Construction of a portable test rig to perform experimental modal analysis on mobile agricultural machinery

Higher working speeds of mobile off-road vehicles arouse dynamical problems like excessive vibrations, which decrease the vehicle lifetime, the driver's comfort, and the working precision. Testing the vibration behaviour of present vehicles is an indispensable source of knowledge for further improvement of vehicle dynamics. More restrictive legislation concerning allowable exposure of drivers to vibrations raises the need for these tests. For small- and medium-sized companies the required know-how and equipment is too expensive and vibration analysis of their products has to be farmed out. Since tests in situ have many advantages, a prototype of a mobile low power test rig for experimental modal analysis on vehicles is described in this paper. The vehicle under test is excited vertically under one of its wheels while the other wheels rest on stationary platforms. The power saving by using an airspring parallel to the actuator is investigated by including a model of the airspring in the equations of motion of the test rig. It turned out that the power saving due to the airspring is proportional to the relation of the acceleration due to gravity to the maximum acceleration of the excitation signal. For small band excitation signals extra power savings can be achieved by connection of the airspring with a pressure vessel which optimal size depends on the frequency content of the excitation signal. Experiments with a prototype of the vertical shaker demonstrate that the different modeshapes of a tractor-sprayer combination can be sufficiently excited and modal analysis was performed successfully. (C) 1999 Academic Press.