Planar Testbed Contact Sensors

This project is a simple set up consisting of a Flexiforce force sensors from Tekscan, a simple circuit mounted on a breadboard, and an Arduino Mega2560.

Flexiforce:

The Flexiforce sensors are force sensors made of pressure-sensitive resistive ink wedged between two polyester sheets. The benefit to using these sensors is that they are easily bended, and can be wrapped around the planar textbed fixels in a way that cheaper piezoelectric pads cannot be. These pegs are only approximately an inch in circumference. The A401 sensors were purchased in a 4-pack from the Tekscan website. Because the Flexiforce sensors are force sensors, and not simply contact sensors, there is potential to develop a fully tactile application using them. This was not done as of yet in this instance because of the difficulty in calibrating the sensors once on the testbed.

Circuit:

The circuit for the sensors is seen is the following schematic. Each Flexiforce sensor is attached to two inverting amplifier stages, along with a 10uF filter capacitor. The first amplification stage provides a gain of -(Rpot / Rvar). The second stage simply inverts the output of the first stage with a gain of -1, in order to provide a positive voltage to be read from the analog-to-digital converter on the Arduino.

Arduino:

The Arduino Mega2560, an open source/open hardware development board, was used in this circuit. Note that the cheaper Arduino Uno could be used in its place. The purpose the Arduino serves is to act a a server that fields requests from MATLAB to read the values at the analog input pins of the board, which are connected to the output of the breadboard-based Flexiforce circuit.

Software Requirements

• Arduino development environment - The Arduino IDE is available here. Installation and use is very straightforward.
• Arduino drivers - Arduino installation information and drivers for Windows and Linux.
• MATLAB - MATLAB R2011a was used for this project, and tested successfully in Microsoft Windows 7. There were issues connecting to the while in Ubuntu 11.04, where the COM port assigned to the Arduino was not accessible through MATLAB. This issue will be investigated later if needed.
• MATLAB Arduino Library: The MATLAB support package for Arduino was used for this project. A few changes, outlined in the README, were put in place to accommodate for the Arduino Mega2560's greater number of pins than the standard Arduino Uno. The altered files is attached to this page, and the rest of the files remain unchanged.

How to Setup the Arduino to talk to MATLAB (1st time)

1. Open the Arduino IDE folder with the executable, in this instance it was arduino_0022, and run the executable.
2. In the IDE, open the arduino_ml folder (the MATLAB support library), and then adiosrv folder. Open the adiosrv.pde file.
3. In the Tools drop down menu, select your version of the Arduino board under Board, and identify the board's COM port (found in Devices and Printers if on Windows) under Serial Port.
4. Select the right-pointing arrow button from the ribbon, which will upload the .pde file to the Arduino, do not unplug anything until the serial lights have stopped blinking on the board.
5. Close the IDE, it is no longer needed.
6. The library README reviews the simple methods for interacting with the Arduino in MATLAB.

Running Experiments

Please see the running experiments on the Planar testbed tech page for instructions on how to use this system with the in-lab planar testbed set up.

Further Reading

For an overview of tactile sensing, please see the Springer Handbook of Robotics, Chapter 19: Force and Tactile Sensors (this draft is posted online).

For more information on the Arduino, if interested in extending this system and working further with the platform, go here.

-- CihanCaglayan - 2011-07-27