Inverse load identification

To be able to simulate and compare variants and concepts using numerical models, realistic dynamic load information is required. These may require experimental data, for example from a test-bed with the drive units or from first prototype hardware. In most cases the loads cannot be measured directly with force sensors. In that case an indirect estimation of the dynamic loads is possible using inverse force identification. This technique relies on the combination of an operational measurement with artificial excitation.

Accelerations, surface velocities, pressures, or strains are measured near the suspected forces as response signals in natural operation of the device or system on the test-bed. In the next step shakers excite the load interface for each possible force and direction, and the same responses are measured and processed into an FRF matrix. Various approaches are possible, but basically the FRF matrix is inverted and multiplied with the vector of operational spectra to obtain force spectra estimates.

On the measurement side a main challenge is controlled artificial excitation at the force interfaces. Instrumented hammer impacts tend to produce an inaccurate FRF matrix and access can be difficult or even impossible.

Externally supported shakers with a force cell, when possible, can cause significant mass loading on light weight structures and stinger modes limit the frequency range.

Qsources developed its dynamically decoupled and compact shakers with load identification and transfer path applications in mind. Smaller than anything else, working well and aligning in any inclination, very low coupled mass in the working frequency range has allowed to extend the scope of inverse load identification limit significantly.

Our partners Siemens and Head Acoustics can advice further and provide software for this process.

The shakers for such application, from small and higher frequency to higher forces and lower frequency range:

Sound power determination

Everyone needs a coffee-break from time to time. And many love the smell of the coffee when being nearby. But having a quiet conversation while the machine is grinding, hissing and pumping is not always easy. So the noise emission of these machines is relevant.

To quantify the overall noise emission created by any machine, device, the sound power level is often used. Less intuitive than sound pressure, but much more relevant because sound pressure is very dependent on the nature of the environment and the distance to the device. Furthermore sound power allows combination with models on sound propagation and sound isolation.

More complex descriptions of noise emission are also possible like equivalent volume acceleration spectra, or the distribution of volume acceleration over the surface of machines, devices. (see automotive applications, ASQ)

For the practical sound power, the measurement in natural operating conditions is possible in dedicated test-chambers, by detailed sound intensity scanning, and by using reference sound sources.

The latter, using sound sources, is the most practical, because it can almost always be performed on-site, in basic labs, or in the natural operating environment. Beside the reference sound source, at least one microphone is needed. The procedure is well described in ISO 3747.

Qsources reference sound sources satisfying ISO 6926 for sound power measurement:

Experimental modal analysis supporting FE analysis

Components and frames are designed to withstand the loads and provide sufficiently high stiffness for accurate operation. The assessment on whether a sufficient level is achieved, within costs, and maintaining light weight is based on FE based models. The most reliable way to verify these FE models, especially when assemblies with multiple interfaces are included, is EMA experimental modal analysis. The knowledge from the correlation between the test results and the numerical models can be used to optimize and assure the effectiveness of the designs. Qsources provides highly accurate, and efficient, excitation devices for EMA applications.

Some shakers for structural analysis from small and higher frequency to higher force level and lower frequencies:

If the parts to analyze are extremely light and flexible, it could be interesting to use sound sources with real time volume signal for excitation. These can equally well be used for for scaled EMA analysis for vibro-acoustics or pure airborne enclosed volumes. These sound sources can, for some difficult applications, also be used as indirect non-contact excitation of structures to determine their structural modes. Volume sound sources for such:

Airborne Source Quantification (ASQ)

Not all phenomena are structure borne, many mid and high frequency noises are mostly air borne. Radiation of the noise from grinders, motors, fans on the outside, or hidden inside the machine. Various approaches to ASQ exist, either using airborne volume acceleration data from complete operating devices, or from test-beds and combining this with acoustic transfer functions from inside the housing to quantify the airborne emission. This can diagnose the situation and provide meaningful requirements for internal drive units, fans, grinders, pumps, etc. while still in a test-bed.

Using reciprocity is even more interesting if the machine is not very large, like a coffee machine. The sound source is placed at some distance of the machine, and a microphones are placed inside the housing of the machine. The thus measured acoustic transfer or acoustic isolation FRF quantify the sensitivity for noise emission from the interior of the housing to the outside.

Some of the Qsources sound sources typically used in ASQ analysis, for direct or reciprocal excitation, from low to higher frequencies: