Portal de Periódicos da CAPES

Flexible Three‐Axial Force Sensor for Soft and Highly Sensitive Artificial Touch

2014; Volume: 26; Issue: 17 Linguagem: Inglês

10.1002/adma.201305064

1521-4095

Lucie Viry, Alessandro Levi, Massimo Totaro, Alessio Mondini, Virgilio Mattoli, Barbara Mazzolai, Lucia Beccai,

Interactive and Immersive Displays

A soft tactile sensor able to detect both normal and tangential forces is fabricated with a simple method using conductive textile. Owing to the multi-layered architecture, the capacitive-based tactile sensor is highly sensitive (less than 10 mg and 8 μm, for minimal detectable weight and displacement, respectively) within a wide normal force range (potentially up to 27 N (400 kPa)) and natural touch-like tangential force ranges (from about 0.5 N to 1.8 N). Being flexible, soft, and low cost, this sensor represents an original approach in the emulation of natural touch. Furthermore, in addition to being flexible and mechanically robust over a wide pressure range, the sensor is also able to detect in-plane forces. We investigated the performance of the device under tangential stimulation while applying a static normal force Fz of, either, 0.5 N or 1 N; thus mimicking the tactile indentation needed to induce a tangential force through horizontal displacement (Figure 3d). During the experiments, observations of the cross section under an optical microscope suggest that no shear stress is induced in the dielectric fluorosilicone film and that the top electrode is free to slide over the bottom set of electrodes (see video S6, Supporting Information). In particular, the latter aspect is due to the low friction properties of the fluorosilicone. Independently of the static normal applied force, a 0.4 N tangential force is needed before a significant capacitance variation can be observed (see Figure 3d). This minimum force could reflect the shear strain induced in the thick PDMS layer during the transmission of the tangential displacement. Indeed, such shear force could be needed to initiate the sliding of the top electrode over the bottom electrodes. Then, starting from about 0.4 N, a second regime is observed in which varies linearly with the applied tangential force. As a higher static normal force must be applied initially to the device, a higher friction force is needed before sliding occurs, and we can observe that, for the same capacitance variation (same overlapping area of the electrodes), doubling the initial load applied to the sensor contributes to a tangential force divided by a factor two. Minimal detected tangential displacements of 8 μm and 14 μm, under 1 N and 0.5 N initial static normal forces, respectively, were evaluated. It should be noted that this result is about one order of magnitude lower than minimal displacements reported in the literature (i.e., 60 μm).13 Linear behavior is observed within the 0.4–1.2 N and 0.8–1.6 N ranges, for 0.5 N and 1 N static normal forces, respectively. The slope represents the sensitivity of the sensor in the tangential mode. The obtained performances were 0.32 ± 0.02 N−1 and 0.34 ± 0.02 N−1 for the 0.5 N and 1 N initial contact force curves, respectively. Hence, we may conclude that, due to the design of the sensor, in this regime the tangential sensitivities are almost independent of the normal component of the applied force, and they are equal to about 0.3 N−1. Furthermore, when increasing the tangential force, a deviation of the response from the linear behavior is observed, suggesting that the overlapping area of the electrodes is not dictated by a sliding mechanism of the two sets of electrodes, rather it is limited by the deformable properties of the whole device, in particular its stretchability. To conclude, the design and the materials used to develop our sensor generate high performing features for the next generation of three-axis soft tactile sensors. Although our design is simple, we have achieved an improved level of tactile information to mimic natural touch. In parallel, the materials used for the multi-layer structure converge in obtaining a soft, flexible and highly sensitive device that offers a low-cost technological approach. New perspectives are thus open for the design of soft and smart interfaces, and for all kinds of applications where natural-like tactile sensing is desired. Importantly, this sensor could target other specific applications than biomimetic touch, in which very low, for instance, heartbeat monitoring (ca. 100 Pa), to very high, for example, foot pressure-distribution monitoring (300–400 kPa), pressure ranges are required. Finally, we expect that the quest for soft and flexible artificial touch systems will steeply increase as soft technological approaches39 are increasingly being developed in many fields such as exploratory robotics, wearable systems, medicine and rehabilitation, personal robotics, entertainment, and gaming. Preparation of Layers: Polydimethylsiloxane (PDMS) (Dow Corning, Sylgard 184; ratio of base to crosslinker, 10:1 by mass) and fluorosilicone (Dow Corning 730) diluted with acetone were degassed and poured on the surface of a silicon wafer previously covered with adhesive tape. After spin-coating and curing at room temperature for around 24 h, membranes of, respectively, 300 μm and 70 μm thickness were obtained. The PDMS membranes were used as external sealing layers whereas the fluorosilicone film was part of the dielectric layering of the sensor. The dielectric constant of DowCorning730 fluorosilicone is 5.5. Textile-Based Sensor Fabrication: Copper/tin coated woven fabric (Zelt fabric – Mindsets Ltd, UK) with an intrinsic resistance of 50 mΩ ◻−1 was cut by laser (VLS 3.50; Universal Laser Systems, Inc., USA), with a resolution of around 0.5 mm, into four bottom electrodes of 5 mm × 5 mm and one 8 mm × 8 mm top electrode. The textile electrodes were positioned over the PDMS membranes and maintained in position with double-sided adhesive tape. The two layers were then assembled face to face and the fluorosilicone dielectric membrane was placed in between. An air gap of approximately 150 μm thickness was naturally formed because of the low surface free energy of fluorosilicone. The bottom electrodes and top electrodes were manually centered with respect to each other. As the capacitance measurement was differential, the variation in electrode positioning did not affect the sensor behavior. This fabrication aspect emphasizes the simple, fast, and low-cost technological fabrication process of the sensor. Experimental Set Up for Measurements: A schematic of the set up is shown in Figure S2 in the Supporting Information. An 8 × 8 mm2 Delrin ® flat probe was mechanically interfaced to a 6-axis load cell (ATI NANO 17 SI-25-0.25, Apex, NC, USA), which acquired force data during the experiment. The alignment of the load cell to the device was determined with an initial rough manual positioning, by means of three orthogonal manual micrometric translation stages with crossed roller bearings (M-105.10, PI, Karlsruhe, Germany) followed by an accurate controlled positioning, by means of two servo-controlled micrometric translation stages: (M-111.1, PI, Karlsruhe, Germany) for the normal direction (z), and (M-126.1 PI, Karlsruhe, Germany) for the shear direction (x), respectively. Cyclic indentation (static mode) and force incrementing (dynamic mode) experiments were performed at constant speed. For tangential characterizations, the probe was glued to the device and a static normal force of 0.5 N or 1 N was initially applied before translating the stage in the x-direction. In this way, sufficient friction was provided in order to generate a displacement and an overlapping area of top and bottom electrodes, and an experimental protocol that approximates a real tactile event was used. Capacitance measurements were performed by means of custom electronics. Each capacitance output signal was acquired by means of a 24-bit capacitance-to-digital converter (CDC) (AD7747, Analog Devices Inc., Nordwood, MA, USA). The input dynamics of the CDC was 16 pF, which was fully used during sensor characterization. In order to minimize the influence of parasitic capacitances (due to wires, connections, etc), the measuring was done in differential mode, using a dummy reference capacitor for each capacitance channel. In addition, to further reduce parasitic capacitances, the device was connected to the electronic board by means of shielded cables. The capacitance resolution was 1 fF, and, because of the strategies described above, the measured rms noise could be reduced to about 4 fF. Therefore, the minimal detectable signal was about 12 fF (three times larger than the rms noise). Data were acquired by means of a 32-bit PIC micro-controller board (PIC32MX460F512L, Microchip Technology Inc., Chandler, AZ, USA) and sent to a PC via USB. The whole system, comprising the translation stages, the load cell, and the capacitance acquisition board, was connected to a PC. A graphical user interface was implemented to acquire force and capacitance measurements simultaneously, and to control the linear translation stages. In order to determine the initial absolute values needed for normalization, each capacitance was measured in absence of externally applied loads by means of a precision LCR meter (E4980A, Agilent Technologies Inc., Santa Clara, CA, USA). Finally, the sensitivity of the sensor was calculated as the slope of the normalized capacitance variation in function of the force, in different force ranges. For the normal force, the equivalent pressure was considered too and, in the present manuscript, the sensitivity is indicated in kPa−1 in order to better compare the resulting data with state-of-the-art sensors in the literature. This study was partially funded by the PLANTOID project (EU-FP7-FET-Open grant n.: 29343). Note: The licence of this manuscript was changed after initial online publication, as of May 2, 2014. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.