Wavelet based denoising technique for liquid level system

Abstract

The purpose of this paper is to employ wavelet based noise removal technique to remove measurement noise from differential pressure transmitter (DPT) output indicating the level of a process tank. The liquid level system (LLS) is approximated as a first order plant with time delay. When connected in closed loop, the LLS is ideally associated with a PI controller, an actuator, a final control element and a level sensor (DP transmitter in present system). There are enough chances of contaminating the output of LLS by the high frequency measurement noise. In this work, an attempt has been made to judge the performance of discrete wavelet transform (DWT) and discrete wavelet packet transform (DWPT) based noise removal technique when applied to LLS. The available denoised response using wavelet based filtering is compared with that of conventional Butterworth filtering method. Applying hard thresholding that typically retains a very small number of coefficients, the results are often smoothed at the cost of loosing information. By retaining larger number of coefficients and shrinking them, the soft thresholding methods usually give more useful results to denoise the signal. Threshold value has been calculated by Visu Shrink method and Minimax method in different cases.