The basics of applying ultrasonic transducers for sensing objects or fluid flow
■ SPL: This defines the magnitude of the audio output at the defined drive level; it can easily reach 100 decibels (dB) or more. Higher SPL offers coverage over greater distances (a typical ultrasound application has a
functions (called a monostatic arrangement) can simplify physical set-up while minimizing space requirements and transducer cost (Figure 2). The PUI Audio UTR-1440K-TT-R (Figure 3), a 40 kHz ultrasonic transceiver, is a viable choice
for this configuration. It has a diameter of just 14.4 millimeters (mm) and a height of 9 mm. It is designed to operate from an AC drive voltage of 140 volts peak-to- peak (V p-p ) and presents a nominal load of 1800 picofarads (pF) to the driver. Its echo sensitivity is better than 200 millivolts (mV) and its directivity is 70° ±15°.
range in the tens of feet). ■ Receiver sensitivity: This
characterizes the voltage output of the piezoelectric transducer at a given SPL. The higher this number, the easier it will be to overcome system noise and provide an accurate reading. ■ Directivity: This defines the spread of the transmitted beam as well as the angular range over which the receiver is most sensitive. Typical values range from 60° to 80° at 40 kHz, usually measured to the angle at which response is 6 dB below the value at the 0° angle.
Figure 2: In a monostatic arrangement, a single transducer is used for both transmit and receive functions. (Image source: Science and Education Publishing Co.)
Positioning the transducers
One of the factors that determines the choice of a transducer is the relative position and orientation of the object being sensed. If the object is directly in front of the source and all or partially at a right angle to the incident energy, some of that impinging energy will be reflected directly back to the source.
In this situation, the use of a single transducer for both transmit and receive
Figure 3: The UTR-1440K-TT-R is a basic 40 kHz ultrasonic transceiver that combines a transmitter and a receiver in a single housing. (Image source: PUI Audio)
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