Electronic travel aid


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Electronic travel aid

  2. 2. INTRODUCTION  An Electronic Travel Aid (ETA) is a form of assistive technology having the purpose of enhancing mobility for the blind pedestrian.  Perhaps the most widely known device is the LaserCane, which is a regular long cane with a built-in laser ranging system. The Mowat Sensor is an example of a pocket- sized device containing an ultrasonic air sonar system. When it detects an obstacle, the device vibrates, thereby signalling the user.  The research problem of designing a better ETA is a tough one. Despite 50 years of effort, no one has been able to design an electronic device that can replace the long cane.
  3. 3. REQUIRMENT  Blind individuals find travelling difficult and hazardous because they cannot easily determine "where" things are, a process otherwise known as "spatial sensing." Thus the problem of mobility can be reframed as a problem in spatial sensing.  The techniques for spatial sensing are well known, radar, sonar, and optical triangulation methods being the most common, and the latter two have been incorporated into a wide variety of previous ETA designs.  However, there are many problems with currently available devices. First, the range-finder technology is unreliable in its detection of step- downs or step-ups, such as curbs.  Secondly, blind users find the sounds of various pitches or tactile vibrations being used to code the spatial information to be confusing and difficult to understand.  Thirdly, most blind users do not find the slight improvement in mobility performance to be worth the extra cost (which can be many thousands of dollars), and the additional worry of maintaining a complex, expensive battery operated system that must be carried around and kept track of.
  4. 4. ON ELECTRONIC TRAVEL AID DESIGN  In order to navigate, we must first know "where" things are, in other words, the spatial positions of objects and other features in the physical world around us. Thus, we can reframe the problem of travelling as initially a problem in spatial perception.  Man is well endowed with a refined set of spatial sensing systems, and listed in order of decreasing range of operation, they include binocular vision, binaural hearing, and active touch.  To compensate for the loss of binocular vision, a blind traveller desires some sort of electronic travel aid (ETA) that can spatially sense what is out there in the environment out to a reasonable distance, and then convey this information in an easily understood format via the remaining senses of hearing and touch. So first, let's consider the basic problem of spatial sensing and how it is normally done. Spatial Sensing Display parameters
  5. 5. SPATIAL SENSING a) First is the basic problem of "sensing" itself. Generally, in order for us to detect an object, it must be a source of energy, either generated by the object itself or reflected. b) Second, we now consider the problem of "spatial" sensing. Our senses, such as the eyes or ears, can infer the direction of an object because we assume that the energy being radiated, travels in straight lines. In the case of the eye, energy arriving from each angular direction, be it azimuth or elevation, is directed by the lens to a two dimensional array of photo sensors on the retina, each direction being mapped to a unique point on the array. However, we live in a three dimensional space. This means that three coordinates are required to specify spatial position, not only azimuth and elevation, but also range, so we can calculate range using binocular vision. Generally speaking, all three quantities need to be known, or your movement will be impaired. In the case of electronic travel aids for the blind, spatial sensing is to be done electronically. Past approaches include ultrasonic air sonar, range finding via laser triangulation, and most recently GPS (global positioning system) using radio triangulation of timing signals from simultaneously viewed satellites. Previous Display Parameter
  6. 6. DISPLAY PARAMETERS  The next problem is how to display the spatial information to the blind user. Since hearing and touch are the two remaining entry points for information to the blind traveller.  We perceive sound as having the following attributes: 1) loudness (intensity), 2) pitch (frequency, repetition rate), 3) phase (with respect to a reference), 4) spatial location (direction and range). Typically, information gets encoded upon sound via by passing it through a physical space of a particular shape. In spatial hearing, the asymmetrical shape of the external ear or pinna will cause a different transformation to be applied to incoming sound, depending on its direction of arrival and range.  In terms of perception through touch, the skin has some similarity to the ear, and can sense vibration stimuli (changing pressures) with attributes including: 1) intensity, 2) frequency, 3) phase (when two or more points are stimulated), 4) waveform shape as a function of time and 5) location (and shape) of stimulation on the skin's surface. Moreover, the above sensations with proprioceptive cues yields "active touch," which means we can discover the shape of an object and feel the changing sensations as a function of position and orientation.  In case of ETAs, a wide variety of electronically generated sound and tactual cues have been used to signal the blind user. In the case of hand held narrow beam devices (ultrasound or laser ranging), azimuth and elevation cues are typically conveyed proprioceptively via hand orientation. Most often, ETAs convey range information either by a vibration or sound that cause changes in frequency or intensity as the detected object gets closer. Previ ous Next
  7. 7. CONVENTIONAL ELECTRONIC TRAVEL AIDS  In the past three decades several electronic travel aids (ETAs) were introduced that aimed at improving their blind users' mobility in terms of safety and speed.  Three fundamental shortcomings can be identified in all ETAs discussed in the foregoing sections:  The user must actively scan the environment to detect obstacles (no scanning is needed with the Sonic guide, but that device doesn't detect obstacles at floor level). This procedure is time-consuming and requires the traveller's constant activity and conscious effort.  The traveller must perform additional measurements when an obstacle is detected, in order to determine the dimensions of the object. A path must then be planned around the obstacle. Again, a time-consuming, conscious effort that reduces the walking speed.  One problem with all ETAs based on acoustic feedback is their interference (called masking) with the blind person's ability to pick up environmental cues through hearing. Some blind travellers can actually detect certain obstacles through sound reflections from such obstacles.
  8. 8. MOBILE ROBOTS AS GUIDES FOR THE BLIND : RECENT TECHNOLOGY  In general terms, one could argue that any mobile robot with obstacle avoidance (and there are tens or even hundreds of different mobile robots with such capabilities) can be used as a guide for the blind. However, mobile robots are inherently unsuited to the task of guiding a pedestrian. The foremost limitation of mobile robots is that they are large, heavy, and incapable of climbing up or down stairs or boardwalks. One might then argue that the blind pedestrian could use ramps and elevators, which are provided in many locations for the use of disabled persons with wheelchairs. However, this approach would actually burden the blind (but mobility-wise perfectly able-bodied traveller) with the additional, severe handicap of limited mobility.
  9. 9. The NavBelt : RECENT DESIGN  The NavBelt is a portable device equipped with ultrasonic sensors and a computer.  The NavBelt provided two modes of operation:  1. In the image mode the NavBelt produced a 120o-wide view of the obstacles ahead of the user (similar to a radar screen image). This image was then translated into a series of directional (stereophonic) audio cues through which the user could determine which directions were blocked by obstacles and which directions were free for travel. The problem with this method lay in the fact that a considerable conscious effort was required to comprehend the audio cues. Because of the resulting slow response time of the test subjects could not travel faster than roughly 0.3 m/sec (1 foot/sec). And even this marginal level of performance required tens of hours of training time.  2. Another mode of operation is called guidance mode. In this mode it was assumed that the system knew the traveller's momentary position and the traveller's desired target location. Under these conditions, the NavBelt needed only generate a single (thus, low-bandwidth) signal that indicated the recommended direction of travel. It was much easier to follow this signal, and walking speeds of 0.6 - 0.9 m/sec (2 - 3 feet/sec) were achieved.
  10. 10. REFERENCES. • Foundation of Orientation and Mobility. • Heyes, D. Anthony. (1984). "The Sonic Pathfinder: A New Electronic Travel Aid." Journal of Visual Impairment and Blindness, 77, 200-202. • Kay, Leslie. (1984). Acoustic Coupling to the Ears in Binaural Sensory Aids. Journal of Visual Impairment and Blindness, 77, 12-16. • Bissitt, D. and Heyes, A. D., 1980, "An Application of Biofeedback in the Rehabilitation of the Blind." Applied Ergonomics, Vol. 11, No. 1, pp. 31-33. • Johann Borenstein and Iwan Ulrich, “ The Guide Cane A Computerized Travel Aid for the Active Guidance of Blind Pedestrians."