Judd Communications Depth Sensor, Analog Version
OPERATION
Ultrasonic simply means sound waves that are above the range of human hearing in frequency. The depth
sensor’s ultrasonic ranging system operates at a frequency of 50 kilohertz (50,000 cycles). The key
component of the ultrasonic system is the transducer. The transducer is first used as a speaker to transmit
an ultrasonic pulse, then it is used as a microphone to listen for the pulse after being reflected off a
surface. By measuring the amount of time that it takes the ultrasonic pulse to travel from the transducer to
a surface and back again, a distance can be calculated.
The Judd Communications Depth Sensor outputs an analog signals that are proportional to the distance
and air temperature being measured. Both measurements are output sequentially through a single output.
The output signal has two ranges: 0 to 2.5 Volts or 0 to 5 Volts. The range is selected by a jumper on the
main circuit board. The output signals can be can be scaled using the following multipliers:
Distance:
Scale
0-2.5 Volt Range
0-5 Volt Range
Inches
196.85 inches/Volt
98.43 inches/Volt
Centimeters
500 cm./Volt
250 cm./Volt
Air Temperature:
Scale
0-2.5 Volt Range
0-5 Volt Range
Degrees Kelvin
200 degrees/Volt
100 degrees/Volt
The JC Depth Sensor performs multiple echo processing. Multiple echo processing dramatically
improves the reliability of measurements. For each measurement cycle, two initial measurements are
made. If the difference between the two distances is less than 1 centimeter, then the second sample is
saved, and output. If the difference between the two samples is greater than 1 centimeter, then the oldest
sample is discarded and another measurement is made and another comparison is made. This retry
algorithm will continue up to a maximum of ten times. When a valid measurement can not made, or no
echo is returned, a value of zero is output. When a zero is output to the digital to analog converter, the
output voltage is typically in the range of 3-5 millivolts.
The speed of sound in air varies with temperature. Graph 1 shows the relation. An accurate air temperature
measurement is required for calculating the speed of sound in air. From this, a correction factor can be
calculated for properly compensating the measured distance. With the air temperature measurement taken
from its built in probe, the JC Depth Sensor outputs a temperature compensated distance.
Speed Of Sound In Air
sos = 331.4(TempK/273)^.5
250
270
290
310
330
350
370
390
-100
-50
0
50
100
Air Temperature degrees C.
Speed of Sound Meters/Second
Graph 1