This engineering manual outlines verified diagnostic workflows, mathematical calculations, and safety clearances for Building Management System (BMS) DDC PID Loop Tuning & BACnet MS/TP Network Architecture. Adheres to national and international building standards including ASHRAE Standard 135 (BACnet) / Guideline 36.
Table of Contents
1. AHU Chilled Water Valve PID Tuning
Direct Digital Control (DDC) loops for Air Handling Unit discharge air temperature frequently hunt due to over-aggressive proportional gain.
Discrete PID Algorithm: u(t) = K_p * e(t) + K_i * ∫ e(t) dt + K_d * (de/dt)
For AHU chilled water modulating valves, recommended baseline values: K_p = 2.5–4.0, Integral Reset Time T_i = 120–180 seconds, Derivative Gain K_d = 0.
2. BACnet MS/TP RS-485 Physical Layer Best Practices
- Daisy-chain topology strictly with 24 AWG shielded twisted pair cable (120 Ohm characteristic impedance).
- Install 120-Ohm end-of-line (EOL) balancing resistors on both physical extremities.
- Cap total device count at 32 nodes per token ring segment at 38,400 or 76,800 baud.
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