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PRIMARY AREAS OF APPLICATION
Where material to be sensed constantly changes physical
properties, eg: Municipal Solid Waste. Reliable sensing does
not depend on any specific physical or electrical
characteristics of the material.
Low bulk density materials
Eg: Textile fibers, onion skins, popcorn, Styrofoam pellets,
puffed cereals. This sensing technique permits sensing
products which are so light that they cannot be reliably
sensed by other means. Reliably senses products with bulk
densities of less than 1/4 lb./cubic foot.
Stringy, fibrous materials
Eg: Yarn, chipped fibers, waste paper, trash. Sensor is
mounted flush with bin wall to prevent material build-up at
sensing point. No moving parts to catch fibers.
Delicate materials
Eg: Puffed cereals, potato chips. Nothing projects into
falling product to crush it, no motion of sensor to fracture
fragile products.
PRINCIPLE OF OPERATION
A pair of matched sensors is connected to an amplifier as
a receiver (microphone) and a transmitter (loudspeaker). When
the amplifier gain is increased to the point where its gain
exceeds the loss in the acoustic path between the sensors,
oscillation (feedback) occurs. An object entering the acoustic
path increases the loss and the oscillation decreases.
In the Sonac® system, one sensor is connected to the Sonac®
amplifier and operates essentially as a loudspeaker. This
transmitting sensor will produce ultrasonic sound waves of the
frequency dictated by the sensors themselves. The other
sensor which we will call the receiving sensor is connected to
the amplifier as a microphone and will deliver to the amplifier
electrical energy from ultrasonic sounds reaching its diaphragm.
The amplifier itself is capable of amplifying the weak sounds
received by the receiving sensor more than 1,000,000 times.
The Sonac® sensors are, by design, quite directional in their
response to sound waves. If the transmitting and receiving
sensors are positioned facing each other and the path between
the two sensors is unobstructed and the electrical gain in the
amplifier is sufficient to overcome the losses in sound energy
across the path between the two sensors, acoustic feedback
will occur. This, of course, cannot be heard, as Sonac® is
designed to operate near 38,000 Hz. This is well above the
range of human hearing.
When the acoustic characteristics of the air path are changed
by variations in temperature, relative humidity, or standing wave
conditions, then the Sonac® system merely adjusts itself to
some new frequency which is optimum for the present path
conditions. In every case the actual frequency of the acoustic
feedback in the Sonac® path is whatever frequency will produce
the least loss across the path. In actual operation, this change
in frequency is limited by the electrical characteristics of the
amplifier and the acoustic properties of the sensors.
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