A successful application of an ultrasonic distance measurement system takes into account the operating principles of the apparatus, environmental factors, and characteristics of the target.

An Introduction to Ultrasonic Sensing

Paul A. Shirley, Massa Products Corp.

UItrasonic ranging and detecting devices use high-frequency sound waves to detect the presence of an object and its range. The systems either measure the echo reflection of the sound from objects or detect the interruption of the sound beam as the objects pass between the transmitter and receiver.

Transducer Configurations

A transmitting transducer sends out a pulse of sound that is detected by a receiving transducer. Figure 1 shows several types of transducer configurations. In Figure 1(A), two transducers are mounted side by side. In this application, ultrasonic energy from the transmitter is reflected by an object and the echo is detected by the receiving transducer. This system measures the elapsed time from when the sound pulse is transmitted to when the echo is detected to determine the exact range of the object from the transducers. The application shown in Figure 1(B) differs only in that a single transducer is used to transmit the sound pulse and receive the echo.

In some applications, such as high-speed counting and mechanical equipment positioning, it may be desirable to position the transducers opposing each other as shown in Figure 1(C). For clarity, the term "sensor" will be used in this article to describe either a single or dual transducer configuration.

Beam Angles and Side Lobes

Ultrasonic transducers are often designed to be directional so that the sound is efficiently transmitted or received only over a certain conical beam angle in front of the sensor. Ultrasonic transducers can be designed to produce any beam angle desired, from narrow (with beam widths of a just few degrees) to virtually omnidirectional. Some narrow beam transducer designs produce side lobes as the sound energy is transmitted; an example is shown in Figure 2(A). Advanced transducer designs eliminate all secondary side lobes and are more desirable for ultrasonic echo ranging; see Figure 2(B).

Different applications may require different beam angles for the sensors. In most cases, however, narrower beam angles are usually preferable to broader ones. A narrow beam angle system will not detect unwanted objects that are not in the insonified path of the transducer. (To insonify means to fill a specific volume with sound from a transducer.) Narrow beam angle systems are also less susceptible to background ultrasonic noise, and the systems will also operate over a greater range.

The beam angle of a transducer, alpha, is defined as the total angle between the points at which the sound power has been reduced to half its peak value. These are commonly referred to as the 3 dB down points. It is often advantageous to compute the spot diameter that is insonified by the ultrasonic beam. To calculate this spot diameter, use the formula:

D = 2 * R * tan(0.5 * alpha) where:
D = spot diameter in inches
R = target range in inches
alpha = total beam angle in degrees

Frequency, Wavelength, and Attenuation

The operating frequency of a transducer, f, is predetermined by mechanical design. It should be selected after considering a number of factors such as transducer size, measurement resolution, background noise, and attenuation and range to the receiving transducer. The wavelength, lambda, of sound becomes shorter as the frequency increases. The relationship between frequency, wavelength and the speed of sound is expressed by: lambda = c/f
where:
lambda = wavelength
c = velocity of sound
f = frequency
Since the speed of sound changes with temperature, we must determine the current speed of sound before we can accurately calculate wavelength. At 0 C, the speed of sound is 13,044 in /s. At other temperatures use: C_T = C₀ sqrt(1+ (T/273)

where:
C_T = speed of sound at a specific temperature
C₀ = speed of sound at 0 C
T = temperature in degrees C

For example, using these two equations, the speed of sound at 25 C will be 13,628 inches/sec. At this temperature, the wavelength of sound at 215 kHz is 0.063 in. Because measurement resolution of ultrasonic systems is improved if the wavelength is shorter, applications requiring high resolution should use a transducer with the highest frequency possible in order to achieve the desired specification. As sound travels through air, its energy attenuates more rapidly if the frequency is increased. The maximum theoretical attenuation for ultrasonic sound (up to 200 kHz) may be calculated by this formula:

a_max = f * 10^-2

where:
a_max = maximum attenuation in dB/ft
f = frequency in kHz

For example, using this equation, sound energy from a 215 kHz transducer would be attenuated a maximum of 2.15 dB/ft as it traveled through air. Attenuation may be less, depending on humidity, but is not so easily defined or calculated. Although attenuation limits the range of higher frequency transducers, there is a bonus: background noise at the same higher frequency is also Iess. Higher frequency ultrasonic sensors therefore, have a much better chance of working in an acoustically noisy environment than do lower frequency sensors. Beam angle also helps to lower background noise interference by limiting the transducer's noise sensitivity to the area defined by the beam angle of the sensor. Some transducer designs utilize a detachable horn. When the horn is attached, the beam angle of the transducer is reduced. This concentration of acoustical energy into a tighter beam increases the range of the sensor and reduces the background noise as well.

Temperature. The velocity of sound in air is 13,044 in./s at 0 C; it is directly proportional to air temperature (see above). As the ambient air temperature increases, the speed of sound also increases. Therefore if a fixed target produces an echo after a certain time delay, and if the temperature drops, the measured time for the echo to retum increases, even though the target has not moved. This happens because the speed of sound decreases, returning an echo more slowly than at the previous, warmer temperature. If varying ambient temperatures are expected in a specific application, compensation in the system for the change in sound speed is recommended.

Air Turbulence and Convection Currents. A particular temperature problem is posed by convection currents that contain many bands of varying temperature. If these bands pass between the sensor and the target, they will abruptly change the speed of sound while present. No type of temperature compensation (either temperature measurement or reference target) will provide complete high-resolution correction at all times under these circumstances. In some applications it may be desirable to install shielding around the sound beam to reduce or eliminate variations due to convection currents. Averaging the return times from a number of echoes will also help reduce the random effect of convection currents. Users addressing applications requiring high accuracy and resolution should evaluate these suggestions carefully.

Temperature variations and wind produce air turbulence that has various effects on the total performance of any ultrasonic sensing system, causing bending and distortion of the sound waves. The narrower the angle of the sound beam and the greater the distance to the target, the greater the turbulence. Additional signal processing may be able to filter data under high turbulence conditions to improve ranging information.

Atmospheric Pressure. Normal changes in atmospheric pressure will have little effect on measurement accuracy. Reliable operation will deteriorates however, in areas of unusually low air pressure, approaching a vacuum.

Humidity. Humidity does not significantly affect the operation of an ultrasonic measuring system. Changes in humidity do have a slight effect, however, on the absorption of sound. If the humidity produces condensation, sensors designed to operate when wet must be used.

Acoustic Interference. Special consideration must be given to environments that contain background noise in the ultrasonic frequency spectrum. For example, air forced through a nozzle, such as air jets used for cleaning machines, generates a whistling sound with harmonics in the ultrasonic range. When in close proximity to a sensor, whether directed at the sensor or not, ultrasonic noise at or around the sensor's frequency may affect system operation. Typically, the level of background noise is lower at higher frequencies, and narrower beam angles work best in areas with a high ultrasonic background noise level. Often a baffle around the noise source will eliminate the problem. Because each application differs, testing for interference is suggested.

Radio Frequency Interference. Another possible source of noise is RFI emitting from SCRs in a variable speed drive. Shielding around the back and sides of the transducer may prevent RFI noise from entering the system.

Splashing Liquids. Splashing liquids should be kept from striking the surface of the sensor, both to protect the sensor from damage if it is not splashproof and to ensure an open path for the sound energy to travel. Sensors used in a splashing environment, however, should be designed to operate when wet.

Mounting orientation is also a consideration in such an environment. Straight-down orientation can cause moisture to form as a large drop on the face of the sensor, reducing the efficiency of the system. Certain applications permit mounting the sensor so that it is aimed lower than horizontal but not pointed straight down; in this orientation, gravity will help to keep moisture from collecting on the face of the sensor.

Two methods may be used to improve the reliability of ultrasonics in the presence of splashing liquids. While beam bouncing (see Figure 3) provides a clever way of keeping the sensor out of the immediate area of splashing liquids, some applications don't lend themselves to this technique. An alternative method involves placing around the sensor a short tube that extends out past its face but not into the actual beam pattern. It is very important that the acoustic beam not be allowed to touch the edge of the tube; if it does, the tube might deteriorate the acoustic performance (see Figure 4).

Target Considerations

Composition. Nearly all targets reflect ultrasonic sound and therefore produce an echo that can be detected. Some textured materials produce a weaker echo, reducing the maximum effective sensing range. The reflectivity of an object is often a function of frequency. Lower frequencies can have reduced reflections from some porous targets, while higher frequencies reflect well from most target materials. Precise performance specifications can often be determined only through experimentation.

Shape. A target of virtually any shape can be detected ultrasonically if sufficient echo returns to the sensor. Targets that are smooth, flat, and perpendicular to the sensor's beam produce stronger echoes than irregularly shaped targets. A larger target relative to sound wavelength will produce a stronger echo than a smaller target until the target is larger than approximately 10 wavelengths across. Therefore, smaller targets are better detected with higher frequency sound. In some applications a specific target shape such as a sphere, cylinder, or internal cube corner can solve alignment problems between the sensor and the target.

Target Orientation to Sensor. To produce the strongest echoes, the sensor's beam should be pointed toward the target. If a smooth, flat target is inclined off perpendicular, some of the echo is deflected away from the sensor and the strength of the echo is reduced. Targets that are smaller than the spot diameter of the transducer beam can usually be inclined more than larger targets. Sensors with larger beam angles will generally produce stronger echoes from flat targets that are not perpendicular to the axis of the sound beam.

Sound waves striking a target with a coarse, irregular surface will diffuse and reflect in many directions. Some of the reflected energy may return to the sensor as a weak but measurable echo. As always, target suitability must be evaluated for each application.

Averaging. Certain applications involve a constantly moving target, such as the surface of agitated liquid in a tank. Analog outputs, which may be averaged by a Programmable Logic Controller or computer, will track the constant movement with little difficulty, but set point outputs might turn on and off unnecessarily as the target hovers around a set point distance. Hysteresis will prevent switched outputs from oscillating to a certain extent, but if the agitation or movement is great enough, the outputs will still switch on and off.

This problem can be easily solved in a number of ways. One method is to delay the decision process by using a time delay relay (TDR). A DC powered on-delay TDR may be directly connected to a set point output and programmed to delay switching on its output until it has received power for a specific length of time. The target will then have to be past the set point distance for that programmed time before the TDR will turn on, activating the primary load. Measurement can also be averaged with a Programmable Logic Controller (PLC) or an averaging digital panel meters. PLCs have done for industrial control what word processing has done for the modern office—they have provided flexibility and have reduced costs by greatly simplifying the wiring and troubleshooting of a complete process system.

Ultrasonic systems have also increased the flexibility of measurement and control systems used in typical applications. One ultrasonic system can often replace multiple photoelectric, capacitive, or mechanical limit switches and at the same time provide additional distance information. The fact that all measurements are made without physical contact with the target improves the lives of both the target and the sensing system.

Air/Ultrasonics • Underwater • Fundamentals of Electroacoustics
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