A PEH powered wireless sensor system with an EAI

6.3.1 Description of the system

Figure 6-8 shows the block diagram of the developed PEH powered wireless sensor system.

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energy from structural vibrations and convert it into electrical energy; a PMM to convert the harvested electrical energy into usable electrical energy and store it for the end-user application; and a custom developed WSCN as the end-user, which senses and wirelessly transmits data to a base station. The novelty of the implemented system is that an EAI block is integrated within the PMM to manage the flow of the harvested energy so as to store it with minimum power consumption, during the non-active phases of the system, and then release it to perform data acquisitions and wireless transmissions. The system is specifically implemented as a whole to approach the capability to continuously power the WSCN entirely from the harvested energy for on-line SHM applications. After energy transduction, the mechanical strain on the substrate that transfers to the piezoelectric transducer appears as an alternating electrical input to the PMM. The PMM rectifies this electrical signal and stores its energy into an internal capacitor bank, which is used to power the WSCN during each active cycle for data acquisitions and wireless transmissions. In the developed system, capacitive energy storage was preferred to rechargeable batteries as capacitors can achieve faster charging intervals, longer lifetime and better performance in harsh conditions (i.e., at very hot/cold temperature). It is worthwhile to mention that the stored energy flows to the WSCN through the EAI, which regulates the voltage supply downstream of it. Indeed, the EAI consists of a voltage sensing device and switching circuitry for monitoring the amount of the stored energy while electric charges are generated by the harvester and accumulate in the capacitor bank for later use as from the concept illustration in Figure 6-7. The custom developed WSCN consists of three units: 1) sensors, to acquire required information from surrounding conditions; 2) microcontroller, to process data from sensors and coordinate data transmissions; and 3) wireless transmitter, to transmit collected and processed data to a base station. When enough energy has been stored, the WSCN is powered through the EAI and a proper length of time slots is allocated to its operation via the microcontroller, for activating the sensors and the transmitter so as to establish a data wireless communication with the base station.

6.3.2 Implementation of the system

Figure 6-9 shows the circuit diagram of the implemented system, including the PMM, the EAI, and the WSCN, which used ultra-low power consumption hardware to minimise the overall power consumption of the system.

Figure 6-9 Circuit diagram of the implemented system

A standard 4-diode bridge configuration was used to implement the AC/DC rectifier of the PMM. Fast switching 1N4148 diodes were arranged to achieve full-wave rectification of the electrical signal generated by the piezoelectric energy harvester. For the energy storage of the PMM, two aluminium electrolytic capacitors of 1 mF capacitance were connected in parallel to the diode bridge output. Voltage across these capacitors was constantly monitored by the EAI through a low current consumption BU4832 voltage detector (ROHM Semiconductor GmbH, Willich-Munchheide – Germany), which was connected from ground to the wireless microcontroller of the WSCN through a 2N7000 N- MOS transistor. The transistor acted as a switch to enable the alternation of the non-active phase and the active phase of the WSCN. When the voltage across the capacitive energy storage reaches the threshold voltage of 3.2 V, the EAI output enables the activation of the wireless microcontroller via restoring its ground connection. Data are acquired and wirelessly transmitted; hence, the stored energy is consumed by the WSCN. The EAI detects if the monitored voltage is under the threshold voltage and, if so, it brings the transistor into a high-impedance state. As this cuts the power supply to the JN5148 wireless

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2.4 GHz IEEE 802.15.4 compliant JN5148 (NXP Semiconductors, Cheshire – Manchester, UK), with features such as 128 kb Random Access Memory (RAM) and 4 Mbit serial flash memory, was selected as the wireless microcontroller of the WSCN due to the low power consumed in active mode for data transmitting. Furthermore, the JN5148 wireless microcontroller offers the possibility to set the clock speed of the Central Processing Unit (CPU) up to 32 MHz, which is the configuration implemented in the developed system. For the purpose of SHM, a ADXL335 3-axis accelerometer (Analog Devices International, Limerick, Ireland, UK), a MCP9700 temperature sensor (Microchip Technology, Inc., Chandler - Arizona, USA), and a GA1A2S100 light detector (Sharp Electronics, Ltd, London, UK) were integrated into the prototype board. Connection from ground through 2N7000 N-MOS transistors was also implemented for each of this system‟s sensors so as to enable them one at a time and only for the duration of that data acquisition cycle, thus consuming less energy. The selected sensors all have very short initialisation time (≤ 1 ms) so as to have low power consumption for data acquisition. Reduction of power consumption was also considered at software and data transmission levels by implementing TDMA (Time Division Multiple Access) protocol on IEEE 802.15.4 star configuration, as it has been described in [231]. 100 bytes of information were serially acquired from the different sensors; then, wirelessly transmitted at +2.5 dBm over three channels of the 16 available in the 2.4 GHz standard frequency range. Data byte collisions and interference issues are better managed as the result of such a multi-channel transmission. Wireless communication was established with the base station at a distance of approximately 12 m from the WSCN, where collected data were received with no indication of data loss.