/n/nCRN 54392 / 54397 Measurement Analysis and Assessment @ University of Salford Vibroacoustics Lab Core Aims . To measure the passive mobility of an assembly using an impact hammer and accelerometers. • To plot this data including validation checks (reciprocity, coherence) and standard error. . To measure and plot the operational velocities when the source part of the assembly is active. Extended Aims (level 7 students only) All the core aims plus: . To calculate the blocked forces the source assembly exerts at the interface. • To validate these blocked forces using the 'on board validation' approach. Background In this lab we introduce modal testing, also called system definition, structural testing, mobility measurement, and Frequency Response Function (FRF) measurement. Although we are using a mechanical device as our test object, there are strong parallels with the Room Acoustics Lab. The mechanical object responds to mechanical excitation by vibrating and radiating sound. In room acoustics we thought of the room as a filter, and similarly we can consider the mechanical structure as a filter whose properties we can extract by measurement. We are using an impulsive excitation here, but the system will filter any mechanical input in essentially the same way. A complication relative to typical Room Acoustics measurement is that we need to measure multiple forcing and multiple response points, thus the system become Multi-Input Multi-Output (MIMO). A diagram and photographs of the apparatus are shown in figures 1 and 2 respectively. Shaker (internal source excitation) Source Ys, fs Z Interface x Isolators Zı y Interface Target Vb Receiver Y Figure 1: Diagram showing the test apparatus in a disassembled state to distinguish: the source assembly (top), which includes a shaker that will be used in the operational case; the isolators (middle); and the receiver assembly (bottom), which includes the on-board validation point vb. Vibroacoustics Lab 1 CRN 54392 / 54397 Measurement Analysis and Assessment @ University of Salford Figure 2: Photo showing the apparatus. Left: top view, plus impact hammer. Right: side view, focussing on source assembly and isolators. Note that these photos do not show the complete set of accelerometers. Lab Brief The measurements will be performed using Dewesoft X software. The process is detailed in a video that can be found on Blackboard. Here is a non-exhaustive checklist to help you while you are in the lab: . Channel Setup > Analog In screen: ensure all sensors are detected (most will require power via IEPE), are functioning correctly and have the correct sensitivity data set (should be read from TEDS for the accelerometers but needs to be entered manually for the hammer). . Channel Setup > Modal Test screen: this is where the FRF measurement is set up. o Left column: Set test method and number of averages appropriately. Ensure the FFT length is set to be a power of 2 (required to match the FFT analysis used to analyse the operational data) and to give a suitable frequency resolution (df). In the Output channels box ensure 'FRF H1', 'Ordinary Coherence' and 'Intermediate Fourier Transforms' are all set to 'Used' (the latter is required so that FFT data of each hit is also saved separately, as is required for the specified analysis). o Excitation tab: There should be one excitation channel: the hammer. Set the trigger threshold, window type, and pre-trigger appropriately. o Response tab: Add all accelerometers and the hammer here (the latter is required so that Intermediate Fourier Transforms Measure > Modal Test screen: this is where the FRF measurement is performed (see video). • . Channel Setup > FFT Analysis screen: this is where the operational measurement is set up. o Enable this for all accelerometers. Set Output spectra to be complex and specify no averaging. . Measure > FFT Analysis screen: this is where the operational measurement is performed (see video). . Analyse > Data Files > Multifile Export: Use this to export your data to Matlab . mat files. Be sure to tick the Real and Imag complex export boxes so that data is exported with phase. The required analysis is specified below. The majority of this follows the tutorial script issued on Blackboard, which serves as a skeleton code for the lab and provides a summary of relevant theory. You are strongly advised to work through that script and complete the missing sections before the lab week. Core Analysis 1. Plot some FRF data checks - coherence and reciprocity for a few point combinations. 2. For one point combination, compute the FRFs for individual hits (using the data saved by the 'Intermediate Fourier Transforms' option) and plot this overlaid on the averaged FRFs output by Dewesoft. Compute Standard Error based on the coherence and use this to add ±95% confidence limits on the averaged FRF line (plotting as additional dashed lines is recommended). 3. Plot the spectrum of the operational velocity data. Please note that you should use the Vibroacoustics Lab Report Proforma if you are doing only the core analysis and submitting a Summary Lab Report. There are also marks for discussion & evaluation (see assessment brief). Vibroacoustics Lab 2 CRN 54392 / 54397 Measurement Analysis and Assessment @ University of Salford Extended Analysis (selected level 7 students only) All the above core analysis, plus: 4. Plot the magnitude of the blocked forces at each connection versus frequency. 5. Plot the result of the 'on-board validation', being the comparison of the directly measured operational vibration response Vb versus that computed via the blocked forces from 2. 6. Repeat step 5, but this time excluding rotations DoFs. For Full Lab Reports you are not required to use the issued Summary Lab Report Proforma. Instead, the report should be more along the lines of a standard academic report - see assessment brief for full details. You are advised to use the Salford report template (available on Blackboard) for this. Note that there are also marks for summarising the background theory, introducing the experiment, and discussion and evaluation. Vibroacoustics Lab 3