System identification method for Ultrasonic Intake Air Flow Meter for engine test bed applications

In this paper a novel system identification method, developed for the needs of an Ultrasonic Intake Air Flow Meter (AVL FLOWSONIX™ Air (FSA)) for engine test bed applications is presented. An intake air flow meter must provide high measurement accuracy (1% of reading) and stable long-term performance over the extended measurement range of (1∶80). Thus, the required accuracy and stability of the measured flow velocity must be better than 0.05 (m/s). The reachable accuracy is strongly influenced by the group delay of the ultrasonic sensors, being a component of the total measured transit time. The goal of this work was the development of an identification method of the actual group delay of the ultrasonic sensors, executable during the regular flow measurement. For this purpose, at first a theoretical investigation of the achievable system accuracy, based on the Cramer-Rao lower bound, has been performed. Further, an approach for system identification based on the fitting of the system response function with a band-pass filter model of the 4-th order has been proposed. Special attention has been given here to the selection of a correct filter model and input parameters of the fitting procedure. A maximum length sequence has been used as the carrier signal. Finally, the proposed approach has been proven by a series of short and long-term zero flow measurements, which have shown the improvement of the accuracy of the flow measurement by at least a factor of 10.