WriterRobot

Open-source tools for controlling the Writing Robot T-A4 from macOS.

ta4-send streams G-code to the robot's GRBL controller one command at a time and waits for each ok response. Every session begins by homing the machine and dwelling briefly.

Machine profile

• Controller: GRBL
• Serial speed: 115200 baud
• Machine home: upper-left
• Paper origin: calibrated per sheet
• X+: right
• Y-: down the paper
• Default pen up: Z0.50
• Default pen down: Z1.70
• Drawing feed: F600

Build

go mod tidy
go test ./...
go build -o ta4-send ./cmd/ta4-send

Calibrate the pen and starting position

Run:

./.build/ta4-send \
  --port /dev/cu.usbmodem201912341 \
  --calibrate

Calibration now has two stages after the machine homes:

1. Pen-down height
   • Up arrow raises the pen by --calibration-step.
   • Down arrow lowers the pen by --calibration-step.
   • Enter accepts the pen-down value and raises the pen.
2. Starting position
   • Arrow keys move X/Y by --position-step (default 1.0 mm).
   • U raises the pen.
   • D lowers the pen, which is useful for checking the exact mark location.
   • Enter saves X/Y and raises the pen.

Escape or Ctrl-C cancels without saving. Example with finer increments:

./.build/ta4-send --port /dev/cu.usbmodem201912341 --calibrate \
  --calibration-step 0.01 --position-step 0.25

The saved JSON configuration contains pen_up, pen_down, start_x, start_y, machine_home_x, machine_home_y, and return_home_on_completion. G-code files may use {{PEN_UP}}, {{PEN_DOWN}}, {{START_X}}, and {{START_Y}} placeholders.

Configured pen positions in G-code

WriterRobot G-code can use these placeholders:

G1 Z{{PEN_UP}} F300
G1 Z{{PEN_DOWN}} F200

ta4-send replaces them with the saved values before sending commands to GRBL. testdata/square.gcode uses these placeholders, so a calibration immediately affects the next square test.

Run the square test

./ta4-send \
  --port /dev/cu.usbmodem201912341 \
  testdata/square.gcode

Every session automatically sends:

$H
G4 P0.300

The sender stops on error: or ALARM: and polls until the machine reports Idle after the last drawing command. On normal completion, the session clears the temporary paper-origin offset, raises the pen, and returns to the configured machine home with a G53 rapid move. The default configuration enables this return home behavior with machine home at X0 Y0; setting return_home_on_completion to false in the config keeps the machine at the drawing completion position.

Plot an SVG

ta4-send can import a local SVG directly:

./.build/ta4-send \
  --port /dev/cu.usbmodem201912341 \
  --svg drawing.svg

Supported SVG geometry is path, line, polyline, polygon, rect, circle, and ellipse. Path import supports M, L, H, V, C, S, Q, T, and Z, including relative lowercase forms. Curves are flattened into line segments before streaming to GRBL.

Useful sizing options:

./.build/ta4-send \
  --port /dev/cu.usbmodem201912341 \
  --svg drawing.svg \
  --svg-fit-width 80 \
  --work-width 100 \
  --work-height 100

SVG coordinates are transformed into WriterRobot program coordinates with X increasing right and Y increasing downward on paper as negative program Y. By default, imported SVG geometry is uniformly scaled to fit within the configured --work-width and --work-height, preserving aspect ratio, then left/top aligned at the calibrated paper origin. The sender preflights the final fitted bounds before homing or moving the machine and rejects unsupported SVG features, malformed coordinates, empty geometry, and drawings outside the configured work area.

SVG fixture suite

Checked-in SVG fixtures live in testdata/svg/. The numbered files each isolate one SVG import or geometry-processing behavior: lines, rectangles, circles, ellipses, polylines, polygons, closed paths, relative paths, cubic and quadratic Bezier flattening, multiple disconnected strokes, transforms, nested transforms, viewBox scaling, and a small signature-like drawing. Invalid and unsupported fixtures in the same directory exercise malformed XML, malformed path data, non-finite coordinates, empty SVGs, text, images, and clip paths.

The standard manual SVG hardware check is:

./.build/ta4-send \
  --port /dev/cu.usbmodem201912341 \
  --svg testdata/svg/hardware-check.svg \
  --work-width 100 \
  --work-height 100

hardware-check.svg contains one rectangle, one circle, one triangle, one cubic Bezier curve, and disconnected paths. Its default plotted geometry bounds are 70 mm wide by 50 mm tall.

For incremental hardware validation, plot these fixtures in order: 02-rectangle.svg, 03-circle.svg, 07-triangle.svg, 08-cubic-bezier.svg, 12-multiple-strokes.svg, 16-simple-signature.svg, then hardware-check.svg.

Internal plotting architecture

The SVG plotting path is deliberately layered:

• internal/svg reads XML, parses supported SVG elements and path commands, and returns neutral source-coordinate vector geometry plus document metadata such as viewBox, width, and height.
• internal/drawing defines neutral geometry types only. It does not know about SVG, G-code, GRBL, machine control, sessions, or CLI flags.
• internal/geometry applies transforms, flattens curves, handles viewBox and SVG sizing, scales or fits geometry, inverts Y into WriterRobot program coordinates, computes bounds, and performs work-area preflight.
• internal/plot turns processed drawings into ordered pen-up, rapid-move, pen-down, and drawing-move operations. It first merges already-contiguous open strokes, then uses constrained nearest-neighbor planning: only the first three remaining source-order strokes are considered, and any stroke bypassed twice becomes mandatory. Open strokes may be drawn in reverse when that endpoint is closer to the current pen position. Closed strokes are never reversed, but they may rotate to enter at the nearest vertex while preserving the original orientation. Equal-distance ties keep the original document order, original direction, and earliest closed-path vertex. Pen-down drawing moves use the configured drawing feed by default. When SVG input is plotted with --signature, the planner enables curvature-aware feed-rate modulation based on a distance-weighted curvature histogram for the whole plan: the lower third uses a slow feed, the middle third uses the configured normal feed, and the upper third uses a fast feed. The current experimental levels are 40%, 100%, and 135% of the configured drawing feed. A small smoothing pass suppresses direct slow/fast chatter. No dwell commands are added, and geometry, route planning, and pen-up travel speeds are unchanged.
• internal/machine, internal/session, and internal/grbl own machine command formatting, session lifecycle, and controller transport.

Safety

The robot moves to its upper-left machine home position immediately after initialization and, by default, returns there after a successful plotting session. Keep the travel path clear and keep a hand near the power switch during calibration and early tests. Lower the pen in small increments to avoid forcing the mechanism into the paper.

Generate the Bezier calibration pattern

Build both commands:

go build -o ta4-send ./cmd/ta4-send
go build -o ta4-bezier-test ./cmd/ta4-bezier-test

Generate the default four-row adaptive-flattening test:

./ta4-bezier-test

This writes testdata/bezier-calibration.gcode. From top to bottom, the rows use tolerances of 0.50, 0.25, 0.10, and 0.05 mm. Each row contains a shallow curve, a tight curve, a continuous S-curve, and a closed loop. The generator prints the number of line segments used for each row.

Send the generated test using the saved pen calibration:

./ta4-send \
  --port /dev/cu.usbmodem201912341 \
  testdata/bezier-calibration.gcode

To test different tolerances or placement:

./ta4-bezier-test \
  --tolerances 0.40,0.20,0.10,0.05 \
  --x 25 \
  --y -20 \
  --row-spacing 35

The --y value identifies the top of the first row. Because the machine homes at the upper-left and moves down the paper with negative Y coordinates, subsequent rows are placed at increasingly negative Y values.

Generate the motion and feed-rate calibration pattern

The generalized build.sh discovers and builds every command under cmd/:

./build.sh

Generate the default six-row motion test:

./.build/ta4-motion-test

This writes testdata/motion-calibration.gcode. From top to bottom, the rows use drawing feed rates of 200, 400, 600, 800, 1000, and 1200 mm/min. Each row draws:

• a horizontal line
• a vertical line
• a 45-degree diagonal
• a sharp-corner box
• a 4 mm circle
• an 18 mm circle
• a tightening spiral
• a continuous sweeping S-curve

Send the pattern using the saved pen calibration:

./.build/ta4-send \
  --port /dev/cu.usbmodem201912341 \
  testdata/motion-calibration.gcode

The rows are intentionally identical except for feed rate. Compare line straightness, corner overshoot, visible vibration, circle roundness, spiral consistency, and hesitation along the S-curve. Stop the machine if a higher-speed row begins to chatter or flex excessively.

Customize placement or feed rates as needed:

./.build/ta4-motion-test \
  --feeds 300,450,600,750,900 \
  --x 20 \
  --y -20 \
  --row-spacing 40

Signature stroke profile

For signature-style SVGs, add --signature to vary drawing feed with local curvature. The default SVG mode uses the configured fixed drawing feed. Signature mode logs the curvature histogram and estimated pen-down time in each feed band so speed variation can be tuned from actual plotted geometry.