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H2: brain transplant, blade off

What you build: a used robot mower whose own electronics are replaced by the OpenMower mainboard, driven by our software: the H1 Pi, GNSS and IMU move inside. At the end it mows your lawn's pattern without cutting, returns to its dock and charges.

Time: about three weekends. Cost: hardware/bom.md, stage H2. Blade: removed from the motor, the connector taped, for the whole of this stage.

Draft: written before the hardware arrived

The steps follow hardware/build-guide.md and our own checkpoints; the times are estimates. The mainboard itself is fitted by OpenMower's own step-by-step guide, not by this page: it's specific to your mower's revision and theirs is the authority. This page will be rewritten from the real build, with photos.

Tools: everything from H1; Torx and Phillips drivers (the mower's screws); cut-resistant gloves; a multimeter; insulating tape; a sturdy box to stand the robot on with its wheels off the ground.

Safety for this stage

  • Disconnect the mower's battery before opening the body, and before connecting or disconnecting anything inside. The pack is a lithium pack: don't puncture, crush or short it, and keep it away from metal tools.
  • Wear cut-resistant gloves whenever you touch the blade, even a blunt one.
  • The blade comes off in step 3 and stays off until H3. Driving tests are done blade-off because a software bug at this stage should cost a flowerbed, not a foot.
  • Wheels-off-the-ground tests first: a robot on a box can't drive into anyone.

1. Choose and check the mower (before buying)

Check the mower's model and revision against the OpenMower compatibility list before you pay for it, and that the mainboard kit version matches it (hardware/bom.md, rows 10–11).

Checkpoint H2-A: the stock mower works

With its own firmware, the mower drives, mows, finds its dock and charges. Record the battery voltage when it's full. Fail: don't convert a mower whose motors, battery or dock are already faulty; you'd never know which half is broken.

2. Open the body (30 min)

Battery disconnected (see the safety box). Follow OpenMower's guide for your mower to remove the top cover. Photograph every connector before you unplug it.

3. Remove the blade (10 min)

Gloves on. Remove the blade from the motor, bag it and label it with the robot's serial. Tape the blade motor's connector (step 4 disconnects it from the board).

Checkpoint H2-B: no blade

Blade off, in its bag; the blade motor connector taped and unplugged. A photo of both goes in the build record.

4. Fit the mainboard (one afternoon)

Follow OpenMower's step-by-step guide for your mower revision. Keep the mower's own stop button, lift and tilt sensors connected: they become inputs to the mainboard's emergency-stop chain, which is the hardware layer under our software.

5. First power-up, wheels off the ground (20 min)

Robot on the box, wheels free. Reconnect the battery. Power on.

Checkpoint H2-C: nothing moves by itself

At power-up no wheel turns. Pressing the stop button and lifting the body both show as emergency-stop in the mainboard's status. Fail: power off, battery out, and recheck the wiring against your photos.

6. Move the H1 electronics in (one afternoon)

Pi, GNSS board and IMU inside; the GNSS antenna on top, as high as the body allows, on the centreline. Measure the antenna's and the IMU's position from the middle of the rear axle (x forward, y left, z up, in metres): those numbers go into the robot description, as they did for the cart.

Checkpoint H2-D: offsets

Antenna and IMU offsets in the build record. With the robot on flat ground, the IMU reads level (roll and pitch within 2°) and RTK reaches FIXED within 5 minutes.

7. Connect our software to the mainboard (one weekend)

Bring up the ROS 2 bridge to the mainboard (OpenMowerNext is the ROS 2 Jazzy reference). The robot needs the same topics as the simulator: /cmd_vel, /joint_states (wheel speeds), /imu, /gps/fix, /battery, and the buttons on /ui/buttons. Then start the safety gate and the UI nodes, exactly as in the sim.

Checkpoint H2-E: the end-of-line test, wheels in the air

Still on the box: run the end-of-line test with blade:=false lidar:=false. Every wheel turns the right way at the right speed, STOP stops it, and the watchdog stops it when the driver goes silent. The three IMU yaw-rate rows fail in the air (the body isn't turning); that's expected here, and only here. Everything else must pass.

8. On the lawn (1 hour)

On the lawn, with the dock placed and its position surveyed with RTK, run the whole end-of-line test (blade:=false lidar:=false) and record the result in the build record.

Checkpoint H2-F: end-of-line PASS

Every automatic row PASS or SKIP (blade, lidar). Fail: don't go on to gate 2; the report says which row.

Stage H2 done: next is verification gate 2: drive, localize, mow the pattern (blade off), dock 10 times out of 10.