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Posted on 01 February 2019

New Record for Micromachined Energy Harvesters

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In the future, we will be surrounded by miniaturized autonomous wireless sensor nodes, integrated into our environment and measuring parameters in buildings, on our body, on machines. Today, these sensor systems still rely on batteries which hinder their downsizing and autonomy.

By Els Parton and Rob van Schaijk, IMEC

 

For some applications – sensing in highly accessible environments – sensor systems may run on batteries. But for other applications, such as continuous machine monitoring, or monitoring in an environment that is difficult to access, batteries are not the best option. Depending on the type of battery, the autonomy of a 100µW module would be limited to a few months, half a year maximum.

Piezoelectric energy harvester

Therefore, researchers worldwide are looking for ways to make the sensor systems harvest energy from their environment (vibration, light, heat). For example, sensor nodes on machines could use the vibrations of the machine’s components as an energy source.

Within the context of its activities at Holst Centre, the research centre IMEC has recently shown a new piezoelectric energy harvester for sensor systems. The new harvester delivers an experimental output power of 60µW, which is a new record for micromachined energy harvesters, and which is enough to drive simple sensor systems that intermittently transfer sensor readings to a master.

Piezoelectric energy harvester 2

The main building blocks of the envisaged autonomous sensor systems are a sensor or actuator, a module to acquire and preprocess the signals, and a wireless radio to send them to a base station. Their autonomy will allow them to operate an indefinite time without battery recharge or connection to a power grid.

Resonance curve for the AIN piezoelectric harvester

Reduce energy consumption + develop energy harvesters

IMEC’s solution is to tackle the energy problem from both sides: consumption and generation. To reduce the energy consumption, IMEC is working on micromodules that run on a minimal amount of energy. The goal is to create microsystems than only need an average of 100μW. As for the energy generation, IMEC looks into generating and storing power at the micro-scale to improve the autonomy of wireless autonomous modules. For generating energy, the choice is to develop micromachined energy harvesters, and to combine these with added energy storage, as backup when the harvester is not active, or to handle peak loads, when the harvester cannot generate enough power.

Dimensions of the piezoelectric energy harvesters are compared

Energy harvesters take their energy from the environment in the form of vibration, ambient light, or heat. They convert this energy into electricity. Each form of energy harvesting has its characteristics and application for which it is best suited. Outdoor sensors, for example, may best be combined with photovoltaic cells, which can generate up to 10mW/cm². Monitoring in machines, on the other hand, will require harvesting the vibrations or heat coming from the machines. Typical for machine components is that they vibrate at constant, predictable frequencies. Tapping into these vibrations could deliver 100μW/cm², which is enough to drive the micropower devices that IMEC envisages.

The working principle

Vibrational energy scavengers make use of electromagnetic, electrostatic, or piezoelectric conversion to generate electrical power. A microsized piezoelectric transducer is simplest to design, and has so far shown the best result. It consists of a cantilever with one or several piezoelectric layers sandwiched between metallic electrodes forming a capacitor. At the tip of the cantilever, there is a seismic mass that will capture the vibrations of the machine to which the scavenger is attached.

The vibrations of the machine cause the mass of the harvester to vibrate. This stretches the piezoelectric layer on the cantilever, generating a voltage across the piezoelectric capacitor. The generated energy is extracted by a resistive load.

These piezoelectric transducers have a resonance frequency that depends on their mass and stiffness of the cantilever. When the machine vibrations cause the transducer to vibrate at this frequency, the transducer will generate its maximum power. For best results, the machine vibrations and the resonance frequency of the harvester should match. This can be done by adapting the mass and cantilever stiffness to the environment in which the harvester will operate.

New record

IMEC’s new vibration harvester consists of a piezoelectric capacitor formed by a Pt electrode, an AIN piezoelectric layer and a top Al electrode. It is fabricated in a silicon-based process using three wafers bonded by SU-8. The resulting harvester delivers an experimental output power of 60μW. It weights only 34mg. The cantilever beam and mass are 6mm long, and the beam is only 5mm wide. The output power was measured at a resonance frequency of 500Hz and an acceleration of 2g. Last year, IMEC already showcased a piezoelectric harvester with a reported 40μW. But this device had a piezoelectric layer fabricated in PZT. The current AIN layer has the advantage that it can be made in a simpler deposition process, compatible with standard CMOS processes, allowing production at a lower cost. Moreover, the PZT device operated at 1.8kHz. The lower resonance frequency of the new harvester – 500 Hz - corresponds with vibration frequencies in, for example, industrial equipment or car tires.

These state-of-the-art piezoelectric harvesters can still be improved along several lines. First, the fabrication process can be improved using SOI wafers. Second, a vacuum package should be designed to eliminate the effect of air damping of the cantilever movement. Third, the load should be optimized, to have a maximum power output. Further out in the future, vibration harvesters will be made which tune their resonance frequency, and optimum power output, to the application. Another option is to make broadband harvesters that generate power at a broad spectrum of vibrations. With their 60μW output power, IMEC’s harvesters are already powerful enough to drive simple wireless sensors that intermittently transfer sensor readings to a master, which is the case in TPMS systems.

 

1) Els Parton is Scientific Editor at IMEC (Leuven, Belgium) – email: Els.Parton@imec.be
 2) Rob Van Schaijk is principal researcher and activity leader of the Micropower program of IMEC at Holst Centre (Eindhoven, The Netherlands) – email: Rob.VanSchaijk@imec-nl.nl

 

 

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