Control Systems: What Are They? (Open-Loop & Closed-Loop Control System Examples)

What is a Control System?

A control system directs a plant or process towards a stated objective. It can include a command, controller, actuator, plant and measurement. Some control systems use feedback to compare the measured output with a reference, while others act without measuring the output.

The definition of a control system therefore depends on its purpose and signal path, not simply on whether it is automated. A human operator can close a feedback loop by observing an output and adjusting an input. An automatic controller can also run open loop when it follows a fixed command without measuring the controlled result.

Control systems are common in appliances, vehicles, manufacturing, power networks and robotics. Engineers use them to regulate quantities such as temperature, speed, position, pressure, voltage and current.

Everyday examples include a thermostat-controlled air conditioner, a refrigerator, a toilet cistern, an electric iron and a vehicle cruise-control system. Each example has a different plant, controlled quantity and performance objective.

Industrial examples include production machinery, transport systems, power converters, power-system controls, spacecraft and robots. Their controllers may use continuous signals, sampled data, logic or a combination of these methods.

The principles of control theory apply when a system’s inputs, outputs, dynamics and objectives can be defined. You can review related terminology with our control system MCQs.

Features of a Control System

A useful model of a feature of a control system relates inputs, internal states and outputs over time. Engineers may express that model with differential equations, state-space equations or transfer functions.

A system is linear when its model satisfies superposition: scaling and adding inputs produces the same scaling and addition in the response. A linear model can still contain dynamics, delays and several state variables.

A nonlinear system does not satisfy superposition across the operating range of interest. Saturation, dead zones, backlash, products of variables and nonlinear device characteristics can all produce nonlinear behaviour.

Requirements of a Good Control System

Accuracy: Control accuracy describes how closely the output follows its reference under stated conditions. Sensor accuracy is one contributor, but modelling error, disturbances, actuator limits and controller design also affect the result.

Negative feedback can reduce tracking error where the loop gain is high enough, but it does not guarantee accuracy at every frequency. A comparison element can act as an error detector by subtracting measured feedback from the reference.

Sensitivity: Sensitivity measures how a closed-loop response changes when a plant parameter, disturbance or other model quantity changes.

Feedback can reduce sensitivity to selected plant variations within part of the frequency range. Designers must specify which changes matter because no practical loop is insensitive to every uncertainty while responding to every command.

Noise: Noise is an unwanted signal in a measurement or control path. A controller should limit its effect on the output and actuator without removing the response needed for useful commands.

Stability must be assessed for the chosen model and operating point. Bounded-input, bounded-output stability means every bounded input produces a bounded output when initial conditions are zero. A system can still have a nonzero zero-input response when stored energy or nonzero initial conditions are present.

Bandwidth: Closed-loop bandwidth indicates the frequency range over which a system meets a stated response level. A wider bandwidth can improve speed, but it can also increase noise response, control effort and sensitivity to unmodelled high-frequency dynamics.

Speed: Rise time and settling time describe different parts of a transient response. A faster response is useful only when overshoot, actuator limits, noise and stability margins remain acceptable.

Oscillation: Overshoot and ringing can reveal lightly damped dynamics. A sustained oscillation does not by itself demonstrate stability, so engineers check closed-loop poles or frequency-domain stability margins.

Types of Control Systems

There are several ways to classify types of control systems. One basic classification asks whether the control action uses a measurement of the controlled output. Position, speed, temperature, pressure, voltage and current can each be controlled with either architecture.

Consider a room heater that produces heat whenever its power supply is switched on. The room is the plant, heater power is the manipulated input, and room temperature is the controlled output.

If a person watches a thermometer and switches the heater off at the desired temperature, the person uses measured output to decide the input. This is a manual closed-loop controller.

When the room cools, the person can switch the heater on again after observing the temperature. The manual control system remains closed loop because its control action depends on feedback, even though a person performs the action.

A timed switch can instead run the heater for preset intervals. That automatic system is open loop for room temperature because it does not measure whether the room is too warm or too cold. Its result changes with outdoor temperature, heat loss and other disturbances.

A thermostat provides closed-loop control. Its sensor measures room temperature, and the controller compares that measurement with the desired value.

The controller switches or modulates the heater to reduce the temperature error. A simple thermostat often uses upper and lower thresholds so that it does not switch rapidly near one temperature.

The timer and thermostat are both an automatic control system, but they have different loop structures. The timer follows a schedule independent of measured room temperature. The thermostat bases its action on that output.

The timed heater therefore illustrates an open-loop control system for temperature. A different sensor might control a safety limit or heater current, but that would not by itself close the room-temperature loop.

In the thermostat case, the control action depends on the difference between desired and measured room temperature. With a negative-feedback sign convention, this difference is the error signal.

The controller acts on the error and changes heater power. Because the measured output affects the control input through a feedback path, this is a closed-loop control system.

Based on whether the controlled output is fed back, there are two main types of control systems:

1. Open-loop control systems
2. Closed-loop control systems

Open Loop Control System

An open-loop control system chooses its control action without using a measurement of the controlled output. Manual operation is not automatically open loop; a person who observes the output and corrects the input forms a feedback path.

The figure below shows a control system block diagram for an open-loop arrangement. The command passes through the controller and plant, but the measured process output does not return to determine the control action.

Practical Examples of Open Loop Control Systems

Examples of open-loop control systems in daily life include:

1. Electric Hand Drier – A presence sensor can close the hand-detection loop, while dryness remains open loop if the machine never measures how dry the hands are.
2. Automatic Washing Machine – A fixed-time programme is open loop for washing result, although modern machines may use feedback for water level, motor speed, load or imbalance.
3. Bread Toaster – A timer-controlled toaster is open loop for browning because it stops after a set time without measuring toast colour.
4. Automatic Tea/Coffee Maker – A fixed-time brewing cycle is open loop for drink strength when it does not measure the resulting concentration.
5. Timer Based Clothes Drier – A timed dryer is open loop for moisture because it stops after a preset duration rather than measuring how dry the clothes are.
6. Light Switch – A basic switch applies the user’s command without measuring room illumination, so the lighting level is controlled open loop.
7. Volume on Stereo System – A fixed volume setting is open loop for sound level if no microphone measures the acoustic output.

Advantages of Open Loop Control Systems

Advantages of open-loop control systems include:

1. It can use fewer sensors and signal-processing components.
2. It can cost less when output measurement would be expensive.
3. Its simpler architecture can make maintenance easier.
4. It avoids instability caused by closing a poorly designed feedback loop, although the plant itself can still be unstable.
5. It can be practical when the output is difficult to measure and the plant is predictable enough for the task.

Disadvantages of Open Loop Control System

Disadvantages of open-loop control systems include:

1. Accuracy depends strongly on the plant model and calibration.
2. Unmeasured disturbances and plant changes can shift the output away from its target.
3. The controller cannot automatically correct an output error that it does not measure.

Closed Loop Control System

A closed-loop control system measures the controlled output or a related state and uses that information to determine its control action. The feedback path closes the signal loop around the controller and plant.

An open-loop control system can become closed loop when a suitable sensor, comparison rule and controller make the measured output affect the input. Well-designed negative feedback can improve tracking and disturbance rejection over a specified frequency range.

A closed loop can be automatic or can include a human operator. The figure below shows an automatic closed-loop arrangement in which a measured output returns through a feedback path and affects the controller input.

Practical Examples of Closed Loop Control System

Examples of closed-loop control systems in daily life include:

1. Automatic Electric Iron – A thermostat measures or responds to soleplate temperature and switches the heating element.
2. Servo Voltage Stabilizer – Its controller uses measured output voltage to adjust the regulator.
3. Water Level Controller – A level measurement controls the inlet flow or pump to keep the reservoir near its target.
4. Missile Launched and Auto Tracked by Radar – A guidance loop can compare estimated target and vehicle states, then command steering corrections.
5. An Air Conditioner – A thermostat compares measured room temperature with its setpoint and commands heating or cooling.
6. Cooling System in Car – A temperature sensor can control a fan or coolant-flow actuator according to engine temperature.

Advantages of Closed Loop Control System

Advantages of closed-loop control systems include:

1. Feedback can improve tracking accuracy within the controller’s designed operating range.
2. Measured output errors and some disturbances can be corrected while the loop has adequate gain and actuator authority.
3. The controller can shape closed-loop bandwidth to meet a stated speed and noise trade-off.
4. Measured feedback supports automatic regulation when operating conditions change.
5. Negative feedback can reduce sensitivity to selected plant-parameter changes over part of the frequency range.
6. A suitable loop can reject some plant-input and output disturbances, although sensor noise needs separate analysis.

Disadvantages of Closed Loop Control System

Disadvantages of closed-loop control systems can include:

1. Sensors, actuators and processing can increase cost.
2. Loop dynamics, uncertainty, delays and actuator limits make design more involved.
3. Sensors and actuators may need calibration and maintenance.
4. Poor damping or inadequate stability margins can cause ringing or oscillation.
5. High performance can demand more actuator effort or amplify measurement noise in some frequency bands.
6. Closing a loop can improve or reduce stability, so the designer must check closed-loop poles and stability margins.

Open Loop vs Closed Loop Control Systems

The table below compares open loop and closed loop control systems.

Sr. No.	Open Loop Control System	Closed Loop Control System
1	Control action does not use the measured controlled output.	Control action uses the measured output or a related state.
2	An output-error comparison is not required.	A reference and feedback measurement are typically compared.
3	Stability depends on the plant and command path.	Stability depends on the complete loop dynamics.
4	It can use fewer components.	It needs a measurement and feedback path.
5	It can cost less when measurement is difficult.	Additional sensors and processing can increase cost.
6	Response bandwidth follows the plant and command shaping.	Feedback can shape bandwidth within physical limits.
7	Accuracy depends on calibration, modelling and disturbances.	Feedback can reduce error within its designed range.
8	Fewer components can reduce maintenance.	Sensors and actuators may need calibration and maintenance.
9	Reliability depends on the plant and required performance.	Reliability depends on loop design and component integrity.
10	Examples: fixed-time toaster, timer-based dryer	Examples: thermostat, servo voltage stabilizer

Feedback Loop in a Closed Loop Control System

Feedback is a common design method in a control system. A measurement of the output or state returns to a comparison point, allowing the controller to act on the difference between the reference and feedback signals.

Disturbances, plant changes and command changes can affect the output. In a feedback system, a sensor measures a relevant quantity and returns a signal through the feedback path.

The summing junction combines the reference and feedback signals to produce an error signal for the controller. The figure below shows a feedback-system block diagram.

Under the sign convention shown, positive feedback adds the feedback signal to the reference, so the error relation is E = R + B. The actual sign must always be read from the summing junction.

For negative feedback under the same convention, the feedback signal is subtracted from the reference and E = R – B. If the forward path is G and feedback path is H, the reference-to-output transfer function is G/(1 + GH).

Effect of Feedback in a Control System

The following labels apply to the figure below:
R = Input signal
E = Error signal
G = Forward path gain
H = Feedback
C = Output signal
B = Feedback signal

Feedback has the following effects on a control system:

1. Negative feedback can reduce tracking error and selected disturbances where loop gain is high enough.
2. The closed-loop gain is G/(1 + GH) for negative feedback and G/(1 – GH) for positive feedback under the stated sign convention.
3. For negative feedback, the sensitivity function is 1/(1 + GH), so high loop gain can reduce sensitivity to selected plant changes in that frequency band.
4. Feedback can improve or reduce stability. Closed-loop poles, gain margin and phase margin must be checked for the actual loop.