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Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

Synthèse de haut-niveau de contrôleurs ultra-faible consommation ...

tel-00553143, version 1

tel-00553143, version 1 - 6 Jan 2011 36 WSN node architectures and low-power microcontrollers Measurement for Wireless Sensor Networks Measured Transduction principle Physical properties Pressure Piezoresistive, capacitive Temperature Thermistor Humidity Resistive, capacitive Flow Pressure change Motion properties Position GPS, contact sensor Velocity Doppler, Hall effect Acceleration Piezoresistive, piezoelectric Presence Tactile Contact switch, capacitive Proximity Hall effect, magnetic, seismic Distance Sonar, radar, magnetic Motion Sonar, radar, acoustic, seismic (vibration) Table 2.1: Some measured quantities and corresponding physical principles used to measure them. Line of Sight (LoS) between sender and receiver. Some examples of RF-based radio transceivers are CC 1000 [132] and CC 2420 [131] from Texas Instruments, and TR 1000 from RFM [120]. 2.2.3 Sensing subsystem WSN nodes may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic and radar which are able to sense a wide variety of environmental conditions. Table 2.1 summarizes some of the physical principles that may be used to measure various quantities (as indicated by Lewis [38]). Actuators are just about as diverse as sensors, yet for the purposes of designing a WSN, they are a bit simpler to take account of: in principle, all that a WSN node can do is to open or close a switch or a relay or to set a value in some way. Whether this controls a motor, a light bulb, or some other physical object, is not really of concern to the way communication protocols are designed. 2.2.4 Power supply subsystem The power supply subsystem consists of a power supply and (possible) DC-DC converter. Power supplies can be of different types but they mostly lie in two major categories: (i) Non-replenishing: mostly consisting of simple batteries and (ii) Selfsufficient: consisting of energy-scavenging mechanism (e.g. solar (photo-voltaic) cells, Piezoelectric and thermal-gradient materials etc.) to harvest the energy and batteries to store it. Moreover, batteries (or other forms of energy storage) alone are not sufficient as a direct power source for a sensor node. One typical problem is the reduction of battery voltage as its capacity drops. This can result in less power delivered to the sensor node circuits, with immediate effects on oscillator frequencies and transmission power. Hence, a DC-DC converter is used to overcome this problem by regulating the voltage delivered to the node circuitry. However, to ensure a constant voltage even

tel-00553143, version 1 - 6 Jan 2011 Power dissipation analysis of a WSN node 37 Sensor / Actuator Sensor Subsystem Computation Subsystem Controller + Memory Power Supply Power Subsystem Solar Cells + Ctrl. Mechanism Location Finder Mobilizer Communication Subsystem Tx Rx Energy-Harvesting Subsystem Figure 2.2: Block diagram of a mobile sensor WSN node. though the battery output voltage drops, the DC-DC converter has to draw increasingly higher current from the battery when the battery is already becoming weak, speeding up battery death [116]. Also, the DC-DC converter does consume energy for its own operation, reducing overall efficiency. However, the advantages of predictable operation during the entire life-cycle of the node can outweigh these disadvantages. In addition to the four basic subsystems discussed-above, a WSN node can contain additional application-specific subsystems. These additional subsystems can be location-finding and mobilization subsystems in a mobile sensor node or an energyharvesting subsystem that works in cooperation with the power-supply subsystem to create an autonomous sensor node w.r.t. the energy needs [5]. Figure shows a scenario where these additional subsystems are included to create a energy-harvesting mobile WSN node. In the next section, we will discuss the power profile and major sources of power dissipation in a WSN node architecture. 2.3 Power dissipation analysis of a WSN node As mentioned earlier, in some applications, replenishment of power sources might be impossible. Sensor node life-time, therefore, is strongly dependent on battery life-time. In a multi-hop sensor network, each node plays a dual role, both of data originator and data router. As a consequence, few node failures can cause significant topological changes and might result in re-routing of packets and re-organization of the network. Hence, power consumption and power management take on additional importance. For these reasons, both power-aware software (such as protocols and algorithms) and hardware design are the current focus of WSN research community. The main task of a sensor node in a sensor field is to detect events, perform quick local data processing, and then transmit the data. Power consumption can hence be divided into three domains: sensing, communication, and data processing.

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