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Digitisers, software for mechanical measurements analysis

16 Sep 2015  | Arthur Pini, Greg Tate, Oliver Rovini

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A great many details arise when using transducers. You should consult the supplier's data sheets, application notes, and recommendations when selecting a transducer for a particular measurement.


Experimental data and analysis
The digitiserNETBOX was controlled using Spectrum's SBench6 software. This is a full featured software tool for acquiring data using the digitiser. It can display the acquired data with proper scaling in mechanical units and it offers numerout signal processing and measurement tools.

Figure 4 shows an example of the acquisition, analysis and measurement of the data in this experiment. It uses SBench 6's parameter measurements, Fast Fourier Transform (FFT), and rescaling capabilities.


Figure 4: The measurement of the vibration and acoustic properties of a small cooling fan instrumented with a tachometer, accelerometer, and microphone.


The screen image shows the tachometer output in the left-most grid. This waveform consists of one pulse per revolution of the fan. The fan speed is read by measuring the frequency of this signal. The Frequency readout in the Info pane on the left centre of the figure reads this frequency as 27.8Hz (revolutions per second). Multiplying this frequency readout by 60 gives the rotational speed of the fan as 1668 revolutions per minute (RPM). Statistical readouts showing the minimum, maximum, and deviation of the frequency appear below the frequency readout.

The accelerometer's output appears in the upper centre grid labelled "Accelerometer Output." A custom scale has been setup using the analogue channel settings to read directly in g's. Measurements of the signals peak to peak and effective (rms) amplitudes appear in the Info pane. This time domain view of the signal is somewhat difficult to interpret so the FFT of this signal is computed and shown in the upper right hand display grid.

The FFT shows the frequency components that make up the acceleration signal. A spectral view provides easier physical interpretation because it separates the various frequency components. The left most peak occurs at 27.8Hz, the rotational frequency of the fan motor. There are also harmonic components at 56, 83, 111, and 140Hz. The third harmonic at 83 is higher than the others because it is also the blade passing frequency. As each of the three fans blades passes the fixed struts that support the motor in the fan housing, they induce vibration into the frame. The large peak at 120Hz is vibration due to the rotating magnetic field in the induction motor.

The microphone output is shown in the centre bottom grid labelled Acoustic Sound Pressure. The data has been rescaled to read in units of pressure, namely Pascals. Measurements in the Info pane show the signal's peak-to-peak and effective amplitudes. As in the case with the vibration signal, the FFT of the acoustic signal provides a good deal of physical insight. Note that the two principal spectral lines are at 84Hz and 168Hz. This is the blade passing frequency and its second harmonic. The primary narrowband acoustic signals are related to the movement of the fan blades. Low-frequency mechanical vibration and broadband "air noise" make up the raised baseline of this FFT.


Conclusion
Mechanical measurements can be easily made with a digitiser, all you need are appropriate transducers and accessories. The basic setup and operation of some simple mechanical measurements have been shown along with the use of some helpful digitiser software tools for analysis of data. This includes parameter measurements in appropriate units of measure and extraction of useful information, like variation in rotational speed, from the raw data.


About the authors
Arthur Pini is a technical support specialist and electrical engineer with over 50 years experience in the electronics test and measurement industry. He has supported oscilloscopes, real-time spectrum analysers, frequency synthesisers, digitisers and arbitrary waveform generators for leading manufacturers.

With over thirty years of experience working in Test and Measurement, Greg Tate specialises in high speed ADC and DAC technology and has extensive knowledge of oscilloscopes, spectrum analysers, arbitrary waveform generators and digitiser products.

As an electronic engineer, Oliver Rovini is working at Spectrum since 1995 now. As head of development, he is responsible for hardware and software developments in the area of high speed digitisers and arbitrary waveform generators for PC platforms like PCIe, PXI, Ethernet.


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