Tony Frank
All projects

Gimbal electronics

Complete electronics for a three-axis gimbal. A mainboard, three round BLDC motor drivers with magnetic encoders and an IMU board, all connected over RS-485. Designed and assembled by me.

  • STM32
  • BLDC
  • RS-485
  • Altium
  • Hardware Design

The system consists of five boards: a mainboard, three identical motor drivers and an IMU. Each board has its own STM32, and they talk to each other over full-duplex RS-485. The Molex Pico-SPOX connectors carry both the bus and the supply to every board.

All five boards side by side: three round motor drivers at the top, the IMU and the mainboard below The complete set: three motor drivers, with the IMU (left) and the mainboard (right) below.

BLDC motor driver

The round drivers sit directly on the motor. The AS5047D magnetic encoder measures the rotor position, the DRV8313 drives the three phases, and the motor current is measured across a shunt.

Freshly assembled motor driver, still in the panel, next to a 2-euro coin Freshly assembled and still in the panel. The 2-euro coin shows the scale.

Function Part
Microcontroller STM32F401CCU6
Motor driver DRV8313, three integrated half-bridges
Rotor position AS5047D, magnetic encoder over SPI
Current sensing AD8418 across a 50 mΩ shunt
Supply LT3470, buck from motor voltage down to 3.3 V
Bus ISL3177, full-duplex RS-485
Memory FM25CL64B, 64 Kbit FRAM

3D view of the motor driver from Altium Fully assembled motor driver The layout in 3D view and the finished board.

Schematic of the BLDC motor driver Schematic of the driver. Click to enlarge.

IMU board “Oculus”

The IMU measures the orientation of the gimbal. It carries two sensors: an MPU6500 on SPI and a BNO055 with on-chip sensor fusion on I²C. It can also output an SBUS signal whose polarity can be switched through an XOR gate.

Function Part
Microcontroller STM32F401CEU6
Sensors MPU6500 (SPI), BNO055 (I²C)
Supply LP2985, LDO to 3.3 V
Bus ISL3177, full-duplex RS-485
SBUS output SN74LVC1G86 as a switchable inverter
Memory FM25CL64B, 64 Kbit FRAM

3D view of the IMU board with the Oculus label Assembled IMU board Layout and assembled board.

Schematic of the IMU board Schematic of the IMU, revision 2.0. Click to enlarge.

Mainboard

The mainboard is the hub. It has five RS-485 ports for the drivers, the IMU and a system connector, an SBUS input for the radio receiver, and it measures input current and voltage. Analog switches select which of the controller’s UARTs goes to which port.

Function Part
Microcontroller STM32F405RGT6
Supply LT3991, buck to 5 V, followed by an LP2985 to 3.3 V
Bus 5 × ISL3177, full-duplex RS-485
UART routing 2 × TS5A3359, 1-of-3 analog switches
Measurement AD8211 across a 30 mΩ shunt, divider for the input voltage
Radio SBUS input with an SN74LVC1G86 as inverter
Memory FM25CL64B, 64 Kbit FRAM

3D view of the mainboard Assembled mainboard Layout and assembled board.

Schematic of the mainboard Schematic of the mainboard. Click to enlarge.

From the bring-up

The red notes in the schematics are fixes from the first bring-up. On the mainboard these include a BOOT0 pin that was tied directly to VDD and had to be connected through a 10 kΩ resistor instead, a change to the LT3991 compensation network, and an additional ferrite between VDDA and VDD.