An Autonomous Sensor with Energy Harvesting Capability for Airflow Speed Measurements A. Flammini, D. Marioli, E. Sardini, M. Serpelloni Department of Information Engineering University of Brescia V. Branze 38, 25123 Brescia, Italy [email protected]
Abstract — In several environments modest ambient airflows are present, for example, in air-conditioning ducts, in outdoor environment, or in moving vehicles. Furthermore, an airflow speed measurement is an important indicator, for the energy efficiency in the regulation of the conditioning implants. An autonomous sensor placed in a duct and powered by an electromechanical generator scavenging energy from the airflow has been designed and tested. The airflow speed is measured trough the rotor frequency of the electromechanical generator. When the external readout unit is active the electromagnetic field is used to power the autonomous sensor system and transmit the data. A set of tests on three airflow harvesters using resistive loads was conducted to demonstrate their energy conversion potentials. An experimental system has been designed and realized demonstrating that the airflow harvester can power the autonomous sensor permitting airflow speed measurements. Keywords - Autonomous sensor; airflow measurement; telemetry; contactless system; power harvesting; energy scavenging, wind energy.
Airflow measurements contribute to determine the indoor air quality and to provide healthy environments for the occupants of the buildings. They are also used for wind measurement in open space, in specific point sometimes difficultly accessible and without any availability of electricity. This paper outlines an airflow sensor that autonomously performs the measuring and saving functions, and transmits data to an external readout unit without requiring any battery. The autonomous sensor is powered by a harvesting system. Without batteries, the system has a positive ecological impact: no replacement costs or disposal problems. In addition the lack of batteries makes easier to measure at any point of a building or in places where there is no electricity. A growing demand for autonomous sensors and autonomous nodes is leading to a subsequent increase in the demand for localized, independent energy harvesting capabilities for each node or sensor [1, 2]. The use of power harvesting modules as a power source presents interesting possibilities, eliminating the batteries. In the literature, airflow harvesters are evaluated for their potential utilization in autonomous measurements. An airflow harvester can be a promising technological solution for producing electricity in remote applications [3-6]. In general, it is necessary for the power consumption of the harvesting electronics to be less than the available power for harvest,
which varies as a function of wind speed. In recent years, several groups have demonstrated small airflow harvesters based on the wind turbine principle. For this purpose, a properly sized small wind turbine is required to exploit the available wind potential for producing electrical energy. In , a wind energy system is evaluated for their potential utilization in urban areas. The techno-economic analysis of such energy systems is reported, as well as their life cycle assessment. In , the paper reports on the design, fabrication, and testing of small scale piezoelectric windmill. The windmill was tested at average wind speed of 4.47 m/s and it provided 5 mW continuous power. In , a 100 mm diameter fan rotor is combined with a brushless DC motor operated as a generator that could deliver up to 28 mW at 5.1 m/s flow rate or 8 mW at 2.5 m/s. More recently, in  a smaller device, with a 4.2-cm diameter rotor, is described. The device delivers powers of 2.4 and 130 mW at flow rates of 5.5 and 12 m/s, respectively. In  a small scale airflow harvester is reported. This device was developed using MEMS technology and was aimed at higher flow-rate applications. It comprises a 12-mm-diameter axialflow turbine integrated with an axial-flux electromagnetic generator. In this paper an autonomous sensor for measuring airflow speeds are presented. This autonomous sensor uses an airflow harvester for powering the sensor. A measure of flow is an important indicator of operational efficiency in the regulation of air conditioning, or in urban areas or in industrial implants [11, 12]. In the proposed application different airflow harvesters, using commercial rotor and generator parts, have been tested to study the power as a function of the load. An experimental setup has been arranged to test the autonomous sensor by measuring: the airflow speed inside a duct, voltage and current that can be generated by the airflow harvester. The applications here considered are those within a room or zone, such as through doorways (open or closed) or passive vents, those between the building and outdoors, and those through mechanical air distribution systems. For internal flow, such as ventilation ducts, the air speed can reach 12 m/s in large ducts, down to 1-2 m/s. For internal air duct applications, tiny windmills have significant advantages. Considering these aspects the autonomous sensor has been designed for low airflow speed.
DESCRIPTION OF AUTONOMOUS SENSOR
In Figure 1, a schematic diagram of the autonomous sensor is shown. A commercial fan is connected to an electromagnetic generator. The power harvested, using the air motion energy, supplies an electronic circuit for the measurements of the same airflow. Therefore, when the autonomous sensor is placed in ducts and a sufficient airflow is present, the electromagnetic generator harvests energy, powering the electronic circuit. In this condition no external power source is necessary, the airflow measurement is performed and the data are saved into non-volatile memory. When necessary, these measurement data can be downloaded by an external unit placed close to the autonomous sensor. Since during this phase the autonomous sensor is supplied by an electromagnetic field generated by the external readout unit, the correct functioning is guaranteed even if no energy comes from the airflow power harvesting system. FAN
ELECTRONIC CIRCUIT ENERGY HARVESTER AIRFLOW
Figure 1. Schematic diagram of the autonomous sensor.
A. Energy harvesters Whereas the intent is to use the device indoors, in air ducts used for heating, ventilating, and air-conditioning, the airflow speed is used to drive a miniature electromagnetic wind turbine to harvest the energy. The adopted components for the airflow harvester are commercially available. Three different electromagnetic devices were tested to be adopted in the harvester: a brushless DC servomotor (AD0612 HB-C76GL Dr. Cooler), a brushless three phase AC servomotor (nuvoDisc 32BF Portescap) and a DC servomotor (1624T 1,4 G9 Faulhaber). The main features are reported in Table1. The generators use a commercially available fan blade as a turbine blade (6 blades of 4 cm). Table 1. Main features of the electromagnetic devices.
brushless DC servomotor brushless 3-phase AC servomotor DC servomotor
Dimension [mm] 27 x 27 x 15
Weight [g] 22.7
16 x 16 x 33
32 x 32 x 22
B. Electronic circuit In Figure 2, the block diagram of the autonomous sensor and the readout unit are reported. The electronic circuit of the autonomous sensor consists of different modules. The transponder (U3280M) is commercialized by Atmel and its working frequency is 125 kHz. This device modulates the magnetic field by a damping stage to transmit data; in particular the OOK modulation and the Manchester code are chosen to modulate the signal. The transponder includes a power management block that handles the switching between the magnetic field and power harvesting supply. This component, able to supply the low-power microcontroller, consists of a rectifier stage for the antenna, a power manager, a damping modulator and a field-gap detection stage for RF communication. Therefore, the autonomous sensor provides the signal transmission through electromagnetic coupling at 125 kHz between the antenna of the transponder and the antenna of the readout unit. The advantage of transmitting an electromagnetic field at such comparatively low frequency is the possibility to more effectively transfer data and energy through different interposed materials without a cabled link. To transmit data, the transponder modulates the magnetic field using a damping stage. One pin of the device (clock extractor) is used to provide a clock signal for the synchronization of data transfer. The transponder interface can also receive data: the readout unit modulates the data with short gaps in the field and a gap-detection circuit reveals these gaps and decodes the signal. The autonomous sensor antenna has an inductance of about 640 µH, while the readout antenna has a diameter of about 120 mm and an inductance of about 3.3 mH. Both are built by wrapped wire. The low-power microcontroller chosen is the 9S08QE128 commercialized from Freescale which offers an 8/12-bit analog to digital converter, 128 KB Flash memory to save the data and two timer units. The low-power microcontroller has a timer unit that permits to synchronize the data transmission. The microcontroller has a low-power configuration: all the unused peripherals are switched off. To maintain the power consumption low, the clock of the microcontroller is 500 kHz during the measurement and transmission, while the clock is 16.4 kHz during stop mode. The airflow speed is measured trough the rotor frequency of the electromechanical generator using an infrared Light Emitting Diode (LED) and an NPN silicon phototransistor (Optek OPB815-WZ). The readout unit consists of a transceiver (U2270B) commercialized by Atmel. This component permits to drive the coil antenna and to demodulate the digital signal. The transceiver is...