Eyes in the back of your back

or, Vibro Vest. Name suggestions encouraged!

Rachel and others are creating a vest that will allow the wearer to feel side-to-side movement behind them. This will be done by a set of motion sensors connected to a touch display (of pager motors, at least initially). The first prototype will have a one dimensional display so that up & down motion will not be detected, but this can probably be added later once some of the more pressing problems are solved. We have not yet made a final determination on the prototype sensors but have had some success with ultrasonic rangefinders.

Current Status

Some functional if rough code for a Parallax Ping))) module has been produced. This module is large and somewhat cumbersome, and appears to be quite inaccurate at distances greater than about 2-3 feet. There is also a pair of MaxSonar-EZ1 modules which look interesting to try as well.

I (Rachel) have a fairly detailed mental model of the armature but have not made it yet.

Code:

/*
 * Based on code found at http://arduino.cc/en/Tutorial/Ping
 */

int pin = 13;
int pingPin = 12;
int ledPin = 11;
double periodFactor = .5; // percent of one second

void setup()                    // run once, when the sketch starts
{
//  pinMode(pin, OUTPUT);      // sets the digital pin as output
  Serial.begin(9600);
  pinMode(ledPin, OUTPUT);
}

void loop()                     // run over and over again
{
  long duration, inches, cm;

  // The PING))) is triggered by a HIGH pulse of 2 or more microseconds.
  // We give a short LOW pulse beforehand to ensure a clean HIGH pulse.
  pinMode(pingPin, OUTPUT);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);
  digitalWrite(pingPin, HIGH);
  delayMicroseconds(5);
  digitalWrite(pingPin, LOW);
  delayMicroseconds(2);

  // The same pin is used to read the signal from the PING))): a HIGH
  // pulse whose duration is the time (in microseconds) from the sending
  // of the ping to the reception of its echo off of an object.
  pinMode(pingPin, INPUT);
  duration = pulseIn(pingPin, HIGH);

  // convert the time into a distance
  inches = microsecondsToInches(duration);
  cm = microsecondsToCentimeters(duration);

  Serial.print(inches);
  Serial.print("in, ");
  Serial.print(cm);
  Serial.print("cm");
  Serial.println();

  pwm(duration);
}

void pwm(long microseconds)
{
  Serial.print(microseconds);
  Serial.println("ms");

  long offCycle = microseconds / 18;
  long onCycle = 1000 - offCycle;
  digitalWrite(ledPin, HIGH);
  delay(onCycle * periodFactor);
  digitalWrite(ledPin, LOW);
  delay(offCycle * periodFactor);
}

long microsecondsToInches(long microseconds)
{
  // According to Parallax's datasheet for the PING))), there are
  // 73.746 microseconds per inch (i.e. sound travels at 1130 feet per
  // second).  This gives the distance travelled by the ping, outbound
  // and return, so we divide by 2 to get the distance of the obstacle.
  // See: http://www.parallax.com/dl/docs/prod/acc/28015-PING-v1.3.pdf
  return microseconds / 74 / 2;
}

long microsecondsToCentimeters(long microseconds)
{
  // The speed of sound is 340 m/s or 29 microseconds per centimeter.
  // The ping travels out and back, so to find the distance of the
  // object we take half of the distance travelled.
  return microseconds / 29 / 2;
}

Next Steps

Next we'd like to see if the MaxSonar-EZ1 device has any better of a range than the Ping))). Someone else has hooked it up to an arduino and posted code: http://www.flatline.net/journal/2007/12/02/arduino-reading-the-maxbotix-ultrasonic-rangefinder/