MODAL TESTING EXCITATION CONSIDERATION
Sentek Dynamics’ MS Series Modal Shakers are proven solutions for Experimental Modal Analysis. The MS Series are designed for laboratory experiments, modal studies and structural analysis and are available in different force rate. They cover a wide range of modal analysis applications. No matter the device under test is a PCB card, automotive component, power machinery, passenger car, airplane, or space craft, one of the MS series modal shaker can be acquired to carry out the test.
There are a few modal testing methods considering different type of excitation used. Commonly known, they are Hammer Impact and Modal Shaker. The Operational Modal Analysis which take use of the Ambient Excitation is not discussed here.
Hammer impact testing uses an Impact Hammer to excite the structure under test. The impulse force applied to the structure is categorized into the broadband range excitation, due to fact it contains the energy up to certain frequency range. The hammer impact test is quick to set up and to carry put, thus is widely used. Also, it has its limitations, and some concerns need to be considered.
Modal shaker testing takes use of the modal shaker to excite the structure under test. There is a list of waveform types can be selected to excite the structure under test. It is more often used for complex or large scale structural testing. Compared to the hammer impact test, it is more repeatable. On the other hand, Modal shaker testing requires more hardware, i.e., modal shaker(s), stinger, more input and output channels from the dynamic signal analyzer. The test setup would require experienced user.
Ambient excitation uses natural excitation of structure, will results in the response only measurement data. This can be used for testing on bridges, buildings, etc., in the civil engineering field. It does not require any boundary condition setup, and no excitation equipment is needed. Due to the lack of knowledge of excitation, unscaled modal data will be resulted. The analysis will require special processing.
Hammer Impact Testing
When using impact hammer to run the modal testing, there are two ways to perform the measurement. One is roving hammer, with a fixed response measurement point, and the excitation point is roving over the measurement DOFs on the structure under test. The FRF signals between the fixed response measurement DOF and excitation DOFs are acquired. And this results in one row of FRF signals.
Equivalently, roving response method can be used. The responses can be measured at several DOFs, while the excitation is fixed at one DOF on the structure under test. With this arrangement, a column of FRF signals will be acquired. With possible multiple responses measured at the same time, more FRF signals can be measured at the same time. This will speed up the testing process. But this will result in the so-called mass loading issue, due to the moving the response accelerometers are moving from one batch of measurement point and direction to the next. To handle this issue, either dummy blocks can be applied to all DOFs are not under measurement; or ultimately take use enough accelerometers to measure all DOFs in one shot.