This article presents a method for preparing and testing the piezoelectric properties and stability of cellular electret based on polypropylene (PP). Introducing 5% mineral filler as a mixture of crystalline silica, colloidal silica, and kaolin to isotactic polypropylene followed by a film stretching process resulted in the formation of a composite cellular structure. To manufacture electrets, the films were polarized at a constant electric field in the range from 100 V/lm to 125 V/lm, in a climatic chamber heated up to 80°C. The durability of the electrets was determined using thermostimulated discharge currents and approximate calculations of depolarization process activation energy. For electrets made of cellular films, the depolarization temperature T m at which the density of the discharge current assumes the highest value was $108°C and the activation energy was 6.25 eV. The response of the polarized composite film to mechanical stress expressed as the piezoelectric constant d 33 was about 3 times higher than for a-PP film of the prevailing atactic phase and poly(vinylidene fluoride) film without a cellular structure. In the range of stress of 1 kPa to 120 kPa it was 135 pC/N for lower stresses and 60 pC/N for higher stresses.
Purpose -This paper aims to present a prototype of the diagnostic system for the examination of the distribution of the force applied by foot to substrate during usual human moving. Presented system is competitive to other currently available devices, thanks to sensors reliability, user-friendly operation manner and design based on cheap parts. The results of examinations are transmitted by radiomodem. Its recording and visualization are possible on either personal or mobile computers. Design/methodology/approach -During selection of the sensors substrate, many polymeric electrets were examined. Polyvinylidene fluoride films were selected, because they have good charge uniformity across the surface, wide range of acceptable temperatures, linear relation between mechanical stress and output signal and high resistance for squeezing. The system measures the charge generated in film. Findings -The pressures are recorded in relation to maximum value; therefore, measuring system does not require calibration. The simultaneous recording of data from all eight sensors allows tracking the signal without distortion. Originality/value -An array of sensors is installed in the shoe insole. The measuring device is fixed to the outer surface of the shoe. Its weight is 75 g. The range of transmission is suitable for examination in the natural environment, outside traditional consulting room. Software is dedicated for analysis of the pressure distribution in every moment of the foot movement. The system is suitable for examination of flat feet, diabetic foot and recovery progress after injures.
The paper presents a dynamic measurement method of the distribution of foot pressure exerted on the ground by a four-point shoe insole, developed by authors, which can be placed in any sport footwear. The value of pressure was measured on the heel, medial midfoot, metatarsal, and great toe by recording values of a generated voltage by sensors which were made of piezoelectric polymer PVDF film 110 µm thick with printed silver electrodes. As confirmed by scanning microscope studies, the foil applied in the sensors is semi-crystalline. The shoe measurement insert consists of two polyester films without piezoelectric properties between them, electroactive polymer sensors were placed. The films were glued together. To match the measuring circuit to the sensors used, two circuits were tested, a voltage measuring circuit with an input resistance of above 1012 Ω (open circuit), and a charge measuring circuit (shorted circuit). The charge measuring circuits with the RC high-pass filter, which attenuates the slow-changing pyroelectric signal was selected as it ensures the desired measurement accuracy. As presented in the paper, as PVDF sensors are very sensitive to any mechanical deformation, it is important to properly design the shoe insole to ensure its correct use during pressure distribution measurements. The measuring system developed by the authors, allows testing of foot pathology for any length of time in a dynamic way.
Parasitic harvesting of energy from environment is being investigated as a possible solution for powering electronic devices. One possible solution is to harvest energy generated from human walking, by placing a piezoelectric elements inside footwear. As the amount of available energy is extremely low, conversion effectiveness is of prime importance. This paper deals with a micropower source, designed to harvest energy from walking, built in a form of a shoe insole containing a piezoelectric material (43 mm thick polyethylene foil). Laboratory investigation of the piezoelectric properties is presented first, followed by a measurement result of an actual generator with 0?6 mW output power. Finally, a modification of the generator, by adding springy elements, is presented, resulting in ninefold increase in power output (to 5?6 mW).
This paper presents investigation of piezoelectric proprieties of polypropylene PP and polyvinylidene fluoride PVDF films at an angle of their application as micropower generators and foot pressure sensors in walking process. Obtaind micropower from single layer is about 1.7W and 5.3W for polypropylene film and about 1.7W and 3.3W for polyvinylidene fluoride film. Obtained voltage from single film layer is 8.9V to 14V for PP film and 2 to 3.4V for PVDF film. Obtained micropower from piezoelectric film and course character of voltage in function of time during walking process, depends from used film and shoe insole construction, where active element was sandwich. Recived data record of voltage, power and foot movement images from measuring system, can be use in dynamic investigations of posture defects.
Article presents research methods and their results for 100μm polyethylene terephthalate PET film, from a certain angle of its application as pressure sensors. Film was polarized at 150V/μm electric field intensity in dwo different temperatures: 60°C and 90°C. Direction of polarization was measured by thermally stimulated depolarization current TSDC. Results shows presence of homo-and heterocharge. Relaxation was calculated by two methods: from relationship ln J (T) = const – W/kT and from relationship (T) = P (T) / J (T). Activation energy for heterocharge calculated by first relationship is 6.14 eV to 6.77 eV and its relaxation time in 24°C is ~1020 years. For second one relationship, relaxation time in 24°C is ~1035 years. Piezoelectric voltage in 33 direction is 74V for termination resistance 1014Ω and 30V for termination resistance 109Ωfor applied stress 10N/cm2 from 10 cm2 surface. Obtained results for studied PET film, encourage to use it in practice as pressure sensors.
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