A time-of-flight sensor helps a robot measure distance by sending out light and timing how long the reflection takes to return. In many robotics systems, the light is infrared, so it is invisible to humans but easy for electronics to detect. This matters because robots need reliable distance information to avoid obstacles, dock with chargers, follow walls, and map nearby spaces.
ToF sensing is especially useful at short range because it can be compact, fast, and precise.
Key Facts
- Distance from round-trip time: d = cΔt/2, where c is the speed of light and Δt is the total travel time.
- Speed of light in air is approximately c = 3.0 x 10^8 m/s.
- Many ToF sensors use modulated infrared light and measure phase shift instead of directly timing a tiny pulse.
- Phase-based distance idea: larger phase shift means the reflected light took longer to return.
- Single-point ToF sensors measure one distance along a narrow field of view, while ToF arrays measure many points to form a depth map.
- Short-range ToF sensors often work best from a few centimeters to a few meters, depending on power, optics, reflectivity, and ambient light.
Vocabulary
- Time-of-flight sensor
- A sensor that measures distance by sending out a signal and measuring how long its reflection takes to return.
- Infrared light
- Electromagnetic radiation with wavelengths longer than visible red light, often used by ToF sensors because it is invisible and easy to generate.
- Modulation
- A controlled change in a signal, such as varying light intensity over time, that helps a sensor identify its own reflected light.
- Phase shift
- The difference in timing between a transmitted wave pattern and the returned wave pattern.
- Depth map
- A grid of distance measurements that represents the 3D shape of nearby objects.
Common Mistakes to Avoid
- Forgetting the factor of 2 in d = cΔt/2 is wrong because the measured time includes the trip to the object and the trip back to the sensor.
- Assuming infrared light is slow is wrong because infrared light is still light and travels at approximately 3.0 x 10^8 m/s in air.
- Treating all surfaces as equally easy to detect is wrong because dark, angled, transparent, or shiny objects can reflect less usable light back to the sensor.
- Confusing a single-point ToF reading with a full 3D image is wrong because one detector gives one distance, while an array is needed to build a depth map.
Practice Questions
- 1 A ToF sensor measures a round-trip light time of 10 ns. Using c = 3.0 x 10^8 m/s, what is the distance to the object?
- 2 A robot is 1.2 m from a wall. What round-trip time should its ToF sensor measure for the reflected light? Use c = 3.0 x 10^8 m/s.
- 3 A robot drives toward a black fabric curtain and its ToF readings become noisy. Explain why the surface can affect the measurement and name one design change that could improve reliability.