Understanding the power benefits of integrated motion detection in image sensors

A feature of the new Hyperlux™ LP image sensors from onsemi is wake-on-motion, which allows the device to operate in a low-power mode until the scene is worth capturing.

Some of the functions that an image sensor performs are not evident at first glance. For instance, a smart doorbell can only detect when a person is walking up to a front door because an image sensor identifies important movements in its field of view. 

These tiny sensors built into the doorbell can clearly display the image of the person or object approaching the doorstep. Normally, the image sensors operate at full performance, at high resolution of 30 frames/s, all the time. Yet these image sensors operate 24/7, drawing charge from the battery all the time. How is it, then, that they can operate reliably without needing frequent battery replacements? 

A feature of the newest Hyperlux LP image sensor family from onsemi provides an answer. The Hyperlux LP image sensors have a built-in feature called Wake-on-Motion (WoM) which enables them to operate in a low-power mode which consumes a small fraction of the power needed in full-performance mode. Once the sensor detects movement, it moves to a higher performance state in less time than it takes to snap a photo. 


Benefits of WoM

The WoM feature in an image sensor enables a vision system to adjust its operation in response to the motion of objects in the scene. In the pre-detect state, the image sensor operates at low resolution and a low frame rate, consuming very little power. 

When motion is detected, the image sensor sends a notification to the host image signal processor or system-on-chip. The host determines whether the motion is relevant to the application; if the image is relevant, it wakes up the entire vision system to go into normal active operation. 

This feature is a considerable benefit as most applications do not need the vision systems to operate in native mode, at full resolution and maximum frame rate, continuously. This state is often needed for only a fraction of the time; for the rest of the time, the sensor can be in the lowest-power pre-detect state.

Fig. 1: Power consumption of the Hyperlux LP image sensor is reduced until motion is detected

In the pre-detect state, Hyperlux LP image sensors consume less than 1% of the power used in native mode. In battery-powered systems, this ability to operate at very low power enables developers to extend charging cycles substantially. 


Applications that use WoM

A wide variety of applications can take advantage of the power-saving benefit of WoM, including:

  • Vision systems used in access control, including video doorbells and biometric access systems
  • Public security and safety systems, including bodycams and surveillance cameras

For example, video doorbells are either battery-powered, or use a wall-power source of less than 20 W. In battery-powered systems, industry practice demands charging cycles of a minimum of 180 days, with the aim to extend the cycle to a full year. Without WoM, the system must process full-resolution images all the time at the chosen frame rate, regardless of whether there is pertinent motion in the scene or not. Operating the video doorbell in the pre-detect state keeps the system operational at the lowest power consumption, but allows it to move to normal operation on demand.

Devices such as surveillance cameras are not normally battery powered, but also benefit from the WoM feature. When operating mostly in the pre-detect state in WoM mode, the surveillance system reduces the bandwidth requirement for data transfer without any loss of awareness of the scene’s context. This, in turn, reduces storage and thermal management requirements and consequently the total cost of ownership of the vision system.

Fig. 2: Image resolution is greatly reduced until motion in the region of interest triggers the wake-on-motion function

The basic WoM capability is enhanced by the Hyperlux LP sensor’s ability to distinguish regions of interest (RoIs) within the total scene. This is useful because, for example, a video doorbell should not wake up just because leaves in a tree are being moved by the wind, or because a bird flies through the field of view. 

To avoid this, the Hyperlux LP image sensor can be targeted at one or more non-contiguous RoIs. This extends still further the power-saving capability of the WoM feature in many industrial and commercial applications.