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ee:hydrophones:start [2018/01/24 20:31] Ryan Summers |
ee:hydrophones:start [2018/01/28 10:45] Ryan Summers |
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| ===== Introduction ===== | ===== Introduction ===== | ||
| - | The purpose of this page is to describe how the arrival times are acquired on a number of hydrophone channels. This documentation does //not// describe how that information is turned into bearings or how it is used in localization. | + | //Note: The purpose of this page is to describe how the arrival times are acquired on a number of hydrophone channels. This documentation does //not// describe how that information is turned into bearings or how it is used in localization.// |
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| - | The arrival time calculations tend to be complicated by noise, which means that it is not possible to trivially detect rising edges. The ping can be thought of as an identical signal arriving slightly later on each of the hydrophones. In order to accurately calculate the time differences between each signal, a [[https://en.wikipedia.org/wiki/Cross-correlation|cross correlation]] is used. A cross correlation essentially slides one signal over the top of another, and at the point where the signals maximally overlap, the amount of shifting needed is equal to the time delay of the signal. | + | The arrival time calculations tend to be complicated by noise, which means that it is not possible to trivially detect rising edges. The ping can be thought of as an identical signal arriving slightly later on each of the hydrophones. In order to accurately calculate the time differences between each signal, a [[https://en.wikipedia.org/wiki/Cross-correlation|cross correlation]] is used. A cross correlation essentially slides one signal over the top of another, and at the point where the signals maximally overlap, the amount of shifting needed is equal to the time delay of the signal. An education GIF of a cross-correlation is available [[https://media.giphy.com/media/VVPKOXc6aY1Lq/source.gif|here]]. |
| Further simplifications can be applied to the problem as well. By ensuring that the hydrophones are close enough, it can be guaranteed that the arrival time difference will never exceed one half wavelength. Therefore, the cross correlation only needs to be completed within +/- half a wavelength. The signals occur at a maximum frequency of approximately 40KHz, which means that the arrival time difference can never exceed approximately 12.5 microseconds. This means that the hydrophones must be spaced within 1.8cm of each other due to the speed of sound in water. | Further simplifications can be applied to the problem as well. By ensuring that the hydrophones are close enough, it can be guaranteed that the arrival time difference will never exceed one half wavelength. Therefore, the cross correlation only needs to be completed within +/- half a wavelength. The signals occur at a maximum frequency of approximately 40KHz, which means that the arrival time difference can never exceed approximately 12.5 microseconds. This means that the hydrophones must be spaced within 1.8cm of each other due to the speed of sound in water. | ||
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| | reset | N/A | Causes the Zynq to perform a software reset. | | | reset | N/A | Causes the Zynq to perform a software reset. | | ||
| | threshold | unsigned int | Sets the HydroZynq ping ADC threshold value. | | | threshold | unsigned int | Sets the HydroZynq ping ADC threshold value. | | ||
| + | | debug | unsigned int | If data is 0, HydroZynq debug mode is disabled. Otherwise, debug mode is enabled. | | ||
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| + | In debug mode, the HydroZynq records for 2.1 seconds, dumps data to the data stream port, and repeats. No correlations are performed. | ||
| ===== Hardware ===== | ===== Hardware ===== | ||
