Sensors: Ultrasonics
The ultrasonics used by the Sheep and Preston are from an old
design in Electronic Engineering Magazine, October 1988 pp 37-40.
They use a ramp generator to make the sensitivity of the reciever
a function of time, so that spurious echos generated by the unit
itself are not picked up. In operation the units are triggered using
a pulse input, which sets the output to low, and at some time later
the output becomes high which indicates that an echo has been recieved.
The CPU attached to the ultrasonics can therefore work out the time
interval between triggering the ultrasonics and recieving the echo.
Voltage supply to the ultrasonics is split between a fixed 5V supply
for the logic (driven from the regulator on the IO board) and a variable
6V supply straight from the battery (to drive the tranmistter at
maximum voltage). The variable supply may be from 6V to 12V but
the receiver may need a resister change and re-callibration if
driven much above 7V.
The ultrasonics are good for measuring distances from 10cm to 6m at
an accuracy of 2%, or 2cm per meter.
The ultrasonic sensors are very simple to code for and to operate, simply
toggle the reset line, and monitor the recieve line until it goes high. Easy.
The time spent from the toggle to detecting the recieve is directly
proportional to the distance of the target that generated the echo.
The ultrasonic sensors I have built are very sensitive so spurious echos
from other ultrasonic sensors
are
a problem. The ramp generator takes care of most of the direct echos
self generated by the particular sensor, however using multiple
ultrasonics causes headaches. The way the Sheep and to some extent Preston
takes care of this is to fire all the ultrasonics all at once so any reflections
from one sensor which end up recieved by another sensor will be, by definition,
an accurate reading.
The PCB masks and a parts list for the ultrasonics will be made available later this year.
If you find them useful in your projects or have a design suggestion drop a note
to jpg@janus.demon.co.uk.
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