Source: onsemi Author: Ganesh Narayanashwamy

    Have you ever thought about how smart doorbells detect someone walking to your doorstep and how they recognize important movements through cameras? The answer is image sensors. These miniature sensors are built into the smart doorbell and always operate in full state (full resolution, 30fps), with the recorded images clearly showing who or what object is approaching your doorstep. Considering that such devices require 24-hour uninterrupted operation, you may want to know how long the version powered by mainstream batteries or other versions can reliably work?

    The newly launched image sensor series Hyperlux LP by onsemi has a built-in "motion wake-up" (WoM) function, which allows the sensor to operate in low-power mode, with power consumption only a small fraction of that in full performance mode. Once motion is detected, these sensors quickly enter a higher performance state, and the entire process takes less time than taking a snapshot. In this article, we will delve into the motion wake-up function, detailing its advantages and suitable application scenarios.
Advantages of WOM
    The Wom function of image sensors allows the visual system to adjust its working state based on the motion of objects in the scene. In Wom mode, the sensor is in a pre detection state (low resolution, low frame rate). When motion is detected, the image sensor determines whether there is indeed a moving object in the scene and sends a notification to the Image Signal Processor (ISP)/System on Chip (SoC). The latter then determines whether the detected motion is relevant to the application scenario, and if so, wakes up the entire visual system to perform the predetermined operation. This feature has a huge purpose, as most applications do not require the visual system to always run in native mode (full resolution, maximum frame rate). During its work process, only a small portion of the time requires the use of native mode. At other times, sensors can be in a pre detection state to minimize losses as much as possible.

    In the pre detection state, the power consumption of the Hyperlux LP image sensor is less than 1/100 of the power consumption in native mode. The visual system is highly sensitive to power consumption, and the Wom function can significantly reduce its power consumption, thereby greatly extending the operating cycle. Whether using limited power sources such as batteries or continuous power supply, the relevant systems can operate at ultra-low power consumption levels. For the former, the charging cycle will be significantly extended, while for the latter power supply method, this feature helps to achieve a differentiated system that combines multiple innovative functions, operates at the edge, and does not require a remote central processing system.

Application scenarios of WOM
    Visual systems used in areas such as access control (such as video doorbells, biometric recognition), public safety and protection (such as law enforcement recorders), and security (such as surveillance cameras) can all utilize Wom functionality. For example, video doorbells are either powered by batteries or wall plugs, with very low power (less than 20W when the power factor is 1). For the former scenario, the current industry trend requires a charging cycle of 180 days, with the ultimate goal of reaching one year. If there is no feature like Wom, the system will have to always process full resolution images at the selected frame rate, regardless of whether there is relevant motion in the scene. But in most cases, this treatment only makes sense when someone walks to the door. With the help of Wom mode, the video doorbell is mostly in a pre detection state, and the system operates at ultra-low power consumption. It only enters normal working mode when motion is detected.

    The Wom function is also beneficial for non battery powered applications such as surveillance cameras. In WoM mode, the dominant operation mode used for pre detection significantly reduces the usage of data bandwidth without causing loss of scene information. This in turn reduces storage and cooling requirements, reducing the total cost of ownership of the visual system.
    The basic application of the WOM function mentioned above has many benefits, but we still need to consider how to optimize it for specific movements. For example, we certainly don't want image sensors and vision systems to be awakened just because the wind blows through the leaves or birds fly by in the scene. It is crucial to keep the sensor focused on the region of interest (ROI) at all times.
    nson's Hyperlux LP series image sensors are equipped with WoM functionality, which can be flexibly applied to multiple non adjacent regions of interest. This feature has been carefully designed to significantly reduce power consumption and enhance the value of visual systems in practical applications.

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