TIDUES5A August 2019 – September 2019
As mentioned above, the acquisition algorithm is run when there is no previous memory of the AbsTOF value or when the algorithm detects an anomaly in two consecutive measurements of AbsTOF. Figure 5 shows a representative captured ADC waveform. The small circles in this figure represent the captured points.
It is important to ensure there is some dead time in the capture prior to the ultrasonic signal. This additional dead time enables the algorithm to operate properly and also ensures the signal is still within the capture window at higher temperatures. The dead time between the start of capture and the signal for a DN-25 pipe should be around 5us.
As given in Equation 6, i is the time index for the signal capture and is the upstream captured signal.
The acquisition algorithm then employs the following steps:
This means the central value is greater than or equal to the two points on either side. The algorithm then uses parabolic interpolation between the for all such i. A parabolic interpolation is chosen (rather than the cosine interpolation for dTOF calculation) for reduced complexity, and the accuracy on AbsTOF estimation is not as tight as the dTOF estimation in terms of contribution to the overall flow error. The algorithm tabulates all these interpolated values. Letting denote the interpolated value these tabulated values can be given by . These interpolated values would now correspond to the "maximum of lobe" values of each wave in Figure 5. These are shown by dark red dots in Figure 7 and Figure 8.
The value iK would now be used as an index for the next signal capture. Figure 7 and Figure 8 show the acquisition process.