A Low Cost Embedded Color Vision SystemAnthony Rowe1, Charles Rosenberg2, Illah Nourbakhsh3Carnegie Mellon University, Pittsburgh, PA, USA1Electrical and Computer Engineering Department, [email protected] Science Department, [email protected] Institute, [email protected] this paper we describe a functioning low cost em-bedded vision system which can perform basic colorblob tracking at 16.7 frames per second. This s ys-tem utilizes a low cost CMOS color camera moduleand all image data is processed by a high speed, lowcost microcontroller. This eliminates the need fora separate frame grabber and high speed host com-puter typically fo und in traditional vision systems.The resulting embedded system makes it possible toutilize simple color vision algorithms in applicationslike small mobile robotics where a tra ditional visionsystem would not be practical.1 IntroductionThere are many examples in the literature of simplecomputer vision algorithms proving to be extremelyuseful in a variety of applications [2], [4], [5], [7], [12],[14]. However the usefulness of these algorithms isoften limited by the cost and complexity of the hard-ware needed to implement them. Such systems tradi-tionally co ns ist of a camera, a frame grabber, and anassociated computer to interface to the frame grab-ber and execute the algorithm. Recent hardware de-velopments now make it possible to greatly simplifyand reduce the cost of these systems. The two de-velopments which we take a dvantage of in this workare low co st CMOS color camera modules and highsp e e d, low cost microcontrollers. A major advantageof C MOS versus CCD camera technology is the abil-ity to integrate additional circuitry on the same dieas the sensor itself. This makes it possible to inte-grate the analog to digital converters and associatedpixel grabbing circuitry so a separate frame grab-ber is not needed. As microcontrollers have be c omemore prevalent their cost has decrease d and their c a-pabilities have increased. This makes it possible toperform simple pixel processing “on the fly” as thepixel values are scanned out of the ca mera mak inga full frame buffer unnecessary in many situations.This sugge sts that it should be possible to team aCMOS camera chip with a low cost microcontrollerand implement a simple vision system.[10] We haveconstructed a functioning s ystem based on this ideawhich we describe in the remainder of this paper.The fully assembled system is commercially availablefor a cost of $109.[9]2 System DetailsOur vision system is designed to provide high-levelinformation extra cted from a camera image to anexternal processor that may, for example, control amobile robot. In a typical s c e nario, an external pro-cessor first configures the vision s ystem’s streamingdata mode, for instance specifying the tracking modefor a particular bo unded set of RGB values. The vi-sion system then processes the data in real time andoutputs high-level information to the external con-sumer. The following sections describe the details ofthe system which we have implemented.Figure 1: The microcontroller board mated with theCMOS camera module. A standard size hobby servois shown for scale.2.1 Hardware SystemThe hardware for our sy stem consists of a three chipdesign. The first two chips are the OV6620 CMOScamera and the SX28 microcontroller. The third chipis a simple level shifter for the RS232 serial da ta. Tokeep the design simple, the data bus, synchroniza-tion pins and configur ation bus from the OV6620are directly connected to the SX28 without the aidof any glue logic. The SX28 waits for incoming datato stream fro m the camera and processes it in realtime. It then re lays the extracted high level informa-tion to the outside world via an as ynchronous serialinterface implemented in software. The complete vi-sion system is 1.75” × 2.25” and less than 2” deepwith the camera module and le ns attached, see Fig-ures 1 and 2. The s ystem operates at 5 volts anddraws a bout 200 milliamperes of current.The image input to the sy stem is provided by anOmnivision OV6620 CMOS camera on a chip.[8] TheCMOS camera is is mounted on a carrier bo ard whichincludes a 4.9 mm F2.8 lens and a few supportingpassive co mponents such a s a 17 MHz clock crystal.Different lenses are available for customizing the op-tics. By itself, the board is free running and willoutput a stream of 8 bit RGB or YCrCb color pix-els along a 8 or 16 bit wide data bus. Synchroniza-tion signals, including a pixel c lock, are then used toread out data and indicate new frames and horizontallines. The CMOS image array contains 101,376 pix-els and supports resolutions of up to 352 × 288 witha maximum refresh rate of 60 frames per second.[8]CMOS camera para meters such as color saturation,brightness, contrast, white balance, exposure time,gain and output modes are programmable using astandard seria l I2C interface. To utilize video datafrom the OV6620 one must properly initialize thecamera and then remain synchronized with each ofits output signals. An independent monochromeanalog output exists that can be used for externalmonitoring of the image. Due to the nonstandardframe rate utilized in our system a multisync displaydevice is necess ary to properly decode the image.The microcontroller that is used to process the videodata is a Ubicom SX28 operating at 75 MHz, modelnumber SX28AC/DP.[13] It is housed in a standard28 pin narrow DIP package . The SX28 is a RISCprocessor and operates at 75 MIPS. It has a 2048word flash programmable E PROM and 136 bytes ofSRAM. Although it has few hardware peripherals, ithas fast a nd deterministic interrupts as well as threeflexible multi-bit I/O ports that allow software toemulate standard hardware peripherals as “virtual”peripherals. Using these virtual peripherals, we im-plemented a serial UART port, a standard hobbyservo PWM output port a nd can control a statusLED in our system. With our hardware design, it isalso possible, using a pass-through PC104 style con-nector to join multiple SX28 vision bo ards on a singlecamera bus. This allows for parallel processing of theimage data in what we call slave mode. Using this“slave mode” two microprocessors can be attached tothe output of a s ingle CMOS camer a, allowing twodifferent image operations to be performed in a fullysynchronized fashion.Figure 2: Detail of the assembled microcontrollerboard, 1.75” × 2.25”. Visible are the microcontrollerat the
View Full Document
Unlocking...