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Low-Cost Fabric Tactile Sensing Skin

We present our design for inexpensive, flexible, and stretchable tactile sensor arrays with low spatial resolution using conductive and resistive fabric. Our design should be relatively straightforward to replicate. Please refer to the following papers for more information about our research on whole-arm tactile sensing and robotic manipulation. If you use our open-hardware tactile sensor design or wish to refer to it, please cite our World Haptics Conference paper.

1. Basic Principle

A single sensing element or taxel (short for tactile pixel) consists of five layers of fabric. The layer in the middle is resistive fabric, which is sandwiched between two layers of conductive fabric. The taxel exhibits piezo-resistive properties. Specifically, the resistance between the two electrodes made of conductive fabric decreases when we apply a force on the sensor.
We use this taxel as part of a resistive voltage divider that we connect to the analog input pin of an Arduino board.

2. Steps to Make a Single Taxel

Having a picture or wiring diagram associated with each step would be geat.

3. Making a Tactile Sensor Array

To make an array of tactile sensors, we change one of the layers of conductive fabric to have multiple discrete electrodes of the desired shapes and sizes. For example, 15 taxels that are rectangles of size 3cm x 5cm.
The second conductive fabric serves as a common ground for all the taxels.
We also use a single sheet of resistive fabric on top of the common ground.

3.1 Simple Tactile Sensing Array

To create a simple array of tactile sensors, we connect the taxels to snap buttons using conductive thread (as shown in the picture below), or put the snap buttons directly on the taxel and use normal wires (method that we used for the PR2). We then connect them to an ADC port on an Arduino board.

3.2 Advanced Tactile Sensing Array

To create an advanced array of tactile sensors, we attach snap buttons on the taxels, but instead of connecting them to an ADC port on an Arduino board, they are connected to an 8-channel ADC chip mounted on the skin. The I2C communication protocol is used to connect the ADC chips to the Arduino. Up to four ADC chips can be daisy-chained and then a single set of Power, Ground, SCL, and SDL wires connect the ADC to the arduino. This helps reduce the wiring from the sleeves to the Arduinos.
The ADC chip, bought from Sparkfun, is mounted on a custom PCB board. The PCB board also houses in-line pullup resistors and connectors. The entire PCB board is stitched on to the sleeve to prevent it from moving, as shown below. Since each ADC chip has 8-channels, eight wires branch out from the PCB, connecting to the snap buttons on eight different taxels.

4. Cody Specific Design

4.1 Simple Taxel Layout

To cover Cody's arm with our tactile sensors, we used one sleeve with 12 taxels in the forearm and 12 taxels in the end-effector region.

Design Files

Click here for all SolidWorks files.
The following are the images showing the taxel arrangement for Cody's sleeve.



5. PR2 Specific Design

5.1 Simple Taxel Layout

To cover the PR2 arm with our tactile sensors, we have different designs for the gripper (10 taxels), forearm (14 taxels), and upper arm (3 taxels).

Design Files

Gripper

Click here for all SolidWorks files

Forearm

Click here for all SolidWorks files

Upper Arm

Click here for all SolidWorks files

5.2 Advanced Taxel Layout

For the advanced taxel layout, the number of taxels on the gripper and the upper arm are not changed. The number of taxels on the forearm were increased to 22 taxels. This was done by adding two taxels on each of the four edges, thus adding a total of eight taxels.

Forearm Design Files

Click here for all SolidWorks files.
The following are the images showing the taxel arrangement for different sides of the forearm.


6. Raw Materials and Equipment

Here are the places where we purchased the different raw materials and equipment:

7. Contributors

Advait Jain, Sarvagya Vaish and Prof. Charles C. Kemp are the main contributors to this project.

8. Support

This work was support by DARPA Maximum Mobility and Manipulation (M3) Contract W911NF-11-1-603.

9. Acknowledgements

Our design is inspired by the rSkin project. We thank Hannah Perner-Wilson and Ian Danforth for helpful discussions.
We also thank Matt Carney and Aaron Edsinger from Meka Robotics for pointing us to the rSkin project.

10. Useful Links