Banner U-GAGE S18U Series—Analog - Manual (Page 2)

U-GAGE ™ S18U Series Sensor — Analog Output
2
P/N 110738 rev. A
Banner Engineering Corp. • Minneapolis, MN U.S.A.
www.bannerengineering.com • Tel: 763.544.3164
Principles of Operation
Ultrasonic sensors emit one or multiple pulses of ultrasonic energy, which travel
through the air at the speed of sound. A portion of this energy reflects off the target
and travels back to the sensor. The sensor measures the total time required for the
energy to reach the target and return to the sensor. The distance to the object is then
calculated using the following formula:
D =
ct
2
To improve accuracy, an ultrasonic sensor may average the results of several pulses
before outputting a new value.
Temperature Effects
The speed of sound is dependent upon the composition, pressure and temperature of
the gas in which it is traveling. For most ultrasonic applications, the composition and
pressure of the gas are relatively fixed, while the temperature may fluctuate.
In air, the speed of sound varies with temperature according to the following
approximation:
C
m/s
= 20 √273 + T
C
Or, in English units:
C
ft/s
= 49 √460 + T
F
Temperature Compensation
Changes in air temperature affect the speed of sound, which in turn affects the distance
reading measured by the sensor. An increase in air temperature shifts both sensing
window limits closer to the sensor. Conversely, a decrease in air temperature shifts
both limits farther away from the sensor. This shift is approximately 3.5% of the limit
distance for a 20° C change in temperature.
The S18U series ultrasonic sensors are temperature compensated. This reduces the
error due to temperature by about 90%. The sensor will maintain its window limits to
within 1.8% over the -20° to +60° C range.
NOTES:
• Exposure to direct sunlight can affect the sensor’s ability to accurately compensate for
changes in temperature.
• If the sensor is measuring across a temperature gradient, the compensation will be
less effective.
• The temperature warmup drift upon power-up is less than 1.7% of the sensing
distance. After 10 minutes, the apparent distance will be within 0.3% of the actual
position. After 25 minutes, the sensing distance will be stable.
D = distance from the sensor to the target
c = speed of sound in air
t = transit time for the ultrasonic pulse
C
m/s
= speed of sound in meters per second
T
C
= temperature in °C
C
ft/s
= speed of sound in feet per second
T
F
= temperature in °F
B