Getting a board shape
Every design starts the same way: get a closed 2D outline into the Board Outline canvas. There are three ways to do that — mix and match, or switch mid-project using the Mode switcher.
Draw
Use the Line, Rectangle, H/V, and Arc tools to sketch your outline directly. When you're done, select the edge where the connector tail should exit and click Set cable edge (it turns orange) — Generate is blocked until this is set, so a stray first stroke can never silently become the connector edge. This isn't just freehand: select a vertex or edge and type an exact Length and Angle in the fields above the canvas to dimension it precisely, like a real CAD program.
DXF
Upload a closed 2D outline exported from AutoCAD, Fusion 360, or similar CAD tools. We automatically pick the largest closed shape in the file (in case it contains extra reference geometry) and scale it to millimeters using the file's unit metadata. A DXF outline has no inherent cable edge — confirm or change it with Set cable edge after import.
STL
Upload a 3D model, then click a face (or several connected faces, or a smoothly curved patch) to select the surface your sensor will cover — or use Draw patch to click points directly on the surface and outline your sensor by hand, letting the app smooth away texture like tread patterns. Unfold selection flattens the result into a 2D outline using a true-scale unfolding algorithm — like unwrapping a physical piece of paper from the part. The result lands in the same Board Outline canvas, where you can still hand-edit it. Like a DXF import, an unfolded outline has no inherent cable edge — confirm or change it with Set cable edge once it loads.
If your selection spans surfaces that don't share a mesh edge (say, a fingertip's front face plus its two sides), the button changes to Join surfaces… and the viewer walks you through the join, one cut at a time: it shows numbered marker dots along each surface's boundary, and you click two matching corner pairs — "this corner attaches to that one" — for each cut. Only after every cut is resolved does the 2D preview appear, with Use this shape handing it to the outline canvas. Picking the corners yourself is deliberate: you can see the real part, so you know which corners truly touch.
Mirroring a design
Grippers come in pairs. Once you have a sensor for the left jaw, Mirror ⇆ in the drawing toolbar flips it into the right-hand version — and Mirror ⇅ flips it top-to-bottom.
The flip happens in place: the shape keeps its exact size and position, so every dimension you typed stays what it was. It moves the whole design together — outline, sensorize zone and any fold lines — so they do not come apart.
Your cable edge survives the flip. It stays the same physical edge of the shape, just on the other side, so you do not have to set it again. (Under the hood a mirror reverses which way a shape winds, which would otherwise move the connector to a different edge without any visible sign; the flip corrects for that.)
Mirror is undoable like any other edit, and mirroring twice returns you exactly where you started.
Sensorize zone
The sensorize zone is an optional second shape you can draw inside your board outline. It constrains where taxels are placed — it does not change your board's outer edge, connector, or routing bounds.
Use it when only part of your board should be sensing area — for example, leaving a clear margin around a mounting hole, or concentrating taxels on a fingertip while leaving the rest of the outline as structural board.
To use it: draw a board outline first (the toggle unlocks once you have one), switch the toolbar target to Sensorize zone, and draw a smaller shape with the same tools. It renders as a dashed green overlay so it's never confused with the board outline.
Parameters glossary
A taxel is one sensing element of the skin; the sensor is a grid of them. Most parameters are straightforward (taxel size, pitch, trace width). Two groups need more explanation: the taxel footprint (the copper of one taxel) and board sizing (four related but distinct mechanisms for making room for the wiring that connects every taxel to the connector).
Sensor grid: size or count
The sensor grid can be specified from either end:
- Set taxel size
- You give Taxel W/H and Pitch X/Y; how many fit is whatever your outline allows. This is the default.
- Set rows & columns
- You give the grid you want and the taxel size needed to fit it is calculated from your outline (its bounding box, minus the edge keepout, divided by the requested counts). Pitch is set equal to the taxel size, so taxels tile edge to edge with no gap. On a non-rectangular outline the taxel size is shrunk in small steps until the requested rows and columns really pack — corners that fall outside the shape are still dropped, so you may get slightly fewer taxels than rows × columns. Available in Grow board to fit wiring mode only: with the outline fixed, the packable area depends on the wiring keep-out band, whose width depends in turn on the row/column counts you are trying to set.
Taxel footprint
Each taxel is a pair of interdigitated copper electrodes — two interleaved comb-like patterns. The piezoresistive layer laid over the top bridges the fingers, so its resistance drops measurably under pressure. The live preview in the Parameters panel shows what one taxel's electrodes look like as you adjust these:
- Taxel edge clearance (
edge_clear_mm)
- Gap from the taxel's own outer boundary to its fingers. Not the same as Edge keepout above, which is a board-level margin from the outline — this one is local to a single taxel's footprint.
- Taxel center clearance (
center_clear_mm)
- How far each finger reaches toward the opposite pad before stopping, controlling the gap down the taxel's center line.
- Via drill / diameter (
via_drill_mm / via_dia_mm)
- The plated hole connecting a taxel's top-layer pad to its routed trace underneath.
Board sizing
- Readout board (
readout_board)
- The OpenTac board the sensor plugs into: 30-pin (up to 15 x 15 taxels, two stacked 16 x 22.8 mm boards) or 60-pin (up to 30 x 30, one 37 x 34.3 mm board). It sets the connector on the sensor's tail.
- 2 or 4 copper layers (
copper_layers)
- On 2 layers, columns run on the top copper and rows on the bottom, and every wire travels around the outside of the sensor field to the connector, so the board needs a wiring band around the field. On 4 layers the rows are wired on two inner layers instead: In1 carries each row straight through the field to the connector, in row order, and In2 joins the pieces of rows a notch cuts in two. Rows then need no band, so with a fixed outline more of it holds taxels. Either choice works with either board mode below. A 4-layer board costs more to fabricate.
- Grow board to fit wiring (
board_mode: "expand")
- The default. Taxels are packed at maximum density inside the shape you drew, then the final board outline is grown outward until every wire fits. No taxel is ever dropped — but the exported board is larger than the shape you drew.
- Keep outline fixed, shrink sensor grid (
board_mode: "fixed_keepout")
- The shape you drew is exactly the board you get. A wiring keep-out band is reserved just inside the outline — wider on the sides that carry more wires, narrow at the top where few pass — and only the area left over is filled with taxels, so taxels that would reach into the wiring zone are never placed. Because the keep-out is a clean geometric band, the surviving taxels stay a regular rectangular grid rather than a shape with holes punched in it. The band is sized from the actual wiring demand and the result is DRC-verified, falling back to a more conservative band if the dense one does not route cleanly. Trade-off: still fewer taxels than grow mode on the same outline.
- Board edge in grow mode
- Set automatically. The final edge is shrink-wrapped about 2 mm around the outermost taxels and wires, with rounded corners. On 4 copper layers that margin is added only where wires run at the edge, mostly near the connector; elsewhere the board keeps the shape you drew.
- Post-route safety net
- After wires are routed, if any wire still pokes outside the board, the board is grown just enough to contain it.
- Tail extension
- A separate, later step — after you Generate, the PCB Preview toolbar lets you draw a custom path for the cable/connector tail, growing the board along that path.
Route editing
After Generate, the PCB Preview toolbar lets you fine-tune the routed board:
- Undo / Reset route / Reset all — step back or discard route edits.
- Redo padding — recompute the board's outer padding after edits, without re-running the full router.
- Tail extension — draw a custom path for the connector's cable/tail instead of a straight edge.
- Chamfer — cut sharp corners on routed traces to a given size.
- Check DRC — re-run the design-rule check (clearance/crossing/board-edge violations) on the current routes.
Exporting
The downloaded ZIP contains manufacturing files you can send to a PCB fab, or open in KiCad:
- A
.kicad_pcb board file and the taxel and connector footprints.
- Gerber files (the industry-standard format PCB fabs use).
- An Excellon drill file for vias.
manifest.json — taxel placements and the scan matrix, for driving the sensor from a microcontroller. It also carries a design block with every setting the board was made from.design.json — the design itself, so you can reopen it here later.case/ — the printed case as STL files: the bottom, the lid with shear-sensing bumps, and the flat and hatched lids. Print the bottom and one lid. case/README.txt says which lid is the primary one and which settings were used.
The case is built when you press Download ZIP, from the settings in the case panel (its defaults if you have not changed them), so the download takes a few seconds longer, more on a large board. The primary lid is the one chosen in the case panel, or the bumped lid if you have not chosen one. If the case cannot be built for your board, the ZIP downloads without it and a note under the button says why.
What to do with the files is on the OpenTac website: Order the sensor for the flex PCB, and the assembly guide from the fit check to live readings.
Reopening a design
Keep the ZIP and you can carry on where you left off: Open in the mode switcher (or Open a saved design on the start screen) takes the ZIP back — no need to unzip it — and puts the board and every setting back on screen, including routes you moved by hand.
If the app has been updated since you downloaded it, the exact board can no longer be guaranteed, so it is rebuilt from its saved settings instead and you are told so. That rebuild reproduces the same board; only hand edits to individual routes are lost.
The sensor plugs into an OpenTac readout board, chosen under Board sizing. The 30-pin board reads up to 15 rows x 15 columns and the 60-pin board up to 30 x 30; "picked automatically" uses the 30-pin board when the grid fits it. The tail ends in the matching 0.5 mm-pitch FPC connector, 30 or 60 positions. A grid larger than 30 x 30 does not fit either board: use a larger taxel pitch or a smaller outline.
The printable 1:1 PDF
Both the ZIP (printable_1to1.pdf) and the Download printable PDF button next to it give you a full-size paper template of your board. Print it, cut it out, and hold it against the real part before you order boards — it takes a minute and catches a wrong dimension that would otherwise arrive as a batch of useless PCBs.
It draws the board's real manufactured edge, including the cable tail and connector — not the shape you drew, which is smaller. The sensor grid is printed faintly inside so you can see where the taxels land.
Print it at 100%. Print dialogs default to "Fit to page" or "Shrink to printable area", which quietly resizes the sheet by a few percent and makes the template wrong. Every page carries a 100 mm ruler: measure it with a real ruler after printing. If it is not exactly 100 mm, reprint at Actual size. The sheet size used (A4 or A3) is printed at the top — pick that size in your print dialog.
Boards too big for one sheet are split across several, with a dashed overlap line on each: trim to the dashed line and butt the next sheet against it.
A bigger connector body can be wider than the cable/tail running into it, which puts a step in the board's cut outline. That step is always rounded with a 1 mm fillet, at the connector tip and at the two inward notches where the tail meets the shoulder, so there are no sharp 90° corners to concentrate stress in the flex PCB.
Printed case and shear bumps
The case is two 3D-printed parts: a bottom with a pocket that the sensor and its piezoresistive layer drop into, and a lid held on by a ribbed rim. No glue and no screws. Both halves stop at the cable edge, so the tail leaves through the open side.
Download ZIP includes the case in its case/ folder: the bottom and all three lids, built with the settings in the case panel (Add a printed case, under the board preview). Use that panel to change the settings, preview the parts, or download a single part.
The lid comes in three versions:
- Flat
- A plain lid. Reads normal force only.
- Hatched
- Cuts or grooves along the boundaries between taxels, so a press lands where you put it instead of spreading to the neighbours. An optional small bump on each taxel concentrates contact from curved objects.
- Shear-sensing bumps
- One bump over each group of four taxels. Press straight down and all four read about the same. Push sideways and the bump tips, pressing harder on the two taxels it leans toward and lighter on the two it leans away from. That difference is the direction of the push.
Bump settings
- Bump height
- How far each bump stands above the lid. Taller tips more for the same sideways push, so shear reads more strongly, but it raises the surface and snags more easily.
- Bump width (% of block)
- How much of its four-taxel block each bump covers. At 100% neighbouring bumps touch.
- Bump taper (top % of base)
- How pointed each bump is. Lower is more cone-like and tips more readily for the same push; 100% is a straight-sided post.
Things to know
- Every taxel gets covered. Bumps are placed on a regular grid of four-taxel blocks. Where the board's edge cuts a block short, that block still gets a full-size bump, so the taxels that are there keep reading direction.
- Non-square taxels get oval bumps. A bump is sized from the block it covers.
- Print it in something soft. TPU (roughly 85–95A) is the usual choice. A rigid plastic will not tip, so it will not read shear.
- Rebuild it if you rebuild the board. The case is cut to fit one exact board outline. Pressing Generate again clears the old case on purpose.
FAQ / troubleshooting
- "No taxels fit in the outline" / similar errors on Generate
- Your outline is too small for the current taxel size, pitch, and edge keepout. Draw a larger outline or reduce those parameters.
- "Sensor fill region does not overlap the board outline"
- The sensorize zone you drew is fully outside the board outline — make sure it's drawn inside the outline shape.
- "No closed outline found — check the DXF contains a single closed polyline/region (no gaps or open ends)" (DXF import)
- The DXF file doesn't contain one clean closed loop of geometry — check for gaps, unconnected segments, or stray extra shapes in the source file.
- "Server: not reachable"
- The server has not answered for 90 seconds. On the hosted site the server sleeps after 15 minutes without use and takes about a minute to start; the page keeps trying every 30 seconds and runs a pending Generate once it answers. If it stays unreachable, reload the page. On your own computer, check that the window running
opentac cad is still open.