Development of a focused ultrasonic array system for pasture biomass estimation : a thesis presented in fulfilment of the requirements for the degree of Master of Engineering at Massey University, Albany, New Zealand

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The ability to accurately measure pasture biomass can significantly impact the profitability of the pasture agriculture industry. One technique that has been used to estimate pasture biomass is to measure pasture height using ultrasonic transducers. It was traditionally achieved using a single ultrasonic transducer with a wide beam angle. Additionally, the previous studies using this method only used the first arrival time of the echo from the top of the grass. However, this can lead to overestimating grass height due to isolated pieces of grass, which may not be directly below the sensor. It does not measure the pasture density. Also, height measurement errors may occur when the sensor is mounted on an agricultural vehicle as the vehicle bounces and tilts. To solve these problems, Legg and Bradley developed a new ultrasonic air-coupled transducer array to estimate the biomass of pastures and achieved good experimental results. However, it was believable that measurement accuracy can be further improved using near field focusing of the transmit and receive arrays. This work describes the development of an ultrasonic array system capable of focusing on the near and far fields for pasture biomass estimation. It extended on the system developed by Legg and Bradley. Angular measurements were made with the array attached to a computer controlled turntable system for different near- and far-field beamforming configurations. It was found that improved beamwidth and dynamic range were obtained when the system focused on the receiver in the near field. Some initial lab measurements were also performed on pasture samples, comparing the effect of using the array's transmit far-field and near-field focusing. The results indicate that focusing the array in the near field improves the performance in detecting the grass, particularly the top, compared with focusing the receiver in the near field and the transmitter in the far field. However, more work is needed, including field trials.