Time of Flight (ToF) Sensors in the Automotive World
How sophisticated ranging and depth mapping enabled by ToF sensing opens a new generation of robotic and autonomous devices
The automotive world has begun to introduce Time-of-Flight (ToF) sensors for ranging and depth mapping into a number of applications in recent years. Naturally, many of these have been in systems for assisted and autonomous driving. But in fact there are many other applications in which the third dimension provided by ToF sensing enhances the application’s operation.
Why ‘third dimension’? Conventional video cameras capture flat, two-dimensional images of the surrounding world. While in principle distance information can be gathered from video images through the application of triangulation techniques, or by depth-of-field calculation, these operations are computationally intensive. They are also prone to error, as the system may be compromised by dim lighting or a lack of contrast. ToF sensors on the other hand can capture detailed distance measurements of multiple objects in their field-of-view to form a 3D depth map of the surrounding area.
ToF sensors work in tandem with their own Infrared (IR) light source which is invisible to the human eye. Illumination provided by a certified Class 1 Vertical-Cavity Surface-Emitting Laser (VCSEL) emitter is completely eye-safe. The provision of a source of illumination means that, unlike a conventional video camera, a ToF sensor-based system works at night as well as during the day. If the ToF sensor has narrowband sensitivity, using only a small part of the IR spectrum, the system is highly resistant to interference from sunlight.
ToF sensors record directly the actual distance of objects in their field-of-view without any intermediate steps required. The ToF sensor avoids the error which arises in video camera systems when a small object nearby looks the same as a large object which is far away. A ToF sensor avoids these ranging errors caused by scale effects, greatly reducing the need for image post-processing. This is a substantial advantage when implementing higher-level functions such as object and face detection, or motion prediction.
Melexis ToF sensors support operation over an extended temperature range. This means that they are suitable for applications in harsh environments, such as cave inspection, industrial process control, inventory and stock inspection, and various types of security-related equipment.
Building on new developments in industry, a promising application is intelligent automation: here, the ToF sensor complements the other sensors in industrial robots or autonomous vehicles on the factory or warehouse floor. An industrial robot which has ToF sensors for object detection and motion prediction can be made fully environment-aware, minimizing the risk of collisions with moving or static objects.
Another interesting use case outside the industrial or automotive spheres is the monitoring of office occupancy. The primary function of this might often be safety, for instance to support appropriate evacuation procedures in the event of a fire. But occupancy monitoring can also enable the optimization of lighting and heating to save energy and reduce cost. Applied on a larger scale, it could also be used in smart city applications.
Automation supported by ToF sensing also has uses in home automation - object detection, face recognition and ranging might be valuable in smart home products such as autonomous vacuum cleaners, robotic lawn mowers and automatic access control for pets.
Sensor Evaluation Kit
Melexis sensor evaluation kits (EVKs) provide a feature-rich base on which OEMs can start the development of their applications. For example, the EVK75027 is a third-generation ToF evaluation kit which features the MLX75027 sensor.
The kit consists of four stacked PCBs. From top to bottom, they are:
- Illumination board featuring 940nm VCSEL emitters providing a 110° horizontal field-of-view
- ToF sensor board featuring the MLX75027
- Interface board
- Processor board
It is possible to detach the top two boards from the bottom two by bypassing the board-to-board connection with an external cable. A graphical user interface provides live depth map visualization, and basic recording, analysis and configuration.
Other key features of the EVK75027 include:
- High-speed operation - up to 35 distance frames/s at full VGA resolution
- Modulation frequencies up to 100MHz
- MATLAB software development kit
- C application programming interface