What is a control system?
A control system is a set of parts that makes a machine or process do what we want, by itself.
We draw it as a block diagram: input → process → output. Information (signals) flows along the arrows.
- Input: what we ask for, or what is measured (a button, a set temperature, a light level).
- Process / controller: decides what to do.
- Output: what happens (a motor turns, a lamp lights, a heater warms).
Open-loop control
In open-loop control the controller does not measure the result. It follows fixed instructions.
Examples: a toaster on a timer, a simple washing-machine program, traffic lights on a fixed timetable, a microwave.
Good: simple and cheap. Bad: cannot correct itself if something changes (a disturbance), like thicker bread or more traffic.
Closed-loop control and feedback
In closed-loop control a sensor measures the output and sends the value back. This is feedback.
The controller compares the measured value with the set point (the target). Error = set point − measured value. It then acts to make the error small.
Negative feedback reduces the error and keeps things steady (thermostat, cruise control, the body keeping 37 °C). Positive feedback makes a change bigger (a microphone squealing near a speaker).
Simple controllers are on/off (heater fully on or off, so the temperature wobbles). Smarter ones change the output smoothly in proportion to the error (proportional control, and PID in industry).
Sensors, controllers and actuators
- Sensors turn a physical quantity into an electrical signal: thermistor (temperature), LDR (light), ultrasonic (distance), switch, infrared.
- Controllers: a transistor or comparator circuit, a microcontroller board (like Arduino or micro:bit) running a program, or a PLC (programmable logic controller), a tough computer for factories.
- Actuators do the work: motors, servos, relays, heaters, lamps, buzzers, solenoid valves.
- Pneumatics uses compressed air: a compressor, valves and cylinders that push and pull. Used in bus doors, factory grippers and dentist chairs. Hydraulics does the same with oil for bigger forces.
Designing a control system: state the need → choose the input sensor → decide the control rule (often as a flowchart or program) → choose the actuator → build → test and improve.
Programmed control, automation, IoT and SCADA
Automation means machines doing a process with little human help, like a bottling line. The control rule is written as a program (flowchart, blocks or code: IF temperature < 20 THEN heater ON).
IoT (Internet of Things): sensors and actuators connected to the internet, so a smart plug or a farm pump can be checked and switched from a phone.
SCADA (Supervisory Control And Data Acquisition) collects data from far-away sites by telemetry (remote measurement) and shows it on control-room screens for power grids, pipelines and water supply.
New trends: AI that learns better control, robots working with people, digital twins (a computer copy of a factory). Networked control also needs cyber security.
Try it
Be the thermostat: fill a cup with warm water and hold a thermometer in it. Your target is 40 °C. Add a little hot or cold water only when the reading is off. You are the sensor, the controller and the actuator. Then try the 3D free play with feedback on and off.
Key formulas and definitions
- Input → Process → Output
- Error = Set point − Measured value
- Open loop: no feedback; Closed loop: feedback from a sensor
- On/off rule: IF measured < set point THEN output ON ELSE OFF
- Sensor → Controller → Actuator
Worked examples
1. A street light turns on at 7 pm every day by a timer. Open or closed loop?
Open loop. It does not measure how dark it is. If it is cloudy at 5 pm, it stays off.
2. Change the street light so it is closed loop. Name the sensor, controller and actuator.
Sensor: LDR measures light level. Controller: a comparator circuit or microcontroller that switches when light is below a threshold. Actuator: the lamp (through a relay). It now responds to real darkness.
3. A room heater has set point 22 °C. The sensor reads 19 °C. Find the error and say what the on/off controller does.
Error = 22 − 19 = 3 °C (positive, too cold). The controller switches the heater ON until the reading reaches 22 °C, then OFF.
Common mistakes
- Thinking any automatic machine is closed loop. If nothing measures the output, it is open loop.
- Mixing up sensor and actuator. Sensors measure (input); actuators move or change things (output).
- Writing error as measured − set point and getting the sign wrong. Use set point − measured.
- Thinking positive feedback means 'good'. It means the change is made bigger, which often makes a system unstable.