project specification

Fabric-based Tactile Sensor

A fabric-based, flexible, and stretchable tactile sensor.

Overview

This design allows for several tactile cells to be embedded in a single sensor patch. It can have an arbitrary perimeter and can cover free-form surfaces.

The flexible tactile sensor remains operational on top of soft padding such as a gel cushion, enabling the construction of a human-like soft tactile skin. The sensor allows pressure measurements to be read from a subtle less than 1 kPa up to high pressures of more than 500 kPa, which easily covers the common range for everyday human manual interactions. Due to a layered construction, the sensor is very robust and can withstand normal forces multiple magnitudes higher than what could be achieved by a human without sustaining damage.

Hardware

We are striving for a better understanding of human grasping and manipulation skills and are convinced that by observing humans we can gain a deeper insight into the control processes involved in manual intelligence. Having only postural information about the hand does not reveal the associated forces and we therefore sought an explicit sensing channel for contact forces during manipulation.

However, we found that there are very few commercially available datagloves with tactile sensing capabilities. The Pinch Glove from Virtual Realities Ltd. and the now discontinued X-IST Data Glove HR3 from No-DNA both only provide tactile sensing at the fingertips. As only a very limited set of human grasps rely solely on the fingertips, a tactile covering of the fingers and palmar side of the hand is needed. Therefore using the fabric sensor we developed a dataglove that allows us to capture tactile patterns from the complete palmar surface of the hand and all fingers. It uses a total of 54 tactile sensitive areas.

Multitaxel Sensor

We discovered that the conductive coating of the covering electrode fabrics were etchable with a ferric chloride (FeCl3) solution, a common etchant used in printed-circuit-board development to remove the copper between tracks. A very mild solution of 30 mol/m3 of FeCl3 dissolved in distilled water produced good results. By etching the areas between desired taxels on one electrode sheet, we can effectively isolate taxels from each other within a single fabric patch, allowing us to build compact multi-taxel flexible fabric sensors.

Tactile Dataglove Construction

We optimized the glove fabric cut-pattern towards a minimum amount of patches. We wanted a large single piece for the palmar side of the hand to simplify taxel placement and achieve low seam stresses. This resulted in a glove design with three fabric patches: a palmar patch carrying all tactile areas, the back of the hand, and a small area between the thumb and the index finger.

For an optimal placement of tactile-sensitive areas on the final glove, we marked desired taxel areas on the palmar patch directly on a human hand coated with a latex skin. The contours and taxel positions on the corresponding latex patches were subsequently digitalized with a scanner.

The three fabric patches that form the glove. Left — the palmar part. Upper middle — patch to cover the connecting area between the thumb and the index finger. Right — the back of the hand.

The contours of the taxels and the fabric patches were imported into a CAD program, and a double sided etch mask was created. On areas that were to be etched, grooves were placed into the mask to facilitate circulation of the etching solution. Using a CAM program, the drive code for the CNC mill was generated based on the CAD design. The two mask halves were then milled from PVC.

The etching rig that separates the 54 taxels in the outer electrode layer of the glove. During the etching process, the heated ferric chloride solution is circulated with a pump and the rig is manually rotated for a more uniform and quicker etch result. The lower right inset displays the resulting etched electrode fabric, where in darker areas the conductive coating has been removed. The 54 inner lighter areas correspond to tactile sensitive regions of the glove and each of them forms one of the sensor electrodes of a corresponding tactile cell.

During etching, the fabric is placed between the mask halves and the heated (≈60 °C) ferric chloride solution is circulated through the milled grooves from one side of the mask through the fabric to the other side with a pump. This effectively etches off the conductive particles in the fabric in areas that the solution reaches. During etching the mask is manually rotated to better distribute the solution and to speed up the process.

To keep the solution only in the desired areas and to avoid the etchant creeping into the material, all mask edges were applied with silicone to ensure a tighter seal. Our system employed a vacuum, which further reduced the possibility of solution creep, and all regions not processed with etchant (taxel areas) were equipped with a vent hole. After approximately 10 min of etching, the fabric was immediately rinsed with fresh water and dried.

The chosen materials allow the rig to be used for a high number of etchings without performance deterioration. After the desired taxels have been separated using the etching process, the layers forming the tactile glove are assembled in a lightly stretched state on a frame. Each glove layer more proximal to the hand is stretched slightly more in the frame to obtain a natural convexity of the finished glove. The etched fabric forms the first layer on the frame and the most distal layer in the final glove. The mesh, piezoresistive sensor and the unetched electrode layers come next.

Left — sewing the layers of the tactile glove together. Right — the glove after removal of excessive material, especially important around finger joints to allow unobstructed finger flexing. An additional seam is applied around the perimeter of the tactile sensor patches to increase durability.

Then, a fine-meshed, elastic, non-conductive fabric with approximately 0.2 mm mesh opening is used as the main glove material, providing good ventilation for the hand. Finally a water soluble embroidery backing is added to allow the sewing head of the machine to advance properly on the stretchable material. The layers are then sewn together around the borders of the tactile sensor patches. Great care needs to be taken to sew only the borders surrounding the taxels to avoid having a permanent load on the tactile sensitive areas. The fabric sensor material between the tactile patches is cut with a fine scissors outside the seam around the tactile patches and removed. This is important to keep the glove as thin as possible around joints it becomes folded.

Specifications

  • Pressure sensing range: 1 kPa to 500 kPa
  • Embedded tactile cells: 54
  • Stretchable knitted fabric: 72% nylon, 28% spandex
  • Volumetric resistivity of the fabric: 20 kΩ·m
  • Taxel area range from: 34 to 130 mm2 in the finger and 195–488 mm2 in the palm
  • Spatial resolution: 7.2–9.6 mm in the fingers and 12.4–29.7 mm in the palm
  • Microcontroller: PIC18F24J50

References

Describes the construction of the fabric tactile sensor in detail. The sensor performance is evaluated and measurement results are given. Describes application for the developed flexible tactile sensor in the form of a tactile dataglove. Discussion on future work.

G Büscher, R Kõiva, C Schürmann, et al. - Robotics and Autonomous Systems Volume 63, Part 3, January 2015.