Sensors
Sensors also called detectors will perform the data acquisition of your robot and will be its senses. Unlike actuators, you will find far more sensors type and most of them are really affordable. Let's name some: vision, light, motion, gravity, orientation, temperature, magnetic field, stretch, glucose, oxygen, humidity, smoke, moisture, vibration, electrical field, sound, etc... We don't have them all. We have focused on the most commons for robotics.