Research On ADCP Frequency Offset Responsive Calibration Method For Low Flow Velocity

ADCP is an instrument for velocity measurement in large-scale waters,such as ocean,based on Dopler’s principle.It has many advantages,such as non-contact,layered measurement and continuous monitoring.At present,there are three main calibration methods for ADCP,namely,walking method,comparison method and towing method.The former two methods are restricted by the calibration environment on site and the minimum ship speed,so their stability is not high and they can not be used to calibrate the small flow velocity,the main disadvantage of the latter is that it is difficult to establish standards and high cost,and there is still no mature measurement method and standard device to calibrate ADCP.Based on the principle of ADCP flow velocity measurement,a set of calibration method and equipment for small flow velocity(0.02 ～ 1m/s)is developed.The frequency offset response is used to receive and send back the frequency offset signal to simulate the water layer echo,and the flow velocity-frequency offset-frequency traceability system is established to calibrate the ADCP current measurement capability.The working principle and flow chart of the calibration system are introduced.This paper presents two methods to realize the high precision frequency offset signal,introduces the frequency offset principle and compares the frequency offset precision,finally chooses the digital frequency conversion method,the frequency offset precision can reach 0.0002 Hz.The underwater propagation model of ADCP signal and the water layer scattering model are established,and the echo signal is simulated and verified by comparing with the measured waveform.The device is designed and built.The structure and design index of the instrument end and the control end are introduced.The hardware structure and function for signal processing are introduced,and the process of signal processing is described in detail.Velocity calibration experiments were carried out in different environments.The experiment in an anechoic tank verifies the feasibility of the calibration system for the small flow rate of 0.05 ～ 1m/s in the laboratory.The maximum uncertainty is 2.3mm/s.The results show that the calibration results are affected by the underwater echo signal,which is compared with the self-checking model in the Zhoushan current,the maximum uncertainty in calibration range is 18.2 mm/s and 14.0 mm/s,respectively.The calibration experiment of small flow velocity in Qingdao Sea area proves that the experimental device has a good calibration ability for the range of0.02 ～ 1m/s small flow velocity in the field water area,the measurement uncertainty of calibration results is all within the range of 5‰ velocity + 5 mm/s.