Measurement of the mass properties of rigid bodies by means of multi-filar pendulums – Influence of test rig flexibility

Mass properties (mass, centre of gravity location and inertia tensor) are commonly defined as attributes of rigid bodies. The measurement of such attributes is usually performed under the hypothesis of rigid behaviour of both the body under test and the test rig. For large or heavy bodies, the forces exchanged between the body and the testing structure may be so high that the rigid behaviour hypothesis is not satisfied. The body under test can often be considered as rigid, so the accuracy of the measurement can be affected by the deformations of the testing structure. In this paper, the effect of the deformation of the testing structure on the measurement of relatively large or heavy bodies is investigated both numerically and experimentally. The InTenso+ Measuring System of the Politecnico di Milano, a special type of multi-filar pendulum is considered relevant as example. A flexible multi-body model of the test rig is employed for assessing the influence of test rig deformations on mass properties measurement. Experimental tests are conducted on a series of bodies to study the uncertainty. Numerical and experimental evidences show that the deformation of the testing structure can alter considerably the results of the measurement. However, this effect can be modelled with a reasonable accuracy. A proper mathematical procedure is developed for use during the measurement to compensate the effects of the undesired test rig deformations. The presented procedure can be effectively employed when the mass properties of very large or heavy bodies have to be measured and the construction of a sufficiently rigid testing structure is impossible in practice