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Sensors and Actuators
Detecting the World and Responding to It
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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.
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.