genarm

A robot arm that draws its landscape.

Part of the Cybernetic Garden — a machine-creature that perceives the territory around it (device pings, signal landscape, eventually a depth camera) and renders what it senses as drawings on paper. Sibling of gen3d (ceramic) and gen2d (plotter).

v1 scope — see it move, draw a square

Three motors, two struts, direct drive, horizontal table, paper taped down. No wrist, no pen lift, no elegance. Assembled and moving first; fancy joints, belts, and perception later.

rotate      pivot        pivot
[base] --- [shoulder] --- [elbow] --- pen
NEMA17      NEMA23        NEMA17
             |mast          30cm 2020     30cm 2020

Architecture

Pi 3B+ (host)                Arduino Uno + CNC shield        Arm
--------------               ------------------------        ---
perception (later)     USB    3x TMC2209 (assumed - verify!)
path planning        ------>  coordinated stepping            3x stepper
inverse kinematics   serial   homing, limits                  2x 30cm strut
host/arm_host.py              E-STOP                          pen
host/arm_ik.py                firmware/genarm_firmware

The host does all thinking (IK, path planning, perception). The Arduino is a dumb, reliable motion executor: it receives joint angles and steps to them. Same pattern as the gen3d ceramic extruder rig.

Geometry

• L1 = L2 = 300 mm pivot-to-pivot (2020 aluminium extrusion)
• Shoulder pivot height H ~ 200 mm above table (19 cm mast + base bearing stack; measure after assembly). Taller = better pen angle + less torque, bigger central dead zone.
• Workspace: annulus roughly 50-588 mm radius (see visualizer)

2020 stock allocation (no cuts needed)

Piece	Job
39 cm x2	upper arm + forearm: pivot inset 9 cm from one end = 30 cm working strut + 9 cm counterweight tail through the pivot
39 cm x2	base H-frame rails (clamp to table)
24 cm x2	base H-frame cross-pieces
19 cm	mast
19 cm	spare / pen boom / endstop mounts

Counterweight = washer stack or chunky bolt in the tail slot; slide along the slot until the arm floats, lock the T-nut.

• IK is closed-form: base aims at the target bearing, shoulder+elbow are a 2-link planar solve (law of cosines), elbow-up only

Open visualizer/index.html in any browser — click the table to move the pen, run test patterns, drag the shoulder-height slider.

Torque (why the motors are assigned this way)

Worst case = arm straight out horizontal.

Joint	Load moment	Motor (from stock)	Plan
Shoulder	~25 kg-cm	57HD4016 NEMA23 (short stack, ~5-9)	counterweight MANDATORY
Elbow	~5 kg-cm	42-40 Creality NEMA17 (~4)	small counterweight stub
Base	~0 (gravity-neutral)	42BYGH610P2-X NEMA17 (long stack)	fine

Spares: 2x 42-40, 42SHDC3025-24B, 42-34 (future pen-lift axis). The 57HD4016 is a short NEMA23 - if a taller one turns up, swap it in.

Counterweight = run the strut past the pivot and hang washers until the arm nearly balances. Costs nothing, halves the problem.

Known v1 limitation: direct drive at 16 microsteps is ~1 mm of pen motion per microstep at full reach. It will draw; fine lines will be coarse. Upgrade #1 is a 3-5:1 GT2 belt reduction on shoulder + base.

Serial protocol (115200 baud, newline-terminated)

Command	Meaning
G <base> <sh> <el>	coordinated move to joint angles (degrees)
H	home all axes
E / D	enable / disable steppers
U / L	pen up / down (future - servo lift)
P	report status + current angles
!	E-STOP: halt + disable immediately

Firmware replies ok when a move completes (host flow control), err ... on rejects. Physical E-STOP button also wired (pin A0 to GND).

Repo layout

visualizer/index.html      standalone IK simulator (no deps, just open it)
host/arm_ik.py             inverse kinematics (stdlib only; python arm_ik.py --test)
host/arm_host.py           Pi/laptop-side controller: streams angles over serial
firmware/genarm_firmware/  Arduino sketch (UNTESTED - written ahead of hardware)

Pi 3B+ setup

sudo apt install python3-serial   # or: pip install pyserial
python3 host/arm_ik.py --test
python3 host/arm_host.py --port /dev/ttyUSB0 --demo square

Arduino appears as /dev/ttyUSB0 (CH340 clone) or /dev/ttyACM0 (genuine). Add user to dialout: sudo usermod -a -G dialout $USER, re-login.

Roadmap

1. IK + visualizer + self-test
2. Verify CNC shield drivers actually are TMC2209 (look for "2209" on the driver boards)
3. Firmware bring-up: motors on bench, no arm attached
4. Cut list, assembly, counterweights, homing switches
5. First drawing: square + circle on paper
6. Pen-lift servo; belt reduction on shoulder + base 6.5 [ ] Leadscrew elbow (linear actuator across the joint, excavator-style): motor mass moves to the shoulder end, self-locking (holds pose unpowered), huge reduction. Angle<->screw-length is a triangle solve added to arm_ik.py.
7. Wrist roll + pen pivot (4th/5th DOF)
8. Perception: device pings / signal landscape / depth cam -> drawing
9. Canvas change mechanism (or whiteboard + wipe)

Status

2026-07-26: IK solved, tested (5000-point roundtrip, worst error 7e-13 mm), visualizer working. Firmware written but never flashed. No metal cut yet.