Demand for autonomous flying vehicles intended for transportation of people and goods is expected to accelerate in the next few years as urban air mobility maturity reaches level-4, a rating implying that hundreds of flights will simultaneously take off from urban aerodromes around the country. Wind sensors available on the vehicles and located in the air space will become a key necessity for ensuring safe navigation. Conventional anemometers suffer from various drawbacks due to their non-aerodynamic construction, high power consumption, complex signal processing, and cost. An airfoil-shaped low drag anemometer is presented for wind speed and direction measurement on tethered systems such as kites, balloons, and drones. The airfoil anemometer is equipped with a flexible, dual-layer capacitive pressure sensor with a polyvinylidene fluoride (PVDF) diaphragm for wind speed and a commercial digital magnetometer for wind direction measurement. The fabrication process for the diaphragm-type capacitive sensor is presented, along with characterization of the sensor in a pressure chamber. The completed sensor is then integrated into a NACA-2412 profile airfoil, along with a commercial magnetometer, for demonstration in a laboratory-scale wind tunnel.
Metal-matrix composites with active components have been investigated as a way to functionalize metals. As opposed to surface-mounted approaches, smart materials embedded in metals can be effectively shielded against the environment while providing in-situ sensing, health monitoring, actuation, or energy harvesting functions. Typical manufacturing approaches can be problematic, however, in that they may physically damage the smart material or degrade its electromechanical properties. For instance, non-resin-based embedment procedures such as powder metallurgy involve isostatic compression and diffusion bonding, leading to high process temperatures and breakdown of the electromechanical properties of the active component to be embedded. This paper presents the development and characterization of an aluminum-matrix composite embedded with piezoelectric polyvinylidene fluoride (PVDF) sensors using ultrasonic additive manufacturing (UAM). UAM incorporates the principles of solid-state, ultrasonic metal welding and subtractive processes to fabricate metal-matrices with seamlessly embedded smart materials and without thermal loading. As implemented in this study, the UAM process uses as-received, commercial Al 6061 tape foilstock and TE Connectivity PVDF film. In order to increase the mechanical coupling between the sensor and the metal-matrix without the aid of adhesives, the PVDF sensor is embedded with an empirically optimized pre-compression defined by the tape foils welded above the sensor. The specimen is characterized by tensile (d31 mode), bending (d31 mode), and compression tests (d33 mode) to evaluate its functional performance. Within the investigated load range, the specimen exhibits open-circuit sensitivities of 4.6 mV/N under uniaxial tension and 9.7 mV/N under compressive impulse tests with better than 95% linearity and frequency bandwidth of several kilohertz. The technology presented in this study could be applied for load and tactile sensing, impact detection and localization, thermal measurements, energy harvesting, and non-destructive testing applications.
KEYWORDS: Ferroelectric polymers, Sensors, Composites, Amplifiers, Microsoft Foundation Class Library, Motion models, Energy harvesting, Mechanical sensors, 3D metrology, Motion measurement
Bistable laminated composites are attractive for morphing structures because they can hold deformed stable shapes without any external energy and require actuation only to switch between shapes. They offer opportunities for shape morphing, energy harvesting, and flow control devices. Piezoelectric macrofiber composites (MFC) embedded in bistable laminates have been demonstrated for actuation and energy harvesting. However, their relatively high stiffness, relatively complex architecture, and arbitrary fiber orientation limit their ability to sense shape change in bistable laminates. There has been little work on the integration of sensing methods to monitor an adaptive structure’s shape; shape sensing has been investigated mainly for the detection of snap-through events. In this paper, we present self-sensing curved bistable laminates layered with piezoelectric PVDF films that can sense smooth changes as well as abrupt snap-through transitions. Measurement of smooth changes in the laminate’s curvature is enabled by an automated drift compensation charge amplifier with an extremely low cutoff frequency of 0.01 mHz. The sensing function is demonstrated on bistable laminates created using mechanical prestress. Two sensor layers are configured in the composite such that one measures change in curvature and the other measures snap-through response. The shapes measured by the sensor in terms of voltage correlate well with the shapes measured with a 3D motion capture system. An analytical model is developed to relate curvature change to voltage output and is found to be in good agreement with the measured curvature-voltage sensitivities. The weakly coupled shapes of the laminate and the low cross sensitivity of PVDF enable real-time measurement of the principal curvatures.
Pressure sensors that can provide both high temporal and spatial resolutions are desired for the measurement of aerodynamic and acoustic events, ultrasonics, and underwater phenomena. Piezoelectric materials are attractive candidates for measuring dynamic pressure due to their high sensitivity, high signal-to-noise ratio, and potential for miniaturization. However, their inability to directly measure static pressure prevents their use in many applications. Due to their strong pyroelectric response, their use is also generally limited to conditions where the rate of temperature change is below the lower cutoff frequency of the measurement system. Polyvinylidene fluoride (PVDF) is a polymer with a high piezoelectric sensitivity which is readily available as a flexible, tough film. Under steady ow conditions, configuring PVDF as a cantilever unimorph provides a higher pressure sensitivity than alternatives such as compressive, doubly clamped, or diaphragm configurations. In this work, we demonstrate a differential aerodynamic pressure sensor based on a cantilever PVDF unimorph that has been optimized to maximize pressure sensitivity for a targeted deflection sensitivity. The sensor is characterized using a laboratory-scale wind tunnel for flows ranging from 0 to 12 ms-1. Near-static measurements are enabled by a compensated charge amplifier with an extremely low cutoff frequency. The pyroelectric voltage generated from changes in the air ow temperature is compensated using a PVDF sensor in compressive mode. Within the tested pressure range of 0 to 80 Pa, the sensor exhibits a proportional response with a sensitivity of 0.97 mV Pa-1.
Piezoelectric elements serve as a preferred candidate for measuring dynamic pressure owing to their high sensitivity, signal-to-noise ratio, high natural frequency, and suitability for miniaturization. Polyvinylidene fluoride (PVDF) is a mechanically tough, flexible, low density polymer commercially available as a film. Being mechanically compliant and minimally invasive to the host structure, PVDF can be conformed to a variety of surfaces using adhesive bonding, thus making it a suitable candidate for surface pressure mapping and acoustic pressure measurement applications. However, PVDF sensors in compressive mode are insufficient for the low frequency and high sensitivity requirements of vehicle surface pressure measurements. Under steady flow conditions, cantilever and clamped-clamped unimorphs with segmented electrode coverage configurations serve as alternative candidates for differential pressure measurements. This paper presents an analytical and computational design framework for optimizing the performance of PVDF unimorphs. Electrode coverage, thickness ratio, and elastic modulus ratio are optimized for cantilever and clamped-clamped configurations for a given sensor geometry. The goal of the optimization procedure is to maximize charge sensitivity of the pressure sensor while minimizing deflection. A closed-form solution is derived for deflection and charge sensitivity of cantilever and clamped-clamped configurations based on Euler-Bernoulli beam theory. For a given deflection sensitivity target and sensor geometry, the charge sensitivity of the optimized cantilever sensor is three orders of magnitude greater than compressive (d33 mode) design and 3.15 times higher than the clamped-clamped configuration with segmented electrodes.
There is a need for high-performance force sensors capable of operating at frequencies near DC while producing a minimal mass penalty. Example application areas include steering wheel sensors, powertrain torque sensors, robotic arms, and minimally invasive surgery. The beta crystallographic phase polyvinylidene fluoride (PVDF) films are suitable for this purpose owing to their large piezoelectric constant. Unlike conventional capacitive sensors, beta crystallographic phase PVDF films exhibit a broad linear range and can potentially be designed to operate without complex electronics or signal processing. A fundamental challenge that prevents the implementation of PVDF in certain high-performance applications is their inability to measure static signals, which results from their first-order electrical impedance. Charge readout algorithms have been implemented which address this issue only partially, as they often require integration of the output signal to obtain the applied force profile, resulting in signal drift and signal processing complexities. In this paper, we propose a straightforward real time drift compensation strategy that is applicable to high output impedance PVDF films. This strategy makes it possible to utilize long sample times with a minimal loss of accuracy; our measurements show that the static output remains within 5% of the original value during half-hour measurements. The sensitivity and full-scale range are shown to be determined by the feedback capacitance of the charge amplifier. A linear model of the PVDF sensor system is developed and validated against experimental measurements, along with benchmark tests against a commercial load cell.
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