05 Zero-calibration (arrow keys; required for DIY builds)

The servo zeros and the IMU accel zeros are calibrated and baked per unit at the factory, so a stock unit needs nothing here. A DIY build must do this page: a hand-assembled arm will almost never sit exactly at the center. Even if the horn was not mounted exactly at the firmware zero, you can fix it here - jog the arm to visually vertical with the arrow keys and press z to declare the current pose q=0. The z command does not use the IMU and works at any pose or horn index.

Come back when: a horn was re-indexed, the arm was disassembled/re-assembled, a servo was replaced, the IMU was replaced/re-mounted (recalibrate the accel), or the arm no longer rests at the visual center after retracting.

z only shifts the software zero; it does not change how the horn index splits the travel (PWM 500-2500 µs). If after zeroing one direction runs out of range (the pulse clamps at 500/2500 µs and the arm cannot reach a required sequence angle), re-index the horn (about 2 teeth on the shoulder) and zero again.

Timing: after the robot is fully assembled and the main firmware is flashed (the DIY path runs 03 → 04 → 05).

5.1 Principle

The firmware defines q=0 as the visual center: arm vertical, both links in one line. The z command is an encoder-style zero: it makes the current joint positions q=0 and prints the corresponding center pulse widths.

5.2 Procedure

1. Connect both the battery and USB (servos powered) and run this in the SR-Support folder with .venv activated:

   python tools/hw_shoulder_range.py

   Close any serial assistant/monitor first (they hold the port); the keys in the window below are read by this tool, no serial assistant is needed. Serial tools / the Windows UartAssist download are on page 03, section 3.3.

   A persistent four-line display updates in place:

   Up/Dn=BigArm  L/R=SmallArm  Sh:fast
   z=SetCenter  k=Read  n=Home  q=Quit
   SmallArm: +0.000 rad (  +0 deg)
   BigArm:   +0.000 rad (  +0 deg)

2. Jog with the arrow keys: ↑/↓ = big arm ±0.05 rad, ←/→ = small arm ±0.05 rad (Shift = ±0.20) until the arm looks vertical and aligned;

3. Press z; the firmware prints:

   zero set: shoulder XXXX us, elbow XXXX us (current pose is now q = 0)

4. Sanity check: press n (home), jog a little, home again - the arm must return to the visual center you just defined.

5.3 Baking (mandatory once satisfied)

z only touches RAM; a reboot restores the old zero. When happy:

1. Write the two pulse values into config.h (SERVO_SHOULDER_CENTER_US / SERVO_CENTER_US);
2. Shift every waypoint of EXEC_REF* and training/b_line/getup_reference_b39.py by the same offsets (shoulder/elbow each −Δ rad);
3. Reflash and re-test all four directions (page 07).

A mis-press is harmless: zero again to overwrite.

5.4 Related commands

Command	Function
z	Current pose becomes q=0 for both joints; prints the new center pulses (RAM only)
k	Read the current center pulses (shoulder/elbow, µs)
o / p	Shoulder center ±10 µs live trim (reset on reboot)

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