This tutorial outlines the procedure for Experimental Modal Analysis (EMA) with ARTeMIS Modal. The considered example is an iron circular plate, which is simply supported on its edges. The plate will be excited with a PCB impact hammer in nine positions, and the resultant output accelerations will be measured in two fixed locations. Input and output data will be collected with NI acquisition system directly with ARTeMIS, and two popular techniques, that is CMIF (Complex Mode Indicator Function) and RFP-Z (Rational Fraction Polynomial in Z domain), will be used to estimate its modal parameters.

First a blank internal storage EMA project is created

In ARTeMIS geometry of the considered structure does not constitute any physical laws and is only intended for the mode shape or operation deflection shape visualization. The geometry of the plate is created by drawing a circular surface, divided to 8 slices. Two nodes are added on top of the surface to represent the location of the measured outputs.

To collect measurements, first an empty placeholder for one measurement session is created

Next, one input channel and two output channels are inserted by pressing the Insert Object Icon.

Since the position of the output is fixed and location of the hammer impact changes from the test setup to test setup, the Fixed Output/Rowing Input type of project is chosen.

Nine locations of the measured input corresponds to nine test setups. To create the corresponding setups the first test setup is replicated eight times.

Each test setup consists of one input and two output channels, respectively measuring the force and the accelerations perpendicular to the surface of the plate. Each of the respective directions is assigned in Assign DOF Information task.

For each channel of each setup a vertical degree of freedom is assigned. It can be viewed that after assigning the two output channels of the second test setup the project automatically changes to multi setup project, and the fixed input channels are labeled as reference channels.

To start the data acquisition, the National Instruments plugin from the Data Acquisition task is selected.

Before the actual data recording, the properties of the modal hammer and the sensors are set. In this example we use PCB Modal Hammer and two B&K Acceleration sensors.

Subsequently, the respective channels are assigned to their setups, with the Auto Assign settings.

Considered EMA modal parameter estimation methods rely on FRF estimation. For that H1 estimator is chosen.

Sampling frequency and the force thresholds for hammer testing are set up in the Hammer Setup settings.

Finally, the EMA test can be started.

After completing the test, resultant time data and spectral densities can be viewed in Measurement Explorer in Manage Measurements.

The singular values the FRF matrix can be viewed in the Prepare Data tab. In fact the recorded measurements are already processed and no further data preparation is required.

First, the modal parameters are estimated with the CMIF algorithm. This comprises hand picking the peaks of the singular values of FRF matrix. whose frequency, mode shape and damping ratio estimate a single degree of freedom system.

It can be viewed that 7 modes are hand picked. Their mode shapes can be animated in the Geometry window, and their details can be viewed after enabling the Details in the Shape Details section of the Properties window.

Next, the modal properties of the plate are estimated with RFP-Z algorithm. RFP-Z is a parametric method, fitting a rational fraction polynomial in Z-domain to the estimated FRFs. Therein, to estimate the modes maximum model order of 50 is chosen. The resultant stabilization diagram of natural frequencies is illustrated on  Figure below.

Automated modal parameters estimated with RFP-Z are similar to hand-picked parameters estimated with CMIF. The results from both estimator can be validated in the Validate tab.

It can be seen that the diagonal of the MAC matrix computed between mode shapes from CMIF and RFP-Z reaches unity for most of the modes. Some of the MAC on the off-diagonal of the MAC matrix are correlated, due to symmetry of the plate and small discretization of the measurements.

This concludes this tutorial.

For more information check out these online video tutorials describing the Impact Testing Module:

or our online material on EMA Module in general: