Two axis balancing carrier mcu version

Two-Axis Balancing Robot

A robot that balances in pitch and roll. The two axes use different hardware: pitch runs a brushless motor through a VESC over CAN, roll runs a brushed motor through an H-bridge on a custom power board. Both close around an STM32G474 and an ICM-42688-P IMU on a custom carrier board. Two custom 4-layer boards, designed in KiCad.

Specifications

Status

Both custom PCBs are designed and the electrical architecture is laid out. Mechanical design is partly finished.

I simulated the pitch axis in Simulink first, and that's what pushed me away from PID and toward LQR. Wheel position and tilt are coupled through the dynamics, so tuning them as separate loops meant the two kept fighting each other. Full state feedback made more sense than trying to manage that interaction by hand.

What I'm working on now is the model itself: deriving the equations of motion with Lagrangian mechanics, linearizing about the upright equilibrium, and designing a discrete-time LQR with Bryson's rule setting the cost weights. That work is all in MATLAB.

Repository layout

Hardware

Both boards are 4-layer. Every copper layer below is plotted from the top and unmirrored, so features line up between layers.

Carrier board

101.6 × 92.9 mm. Holds the Nucleo-G474RE and everything the control loop reads.

The ICM-42688-P is on SPI (CS, SCLK, SDI, SDO) with INT1 and INT2 brought out, so the loop can run off data-ready rather than polling. An SN65HVD230 with 120 Ω termination puts the board on the CAN bus to the VESC. A MIC5219-3.3 makes the 3.3 V rail from the 5 V coming off the power board.

Roll encoder channels come in here on ENC_A / ENC_B. Motor commands (AIN1, AIN2, PWMA, STBY) and the battery signals go back to the power board over a 2×5 IDC ribbon. There are 13 test points, and the SPI, UART, encoder and rail pads are all labelled on the silkscreen for bring-up.

Power board

50.0 × 42.9 mm. Takes the battery in and handles everything that touches VBAT.

Battery comes in on an XT60, behind a 3 A fuse, an AO3401A P-channel MOSFET for reverse polarity, and a 10 V zener. An RPL-1.0-R module makes the 5 V rail and reports Buck_PG back to the MCU. A TB6612FNG dual H-bridge drives the roll motor on MOTOR_P / MOTOR_N.

Battery monitoring is done on this board rather than in firmware. A divider gives BAT_LEV, an LM393 compares it against BAT_REF, and BAT_LOW goes back to the carrier board. The MCU sees both the raw level and a latched low flag.

Connectors

The 2×5 ribbon is the only link between the two boards. It carries the 5 V rail and ground, the four TB6612FNG control lines (AIN1, AIN2, PWMA, STBY) and the two battery monitoring signals (BAT_LEV, BAT_LOW). VBAT never reaches the carrier board, which only ever sees 5 V.