Sometimes, more than just a number is needed. Time waveform capture and PC software offer the ability to look at the captured waveforms and apply signal processing effects to them. Effects applied to the time displacement waveform ranges from rate of change for velocity and acceleration to fast Fourier transforms (FFTs) and spectrograms for frequency analysis.
Recording the sounds to play back the audio later allows sharing of information with others for training and further analysis. Some industrial plants require double hearing protection that makes on-site listening difficult. But audio signal recording is not affected by the noise and allows listening and analysis later in the office. The amplitude of some ultrasonic signals, such as slow speed bearings, can be too small for a stable dB reading, but can be heard, recorded, amplified and analyzed.
The one drawback (sometimes major benefit) to PC audio software is you have to go back to the office for listening and analysis.
WHY CAPTURE WAVEFORMS? |
1. Signals too small for dB reading 2. Slow speed bearings 3. Training 4. Human factor – we all hear differently 5. Critical equipment history/baseline/fault analysis |
AUDIO CAPTURE DEVICES
Today, some of the top-of-the-line ultrasonic devices have onboard recording and display capabilities.
There are also third party add-ons for capturing the time waveform for audio playback, visual display and signal processing. These choices include: digital voice recorders, oscilloscopes, vibration spectrum analyzers, and PC-soundcard- software combinations. There are pros and cons to each hardware and software combination.
BUILT-IN RECORDING CAPABILITIES
Some manufacturers of ultrasonic detectors include the capability for on-site time waveform display and recording of the output audio signal with the capability of processing the time waveform to the frequency domain. Other equipment makers’ devices can capture the waveform internally, but must be taken back to the office PC for viewing and software manipulation.
Figure 1: UE Systems 15,000 displaying time waveform and FFT
EXTERNAL RECORDING DEVICES
For companies that cannot afford the latest equipment or who already have existing ultrasonic detectors, there are several external devices that can plug in to the headphone output jack to display and capture the audio signal for analysis. There are advantages and disadvantages to each method, such as whether the waveform can be viewed on-site or taken back to the office, whether the captured audio can be replayed and emailed, and whether the maximum recording time length is measured in seconds or hours.
The four devices commonly used to record the audio headphone output are: vibration spectrum analyzers, oscilloscopes, digital voice recorders and PC-soundcard-software.
VIBRATION SPECTRUM ANALYZER
Most maintenance departments performing ultrasound scans already have vibration spectrum analyzers (VSAs) in their possession. The analyzer can be connected to the ultrasonic detector output to display the time waveform and convert that to a spectrum in the frequency domain. Spectrums present a complex sound waveform in terms of its component frequencies. The sensor input settings of the VSA are important. The VSA normally supplies power to accelerometers. This power must be disabled (turned off) when connecting to an ultrasonic detector.
OSCILLOSCOPE
An oscilloscope is an instrument for viewing a graphical representation of how a signal’s amplitude changes over time. The waveform is graphed on an X versus Y axis. The horizontal X-axis represents time. The vertical Y-axis represents voltage. The graph illustrates how the signal changes over time. There are adjustments for the scaling of both axes and triggering of the waveform. Today's handheld portable oscilloscopes offer an affordable and easy alternative for viewing the time waveform on-site for analysis.
DIGITAL VOICE RECORDER
What’s beneficial about using a digital voice recorder is the long recording times possible (hours), the ability to play back the audio for others and the ability to dictate notes directly into the audio recording. A few of the most important settings for repeatability are the microphone setting (e.g., low or high), sampling rates, and audio recording format (e.g., wav, mp3, wma).
PC - SOUND CARD - SOFTWARE
A PC and a sound card can be used with audio recording software to capture the ultrasonic detector headphone output. The PC recording volume input slider makes repeatable amplitude measurements difficult. We use precise voltage dividers to interface between the detector output and the PC input and then set the recording volume slider to maximum. Environmental concerns must be addressed when using a PC in the plant and must be weighed against benefits, such as audio playback and nearly unlimited recording times.
INTERCONNECTING CABLES
Four areas must be addressed in the connection and setup of external equipment. They are: cabling and interconnecting hardware, voltage levels, equipment settings and calibration/testing.
Custom-built cables are often required to interconnect the ultrasonic detector output to the input of the various capture devices. Depending on the manufacturer of the ultrasonic equipment, you may need a 1/8-inch or 1/4-inch phono connector for the ultrasonic detector output. The input of the vibration spectrum analyzer may be a BNC connector. The input to digital voice recorders or PC-sound card-line in/microphone input are 1/8-inch connectors. The maximum output voltage of the ultrasonic detector must not exceed and should be matched to the maximum input voltage of the recording device. For example, if the maximum output of the ultrasonic detector is 8 volts peak-to-peak (p-p) and the maximum input voltage of the VSA is 2 volts p-p, then a 4:1 voltage divider is needed and can be constructed with resistors. A digital voice recorder may need a 500:1 or a 1000:1 voltage divider.
If the signals are not matched, clipping of the signal can result or the signal may be too small to be usable. Both situations render any post processing results, such as FFTs, unreliable.
Ultrasonic equipment settings, such as sensitivity/amplification/volume control and frequency tuning, must be recorded for repeatable and comparable measurements.
An ultrasonic tone generator is an invaluable tool for testing and calibrating. We use an ultrasonic tone generator before and after any serious waveform recording to ensure all equipment is connected and working properly. The tone generator transmits alternating bursts of two different frequencies. The time waveform will display two repeatable sine waves and the frequency domain will show distinct lines.
COMPUTER SOFTWARE FOR ANALYSIS
Add-on PC software offers a wide range of prices and capabilities for analysis, from simple displays of the time waveform to applying sophisticated signal processing effects. These effects can include taking the first and second derivative of the time displacement waveform and converting to velocity and acceleration waveforms. Other effects include transformation to the frequency domain for spectrum plots and spectrograms. A spectrum plot takes the audio in blocks of samples, does the FFT and averages all the blocks together to produce one picture. A spectrogram shows how the amount of energy in the different frequency bands change over time and can be described as video versus a photo.
Figure 2: The display to the left shows the two sine waves of a tone ultrasonic generator. The display to the right shows clipping because the amplification of the ultrasonic detector is set too high.
Documenting the various settings involved with time waveform recordings are required for proper analysis. You may want to document and build a library of waveform signatures and measurements for future reference and comparison purposes. Having a reference to compare can prove invaluable with future troubleshooting. Capture reference waveforms:
- Before and after installation.
- After changes or additions.
- After repairs or upgrades.
Remember, it all starts with assessing which equipment is worth the time and effort of capturing waveforms. One of the benefits of ultrasonic detectors is the simplicity of use. Sometimes, a number is all that's needed for comparison. But there are other times when a picture is worth a thousand words.
