1. Objective
The goal of this project is to design a simple HVAC (Heating, Ventilation, and Air Conditioning) control system. We will simulate this system using MATLAB Simulink.
The model will automatically switch Heating or Cooling ON or OFF based on the cabin temperature.
2. System Requirements
| Condition | Action |
|---|---|
| Temperature > 24°C | Turn ON Cooling |
| Temperature < 20°C | Turn ON Heating |
| 20°C ≤ Temperature ≤ 24°C | Maintain previous state (deadband) |
| Desired Cabin Temperature | 22°C |
This means:
- If the cabin is too hot, cooling should start.
- If the cabin is too cold, heating should start.
- Between 20°C and 24°C, no action is taken; the system holds the last state.
3. Concept
The Unit Delay latch is used to store the last output. It remembers whether the system was in the Heating or Cooling state. This occurs when the temperature is within the deadband range (20–24°C).
This helps avoid frequent ON/OFF switching of the HVAC system, which improves stability and saves energy.
4. Model Blocks Overview
| Block | Purpose |
|---|---|
| Constant | Set the desired temperature value (22°C). |
| In1 (Input) | Input temperature signal. |
| Relational Operator | Compare input temperature with limits (20°C and 24°C). |
| Logical Operator | Combine logical conditions (AND, OR). |
| Switch | Choose between Heating or Cooling based on comparison results. |
| Unit Delay | Holds the previous output when inside deadband. |
| Out1 (Output) | Shows final control output (Heating = 1, Cooling = 0). |
5. Model Logic Explanation
- Input Temperature (T_in) is compared with 20°C and 24°C.
- Three possible conditions are checked:
- If T_in < 20°C → Heating ON.
- If T_in > 24°C → Cooling ON.
- If T_in is between 20°C and 24°C, the Unit Delay block keeps the previous output.
- The Switch block selects the new state based on these conditions.
- The Unit Delay output feeds back to maintain the last output when the temperature is within the deadband range.
6. MATLAB SIMULINK LOGIC
Output
7. Test Results
| Temperature (°C) | Expected Output | Description |
|---|---|---|
| 18 | Heating ON | Below 20°C |
| 22 | Hold State | Within deadband |
| 26 | Cooling ON | Above 24°C |
8. Conclusion
The HVAC system model successfully:
- Turns ON Heating when temperature < 20°C.
- Turns ON Cooling when temperature > 24°C.
- Maintains previous state between 20–24°C.
This approach using Unit Delay latch ensures smooth operation and avoids unnecessary switching.