Bus AC Controller

Project Background & Mission Objectives

Commercial transit buses experience extreme thermal and vibrational stress. Standard consumer HVAC microcontrollers routinely suffer from electrical transients caused by alternator load-dumps, heavy inductive blower relays, and multi-zone temperature imbalances across 40-passenger cabins.

Embedded India was commissioned to architect a turnkey electronic climate controller conforming to strict automotive durability standards.

System Architecture & Hardware Engineering

+-----------------------------------------------------------+
|               Vehicle CAN-FD / SAE J1939 Bus              |
+-----------------------------+-----------------------------+
                              |
                     +--------v--------+
                     | Isolated CAN-FD |
                     |   Transceiver   |
                     +--------+--------+
                              |
+-----------------------------v-----------------------------+
|               STM32F405 168MHz Cortex-M4F                 |
|             Hardware Floating Point Unit (FPU)            |
|             FreeRTOS Deterministic Scheduler              |
+--------------+-----------------------------+--------------+
               |                             |
      +--------v--------+           +--------v--------+
      |  12-Bit ADC NTC |           | High-Current    |
      |   Signal Chain  |           | Gate Drivers    |
      +--------+--------+           +--------+--------+
               |                             |
         Cabin Probes              Solid-State Blower Relays

Key Engineering Achievements

1. Active ISO 7637-2 Load-Dump Surge Protection: Integrated TVS clamping and discrete MOSFET reverse-polarity suppression withstands +87V/400ms alternator transients without microcontroller reset.
2. Deterministic Dual-Zone PID Control: Implemented parallel FreeRTOS tasks to dynamically regulate evaporator temperature and variable expansion valves with ±0.5°C cabin precision.
3. Automated Diagnostic Logging: Non-volatile SPI FRAM stores real-time error codes, compressor run hours, and refrigerant pressure spikes for OBD-II vehicle maintenance tool readouts.
4. HIL Verification: 100% of manufactured production units pass automated Hardware-in-the-Loop functional tests before packaging.