Motors

The car uses JGB37-520 DC drive motors rated for 12 V and 550 RPM. They are driven by a Yahboom AT8236 Motor Controller, which the Raspberry Pi 5 controls with PWM on four GPIO pins. Steering is separate (a servo on the PCA9685); these motors only provide thrust.

Parts (Amazon)

Motor Specification

Specification Value
Model JGB37-520
Type DC geared motor
Rated voltage 12 V DC
Rated speed 550 RPM

Wiring

Left and right sides each have a forward and a backward PWM channel (from config.py):

Channel GPIO pin
Right forward 19
Right backward 20
Left forward 25
Left backward 13

Each channel is a PWMOutputDevice at 1 kHz (hardware.py). To drive forward, the forward pins get a PWM duty and the backward pins stay at 0; reverse swaps them.

How It Works

  • The control loop computes a motor PWM value (0..1) from throttle, cruise/PID, or autonomy, clamped by acceleration/brake rates, and writes it to the AT8236 Motor Controller through the four GPIO channels.
  • Per-side scaling constants LEFT_MOTOR_PWM_SCALE and RIGHT_MOTOR_PWM_SCALE (both 1.0 currently) can trim a measured drive-force mismatch. A pull can also come from steering alignment, linkage load, tires, surface, or payload, so motor scaling should change only after a restrained/straight-line test isolates motor thrust as the cause.
  • Partial braking ramps PWM toward zero using the configured brake rates. A full brake force (including an AEB emergency stop) bypasses that ramp, forces motor PWM to zero, and drives both AT8236 Motor Controller inputs high on each side for active brake mode.

Why This Choice

  • The AT8236 Motor Controller is sized for this chassis and exposes simple PWM/direction control that maps cleanly onto four GPIO pins.
  • Keeping optional left and right motor scaling separate from steering calibration makes the two effects measurable instead of hiding one inside the other.

Verify Before a Run

  • Bench test: the Hall-effect wheel-speed sensor (code/test_files/sensors/hall_sensor_test.py) confirms wheel rotation and feeds the speed estimate.
  • A car that pulls with logical steering centered is a symptom, not a diagnosis. Compare physical wheel alignment, free-wheel behavior, loaded motor speed/current, tire condition, surface, and payload before changing either PWM scale or steering trim.