Rainer Petersen, Senior Application Engineer, MELEXIS
Time-of-flight (ToF) sensors are being increasingly used for a variety of in-cabin automotive monitoring applications, including monitoring the gaze direction and head position of the driver. A ToF device is a sensor that detects wavelengths of light. Light waves are modulated at high frequencies—in the lower to upper MHz range—and are reflected by an object of interest. A fraction of the original signal is sent back to the sensor and processed. The traveling time from the emitter to object and back to the receiver means that that reflected signal is phase-shifted with respect to the modulation signal. The phase shift is also proportional to the distance, and this enables the distance from the TOF sensor to the object to be determined.
The TOF signal is emitted in bursts, and each burst contains pulses that are modulated with different phases. This enables any biases and ambiguity to be removed from the readings. The shorter bursts reduce motion blur via a higher signal-to-noise ratio. The longer ones increase the absolute amount of return signal, as well as the range and detectability of low-reflection objects in some cases.
Despite the benefits of ToF systems, there are still some inherent challenges. The detection range of ToF sensors depends on both the modulation frequency of the device and the signal-to-noise ratio. After hitting the object, the reflected signal strength decreases with the square of the distance, so they are more accurate at shorter distances. The noise of ToF sensors comes from intrinsic sources such as sensor noise and illumination noise.
Melexis has recently introduced theMLX75027 VGA and MLX75026 QVGA ToF sensor, which can achieve higher, less ambiguous resolutions at long distances using lower modulation frequencies. Moreover, the sensors have been certified for use in operation ranges of -40 to 105 °C, making them suitable for automotive applications. But it’s not just the sensor that is important in these measurements. Another vital component is the illumination unit, which consists of the light emitting components and the accompanying electronic driver circuitry.
Dealing with illumination challenges
The illumination unit in ToF sensors is crucial, and if the right component is chosen, it can help overcome the illumination noise present in some readings. Infrared (IR) light sources offer the best performance. There are currently two competing solid-state IR light technologies that are worth talking about. These are LEDs and VCSELs. Like any technology, both options have inherent advantages and disadvantages.
On the one hand, LEDs are an older technology, so they are more mature. However, there has been a lot of effort put into mass manufacturing VCSELs. So, at this stage, both are capable of large-scale integration in high-tech applications. Both technologies also show similar current drive capabilities, and because the illumination pattern of a VCSEL is “LED-like”, the eye safety of both technologies is comparable.
LEDs have advantages over VCSELs in a few areas. The first one is cost. Currently, LEDs are cheaper to manufacture than VCSELs. Still, the mass manufacturing efforts of VCSELs are bringing the price down, and this advantage is becoming narrower and narrower, to the point where it may need to be revised soon. LEDs also perform better when it comes to the power output stability, as the output of a VCSEL is 50% more sensitive to temperature.
There are also some advantages that VCSELs possess over LEDs. One of the key benefits of using VCSELs is their sunlight immunity. Ambient light (such as daytime sunlight and headlights) can interfere with LED-based tracking devices because they operate over a wide spectrum. Because VCSELs operate in a narrower spectrum, a narrower spectral filter can be employed, leading to an increased signal-to-noise ratio, sensitivity, and resilience to changing environmental conditions—such as those associated with driving.
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Figure 2: TOF 3D image: VCSELs unaffected by sunlight
Even though LEDs are more mature, the best VCSEL devices out there currently offer better efficiencies than the best LEDs. While LEDs have a better spectral width, the spectral stability of VCSELs over a broader temperature range is much better. Finally, VCSELs have a much quicker rise and fall time, so they can achieve a much higher resolution than LEDs.
Even though both illumination sources have their own benefits, when it comes to in-cabin monitoring applications, the advantages of VCSELs outweigh the offerings from LEDs. Overall, this is due to VCSELs possessing a narrow output spectrum, low spectral temperature coefficient and high modulation capability.
Selecting a VCSEL driver
When you are looking to select the ideal illumination driver electronics for your application, there are several factors you should consider, and all are important.
The first consideration (and one of the most important for in-cabin monitoring) is to check that the VCSEL driver is automotive qualified. There are several ToF drivers on the market, but very few of them are automotive grade. The Lumentum driver is one solution that is qualified for automotive-grade applications but using a drive circuit with discrete automotive-grade components is an alternative solution.
Another thing that should be considered is the lead time, and ideally you want a short lead time. There are some off-the-shelf components—from capacitors to diodes and field effect transistors (FETs)—that are automotive grade. By using easy-to-source components, a prototype illuminator sample can be designed and fabricated within a few weeks. You’ll also want to check that the system you build can support modulation frequencies up to 100MHz .
Another aspect that should be considered is the flexibility of the design. Using discrete components can offer greater flexibility and can be customized towards different design requirements. Such systems can then be tailored towards driving conditions, peak current, or pulse shape. The use of modular architectures allows you to easily customize the rise times and optimize the power efficiency of the driver. Additionally, if you use discrete components over a single-chip driver, then you can often achieve a better performance thanks to the drive circuit having a lower resistance. For automotive applications, this can be used to tailor the driver so that it can handle higher peak currents.
Because the IR illumination component is a vital part of ToF sensor systems, choosing a system with the right wavelength characteristics for the intended application is key. This includes determining the correct spectral emission, optical filtering and amplitude characteristics that will block the background solar radiation in the car—as the in-cabin solar radiation will vary in different types of vehicles.
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Figure 3: TOF sensors system diagram
Finally, with any instrument where there is an illuminating source, eye safety is vital. The regulations of the laser need to be applied according to the laser safety standard IEC 60825-1:2014. This is a complex area to navigate, but in short, there are many aspects that you will need to consider. These include the laser safety class, the source brilliance, the shape of the light source and how it is imaged on the retina in someone’s eye, the brightness of the light at different angles, the eye pupil diameter of people using the instrument (wider pupils let in more light), the laser wavelength, the maximum and minimum source distances, and the average laser power (including when the laser is pulsed).
In-cabin driver monitoring
There are several areas where ToF VCSEL systems can be used to monitor the driver in automobile cabins to help provide an increased level of comfort and safety. On the one hand, these systems can be used to monitor the levels of driver distraction and detect drowsiness. On the other hand, the same sensor can simultaneously be used to monitor the cabin directly and measure head and body position for airbag control. The same sensor can also perform additional functions, including gesture control. So, while these sensor systems can utilize either a single camera or a series of cameras, only a single sensor system is required for simultaneous driver and in-cabin monitoring.*
Increased safety for all
The combination of ToF sensors to monitor the cabin and the driver simultaneously and the ability to sense both highly reflective and non-reflective objects enables the driver to feel a lot safer while on the road. These systems are not only dependent on the sensitivity of the sensor itself, but on the effectiveness of the illumination source. While LEDs have been around for longer, the benefits offered by VCSELs are much greater, and because a lot of effort has gone into manufacturing VCSELs on a large scale, they should soon be the clear choice from an economic standpoint as well, and not just from a performance perspective.