Autonomous sensor and its application
An autonomous sensor is a device that is generally able to perform its task without being connected to the interrogation unit. Its power supply is integrated in the device, and very often a harvesting mechanism is used for its energy production and is able to totally or partially power the
device operation. There has been broad and rapid evolution in the field of autonomous sensors. Emerging application fields, the need to increase the life of electronic devices, increased computational capabilities that require more energy and drastic reduction in device volume have been drivers of this
field. Power management and wireless connection are becoming major issues in
many applications. Research communities all over the world are working to find solutions and harvesting methods to optimize the power issue in sensors with the specific goal of implementing autonomous sensors
classification of autonomous sensors into “passive autonomous sensors” and “self powered autonomous sensors” is introduced.
“Passive autonomous sensors” are defined those that are just
passive elements, interrogated wirelessly by a readout unit.
“Self-powered autonomous sensors” are those that have a power-harvesting module or are supplied power by an electromagnetic field. Here we discuss these concepts and two examples: an autonomous sensing device and an energy harvester.
Examples of autonomous sensor applications are all around
◗ Wireless networks that have low installation costs, are easy to maintain, and have sensor nodes with long life;
◗ implants for human beings which are characterized by difficult system accessibility;
◗ Applications where wiring is not allowed because of mechanical constraints;
◗ Systems that measure in harsh environments characterized by high temperature or corrosive atmosphere; and
◗ Systems that monitor large environments for a smart home or Ambient Assisted Living purpose where flexibility
Architectures of Passive Autonomous Sensors
A general architecture of a measurement system based on a passive autonomous sensor is shown in Figure below is the passive autonomous sensor is the sensing element in the harsh or remote area, while the readout unit is placed in the safety zone. The two elements are connected by a wireless communication exploiting an electric-magnetic, optic or acoustic link. Between the sensing element and the readout unit there is usually a barrier whose characteristics (mainly material and geometry) influence the system’s performance. The sensing element is a passive device that does not require any power supply.The quantity under measurement is usually seen as reflected impedance by the front-end electronics
contained into the readout unit.
Application of passive autonomous sensor for humidity measurement
Measurements of relative humidity (RH) in hermetic environments, for example in logistic and biomedical fields, can be executed wirelessly by passive autonomous sensors. In below Figure is an example of autonomous sensor and readout system for this application are schematically represented.
The passive autonomous sensor is a standalone planar inductor, fabricated in PCB technology windings with an external diameter of 50 mm covered by polyethylene glycol (PEG). The relative humidity (RH) variations change the dielectric of the polymer deposited over the inductor causing a variation of the parasitic capacitance.A conditioning
circuit individuates the three resonant frequencies and a microprocessor calculates the RH and compensates the distance variation. The readout system consists of different functional blocks:
One generates the sinusoidal reference signal, the second measures the impedance module and the third calculates RH. The sensor has been characterized using the experimental setup shown in figure
The sensor is positioned inside a Plexiglas chamber, which is used as a hermetic
container for damp air. In the chamber there is a hygrometric sensor (HIH-3610) for reference measurements. The inductances are positioned parallel and their axes are coincident. The three resonant frequencies are monitored by an impedance analyzer (HP4194A), connected to the readout inductor or alternatively, to the dedicated electronics. The damp air that flows inside the chamber is produced by the system that controls the mixture of the two gaseous fluids by two flux-meters. The distance of the readout from the sensor is controlled by a micrometric screw with resolution 10 μm and runs up to 25 mm. The three resonant frequencies have been measured at a distance of 20 mm and,
according to the 3-Resonancies Method
Passive Autonomous Sensor for high temperature measurements
A passive autonomous sensor for high temperature measurement is represented schematically in figure below the sensor is a hybrid MEMS composed by a novel MEMS temperature sensor developed using the Metal MUMPs process and a planar inductor
realized in thick film technology by screen printing over an alumina substrate a conductive ink in a spiral shape.
The MEMS temperature sensor exploits a cascade of 36 bent beam structures. The single structure
is composed by a V-shaped beam anchored at two ends as reported in the enlargement in the upper
Sensors 2009, 9
part of Figure The temperature variation induces a thermal expansion of the structure generating a displacement of the central apex, which is connected to an inter digitized comb. The device is built directly over a silicon nitride isolation layer with a nickel and gold structural layer. The maximum operating temperature is due to the maximum operating limit of nickel (350 °C). The MEMS capacitor is coupled to an embedded coil inductor and the equivalent LC circuit has a resonance frequency which depends on the temperature to be measured. An external inductor can be applied to the external
part of the barrier delimiting the harsh zone. The two inductors represent an inductive telemetric system. In the experimental set-up the oven, in which the autonomous sensor is positioned, has a windows of tempered glass with a thickness of 8 mm.
Self powered Autonomous sensor
Self-powered autonomous sensors are autonomous devices having the capability and functionality of a stand-alone measurement unit, even if the readout unit is not close. Self-powered autonomous sensors should be able to execute measurements, store the
measurement data and send these values to the readout unit. In literature examples of self-power autonomous sensors are being increasingly reported. Many designing use batteries as the power source. Physical and chemical sensors for logistic data-logging applications to evaluate food quality and freshness are discussed. In recent results of autonomous sensor can use for brain stimulation and neuronal activity recording are reported.
patient vital sign data in a hospital.
all the internal modules are supplied by a power harvesting module or by the
electromagnetic field of a wireless link. Since the possibility of substituting batteries with harvesting system is greatly attractive from an ecological point of view, our analysis will be concentrated only on autonomous sensors equipped with harvesting systems. These self-powered autonomous sensors consist of one or more sensing elements and different modules: front-end electronics, an analog-to digital converter, an elaboration unit to manage the internal tasks, power management, a wireless transceiver and storage memories. In Figure 3 a block diagram of a self-power autonomous sensor is
shown. Common characteristics can be extracted: very low-power design, stand-alone configuration, minimal control and communication circuits in order to achieve the smallest and most easily attachable form.
. Block diagram of self-powered autonomous sensors.
Self‐Powered Auto n Sensor
Remote Area Accessible Area
Self-powered autonomous sensors require specific level of voltage and current supply obtainable by an appropriate power management block. Usually the power management circuit has a dedicated DC–DC converter or charge pump to match the output electric impedance of the generator with the characteristics of the circuit load realizing a maximum power transfer. In a characterization of thermoelectric modules connected to a charge pump has been described. Since the power harvesting can work intermittently or the autonomous sensor can require much higher energy than that available from the power harvesting block, energy storage elements can be useful. In the literature different
emerging types of energy harvesting for small-scale devices are reported: thermoelectric, vibration-to electric, and radiofrequency RF power conversion some published power harvesters for self-power autonomous sensors.
Self-Powered Autonomous Sensors with Thermoelectric or Airflow Generator
Two self powered autonomous sensors are presented in this paragraph, one measures the
temperature and the second the wind velocity. They use a thermoelectric and an electromechanical harvesting system. In the second example the measured parameter is supplied by the same energy used by the power harvester. Temperature values along the section of heating plant are important indicators to control the energy efficiency in the regulation of the thermal comfort . An autonomous sensor system consisting of a low power microprocessor, a 125 kHz RF-ID transponder, a low-power temperature sensor and an energy harvesting module has been developed for temperature measurement
of walled-in pipes Figure below reports in (a) the experimental set-up for the testing of the self-power
autonomous sensors, while in (b) the block diagram of the sensor and readout system.
If the autonomous sensor is placed on the hot pipes, a thermoelectric generator harvests energy, powering the autonomous sensor that periodically performs the temperature measurement and saves the data in non-volatile memory. A time stamp associated to the single data can be saved, but if the thermal gradient is not sufficient to guarantee continuously the power on, it can be lost, but the data are not overwritten. When the remote unit is close to the autonomous sensor, it generates an electromagnetic field exploited by the autonomous sensor to power its circuits and to communicate the
stored measurement data to the same remote unit. In this way the autonomous sensor harvests two types of energy always available for the function it has to execute: it measures and stores the temperature when the same temperature is high, and thermal energy is available and communicates the stored data when the remote unit is close.
The thermoelectric generator produces electrical power directly from temperature differences using the Seebeck effect. When a temperature difference ΔT is applied between the TEG faces, an opencircuit
output voltage VG is generated according to the following equation
where α is the Seebeck coefficient of the TEG materials (368 μV/°C), N is the number of
thermocouples (254) and ΔT is the temperature difference applied. The used TEG is the module
TGM-254-1.0-1.3 by Kryotherm with dimensions of (40x40x3.6) mm3.
The output power delivered by the thermoelectric module to the load depends on the difference between the heat rates that flow from the waste heat source to the hot junctions and from the cold junctions to the environment. On matched-load conditions, the output power and the output voltage are about 27 mW and 0.9 V, respectively, with a temperature difference of about 9 °C. The thermoelectric generator output is directly connected to the DC-DC converter (TPS61200) that assures the voltage level required by the electronic circuits. During the measuring and saving data operations the current consumption is about 400 microA at 2.1 V, corresponding to about 840 microW. While, during the telemetric communications, the current consumption of the microprocessor (9S08QE128), the sensor (LM94022) and the transceiver (U3280M) is about 220 microA at 2.58 V, corresponding to about 570 microW.
An experimental set-up described in has been arranged to test the developed autonomous
sensors. The experimental system consists of a flue, in which hot air is fluxed throughout. The heater system represents a simplified model of a generic building heating plant. The hot air is conveyed into a metallic pipe, heating its surfaces. The pipe is made of enameled iron and has a thickness of 1 mm; a square cross section with sides of 100 mm and a length of 1 m. External temperature distribution along the flue was measured using five NTC thermistors placed every 20 cm from the lower end of the pipe.
Two autonomous sensors were placed on the external side of the flue respectively 20 and 60 cm fromthe lower end of the pipe, near NTC sensors. During the test the temperature measured by both the reference and autonomous sensors, the voltage generated by thermoelectric generators have been evaluated. Figure 10 shows the temperature values as a function of time. The experimental results show the functioning of the system during measuring, saving and data transfer operations. Thetemperature data measured by the autonomous sensors agrees with those of the reference sensors being
the maximum temperature difference of about 3.4 °C.
The telemetric communication has been characterized interposing between the readout system andthe autonomous sensor layers of different material. Metallic layers have been tested as well, but asexpected, they do not permit the communication. The tested materials are: polystyrene (thickness 6cm), polyurethane (6 cm), wood (4 cm), glass wool (4 cm), red brick (5 cm) and tiles (4 cm). In Figure the voltage generated by the transponder is measured as a function of the distance between the autonomous sensor and remote unit. The normal working operations are executed with transponder voltage supply over 1.8 V. As expected, the air curve presents the highest readout distance, while the
other curves present a readout distance of few centimeters less.
In several environments modest ambient flows are present, for example, in air-conditioning ducts, in outdoor environment, or in moving vehicles. A flow measurement is an important indicator to control the energy efficiency in the regulation of the conditioning implants as well . An autonomous sensor placed inside the pipes and powered by an electromechanical generator scavenging energy from the airflow has been designed and tested (Figure 12a). The adopted block diagram is similar to that of the autonomous sensor for temperature monitoring described above, while the airflow is measured trough the rotor frequency of the electromechanical generator. Moreover, when the readout unit is active the electromagnetic field is used to power the autonomous sensor system and
to communicate the data. In the literature the use of wind turbines or airflow generators to power autonomous sensors. The available theoretical airflow power can be calculated
with the kinetic energy.