Control systems are the hidden decision makers inside many engineered devices, from thermostats and washing machines to robots and aircraft. An open-loop controller sends commands without checking whether the output actually reached the goal. A closed-loop controller measures the output and uses feedback to reduce the difference between the desired and actual result.
Understanding the difference matters because feedback can improve accuracy, reject disturbances, and change how stable a system is.
Key Facts
- Open-loop control: input command -> controller -> actuator -> process -> output, with no measurement returned.
- Closed-loop control: reference -> controller -> process -> output, and a sensor feeds output information back to compare with the reference.
- Error signal: e(t) = r(t) - y(t), where r(t) is the desired reference and y(t) is the measured output.
- Negative feedback acts to reduce error by adjusting the control input in the direction that brings y(t) closer to r(t).
- Simple proportional control: u(t) = Kp e(t), where larger Kp usually gives faster response but can cause overshoot or instability.
- Open-loop systems are often cheaper and simpler, while closed-loop systems are usually more accurate but require sensors, tuning, and stability checks.
Vocabulary
- Open-loop control
- A control method that applies a command without measuring the actual output or correcting errors.
- Closed-loop control
- A control method that measures the output and uses feedback to adjust the command.
- Feedback
- Information about a system output that is returned to the controller to influence future action.
- Disturbance
- An outside effect that pushes a system away from its desired behavior.
- Stability
- The ability of a system to settle to a bounded, controlled response instead of growing or oscillating uncontrollably.
Common Mistakes to Avoid
- Calling any automatic system closed-loop, because automation alone does not mean the output is measured and used for correction.
- Ignoring sensor error, because a closed-loop controller can only correct based on the measurement it receives, not the true value directly.
- Assuming higher gain is always better, because too much gain can create overshoot, oscillation, or instability.
- Forgetting disturbances in open-loop analysis, because an open-loop controller cannot detect or correct changes such as load, friction, wind, or temperature loss.
Practice Questions
- 1 A motor speed controller has a reference speed of 1200 rpm and a measured speed of 1110 rpm. Calculate the error e = r - y.
- 2 A proportional controller uses u = Kp e with Kp = 0.8 V/°C. If a thermostat setpoint is 22°C and the measured room temperature is 18°C, what control voltage u is produced?
- 3 A toaster runs its heater for a fixed time, while an oven thermostat measures temperature and turns heating on or off. Identify which is open-loop and which is closed-loop, and explain why feedback matters for each case.