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:

Cm/s = 20 √273 + TC

Or, in English units:

Cft/s = 49 √460 + TF

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

Cm/s = speed of sound in meters per second

TC = temperature in °C

Cft/s = speed of sound in feet per second

TF = temperature in °F

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