MEMS climate sensor for crops in greenhouses

We have developed and fabricated a multi-sensor chip for greenhouse applications and demonstrated the functionality under controlled conditions. The sensor consists of a humidity sensor, temperature sensor and three photodiodes sensitive to blue, red and white light, respectively. The humidity sensor responds linearly with humidity with a full scale change of 5.6 pF. The best performing design measures a relative change of 48%. The temperature sensor responds linearly with temperature with a temperature coefficient of resistance of 3.95 × 10-3 K-1 and a sensitivity of 26.5 ¿C-1. The three photodiodes have been characterized and show an almost ideal diode behavior with an ideality factor of 1.27 and a series resistance of 14.9 . The diodes are sensitive to blue, red and white light with the measured quantum efficiencies of 69%, 81% and 68%, respectively. The temperature and humidity sensors have further been tested on plants in a greenhouse, demonstrating that individual plant behavior can be monitored. (Some figures in this article are in colour only in the electronic version) 1. Introduction Throughout time, horticulture has been a subject of intensive development and optimization in the attempt to produce more with less investment. In the northern hemisphere, greenhouses are of major importance for horticulture production, mainly because the environment can be controlled and crops can be protected during winter. By controlling specific parameters of the climate inside a greenhouse, plant growth and energy use can be optimized. Factors such as irrigation and air humidity have huge impact on the health and survival rate of the plants [1–3]. The CO2 content of the air as well as the temperature is important factors for the growth rate. Furthermore, the photosynthesis that takes place inside the plant is crucial for the growth and since photosynthetically active radiation (PAR), ranging from 400 to 700 nm, is the only energy source for this process, PAR is a very important parameter to control. Research has shown that specific wavelengths of light in the visible range can alter the height of plants. The architecture of plants can thereby be manipulated by light control [4, 5]. Horticultural production has become more and more automatic and computerized, where bulky discrete sensors are placed at strategic places monitoring vital parameters and sending data to computers that alter the environment if necessary. These sensors often give very precise measurements of the macro-climate in the greenhouse. However, there is an increasing interest for monitoring the environment near the individual plants. Wireless sensor network (WSN) solutions already exist where sensors, measuring humidity and salt concentration of the soil, report through wireless communication to a control unit [6]. To monitor the micro-climate on individual leaves or stems of the plants, still smaller sensors are desirable. The sensors should be sufficiently small to be mounted on the plants leaves without harming the plants and still be able to 0960-1317/10/085021+07$30.00 1 © 2010 IOP Publishing Ltd Printed in the UK & the USA