Banner WORLD-BEAM QS18 Ultrasonic Sensors - Manual (Page 2)

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
D = distance from the sensor to the target
c = speed of sound in air
t = transit time for the ultrasonic pulse
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:
In metric units: C
m/s
= 20 √273 + T
C
In English units:
ft/s
= 49 √460 + T
F
C
C
m/s
= speed of sound in meters per second C
ft/s
= speed of sound in feet per second
T
C
= temperature in °C T
F
= temperature in °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 QS18U series ultrasonic sensors are temperature compensated This reduces the error due to temperature by about 90%. The sen-
sor will maintain its window limits to within 1.8% over the -20° to +60° C (−4° to +140° F) range.
NOTE:
• Exposure to direct sunlight can affect the sensor’s ability to accurately compensate for changes in temper-
ature.
• 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 7% of the sensing distance. After 5 minutes, the
apparent switchpoint will be within 0.6% of the actual position. After 25 minutes, the sensing position will
be stable.
Sensor Programming
Push Button
(IP67 models only)
Yellow/Red
TEACH/Output
Indicator LED Green/Red
Power/Signal
Strength LED
Figure 1. Sensor Features
Two TEACH methods may be used to program the sensor:
• Teach individual minimum and maximum limits, or
• Use Auto-Window feature to center a sensing window around the taught position
The sensor may be programmed either via its push button, or via a remote switch. Remote
programming also may be used to disable the push button, preventing unauthorized per-
sonnel from adjusting the programming settings. To access this feature, connect the white
wire of the sensor to 0V dc, with a remote programming switch between the sensor and
the voltage.
Programming is accomplished by following the sequence of input pulses (see programming procedures ). The duration of each pulse
(corresponding to a push button “click”), and the period between multiple pulses, are defined as “T: 0.04 seconds < T < 0.8 seconds."
WORLD-BEAM QS18U Ultrasonic Sensors
2 www.bannerengineering.com - tel: 763-544-3164 P/N 119287 Rev. H