Robotics sensors help a robot detect distance, obstacles, surfaces, and motion in its environment. This cheat sheet covers three common sensing methods: LiDAR, ultrasonic, and infrared sensing. Students need these ideas to choose the right sensor, calculate distance, and understand why readings change in real robot systems.
It is designed as a quick classroom reference for building, testing, and troubleshooting robots.
Key Facts
- Time of flight distance is calculated with d = v t / 2 when a signal travels to an object and reflects back.
- For ultrasonic sensors in air, use the speed of sound v ≈ 343 m/s at 20°C, so d = 343 t / 2 in meters when t is in seconds.
- LiDAR usually uses light pulses, so its signal speed is approximately c = 3.00 x 10^8 m/s.
- Infrared distance sensors often estimate distance from reflected IR intensity or angle, so their output may be nonlinear.
- Sensor resolution is the smallest change in distance or signal that the sensor can reliably detect.
- Sensor range is the minimum to maximum distance over which the sensor gives useful readings.
- A narrow field of view gives more precise direction information, while a wide field of view detects objects over a larger area.
- Sensor fusion combines readings from multiple sensors to improve reliability, such as using LiDAR for mapping and ultrasonic sensing for close obstacles.
Vocabulary
- LiDAR
- LiDAR is a sensing method that uses laser light to measure distance and often creates maps of surrounding objects.
- Ultrasonic sensor
- An ultrasonic sensor measures distance by sending high-frequency sound waves and timing the returning echo.
- Infrared sensor
- An infrared sensor detects infrared light, often to sense nearby objects, lines, edges, or reflected light intensity.
- Time of flight
- Time of flight is the time a signal takes to travel from a sensor to a target and back or to another receiver.
- Field of view
- Field of view is the angular area a sensor can detect at one time.
- Calibration
- Calibration is the process of comparing sensor readings with known values and adjusting the system to improve accuracy.
Common Mistakes to Avoid
- Forgetting to divide by 2 in echo distance calculations is wrong because the measured time includes the trip to the object and the return trip.
- Using the speed of light for an ultrasonic sensor is wrong because ultrasonic sensors use sound waves, not light waves.
- Assuming IR sensors give perfectly linear distance readings is wrong because reflected infrared intensity depends on distance, surface color, angle, and sensor design.
- Ignoring surface material is a mistake because shiny, dark, soft, transparent, or angled surfaces can reflect or absorb signals differently.
- Trusting one noisy reading is a mistake because real sensors have random noise, so robots should filter, average, or confirm important measurements.
Practice Questions
- 1 An ultrasonic sensor measures an echo time of 0.020 s. Using v = 343 m/s, how far away is the object?
- 2 A LiDAR pulse returns after 40 ns. Using c = 3.00 x 10^8 m/s, calculate the distance to the object.
- 3 An IR sensor gives voltage readings of 2.8 V at 10 cm and 1.1 V at 30 cm. What does this suggest about how reflected IR changes with distance?
- 4 A robot must detect glass walls, dark furniture, and close obstacles in a hallway. Explain which sensor or sensor combination would be most reliable and why.