Global shutter image sensor simplifies driver monitoring system design

Using 3D chip technology, STMicroelectronics has developed the VB56G4A, global-shutter sensor for vehicle driver monitoring safety systems.

It is the company’s second generation automotive global-shutter image sensor. Driver monitoring systems (DMS) continually watch the driver’s head movements to recognise signs of drowsiness and distraction for vehicle systems to generate safety warnings.

Global-shutter sensor offers big advantages over rolling-shutter imagers. By simultaneously exposing all pixels to the image, a global-shutter sensor allows simple synchronisation with near infra red (NIR) illumination, improving the illumination-subsystem power budget. 

The VB56G4A sensor achieves high quantum efficiency (QE), reaching 24 per cent at 940nm NIR wavelength, with linear dynamic range up to 60dB. This enables a simple low-power, non-visible LED emitter to provide adequate illumination for the sensor. Operating outside the visible spectrum also ensures consistent response in day or night driving and in bright or overcast conditions, said ST

The sensor’s high QE, combined with a pixel size of just 2.6 micron, helps optimise total power consumption and camera size. In addition, integrated automatic exposure control eases use and simplifies the application software design by minimising system interaction with the sensor.

The sensor also provides flexible operating modes that help optimise system features and performance. These include programmable sequences of four-frame contexts, illumination control outputs synchronised with sensor integration periods, an input for an external frame-start signal, automatic dark calibration, dynamic defective-pixel correction, image cropping, and a mirror / flip-image readout.

External connections include eight programmable general-purpose I/O (GPIO) pins and a dual-lane MIPI CSI-2 transmitter interface operating up to 1.5 Gbits per lane. The sensor can operate at up to 88 frames per second (fps) at full resolution and typical power consumption is 145mW at 60 fps.

It is estimated that around 95 per cent of driving accidents result from human error leading to the introduction of legislation in Europe that DMS will be mandatory in all new car models from 2024 and for existing models in 2026. In Europe, there were nearly 19,000 accident fatalities in 20201 and twice as many in the US, leading the National Transportation Safety Board (NTSB) to recommend DMS for all semi-autonomous vehicles.

“Drivers may not realize that they are unsafe to drive due to tiredness or distraction. DMS removes uncertainty by detecting the problem automatically, which protects all occupants as well as others traveling on the road,” said Eric Aussedat, executive vice president, imaging sub-group general manager, STMicroelectronics. The VB56G4A is sensitive and compact to simplify DMS hardware and reduce overall system cost, he added. 

The VB56G4A global shutter sensor uses ST’s 3D-stacked back-side illuminated (BSI-3D) image sensors. These are more sensitive, smaller, and more reliable than conventional front-side illuminated (FSI) sensors typically used in DMS, said the company.

ST is supplying samples of the sensor to lead customers now and mass production is scheduled for the beginning of 2023 for adoption in model year 2024 vehicles.

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