Magnetorheological Device Development and Testing: Student Projects Augmenting Research

Journal of Engineering Technology, Spring 2004 by Vavreck, Andrew N

System Description

As pari of a capstone design project, an electromechanical ET student designed and constructed a pneumatic test bed for commercially available linear MR dampers, as shown in figure 5.

The test bed uses a pneumatic cylinder with a 3-inch bore to provide displacement of the damper. An inline load cell measures applied force while a linear variable differential transformer (LVDT) measures displacement. The cylinder is controlled by pneumatic directional valves that are activated by a solenoid, with spring returns on both the valves and the cylinder to assist with stroke stability. A function generator signals a relay that opens and closes these valves. The function generator also controls the waveform and frequency of the system, and air pressure set through the regulator controls the cylinder's stroke. Valve bleeds provide some adjustment for centering of the stroke range and for the shape of the displacement waveform. In addition, a limited amount of displacement control can be achieved by varying the type of waveform (sinusoid, saw-tooth, or square) with the function generator, though it is simply not possible to produce a clean sinusoid with such a simple device. Even with the cylinder's return springs, it is difficult to stabilize the displacement; with no automatic control, the system tends to drift slightly. The load cell and LVDT signals are passed to a PC-based data acquisition board, and LabView software is used to log data into a spreadsheet file for later analysis.

Given the limitations on controlling the shape of the applied displacement waveform, techniques that rely on sinusoidal excitation for damper characterization can not be used with the pneumatic test bed, but those methods with less rigorous input displacement calculations will be unaffected, including a scheme developed by the author that involves manipulation of time-series data to estimate damper parameters.6 Data collected with the pneumatic test bed will serve to verify this damper parameter estimation approach.

To estimate damper parameters, data were taken at various frequencies, displacement levels, and damper currents. A compressor with the regulator set at 80 psig supplied air pressure to the cylinder. Force-displacement data from a series of experiments are shown in figures 6 through 14. Here, positive force is compression while positive displacement is extension (the curve is circumscribed clockwise). Time-series force and displacement data are shown in figures 15 and 16, illustrating both the nonsinusoidal nature of the displacement and the nonlinear response of the damper.

The peak force generated in this system increases significantly when the current is increased from O to 0.5 A, essentially to the same level reached at 1.0 A. These force-displacement plots exhibit the typical nonlinear behavior of the MR damper. The test bed generally produces lower displacements at higher damper currents, and the graphs reveal a more sinusoidal displacement at higher frequencies and currents as well as a correspondingly more symmetrical force response.