COVER STORY
Overview of Time of Flight system design
Paul O’Sullivan, System Engineer, and Nicolas Le Dortz, System Engineering Manager, both at Analog Devices, explain how Time of Flight approach makes for super-precise cameras required in machine vision applications – especially when these machines work in close proximity to human operators
M
any machine vision applications now require high-resolution 3D depth images to replace or
augment standard 2D imaging. These solutions rely on the 3D camera to provide reliable depth information to guarantee safety, especially when machines are operating in close proximity to humans. The cameras also need to provide reliable depth information while operating in challenging environments, such as in large spaces with highly refl ective surfaces and in the presence of other moving objects. Many products to date have used low-
resolution range-fi nder type solutions to provide depth information to augment 2D imaging. However, this approach has many limitations. For applications that benefi t from higher-resolution 3D depth information, CW CMOS Time of Flight (ToF) cameras provide the highest performance solutions on the market. Some of the system features enabled by high-resolution CW ToF sensor technology are described in more detail in Table 1. These system features also
Figure 1: Overview of Continuous Wave Time of Flight sensor technology
translate to consumer use cases such as video bokeh eff ects, facial authentication, and measurement applications, as well as automotive use cases such as driver alertness monitoring and automated in- cabin confi guration.
Table 1: Continuous Wave Time of Flight (CW ToF) system features
Continuous wave CMOS camera overview A depth camera is one where each pixel outputs the distance between the camera and the scene. One technique to measure depth is to calculate the time it takes for the light to travel from a light source on the camera to a refl ective surface and back to the camera. This travel time is commonly referred to as time of fl ight. A ToF camera is comprised of several elements (see Figure 1) including: • A light source, such as a vertical cavity surface emitting laser (VCSEL) or edge- emitting laser diode, that emits light in the near infrared domain. The most commonly used wavelengths are 850nm and 940nm. The light source is usually a diff use source (fl ood illumination) that emits a beam of light with a certain divergence (aka, fi eld of illumination or FOI) to illuminate the scene in front of the camera. • A laser driver that modulates the intensity of the light emitted by the light source. • A sensor with a pixel array that collects the returning light from the scene
12 October 2021 | Automation
automationmagazine.co.uk
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