The Bit

Good question.

I lightly modified harbor freight snap ring pliers for a bit more travel. Just remove a bit of metal from the inner surface so they can open further without interference.

After folding back the silicone neck and then expanding it with the pliers, the boards can easily be loaded.

The barometer is only working sporadically...

I have tried:

Replacing the sensor,

Changing the pull-up resistor values,

Changing the power supply voltage,

Changing the measurement timing...

But no dice. Here is proof that is does work sometimes though:

It looks like I am not alone with this issue - these guys ran into the same problem, but ended up just switching to a different type of sensor: PiAQ

To provide a more open and accessible way to interface the bit, the beta prototype uses a CJMCU Beetle ATmega32u4 breakout. I wanted to use a fully open hardware DigiSpark, but it was not able to handle the size of the firmware - It may be possible to heavily optimize the code and make it work eventually.

The Beetle runs on 5V, so a generic level translator was required to interface the sensors. I2C pull-up resistors were added directly to the translator board:

 The translator joined up with the Beetle:

The two resistors on the beetle are supposed to help keep the analog force sensor voltage mid-range when the force sensors are unloaded (their resistance can go to Mega-ohms)

The happy family:

After trying out several combinations, I decided on the following sensors for the beta prototype:

Optical surface tracking

Time of flight laser displacement

Barometric pressure+temperature

Differential force (front to back)

Partially Assembled:

Getting Closer:

Buttoned Up:

Top side (showing front and back force sensors):

The two force sensors form a voltage divider allowing one analog pin to effectively monitor both. (Idle output voltage is half full scale)

Some applications may benefit from joystick-like control. This can be accomplished with three force sensors arranged in a triangle - here is a mockup on the left:

Why not measure how far the tongue is from the bit?

How about a VL6180X laser time of flight displacement sensor:

Needs to be chopped down a bit:

Mounted stand-alone:

I found a nice analog output pressure sensor in my parts bin and decided to add it on. It should be possible to use it for suck-puff type input.

mp3h6115a

Attaching one to a breakout board:

It hangs off one side, but those pins are not needed... so I clipped them off:

All tucked in:

A single turn 10K potentiometer is used in combination with the bite force sensor to create an adjustable voltage divider. This will probably be replaced with a fixed value resistor eventually, but it was helpful to have an adjustable value during testing. Here you can see the potentiometer zip tied to the back of the Teensy 3.1.

A Teensy 3.1 microcontroller board is used to interface the sensor and emulate a USB mouse. 4.7K resistors have been added to the i2c pins.

Signals:
yellow: 3.3V
green: GND
blue: Force sensor analog value
white: SCLK
purple: SDA

I added a conductive rubber pressure sensor to measure bite force.

This allows tracking to be enabled with light bite force, and registers clicks with a stronger bite. 

The force sensor is connected to the 10K potentiometer to create an adjustable voltage divider that connects to an analog input pin on the Teensy 3.1.