Uncrewed Systems Technology 049 - April/May 2023

42 a secondary sensor alongside radar or Lidar that can quickly identify an area in the field of view for themain sensor to focus on. Outside the vehicle, EVSs support automated electronic braking systems using the low latency and intrinsically high dynamic range of the technology. They also support scanning Lidar systems, which come with a resolution/accuracy trade-off by detecting multiple, arbitrarily shaped regions of interest on which a Lidar can focus its attention. However, this is a new approach, and sensor manufacturers and Tier 1 suppliers need new systemarchitectures in order to develop these use cases to a level of reliability and accuracy that meets automotive standards. EVSs are also increasingly interesting for UAVs for camera-based collision detection systems as a result of the low power consumption and fast response, which reduces the battery payload required. The latest EVS combines the pixel array, electronics and ML processing on three stacked wafers to achieve a high- performance, low-power system. The key element in the design is the temporal contrast detection, which is amore complex algorithm than the processing currently used on image sensors. This is a relativemeasurement, and is encoded by ‘events’ – a combination of the x-y pixel location, event time stamp and the polarity – indicatingwhether an increase or decrease in illuminance has been detected. Many applications require EVSs to be used alongside a traditional CMOS image sensor (CIS) to get absolute measurements of the illuminance. Dual-sensor configurations have several shortcomings though, such as the parallax error from the camera collocation, complexities in synchronisation, and the added cost from the need for two pairs of lenses and more complex packages. An early hybrid EVS-CIS sensor uses the photocurrent (the current from the photodiode) for the time-continuous front-end circuit in the EVS, and then integrates the photocurrent on a sampling capacitor for the CIS. However, this approach is not easily scalable, as the CIS pixel pitch is limited by the EVS pixel pitch, which is far larger as a result of the larger pixel processing electronics. Also, storing the charge from the pixel for EVSs does not benefit from April/May 2023 | Uncrewed Systems Technology The latest event-driven sensor is based on a three-wafer stack (Courtesy of Omnivision) The readout circuitry of an EVS (Courtesy of Omnivision)