Vol.3 No.3 2010
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Research paper : Development of a sensor system for animal watching to keep human health and food safety (T. Itoh et al.)−227−Synthesiology - English edition Vol.3 No.3 (2010) 3 Strategy for achieving ultra low power consumption of the nodesTo achieve a small (can be attached to the wing), lightweight (about 1 g), flexible and maintenance-free (2-year lifespan) wireless sensor node, we devised a new digital sensor using the MEMS switch[6][7]. This digital MEMS sensor is composed of an array of switch sensors, is capable of directly outputting digital signals without an AD conversion circuit, and can be used as a start-up trigger of the wireless sensor node in sleep (clock stop) modes. The individual switches that compose the sensor are micromechanical switches that use the MEMS technology, and do not require power for ON/OFF of the switch itself. Specific explanation will be provided using the digital bimetal thermosensor and digital piezoelectric accelerometer developed in this research.Figure 4 shows the schematic diagram of the digital bimetal thermosensor[7]. It is very simple in principle, and is composed of an array of bimetal cantilevers that come in contact with the opposite electrode above a certain temperature. Since the normal body temperature for chickens is about 41 °C, it can be considered that a fever is occurring due to some health abnormality if the temperature rises above 42.5 °C as in the case of infection by the Yokohama strain, as shown in Fig. 7. Therefore, a bimetal cantilever that turns ON at 42.5 °C or above is installed. When the contact is ON due to temperature increase, the node wakes up from sleep, and transmits a message that includes only the fact that the contact is ON as the sensing information. This is the basic thinking of the event-driven concept. In the case of the intermittent operation mode using a timer, the detection timing is determined by the set interval regardless of the occurrence of the temperature increase event, while in the event-driven node, the moment of temperature increase can be detected instead of the temperature value. The event-driven type can be used to shift the node from the sleep mode to the emergency time-driven measurement mode, instead of sending one transmission at the occurrence of the event. For the event, the individual difference can be considered, as well as multiple bimetal cantilevers can be installed for each temperature value setting to detect the rough temperature change. For example, if the multiple bimetal cantilevers of different sizes are installed and the ON temperatures are set at certain intervals such as 0.5 °C, it can be used as a digital thermosensor. The meaning of “digital” here is that it is possible to detect the digital signal of 1100, or the four switches of ON, ON, OFF, OFF, if the sensor output is the digital signal itself, and this can be included in the message without alteration. The ON detection of a mechanical switch can be done with very low power, and the standby power of the sensor is basically the power for the semiconductor switch. While the bimetal switch can be fabricated without the MEMS technology, the use of MEMS technology is vital in order to achieve the downsizing and cost reduction of the bimetal thermosensor, since the multiple three-dimensional microstructure can be realized at once on a silicon wafer without an assembly process.The activity level sensor can be constructed from the mechanical switch that turns ON when the acceleration of certain level or higher is inputted, but we developed a sensor where piezoelectric thin-film is formed over the cantilever, as shown in Fig. 5. Here, the technological details will not be provided[8][9], but the power is generated by the piezoelectric effect when the cantilever is activated, and the transistor can be turned ON or OFF using this power. In theory, a digital accelerometer with zero-power consumption is possible. As in the bimetal thermosensor, it is possible to arrange the cantilevers with different sensitivity, but it is also possible to arrange a series of the same cantilevers[6] or devise the circuit to extract the digital output that corresponds to a certain acceleration threshold with one cantilever[10]. From the result of the experimental infection using the experimental node that will be described in chapter 5, it is known that health abnormality can be detected 10 hours beforehand[3] by counting the number of occurrences of accelerations surpassing the threshold within a certain time (30 minutes, for example) and by comparing the number with the number for 24 hours Fig. 3 Overall picture of the development process (outline of the scenario)Table 3 Main elemental technologies for the chicken health monitoring system Development ofprototype sensor node,and wirelessnetwork systemExperimental farm(poultry house) experiment(prefectural livestockresearch center)Prototype of working sensor node and systemDemonstration at experimental farmDevelopment of animalexperiment sensor nodeExperimental animal infection (NIAH)Development ofcustom devices(MEMS sensors,custom RF-IC)Selection of appropriate event, setting of thresholdExtraction of issues in actual practiceMEMS fieldsInformationfieldsLife sciencefieldsPoultry house monitoring systemDirect conversion receiver systemFlexible node packaging technologyminiaturized antenna (315 MHz)Custom RF-IC (event-driven type)Digital piezoelectric accelerometer (activity level sensor)Digital bimetal thermosensorULP wireless sensor node

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