SAMD11 Flexible Capacitor

This project demonstrates qualitative angle/force reading using the peripheral touch controller (PTC) of a SAMD11 on a flexural capacitance pad in front of a ground plane.

The copper on top of the flexural joint allows the ground plane to be connected despite the flexing. On a FR1 board, the bend seems reliable and durable.

Design

Schematic

The design is as simple as it gets: a 3.3V regulator feeding the D11, a 2x2 SWD connector for flashing and a set of 4 LED + resistor for visual feedback. The capacitance pad is merely a tag, and has to be manually added in the design as a copper pour.

Board layout

Making the flexure in KiCAD was tedious as it involved copy/pasting one flexure element and lining it up perfectly on the grid.

Here are subtleties in the design:

• The GND pad (top) and capacitance pad are kept separated as much as possible, otherwise the capacitance at rest would become non-negligible
• Cleared copper zones on the top and are meant to hold the flexure with fingers without impacting the capacitance readings.

Making

Milling

The milling was performed in around 30 minutes with the following operations:

• Two isolation passes with a 0.4mm flat end mill (depth 0.1mm)
• Cut of the flexure with a 0.4mm mill (depth 1.6mm, steps of 0.25mm)
• Copper clearing pass with a 0.8mm flat end mill (depth 0.1mm)
• Edge cute with a 0.8mm mill (depth 1.6mm, steps of 0.45mm)

Soldering

The finished board shows no unwanted copper thanks to the clearing pass with a 0.8mm mill. The board is cleaned with soap and water and thoroughly dried before soldering.

I install the SAMD11 first as it has the tighted pin pich among all components. The last components I add are the LEDs and resistors.

Flexure

Bending the flexure is notn only fun, it also seems reliable on the long run. I suspect that a FR4 board wouldn't be as compliant as the FR1 I've used here.

The bend can reache a full 180. Note that I avoid touching or hovering the pads with my fingers, as this would affect the reading.

Code

The code has three main purposes:

• Setting up the PTC for reading the capacitance continuously
• Sending the values to the host computer through the CDC serial, for furhter analysis
• Lighting up the LEDs for a fun visualization of the capacitance's reading. For a nicer effect, PWM is used to modulate the current LED in the gauge. When it's fully lit, the next LED is enabled, etc.

The full code is available here.

It makes use of the Fab SAM Arduino Core, but without a bootloader to save 4kb of flash. The binary is compiled in the IDE with "NO_BOOTLOADER" under the setting "Bootloader size". The compiled binary can then be flashed using a CMSIS-DAP adapter, for instance using edbg:

edbg -ebpv -t samd21 -f d11_flex_freetouch.ino.bin

Adafruit Freetouch

The code makes intensive use of the Adafruit Freetouch library, which is a clever reverse engineering of the Qtouch hardware included in the SAMD family. The library must be installed separately, in this case you need my modified fork available here.

Signal processing

The raw signal is a 10 bit value, that increases with the capacitance. It saturates at 1023 whenever a finger or the other pad is touching.

By flexing at a constant rate, I was able to extract a very crude curve.

For the purpose of this project, this was enough to provide a lookup table with 6 samples:

uint32_t lookup_x[] = {440, 447, 456, 487, 516, 551};
uint32_t lookup_y[] = {0,  342,  584,  868,  980, 1023};

By interpolating this lookup table based on a reading, a linearized 10 bit value is provided, and the LEDs are lit up according to that converted value.

Demo

Downloads

PNG files (1000 DPI)

Traces:

Interior:

Traces + exterior:

Composite:

KiCAD 6 files

• D11_flexible_capa.zip

Firmware

• d11_flex_freetouch.ino.bin

Code

The code is available here.

License

This project is provided under the MIT License.