22-12-2010, 11:45 AM
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Introduction
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 thedevice 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
us in:
◗ 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 the sensor is a hybrid MEMS composed by a novel MEMS temperature sensor (above in
Figure 7a) developed using the Metal MUMPs process [42] and a planar inductor realized in thick film technology by screen printing over an alumina substrate a conductive ink in aspiral 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