Build a sensor¶

An OpenTac sensor combines a flex PCB and a piezoresistive layer inside a printed TPU case. The case comprises a tray and a lid; the lid can be flat, hatched, or fitted with bumps for shear sensing. OpenTac-CAD generates the board, case and cutting template from the same geometry, so the sensing layers and mechanical features align during assembly. The flex tail connects the assembled sensor to a readout board.

The parts of one sensor laid out on a desk: the printed TPU tray, the flex PCB with its tail, the black piezoresistive textile cut to shape, and the printed bump sheet. — The four layers of one sensor, from left: printed tray, flex PCB, piezoresistive textile, bump sheet (the lid).

The assembly guide follows the complete procedure: verify the fit with a paper template, print the case, cut the piezoresistive layer, assemble the sensor, and inspect its readings. The remaining pages cover PCB ordering, materials and the readout boards.

Assembly guide

The printed case¶

The case fixes the relative positions of the PCB, piezoresistive layer and bump sheet. Its bottom tray follows the board outline, while the lid places one bump over each 2×2 group of taxels. Generating both from the same design removes the need for a separate alignment jig.

Alignment matters because shear is estimated from differences in loading across the four taxels beneath a bump. Tangential force tilts the bump and shifts load toward one side (How it works). If the bump is off-centre, even a normal load can produce unequal readings and an apparent shear response.

Close-up of a printed bump sheet seated on a flex PCB: each bump's cross-shaped base sits in the gaps between the gold taxel electrodes. — The bumped lid on its board. The cross-shaped base of each bump seats in the gaps between taxels, which centres the bump over its 2×2 group.

Download ZIP in OpenTac-CAD puts the case in the ZIP's case/ folder: the bottom, the bumped lid as the primary one, the flat and hatched lids, and a README with the settings used (What the export contains). The case panel in OpenTac-CAD (Add a printed case) previews the parts and changes their settings. The ZIPs of the ready-to-order designs include it too. Printing and fitting are in the assembly guide: Print the case and Assemble.

Fabrication and assembly sequence¶

	Step	Time	Page
1	Design the sensor and download the ZIP: board files and the printed case	minutes	Design, What the export contains
2	Print the 1:1 PDF and check the fit on the part	10 min	Assembly
3	Order the flex PCB	days to weeks, shipping	Order the sensor
4	Order a readout board, if you don't have one	days to weeks, shipping	Order the readout board; files in the repository's `hardware/` folder
5	Get the piezoresistive layer, TPU and tape		Materials and accessories
6	Print the case from the ZIP's `case/` folder: the tray and the bumped lid, in TPU	a few hours	The printed case, Assembly
7	Cut the piezoresistive layer	10 min	Assembly
8	Assemble and plug in	5 min	Assembly
9	Flash the readout board and read the sensor	10 min	Assembly, Flash

PCB fabrication and shipping usually dominate the elapsed time. Order the sensor and readout boards together, and print the case while they are being manufactured.