Self-Powered Camera

The goal of this project is to enable the camera to function as an untethered, stand-alone device that can produce visual information. To make this goal a reality, the camera must be able to generate all the power it needs to function. Since cameras measure light, and light can be used to generate energy, we seek to develop image sensors that can not only capture images but also generate the power needed to do so. As a first step in this direction, we have developed a fully self-powered camera that produces video indefinitely for a well-lit indoor scene. We refer to a camera operating in this mode as an "eternal camera." We believe such a device can play a vital role in emerging fields such as wearable imaging, sensor networks, smart environments, and the Internet of things.

At the core of our image sensor design is a simple pixel circuit, where the pixel's photodiode can be used to not only measure the incident light level, but also to convert the incident light into electrical energy. A sensor architecture is developed where, during each image capture cycle, the pixels are used first to record and read out the image and then used to harvest energy and charge the sensor's power supply. We have conducted several experiments using off-the-shelf discrete components to validate the practical feasibility of our approach. We first developed a single pixel based on our design and used it to physically scan images of scenes. Next, we developed a fully self-powered camera that produces 30x40 images. The camera uses a supercap rather than an external source as its power supply. For a scene that is around 300 lux in brightness, the voltage across the supercap remains well above the minimum needed for the camera to indefinitely produce an image per second. For scenarios where scene brightness may vary dramatically, we have developed an adaptive algorithm that adjusts the framerate of the camera based on the voltage of the supercap and the brightness of the scene. Finally, we have analyzed the light gathering and harvesting properties of our design and explain why we believe it could lead to a fully self-powered solid-state image sensor that produces a useful resolution and framerate. The research was funded by ONR.

Publications

"Towards Self-Powered Cameras,"
S.K. Nayar, D.C. Sims and M. Fridberg,
IEEE International Conference on Computational Photography (ICCP),
pp. 1-10, Apr. 2015.
[PDF] [bib] [©]

Images

  Conventional Pixel Design:

This figure shows the structure of a three-transistor pixel that is commonly used in conventional image sensors. In this case, the photodiode PD is reverse-biased (in photoconductivemode). When light is incidenton PD, current begins to flow through it and the voltage across it drops by an amount that is proportional to the incident light energy and the exposure time. In addition, since PD is reverse-biased, its output voltage is affected not only by the incident light but also by dark current, which, although very small, exists even when there is no light.

     
  Self-Powered Pixel Design:

This figure shows the pixel design we use in our power-harvesting sensor. The photodiode PD is operated in photovoltaic mode with zero bias. The voltage of the anode of PD increases to a level proportionate to the incident lightenergy. In this case PD draws zero power to produce a voltage proportionate to the incident light, and since it is not biased it does not produce any dark current. An important feature of the design is that emitter of transistor Q1 can be switched between ground(for resetting) and a power supply (for harvesting).

     
  Sensor Architecture:

This figure shows the architecture of our self-powered image sensor. Each pixel has the structure shown in previous figure. In addition to the 2D array of pixels, the sensor includes a harvesting power supply, a microcontroller, an analog-to-digital convertor (ADC) for each sensor row, and transistors for resetting all the pixels and harvesting energy from all the pixels, respectively. The microcontroller is not shown in the figure, but is programmed to control the ADCs and to generate the signals for readout, resetting and harvesting.

     
  Power Harvesting Image Sensor:

This sensor array has 30x40 photodiodes. Since the photodiodes have leads on two of their sides, each one was oriented at 45 degrees so as to achieve a lower pixel pitch. Also seen on the front of the board are the readout transistors and the microcontroller. On the backside of the circuit board is an array of two-switch packages, each package including the two transistors used in the corresponding pixel. The backside also includes the global reset and harvest switches and the harvesting power supply. Note that the board does not have a battery but instead a supercap, which is charged to start the camera but is recharged using just energy harvested from the pixels.

     
  Self-Powered Camera:

The complete self-powered camera system that includes the sensor array shown in the previous figure and a lens with an effective F-number N = 3:5. The camera does not have a battery but instead a supercap, which is charged to start the camera but is recharged using just energy harvested from the pixels. For a scene that is roughly 300 lux in brightness the camera can produce an image per second, indefinitely.

     
  Power Experiment:

The setup used to evaluate the power performance of the camera. The brightness of the scene is controlled by varying the intensity of the light source using a dimmer. The scene brightness was measured using a light meter placed right next to the lens of the camera and facing the scene. The camera was started-up by connecting its supercap to an external power source, set its voltage to 2.74 V, and then disconnecting the source. Each second, an image of the scene is recorded along with the voltage across the supercap.

     
  Power Performance:

This plot shows scene brightness (in orange) and supercap voltage (in blue) plotted as a function of time over a period of 80 minutes. As the orange plot shows, the scene brightness was ramped from 150 lux to 1150 lux over a period of 20 min, kept constant at 0 lux for the next 20 min, stepped up to 1000 lux for the next 12 min, and dropped to 200 lux for the last 28 min. The blue plot shows the voltage across the supercap as a function of time. Due to harvesting, the supercap voltage increases and decreases with the scene brightness, and at asteady brightness of 200 lux the voltage stabilizes at around 3 V, which is well above the minimum of 2.5 V needed for the camera to function.

     

Videos

  The Eternal Camera:

A video detailing the motivations behind and design of this self-powered camera.

     
  Face:

A video of a person captured by the self-powered camera. Each image has 30x40 pixels and the framerate is 1 image per second.

     
  Mannequin:

A video of a rotating mannequin with a black board placed on one of its sides. Each image includes the voltage across the supercap, which is shown in red when it drops with respect to the previous image and green when it rises.