Powering Sensor Node using Macro Fiber Composite.

Transcription

1 Powering Sensor Node using Macro Fiber Composite. Henckel, J.T.; Sørensen, T, Vuckovic, D. Delta Madsen, J. Technical University of Denmark, DTU August 31, 2011 Abstract This paper proposes a way to power a sensor network by harvesting ambient energy using Macro Fiber Composite (MFC). By making the sensor network self-sufficient with energy, the expenses for maintenance are greatly reduced. A devise that transfer stresses created by wind pressure into two MFC s are build and tested in an outdoor environment. In a period of 10 hours under low speed wind conditions (1-7 m/s) the devise harvested enough energy to allow a sensor node to record and transmit data 19 times. This proves that the design is capable of powering a sensor node even under low speed wind conditions. Introduction In this paper, the possibility of powering a sensor network by harvesting ambient energy using Macro Fiber Composite (MFC) is investigated. The sensor network consists of many individual sensor nodes that can record and transmit data. The current sensor nodes need 0.25 mj mj to read a sensor and transmit the result. Instead of using other energy harvesting methods such as solar cells and windmills, which decrease their efficiency due to respectively soiling and mechanical wear and tear, MFC s are used to harvest wind power. By making the sensor network selfsufficient with energy, the need for maintenance and hence the running cost are greatly reduced or even eliminated. This will allow the network to be set up in hard to reach locations for long periods of time. A MFC is a piezoelectric actuator developed in 1996 by NASA Langley Research Center (LaRC). The MFC is based on the piezoelectric ceramic material PZT (lead Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 1

2 zirconate titanate). Under pressure, PZT develops a voltage over two of it s faces, making it useful for converting mechanical energy into electrical energy. The full MFC consist of a PZT-core enclosed in a protective plastic body, making it very robust. The MFC s used are shaped like thin strips with the overall dimensions 113mm 8mm 0.3mm. The MFC s energy output are largest when the pressure on the PZT core is applied and removed at high frequencies, but power is also produced at lower frequencies. The full bandwidth is 0Hz to 3MHz [3]. Very little literature on the subject of using MFC s for energy harvesting purposes can be found, non of which is relevant for this study. The procedure used is therefore based on the trial and error principle with no actual theoretical background. To use MFC s to harvest energy, they must be attaches to some kind of structure that apply stresses to the PZT-core. The devise should have no moving parts such as bearings, hinges, tracks, etc. since these tend to jam at some point due to wear and tear. A devise with no moving parts was designed and tested in both the laboratory and in an outdoor environment. The results from the outdoor test are used as a basis for validating the further development of the design. Methods In this section, the methodology behind the development of the energy harvesting devise is explained. Mechanical design of energy harvesting devise The MFC produces most energy when subjected to longitudinal tension. This stress-state is obtained by attaching the MFC to a plexiglass rod and then subject the rod to bending. The stresses normal to the cross sectional surface of the rod will vary from negative to positive over the thickness of the rod (figure 1). These stresses are transfered to the PZT-core in the MFC by strong epoxy glue. The bending moment is created by a circular plate attached in one end of the rod. When the rod is placed in an air flow, the drag force on the plate will induce a bending moment in the rod. The end opposite to the plate is fixed (figure 1). Due to the oblonged shape of the devise is is referred to as the spear. To double the energy output, two MFC s were attached on opposite sides of the rod. When a gust of wind hit the plate, the rod will start vibrating and hence subject the MFC s to cyclic tension and compression at a decreasing rate. Due to the cyclic tension and compression of the MFC s, the output current changes direction. The spear is very easy to manufacture due to it s simple design. The most difficult step is the attachment of MFC s to the rod. Even here, the required level of precision is very low making the spear design very suitable for fast mass production. Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 2

3 Wind direction Segment of plexiglass rod where the MFC is attached Circular plate to catch the wind Cross sectional stress distribution Plexiglass rod MFC Figure 1: A sketch of the spear with enlarged segment about the MFC Electrical design of energy harvesting devise To collect and store the energy created by the two MFC s, the EH300 1 circuit board is used. This devise basically consist of a bridge rectifier that converts an alternating current into a direct current. The energy is then stored in a 1000 µf capacitor. When the capacitor holds a voltage of 3.6V it discharges until it holds 1.8V. The ouput signal from The EH300 with a steady energy supply can be seen in figure 2. The EH300 requires an input voltage of at least 4V to charge the capacitor. To obtain the largest possible input voltage to the EH300, the MFC s are connected in parallel. Since the two MFC s are attached on opposite sides on the rod, their output currents have opposite directions. To avoid the currents from canceling each other out in the parallel connection, the output terminals on the MFC s must be attached positive to negative. Figure 2: Output signal from EH300 when connected to a constant power supply. V H = 3.6V and V L = 1.8V. The image is a modified version of an image taken from [2] Expected energy production Every time the EH300 discharges, the amount of energy released is 1.62mJ. This is calculated using equation 1. 1 EH is short for Energy Harvester Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 3

4 E = 1 2 CU 2 = F (3.6V 1.8V ) 2 = 1.62mJ (1) The amount of energy the sensor/transmitter circuit board needs to read a sensor and transmit data to another node is 0.25mJ-1.5mJ, so for every discharge from the EH300, data can be recorded and transmitted. Field deployment To test whether the spear was capable of harvesting enough energy for recording and transmitting data on a regular basis, the spear was placed on a rooftop in a period of 10 hours under low speed wind conditions (1-7 m/s) [1]. During this time, the spear charged the EH300 s 1000 µf capacitor from 1.8V to 3.6V 19 times corresponding to approximately one reading and transmission every half hour. In figure 3, the time between every energy discharge is shown. Figure 3: Charge cycles between 13:00 and 23:00, Lyngby, Denmark, The data was collected by connecting the EH300 to an Arduino 2 which registers the energy discharges. The Arduino communicated with a computer logging the time and date of the discharges. Conclusion The outdoor test of the spear showed a large potential for further development of the concept. Even at a non-optimized stage, the spear managed to produce enough energy for a sensor on average to perform one data reading and -transmission every half hour in relatively low wind-speeds. This mean that the spear can harvest a sufficient amount of energy in environments where strong wind is rare, such as in forests and between large buildings. The spear did not take any visible damage from the cyclic bending action which suggests that that the concept with no moving parts can work for a long period of time without failing due to mechanical wear and tear. Further tests will have to be conducted to confirm this. Since the sensor itself can be made rather small, it can be mounted directly on the spear making a full sensor node very easy to handle. All a person has to do for installing a fully functional the node is fixing the bottom end of the spear by for instance sticking it in the ground. 2 A microcontroller board taking input from various sensors or switches and communicating them to a computer Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 4

5 Next step in the development of the current design is choice of rod-material and optimization of the placement of the MFC s. Future perspectives The use of MFC s for energy harvesting purposes opens up new opportunities for retrofitting already existing technologies to run on ambient renewable energy. This means that technologies can be deployed in locations for long periods of time without having to bring large, heavy power supplies. The broad bandwidth of witch the MFC can produce power enables the possibility for energy harvesting in many different environments. MFC s can for instance be used to harvest energy from biological movement, such as a walking human, by mounting MFC s in the clothing or the walking pavement harvesting energy from the vibrations form the body and pressure changes in the pavement. Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 5

6 References [1] Meteorological Institute of Denmark. [2] Data sheet for EH Accessed April 13th, [3] Smart Material web page. Accessed July 19th, Henckel, J.T., Sørensen, T, Vuckovic, D., Madsen, J. 6