially the same response from the remote transducers on the test specimen in the laboratory situation as was found in the operating environment. 
     As those skilled in the art would appreciate, this process of obtaining suitable drive signals is not altered when the remote transducers are not physically remote from the test system inputs (e.g. the case where “remote” transducers are the feedback variables, such as force or motion, of the vibration system controller). 
     Although the above-described system and method for obtaining drive signals for a vibration system has enjoyed substantial success, there is a continuing need to improve such systems. In particular, there is a need to improve models of the physical system and the process for obtaining the drive signals. 
     SUMMARY 
     This Summary and the Abstract herein are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the Background. 
     A first aspect of the present invention is a test that includes a physical test rig having a compliant actuator assembly responsive to a drive signal and a test specimen operably connected to the compliant actuator assembly. A non-transitory computer storage device is provided and is configured to operate with a processor to execute instructions stored thereon so as to apply a test drive signal to the physical test rig. An actual response signal of the physical test rig and the test specimen to the test drive signal is obtained and an error as a function of the actual response signal and a selected response signal is calculated. If the error has not reached a selected threshold a new drive signal based on the error and a relaxation gain factor is obtained. The new drive signal is obtained and applied until the error reaches the selected threshold. 
     A second aspect is a method for operating a test system that includes applying a test drive signal to a physical test rig having a compliant actuator assembly for imparting loads to a test specimen. An actual response signal of the physical test rig and the test specimen to the test drive signal is obtained and an error as a function of the actual response signal and a selected response signal is calculated. If the error has not reached a selected threshold a new drive signal based on the error and a relaxation gain factor is obtained. The new drive signal is obtained and applied until the error reaches the selected threshold. 
     One or more of the following features can be provided in further embodiments of the aspects described above. 
     The relaxation gain factor is greater than 0.5, and preferably is greater than 0.65, and more preferably is greater than 0.75, and yet even more preferably is greater than 0.8. By being able to use a relaxation gain factor than that previously used before, the overall number of iterations necessary to obtain the drive using an iterative process such as those discussed below has been significantly reduced when compared to a test system not having such compliant actuator assemblies. 
     The method and test system are not limited by the type of model used. For example and without limitation, a linear or non-linear model can be configured for use with the physical rig and the test specimen, and wherein the new drive signal is obtained based on the error, the linear or nonlinear model and the relaxation gain factor. 
     The compliant actuator assembly can comprise one or more actuators each having a spring connecting the actuator to the test specimen to provide compliance; and/or include an accumulator. The accumulator can be fluidly or mechanically coupled to each of the chambers or piston of a double-acting actuator. The accumulator(s) introduce a spring effect to an otherwise substantially rigid actuator. Each accumulator can include a first portion of compressible fluid (typically, a gas such as nitrogen, mechanical spring, or other resilient media or device,) and a second portion that is filled with a liquid, which compared to the gas, is substantially incompressible. The second portion of each accumulator  164  is fluidly coupled to a bore or mechanically coupled to the piston. Commonly, a diaphragm (or equivalent separating device such as a piston) is provided in each accumulator to maintain separation of the spring device or media and the liquid. Using hydraulic accumulators, typically but not exclusively pre-charged with nitrogen gas or mechanical elements, allows tuning of the spring stiffness (i.e. the compliance) of the actuator assembly to match requirements of a specific test specimen. 
     The compliance of the compliant actuator assembly can be adjustable, and/or, if desired, the compliant actuator assembly is more compliant than the test specimen in one or more degrees of freedom. 
     Other design considerations of the actuator assembly can also be used to obtain desired performance. For instance, any or all of the area ratios between the accumulator effective area and the area of the piston, the mass of the accumulator piston and/or the velocity of oil entering/exiting the accumulator can be used to tune the compliance to be effective at low frequencies yet become substantially inert or at least substantially stiffer at higher frequencies that warrant less compliance and more stiffness. 
     A particular advantage of the method and test system described above in any of the foregoing embodiments is that the test specimen in the test system can be replaced with a new test specimen being similar but different than the test specimen. The drive signal that corresponds to the error reaching the selected threshold is applied to conduct testing on the new test specimen. In prior art systems, a new drive signal would need to be generated, which takes a considerable amount of time. Instead, because of the compliant actuator assembly the same drive signal can be used on similar but different test specimens. 
     As used herein, “similar but different test specimens” are test specimens having the overall same structure for use in the test specimen, but each similar but different test specimen is different in at least one respect such as but not limited to different structure, element, material, operating parameter characteristic, value, setting or adjustment. Stated in another way, two test specimens are similar but different if test results obtained from each test specimen are suitable when the same drive signal is used to test each of the test specimens. If the same afore-mentioned test specimens are used in the test system that is otherwise substantially the same but does not include one or more compliant actuator assemblies and the test results obtained would not be suitable if the same drive signal is applied to each test specimen, then the two test specimens are similar but different. 
     The method and test system are particularly advantageous for a test specimen that is at least a portion of a vehicle, wherein at least one of the compliant actuator assembly is configured to apply a load upon said at least a portion of the vehicle, particularly in a direction corresponding substantially to forward motion or motion being lateral to forward motion of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a prior art test system. 
         FIG. 2  is a schematic diagram of a suitable computing environment. 
         FIG. 3A  is a flow chart illustrating the steps involved in an identification phase of a prior art method of vibration testing. 
         FIG. 3B  is a flow chart illustrating the steps involved in an iterative phase of a prior art method of vibration testing. 
         FIG. 3C  is a flow chart illustrating the steps involved in another iterative phase of a prior art method of vibration testing. 
         FIG. 4A  is a detailed block diagram of a prior art iterative process for obtaining drive signals for a vibration system with an adjuster. 
         FIG. 4B  is a detailed block diagram of another prior art iterative process for obtaining drive signals for a vibration system with the adjuster of the present invention. 
         FIG. 5  is a schematic block diagram of a test system having an aspect of the invention. 
         FIG. 6  is a schematic diagram of a compliant actuator assembly. 
         FIG. 7  is a schematic block diagram of a physical test rig having an aspect of the invention. 
         FIG. 8  is a schematic block diagram illustrating an identification phase of a prior art method of vibration testing. 
         FIG. 9  is a schematic block diagram illustrating an iterative phase of a prior art method of vibration testing. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  illustrates a physical system  10 . The physical system  10  generally includes a vibration system  13  comprising a servo controller  14  and an actuator  15 . In the schematic illustration of  FIG. 1 , the actuator  15  represents one or more actuators that are coupled through a suitable mechanical interface  16  to a test specimen  18 . The servo controller  14  provides an actuator command signal  19  to the actuator  15 , which in turn, excites the test specimen  18 . Suitable feedback  15 A is provided from the actuator  15  to the servo controller  14 . One or more remote transducers  20  on the test specimen  18 , such as displacement sensors, strain gauges, accelerometers, or the like, provide a measured or actual response  21 . A physical system controller  23  receives the actual response  21  as feedback to compute a drive  17  as input to the physical system  10 . In one embodiment of an exemplary iterative process discussed below, the physical system controller  23  generates the drive  17  for the physical system  10  based on the comparison of a desired response provided at  22  and the actual response  21  of the remote transducer  20  on the test specimen  18 . Although illustrated in  FIG. 1  for the single channel case, multiple channel embodiments with response  21  comprising N response components and the drive  17  comprising M drive components are typical and considered another embodiment of the present invention. 
       FIG. 2  and the related discussion provide a brief, general description of a suitable computing environment in which the invention may be implemented. Although not required, the physical system controller  23  will be described, at least in part, in the general context of computer-executable instructions, such as program modules, being executed by a computer  30 . Generally, program modules include routine programs, objects, components, data structures, etc., which perform particular tasks or implement particular abstract data types. The program modules are illustrated below using block diagrams and flowcharts. Those skilled in the art can implement the block diagrams and flowcharts to computer-executable instructions. Moreover, those skilled in the art will appreciate that the invention may be practiced with other computer system configurations, including multi-processor systems, networked personal computers, mini computers, main frame computers, and the like. The invention may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computer environment, program modules may be located in both local and remote memory storage devices. 
     The computer  30  illustrated in  FIG. 2  comprises a conventional personal or desktop computer having a central processing unit (CPU)  32 , memory  34  and a system bus  36 , which couples various system components, including the memory  34  to the CPU  32 . The system bus  36  may be any of several types of bus structures including a memory bus or a memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The memory  34  includes read only memory (ROM) and random access memory (RAM). A basic input/output (BIOS) containing the basic routine that helps to transfer information between elements within the computer  30 , such as during start-up, is stored in ROM. Non-transitory computer readable storage devices  38 , such as a hard disk, an optical disk drive, ROM, RAM, flash memory cards, digital video disks etc., are coupled to the system bus  36  and are used for storage of programs and data. Commonly, programs are loaded into memory  34  from at least one of the storage devices  38  with or without accompanying data. 
     An input device  40  such as a keyboard, pointing device (mouse), or the like, allows the user to provide commands to the computer  30 . A monitor  42  or other type of output device is further connected to the system bus  36  via a suitable interface and provides feedback to the user. The desired response  22  can be provided as an input to the computer  30  through a communications link, such as a modem, or through the removable media of the storage devices  38 . The drive signals  17  are provided to the physical system  10  of  FIG. 1  based on program modules executed by the computer  30  and through a suitable interface  44  coupling the computer  30  to the vibration system  13 . The interface  44  also receives the actual response  21 . 
     Before describing the present invention, it may also be helpful to review, in detail, an exemplary known method for modeling the physical system  10  and obtaining the drive  17  to be applied thereto. Although described below with respect to a test vehicle, it should be understood that this prior art method and the present invention discussed below are not confined to testing only vehicles, but can be used on other processes, types of test specimens and substructures or components thereof. In addition, the description is done assuming spectral analysis based modeling estimation and implementation though operations can be carried by several other mathematical techniques (e.g. Adaptive Inverse Control (AIC) type models, parametric regression techniques such as Auto Regressive Exogenous (ARX) and State Space types of models, or combinations thereof). 
     Referring to  FIG. 3A , at step  52 , the test vehicle is instrumented with the remote transducers  20 . At step  54 , the vehicle is subjected to the field operating environment of interest and the remote transducer responses are measured and recorded. For instance, the vehicle can be driven on a road or test track. The measured remote transducer responses, typically analog, are stored in the computer  30  in a digital format through analog-to-digital converters, as is commonly known. 
     Next, in an identification phase, the input/output model of the physical system  10  is determined. This procedure includes providing drive  17  as an input to the physical system  10  and measuring the remote transducer response  21  as an output at step  56 . The drive  17  used for model estimation can be random “white noise” having frequency components over a selected bandwidth. At step  58 , an estimate of the model of the physical system  10  is calculated based on the input drive applied and the remote transducer response obtained at step  56 . In one embodiment, this is commonly known as the “frequency response function” (FRF). Mathematically, the FRF is a N×M matrix wherein each element is a frequency dependent complex variable (gain and phase versus frequency). The columns of the matrix correspond to the inputs, while the rows correspond to the outputs. As appreciated by those skilled in the art, the FRF may also be obtained directly from prior tests using the physical system  10  or other systems substantially similar to the physical system  10 . 
     An inverse model H(f) −1  is needed to determine the physical drive  17  as a function of the remote responses at step  60 . As appreciated by those skilled in the art, the inverse model can be calculated directly. Also, the term “inverse” model as used herein includes a M×N “pseudo-inverse” model for a non-square N×M system. Furthermore, different forward models H and the inverse models H(f) −1  can be used such as regions with “brakes on” and “brakes off” in a spindle coupled vehicle test system. At this point in the prior art, the method enters an iterative phase, illustrated in  FIGS. 3B and 4A , to obtain drive  17  which produces actual response  21  that ideally replicates the desired remote transducer response  22  (hereinafter “desired response”). The inverse physical system model H(f) −1  is represented at  72 , while physical system (vibration system, test vehicle, remote transducers and instrumentation) is represented at  10 . Referring to  FIG. 3B , at step  78 , the inverse model  72  is applied to a target response correction  77  in order to determine an initial drive  17  x 1 (t). The target response correction  77  can be the desired response  22  for the initial drive, though most often it is reduced by a relaxation gain factor  95 . The calculated drive  17  x 1 (t) from the inverse model  72  is then applied to the physical system  10  at step  80 . The actual remote transducer response  21  (hereinafter “actual response”) y 1 (t) of the physical system  10  to the applied drive  17  x 1 (t) is then obtained at step  86 . If the complete physical system  10  is linear (allowing a relaxation gain  95  of unity), then the initial drive  17  x 1 (t) could be used as the required drive. However, since physical systems are typically non-linear, the correct drive  17  has to be arrived at by an iterative process. (As appreciated by those skilled in the art, drive  17  used in previous tests for a similar physical system may be used as the initial drive.) 
     The iterative process involves recording the first actual response y 1 (t) resulting from the initial drive x 1 (t) and comparing it with the desired response  22  and calculating a response error  89  Δy 1  as the difference at step  88 . (The first actual response signal y 1 (t) is provided at  87  in  FIG. 4A .) The response error  89  Δy 1  is compared to a preselected threshold at step  90  and if the response error  89  exceeds the threshold an iteration is performed. Specifically the response error  89  Δy 1  is reduced by the relaxation gain factor  95  to provide the new target response correction  77 . In this embodiment, the inverse transfer function H(f) −1  is applied to the new target response correction  77  to create a drive correction Δx 2    94  (step  91 ) that is added to the first drive x 1 (t)  17 A to give a second drive x 2 (t)  17  at step  92 . The iteration process (steps  80 - 92 ) is repeated until the response error  89  is brought down below the preselected threshold on all channels of the response. The last drive  17 , which produced a response  21 , that was within the predetermined threshold of the desired response  22 , can then be used to perform specimen testing. 
     As described, the response error  89  Δy is commonly reduced by the relaxation gain factor (or iteration gain)  95  to form the target response correction  77 . The iteration gain  95  stabilizes the iterative process and trades off rate-of-convergence against iteration overshoot. Furthermore, the iteration gain  95  minimizes the possibility that the test vehicle will be overloaded during the iteration process due to non-linearities present in the physical system  10 . As appreciated by those skilled in the art, an iteration gain can be applied to the drive correction  94  Δx and/or the response error  89 . It should be noted in  FIG. 4A  that storage devices  38  can be used to store the desired response  22 , the actual responses  21  and previous drives  17 A during the iterative process. Of course, memory  34  can also be used. Also, a dashed line  93  indicates that the inverse model  72  is an estimate of the inverse of the physical system  10 . The block diagram of  FIG. 4A , as discussed above, can be implemented by those skilled in the art using commercially available software modules such as included with RPCIII™ from MTS Systems Corporation of Eden Prairie, Minn. 
     At this point, a modified method of the prior art for calculating the drive can also be discussed. The modified prior art method includes the steps of the identification phase illustrated in  FIG. 3A  and many of the steps of the iterative phase illustrated in  FIG. 3B . For convenience, the iterative steps of the modified method are illustrated in  FIG. 3C  and the block diagram as illustrated in  FIG. 4B . As illustrated in  FIG. 4B , the calculation of the target response correction  77  is identical. However, if the response error  89  between the actual response  21  and the desired response  22  is greater than a selected threshold, then the target response correction  77  is added to a previous target response  79 A at step  97  to obtain a new target response  79  for the current iteration. The inverse model  72  is applied to the target response  79  to obtain the new drive  17 . As illustrated in  FIG. 4B , the iteration gain  95  can be used for the reasons discussed above. 
       FIGS. 4A and 4B  generally illustrate another type of iterative process that includes an adjuster  100  that operates during each step of the iterative process, to improve the physical system inverse model  72 . This process is described in detail in U.S. Pat. No. 7,031,949, which is hereby incorporated by reference in it&#39;s entirety. Generally, as illustrated in  FIG. 4A , the adjuster  100  corrects the inverse model  72  which receives the target response correction  77  directly as a simple function of the response error  89  (i.e. without previous target information  79 A of  FIG. 4B ) and where the physical system drive  17  comprises drive correction  94  in combination with a previous drive  17 A. Conversely, as illustrated in  FIG. 4B , the inverse model  72  receives the target response  79  as the combination of the target response correction  77  and the previous target response  79 A, and drive  17  is directly obtained by applying the inverse model  72 . In the case of  FIG. 4B , the adjuster  100  corrects the inverse model  72  in a conceptually identical fashion as in  FIG. 4A . However, the configurations of  FIGS. 4A and 4B  render different signals available to the virtual identity modeling process described in U.S. Pat. No. 7,031,949, each with inherent situational advantages. The adjuster  100  can also operate in an iterative manner. 
     Generally, an aspect of the invention is schematically illustrated in  FIG. 5 , which is similar to  FIG. 1 ; however, actuator  15  has been replaced with a compliant actuator assembly  150 . When embodied in a test rig to generate loads upon the test specimen simulating actual loads seen by the test specimen, the compliant actuator assembly  150  must be able to generate high loads at high frequencies commonly seen in such systems. However, the compliant actuator assembly  150  exhibits a low stiffness spring characteristic such that displacement of the test specimen  18  can be accommodated. In the schematic diagram of  FIG. 5 , such characteristics may not be appreciated; however when applied in a test system for applying loads in multiple degrees of freedom such as a road simulator having one or more vehicle spindles each with a vehicle spindle test fixture  200  illustrated in  FIG. 7  that applies loads to the vehicle spindle to simulate the vehicle traveling along a course, such compliance in one or more degrees of freedom, and in particular for horizontal loads (those in the direction of simulated forward motion of the vehicle and lateral thereto and also camber and steer moments), has been found to be very advantageous. Providing compliant actuator assemblies in the load path for one or more horizontal loads when testing a vehicle may allow the tester to test vehicle components having the same function but different characteristics, for example axle or stabilizer bushings of different stiffness without having to record unique test data at step  54  and generate unique drives using the iterative process such as described above for each different bushing. 
     Although illustrated in  FIG. 7  in the form of a test system that is coupled to a vehicle spindle, this is but one example. Other multi-degree of freedom actuator based load applying test systems include, but are not limited to, steering gear test systems, steering knuckle test systems, control arm test systems and, in general, any application where specimen or fixture motion imparts disturbances into and/or between control channels. 
     A first embodiment of a compliant actuator assembly  150  is schematically illustrated in  FIG. 6 . A piston  158  is slidable in a cylinder or bore  155 . The piston  158  and the bore  155  operate as a double-acting hydraulic actuator, either single ended or double ended depending upon the test system design. A flow control valve  159  comprising part of servo controller  14  is fluidly coupled to the bore  155  and selectively provides hydraulic fluid to the bore  155  to displace the piston  158 . An accumulator  164  is fluidly or mechanically coupled to each of the chambers or piston of the double-acting actuator. The accumulators  164  introduce a spring effect to an otherwise substantially rigid actuator. Each accumulator  164  includes a first portion  165  of compressible fluid (typically, a gas such as nitrogen, mechanical spring, or other resilient media or device,) and a second portion  167  that is filled with a liquid, which compared to the gas, is substantially incompressible. The second portion  167  of each accumulator  164  is fluidly coupled to the bore  155  or mechanically coupled to the piston  158 . Commonly, a diaphragm  169  (or equivalent separating device such as a piston) is provided in each accumulator  164  to maintain separation of the spring device or media and the liquid. Using hydraulic accumulators  164 , typically but not exclusively pre-charged with nitrogen gas or mechanical elements, allows tuning of the spring stiffness (i.e. the compliance) of the actuator assembly  150  to match requirements of a specific test specimen. 
     Other design considerations of the actuator assembly  150  can also be used to obtain desired performance. For instance, any or all of the area ratios between the accumulator  164  effective area and the area of the piston  158 , the mass of the accumulator piston and/or the velocity of oil entering/exiting the accumulator  164  can be used to tune the compliance to be effective at low frequencies yet become substantially inert or at least substantially stiffer at higher frequencies that warrant less compliance and more stiffness. 
     U.S. Pat. No. 6,457,369 discloses other forms of actuators (linear or partially rotatory) using volumes of compressible gas to provide gas springs which can be used in the present invention, and as such is incorporated herein by reference in its entirety. It should be noted the compliant actuators described in U.S. Pat. No. 6,457,369 however are not used in the manner as taught herein. In U.S. Pat. No. 6,457,369 the compliant actuators are used to provide a high static or low frequency load that is also compliant to higher frequency input disturbances. However, some control techniques particularly with respect to hydraulic powering up or shutting down as described therein can be incorporated, if desired. 
     As indicated above, a compliant actuator assembly is particularly advantageous in a multiple degree of freedom (multiaxial) test system such as test system  200  illustrated in  FIG. 7 . Test system  200  is described in detail in U.S. Pat. No. 6,640,638, which is hereby incorporated by reference in its entirety, but nevertheless is one form of a road simulator. 
     Referring to  FIG. 7  and the schematic representation thereof, the vehicle spindle test fixture  200  is exemplary of a system designed to apply linear force and rotational moments to a spindle of a vehicle, not shown. The vehicle spindle test fixture  200  includes a wheel adapter housing  216  that is fixed to the vehicle spindle in a conventional manner. A first loading assembly  213  includes the wheel adapter housing  216  and a pair of vertically extending loading links or struts  220 . Generally, the first loading assembly  213  applies loads to the spindle, in directions along one or both of two mutually perpendicular axes  222  and  224 , with actuator assemblies  223  and  225 , respectively. In addition, the first loading assembly  213  can apply a moment or torque about an axis  226  that is mutually perpendicular to axes  222  and  224  using actuator assembly  227 . 
     In the exemplary embodiment, the test fixture  200  also includes a second loading assembly  215 . The second loading assembly  215  comprises a plurality, of struts  217  and at least one of actuator assemblies  219 A,  219 B and  229 . Generally, the second loading assembly  215  can apply a force substantially along the axis  226  using actuator assembly  229  as well as a moment about axis  224  using actuator assemblies  219 A and  219 B, and a moment about an axis parallel to the axis  222  using actuator assemblies  219 A,  219 B and  229 . 
     Each of the actuator assemblies of  FIG. 7  comprise a second form of a compliant actuator assembly in that each actuator assembly includes a spring element  240  operably coupled in series with the associated actuator, which could be hydraulic or electric. The spring element can comprise a mechanical spring (e.g. coil spring) or a gas or pneumatic spring. Although illustrated in  FIG. 7  as a spring element connected in series to two struts, it should be understood that that spring element may be incorporated anywhere along the load path from the actuator to the coupling to the specimen such as but not limited to being incorporated in a portion of any lever arm in the load path, or to a portion of the lever arm to provide compliant pivoting of the lever arm, or in any coupling in the load path. Commonly, the spring element would provide an axial spring effect, which can include a spring operably coupled to a lever arm to allow the pivot point of the lever arm to move with compliance as well as an axial spring element  240  as illustrated. Stated yet another way, one aspect of the present invention is to provide compliant actuator assemblies such that the stiffness of the test system is substantially less than the stiffness of the test specimen. 
     It should be noted that in comparison to the mechanical springs  240 , the compliant actuator  150  having the compliance elements operably coupled between the double-acting actuator ends, or the fixed and single acting actuator end, may be advantageous because compliance of the actuator assembly is “inside” the control loop (signal lines  19  and  15 A in  FIG. 5 ), thereby still providing closed loop control of the resulting motions, which may reduce or eliminate uncontrolled resonant response. 
     A particular advantage of including a compliant actuator or assemblies in the test system is that a new drive may not be needed for testing a plurality of “similar but different” test specimens. Often, test systems of the prior art a new drive using an iterative process as that described above for each similar but different test specimen to be tested based on each corresponding test specimen&#39;s unique response data collected recorded at step  54  in  FIG. 3A . However, installing each similar but different test specimen in it operating environment such as a vehicle in step  52  and recording the data is very costly in both labor and time. Likewise, generating a drive using the iterative process based on the unique recorded data is also typically very time consuming and due to the nature of the iterative process causes wear upon the test specimen and/or the test system. It has been found use of one or more compliant actuator assemblies can reduce system sensitivity, particularly to specimen induced motion, thus increasing control loop disturbance rejection capability, and allowing the same drive to be used for the plurality of similar but different test specimens. 
     The compliant actuator assemblies also help perform testing upon a test specimen that from time to time exhibits different characteristics during testing. The compliant load assemblies also can keep the applied forces or loads more consistent over time. 
     It should be noted yet another significant advantage that has been achieved with the use of the compliant actuator assemblies in the test system is that the overall number of iterations necessary to obtain the drive using an iterative process such as those discussed above has been significantly reduced when compared to a test system not having such compliant actuator assemblies. Commonly, for the reasons discussed above the iteration gain or relaxation gain factor  95  must be kept small, for example on the order of 0.3 so that overshoot does not occur and damage the test specimen. Since the relaxation gain factor is small the number of iterations required to obtain the final drive is quite large, for example, 30 iterations. It is not uncommon for each iteration to take an hour or more for test systems such as a road simulator; hence to converge upon the final drive may easily take 30 hours or more. However, use of the compliant actuator assemblies that in effect allows the test system to be substantially less stiff than the test specimen (in at least some degrees of freedom, like the horizontal channels with complete vehicle spindle coupled road simulators, or partial vehicle testing that use one or two spindle coupled road simulators, for example, to test the rear axle/suspension of a vehicle, or one corner of a vehicle suspension, or a directly coupled component test specimen such as an engine mount connected to one or more compliant actuator assemblies) allows relaxation gain factors greater than about 0.5 to be used, and in a further embodiment greater than about 0.65 to be used, and in yet a further embodiment greater than about 0.75 to be used, and in yet other further embodiments greater than about 0.8 to be used. Use of a larger relaxation gain factor drastically cuts the number of iterations required to converge upon the final drive, thereby saving considerable time and expense, where as the relaxation gain factor increases the number of iterations needed generally decreases; therefore any increase in the relaxation gain factor during can provide significant advantages since the number of iterations decreases. 
     At this point it should also be noted that afore-mentioned advantages are obtained for any type of model that is used during processing or calculation to arrive at a new drive signal. The type of model used is not important because it is the reduction in the number of iterations that has been achieved with the use of one or more compliant actuator assemblies in the test system. Therefore, the invention is not limited to the exemplary test system methodologies used during iteration of the drive signals, but rather can be used with, for example, both linear and non-linear models. 
     Yet another difference between the prior art test systems and methods and the present test system and method having compliant actuator assemblies is that one can adjust the compliance of the test system (physical test rig) to have a selected compliance or a selected stiffness relative to the test specimen, for example, by adjusting the test system to be much softer than the test specimen, such as 10% as stiff as the test specimen (in at least some degrees of freedom). This again allows a larger relaxation gain factor to be used, thereby reducing the number of iterations. Such adjustments to the stiffness or compliance of the test system may allow the relaxation gain factor to be independent of similar test specimens, for example, if the road simulator was adjusted to have 10% of the stiffness of a car in one test and 10% the stiffness of a truck in another, the same number or nearly the same number of iterations may be needed for each vehicle. 
     Other exemplary iterative processes and embodiments that can benefit from aspects of the present invention are described in U.S. Pat. No. 8,135,556; U.S. Published Patent Application US 2013/0304441A1; and US patent application entitled “Methods and Systems for Testing Coupled Hybrid Dynamic Systems,” filed on even date herewith, all of which are hereby incorporated by reference in its entirety. 
     Generally, the afore-mentioned patent and applications provide arrangements for controlling simulation of a coupled hybrid dynamic system. In one exemplary arrangement, the arrangement comprises a physical test rig configured to drive a physical structural component of the system and to generate a test rig response as a result of applying a drive signal input to the test rig. A processor is configured with a virtual model of the complementary system (herein in also “virtual model”) to the physical component (i.e. the virtual model of the complementary system and the physical component comprises the complete hybrid dynamic system). The processor receives a first part of a test rig response as an input and generates a model response of the complementary system using the first part of the received test rig response and a virtual drive as inputs. The processor is further configured to compare a different, second part of the test rig response with the corresponding response from virtual model of the complementary system to form a difference, the difference being used to form a system dynamic response model which will be used to generate the test rig drive signal. 
     In an embodiment, the processor is further configured to generate the test drive signal, receive the test rig response, generate a response from the virtual model of the complementary system, and compare the test rig response with the response from the virtual model of the complementary system to generate a hybrid simulation process error. The error is then reduced using an inverse of the system dynamic response model, in an iterative fashion until the difference between the response from the virtual model of the complementary system and the test rig response is below a defined threshold. 
       FIG. 8  depicts an exemplary arrangement for controlling the simulation for a coupled hybrid dynamic system, where it should be understood aspects of the present invention are not limited to the exemplary arrangement herein described, but rather can also be applied to any of the other arrangements in the above-identified patent and patent applications. 
     In the exemplary arrangement, a complementary vehicle model  370  is provided in suitable non-transitory computer readable media such as a hard disk of a computer and accessible by a processor. The model of a vehicle is exemplary only, however, as other systems may be modeled without departing from the present disclosure. Also, for purposes of explanation, the physical component is a strut employed in a vehicle suspension system. Other components may be tested, as the strut is an example only of a physical component, including but not limited to testing of a complete vehicle less actual tires and wheels as described in the above-identified patent application. A test rig  372  is also provided that accepts drive(s) and provides response(s) to any of the compliant actuator assemblies discussed above, which are a part of the test rig  372 . In this example, the test rig  372  is configured to test a physical strut mounted within the test rig  372 . However, the test rig  372  may be configured to test other structural components. The test rig  372  has a rig controller  374 . 
     The arrangement forms or ascertains a system dynamic response model that can be employed to generate a drive signal used to drive the test rig  372 . The system dynamic response model  376  may be a frequency response function (FRF), as one example. The system dynamic response model  376  may also be determined, or calculated, by the same processor on which the model  370  of the complementary is run. However, a system dynamic response model  376  may also be determined and calculated on a separate processor. 
       FIG. 8  depicts the arrangement and steps to form the system dynamic response model  376 . This can be termed the system response modeling step. This system dynamic response model  376  can be employed in the iterative process of  FIG. 9 , described later. In  FIG. 8 , a random test rig drive  378  is played into the test rig  372  that has a vehicle component  380  (such as a strut) installed. The random test rig drive  378  may be a generic drive, such as a random amplitude, broadband frequency drive. Two responses are measured in the disclosed embodiment although the arrangement is not limited to two responses. One of these responses, such as a random test rig force signal  382 , is to be applied to the vehicle model  370  of the complementary system. The other response, such as a random rig displacement  384 , is a response to be compared to the response of the virtual model  370  of the complementary system. In the disclosed embodiment of  FIG. 8 , the first response  382  is the force exerted by the strut on the test rig  372 , while the second response  384  is the displacement of the strut  380 , which can also be provided as an input to the rig controller  374 . It is to be noted that the force and displacement signals are exemplary only, as other response signals may be provided from the test rig  372 . 
     The response from the test rig  372 , such as the random rig force  382 , is supplied as an input to form a random model drive  386  to the virtual vehicle model  370  of the complementary system. The virtual vehicle model  370  of the complementary system excludes the component under test, in this case the strut  380 . The virtual vehicle model  370  of the complementary system responds to the random model drive input signal  386  with a random model response signal  88 , in this case a displacement. 
     In the third step of the process, the random response  88  of the virtual model  370  of the complementary system is compared to the associated test rig random response  384 . A comparison  390  is performed to form a random response difference  392  (herein by example a displacement). The relationship between the random response difference  392  and the random rig drive  378  establishes the system dynamic response model  376 . The system dynamic response model  376  will be inverted and used for test rig drive prediction in the iterative simulation control process of  FIG. 2 . 
     The determination of the system dynamic response model  376  may be done in an offline process, such that high powered and high speed computing capabilities are not required. Further, since there is no need to acquire data, any component can be tested without previous knowledge of how that component is going to respond within a virtual model, or in a physical environment. The offline measurement of the system dynamic response model  376  measures the sensitivity of the difference in response  88  of the virtual model of the complementary system and rig response  384  to the rig inputs when the component  380  is in the physical system. Once the relationship between rig drive  378  and system response difference  392  has been modeled, an offline iteration process is performed, as seen in  FIG. 2 . This may be considered as the test drive development step. 
     In the iterative process of  FIG. 2 , which is an offline iteration, the virtual model  370  of the complementary system, which excludes the test component  380 , is operated. In the exemplary embodiment, the virtual model  370  is the complementary system of a virtual vehicle and the test component that is excluded is the strut  380 . The virtual vehicle is driven over a test road, to generate a response  400  of the virtual model  370  of the complementary system. As an example, the response  400  may represent a displacement of the strut  380 , although since the strut  380  is not actually present, it is really the displacement of the space that would be occupied by the strut  380  that is measured by the response  400 . An additional input to the virtual model  370  of the complementary system, in addition to the virtual test road input, is shown as reference numeral  398 . The additional model input  398  to the vehicle model  370  of the complementary system is based on the test rig response  394  from the test rig  372 . The additional model input  398 , such as the force measured at the test rig  372  is applied simultaneously to the vehicle model  370  during testing. For an initial iteration (N=0), the input  398  to the virtual model  370  of the complementary system will typically be at zero. 
     The response  400  of the virtual model  370  of the complementary system is compared to the test rig response  396  from the test rig  372 . This test rig response  396  must also be a displacement, if the response  400  of the virtual model  370  of the complementary system is a displacement. A comparison of  402  is made between the test rig response  396  and the response  400  of the virtual model  370  of the complementary system to form a response difference  403 . 
     The response difference  403 , in this case a displacement difference, is compared to a desired difference  404 . Typically, the desired difference  404  will be set at zero for an iterative control process. In further embodiments, however, other desired differences may be employed without departing from the scope of the present disclosure. 
     The comparison  406  between the response difference  403  and the desired difference  404  produces a simulation error  407  used by the inverse (FRF-1) of the system dynamic response model  376  that was previously determined in the steps shown in  FIG. 1 . The inverse of the system dynamic response model  376  is depicted as reference numeral  408  in  FIG. 2 . A drive correction  409  is added to the previous test rig drive signal  410  at  412  to generate the next test rig drive signal  414 . Typically, the simulation error  407  is reduced by a relaxation gain factor. The relaxation gain factor (or iteration gain) stabilizes the iterative process and trades off rate-of-convergence against iteration overshoot. Furthermore, the iteration gain minimizes the possibility that the test component will be overloaded during the iteration process due to non-linearities present in the physical system. As appreciated by those skilled in the art, the iteration gain can be applied to the drive correction  409 , if so desired. 
     The next test rig drive signal  414  is applied to the test rig  372  and first and second responses are measured. The response  394  to be applied to the vehicle model  370  generates via the processor and the virtual model  370  of the complementary system, a response  400  that is compared to test rig response  396 . The process is repeated iteratively (represented by arrows  397  and  399 ) until the resulting simulation error  407  is reduced to a desired tolerance value. 
     The processing of the vehicle model  370  and the determination of the final test rig drive signal  414  is capable of being performed within a single processor. However, in certain embodiments, multiple processors may be employed. Also, it should be understood that the process for determining the simulation error  407  and the determination of the test rig drive signal  414  may be performed offline. 
     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above as has been held by the courts. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.