Banner U-GAGE Sonic OMNI-BEAM Ultrasonic Sensors - Manual (Page 6)

Banner Engineering Corp., 9714 10th Ave. No., Minneapolis, MN 55441 Telephone: (612) 544-3164 FAX (applications): (612) 544-3573
Ultrasonics are sound waves of frequencies above the range of human
hearing. In sonic OMNI-BEAM™ sensors, ultrasonic waves are
produced by a vibrating object called a piezoelectric transducer. This
transducer is part of an electrical circuit, and "rings" when an ac voltage
"spike" is applied to it. This ringing compresses and expands the air
molecules in front of the sensor, sending "waves" of ultrasonic sound
outward from the transducer's face. The transducer is not constantly
transmitting ultrasonic sound, but is switched "on" and "off" at a regular
rate. During the "off" times, the transducer acts as a receiver and listens
for ultrasonic reflections from objects in its path.
A basic knowledge of how ultrasonic waves behave in air, which is
presented below, can be of help in using ultrasonic sensors successfully.
(1) The intensity of ultrasonic sound decreases at a rate equal to the
square of the distance from the sound source.
For example, if the intensity of ultrasonic sound at a distance of 1' in
front of the sensor is designated as "1", then the intensity at 3 times that
distance is (1/3)
2 , or 1/9th. If these waves are reflected back to the
sensor, the intensity of the waves decreases again by the square of the
distance. The stronger the generated ultrasonic waves, the stronger
will be the returned waves. And, the more efficient the object is as a
reflector of ultrasonic waves, the stronger will be the returned waves.
(2) Ultrasonic waves are affected by the size, density, orientation,
shape, surface, and location of the object being sensed.
a) Size of the object. At a given distance in front of the sensor, a large
object reflects more ultrasonic energy than does a smaller, otherwise
identical object at the same position, and so is more easily sensed. The
recommended object size for the sonic OMNI-BEAM sensor is 1 square
inch of reflective surface area presented to the sensor for each inch of
sensing distance. EXAMPLE: at a sensing distance of 25 inches, the
object should be 25 square inches (5 inches x 5 inches) in size. This is
an "average figure", and is influenced by other characteristics of the
object being sensed.
b) Density of the object. Density is the mass of an object per unit of
volume. The more dense the object being sensed, the stronger is the
sound reflection, and the more reliably the object can be sensed. For
example, a wall covered with hardboard paneling reflects sound more
efficiently than does a wall of foam insulation panels. The hardboard
paneling is denser than the foam. Note that water and other liquids
(although certainly not solid) are nonetheless denser (and better reflec-
tors of ultrasound) than are materials like foam.
Behavior of Ultrasonic Waves
from perpendicular often produce adequate reflections. Some materials
may actually produce just as good reflections when sensed "at an angle"
as when sensed "straight on". This phenomenon allows a degree of
freedom in choosing a sensor mounting location for some applications.
Some trial-and-error experimentation may be required.
d) Location of the object within the sensor's response pattern. The
ultrasonic signal radiated from the sonic OMNI-BEAM is strongest
along the "sensing axis" and drops off with increasing angle away from
the axis. Objects are most reliably sensed when they are as close as
possible to the sensing axis.
e) Location of sidewalls with respect to the beam pattern. Sidewalls
located close to the sensing axis may sometimes cause unwanted signals
to be reflected back to the sensor. Unwanted reflections may also occur
from deposits of material adhering to the sidewalls of hoppers, bins, etc.
If possible, align the sensor so that its beam will not encounter sidewalls,
and try to keep sidewalls free of buildup.
(3) Extreme environmental conditions may affect ultrasonic sens-
ing. Temperature, thermal air currents, wind, humidity, and atmos-
pheric pressure all exert some effect on ultrasonic waves.
a) The speed of sound increases and decreases slightly with in-
creases and decreases in ambient temperature. A large temperature
increase will "move" the object slightly towards the sensor. A large
decrease will "move" the object slightly away from the sensor.
The amount of shift is 3.5% of the sensing distance and window limits
for every 20
°
C of temperature change. It is a good idea to set the
sensing window limits when the ambient temperature is midway in
the expected environmental operating temperature range of the
sensor. Whenever possible, adjust the sensing window so that the
object(s) to be sensed will pass through the midpoint of the window.
Fluctuations in the speed of sound and signal strength can occur when
hot objects are sensed or when the air temperature between the sensor
and the object fluctuates. A small fan blowing along the sensing axis
helps to thermally stabilize the sensing path.
b) Care should be taken to shield ultrasonic sensors from sustained,
loud sounds such as factory whistles and similar sources. Sound
sources produce harmonics (sounds at frequencies above the funda-
mental frequency of the source). Harmonics may fall in the ultrasonic
range and "confuse" ultrasonic sensors. High pressure air blasts are
especially good producers of harmonics in the ultrasonic range. Since
sound waves travel in a straight line from the harmonic source to the
sensor, the solution is simple: a wall or baffle placed between the
sensor and the harmonic source is nearly always sufficient. This tactic
can also help prevent possible interference between ultrasonic sensors.
c) Humidity: extreme changes in humidity influence ultrasonic sensing
by a maximum of 2% of the sensing distance or window limits. While
the speed of sound increases with increasing humidity, heavy fog
increases sound absorption and reduces sensing range.
d) Atmospheric pressure: a 5% increase in atmospheric pressure
increases the speed of sound by 0.6%. A 5% decrease in pressure slows
the speed of sound by 0.6%.
e) Condensation or other contamination on the transducer face can
seriously impede sensor performance, and should be avoided.
Condensation or particulates on the transducer dampen its movement.
Most contamination can be prevented by mounting the sensor in the
driest, cleanest location possible that still allows reliable sensing
performance in a given application. Never mount the sensor "face
up" in areas where contamination might be a problem.
BASIC THEORY OF ULTRASONIC SENSING
Basic Guidelines for Object Density and Surface:
1) The greater the density of the object, the stronger the reflection.
2) The smoother the surface of the object, the stronger the reflection.
c) Object orientation, shape, and surface characteristics. Ultrasonic
waves follow the same laws of reflection as do light waves. The angle
of incidence equals the angle of reflection. This means that ultrasonic
waves are reflected from a smooth, flat surface at an equal and opposite
angle to the incoming angle. A perfectly flat object that is exactly
perpendicular to the direction of travel (the "axis") of the ultrasonic
waves will reflect the waves back along the same path.
As the object's reflecting surface is tilted away from the axis of the
waves, however, less and less of the ultrasonic signal is reflected back
to the sensor. Eventually the point is reached beyond which the object
can no longer be sensed. When attempting to sense an object with a flat,
smooth surface, the angle of the reflecting surface to the sensing axis
should never be more than 3
°
off of perpendicular.
Irregularly shaped objects and aggregate matter (coal, ore, sand, etc.)
have many reflecting faces of many different angles. Although these
faces scatter much of the ultrasonic energy away from the sensor,
enough sound energy may be reflected back to the sensor for reliable
sensing. In fact, due to the large number of reflecting surfaces, the
"perpendicularity requirement" described above may not be nearly as
critical for these materials. Sensor angles of up to several degrees away
WARRANTY: Banner Engineering Corporation warrants its products to be free
from defects for one year. Banner Engineering Corporation will repair or replace,
free of charge, any product of its manufacture found to be defective at the time it
is returned to the factory during the warranty period. This warranty does not cover
damage or liability for the improper application of Banner products. This warranty
is in lieu of any other warranty either expressed or implied.