Contactless power and data transmission for underwater sensor nodes
© Huang et al.; licensee Springer. 2013
Received: 6 January 2013
Accepted: 25 February 2013
Published: 20 March 2013
When the technology of wireless sensor network is applied to ocean monitoring, the ocean underwater monitoring system will become a distributed network system. That could improve the level of ocean monitoring significantly. But most of the current ocean monitoring equipments work independently, and it is difficult to form them into a sensor network. To set up a sensor network, electric energy supply and data transmission are the chief problem, especially for the underwater sensor nodes. To solve this problem, a new method for contactless power and data transmission is proposed. It operates with the principle of electromagnetic coupling. Power and data are transmitted through a mutual channel. This channel is realized by the structure of a two-stage electromagnetic coupler. Here, steel rope is used as the closed steel ring to compose an electromagnetic coupler. Since steel rope is widely used in the current underwater equipments, the transmission channel of power and data can be realized without changing the existing structure. Furthermore, because of contactless transmission the sensor network has good expansibility and convenient configuration of its nodes for underwater applications. A voltage transformation model for the two-stage electromagnetic coupler is constructed and the transmitting principle of power and data is analyzed based on the model. According to the energy transmission efficiency and the bit error rate, an experimental prototype is designed and fabricated. Finally, the prototype testing of power transmission, data transmission, and reactive power compensation are carried out. The experimental results show that the proposed method for contactless power and data transmission is feasible and it is suitable for the application of underwater sensor nodes.
KeywordsPower and data transmission Electromagnetic coupling Sensor nodes Reactive power compensation
The ocean underwater monitoring system is a main part of ocean stereo monitoring network. It can obtain the data of ocean dynamics parameters and environmental elements, and it works in real-time, continuously and rapidly. That data include salinity, temperature, dissolved oxygen, chlorophyll, heavy metal, ocean current, and so on. The ocean underwater monitoring is characterized by the vast area and too many parameters. The monitoring system should meet the requirements of stereo monitoring with the characteristics of multipoint, multiple-profile, and network . Therefore, it is important to introduce the technology of wireless sensor network (WSN) into the ocean monitoring system. With WSN, a distributed network system for ocean monitoring can be constructed. That will greatly improve the monitoring level .
The current monitoring systems usually operate in the mode of single-point or scanning, such as buoy, submersible buoy, underwater robot, shipborne equipments, and so on. When in scanning mode, the single-point equipment can realize a kind of multi-point and multi-profile monitoring by navigation. Although most of these equipments have the function of communication, they work as independent system and it is difficult to form them into a sensor network. As a sensor node, the underwater equipment has to realize the demand of long-term, networking and autonomous work. Therefore, power feeding and data transmission are the chief problem for this application. In this area, some related research works have been conducted. Because most of the current underwater equipments use batteries as the power, the technology of low-power design gets much attention, such as the low-power underwater seismograph designed by Manuel et al. . Tomisa et al.  utilized solar energy to power the ocean buoy with low maintenance cost. Ahnet al.  researched a new kind of buoy, in which wave energy was converted into electric power. Some other research on energy conversion for ocean applications was also achieved, such as tidal energy  and ocean thermal energy conversion . In underwater data transmission area, acoustic communication is the uppermost method at present . Jane et al.  employed communication cables to transfer data between buoy and sensors.
This article proposes a new method for power feeding and data transmission based on electromagnetic coupling. It implements power and data transmission with a closed steel ring, which is widely used in the existing underwater equipments. Here, the closed steel ring serves as a transmission channel, and also a supporting structure to fasten the underwater sensors. Based on the proposed method, an experimental prototype is designed, which realizes contactless power and data transmission in the same channel for the underwater sensors.
2. System structure
2.1. The structure of underwater sensor network
2.2. The cluster based on contactless power and data transmission
Because of electromagnetic coupling, there is no tight electrical contact between the closed steel ring and the sensor nodes. The sensor nodes can be fixed at arbitrary depth along the closed steel ring. It is easy to mount the sensor nodes. For instance, temperature sensor, salinity sensor, and ocean current sensor are arranged at different depth. That contributes to the good expansibility of the sensor network.
3. The method for contactless power and data transmission
3.1. Transmission principle based on electromagnetic coupling
where N3 is the number of turns of coil 3 and k2 is the coupling coefficient of transformer B.
3.2. Implementation of power and data transmission
Equation (5) gives the voltage transformation model of the two-stage coupler. That is the model for delivering electric energy. Both of the monitoring platform and underwater sensor should be powered with DC, while electromagnetic coupler needs AC. So, the energy delivery is a conversion process of DC/AC–AC/DC.
In Equation (5), ui can be denoted as u i (t) = f(t) cos (ωt), where f(t) is the data to be sent. Accordingly, u3 would include the information of f(t). Here, the data signal is modulated by the carrier cos (ωt) to fit the transmission channel. Consequently, the transmission distance can be tens of meters or even hundreds of meters by signal modulation.
4. Prototype design and experiments
4.1. Prototype design
In terms of Equation (5), the coupling coefficient has a great influence on the transmission efficiency. It is the key parameter to design the prototype. The material and structure of the coupler determine the coupling coefficient.
The higher permeability of the coupler is conducive to the reduction of the magnetic leakage and the improvement of the coupling efficiency. The transmission efficiency is also related to the loss of the coupler. Different materials have different loss value per unit volume. Therefore, the electromagnetic coupler material should have such properties: (1) soft magnetic material, (2) small loss in highfrequency, (3) great initial permeability, and (4) great saturation magnetic induction. According to the above requirements, Fe-based nano-crystal material is selected to design the coupler.
4.2. Experiments on the prototype
The results of power and data transmission experiments
Bit error rate (%)
In the ideal condition, the power factor of the resonance circuit is 1 and the reactive power is zero.
The above experiments prove that the method for power and data transmission based on electromagnetic coupling is feasible. For data transmission, the bit error rate is 0% when the transmission distance is either 2 or 10 m. That is due to good anti-interference ability of FSK modulation. If the transmission distance further increases, the bit error rate should still keep to a low level. For power transmission, the transmission efficiency declines with the increase of the transmission distance. Reactive power compensation plays an important role in improving the transmission efficiency. In addition, some other factors influence the transmission efficiency, such as the area of the closed steel ring, wattful loss, inverter efficiency, and the accuracy of the compensation model parameters. Those factors need further study. Therefore, it is feasible to further improve the transmission efficiency based on the present prototype.
The underwater sensor nodes should meet the requirements of long-term, networking and autonomous working. Therefore, the key to constructing a distributed sensor network is to solve the problem of power and data transmission. The principle of electromagnetic coupling is innovatively applied to contactless power and data transmission for underwater sensor nodes. With a two-stage electromagnetic coupler and closed steel ring structure, power and data are transmitted on the same channel. The voltage transformation model is established and the transmission process is analyzed. An experimental prototype is designed and fabricated. The results of test experiments prove that the presented method is feasible. This research provides a new method of power and data transmission for underwater sensor nodes. That method can directly be used in the existing underwater monitoring equipments, which will improve the ocean monitoring performance of real-time, maintenance-free, and expansibility.
This study was supported by the National Natural Science Foundation of China (60972129) and the Science and Technology Support Project (State Key Laboratory of Mechatronical Engineering and Control).
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