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In the past, we often use the level measurement instruments from the user's point of view, to discuss how to improve the measurement accuracy of the instrument, for example, from the installation and maintenance of the commissioning of these aspects. Today we will be from the manufacturers of instrumentation from the point of view, to analyze how to improve the accuracy of the level measurement instruments, in this paper we will focus on a data acquisition method, this method is important to improve the measurement accuracy and response speed of the instrument.
The data acquisition processing method that we are going to share today is to use a low-power CPU with the AD module, DMA module, and timer module that comes with the CPU. When acquiring waveforms, the TA timer is used to start timing, start AD conversion, and after AD conversion is completed, start DMA to transfer data.
(Image source: Internet)
First of all, coarse sampling, sampling the original echo signal at a relatively low sampling rate, using a timer to reach 25K time to generate an interrupt signal to start the AD conversion, AD conversion is completed interrupt flag triggers the DMA transfer of data, as well as RAM inside the DMACPU information to the domain, when the amount of data transferred by the DMA reaches 1,000, the DMA interrupt to complete the data transfer, complete the original echo of the Coarse sampling, give the data processing flag, the CPU processes the data, generates false echoes, calculates the echo curve, determines the starting point position and the real effective echo position, and calculates the delay time required for the starting point and the effective waveform position. Secondly fine picking the starting position, that is, sampling the starting position of the original echo signal at a higher sampling frequency, according to the delay time calculated during coarse sampling, the delay time is in the front, the delay time is on the top, the control AD, timing TA, DMA module scores close to the starting point of the 80 points, the sampling frequency is higher. Fine-picked effective waveform position, that is, the effective waveform position of the original echo signal is sampled at a higher sampling frequency. Time to select the calculated delay time first delay, after the end of the delay time, control AD, timer TA, DMA module in the sampling frequency higher effective waveform location near the 80 points. After the acquisition is completed, set the data calculation flag and start the calculation. Finally, it is data calculation, according to the starting point, effective waveform and cumulative delay time, according to which the exact starting point position and the material reflection point position are calculated, the difference between the reflection point position and the starting point position is the corresponding time difference, the reflection distance, and then the actual level is calculated accordingly to the sampling frequency.
In summary, the approximate position of the effective signal space is determined by coarse sampling, at which time the accuracy is low, and then only this part of the standing wave pattern is collected by high sampling frequency and the calculation process is performed. The measurement accuracy of the level measuring instruments is improved on the basis that the amount of collected data does not increase too much. In general, after using the coarse mining and fine mining of this square transmission, reduces the CPU's computational task, greatly improves the level measuring instruments' response speed and measurement accuracy.
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