Most CRT power wheelchairs including those from large manufacturers like Quantum and Permobil have a standard documented way to interface alternative driver controls or specialty input devices. These controls could be proportional or non-proportional (switch). Many examples are available from 3rd party manufactures such as Adaptive Switch Laboratories and Stealth Products. If necessary we can also build our own alternative driver controls and plug them in the same way. Below is a video of a quick demo I put together of a wink based system.
WARNING: Extreme care, attention, testing and supervision must be used with any drive control setup but especially a fully custom one. When applicable, an emergency stop switch that the user can activate must be in place and/or an aide must always be nearby and ready to stop the power wheelchair should something go wrong.
In that demo my left eye has the functions of initially unlocking the system and changing to the next drive command (left, right, reverse). The current drive command is reset to forward after any action. The current drive command is triggered and sustained with my right eye.
The breadboard circuit I assembled connects to the 9-pin connector on the Q-Logic enhanced display. Being able to interface with a Q-Logic this way covers Quantum chairs. This same 9-pin connector exists on the R-Net enhanced display which covers many other brands including Permobil. For non-proportional (digital) mode this 9-pin connector has the same pinout across brands. The MCU pulls pins low on the connector through a Darlington transistor array. The MCU could drive the pins directly, but if it lost power it would then inadvertently pull the pins low through its ESD diodes.
For this demo I used VCNL4000 proximity sensors read by a Teensy 3.5. I had it on hand and it had the 2 I2C ports I needed. Then the 9-pin connector is driven through a ULN2803 transistor array. In this setup winking either eye causes enough facial movement to trigger the corresponding proximity sensor. This or a similar setup could be used for a number of disabilities especially where other switches and switch sites are either unreliable or too difficult for the user.