![]() To those who are much younger than me, I need to say that the hard disc in my PC in 1990 was an impressive 10MB. The processors were too slow to record the raw signal, and the memory was too small to save such a large amount of data. The readings were saved, but the “raw record” was not. The tape recorders were quickly abandoned as all of their readings could be saved at this stage to the memory of the portable measurement device. In the 1980s the first “portable analysers” appeared on the market and they caused a revolution in the field of vibration diagnostics – especially in route measurements. This device was even bigger and heavier than the tape recorder! To analyse the recording a separate “analyser” was needed. Back then they were huge and heavy devices, so a strong guy was needed to perform the route with this equipment. When I started to work in the field of vibration diagnostics a long time ago in the 1980s, tape recorders were used for signal recording. All required readings (for example overall values, spectrum and time waveform) can be processed from the record in the computer later. The record can be required also in route collection. It is better to make a recording of the whole cycle and then pick up the required part of it and analyse it. The whole cycle can take just a few seconds. An elevator engine is another example first here is a short run-up, then the elevator runs constantly for a while, then it stops. This can be caused by change of power or other similar condition. This approach would have eliminated the need for the preamplitude trigger that resulted in the disastrous experience described above.Īnother situation, when it is better to record the raw signal, is a measurement of a machine that continuously changes its state. In this case, the recording would have been started manually and allowed to run maybe up to a few minutes before the explosives detonated and then manually stopped. This was an example of a perfect situation when a recording should have been done of the whole event and post analysed. When we jokingly suggested performing the detonation once again, the customer did not find the idea funny at all. Trust me – it was very hard to explain to the customer why we could not perform the analysis. ![]() This meant that we could not perform the analysis as ordered by our customer. We had lost the whole beginning of the detonation signal and the measurement had started 250ms after the wave reached the sensor. Instead of a 250ms pre-trigger we had set up a 250ms post-trigger! We opened the setup menu in the analyser with a trembling hands and immediately realised what had happened. There were high blast amplitudes present from the very beginning of the measurement (in the first sample as well) and they decreased with time. There was no expected low noise at the beginning with the sudden increase from the blast energy. When we viewed the signal we instantly knew something had gone wrong. ![]() We were all nervous as we counted down to detonation we held our breath and the explosives were detonated. Naturally we had used a pre-trigger (250ms), so we could see the noise just before the detonation. We had set up the amplitude trigger so that the wave from the detonation coming to the first sensor would start the measurement. The required measurement report should have contained time waveform from each sensor and a graph showing the energy decay curve with respect to the distance from the detonation centre. Moreover, the measurements needed to be taken synchronously from many sensors placed in different spots at various distances from the point of detonation. The preparation had taken the whole day due to the sheer volume of explosives involved. My personal experience happened when we were supposed to measure explosion vibrations in a quarry. The risk of setting up the measurement parameters inappropriately or making a mistake in setting up phase can be quite high, especially with the added pressure of knowing that the measurement cannot be repeated. Run up and coast down are the typical examples of this situation. The reason is that the running time of the machine is too short to be able to take the readings repeatedly. WHEN TAKING measurements and analysing the vibration directly on site we can find ourselves in situations where time does not permit trying various measurement types with various parameters. ![]() ![]() This article describes how technology that was thought to be obsolete is again in action. ![]()
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