Deriving Hydromount-Model Parameters with the Isolator-Parameter Identification Tool
In the first part of this tutorial, you used the Adams Car Ride Component-Model Test Rig to calculate the dynamic stiffness and loss-angle characteristics of a hydromount for a range of sinusoidal excitation frequencies and amplitudes that you specified. The Adams Car Ride Hydromount Component Model uses eight parameters to model a hydromount. These eight parameters for a hydromount produce the response you viewed in the first part of the tutorial. These parameters, however, must be obtained before the model can be used.
To obtain the parameters, you use the Adams Car Ride Isolator-Parameter Identification Tool (IPIT). Given a set of hydromount test data, the IPIT finds values for the eight model parameters, such that the model reproduces the test data within a specified error tolerance. In this part of the tutorial, you will use the IPIT to do that.
Opening the Hydromount Property File
To run IPIT, you must launch IPIT and open the file containing the hydromount test data. The IPIT uses this as input for the parameter derivation. Before running IPIT, however, you should know something about the structure of a hydromount property file. So, first you’ll open an example property file and take a look at the contents.
To view the parameter file:
1. If Adams Car Ride is not running, start it as explained in Starting Adams Car Ride.
2. If the component_hydro_bushing_example.asy is still open, skip to step 3. Otherwise, open the component_hydro_bushing_example.asy as you did in Opening the Assembly.
3. If the component_hydro_bushing_example.asy is not in view:
■From the View menu, select Assembly.
■From the Assembly pull-down menu, select component_hydro_bushing_example.asy.
■Select OK.
4. Right-click the hydromount, which is the dark gray cylinder in the center of the screen, point to Hydro_bushing: component_hydro_bushing.bgs_b1, and then select Modify.
Adams Car Ride displays the Modify Hydro Bushing dialog box. Note that there are two Property File labels in the dialog box:
■The text box adjacent to the first label displays the path to the file currently being used for the hydromount: mdids://aride_shared/hydro_bushing.tbl/hyd_bus001.hbu.
■The second one doesn’t have an associated text box, but displays an Apply Property File button to the right of the View Property File tool 
.
. 5. Select the View Property File tool to the right of the second Property File label.
The Information window appears, displaying the contents of the property file. Look through the file and note the six blocks of data: [MDI_HEADER], [UNITS], [GENERAL], [HYDRO_PARAMETERS], [HYDRO_IDENTIFICATION_DATA], and [HYDRO_TEST_DATA]. We will focus on the last three blocks:
■The [HYDRO_PARAMETERS] block contains the eight parameters used for the hydromount model. These were used when you performed the simulations in the first part of this tutorial.
■The data in the [HYDRO_TEST_DATA] block is meant to be obtained directly from testing a real hydromount in the laboratory. It is not used directly by the hydromount model. It is, however, used by the IPIT.
■The data in the [HYDRO_IDENTIFICATION_DATA] block is calculated using the hydromount model with the parameters in the [HYDRO_PARAMETERS] block. There is exactly one data point for each data point in the [HYDRO_TEST_DATA] block. Ideally, the [HYDRO_IDENTIFICATION_DATA] block and the [HYDRO_TEST_DATA] would be identical. This would be the case if the hydromount model were perfect and the parameters in the [HYDRO_PARAMETERS] block were chosen perfectly. It is the job of IPIT to choose the parameters so that the identification data matches the test data (within user-specified tolerances).
At a minimum, IPIT requires only the [HYDRO_TEST_DATA] block to run (along with the [MDI_HEADER], [UNITS], and [GENERAL] blocks). In this case, it starts with all parameters set to zero. An optimizer within IPIT tries to select parameters so that identification data matches test data within user-specified tolerances. When the optimizer is done, the IPIT saves a new property file. This file contains parameters it derived and identification data implied by the model given those parameters.
You can then use this new file as input to the IPIT on a subsequent run. In this case, IPIT starts the optimization with the parameters in the [HYDRO_PARAMETERS] block, so it can begin where it left off. This is useful if you want to derive new parameters that match test data with tighter tolerances.
Next you will make a new property file without any parameters to use as input to IPIT, and then you will run IPIT with this file.
6. Close the Modify Hydro Bushing dialog box.
To make a new property file and run the IPIT:
1. Using a text editor, open the file mdids://aride_shared/hydro_bushing.tbl/hyd_bus001.hbu. Save a copy called mdids://aride_shared/hydro_bushing.tbl/hyd_bus001_test_data_only.hbu. If not already opened, open mdids://aride_shared/hydro_bushing.tbl/hyd_bus001_test_data_only.hbu in a text editor.
2. Delete the blocks [HYDRO_PARAMETERS] and [HYDRO_IDENTIFICATION_DATA] and all associated data, including the $---... line that starts each block.
3. Change the name of BUSHING_PROPERTY_FILE to mdids://aride_shared/hydr_bushings.tbl/mdi_0001_test_data_only.bus.
4. Save mdids://aride_shared/hydro_bushing.tbl/hyd_bus001_test_data_only.hbu, and then exit the text editor.
Now you are ready to run IPIT.
5. Note that you can launch the IPIT tool from the interface. From the Ride menu, point to Tools, and then select Isolator-Parameter Identification).
6. Select Load, and then browse for the file you just created: mdids://aride_shared/hydro_bushing.tbl/hyd_bus001_test_data_only.hbu.
After the file loads, you will see that all parameters are set to zero. The Plot tab should be active. In it you will see two scatter plots of the test data: one for dynamic stiffness (the top plot) and one for loss angle (the bottom plot).
7. Select the Data tab. You should see the contents of the file you just created.
8. Select the Plot tab to display the plots again. Leave the defaults values for the Error Control parameters unchanged.
9. Select Start Optimization.
The optimizer begins running. As it progresses, you will see solid-line plots of the identification data, generated with the current values of the hydromount parameters, superimposed upon the test data.
With the default error control parameters and the sample test data, it takes quite a while for the optimization to finish.
The optimizer will stop after completing the optimizer has found the optimal solution. At the bottom of the dialog box, you will see the following message: Hydromount HBU optimization (1) finished, objective function value:
10. When the message at the bottom of the dialog box changes to Ready to Start Optimization, or Calculate Frequency Response data, select Save.
By default, IPIT makes a file name for output that is the same as the input file name, but with _out appended to the root name.
11. Select Load and load the output file you just saved: mdids://aride_shared/hydro_bushing.tbl/hyd_bus001_test_data_only_out.hbu.
You will see the same parameters that were just calculated by the optimizer.
12. Select the Data tab.
You will see that the file created as output contains the hydromount parameters and the identification data.
13. Select Quit.
14. When asked to save your data, select No.
15. Exit the program.
Note: | One can run a second Hydromount optimization step (2: Hydromount (time domain)), which is running an optimization using simulations of the component testrig. This optimization takes longer, but may decrease the differences in between the hydromount model and the testdata. |