Persons that are visually impaired have great difficulty navigating andavoiding obstacles as they walk even when using a cane or seeing eye dogand especially under low light levels.
Creating a portable, low-cost, assistive device to aid the visuallyimpaired is the goal of the NSF-sponsored Wearable Low-Vision Aid (WLVA)project. The prototype WLVA uses machine vision to identify walkinghazards and a see-through head-mounted scanning fiber display to presenticons indicating the location of potential hazards. The scanning fiberdisplay projects laser light through a vibrating optical fiber in orderto project an image onto the retina. In this report we describe theengineering of a low-cost portable WLVA that incorporates infrared (IR)illumination and efficient machine vision algorithms to identifypotential walking hazards and a scanning fiber display to present brighticons to warn the user.
The software includes a machine vision program run on the laptopcomputer to identify potential collisions, an embedded processor and program tocontrol the scanning fiber display, and a graphical user interface tofacilitate setting parameters for the embedded processor and generatingeasily recognizable icons.
The scanning fiber display consists of two bimorph piezoelectricactuators, the optical fiber, and lenses. The small core optical fiberis attached to the end of the fast scan piezo, which is coupledorthogonally to the slow scan piezo. The slow scan piezo is cut tovibrate at its first mode of resonant vibration. The fast scan piezoand slow scan piezo are driven independently at 3 kHz and 60 Hz,respectively, to create a raster scan pattern. The other end of theoptical fiber is connected to a laser diode, which modulates the lightintensity synchronously with the fiber position in order to createmultiple low-resolution icons.
The current display creates a scanned object plane of approximately4mm×3mm with 100×28 separated pixels, displayed at 30 framesper second. The scanning fiber display is constructed by hand usingcomponents that cost less than one dollar. The bimorph piezos are cutto an appropriate length and width using a rotary cutter, and gluedtogether. The optical fiber is chemically etched to reduce the fiberdiameter and improve scanning dynamics. The optical fiber is glued tothe tip of the fast scan piezo. The vibrating end of the fiber istrimmed with a CO2 laser in order to achieve maximumdeflection at the first mode of vibratory resonance for the opticalfiber cantilever.
The scanning fiber display creates an audible hum at the resonantfrequency of the fast scan piezo (3kHz). Mounting a second piezo nextto the fast scan piezo and vibrating it at the same frequency, but outof phase, results in an interference pattern that significantlyattenuates the hum. Preliminary tests have recorded a 14 dB decrease innoise by using this method.
A 1.5 inch Delrin tube supports the fiber scanner and allows adjustmentof the lenses. The scanned laser image is reflected onto the user'sretina by a small mirror (or beam-splitter for optical see-through mode)mounted to the end of the tube. By allowing the WLVA user to see theirsurroundings at all times, situational awareness is maintained. Thesmall mirror inset or see-through beam-splitter design allows thedisplayed icon to augment the user's vision. The high-brightness of thelaser diode or future LED source make this display suitable for use inoutdoor conditions with these various display modes.
A color video camera with a ring of 24 IR LEDs is mounted on the rightside of the HMD. An optical filter is mounted in front of the cameralens to block visible light. Custom circuitry synchronizes illuminationof the IR LEDs with alternating video frames. The video is captured andprocessed in real time by the laptop computer to identify and locatepotential hazards up to 12 feet away. The head mounted video camera isangled down slightly to capture hazards from ground level up to headlevel.
A knob is located behind the camera that allows the user to reduce thebrightness of the display by adjusting power to the light source fordarker indoor or nighttime use. Currently, the display brightness isset for upcoming hazard avoidance testing in indoor lightingconditions according to our approved human subjects testing protocolinvolving low vision volunteers.
The backpack mounted equipment includes a laptop computer and analuminum case containing control hardware and batteries. The totalweight of the backpack equipment is 4.5 kg.
The laptop computer is a Dell Latitude with a 1.8 GHz processor. Avideo capture card (Dazzle Digital Video Creator 80) captures video at arate of 30 frames per second. The laptop communicates with the embeddedprocessor through the serial port.
Custom hardware was developed to control the scanning fiber display sothe laptop computer could be dedicated to time-critical machine-visionhazard detection algorithms. The hardware used to control the scanningfiber display consists of an embedded processor, a first-in first-out(FIFO) frame buffer, and other discrete components. An Atmel ATMEGA128generates the frequencies to drive the piezos, the synchronizationsignals to keep the frame aligned, and handles communication with thelaptop and the FIFO frame buffer.
The scanning fiber display generates binary (uniform brightness) imagesbut could easily be adapted to display gray-scale images. Static icons(a single frame projected repeatedly) or dynamic icons (a sequence ofseveral frames) can be displayed. The FIFO frame buffer (IDT 7208)facilitates storage and retransmission of the pixel data (icons ortext). The pixel clock frequency (laser modulation frequency) isdivided down by the ATMEGA 128 programmable counters to generate thehorizontal scan frequency and the vertical scan frequency. This methodkeeps all of the clock signals in phase and eliminates the phase lockedloop used in the original bench-top prototype. The ATMEGA 128programmable counters also generate horizontal and verticalsynchronization pulses that are used to adjust the frame alignment. AThorLabs (LPS-3224-635) 3mW pigtailed red laser diode (633nm wavelength)is the light source. For safety reasons the laser diode is driven atonly 5% of its rated power during development creating an intensity of230uW/cm2. A reading performance study in the see-through mode showedblue (458 nm) light may be easier for low vision subjects to see in theaugmented display mode. The light source could be changed to a bluelaser diode at additional cost. LED's brightness has increased rapidlyin recent years, making pigtailed LEDs the preferred light source in thefuture.
The WLVA software includes machine vision programs running on the laptopcomputer and a scanning fiber display control program running on theembedded processor with a GUI that facilitates initial setup of theembedded processor.
The embedded software facilitates loading icons into the FIFO framebuffer, selects which icon to display, and controls the parameters foradjusting the scanning fiber display. During development or usercustomization, icons are transferred from the laptop computer to theembedded processor through a serial link, and then to the FIFO framebuffer for projection. The embedded software adjusts the framealignment by changing the timing of data output by the FIFO framebuffer. After the customization is complete, several sets of icons arestored in the non-volatile program memory of the embedded processor.Several different hazard icons can be customized to the user'spreference and environment. The icons can be displayed in variouslocations in the visual field to warn the user of the location andproximity of potential walking hazards.
The WLVA prototype is the first system to use a scanning fiber displayin a wearable low vision aid. The IR flash illumination providesenhanced functionality at night, efficient hazard detection capability,and early warning for a broad range of hazards. The scanning fiberdisplay's high brightness, lightweight, see-through design, and low costmake it an ideal choice for this application. The scanning fiber withlaser diode source makes it possible to augment a person's visionwithout obscuring their natural visual ability. Although the icons havea low pixel count, the user can pre-select the icons for specificwarnings making recognition rapid. This enhancement of the visualsystem is preferred to using an auditory warning that could obscurenatural auditory cues that are critical to the visually impaired.
The WLVA system is designed for low cost and versatility. The monocularnear-eye display can be added to a pair of spectacle lenses withfiber-scanning display components costing less than onedollar. Therefore the augmented display can be considered disposableeliminating the need for repair. The laptop computer is commercialoff-the shelf technology that can be used in fixed locations like atschool, work, and home when not being used for machine vision hazarddetection.
The laptop computer could be interfaced with a GPS unit to displaynavigational aids or an optical character reader for text-to-speechconversion.
Testing of the WLVA system and its hazard detection and identificationcapabilities is expected to begin in early 2004 at the University ofWashington with low vision volunteers.
We have prepared illustrations of how the WLVA might help the user to avoida large pot anda bicycle rack.There is also a first-person perspective of avoiding the pot.These illustrations are also available inWindows Media format:the pot,the bike rack, andthe first-person view of the pot.
An illustration of a former, bulkier design is alsoavailable.
Two graduate students received their MS in mechanical engineering andfive undergraduates participated in this research project, most allbeing first authors of journal or conference papers. All studentsobtain the unique experience of designing and testing the assistivedevices with our HITLab staff, one of whom is legally blind.
Subject to continued funding, the WLVA hardware will undergo asignificant reduction in size, using a single tubular piezoelectricactuator less than 2mm in diameter to generate over 50 times more pixelswhile maintaining its extreme low cost. A printed circuit board hasbeen designed to significantly reduce the weight and size of thebackpack electronics as well.
This research was funded by NSF Research Grant to Eric Seibel #9978888,Research to Aid Persons with Disabilities, with REU supplements allowingundergraduate students to contribute significantly.
Eric Seibel <eseibel
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