Page 330 - Amphibionics : Build Your Own Biologically Inspired Robot

Amphibionics 07 3/24/03 9:13 AM Page 309
Chapter 7 / Turtletron: Build Your Own Robotic Turtle
then bounces back. Since we know that sound travels through air
at approximately 1129 feet per second when the temperature is 21
degrees Celsius, we can accurately determine distance by measur-
ing the amount of time between the transmission of the pulse and
the return echo. When the temperature drops, the speed of sound
through air slows down. If a temperature sensor was added, an
algorithm to determine distance based on the speed of sound
through air could take the surrounding temperature into account
and adjust for differences.
The PicBasic Pro command called PULSIN returns the round trip
echo time in 10 µs units when using a 4-MHz oscillator. Since the
pulse width has a 10 µs resolution per increment, that means that
if PULSIN returns a value of 1, then 10 µs have elapsed. The fac-
tors to convert the raw data to inches and centimeters given in the
SRF04 manual are 74 for inches (73.746 µs per 1 inch) and 29 for
centimeters (29.033 µs per 1 cm) based on the Basic Stamps
PULSIN command returning values in 2 µs increments. In the
SRF04 manual, the calculation to determine the distance is not
divided in half to take into account the return time of the pulse
because the sample program is for the Basic Stamp II, which
returns PULSIN values in 2 µs increments. Because the PULSIN
command with PicBasic Pro is returning values in increments of 10
µs, the conversion factor will need to be divided by 5, so that we
get the correct value based on our 10 µs increment. Taking the
PULSIN increment timing difference into account gives us an
approximate conversion factor of 15 for inches and 6 for centime-
ters. Testing with the ranger indicated that the raw value returned
by PULSIN when an object was 12 inches away was 180. One hun-
dred and eighty divided by the inch conversion factor of 15 gives
us the correct distance of 12 inches.
In order to test the SRF04 sonar ranger, a program will be written
to produce audible tones, based on the distance of an object from
the device. Compile the sonar-test.bas code listed in Program 7.5,

309