Sensors & actuators

You feel warmth with your skin, see light with your eyes and move with your muscles. Machines can do this too, just with different parts. Their senses are called sensors, their muscles are called actuators. Thanks to sensors, a lift knows which floor it is on, and thanks to actuators it travels upwards. Without both, every machine would be blind and motionless.

The senses of machines

A sensor is a component that turns a property of the environment into an electrical signal. A temperature sensor contains a material whose electrical resistance changes with heat. A light sensor lets more current through the brighter it is. A pressure sensor deforms a tiny bit under load and changes its signal as it does. There is a sensor for almost everything: humidity, acceleration, magnetic fields, distance, even for single gases in the air.

The measurement becomes a number

The signal from a sensor is at first just a voltage, for example 0 to 5 volts. So that a chip can work with it, an analog to digital converter translates the voltage into a number. In this example 0 volts becomes 0, 5 volts becomes 1000, and every voltage in between gets its own number, evenly spread across the range. Only now can a program compare and decide: if the value is above 700, it is too hot, so fan on. For machines, measuring always means turning the world into numbers.

Actuators, the muscles

Actuators take the reverse step: they turn an electrical signal back into an effect. A motor turns it into rotation, a loudspeaker into sound, a heater into warmth, an LED into light, a valve opens or closes a water path. Servo motors are especially useful: they rotate to exactly the angle the program sets and hold it. This is how robot arms, the steering of model cars and the flaps on aircraft wings move.

Working together: the control loop

Sensors and actuators become really powerful as a team. A thermostat measures the room temperature, compares it with the target value and switches the heating on or off. The heating changes the temperature, the sensor measures again, and everything starts over. Such a circle of measuring, deciding and acting is called a control loop. Your body works this way too: if you get too warm, your nerves report it and your skin starts to sweat.