The power consumption of vision system is optimized with motion wake function

How does a smart doorbell detect someone at your door? How do you recognize important actions through the camera? The answer is image sensors. These tiny sensors are built into the smart doorbell and always operate at full state (full resolution, 30fps), where the recorded images can clearly show what person or object is approaching your door. Given that such devices need to run 24 hours a day, you might wonder how long versions powered by mainstream batteries or others can reliably work.

onsemi's new family of image sensors, the HyperluxTM LP, has a built-in "motion wake up" (WoM) function that allows the sensor to operate in low-power mode at a fraction of the power consumed in full-performance mode. Once motion is detected, these sensors quickly move into a higher performance state, and the entire process takes less time than taking a snapshot. In this article, we will explore the motion wake function in depth, and introduce its advantages and suitable application scenarios in detail.

The advantages of WOM

The WoM function of the image sensor allows the vision system to adjust its working state according to the movement 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 if there is indeed a moving object in the scene and sends a notification to the image Signal processor (ISP)/ System-on-a-chip (SoC). The latter then determines whether the detected motion is relevant to the application scene and, if so, awakens the entire visual system to perform the predetermined action. This feature is extremely useful because most applications do not require the vision system to always run in native mode (full resolution, maximum frame rate). Only a small fraction of the time in its working process requires native mode. The rest of the time, the sensor can be in a pre-detection state in order to minimize losses.

In the pre-detected state, the Hyperlux LP image sensor consumes less than 1/100 of the power consumed in native mode. Vision systems are very sensitive to power consumption, and the WoM function significantly reduces their power consumption, which greatly extends the operating cycle. Whether using a limited power source, such as a battery, or a continuous power supply, the relevant systems can operate at ultra-low power consumption levels. For the former, the charge cycle will be significantly extended, while for the latter, this function 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

Vision systems used in areas such as access control (such as visual doorbells, biometrics), public safety and protection (such as law enforcement recorders), and security (such as surveillance cameras) can take advantage of WoM capabilities. For example, the visual doorbell is either battery powered or wall plugged, and the power is very low (less than 20W with a power factor of 1). For the former case, current industry trends call for charging cycles up to 180 days, with the ultimate goal of reaching one year. Without a feature like WoM, the system would always have to process full-resolution images at the selected frame rate, regardless of whether there is associated motion in the scene. But in most cases, this treatment only makes sense when someone walks up to the door. With the WoM mode, the visual doorbell is mostly in the pre-detection state, the system operates with ultra-low power consumption, and only when motion is detected, it enters the normal working state.

WoM is also useful for non-battery powered applications such as surveillance cameras. In WoM mode, the dominant operation mode of pre-detection state greatly reduces the data bandwidth usage without causing the loss of scene information. This, in turn, reduces storage and cooling requirements, reducing the total cost of ownership of the vision system.

There are many benefits to the basic application of the above WOM features, but we also need to consider how to optimize for specific sports. For example, we certainly don't want image sensors and vision systems to wake up just because the wind is blowing leaves or a bird is flying by in a scene. Keeping the sensor focused on the area of Interest (ROI) is critical.

The Hyperlux LP family of image sensors from On is equipped with WoM functionality for flexible application to multiple non-adjacent regions of interest. This feature has been carefully designed to significantly reduce power consumption and significantly increase the value of vision systems in practical applications.