The Attitude Control PCB — also known as the Attitude and Heading Reference System (AHRS) board — fuses data from gyroscopes, accelerometers, and magnetometers to compute the UAV's 3D orientation (roll, pitch, yaw) in real time. Unlike the inner-loop stability board that focuses on rate control, the attitude board maintains absolute orientation reference with respect to Earth's gravity and magnetic field vectors. This blog examines the specialized PCB design challenges of building a magnetic-field-sensitive, multi-sensor orientation platform.
AHRS Sensor Suite & Placement
A complete AHRS attitude board integrates three sensor types in a tightly-coupled physical arrangement:
3-axis gyroscope: Measures angular velocity with a full-scale range of ±2000°/s and bias instability below 10°/hr. Placed at the board's vibration node (mechanical center) to minimize vibration-induced noise.
3-axis accelerometer: Measures linear acceleration and gravity vector with ±16g range. Must be co-located within 2 mm of the gyroscope to minimize lever-arm errors in the sensor fusion algorithm.
3-axis magnetometer: Measures Earth's magnetic field (typically 25–65 µT) to establish heading reference. This sensor drives the most stringent PCB layout constraints on the board.
Magnetometer Layout: The Hardest Part
The magnetometer — typically a PNI RM3100 or Honeywell HMC5983 — must detect magnetic field vectors with sub-microtesla resolution. Yet the PCB itself generates static and dynamic magnetic interference from several sources:
Current-carrying traces: Every amp of DC current through a trace generates a magnetic field at the sensor. A 1A trace 10 mm from the magnetometer induces approximately 20 µT — nearly equal to Earth's field.
Ferromagnetic components: Nickel barrier layers in capacitors, steel leads on connectors, and even the ENIG nickel layer itself create permanent magnetic offsets (hard-iron distortion).
Eddy currents: Copper pours near the magnetometer induce time-varying magnetic fields when board-level currents change.
Mitigation strategies on the PCB include:
Magnetometer at board edge: Placing the sensor at least 15 mm from any high-current trace or ferromagnetic component, often on a dedicated board-edge tab.
Copper pour removal: A circular keepout zone (radius ≥ 8 mm) around the magnetometer with no copper on any layer. This eliminates both hard-iron and eddy-current distortion from the PCB itself.
Non-magnetic solder: SAC305 solder alloys are inherently non-magnetic; avoid tin-lead solders with ferromagnetic impurities.
ENIG thickness control: Nickel plating thickness is limited to 3–5 µm to minimize ferromagnetic nickel contribution.
Sensor Fusion Processor
The board's MCU — commonly an STM32F4 or Cortex-M7 running at 180+ MHz — executes an Extended Kalman Filter (EKF) or Madgwick/Mahony filter at 500–1000 Hz. The processor interfaces with all three sensors via SPI, requiring 3 dedicated SPI buses or a single bus with 3 chip selects. SPI traces are length-matched to within 2 mm and routed with controlled 50 Ω impedance. A dedicated hardware floating-point unit (FPU) is essential for real-time quaternion computation.
Temperature Compensation Hardware
MEMS sensors exhibit temperature-dependent bias and scale factor errors. The attitude control board incorporates a precision digital temperature sensor (TMP117, ±0.1°C accuracy) placed equidistant from all three MEMS sensors. A factory calibration routine heats the board from -20°C to +85°C while recording sensor outputs at 1°C intervals; resulting compensation coefficients are stored in an onboard EEPROM and applied by the fusion algorithm.
IMU-to-Board Alignment
During assembly, the MEMS sensor packages may be rotated by up to ±1° relative to the PCB fiducial axes. The board design includes optical alignment fiducials placed exactly 50 mm from the IMU center along X and Y axes. Post-assembly, a pick-and-place alignment measurement provides the rotation matrix between the sensor frame and the board frame, which is burned into EEPROM for runtime correction.
Conclusion
The Attitude Control PCB demonstrates that sensor physics directly dictates PCB layout rules. Every copper polygon, every component selection, and every nickel-plated pad influences the magnetometer's ability to resolve Earth's 50 µT field against a background of board-generated interference. Superb Tech's precision fabrication — including controlled ENIG thickness, copper-free keepout zones, and tight fiducial registration — ensures your attitude board achieves its specified heading accuracy.
