Design, fabrication and testing of a low cost Trunk Diameter Variation (TDV) measurement system based on an ATmega 328/P microcontroller

A fast-responding inexpensive dendrometer is needed for measuring daily or hourly plant growth responses to water stress. This study reports on a low-cost microcontroller driven Trunk Diameter Variations (TDV) measurement system that was designed and constructed for automating the measurement and recording of plant stem diameter. The TDV system comprises of an ATmega 328/P microcontroller, which forms the heart of the controlling circuit, a real time clock for time stamping measurements, liquid crystal display for displaying purposes and an external secure digital card and shield for storing measured data. The stem diameter variations are measured by a caliper-type sensor based on a full bridge strain gauge that is attached to a flexible arm of mild steel. When bearing bending strain, the excited full bridge strain gauge outputs a voltage directly proportional to the bending strain, hence linear displacement. The TDV sensor displacement calibration procedure was carried out using an inside micrometer from a calibration kit. The calibration of the sensor indicated a linear relationship between the displacement and the sensor's output voltage with a high coefficient of determination value of 0.998, calibration multiplier of 0.426 mm V-1 and an offset output voltage of 0.688 V. The TDV sensor was mounted on a smooth cylindrical dried wood away from direct sunlight and rain while air temperature was measured close to the TDV installation. Temperature sensitivity test results show that the accuracy of the sensor above 20 degrees C is 0.031 mm and for temperatures below 20 degrees C the accuracy is above 0.050 mm. The TDV system was tested on a tomato plant for 42 days, Fountain tree for three weeks and on a Citrus tree for one week under greenhouse and open-field conditions in order to evaluate its performance and suitability under different plant species and climatic conditions. The TDV system proved to be robust and produced valid results on the plant's physiological measurements, with a good storage of the measured data over the experimental period. We concluded that the TDV system is suitable for stem diameter variations measurements and water and fertilizer stress monitoring on herbaceous and woody stem plants due to its fast response (hourly or less), ease of construction and installation, and low cost (< US$60).