The wireless networks allow to obtain information in real time

Many of the current applications of the electronics equipment dealers equipment dealers depend on sensors that obtain the necessary signals to monitor or control processes of type so miscellaneous like the number of vehicles in a parking or the lighting of the screen of a mobile telephone. When the information uses in a distinct point of that where has obtained , for example because it is inaccessible to the user or because the information wished obtains when combining that proportionate by multiple sensors dispersos, arises the problem of the communication of the signals of the sensor to the final processor. When said communication can not base totally in a pre-existing network, the solution is to implant a wireless network that communicate the sensors to said network, and sometimes also between himself. This situation gives with frequency in applications agroambientales because these are used to implement in zones with low population density and therefore of networks of communication

An electronic sensor is all device that offer an electrical exit from a physical or chemical magnitude. This term suggests a capacity of extension of our senses, that is some regarding the number of magnitudes that can perceive directly with our sight, hearing, smell like and touch; but said extension is less true regarding any of the capacities of perception of the machines, that still do not surpass the ones of the living beings. The sensors base in the change of geometry in an electronic component or in the changes that experience the properties of the materials that constitute them when they are exposed to the magnitude that wishes measure. The signals obtained from said changes are very small and to communicate them it is necessary to process them previously, such as it indicates in the figure 1. To be able to establish a bisteeringal communication, the emisor has to act alternatively like receptor and vice versa (transceiver).

The electronic circuits of processing and communication, and often also the sensor, need an electrical supply to be able to work. In the wireless networks of sensors, the most usual solution is the catchment of solar power because in permanent remote installations the frequent replacement of primary batteries would be a solution ineficiente. A suitable management of the power attracted allows to feed the group with solar cells of dimensions very reduced, and involves the design of electronic circuits of very low consumption that attain that the solar cell work always in his point of maximum performance. The source of feeding of the receptor can base also in solar power, or be connected to the network of electrical supply.

The efficient management of the power comports also a suitable selection of the frequency with that realizar the measurements, because when it is not measuring, the sensor and the circuits associated desconectan to save power. The minimum time between measurements will come determined by the dynamics of the magnitude that wishes measure. Analogously, and with greater reason because of the high consumption of the transceiver with regard to the of the sensor and the processor, the results of the measurements is preferable to transmit them by packages if it is not indispensable to transmit them just after having them obtained.

An effective communication demands some rules that constitute the ‘protocol': formats of the signals, detection and correction of errors, access to the media (here, the air), that is to say, arbitrate who acts like emisor and receptor in each moment, etc. To happen information between systems that have distinct protocols employ ‘runways' or ‘bridges' (gateways, bridges), and this is used to to be the case in the networks of sensors in applications agroambientales because the information obtained finish being communicated finally through a network of pre-existing communication.

To avoid that the systems of radiocommunication in the air interfere between yes, there is a world-wide regulation that assigns bands of frequency to the distinct applications: broadcast, radionavegación, television, radioenlaces, maritime communications, military communications, industrial applications, scientific and medical (ISM), etc. For the applications ISM there is assigned diverse free ‘bands', that is to say, is not necessary to ask permission to operate in them if the power transmitted does not exceed some limits established. These bands depend on the geographic zone. The result of this regulation is a big variety of protocols standardised to work in each one of the bands ISM without interfering mutually. The table 1 collects some basic characteristics of the most used norms.

The frequency of transmission influences in the capacity of the channel: to greater frequency, more channels can transmit , or can transmit data to a greater speed for a determinate number of channels, and lower is the size of the necessary antennas. But, for a same power transmitted, the scope diminishes when increasing the frequency because the obstacles on the way of propagation of the waves absorb more the signals. The figure 2 sample the relation between power and speed of transmission of data according to the extension of the geographic area cover.

The applications foreseen initially in each norm are chords with his bandwidth (speed of transmission): ZigBee for monitoring, Wi-Fi to transmit voice and data, Bluetooth to substitute cables between multimedia equipment, WiMax to increase the bandwidth in the mobile communications, etc. Nevertheless, the wide use of all these standards, especially when they apply in products of consumption, does that the price of the necessary integrated circuits to implement them decrezca until the point that his use in industrial applications was a reasonable option, when by his lower volume of market would not justify the design of specific integrated circuits.

Another important decision in the wireless networks of sensors is his topology, that is to say, the space disposal of the nodes of the network and the functionality of each one of them, because it determines the area covered, the reliability of the communication and the consumption of power. The figure 3 sample three common disposals in wireless networks of sensors. In a network in star, like the used in mobile telephony, Bluetooth and Wi-Fi, all the traffic goes through a point of access (runway), that is a node with greater capacity that links with another network of communications, fixed or without threads. In a network arborescente or in pendant (cluster tree), each node communicates only with the most next node, so that the nearest nodes to the point of access bear a greater traffic, and therefore they consume more power. A failure (‘fall') in one of these nodes leaves incomunicados to all the nodes that are ‘underneath' of him. In a network in mesh (mesh), each node can communicate with all his neighbours (more or less next), so that it is possible to define routes of communication in function of multiple parameters, as for example the state of load of the battery in a concrete node. The maximum number of nodes that can connect to a same server depends on the norm: 7 in Bluetooth, 32 in Wi-Fi and until 64.000 in ZigBee.

The applications agroambientales can benefit also of these technologies developed initially for applications with a number of implantations very high. Like this, for example, the designated agriculture of precision aspires to manage the variability of the local conditions to contribute only the resources (seeds, water, fertilizante, phytosanitary ware products…) necessary in each place and moment, and to predict the harvests. This demands to know the local conditions of the floor, meteorological and environingingmental, and the evolution of the crops. The remote detection from satellites or aeroplanes contributes part of the necessary information, but no on variables such as the nitrogen in the floor or the wind speed. Besides, only it is economically justifiable in exploitations very big. The wireless networks of sensors can contribute said local information.

To continuation describe two applications of type very distinct that they are a sample of the possibilities of this cream technology to the protect of the development of the mobile telecommunications and the personal electronic devices.

The salinity of the water has a negative repercussion in the production of rice. In the Mediterranean zone, the extraction equipment equipment of water of the subsoil does that the water of the sea go in in the phreatic layers with the consequent salinización of the water and of the floor attach, reducing the harvests. The solution is to irrigate abundantly with freshwater that avoid that the salt water of the subsoil rise until the roots of the rice. In the zones arroceras that irrigate with water of river where the tide or the estiaje combined with the wind cause one coins saltworks that can penetrate several quilómetros waters on top of the mouth of the river, the irrigation with water of said coins would cause also grave damages to the rice. Therefore, the knowledge of the salinity of the water is essential for an efficient management of the water of irrigation and protect the rice.

The usual method to determine quickly the salinity of the water is to measure his electrical conductivity: what more salt, more ions and therefore better is the conductivity. But in the market there are not systems that allow to measure said conductivity continuously in a paddy, between other reasons because the reading of the value measured by the current instruments is presented usually in a small alphanumeric screen (figure 4). Besides, the probe is designed to be sustained manually inside the water, and clean it after each use. What would interest is an able system to measure continuously the conductivity and transmit the result to a centre of follow-up so that it could , for example, detain the saltwater pumping to the channels of irrigation or increase the discharge of these to accelerate the process of ‘wash' of the floor.

To face this problem, the European Union has funded a project of investigation of two years designated Smart Paddy whose aim was to develop a wireless network of autonomous sensors that measured in real time the conductivity of the water in the drain of the paddies, channels of irrigation, stations of pumping and channels of drainage, and presented this information through a web application. Of this form would avoid the frequent travel to the field to detect possible symptoms of weakening of the plants by fault of the salinity of the water. So that the solution was economically viable, considered that each point of measure did not have to be much more expensive that a manual measurer of provision metrológicas comparable to the ones of the instruments that they have some agriculturalists or centres of support.

With these specifications of design, the best solution was a wireless network of autonomous sensors of electrical conductivity where the sensor of conductivity of each node had to be much more economic that the current measurers because it would have to incorporate besides a transceiver and a system feeding based in solar cells, and had to work properly during the more than 5 months that lasts the crop of the rice in the Mediterranean area. This involved that it was necessary to guarantee that the measurements went correct during all this time in spite of the fluctuations of the temperature of the water, that influence in his electrical conductivity, and of the immersion continued, or at least detect any deterioration of the electrodes of measure like result of the immersion and that it could affect to the result.

This commitment between good provision, reliability and robustness on the one hand and low cost by another, is usual in networks of sensors. The design of a node sensor based in conventional sensors, whose provision metrológicas easily exceed the needs of the problem but whose consumption and factor of form have not been foreseen for these applications, hardly allows to achieve the cost wished. Besides, if the excessive consumption translates in solar panels of considerable dimensions, the risk of theft or vandalism increases considerably.

The best form to resolve the commitment posed is by means of innovation. In Smart Paddy has designed a measurer of conductivity of the water with compensation of the effect of the temperature, that employs only two electrodes, which simplifies the electronic design in front of the probes of four electrodes, without that the result remain affected by the possible changes of impedance of the electrodes, and that besides allows to detect a possible deterioration of these. The result is the probe showed in the figure 5, where can appreciate that the solar signpost, had in the top of the probe, covers less than the half of the available surface. This solution is applicable also in the reuse of waste water and to control the salinity of the water and effluents in other industries.

Regarding the topology of the network, since the key point was the validation of the sensor and obtain the necessary information for a future prototype pre-industrial, used communication in the band of 868-870 MHz, a network in star with an upper scope to the 500 m in open field, an own protocol, and a runway fed from the electrical network.

The composting is the natural way to recycle the organic waste to turn into them credit. It is a biological process in which the microorganisms, bacteria and funguses decompose the organic matter. Since roughly between the 45 % and 55 % of the waste generated by the action of the man are of organic matter, the composting plays and will play a very significant paper in the waste treatment in the next years.

The process of composting has to be very controlled if it wishes optimise the quality of the compost resultant and minimise negative appearances as they are the apparition of smells or of germs no wished. The efficiency of the process depends on the environingingmental conditions presents in the system: the quantity of oxygen, the profile of temperature and humidity in the different phases of the process, and the size and the activity of the microbial populations.

Whereas the main physical parameters, biological and chemists that affect to the process of composting are very known, the available technological solutions for his follow-up and control are very limited. Two of these essential parameters, the temperature and the humidity in the interior of the material only measure , in the actuality, of punctual form, so much temporary like espacialmente. The temperature measures generally manually by means of the insertion of a probe, what causes a human and economic effort important that prevents his continuous use. Besides, automate these probes can not being a viable solution in front of the periodic need to control the aeration by means of the volteo of the batteries of material. In the case of the humidity, at present the usual is to estimate it by means of the extraction equipment equipment of samples that have to be analysed in a laboratory.

Exists, then , a clear need to provide to the operators of composting a better technology for the control of the process, especially in sight to some increasingly strict regulations and to the demand by part of the market of a compost of greater quality. The project Compoball, subsidised by the European union in the Seventh Program Marco, gives answer to this need with the development of a network of autonomous sensors, that measures of continuous form temperature and humidity from the interior of the material during all the process of composting (upper to one hundred days). To achieve it have had to surpass several technological challenges that looked for to adapt the network of sensors to the hard conditions of environingingmental work (upper temperatures to 80 °C and high humidities), and present high resistance to chemical attacks, created by the own process of decomposition of the material, and big resistance to the hits and movements generated by the periodic process of volteo of the material. The result has been some nodes sensors in shape of ball, protected with rigid polyurethane, that communicate the data of temperature and humidity measured by means of wireless communication in band ISM (434 MHz), and that present a scope of until 5 m in the interior of the compost. To guarantee his estanqueidad does not do lacking to open the nodes for recargar the batteries since these are recargadas with methods inductivos. His cost is sufficiently low to attain a network with an important number of nodes that allow to characterise perfectly the material in spite of his big heterogeneity.

Appears 6. architecture of the system Compoball for the measure of the temperature and humidity from the interior of the batteries of composting.

The wireless networks of sensors are a technology that extends the concept of measures distributed to situations where the power of calculation and the available electrical power in each node are small, but where the capacity of communication of the nodes and his organisation allow to transport, with an affordable cost, the information until a checkpoint that can be very far of the zone of measure. This allows to obtain in real time information that before only could obtain by means of recorder of remote data (date loggers).

Because of his lower cost of installation, material and time with regard to the networks wired up, and the possibility of autoconfigurarse to accept new nodes, fixed or mobile, or do without any of them, the wireless networks of sensors go penetrating in all the productive sectors with a speed chord to the peculiarities of each sector, such as it has sucedido with other technologies. An excessive speed, promoted by the dynamics of the technology in himself same, can carry to failures rotundos. But a too slow speed, usually fruit of the comprehensible resistance to the change and of the prudence in front of the unknown, can be also an unambiguous symptom of a loss of competitiveness that does unavoidable the disappearance in the near future.

In any case, the applications agroambientales, the same that the other, only will be able to justify if the net profit surpasses the cost, and when there are a lot of points of measure it is necessary to design nodes sensors that can that they have to base in new principles of measure, perhaps taking advantage of microsensores developed for other applications, new methods of processing of the signals or both and, in any case, in efficient methods to condition the power attracted with solar panels of dimensions reduced or very reduced.