Embedded Front-End for Xylem Sap Velocity Measurement with Single Pulse-Heated NTC Probe

摘要

In precision agriculture, plants' water-stress management is crucial for preventing damages from drought, optimizing the irrigation costs, and increasing the yield quality. A common technique for quantifying the plants' water-stress is measurement of xylem sap flow using nailed-in thermally excited flow sensors. We present an embedded sensor front-end for a minimally-invasive, low-power, intermittently pulse-heated sap velocity sensor. The design features a single negative temperature coefficient (NTC) thermistor probe, used simultaneously for heating and temperature measurement. Design parameters were assessed by finite-element thermal modelling and verified by laboratory experiments. The sensor front-end enables the measurement of sap velocity from a cool-down time after a single, short heating pulse (25 mJ). The proposed technique is especially suitable for measuring slow flows, enabling the resolution better than 1 μm/s at flow velocities lower than 0.4 mm/s. System's maximal measurement frequency is 6 readings per minute. It enables hourly xylem sap velocity monitoring while consuming only 118 μW.