L2 Technician Curriculum

Five sections covering diagnostic tools, actuator systems, sensor systems, software, and repair procedures.

Section 1: Diagnostic Tools and Signal Analysis

L2 technicians use electronic diagnostic tools to identify faults that are invisible to visual inspection.

Multimeter (CAT III, 600V Rated)

Bus voltage measurement specifications:

12V bus (legacy sensors, some peripheral loads): expect 11.5-12.6V under load
24V bus (mid-power actuators, computing boards): expect 23.0-25.2V
48V bus (primary humanoid drive actuators): expect 46.0-50.4V

Motor winding resistance: measure between any two motor terminals with motor disconnected:

Healthy winding resistance range: 0.1 - 2.0 Ohm depending on motor size
Open circuit (infinite resistance): wire break or burned winding
Short to ground (< 0.1 Ohm): insulation failure — motor must be replaced

Oscilloscope (PWM Analysis)

Motor control signals use PWM (Pulse Width Modulation):

Standard servo PWM: 50Hz carrier, 1-2ms pulse width (1ms = full reverse, 1.5ms = stop, 2ms = full forward)
High-frequency FOC drive: 8-20kHz switching frequency; use oscilloscope to verify switching waveform quality
Faulty drive symptoms: asymmetric PWM duty cycles, ringing at switching transitions, flatline (no PWM = driver fault)

CAN Bus Diagnostics (candump)

# List available CAN interfaces
ip link show type can

# Capture live CAN traffic (save to file)
candump -l can0

# View traffic on screen
candump can0

# Filter to specific arbitration ID (e.g., 0x141 = Unitree G1 right knee motor)
candump can0 | grep ' 141 '

CANopen rates on humanoid platforms: 1 Mbit/s typical. Set interface before capture:

ip link set can0 type can bitrate 1000000
ip link set can0 up

Arbitration ID structure on Unitree platforms:

IDs 0x001-0x007: NMT (network management)
IDs 0x181-0x1FF: PDO Transmit (robot publishing sensor data)
IDs 0x201-0x27F: PDO Receive (operator sending commands)
IDs 0x581-0x5FF: SDO (parameter access, configuration)

ROS 2 Diagnostics

# List all active nodes
ros2 node list

# List all topics with types
ros2 topic list -t

# Check publish rate on a topic
ros2 topic hz /joint_states

# Echo topic data (Ctrl+C to stop)
ros2 topic echo /joint_states

# Check for communication issues
ros2 doctor

# View topic info (publishers, subscribers, QoS)
ros2 topic info /imu/data -v

Key G1 topics:

/joint_states — 500Hz, all 43 joint positions, velocities, torques
/imu/data — 200Hz, acceleration and angular velocity
/camera/depth — 30fps, depth image from front stereo camera
/diagnostics — 10Hz, hardware health summary

Section 2: Actuator Systems

BLDC Motor Principles

Brushless DC motors (BLDC) dominate robotic actuators due to high power density and long service life. Key principles:

Three-phase electrical drive; motor controller (ESC or servo driver) commutates phases electronically
Position sensing via encoder or Hall effect sensors determines commutation angle
Field-oriented control (FOC) enables smooth torque control at all speeds

Harmonic Drives

Harmonic drives provide high gear reduction with zero backlash, critical for precise joint control:

Components: wave generator (input shaft with elliptical cam), flex spline (thin-walled gear, output), circular spline (rigid ring gear, fixed)
Gear ratios: 50:1 to 160:1 typical for humanoid joint applications
Advantages: zero backlash, high torque-to-weight ratio, compact
Failure modes: flex spline fatigue fracture (catastrophic, usually preceded by increasing noise), tooth wear (gradual, detectable by backlash measurement)

Joint Telemetry — Unitree G1 Knee Example

The G1 knee joint (a 43 DOF platform uses knees as highest-load joints):

Position range: +/- 2.7 radians from center
Velocity range: +/- 10 rad/s maximum
Torque range: +/- 45 Nm continuous

Telemetry interpretation:

Position spikes (single frame outlier): encoder noise — normal if isolated
Velocity exceeding 10 rad/s: flag for investigation — may indicate control fault
Torque above 40 Nm sustained: overload condition — check for mechanical binding

Bearing Defect Frequency Formula

Surface defects on bearing races produce vibration at characteristic frequencies:

BPFO (ball pass frequency outer race) = (N/2) * RPM/60 * (1 - d/D * cos(a))
Where N = number of balls, d = ball diameter, D = pitch diameter, a = contact angle
Use an accelerometer + FFT to identify BPFO peaks indicating outer race wear

Section 3: Sensor Systems

IMU (Inertial Measurement Unit)

Common IMU: MPU-9250 (or equivalent) used in many humanoid platforms:

Accelerometer range: +/- 16g
Gyroscope range: +/- 2000 degrees/second
Magnetometer: +/- 4800 uT

Calibration procedure:

Place robot on a stable, level surface
Run ros2 topic echo /imu/data — verify [0, 0, 9.81 +/- 0.1 m/s2] on Z axis at rest
If Z-axis reading deviates > 0.1 m/s2, run platform IMU calibration tool (platform-specific)
After calibration, record calibration timestamp in TechMedix

Force/Torque Sensors

Wrist and foot F/T sensors enable contact detection and safe interaction:

Six-axis measurement: Fx, Fy, Fz (forces in N) and Mx, My, Mz (moments in Nm)
Zero-point calibration: with sensor unloaded, values should be < 0.5N / < 0.05 Nm
Drift over time: recalibrate at 500-hour intervals

Camera Cleaning

Use only lens-grade microfiber cloths and isopropyl alcohol (>90%)
Apply alcohol to cloth, not lens
Single-pass wipe from center outward — do not circular-scrub (introduces micro-scratches)
Clean stereo cameras as a pair — dirty left lens with clean right lens causes depth calibration errors

LiDAR Point Cloud Density

Healthy LiDAR: 100,000+ points/second at 10-15Hz scan rate. Degraded cloud density indicates:

Lens contamination: clean aperture with compressed air first, then optical cloth
Internal mirror fault: service required (L3+)
Environmental absorption: normal near glass, matte black surfaces, and heavy rain

Section 4: Software and Firmware

Firmware Update Procedure

Standard procedure for any BCR-supported platform:

Verify current firmware version: ros2 param get /firmware_manager current_version
Download target firmware from BCR firmware repository (verify SHA256 hash before flashing)
Put robot in firmware update mode (platform-specific command)
Flash firmware using OEM updater tool
Verify update: re-check version after reboot
Run post-update validation: joint range of motion test, IMU calibration check, network connectivity

Never flash firmware to a robot with battery below 40% SoC. Power loss during flash can brick the controller.

ROS 2 Node Health

Healthy node indicators:

Node publishing at specified frequency (+/- 10%)
No error messages in ros2 doctor output
No stuck subscribers (old message timestamps in topic info)

Unhealthy indicators:

Topic publishing at < 80% of expected rate: check CPU load, bus utilization
Node crash/restart: check journalctl -u ros2_* for error logs
Missing node: check if launch file is running correctly

Section 5: Repair Procedures

Actuator R&R Torque Sequence

When replacing a joint actuator module:

Confirm LOTO — hardwired E-stop, battery disconnected, network isolated
Support the affected limb mechanically before disconnecting actuator
Disconnect motor power and encoder connectors in that order
Remove mounting fasteners in reverse torque sequence (center-out pattern)
Install new actuator, hand-tighten all fasteners before any torquing
Torque in center-out pattern to specification (see platform-specific values)
Reconnect encoder before motor power
Power on and verify joint registers correct position

Cable Management

Minimum bend radius: 10x cable outer diameter (OD)
After R&R, verify cables are not pinched at joint extremes — manually articulate through full range while observing cable routing
Use OEM-specified cable tie positions — do not add ties that restrict intended flex points

Post-Repair Validation

Every actuator R&R requires TechMedix post-repair validation log:

Joint position range: verify reaches specification endpoints (+/- 2.7 rad for G1 knee)
Joint velocity: verify able to reach 5 rad/s in a controlled move
Torque calibration: verify torque reading matches calibration fixture
Temperature at 10 minutes of operation: must not exceed 60C
No new fault codes generated

All validation data must be uploaded to TechMedix before work order is closed.