HVAC Control Systems for Commercial Buildings

PROGRAM HVAC_TEMPERATURE_CONTROL_WITH_PID
VAR
    // Inputs
    room_temp : REAL;          // Current temperature °C
    temp_setpoint : REAL := 22.0;  // Desired temperature
    outdoor_temp : REAL;
    occupancy : BOOL;

    // Outputs
    heating_valve : REAL;      // 0-100% valve position
    cooling_valve : REAL;
    fan_speed : REAL;          // 0-100%

    // PID Variables
    error : REAL;
    integral : REAL := 0.0;
    last_error : REAL := 0.0;
    derivative : REAL;

    // PID Gains
    Kp : REAL := 5.0;
    Ki : REAL := 0.5;
    Kd : REAL := 1.0;

    // Control Output
    pid_output : REAL;


    // Building Automation
    Occupancy_Count : INT;
    Occupancy_Detected : BOOL;
    Office_Hours : BOOL;
    Weekend_Mode : BOOL;

    // Energy Management
    Demand_Response_Active : BOOL;
    Peak_Shaving_Mode : BOOL;
    Load_Shedding_Level : INT;
    Utility_Rate_Period : STRING[20];  // 'PEAK', 'OFF_PEAK', 'SHOULDER'

    // Scheduling
    Schedule_Override : BOOL;
    Scheduled_Start : TIME;
    Scheduled_Stop : TIME;
    Holiday_Mode : BOOL;

    // Comfort Optimization
    CO2_Level : REAL;  // ppm
    Lighting_Level : REAL;  // Lux
    Adaptive_Control_Active : BOOL;
END_VAR

// ==========================================
// BASE APPLICATION LOGIC
// ==========================================

// Calculate PID error
error := temp_setpoint - room_temp;

// Integral term with anti-windup
integral := integral + (error * 0.1);
integral := LIMIT(-50.0, integral, 50.0);

// Derivative term
derivative := (error - last_error) / 0.1;
last_error := error;

// PID calculation
pid_output := (Kp * error) + (Ki * integral) + (Kd * derivative);

// Map PID output to heating/cooling
IF pid_output > 0.0 THEN
    heating_valve := LIMIT(0.0, pid_output, 100.0);
    cooling_valve := 0.0;
ELSE
    heating_valve := 0.0;
    cooling_valve := LIMIT(0.0, ABS(pid_output), 100.0);
END_IF;

// Fan speed based on demand and occupancy
IF occupancy THEN
    fan_speed := LIMIT(30.0, ABS(pid_output) * 1.5, 100.0);
ELSE
    fan_speed := 20.0;  // Minimum circulation
END_IF;

// ==========================================
// COMMERCIAL BUILDINGS SPECIFIC LOGIC
// ==========================================

    // Occupancy-Based Control
    IF NOT Occupancy_Detected AND NOT Office_Hours THEN
        // Setback mode - reduce HVAC, lighting
        Temperature_Setpoint := Setback_Temperature;
        Lighting_Level := 10.0;  // Security lighting only
    ELSE
        Temperature_Setpoint := Comfort_Temperature;
        Lighting_Level := 100.0;
    END_IF;

    // Demand Response Integration
    IF Demand_Response_Active THEN
        CASE Load_Shedding_Level OF
            1: // Moderate reduction
                Temperature_Setpoint := Temperature_Setpoint + 2.0;
            2: // Significant reduction
                Temperature_Setpoint := Temperature_Setpoint + 4.0;
                Non_Essential_Loads := FALSE;
        END_CASE;
    END_IF;

    // Time-of-Use Rate Optimization
    IF Utility_Rate_Period = 'PEAK' THEN
        // Minimize usage during peak rate hours
        Pre_Cool_Complete := TRUE;
        Peak_Shaving_Mode := TRUE;
    END_IF;

    // CO2-Based Ventilation
    IF CO2_Level > 1000.0 THEN  // ppm
        Outside_Air_Damper := MIN(Outside_Air_Damper + 10.0, 100.0);
    END_IF;

// ==========================================
// COMMERCIAL BUILDINGS SAFETY INTERLOCKS
// ==========================================

    Production_Allowed := NOT Emergency_Stop
                          AND Building_Systems_Normal;