pick-up to receive the reflected waves. Two reflected pulses are received in quick succession. The first comes from the reference surface as this travels the shorter distance. The second comes from the measurement surface. The more viscous the oil, the more energy is absorbed, making this reflected pulse smaller in amplitude. The ratio of the pulse amplitudes therefore provides a measure of the oil's viscosity.
Interface
The original hardware was replaced with a PC-based oscilloscope/signal generator, which is connected via USB. The unit was supplied with a DLL file to allow communications with application software. In the LabVIEW application, this was embedded in a “Call Library Function” node wherever there was a need to send or receive data, either to configure the unit or read the sampled values. Figure 2 shows the GUI. The Waveform Graph shows the reflected reference and viscosity pulses. The vertical-line cursors define the measurement windows within the pulse, the horizontal lines giving the average value within those windows. The user can control these positions using the slider controls under the Graph to best match the measurement window to the pulse shape.
The controls in the top-left corner control the operating frequency. By adjusting the frequency manually, the user can identify the range of frequency of interest for more detailed analysis. This is also useful for setting measurement window boundaries to appropriate positions such that the correct parts of the waveform are measured across the frequencies of interest. The user can then specify the start, end, step size and
number of readings to average for an automatic sweep of frequencies that are logged to a results file (CSV format) specified at the bottom left of the screen. This file contains time and date information, allowing data to be tied to a particular test-run of the sensor. This can be important, as the sensors exhibit an initial settling behaviour where their response changes over a number of thermal cycles. The file can then be loaded into a spreadsheet program for further manipulation and to produce graphs. The “WRITE TO FILE” button allows individual manual readings to be captured to the results file.
The calculated ratio, which decreases with increasing viscosity, is displayed on the left-hand side of the GUI. The raw value is displayed, along with a version that is smoothed using a Moving Average filter. Below this is the oil temperature, measured using a thermocouple. This is logged alongside the viscosity data, and a temperature chart on a separate Tab Control page shows a historical display of the temperature. This serves as a utility when stepping the operating temperature to show when the oil temperature has stabilised at the next setpoint.
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