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A Video Camera that Powers Itself Using the Same Light that Delivered the Image
Self-powered camera. A video camera based on a self-powered image sensor can run indefinitely without an external power supply. Credit: Self-powered camera A video camera based on a self-powered image sensor can run indefinitely without an external power supply.
Video produced by self-powered camera
Columbia Engineering Professor invents a video camera that runs without a battery
Newswise -- New York, NY--April 15, 2015--A research team led by Shree K. Nayar, T.C. Chang Professor of Computer Science at Columbia Engineering, has invented a prototype video camera that is the first to be fully self-powered--it can produce an image each second, indefinitely, of a well-lit indoor scene. They designed a pixel that can not only measure incident light but also convert the incident light into electric power. The team is presenting its work at the International Conference on Computational Photography at Rice University in Houston, April 24 to 26.
"We are in the middle of a digital imaging revolution," says Nayar, who directs the Computer Vision Laboratory at Columbia Engineering. He notes that in the last year alone, approximately two billion cameras of various types were sold worldwide. "I think we have just seen the tip of the iceberg. Digital imaging is expected to enable many emerging fields including wearable devices, sensor networks, smart environments, personalized medicine, and the Internet of Things. A camera that can function as an untethered device forever--without any external power supply--would be incredibly useful."
A leading researcher in computational imaging, Nayar realized that although digital cameras and solar panels have different purposes - one measures light while the other converts light to power - both are constructed from essentially the same components. At the heart of any digital camera is an image sensor, a chip with millions of pixels. The key enabling device in a pixel is the photodiode, which produces an electric current when exposed to light. This mechanism enables each pixel to measure the intensity of light falling on it. The same photodiode is also used in solar panels to convert incident light to electric power. The photodiode in a camera pixel is used in the photoconductive mode, while in a solar cell it is used in the photovoltaic model.
Nayar, working with research engineer Daniel Sims BS'14 and consultant Mikhail Fridberg of ADSP Consulting, used off-the-shelf components to fabricate an image sensor with 30x40 pixels. In his prototype camera, which is housed in a 3D printed body, each pixel's photodiode is always operated in the photovoltaic mode.
The pixel design is very simple, and uses just two transistors. During each image capture cycle, the pixels are used first to record and read out the image and then to harvest energy and charge the sensor's power supply--the image sensor continuously toggles between image capture and power harvesting modes. When the camera is not used to capture images, it can be used to generate power for other devices, such as a phone or a watch.
Nayar notes that the image sensor could use a rechargeable battery and charge it via its harvesting capability: "But we took an extreme approach to demonstrate that the sensor is indeed truly self-powered and used just a capacitor to store the harvested energy."
"A few different designs for image sensors that can harvest energy have been proposed in the past. However, our prototype is the first demonstration of a fully self-powered video camera," he continues. "And, even though we've used off-the-shelf components to demonstrate our design, our sensor architecture easily lends itself to a compact solid-state imaging chip. We believe our results are a significant step forward in developing an entirely new generation of cameras that can function for a very long duration--ideally, forever--without being externally powered."
The research was funded by Office of Naval Research.
We have designed 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.
"Towards Self-Powered Cameras,"
S. K. Nayar, D. C. Sims, M. Fridberg,
Proceeding of the International Conference on Computational Photography (ICCP),
pp.1-10, Apr, 2015.
[PDF] [bib] [�]
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).
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.
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.
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.
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.
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