Making a sensor takes five steps:
- Buy MEMS barometers and design a printed circuit board (PCB)
- Solder barometers to PCB using standard industry practices or (on a budget) maker tricks
- Design a mold that holds the sensors and restrains liquid urathane rubber
- Pour rubber over barometers and vacuum degass them to bring the rubber into the sensors
- Let cure and demold completed sensor
On the electrical side, signals flow from barometers, to "traffic cop" microcontrollers (ATTINYs), to the main host (XMEGA).
The barometer chips datasheet are hard-wired with a single I2C address. To circumvent the conflict this causes, a "traffic cop" microcontroller manages each group of 5 sensors by enabling/disabling a reset pin on the sensor packages. This allows up to 40 sensors (in the current implementation) to speak on the same I2C bus.
The four-wire I2C bus is connected to either a commercial I2C to USB dongle (we use this one) or to a custom high-speed USB interface that reads at 100Hz.
Plans and Source Code
This is the alpha-release of the plans — they are provided as-is, with no implied warranty, but please download them, check them out, and let us know what you think at ytenzer_at_seas.harvard._edu and ljentoft_seas.harvard._edu (remove underscores).
- Array size - 5x8 sensors
- Sensor spacing - 6mm
- Weight - 11.6g (for the full board, which includes usb interface and programming pins)
Design and Interface
Eagle Files — use these to modify schematics, layout, and print PCBs using services such as seeed studio's fusion-pcb service. They were designed in Eagle 6.0.0—if you run into problems opening the files, check your version. Sparkfun.com has some great tutorials on how to use the software. A few notes are listed here
Firmware files — use these to program the addressing microchips on the tactile arrays, and the microcontroller on the host USB board.
Plastics — we cast the sensors under 6mm of Vytaflex 20 from Smoothon Polymers, Inc. Degassing after pouring over the barometers is extremely important to maintain sensitivity (you can use a standard vacuum pump at roughly -760mm Hg for about 5 minutes).