210  Processes and Systems: Open vs Closed Loop Control

210.1 Comparing Open and Closed Loop Systems

Understanding the trade-offs between open-loop and closed-loop systems is crucial for IoT system design.

210.1.1 Advantages and Disadvantages

Question 8: Comparing open-loop versus closed-loop irrigation control over 30 days in summer. Open-loop waters 10 minutes daily. Closed-loop uses soil moisture sensor (waters when < 30%, stops at 70%). Which statement is most accurate?

πŸ’‘ Explanation: Open-loop waste: Waters daily regardless of conditions β†’ After rain, over-waters β†’ During heat, under-waters β†’ Total: 300 minutes (30 days Γ— 10 min). Closed-loop efficiency: Rainy days (5 days): no watering needed. Hot days (10 days): waters 15 min. Normal days (15 days): waters 8 min. Total: (0Γ—5) + (15Γ—10) + (8Γ—15) = 270 minutes saved, better moisture maintenance. Water savings: 40-60% typical in real deployments. Crop yield: Improved by 10-20% (optimal moisture always maintained). ROI: Sensor cost ($50) recovered in water savings within one season. This demonstrates feedback control’s practical superiority.

210.2 Open-Loop Systems

Advantages: - Simple design and implementation - Lower cost (no feedback sensors needed) - Faster response (no feedback processing delay) - No stability issues or oscillations - Lower power consumption

Disadvantages: - Cannot self-correct errors - critical limitation - No knowledge of output condition - Sensitive to disturbances and variations - Cannot adapt to changing conditions - Accuracy depends entirely on calibration - Output may drift over time

Best Used When: - Output is highly predictable - Disturbances are minimal or absent - Cost is primary constraint - Simple data collection (sensing only) - Speed is critical and accuracy is not

ImportantClosed-Loop Systems

Advantages: - Automatic error correction - key benefit - Maintains desired output despite disturbances - Reduced sensitivity to component variations - Can use inexpensive, less accurate components - Adapts to changing conditions - Improved accuracy and stability

Disadvantages: - More complex design - Higher cost (feedback sensors and processing) - Potential stability problems if poorly designed - Can oscillate around set point - Higher power consumption - Slower response due to feedback processing

Best Used When: - Precision control required - Environment is unpredictable - Disturbances are likely - Safety is critical - Long-term stability needed - Self-regulation is valuable

210.2.1 Decision Matrix

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graph TD
    Start{{"Control System<br/>Design Decision"}} --> Q1{"Precision<br/>Control<br/>Required?"}
    Q1 -->|Yes| Q2{"Environment<br/>Predictable?"}
    Q1 -->|No| Q3{"Cost<br/>Constraint?"}

    Q2 -->|No| CL["Use<br/>Closed-Loop<br/>(Feedback)"]
    Q2 -->|Yes| Q4{"Disturbances<br/>Present?"}

    Q3 -->|High| OL["Use<br/>Open-Loop<br/>(No Feedback)"]
    Q3 -->|Low| Q2

    Q4 -->|Yes| CL
    Q4 -->|No| Q5{"Safety<br/>Critical?"}

    Q5 -->|Yes| CL
    Q5 -->|No| OL

    style Start fill:#16A085,stroke:#2C3E50,stroke-width:3px,color:#fff
    style CL fill:#2C3E50,stroke:#16A085,stroke-width:3px,color:#fff
    style OL fill:#E67E22,stroke:#16A085,stroke-width:3px,color:#fff

Figure 210.1: Decision tree: when to use open-loop vs closed-loop

Open-Loop vs Closed-Loop Decision Tree: Precision requirements, environmental predictability, disturbances, cost constraints, and safety considerations determine appropriate control architecture. Most IoT control applications benefit from closed-loop feedback.

TipExample: Water Quality Monitoring with Feedback

Consider an IoT water quality monitoring system for an aquarium:

Open-Loop Approach (Inadequate): - Oxygen sensor detects low Oβ‚‚ level - Data sent to cloud - No local action taken - Fish may die before human intervention

Closed-Loop Approach (Recommended):

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flowchart LR
    SP["Setpoint:<br/>Oβ‚‚ = 7 mg/L"] --> Comp["Compare"]
    Sensor["Oβ‚‚ Sensor:<br/>Measures<br/>6.2 mg/L"] --> Comp
    Comp -->|Error: +0.8 mg/L<br/>Need More Oβ‚‚| MCU["Microcontroller<br/>PID Controller"]
    MCU -->|Increase<br/>Pump Speed| Pump["Aerator<br/>Pump"]
    Pump -->|Add Oxygen| Tank["Aquarium<br/>Water"]
    Tank -.->|Feedback:<br/>Measure Oβ‚‚| Sensor
    Tank -->|Also Send Data| Cloud["Cloud<br/>Monitoring"]

    style SP fill:#16A085,stroke:#2C3E50,stroke-width:2px,color:#fff
    style Comp fill:#E67E22,stroke:#16A085,stroke-width:2px,color:#fff
    style MCU fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style Pump fill:#E67E22,stroke:#16A085,stroke-width:2px,color:#fff
    style Sensor fill:#2C3E50,stroke:#16A085,stroke-width:2px,color:#fff
    style Tank fill:#7F8C8D,stroke:#16A085,stroke-width:2px,color:#fff

Figure 210.2: Aquarium O2 control with PID aerator feedback loop

Aquarium Oβ‚‚ Control Closed-Loop System: Local microcontroller continuously monitors dissolved oxygen, compares to setpoint (7 mg/L), and adjusts aerator pump speed to maintain safe levels. Cloud monitoring provides secondary oversight but doesn’t delay critical control action.

Operation: 1. Set point: Oβ‚‚ = 7 mg/L 2. Sensor measures: Oβ‚‚ = 6.2 mg/L 3. Error: +0.8 mg/L (need more oxygen) 4. Controller increases aerator pump speed 5. Oβ‚‚ level rises toward target 6. Controller reduces pump speed as error decreases 7. System maintains stable Oβ‚‚ level 8. Cloud also receives data for long-term monitoring

Error Signal Calculation:

Error = Set Point - Measured Value
Error = 7.0 mg/L - 6.2 mg/L = +0.8 mg/L

Positive error β†’ Increase aerator output
Negative error β†’ Decrease aerator output
Zero error β†’ Maintain current output

210.2.2 Practical IoT Design Considerations

When designing IoT systems, consider:

Sensor Placement: - Feedback sensors must accurately measure the controlled variable - Minimize latency between actual change and sensor detection - Consider sensor accuracy requirements vs. cost

Communication Delays: - For distributed systems, network latency affects feedback loop performance - Long delays can cause instability - May need to implement local closed-loop with cloud monitoring

Failure Modes: - What happens if feedback sensor fails? - Should system fail-safe (shut down) or continue open-loop? - Redundant sensors for critical applications?

Power Constraints: - Closed-loop systems consume more power - May need to alternate between open and closed loop operation - Sleep modes between control actions