Sensors and actuators are the basic parts that let engineering systems interact with the real world. A sensor measures a physical quantity such as temperature, light, position, or pressure and turns it into a usable signal. An actuator takes a control signal and produces a physical effect such as motion, force, heat, or flow.
Together they make automation, robotics, vehicles, medical devices, and smart appliances possible.
In a control system, the sensor sends information to a controller, which compares the measured value to a desired target and decides what action to take. The actuator then changes the system, and the sensor measures the result again, creating feedback. This loop helps systems stay accurate, stable, and responsive even when conditions change.
Engineers choose sensors and actuators based on range, sensitivity, speed, power, accuracy, and cost.
Understanding Sensors and Actuators
Different sensors use different physical effects. A thermistor changes its electrical resistance as temperature changes. A light dependent resistor responds to light in a similar way.
A microphone contains a thin moving surface that creates a tiny changing voltage when sound waves move it. An ultrasonic distance sensor sends out a sound pulse, then measures the time until an echo returns. A position encoder often uses patterns of light or magnetic fields to count movement.
The controller cannot always use a raw sensor signal directly. Small signals may need amplification.
Noisy signals may need filtering. An analogue signal may need an analogue to digital converter before a microcontroller can read it.
A sensor reading is never perfectly true. Accuracy describes how close a reading is to the real value. Precision describes how consistently a sensor gives the same reading.
Resolution is the smallest change it can detect. A bathroom scale may show changes of one tenth of a kilogram, yet still be offset by half a kilogram. Calibration finds and corrects this offset by comparing the sensor with a trusted reference.
Engineers must consider drift too. Drift is a slow change in readings caused by age, heat, moisture, or repeated use. This matters in weather stations, medical equipment, and factory measurement systems.
Actuators often need much more electrical power than the controller can supply. A microcontroller pin can send a small control signal, but it cannot directly run a large motor. A driver circuit acts as the power stage between them.
It may use a transistor, relay, or motor driver chip. Motors can draw a high current when starting or when blocked. This can overheat wires or damage components if the circuit is poorly designed.
A diode is often placed across a relay coil or motor circuit to handle the voltage spike created when current is switched off. Servos use built in electronics to move toward a commanded angle, while stepper motors move in counted steps.
Feedback must be designed carefully because a system can react too slowly or too strongly. A room thermostat with a long delay may heat the room beyond its target before the sensor notices. If its response is too aggressive, the heater may switch rapidly on and off.
This wastes energy and wears out parts. Engineers may add a small allowed band around the target so the system does not react to every tiny change. Students meet these ideas in automatic doors, phone screen brightness, washing machines, game controllers, drones, and car parking sensors.
When building a project, test the full system under realistic conditions. Check sensor placement, moving friction, battery voltage, response delay, and safe limits for force, temperature, or speed.
Key Facts
- Sensor: physical input -> electrical signal; actuator: electrical signal -> physical output.
- A basic control loop is: measured value -> controller -> actuator -> plant -> sensor -> measured value.
- Control error is e = setpoint - measured value.
- For a proportional controller, control signal u = Kp e.
- Electrical power used by many actuators is P = VI.
- Common sensor outputs include voltage, current, resistance, capacitance, or digital data.
Vocabulary
- Sensor
- A sensor is a device that detects a physical quantity and converts it into a signal that can be measured or processed.
- Actuator
- An actuator is a device that receives a control signal and produces a physical action such as movement, force, or heating.
- Controller
- A controller is the part of a system that uses sensor data to decide how the actuator should respond.
- Feedback
- Feedback is the process of using the system's measured output to adjust its behavior and reduce error.
- Setpoint
- A setpoint is the desired target value that a control system tries to maintain.
Common Mistakes to Avoid
- Confusing sensors with actuators, because sensors only measure conditions while actuators create physical changes in the system.
- Ignoring units in sensor signals, because a voltage reading, temperature value, and pressure value are not interchangeable and must be interpreted correctly.
- Assuming bigger actuator power always gives better control, because too much force or speed can cause overshoot, instability, or wasted energy.
- Forgetting the feedback path, because without comparing the measured output to the setpoint a system cannot automatically correct errors.
Practice Questions
- 1 A temperature controller has a setpoint of 22 degrees C and the sensor reads 18 degrees C. Find the control error e = setpoint - measured value.
- 2 An electric actuator operates at 12 V and draws 2.5 A. Calculate its electrical power using P = VI.
- 3 A room gets colder when a window is opened. Explain how a closed-loop heating system with a temperature sensor and heater actuator responds differently from an open-loop system.