Wireless sensor networks flexible, easy to implement, and sensors will be placed in areas close to the original, so it's very enticing prospect, can be widely used in bio-medical, industrial, construction (buildings and bridges in the embedded sensors ), Consumer electronics and defence of. However, the new technology will give rise to new problems, mainly wireless sensor nodes of the supply problems. Sensor nodes is the need for long-term work, to keep the replacement battery is time-consuming and laborious, in many occasions is also unrealistic, it is usually not as portable products as often charge. Therefore, the search for a new, suitable for the use of wireless sensor nodes of power is imperative.。 In a number of power, the battery is still the people's choice is mainly a one-time high energy density batteries, the main limitation is its size and cost. In addition, people are more effective research and development of alternative products, so that a class from the surrounding environment in the energy extraction of energy devices by people of all ages. In this paper, small, low-power supply solutions.
Power sensor nodes on the request
In introducing the various power solutions, wireless sensor nodes on the power requirements for some very brief analysis is necessary. Wireless sensor networks with different communications network, has its own characteristics: sensors to monitor temperature, pressure and biological parameters, low data rate, the network utilization rate is not high parameters. This is the calculation of sensor nodes the important basis for power.
Figure 1 vibration electromagnetic generators
2 piezoelectric crystal-generation devices
Figure 3 vibration-section of variable capacitor
As we all know, wireless sensor nodes by the sensors, signal conditioning circuits and components of the RF transceiver. Transceiver power consumption is the major assumption that the average distance between nodes for the 10 m, corresponding wireless transmitter should work in 0 dbm level, the peak power should be between 2-3 mW. The use of ultra-low-power technology, power consumption will not be greater than the receiver 1 mW. There is their own sensors and signal processing circuit power consumption. Both temperature and pressure parameters can be resistance to measure the voltage on its size only to overcome resistance thermal noise (≈ 10-20 W / Hz, at room temperature), power consumption can be neglected. Signal conditioning circuit is A / D converter, it was reported that has successfully developed a 0.5 V, 1mW of successive approximation ADC, its rate of 4 KC / S, we have requested on. Based on the above analysis, the largest peak power is set at 5 mW very reasonable. The largest 100 kb / s wireless data rate and the average of each node 1 kb / s communications load, and each node of about 1 percent of the duty cycle, the average power consumption only 50 μ W. Nodes do not send / receive data, the actual power consumption is low-speed circuit timers, channel monitoring and synchronization circuit nodes, the use of advanced "wake-up technology" and semi-synchronous beacon technology, each node average "standby" power 50 μ W is about. In this way, each node of power requirements for an average of 100 μ W. If the nodes in the distance about 1 m, can further reduce power consumption.
Energy-based power
Currently, wireless devices commonly used by power or batteries, the preferred low-leakage, low cost and high energy density of one-time battery. Application of the sensor nodes, battery life, at least in more than a year, corresponding to each μ W about 32 J energy, lithium-based batteries can provide 1400-3600 J / CC energy, use 1 CC raw materials, batteries may work in principle a few years time. Thus, although life expectancy is limited, there leakage, the temperature would reduce the actual life, the one-time battery is still a very attractive power source.
Another option is the use of fuel supply, people's eyes is that it is a high energy density, such as the energy density of methanol as high as 17.6 KJ / CC. Low-temperature fuel cell work, no moving parts. Small battery is a direct methanol fuel cell, in such devices, methanol and water in the anode a chemical reaction, a free electron and proton, the latter through the polymer film after another in the cathode-water oxidation and reduction. The power level has been reported as high as 47 mW/cm2.
R & D of fuel engine is. This new engine of a new cycle of thermal dynamics, thermal energy conversion to mechanical energy and then through the films of piezoelectric generator will be converted into mechanical energy electric power. Small engine from a cylinder of liquid and gas components, with films sealed at both ends of the cylinder, one of which is the piezoelectric-type films. As the heat flow in and out of the cylinder, the size of the bubble and the subsequent expansion of compression, role in the piezoelectric film on its deformation, resulting in voltage. This engine can be fully produced microelectronic technology, volume production.
Set to power -
Set to-power is extracted from the surrounding environment in the energy and power directly from the device. Wireless sensor nodes in the environment, a number of energy can be used, such as light, infrared radiation, electromagnetic fields, temperature, air flow and vibration, and so on. Solar cells is a perfect solution, is also a fairly mature and compatible with electronic circuit technology. 。 At present the available power level has reached several mV/cm2. Of course, its shortcomings is the sensor must have a good light, the correct orientation and no obstructions, the practical application of a certain limit.
From the electromagnetic radiation of access to energy is also subject to geographical location restrictions. In the VHF and UHF band, small wireless work at a reasonable efficiency in the field 10-2-103 V / m between. Power density similar to that for the E2 / Z. (E is induced voltage, Z. = 377 W Wireless is the impedance of free space). Of 10 V / m or 1 V / m induced voltage, the corresponding power density is about 26 mW/cm2 or 2.6 mW/cm2. That is to say, to succeed from the electromagnetic radiation in the surrounding access to sufficient energy, wireless sensor voltage required to several V / m. Removal of regret, even in the cities have not so strong environmental radiation, unless the wireless base stations in the vicinity.
Wind power is also a general can use the momentum, the researchers developed a windmill power generation known as the piezoelectric device, the principle of rotation is to allow the windmill to generate piezoelectric crystal deformation and voltage. Development of a prototype by 10 cm windmill and 12 double-layer piezoelectric crystal (60 × 20 × 0.5mm3) reactor components, piezoelectric crystal arrangements or stacked into a ring-shaped. Wind driven rotating spiral pulp, pulp spiral cam shaft so that the stack of piezoelectric crystal deformation. 5-10mph airflow can generate 5-10 mW power. This power of the earthquake detection field visits and practical application value.
Vibration everywhere, this tiny mechanical energy is not for people in the past by the attention, but it has in the micro-power sources in its special value. Extract energy from the vibration of the devices are static, electromagnetic and piezoelectric of various forms. The use of a vibration unit is set to electromagnetic MEMS resonator generators. The device's basic structure as shown in Figure 1 a, Figure 1 b is the diagram. It's from top to bottom with four magnetic body, placed the middle of a coil, the former containing boron silicate glass in the home-based corrosion to pit for a while, the coil is located in the cantilever-racket-shaped silicon. Coil vibration induced voltage, the power to:
P＝ma2Q/8ω P = ma2Q / 8 ω
- M in the coil is the quality of inertia, a Acceleration is imported, ω is the resonant frequency, Q is open quality factor. 8g。 The design of silicon plate measuring 3.7 mm wide, 3 mm long, is the separation of the copper coils, diameter 2.4 mm, diameter of 0.6 mm, and quality is 0.028 g. 。 Acceleration of fixed, the power to reduce the frequency. 。 In addition, the micro-processing device, Q value greatly depends on the air damping effect, in order to avoid excessive wear and tear, the electrical work in a vacuum is necessary. According to the calculation, assuming 1000 Q value and 1 KHz resonant frequency, 9.81 ms-2 acceleration can produce 52 mW power, which can be coupled to 26 mW on the payload.
Cantilever vibration generating device also conducted a lot of research. Figure 2 a dual-plate piezoelectric single cantilevered power setup. Piezoelectric crystal plate one end fixed, under certain pre-load, plus other end of a heavy objects, to maintain the quality of films placed in the vibration of the drive, piezoelectric plate was the initial acceleration, a displacement, deformation, will provide the electrical voltage exchanges . Voltage and the size of the piezoelectric crystal permittivity, the flexibility factor, coupling coefficient, but also with the outside of the frequency and acceleration, when the external device with the oscillation frequency of the same resonant frequency, the greatest power output. Generally speaking, the resonant frequency bandwidth is relatively narrow, in order to broaden the resonant frequency bandwidth, can be made into fixed at both ends of the cantilever structure (see Figure 2 b), fixed at both ends of the axial and a pre-load heavy objects on The middle, can also be distributed in a number of weights on the cantilever. Analysis showed that the 2.5 m/s2, 120Hz source of vibration, 1 cm3 design can produce 375 mW power. In order to achieve the integration of integrated micro-sensor nodes, the performance of piezoelectric material and the production process by a number of requirements: 1) piezoelectric crystal with a good interface performance, with growth so that the substrate electrode and a good contact, and there is The full performance of the material can be used to generate voltage, 2) the power source of growth, production and integration should be in line with standard micro-processing technology; 3) the design of integrated devices (with the best piezoelectric crystal layer) in the surrounding environment should be inspired by the vibration Maximize the output voltage.
Table 1 the human body sensor data rate
In addition, developed a micro-machining, mechanical - static electricity power generator. This is the second of these two vibration generators can then extract a way, the structure is a relatively simple way. Electromagnetic generators can use the magnetic field, corresponding, electrostatic generators using electrostatic energy, the core of the device is a variable capacitor, the capacitor overcome the vibration of the two flat-panel formed between the electric field, the mechanical energy to transform the available power. Variable capacitor section as shown in Figure 3. Quartz substrate etching forming the metal film and copper above the polymer films were formed capacitance of the fixed plate and swing the plate. Polymer film on the roof overhang in silicon and quartz substrate, and placed a gold film, a rhythmical swing. Films can swing from top to bottom of the swing to extreme positions and, under the two-connected contacts, respectively, constitute the input and output. Contacts, in connection with the substrate, the largest capacitor charge at the same time; contact with the silicon-on connection, while the minimum power output capacitor. Initial measurements showed that the variable capacitor from 100 pf changes to about 1 pf. 10 Hz in frequency, work in constant charge mode, the voltage to increase almost 100 times. If the input voltage to 26 V, the output capacitor as high as 2.3 KV, equivalent to the energy transformation for the weekly rate of 2.4 mJ, or 24 mW.
Closing remarks
Power has become wider use of wireless sensor networks in major bottleneck, which triggered a new round of exploration of the micro, micro-power power craze. Analysis showed that: There is no single energy can solve this complex problem. Although now the preferred one-off or the battery, but on many occasions, from the environment to extract energy is the most viable option. The use of environmental programmes which visual around, and inexhaustible. In these technologies, people more optimistic about extracting energy from the vibration of the power installations, and conducted various research and development. With the improvement of performance materials and devices designed to improve, with the micro-electronics technology and the continuous progress of MEMS technology, have every reason to believe that the ultimate realization of integration and the integration of self-powered wireless sensor nodes goal.