Piezoelectric Tactile Sensor Research Articles

A high-sensitivity flexible piezoelectric tactile sensor utilizing an innovative rigid-in-soft structure

Piezoelectric tactile sensors are widely adopted to detect vibration and other stimuli. Among them, Polyvinylidene fluoride (PVDF) has the advantage of high flexibility, fast response and low cost. However, its sensitivity is restricted by the traditional d33 working mode. Herein, we proposed a rigid-in-soft structure with truncated pyramid shape and soft bottom layer to enhance the force-transmission efficiency. This design synergizes the d33 working mode and d31 working mode, thereby surpassing the sensitivity limit. The mechanism has been analyzed using Finite Element Modeling (FEM), and experiment results indicate that rigid-in-soft tactile sensor exhibits a sensitivity of 35.6 mV/N, along with a linear force detection range of 1–11 N. The sensitivity is approximately 1.7 times higher than that of control sensor without rigid pillar. Furthermore, the sensor displays a great flexibility and reliability, making it highly promising for detecting dynamic stimuli in robotics, such as slip and vibration.

Design of Piezoelectric Tactile Sensors for Elastic Detection of Tumor Tissues

Design of Piezoelectric Tactile Sensors for Elastic Detection of Tumor Tissues

PVDF‐Based Flexible Piezoelectric Tactile Sensors: Review

AbstractWith the advent of the intelligence era, flexible piezoelectric tactile sensors are the key components for sensing information and sending signals. The performance parameters of piezoelectric sensors are closely related to the selection of piezoelectric materials. Compared with conventional piezoelectric materials, polyvinylidene fluoride (PVDF) has significant advantages in flexibility, stretchability, chemical stability, electromechanical conversion, ease of use, and acoustic propagation impedance. As a result, PVDF has become a popular topic of research. This paper introduces the working principle of piezoelectric materials and flexible piezoelectric sensors based on PVDF materials and details the research progress of different materials based on PVDF for the preparation of flexible piezoelectric tactile sensors, such as: PVDF and its copolymers for the preparation of flexible piezoelectric sensors and PVDF doped with other functional materials for the preparation of flexible piezoelectric sensors. In this paper, the advantages and disadvantages of different materials and the development prospects of PVDF flexible piezoelectric sensors are summarized to provide some insights for high‐performance flexible PVDF piezoelectric sensors.

Recent Progress in Advanced Tactile Sensing Technologies for Soft Grippers

AbstractTactile sensing technology is crucial for soft grippers. Soft grippers equipped with intelligent tactile sensing systems based on various sensors can interact safely with the unstructured environments and obtain precise properties of objects (e.g., size and shape). It is essential to develop state‐of‐the‐art sensing technologies for soft grippers to handle different grasping tasks. In this review, the development of tactile sensing techniques for robotic hands is first introduced. Then, the principles and structures of different types of sensors normally adopted in soft grippers, including capacitive tactile sensors, piezoresistive tactile sensors, piezoelectric tactile sensors, fiber Bragg grating (FBG) sensors, vision‐based tactile sensors, triboelectric tactile sensors, and other advanced sensors developed recently are briefly presented. Furthermore, sensing modalities and methodologies for soft grippers are also described in aspects of force measurement, perception of object properties, slip detection, and fusion of perception. The application scenarios of soft grippers are also summarized based on these advanced sensing technologies. Finally, the challenges of tactile sensing technologies for soft grippers that need to be tackled are discussed and perspectives in addressing these challenges are pointed out.

A flexible and ultra-highly sensitive tactile sensor based on Mg-doped ZnO nanorods for human vital signs and activity monitoring

Development of high-performance nano-ZnO based piezoelectric tactile sensors with outstanding sensitivity, good linearity, flexibility, durability and biocompatibility has a great significance for smart robotics, human healthcare devices, smart sensors and electronic skins. A flexible piezoelectric pressure sensor with a sandwich structure was fabricated by using PI/Cu film as top and bottom electrodes, Mg-doped ZnO nanorods (MZO NRs) as sensing layer and polydimethylsiloxane polymer as charge coupling layer. The MZO NRs were synthesized by a one-step hydrothermal method and their structure, morphology and defect states were characterized by using scanning electron microscopy, atomic force microscope and x-ray photoelectron spectroscopy. The piezoelectric properties of the flexible tactile sensors based on MZO NRs hydrothermally synthesized from different Mg2+/Zn2+ molar ratios were investigated comparatively by evaluating their force response actions. The results indicate that the sensors exhibit a linear response in the range of 0–1 N. The device based on MZO NRs can achieve an ultra-high sensitivity of 3.1 V N−1 (308.5 mV kpa−1) due to higher piezoelectric coefficient of MZO NRs, which is about five times higher than that of the undoped ZnO NRs sensor. Moreover, the sensor shows excellent stability and durability by loading 2500 cycles of pressing. It is demonstrated that the sensor can used to accurately monitor human motion status and vital signs in real time, such as facial motion, joint movement, wrist pulse, cervical artery, and heartbeat.

The electro-mechanics of piezoelectric coating structure under frictional contacts

In papers of the piezoelectric coating/substrate system under smooth spherical contact (DOI: 10.1177/1081 286519 865773), conical contact (DOI: 10.1016/j.ijsolstr.2019.08.031), and cylinder flat head contact (DOI: 10.1007/s11012-019-01085-1), Green’s function method has been applied to solve the problem of piezoelectric friction contact. We deduced the analytical expression of piezoelectric coating structure, which is expressed by elementary functions. The numerical results highlight the best thickness to maintain a higher potential difference and a critical potential difference. This study has guiding significance for the future research and analysis of friction signals in intelligent manufacturing, such as piezoelectric tactile sensors.

Fabrication, Characterization, and Signal Processing Optimization of Flexible and Wearable Piezoelectric Tactile Sensors

Piezoelectric microelectromechanical systems (MEMS) meet the growing demand for sensors with small sizes and low power consumption. For tactile applications and pressure measurements, they are applied in various fields from robotics to healthcare. In this context, flexible devices are very important for their high responsivity and ability to conform to the analyzed surface. This work reports on miniaturized flexible and compliant piezoelectric devices to increase the number of integrable sensors for detecting and discriminating localized pressures and contacts per unit area with minimal crosstalk. For this purpose, a series of Aluminium Nitride-based (AlN) piezoelectric sensors with different diameters (from 5 μm to 500 μm) was realized and the generated electrical signal by sensor deformation was amplified by a differential voltage amplifier circuit. By the analysis of the shape of the piezoelectric signal as a function of the applied pressure, oscillations due to piezoelectric deformations, superimposed on the initial peak signal corresponding to the touch event, have been observed. The integral of the electrical signal was calculated for the most accurate representation to describe the sensor’s response. The best responsivity was obtained in samples with diameters of 200 μm and 500 μm. Furthermore, it was also found that the minimum distance between the edges of closed sensors, observing crosstalk below -20 dB, was around 500 μm.

Optimization of PVDF nanocomposite based flexible piezoelectric tactile sensors: A comparative investigation

Optimization of PVDF nanocomposite based flexible piezoelectric tactile sensors: A comparative investigation

텍스처 인지를 위한 PZT/Epoxy 나노 복합소재 기반 유연 압전 촉각센서

텍스처 인지를 위한 PZT/Epoxy 나노 복합소재 기반 유연 압전 촉각센서

Flexible Piezoelectric Tactile Sensor With Cilia-Inspired Structures Based on Electrospun PVDF/Fe3O4 Nanofibers

Flexible tactile sensors have promising applications at the field of intelligent robotics. However, manufacturing flexible tactile sensors with excellent stability and high sensitivity remain a great challenge. In the present work, polyvinylidene fluoride (PVDF)-based nanofibers are prepared by electrospinning loaded with Fe3O4 nanoparticles (NPs). X-ray diffraction (XRD) and attenuated total reflection Fourier transform infrared spectroscope (ATR-FTIR) results indicate that the addition of Fe3O4 NPs promotes the generation of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -phase in PVDF polymer, which greatly improves piezoelectric properties of composite nanofibers. Simultaneously, a flexible piezoelectric tactile sensor with cilia-inspired structures (PTSCS) based on PVDF/Fe3O4 nanofibers is proposed, which consists of the substrate with cilia structures, Ag electrodes, composite nanofiber membrane, and nanofiber protective layer. The PTSCS demonstrates a higher output voltage compared to sensors with bump and planar structures, which also exhibits high robustness, with a stable output after 30000 cycles of vibration testing. More importantly, the PTSCS demonstrates superior performance in recognizing surface texture and temperature of objects. The flexible PTSCS has potential use in areas such as intelligent soft robotics and material recognition.

A Polymeric Piezoelectric Tactile Sensor Fabricated by 3D Printing and Laser Micromachining for Hardness Differentiation during Palpation.

Tactile sensors are important bionic microelectromechanical systems that are used to implement an artificial sense of touch for medical electronics. Compared with the natural sense of touch, this artificial sense of touch provides more quantitative information, augmenting the objective aspects of several medical operations, such as palpation-based diagnosis. Tactile sensors can be effectively used for hardness differentiation during the palpation process. Since palpation requires direct physical contact with patients, medical safety concerns are alleviated if the sensors used can be made disposable. In this respect, the low-cost, rapid fabrication of tactile sensors based on polymers is a possible alternative. The present work uses the 3D printing of elastic resins and the laser micromachining of piezoelectric polymeric films to make a low-cost tactile sensor for hardness differentiation through palpation. The fabricated tactile sensor has a sensitivity of 1.52 V/mm to mechanical deformation at the vertical direction, a sensitivity of 11.72 mV/HA in sensing material hardness with a pressing depth of 500 µm for palpation, and a validated capability to detect rigid objects buried in a soft tissue phantom. Its performance is comparable with existing piezoelectric tactile sensors for similar applications. In addition, the tactile sensor has the additional advantage of providing a simpler microfabrication process.

High performance waterproof-breathable fully flexible tactile sensor based on piezotronics coupled OFET

As obtaining flexible tactile sensors with high sensitivity, waterproofness and breathability at the same time is still a huge challenge, which limits their application in advanced fields such as human prosthetics and robotic arms. Herein, a fully flexible piezoelectric tactile sensor with high-performance, energy-efficient, waterproof, and breathable was constructed by capacitive coupling the β-PVDF nanofiber membrane (NM) with electro-mechanical conversion function and organic field effect transistor (OFET) device with signal amplification function. The inherent waterproofed β-phase PVDF NM was fabricated by advanced electrospinning process, such morphology with thousands of micropores not only gives the device excellent breathability, but also increases the deformation tolerance and accelerates the respond/recovery time, so that the obtained device can be bent at will without affecting its performance. At the same time, under the guidance of pizotronics, the charge generated by PVDF membrane based on piezoelectric effect polarization was sufficient to drive the OFET device, the as-fabricated sensor shows great sensing performance in monitoring human physiological signals without applying a gate voltage. Such device exhibits excellent pressure sensitivity, detection limit, and response time of 7.94 KPa-1, 48.54 Pa and 105 ms, respectively, which may become promising device in smart electronic skins, human motion detection, and human−machine interaction.

Finger-inspired rigid-soft hybrid tactile sensor with superior sensitivity at high frequency

Among kinds of flexible tactile sensors, piezoelectric tactile sensor has the advantage of fast response for dynamic force detection. However, it suffers from low sensitivity at high-frequency dynamic stimuli. Here, inspired by finger structure—rigid skeleton embedded in muscle, we report a piezoelectric tactile sensor using a rigid-soft hybrid force-transmission-layer in combination with a soft bottom substrate, which not only greatly enhances the force transmission, but also triggers a significantly magnified effect in d31 working mode of the piezoelectric sensory layer, instead of conventional d33 mode. Experiments show that this sensor exhibits a super-high sensitivity of 346.5 pC N−1 (@ 30 Hz), wide bandwidth of 5–600 Hz and a linear force detection range of 0.009–4.3 N, which is ~17 times the theoretical sensitivity of d33 mode. Furthermore, the sensor is able to detect multiple force directions with high reliability, and shows great potential in robotic dynamic tactile sensing.

All electrospun fabrics based piezoelectric tactile sensor

Tactile sensors have been widely used in the areas of health monitoring and intelligent human-machine interface. Flexible tactile sensors based on nanofiber mats made by electrospinning can meet the requirements of comfortability and breathability for wearing the body very well. Here, we developed a flexible and self-powered tactile sensor that was sandwich assembled by electrospun organic electrodes and a piezoelectric layer. The metal-free organic electrodes of thermal plastic polyurethane (PU) nanofibers decorated with multi-walled carbon nanotubes were fabricated by electrospinning followed by ultrasonication treatment. The electrospun polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) mat was utilized as the piezoelectric layer, and it was found that the piezoelectric performance of PVDF-TrFE nanofiber mat added with barium titanate (BaTiO3) nanoparticles was enhanced about 187% than that of the pure PVDF-TrFE nanofiber mat. For practical application, the as-prepared piezoelectric tactile sensor exhibited an approximative linear relationship between the external force and the electrical output. Then the array of fabricated sensors was attached to the fingertips of a glove to grab a cup of water for tactile sensing, and the mass of water can be directly estimated according to the outputs of the sensor array. Attributed to the integrated merits of good flexibility, enhanced piezoelectric performance, light weight, and efficient gas permeability, the developed tactile sensor could be widely used as wearable devices for robot execution end or prosthesis for tactile feedback.

A Self‐Powered, Single‐Mode Tactile Sensor Based on Sensory Adaptation Using Piezoelectric‐Driven Ion Migration

AbstractA piezoelectric tactile sensor is beneficial for creating a self‐powered system with a compact design, which is essential in electronic‐skin technology. However, piezoelectricity is only capable of dynamic pressure detection because it responds to sudden environmental changes. Since it is common to add another sensing unit to detect static pressure that accompanies bulkiness, including a measuring apparatus, we demonstrate a self‐powered, single‐mode piezoelectric tactile sensor by fabricating a piezoelectric gel through the electrospinning technique. As piezoelectricity senses the dynamic pressure without an external power supply, ions detect the static pressure by maintaining the potential difference upon sustained pressure. Since each component outputs a voltage signal of the same type but different profiles upon pressure, it is possible to distinguish dynamic and static pressure in a single mode. Moreover, inspired by the sensory adaptation of mammalian skin, an ion‐assisted piezoelectric tactile sensor efficiently detects concurrently stacked stimuli by decreasing the output signal for sustained stimuli. The sensitivity for superimposed pressure upon initial 14.7 kPa increases by more than four times compared to that without sensory adaptation for both dynamic and static pressure.

Flexible Self-Powered Tactile Sensors Based on Hydrothermally Grown ZnO Nanorods

Flexible tactile sensors with high sensitivity and good flexibility are urgently required in modern biomimetic robots’ electronic skin, and artificial intelligence interaction. In this work, a flexible piezoelectric tactile sensor was fabricated using hydrothermally grown ZnO nanorods (ZnO NRs) on Cu-coated polyimide (PI) substrate. Structural and morphological properties of the ZnO NRs were investigated by SEM, XRD. The piezoelectric response properties of the sensors by using the pasted Cu/PI electrode and directly screen-printed Ag electrode were compared. The results show the sensor with the screen-printed Ag electrode exhibits a high sensitivity of 974 mV/N, which is higher than the pasted Cu/PI electrode, and the response of all sensors to the applied pressure is almost linear in the range of 0.1-1 N. Meanwhile, the sensor can be employed to detect human movements such as bending/stretching motion of fingers and the vibration of the vocal cords and pulse, and realize the conversion from mechanical energy to electrical energy. It is demonstrated that the sensor has great potential applications in wearable health monitoring systems and self-powered devices.

3D Conformal Fabrication of Piezoceramic Films.

Piezoceramic films are an essential class of energy‐conversion materials that have been widely used in the electronics industry. Although current methods create a great freedom for fabricating high‐quality piezoceramic films, it requires well‐controlled synthesis conditions, including special high‐cost equipment and planar substrates particularly. The limited substrate selections hinder the applications of piezoceramic films in 3D conformal structures where most objects possess complex curvilinear surfaces. To overcome such limitations, a fast, energy‐efficient, and cost‐effective approach, named flame treated spray (FTS) coating, is developed for preparing piezoceramic films on free‐form surfaces. The flame treatment significantly enhances the hydrophilicity of a substrate, assisting in forming a uniform and continuous thin film. The followed spray coating deposits hundreds of nanometers to several micrometers thick films on 3D free‐form surfaces. Given the size controllability and arbitrary surface compatibility of the FTS method, a highly conformal piezoelectric tactile sensor array (4 × 4) is assembled on a spherical surface for mimicking robot fingers and an on‐site thin‐film sensor on the wing of an aircraft model to monitor the vibration in real‐time during flight. The FTS film deposition offers a highly promising methodology for the application of functional thin‐film from micro‐ to marcoscale devices, regardless of conformal problems.

Comparison of piezoelectric responses of flexible tactile sensors based on hydrothermally-grown ZnO nanorods on ZnO seed layers with different thicknesses

One peculiar property of ZnO nanorods (NRs) is their piezoelectricity, which opens up a wealth of possibilities in the fields of piezoelectric electronics and piezoelectric optoelectronics, such as pressure sensors, actuators, energy-capture and energy-storage devices. For further clarifying the relation between the piezoelectric properties and structural morphology of ZnO NRs, the effect of their aspect ratio on piezoelectric response was analyzed by simulation method. A piezoelectric tactile sensor with ZnO NRs as a sensitive layer hydrothermally grown on a sputtered ZnO seed layer (SL) was fabricated on a polydimethylsiloxane (PDMS) substrate. The effects of the SL thickness on the structural properties and morphology of ZnO NRs were investigated, and a comparative study on the piezoelectric sensing of the tactile sensors with different SL thicknesses was done. The results show that the ZnO SL thickness has a clear influence on the structural morphology of the ZnO NRs and the sensitivity of the ZnO-NRs-based sensor, and the sensor with a SL thickness of 80 nm has a maximum sensitivity of 524.5 mV/N due to the high aspect ratio. Moreover, the sensors have a good linear response to external pressure within the range of less than 1 N, and so exhibit excellent application prospects in tactile perception and detection of weak dynamic force.

Sensitivity enhancement of the tactile sensor based on hydrothermally grown ZnO nanorods modified by catalytic Au nanoparticles

Flexible piezoelectric tactile sensor with transverse planar electrodes based on hydrothermally grown ZnO nanorods (ZnO NRs) was presented by using polydimethylsiloxane (PDMS) as flexible substrate and packaging material. The effects of the content of gold nanoparticles (AuNPs) added into the precursor solution on the structural morphology of ZnO NRs and on the piezoelectric properties of the ZnO NRs tactile sensor were investigated. Tactile sensors show a linear piezoelectric response in the pressure range of 0–1 N, and the sensor for the precursor solution with AuNPs of 100 μl shows a high sensitivity of 1.42 V N−1 due to the large aspect ratio of the ZnO NRs, indicating that a small amount of AuNPs addition can optimize the structural morphology of ZnO NRs and thus improve the piezoelectric response of the sensor. Meanwhile, the sensor is employed to monitor human information in real-time such as bending/stretching motion of finger and distinguish various objects.

Fabrication and Optical Properties of Transparent P(VDF-TrFE) Ultrathin Films.

The films of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) are widely used in piezoelectric tactile sensors, vibration energy harvesters, optical frequency conversion materials and organic photo-voltaic devices because of high electroactive, good optical and nonlinear optical properties, respectively. In this work, the multilayer structured ultrathin films were fabricated by the Langmuir–Blodgett technique, and the thickness per layer can be controlled accurately. It was found that as the collapse pressure of P(VDF-TrFE) (25:75) and the optimal dipping value are 60~70 mN/m and 15 mN/m, respectively, a high-density film can be obtained due to the compression of molecules. The surface topography and optical properties of the LB films were characterized by X-ray diffraction, white light interferometer and variable-angle spectrum ellipsometer. It was observed that the films are transparent in the visible region and IR-band, but show a high absorption in the UV band. Besides, the transmittance of the films ranges from 50% to 85% in the visible region, and it linearly decreases with the number of monolayers. The average thickness of per deposition layer is 2.447 nm, 2.688 nm and 2.072 nm, respectively, under three measurement methods. The calculated refractive index ranged from 1.443 to 1.598 (600~650 nm) by the Cauchy-model.