Research On Acoustic Doppler Current Profile Technology

The Acoustic Doppler Current Profiler(ADCP)is an emerging device that can directly measure the velocity of layered profiles featured by short measuring time and wide measuring range.With the development of broadband ADCP,the use of coded signals further enhances the ADCP's measurement performance,contributing to high temporal and spatial resolution.Traditional ADCP processing systems usually use DSP and analog demodulation filtering systems.In the process of the analog demodulation filtering,additional noises will be introduced and the demodulated quadrature signals will be hardly fully synchronized.However,the use of a fully digital echo processing system will greatly increase the computing load of the DSP chip.In addition,large amounts of data at different depths require large-capacity FIFO buffers for storage,while also reducing measurement speed.Field programmable gate array(FPGA)has developed rapidly in recent years.In the application of ADCP,FPGA has great advantages in realizing the synchronization of signal transmission and multi-channel acquisition.At the same time,in the data processing,the design of the FPGA can be freely traded between time and space resources according to specific needs.This paper first establishes the acoustic Doppler shift calculation model of ADCP,and then proposes FPGA-based ADCP digital processing system.As a comparison scheme of DSP,the main idea of FPGA design is low resource consumption and high throughput.Considering that digital filtering is the main operation addition after full digitization,the system starts with the optimized CIC+ISOP filter bank to construct a step-by-step optimized digital signal processing chain,including demodulation,filtering,complex autocorrelation and phase angle solution after echo acquisition.Finally,this paper establishes the software and hardware platform for ADCP transmission acquisition and processing.The hardware platform includes a transmitting circuit composed of DDS,power amplification,transducer matching,and a receiving circuit composed of echo isolation,gain voltage controlled amplification,A/D acquisition,and other related marine related parameter measuring circuits.The software platform includes a specific FPGA implementation of a state machine based on USB communication and digital signal processing modules.After the platform is built,the experiment is verified in the water tank in which a stratified profile flow rate measurement of the water body is achieved.