text: Tacit: Wrist-mounted sonar for the visually impaired
  Tacit: Wrist-mounted sonar for...
  The prototype Tacit wrist-mounted sonar device designed by Steve Hoefer
  The internal electronics of Tacit
  How Tacit works
  We've seen a number of devices - such as the UltraCane and EYE 21 system - that combine sonar and haptic or audio feedback to let the visually impaired "see" their surroundings through the senses of touch or hearing. Tacit is a similar device that also uses sonar to measure the distance to objects and provide users with a 'view" of their surroundings through haptic feedback. But unlike previous devices we've looked at, Tacit is mounted on the wrist so it doesn't impair a user's hearing or interfere with the use of other assistance devices such as canes.
  The prototype Tacit device developed by Steve Hoefer at Grathio Labs consists of four ultrasonic range finding sensors connected to a microcontroller that calculates the distance to objects by reading how long it takes for the ultrasonic pulses to return to the device. The distances are then translated into force feedback on the back of the user's wrist through servo motors, with the force of the pressure indicating how near or far the detected objects are. By waving their arm around an area, the wearer is able to get the lay of the land and navigate complex environments.
  In coming up with the prototype Hoefer says he considered various designs and the use of various technologies. His initial version was a headband that had ultrasonic sensors and vibration motors embedded around its circumference. However, he found that the most dangerous obstacles weren't positioned at head level and that having vibrating motors pounding against your skull isn't the most enjoyable experience. Placing the device on the hand also lets the wearer easily point it in any direction.
  He also experimented with infrared sensors, which are smaller and easier to focus than the ultrasonic sensors, but interference from sunlight, remote controls, security cameras and absorbent surfaces confused the sensors. He says he will do some more tests with infrared using different wavelengths and polarizing filters, but for the time being the ultrasonic sensors, which have a range of 1-inch to 10 feet (2.5 cm to 3 m), are the most cost effective option. Lasers, which would be the most accurate, were also rejected as being too expensive.
  Hoefer also went through a number of iterations in designing the gauntlet that supports the electronics. To allow the wearer to retain their sense of touch in not only their fingers but also their palm, gloves were rejected in favor of a design that uses a loop that slips over the wearer's middle finger and a Velcro wrist strap. The resultant one-size-fits-all prototype, which is made from neoprene, is easy to put on and can be worn on the left or right hand.
  Although Hoefer admits the current Tacit prototype isn't perfect and is a little bulky, he says it works and could easily be made to be about half the current size. Additionally, the replaceable batteries currently used to power the device would be replaced with rechargeable batteries with a blind-friendly charging method, such as wireless induction charging or a magnetically-aligning power plug.
  One of Hoefer's main aims in developing an assistance device for the visually impaired was to keep the costs down. With the cost of the materials used in his prototype totaling around US$65 he seems to have achieved this goal.
  Hoefer says he will continue to refine the device and provides instructions on how to build Tacit on the Tacit Project page. He has also released the Tacit circuit and software under a Creative Commons BY-NC-SA license.