EFT_Stage 2

Development of Effective Force Testing

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     A simple single-degree-of-freedom (SDOF) structural model was tested for the development stage.  Effective forces were applied to the structure using a 35 kip hydraulic actuator equipped with a 90gpm three-stage servovalve. The velocity feedback compensation was implemented using a dSpace DS1102 DSP controller with a TI TMS320C31 floating-point digital signal processor with a 2 kHz sampling rate.

   The effect of natural velocity feedback on the EFT method was demonstrated with the direct implementation of EFT. In the velocity feedback compensation, the effect was compensated by modifying the command to the servovalve. The compensation signal was determined as the product of the measured piston/structure velocity and the piston area. The compensation signal was multiplied by the inverse of the forward system dynamics, and then added to the force command. The modified command signal compensates for the effect of the piston motion after going through the forward dynamics.

   Nonlinearities of the servo-system can have significant impact on the performance of the velocity feedback compensation. The servo-system nonlinearities become significant when a test involves large forces and/or large velocities compared to the actuator/servovalve capacities, which usually cause large hydraulic flow demands. Hence, nonlinear velocity feedback compensation is necessary to extend the EFT method to fully utilize the equipment. The compensation is independent of the structural properties (i.e., damping and stiffness) and their changes during a test.

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Pre-test

1. Static loading test to determine the structural stiffness
2. Free vibration test to identify the natural frequency and damping properties

Tests with elastic configuration

1. Direct implementation of Effective Force Testing
2. Small amplitude EFT test with linear velocity feedback compensation

Tests with bi-linear elastic systems

1. Large amplitude EFT test with linear velocity feedback compensation
2. Nonlinear EFT test with nonlinear velocity feedback compensation

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Nonlinear servo-system models

   The testing system model with velocity feedback compensation was used to show the natural velocity feedback, and to explain the velocity feedback compensation.

   The servovalve controller, the servovalve dynamics, and nonlinear servovalve flow properties are critical to the velocity feedback compensation.   Detailed models for these system components were derived to facilitate the design of velocity feedback compensation schemes.

System parameter identification

   Detailed procedures were developed to determine the nonlinear servovalve flow gain, servovalve response delay, and controller gain setting.

Velocity feedback compensation

   The velocity feedback compensation requires the inverse of the servovalve components. A  digital signal processor was used to implement this scheme.  When the effective force relatively small compared to the actuator capacity, and the hydraulic demand is low compared to the servovalve flow capacity, a linear version of the compensation scheme is viable, which includes a proportional gain and a phase adjuster, and can be implemented using an analog circuit board.