Control, Gateways & Networked Systems · Study deck

Open-Loop and Closed-Loop Systems

Picture a heater that runs for ten minutes even after the room becomes warm.

Gateway Gus is your guide for this deck.

openclosedloop

After studying this chapter

Learning objectives

You will be able to:

Explain the practical difference between command-only control and feedback control.
Identify when an open-loop design is acceptable for an IoT system.
Identify when a closed-loop design is required because the system must correct error.
Describe how disturbances, sensor quality, actuator limits, and placement affect loop design.

Distributed IoT closed-loop control: a fish-farm dissolved-oxygen example

Major section

Start With the Missing Measurement

The difference between open-loop and closed-loop control is easiest to see when a measurement disappears.

A timed heater can run for ten minutes whether the room is warm or cold; a closed-loop heater checks the temperature and changes course.
If you cannot measure it in time, the control shape must acknowledge that gap.
IoT teams make this choice constantly when deciding whether a command should trust a schedule, a model, or live feedback.

Major section

In 60 Seconds · MVU: Minimum Viable Understanding

Closed-loop control measures the process variable, compares it with the setpoint, and adjusts the next command from the error.

Closed loop acts from measurement.
Adding feedback improves adaptation, but it also adds sensor, timing, stability, and fallback responsibilities.
Why it matters: A controller cannot correct an error it does not measure.
Key takeaway: Many durable IoT systems combine both: an open-loop schedule or model provides a baseline, and a closed-loop correction trims the output when measurements are trustworthy.

Use the choice map as an architecture gate: first decide whether output error is acceptable, then decide whether the right variable can be measured quickly enough for correction. Partial measurement, unreliable timing, or fallback requirements point to a hybrid record instead of a vague closed-loop label.

Major section

The Two Control Shapes

The observer can inspect or log the result, but there is no arrow returning the measured output to the controller.

That absent return path is the meaning of open loop: the next command depends on the schedule, setting, or model rather than on measured error.
This establishes the baseline for the chapter's running question about whether process variation is observable and correctable.
It adds a measured process variable and an error calculation, allowing the next command to respond to the physical result rather than merely repeating the planned input.

Major section

How to Choose · Control Family Addendum

The choice is not "simple versus smart." It is a fit decision based on observability, consequence, timing, and expected variation.

The process is repeatable over the operating range.
The output does not need tight correction.
The actuator effect is easy to predict.
Failure is visible before it becomes severe.

Major section

Disturbances and Observability

A disturbance is any influence that changes the process output without being the controller command.

Wind, load, supply variation, occupancy, friction, and material changes are common examples.
The loop can react only after the sensor sees their effect.
This ties control of a disturbance to what the system can see and how soon it can act.

A clean-room PID loop compares the temperature setpoint with measured temperature, computes error, drives an HVAC actuator, observes the plant output, and compensates for door-opening and equipment-heat disturbances.

Major section

Disturbances and Observability (continued)

The clean-room diagram in Figure: A clean-room PID loop compares the temperature setpoint shows why feedback only helps with disturbances that become visible in time for the actuator to counter them.

The test must cover sensor place, sample time, actuator range, and a safe state for stale or wrong feedback.
Closed loop does not magically solve every disturbance.
It helps only when the loop can observe the output, compute error, and act quickly enough for the process.

Major section

Hybrid Feedback Timing Addendum

Hybrid IoT control is common: a remote service sets policy, an edge layer coordinates nearby equipment, and a local loop performs time-sensitive correction.

The design is closed-loop at the physical process when measured output changes the next actuation, whether the controller is local or remote.
The decision record should compare runtime, process excursions, sensor reliability, and fallback behavior in the actual installation.

Major section

Placement in IoT Systems

Feedback can be local, distributed, or supervisory.

The correct placement depends on how quickly the process needs correction and whether the system can continue when the network is unavailable.
Sensor, controller, and actuator are near the process.
Measurement and actuation are separated across devices.
Useful when one node observes what another node affects.

Major section

Control Decision Record

A control decision should leave a short record.

The first fields define what the loop may influence; the middle fields bound authority and failure exposure; the final fields make the choice testable.
Minimum, maximum, and rate limits.
This prevents a later maintainer from guessing why a loop was left open, closed locally, or split across layers.

Major section

Worked Example: Pumped Tank Level · Common Pitfalls

This works only if the outlet flow and starting level are predictable.

Closed-loop design: A level sensor reports the tank level.
The loop can correct outlet variation, but it must handle sensor faults, pump saturation, and delayed level response.
Ignoring Actuator Limits.

Major section

Key Concepts

The chapter's vocabulary describes one causal path: a controller compares a setpoint with a process variable, acts within its authority, and encounters disturbances across a chosen loop boundary.

A process variable is useful only when its sensor location and timing reveal the error the actuator can correct.
A disturbance matters when it changes that variable outside the command path.
Loop placement then determines whether the measurement, decision, actuation, and fallback remain available within the required response time.
Process variable: The measured output that represents process behavior.

Major section

Key Concepts (continued)

Closed-loop control: Control that uses measured output to reduce error between a target and a process variable.

The definitions summarise those relationships; they do not replace the control-path and failure evidence developed above.
Disturbance: A factor outside the controller command that changes process output.
Loop placement: The architectural boundary that determines where sensing, control, actuation, and fallback behavior run.

Major section

Overview: Loop Choice Is A Risk Boundary

An open-loop design promises that the command plan is good enough until inspection.

A closed-loop design promises that a measured process variable can safely change the next actuator command.
The risk boundary is the moment the process drifts.
If the drift is slow, visible, and low consequence, open loop may be acceptable.

Major section

Hybrid Loop Record

Many IoT systems should not be labeled simply open loop or closed loop.

They combine a baseline schedule, local correction, remote setpoint updates, alarms, and fallback states.

Major section

Under the Hood: Delay Can Destabilize a Closed Loop

A feedback path can remain closed even when delay makes the response too slow to regulate safely.

Delay does not merely make a loop slower.
The local controller can keep a bounded state near the process, while the slower layer changes policy, records evidence, and reopens the decision when operating conditions move outside the original proof.

Major section

Summary

Closed-loop systems measure output and correct error, which makes them better for drift, disturbances, and accuracy requirements.

Robust IoT designs often use a hybrid pattern: a baseline command plan, local feedback when measurements are valid, and documented fallback behavior when measurement or communication fails.

Deck summary

Key takeaways

The difference between open-loop and closed-loop control is easiest to see when a measurement disappears.

Closed-loop control measures the process variable, compares it with the setpoint, and adjusts the next command from the error.
The observer can inspect or log the result, but there is no arrow returning the measured output to the controller.
The choice is not "simple versus smart." It is a fit decision based on observability, consequence, timing, and expected variation.
A disturbance is any influence that changes the process output without being the controller command.

Retrieval practice

Recall check 1 of 3

Gateway Gus says: answer from memory, then check your reasoning.

Q1A fan must hold cabinet temperature near a target while heat load varies unpredictably as processors wake and sleep. What is the best first loop choice?

AOpen loop, because a fixed fan schedule is simpler to build and inspect

BOpen loop, because processor heat load is fully predictable during operation

CNo control loop, because cabinet temperature self-regulates under varying load

DClosed loop, because measured temperature shapes the next fan command

Show answer

Answer: D A varying heat load needs feedback correction, so a closed loop that adjusts the fan from measured temperature is the right choice.

Retrieval practice

Recall check 2 of 3

Gateway Gus says: answer from memory, then check your reasoning.

Q2A fan must hold cabinet temperature near a target while heat load changes when processors wake and sleep. Which control shape is the best first loop choice?

AClosed loop, adjusting fan command from measured cabinet temperature.

BUse a fixed open-loop fan speed because one setting covers the expected heat load.

COpen loop with no owner because fan control is outside architecture.

DRemote-only loop, delaying fan changes until central history arrives.

Show answer

Answer: A When the output must stay near a target despite changing load, measured feedback should influence the next actuator command.

Retrieval practice

Recall check 3 of 3

Gateway Gus says: answer from memory, then check your reasoning.

Q3A refrigerated cabinet runs a fixed fan schedule, but door openings now vary by hour. The cloud service can suggest setpoints, yet network delays are sometimes several seconds. Which design record best fits the risk?

AUse a hybrid record: baseline fan plan, local temperature loop, slower cloud setpoints, and stale-command/sensor-fault fallback.

BUse remote-only closed loop because cloud feedback is valid once a sensor reading arrives, even with seconds of delay.

CStay open loop with no measurement proof because the old schedule already cooled one operating profile.

DRemove the baseline schedule so measurement or communication failure leaves no bounded behavior.

Show answer

Answer: A A hybrid loop record separates baseline command behavior, local closed-loop correction, supervisory setpoint changes, and fallback rules so delay or stale measurements do not silently undermine stabilizing control.

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Answers

Answer key.

D · A varying heat load needs feedback correction, so a closed loop that adjusts the fan from measured temperature is the right choice.
A · When the output must stay near a target despite changing load, measured feedback should influence the next actuator command.
A · A hybrid loop record separates baseline command behavior, local closed-loop correction, supervisory setpoint changes, and fallback rules so delay or stale measurements do not silently undermine stabilizing control.