On the direct or indirect measurement of force in vibration testing

Direct and indirect methods for force measurement in vibration testing are examined for their implications on the structure under test. The dynamics of the excitation system modify both the structure and the force transmitted from the exciter. For direct force measurements, it is suggested that the 'conventional' way of mounting a force transducer may not be the most appropriate when there is significant lateral motion of the transducer. Alternative arrangements of the force transducer are suggested. The concept of indirect force measurement from the current flowing in the exciter coil is introduced and the consequences highlighted. Differences between constant voltage and constant current power amplifiers are explained together with their advantages and disadvantages for vibration testing. If the 'structure' can be re-defined to include the moving parts of the excitation system (mass, stiffness and damping) and a constant current amplifier is used then there will be no force 'drop-out' at resonance with indirect force measurement. However, there will always be some force 'drop-out' at resonance when using direct force measurement methods, whichever form of power amplifier is employed. FRF equations are developed using the indirect measurement of force for a SDoF system coupled to an exciter in which (a) the body is grounded and (b) the body is flexibly mounted. Some numerical examples are used to illustrate differences between the true FRFs of these systems and those derived with indirect force measurement. The variation of the applied force through a structural resonance is also presented. Extension of these findings to multiple input and multiple output vibration tests is discussed with particular reference to the use of indirect force measurement for force appropriation vibration test techniques.