Pit Pillatsch

A scalable piezoelectric impulse-excited energy harvester for human body excitation

Harvesting energy from low-frequency and non-harmonic excitations typical of human motion presents specific challenges. While resonant devices do have an advantage in environments where the excitation frequency is constant, and while they can make use of the entire proof mass travel range in the case of excitation amplitudes that are smaller than the internal displacement limit, they are not suitable for body applications since the frequencies are random and the amplitudes tend to be larger than the device size. In this paper a piezoelectric, impulse-excited approach is presented. A cylindrical proof mass actuates an array of piezoelectric bi-morph beams through magnetic attraction. After the initial excitation these transducers are left to vibrate at their natural frequency. This increases the operational frequency range as well as the electromechanical coupling. The principle of impulse excitation is discussed and a centimetre-scale functional model is introduced as a proof of concept. The obtained data show the influence of varying the frequency, acceleration and proof mass. Finally, a commercially available integrated circuit for voltage regulation is tested. At a frequency of 2 Hz and an acceleration of 2.7 m s−2 a maximal power output of 2.1 mW was achieved.

A wearable piezoelectric rotational energy harvester

This paper discusses the operating principle of a rotational energy harvester for body motion with an eccentric proof mass. A mathematical analysis for the rotor motion under different excitations is performed and the gravitational and inertial operation explained. The transducing mechanism works on the principle of frequency up-conversion that is now widely used to harvest low frequency vibration more efficiently, and uses a piezoelectric beam and magnetic coupling. A miniaturized device with an overall size similar to that of a wristwatch is introduced. The fabrication is entirely done using standard milling and turning processes. Experimental results for this device show significant improvement in the attachment of the piezoelectric beam compared to a previous prototype. Furthermore, there is a good match between the magnetic forces and the proof mass for the tested excitations. A disadvantage of the miniaturized prototype is the higher stiffness of the piezo beam, preventing free oscillation after actuation. Modifications to counteract this problem are provided and experimentally validated.

Magnetic plucking of piezoelectric beams for frequency up-converting energy harvesters

In the field of energy harvesting from low-frequency random excitation, a technique known as piezoelectric beam plucking or frequency up-conversion has seen increasing interest. This paper presents an experimentally validated model to calculate the voltage response and power output when actuating piezoelectric beams via a pair of magnets. Using magnetic coupling avoids impact on the brittle piezoceramic material. The relevant equations are derived for the piezoelectric beam, and two different approaches to include the magnetic interaction are presented and discussed. After comparing the models to experimental results, their use in predicting the response to changes in the system, e.g. using thicker magnets, and determination of the optimal load resistance and power output with regards to frequency, are investigated.

Real World Testing Of A Piezoelectric Rotational Energy Harvester For Human Motion

Harvesting energy from human motion is challenging because the frequencies are generally low and random compared to industrial machinery that vibrates at much higher frequencies. One of the most promising and popular strategies to overcome this is frequency up-conversion. The transducing element is actuated at its optimal frequency of operation, higher than the source excitation frequency, through some kind of catch and release mechanism. This is beneficial for efficient power generation. Such devices have now been investigated for a few years and this paper takes a previously introduced piezoelectric rotational harvester, relying on beam plucking for the energy conversion, to the next step by testing the device during a half marathon race. The prototype and data acquisition system are described in detail and the experimental results presented. A comparison of the input excitation, based on an accelerometer readout, and the output voltage of the piezoelectric beam, recorded at the same time, confirm the successful implementation of the system. For a device functional volume of 1.85 cm3, a maximum power output of 7 μW was achieved when the system was worn on the upper arm. However, degradation of the piezoelectric material meant that the performance dropped rapidly from this initial level; this requires further research. Furthermore, the need for intermediate energy storage solutions is discussed, as human motion harvesters only generate power as long as the wearer is actually moving.

A scalable piezoelectric impulse-excited generator for random low frequency excitation

This paper introduces an impulse excited piezoelectric energy harvesting prototype. The device is aimed at large amplitude, low frequency excitation typical of human body motion. A rolling, external proof mass actuates an array of piezoelectric cantilevers that form a distributed transduction mechanism. After initial deflection, the beams vibrate at their natural frequency. This allows for improved electromechanical coupling and large operational bandwidth. Measurements are presented to include different excitations and proof masses and an IC for voltage regulation is evaluated. At an excitation frequency of 2 Hz and an acceleration of 2.72 m/s2 a power output of 2.1 mW was achieved.

Self-tuning behavior of a clamped-clamped beam with sliding proof mass for broadband energy harvesting

Real world systems rarely vibrate at a single resonance frequency and the frequencies drift over time. Tunable devices exist, but generally need additional energy to achieve frequency adaptation. This means that the benefits in power output from this tuning need to be large enough to power the mechanism itself. Passively self-tuning systems go into resonance without requiring active control. This paper focuses on a passively self-tuning system with a proof mass that can slide freely along a clamped-clamped beam. Under external vibration, the slider moves along the beam until the system goes into resonance. A proof-of-concept design is introduced using either a copper or a steel beam and a 3D-printed ABS thermoplastic proof mass. Successful self-tuning is demonstrated in both cases. The frequencies range from 80 – 140 Hz at accelerations as low as 0.007 g rms. Results show the resonance of the beam and the position of the slider along the beam with time. Furthermore, the dynamic magnification and the proof mass position at resonance are discussed, together with the inherent non-linearities of double-clamped beam resonators. The findings support the hypothesis that the effect of the ratio between proof mass and beam mass outweighs the Duffing spring stiffening effects.

Piezoelectric Rotational Energy Harvester for Body Sensors Using an Oscillating Mass

A rotational energy harvester for human body applications is presented in this paper. An oscillating mass, similar to those found in wristwatches is used as a proof mass to act on a piezoelectric impulse excited transduction mechanism that is particularly well suited for these low-frequency, non-harmonic vibrations. The electromechanical coupling is enhanced by letting a piezoelectric beam vibrate at its natural frequency after an initial excitation. The plucking of the beam is achieved by a completely contact less magnetic coupling, beneficial for the longevity of the device. The potential advantages of rotary harvesters are discussed and a first design is introduced. The measurement results demonstrate the successful implementation and make it possible to investigate the influence of different factors on the power output. At a frequency of 2 Hz a maximal power of 2.6 microwatt was achieved when tested on a rocking table.

Wireless sensors for automated control of total incombustible content (TIC) of dust deposited in underground coal mines

This paper presents ongoing research towards a low-cost/low-power wirelessly enabled distributed sensing system that can be placed throughout underground coal mines to potentially measure the total incombustible content (TIC) of the deposited dust. Underground coal mining operations produce finely divided coal dust, called float dust, which deposits throughout the mine. This combustible material can be feedstock for coal dust explosions. In the U.S., Mine Safety and Health Administration (MSHA) standard 23789 dictates that a TIC of 80% or above has to be maintained in coal mine return airways. However, current best practices for collection TIC measurements involve manual sampling and laboratory procedures. In this work, we describe work towards developing a new wireless sensor network (WSN) consisting of low-cost/low-power sensor modules that use a variety of optical and microfabricated sensors to continuously monitor the TIC of the accumulated dust. The information is then transmitted off-board through a reliable ad-hoc wireless network. Called Sensors for Automated Control of Coal Dust (SACCD), this system can potentially, for example, be used to automate the control of rock-dusting equipment to maintain TIC at acceptable levels.

Degradation of Piezoelectric Materials for Energy Harvesting Applications

The purpose of energy harvesting is to provide long term alternatives to replaceable batteries across a number of applications. Piezoelectric vibration harvesting provides advantages over other transduction methods due to the ability to generate large voltages even on a small scale. However, the operation in energy harvesting is different from typical sensors or actuators. The applied stress is often at the material limit in order to generate the maximum power output. Under these conditions, the degradation of the materials becomes an important factor for long term deployment. In this work bimorph piezoelectric beams were sub jected to lifetime testing through electromagnetic tip actuation for a large number of cycles. The results of two measurement series at different amplitudes are discussed. The dominant effect observed was a shift in mechanical resonance frequencies of the beams which could be very detrimental to resonant harvesters.

MEMS-based capacitive pressure sensors with pre-stressed sensing diaphragms

This paper presents a MEMS-based capacitive pressure sensor with pre-stressed sensing diaphragms for achieving a linear response with applied pressure. The sensor is designed to work in touch-mode by using sensing pressure diaphragms with compressive residual stress on top of insulated counter electrodes. FEM (Finite Element Method) calculation shows that a sensing diaphragm with residual stress can provide superior linear response than a stress-free diaphragm. The device is fabricated on a Si substrate using surface micromachining. LPCVD (low pressure chemical vapor deposition) oxide is used to seal the pressure cavity and to form the dielectric insulation layer. The experimental results show that the MEMS pressure sensor responds linearly in the pressure range of 16-215 psi with a sensitivity of 0.092 pF/psi and full scale nonlinearity of 3.3% without compensation.