An alternative MIMO FRF estimation method using pneumatic exciters

Abstract

As an alternative to shaker and impact hammer excitation, a new method of excitation using small, single acting pneumatic cylinders is being investigated. In this method, the structure is excited by the piston of pneumatic cylinders on which a load cell and different types of impact tips can be mounted. In contrast to a shaker test, there is no mass loading of structure in this set up as the load cell ismounted on the piston of cylinder with no connection to test structure. Solenoid valves regulate air flow through the cylinders and thus control the timing and duration of impacts. An electrical circuit consisting of a transistor switch is used to actuate the solenoid valves. A system of independent, random duration and interval pulses (digital time series) is sent to each exciter system to be able to estimate Multiple Input (MI) Frequency Response Functions (FRFs). In this way, the excitation signal is more like a random input than an impact input (where only one impact per measurement ensemble is taken). Since, the random pulse sequences sent to each exciter system are uncorrelated and the cylinders and structure are not connected, it will be always possible to uncouple the forces involved. This approach can also be used for single input impact testing as there will be better repeatability in position, line of action and magnitude of impacts. This method will be validated by comparing FRFs and modal parameters obtained by it with those of traditional impact test.