The Electrotactile Braille Display (ETBD) is a refreshable braille display that uses electrical pulses to simulate the feeling of braille dots on the user's fingertips. Because it has no moving parts, the ETBD has the potential to be much lighter, thinner, and cheaper compared to mechanical braille displays, which use physical braille pins.
In electrical stimulation, an electric current is sent across certain nerves or receptors to control their behavior and, in some cases, to imitate physical textures. This concept is often used in physical therapy to reduce pain or to rehabilitate muscles.
In the current version of the ETBD, electrical pulses enter the finger through small copper pads on the reader's surface and exit through a ring that the user wears. This way, the current is guaranteed to stimulate the Meissner's corpuscles (which sense indentation) and Merkel's disks (which sense pressure) under the skin instead of only passing through the surface.
The PWM signal from pin D10 on the Arduino first passes through an RC circuit, the output voltage of which is determined by the PWM signal's duty cycle. This output voltage is fed into a power op-amp, which is wired to output the exact voltage it receives but with a larger current in order to drive the transformer. The transformer then significantly amplifies the voltage (but divides the current by the same factor) and sends electrical pulses to the fingertip. After exiting the fingertip, the current passes through a resistor, the voltage across which is read by pin A6 and used during feedback control to ensure a consistent stimulation.
The ETBD uses a proportional-integral-derivative (PID) feedback loop to maintain a constant current through the user's finger, no matter the fingertip resistance.
The proportional component tries to minimize the error between the current voltage and the target voltage. The integral component depends on the total error accumulated over time, which helps the output voltage reach the target faster. However, this on its own often leads to overshooting, so the derivative component (based on the change in error at every step) is needed to dampen the system.
The feeling it produced was very intense, sharp, and inconsistent.
More robust and controllable, and uses PWM signals fine-tune output voltage.
Switched from protoboard to PCB. Did not use a ring, so the stimulation felt very weak.
Implemented a feedback loop to maintain consistent stimulation across various fingertip resistances.