Time-of-Flight Sensors for In-Cabin Vehicle Safety

As vehicles become more sophisticated, they rely on effective sensing to allow the automated systems to assist drivers and, ultimately, to fully control them. This article looks at time-of-flight (ToF) sensing technology and how it can improve safety, comfort, and convenience.

While many different types of sensor are being deployed, non-contact sensing is one of the most important as it allows sensing of the vehicle’s environment, facilitating object avoidance and more. However, a recent focus has been put on in-cabin sensing of vehicle occupants for safety reasons.

 

Principles of ToF Sensing the the Vehicle Cabin

Time-of-Flight (ToF) sensing allows objects to be detected and provides information about their position and movement within a three-dimensional space. It also supports object identification by detecting object shape, size, and orientation.

The technology relies upon light being reflected by the object being detected with a light source such as an LED array or VCSEL device being used in conjunction with beam-forming optics to produce a light source that illuminates the scene. An advanced sensing IC detects and measures the light reflected from any object in range.

 

Figure 1. Block diagram of a typical ToF sensing system showing the light source, sensor, and companion chip.

 

Direct ToF vs. Indirect ToF

Light travels very rapidly at a known speed; by measuring the time taken for reflected light to reach the sensor (the ‘time of flight’ for the light), the distance of the object can be calculated. This approach is known as ‘direct ToF’ (dToF) as the time is calculated directly from a very accurate time base and is preferred for applications where long distances are involved, such as LIDAR. dToF is generally considered a low-resolution technology, requiring complex (and expensive) mechanical scanning to achieve high resolutions.

Instead of using absolute time, indirect ToF (iToF) calculates distance based on a phase shift to a known reference signal. This technique is far better suited to high-resolution applications and, with modern CMOS pixel arrays, can generate a real-time 3D video at QVGA resolutions and higher.

iToF uniquely combines medium- and high-resolution depth and amplitude (greyscale) mapping allowing complex identifications to be performed (such as people or objects), even with no color contrast between the object and its surroundings. These benefits make iToF very useful in many forward-looking applications where other technologies have limitations.

In relatively close proximity (around 5m) ToF allows objects and free spaces to be identified, meaning that ADAS systems can intelligently predict how an object may move (if at all) and take the appropriate avoiding action. Melexis iToF is relatively unaffected by changes in temperature and light to meet the challenges of external applications such as these.

In-Cabin Applications for ToF Sensing

One of the main drivers for achieving the ultimate goal of fully autonomous vehicles is to increase road safety. A recent report by the NHTSA estimated that over 90% of all accidents are due to driver errors, so eliminating these will make for much safer roads. While mass-produced fully automated vehicles remain some way in the future, iToF can make a significant contribution to road safety by monitoring the driver and his / her behavior.

Driver fatigue is a significant issue and the high resolution attainable with iToF is able to see if the driver has their eyes on the road ahead, whether they are yawning excessively, or even struggling to keep their eyes open. Detecting each of these and suggesting (or even enforcing) a break can potentially avoid accidents and save lives. Other driver behaviors such as not holding the steering wheel properly, eating while driving or using a mobile device in a non-hands-free manner can also be identified and a warning issued or action taken, ultimately bringing the vehicle to a safe stop if needs be.

Airbags have saved many, many lives and are a valuable feature in almost all vehicles these days. However, there have been some cases, especially with infants or the elderly where they have caused injury or worse. ToF is able to detect the size of and estimate the weight of passengers, modifying the airbag deployment as necessary. In the event that there is no passenger in the seat, ToF can prevent unnecessary airbag deployment.

Many modern hybrid vehicles will start and run the internal combustion engine to charge the batteries when they are almost depleted. As it is easy to leave a vehicle with the ignition ‘on’ as the old-fashioned ignition key is a thing of the past, a vehicle can automatically start when unattended. This is potentially dangerous, especially in a confined space, but can easily be prevented by ToF-based occupant detection.

Alongside the improvements in vehicle safety, the same ToF system is also able to add a whole range of comfort and convenience inside the cabin for the benefit of drivers and passengers. For example, seats could be moved and seat belts could be brought closer when a passenger gets into the vehicle, storage compartments could be illuminated when a hand reaches in that direction or the operation of the infotainment system could be modified based upon the number and location of vehicle occupants.

As vehicles become more sophisticated, the human-machine interfaces (HMI) in the cockpit must become more complex. A ToF sensor in conjunction with a light projector could provide a control panel on any available surface, providing greater convenience and flexibility.

The Latest ToF Technology

Melexis’ second-generation chipset comprises a dedicated ToF sensor and a companion chip that controls the system and interfaces with the external microcontroller or serializer.

 

Figure 2. The major functional elements of Melexis’ 2nd generation ToF solution.

 

While retaining the same compact 5.5 mm x 6.5 mm package and optical format as previous generations, the new chipset offers double the sensitivity and pixel-level gain selection that improves low light performance. 940 nm illumination support allows the system to work invisibly to humans, which is invaluable for night-time operation in vehicle cabins and take advantages of the lower sunlight emission at this wavelength.

The new sensor is more efficient than before, requiring 30% less power, which generates less heat and saves cost in the power supply. An improved signal-to-noise ratio allows the system to work over a 65% greater distance with the same illumination level, or over the same distance as previous generations with less illumination. This reduces the cost associated with light sources such as LED or VCSELs. A dual-head camera can now be built with a single companion chip.

 

Figure 3a and 3b. An improved signal-to-noise ratio improves performance and reduces illumination requirements.

 

A new feature, pixel binning, allows for four (2x2) or sixteen (4x4) pixels to be merged where less resolution is required, reducing data throughput and allowing a lower cost host processor to be used.

 

In-Cabin ToF Sensing for Vehicle Design Challenges

In-cabin ToF sensing will allow vehicle manufacturers to meet forthcoming legislation including the NCAP 2025 proposals relating to occupant detection and also, improve the experience of vehicle occupants at the same time.

Melexis has developed ToF solutions for more than a decade and its second-generation solution delivers improved performance, lower power operation, and reduced system cost which will significantly ease the challenges that designers face.

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