Control Systems

Explain what a control system is using a supermarket automatic door: what it detects, what it decides, and what it does, and why we say it works "without anyone flicking a switch".

Every control system has three parts: inputs, control unit, and outputs. Explain what each one does and give an example of all three in a remote-controlled fan.

The figure shows the general chain of a control system, with its central link covered.

Name the missing link, explain what it does with the information it receives from the inputs, and describe a complete control system from your daily life, identifying its three parts.

State what quantity each of these input sensors measures: a motion sensor, an LDR, a thermometer, and a push button, and name a household appliance that uses each one.

State what each of these output actuators does: a motor, a light bulb, a buzzer, and a door lock electromagnet, and describe a situation in which the same system needs two of them at the same time.

Compare a tap that you open and close by hand with a heating thermostat. Explain which of the two checks the result of its action (whether the house is already warm) and what it means that this system "corrects itself".

The figure shows two block diagrams of control systems, A and B. Only one of them feeds information from the output back to the input. In type B, the circle is a point that compares the input with what arrives along the path marked with ?.

1. a) Say which of the two systems is open-loop and which is closed-loop.

1. b) Name what travels along the path marked with ?.

1. c) Give an everyday example of each type of loop.

State whether these statements are true or false and justify each answer: (a) a traffic light that changes based only on time is a closed-loop system; (b) a washing machine measures water and temperature, so it has feedback; (c) in a closed loop, the system checks what it has achieved; (d) a fan without a sensor automatically corrects the room temperature.

The figure shows the block diagram for heating control in a house.

The covered element compares the desired value with the actual value. Explain which two pieces of data it compares, what command it sends to the boiler when the house is at $19\celsius$ and when it is at $23\celsius$, and why this system is closed-loop.

A traditional traffic light changes based on time, without knowing if there are cars waiting. Explain why it is an open-loop system, what happens in a traffic jam or in the early morning, and what sensors you would add to make it closer to a closed-loop system, describing the change in behavior.

Classify these four devices as open-loop or closed-loop and justify each one: a timed toaster, an iron with a thermostat, a fan turned on manually, and air conditioning with a set temperature.

Analyse the washing machine as a control system: list two inputs (water level, temperature, time), the control unit (its program), and two outputs (drum motor, heating element, solenoid valve). Explain why it is a closed-loop system even if you do not see it "think".

Draw (describing block by block) the diagram of an automatic garage door: what input activates it (remote control or sensor), what the control unit does, what output moves the door, and what safety sensor prevents it from closing if there is something underneath. Is it open-loop or closed-loop in each of its two functions?

You can do it on paper and send a photo with your answer.

Topic challenge. When you adjust the temperature of the shower, you yourself work as a control loop. Analyze that process and answer:

1. a) Identify the sensor: which part of your body detects the water temperature.

1. b) Identify the setpoint: what value or reference you want to achieve.

1. c) Identify the actuator: which part of your body changes the water that comes out.

1. d) Identify the feedback: what you do after acting to check the result and decide whether you need to correct again.

1. e) Explain what happens when someone opens another tap in the house while you are showering (for example, the water temperature or flow changes) and how the control loop reacts to correct it.

What will be assessed: that you correctly identify each element of the loop, that you relate it to the shower and that, in part e), you explain the disturbance and the correction you make.

In a doorbell, the push button and the chime play different roles. Classify each as an input or output, explain why the doorbell is not a feedback system, and what would turn it into one (for example, a light confirming to the visitor that it has rung).

Automated systems provide safety, convenience, and savings. Give a real example of each advantage (a barrier that opens automatically, automatic stair lighting, scheduled drip irrigation) and explain in one of them what would happen if the system failed at the worst possible moment.

A farmer claims: "I have programmed a timer so that the sprinklers water every day at 7:00 for 20 minutes; since it works autonomously without my intervention, it is a perfect closed-loop automatic system."

1. a) Refute his claim by explaining why his system is actually an open-loop one and what two serious water-efficiency or plant-health problems it will cause throughout the year.
2. b) Redesign the system to turn it into a closed-loop system: draw the block diagram clearly identifying the setpoint, the measured signal, the necessary sensor, the controller, the actuator, and the process.
3. c) Predict and justify how each of the two systems will react if an unexpected storm of 30 l/m² occurs right at 6:55.

In an apartment building, the staircase lighting is controlled by a staircase timer switch: when a push button is pressed on any floor, the lights stay on for exactly 90 seconds.

1. a) Justify using control systems theory why this timer operates in an open loop and why it is inefficient in opposite real-world situations (a resident who takes 3 minutes to go up carrying heavy bags versus another who goes down in 15 seconds).
2. b) Design a feedback solution that solves both problems. Specify which sensor or combination of sensors you would use, what strategic locations you would place them in to avoid "blind spots", and what the turn-off logic would be.
3. c) Predict what failure your design might experience if a person remains completely still on the landing searching for their keys in the dark, and propose how you would program the controller to prevent them from being left in the dark.

The sales brochure for a digital programmable thermostat states: "Replace your old rotary analogue thermostat with this programmable model: by incorporating hourly and weekly programming, your boiler will go from being an open-loop system to a closed-loop one, achieving up to 30% savings."

1. a) Critically evaluate the brochure's technical claim: is it true that a classic rotary thermostat operates in an open loop? Justify whether changing the device modifies the loop topology or merely adds new variables to the setpoint.
2. b) Draw the block diagram of the system with the programmable thermostat, indicating all inputs processed by the control unit (physical and programming), the comparator, the actuator, and the feedback.
3. c) Justify in detail through what control mechanism the programmable thermostat manages to reduce energy consumption during a day when the home remains empty from 8:00 to 17:00.

A pedestrian traffic light with a push button has been installed on an avenue. A Technology student argues: "Since the traffic light has a push button through which it receives information from the environment to change to red, it is a closed-loop system."

1. a) Justify your refutation of the student's statement: explain the fundamental difference between an "external activation signal" and a "feedback signal" at this crossing.
2. b) Draw the block diagram of the current traffic light identifying the input, the controller, and the outputs (both visual and acoustic).
3. c) Design a technical modification that turns the crossing into a true closed-loop system capable of adapting the duration of the pedestrian green phase to the actual speed of the people crossing (for example, elderly people or people in wheelchairs). What sensor would you add, what would it measure, and how would the timing vary?

You are tasked with the logical design of the control system that regulates an intersection between two perpendicular streets (Street A and Street B), each with its own traffic light.

1. a) Model the intersection as a single multivariable control system: explain why it is unacceptable to program two independent traffic lights with uncoupled timers and what mutual interlock logical condition must be imposed between the outputs.
2. b) Assume that a current sensor detects that the red lamp on Street A has burnt out. Analyse the risk this poses and indicate what immediate protocol the control unit must execute on both streets.
3. c) Justify technically whether, in the event of a major control unit failure, it is safer for the hardware circuit to switch to flashing amber in all directions or to solid red in all directions, taking into account traffic flow and the highway code.

A popular science article states: "Any automatic device operates via a closed loop, since it performs its job autonomously without a person intervening in each cycle."

1. a) Prove that this statement is false by proposing two examples of everyday automated systems: one operating in an open loop and another in a closed loop. Justify the classification of each in detail based on the presence or absence of feedback.
2. b) For the open-loop system you proposed, describe a specific external disturbance that causes faulty operation or energy waste.
3. c) Design the necessary modifications (sensors, physical quantity to measure, and new decision logic) to transform that open-loop system into a closed-loop system capable of correcting that disturbance.

You must design the bioclimatic control system for a high-tech greenhouse where sensitive plants are grown. The enclosure needs to maintain a temperature between 20 °C and 24 °C and a relative humidity between 60 % and 70 %.

1. a) Identify the necessary sensors and the actuators you must install to control the enclosure's air and ventilation.
2. b) During a hot and very dry summer midday, the temperature rises to 31 °C and the humidity drops to 45 %. Explain the operational conflict that arises if the temperature loop and the humidity loop act simultaneously and decoupled from each other.
3. c) Design the control strategy (using a flowchart or pseudocode with priority conditions) that the microcontroller must execute to resolve this conflict, ensuring the survival of the plants.

A vehicle is travelling along a motorway with the cruise control set to 120 km/h.

1. a) Draw the block diagram of the system, identifying: setpoint, error signal, controller, actuator, plant or process, output variable, sensor, and feedback path.
2. b) Analyse the system's response when the car unexpectedly encounters an uphill slope of 7 %: describe step by step how the measured variable changes, what calculation the comparator performs, and how the actuator reacts to compensate for the disturbance.
3. c) From the point of view of machine safety and critical control systems, justify why any pressure on the brake pedal must instantly interrupt the setpoint via a direct hardware interrupt rather than through a routine check in the microcontroller's program.