A MCM-based Micro-system for Soil Moisture Measurements A. Valente 1 , Raul Morais 1 , Carlos Couto 2 , J. H. Correia 2 1 UTAD University, Dept. of Engineering, Apartado 202, 5001 911, Vila Real, Portugal, avalente@utad.pt 2 Minho University, Dept. of Industrial Electronics, Guimarães, Portugal SUMMARY where X m , X o , and θ v are the mineral, organic, and water fractions of the soil, respectively. The leading coefficients represent the volumetric heat capacity (MJm -3 ºC -1 ) of each soil constituent. When a pulse of heat is applied during a fixed interval of time to the heater probe, the maximum rise in temperature (∆T m ) at some distance from the heater is measured. The relationship between the ρc p and ∆T m is [1], This work presents a Multi-Chip-Module (MCM) based microsystem for irrigation control in agriculture. The proposed microsystem includes a soil moisture microsensor, an analog-to-digital converter, signal processing circuits (digital filtering and sensor interface) and a RF 433MHz transmitter. A heat- -pulse technique is used to determine the volumetric heat capacity and hence the water content of a porous media, such as the soil. This method is based on the Joule effect (heater probe) and in Seebeck effect (temperature probe). By using CMOS standard processes (low-cost) a network could be implemented in order to achieve an accurate measurement of the soil moisture at the plant root level. 2 p m q c er T ρ π = ∆ (2) where, q (Jm -1 ) is the heat applied per unit length of the heater, e is the base of natural logarithms, and r (m) is the distance between the heat and temperature probes. Substituting Eq. (1) into Eq. (2) and rearranging yields an expression that shows the relationship between θ v and ∆T m , Keywords: Soil moisture sensor, integrated micro- sensor, heat capacity sensor, sigma-delta converter. ( ) 2 1.92 2.50 4.18 m m o q T er X X v π θ ∆ = + + (3) Subject category: Applications INTRODUCTION MACROSENSOR The economics factors and the desire to minimize resource over-consumption have increased the requirements for irrigation management systems to manage water more efficiently. This need has led to an explosion in the range of equipment available for measuring soil water status. A heat-pulse macro device [2], illustrated schematically in Figure 1, was used in this study to test the dual-probe heat-pulse method. Today, a large number of sensors, based on different methods: nuclear, electromagnetic, tensiometric, capacitance, are available for measuring soil moisture. Generally, these methods present several drawbacks in irrigation management systems: soil dependency, inaccuracy and high cost. Therefore, the development of a low-cost miniaturized system with electronics, network solution, and external communications that could be implemented next to the plant roots will be a breakthrough. Fig. 1: Sketch of the soil moisture macrosensor THEORY SYSTEM OVERVIEW The heat capacity of soil, ρc p , is evaluated by adding the volumetric heat capacities of the soil constituents: The multi-chip module is basically made of three blocks, schematically represented in Figure 2: a micromachined sensor, a mixed signal interface, and a wireless front-end. 1.92 2.51 4.18 p m o c X X v ρ θ = + + (1) 780 TP79 The 16 th European Conference on Solid-State Transducers | September 15-18, 2002, Prague, Czech Republic Applications