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Exotic Ultrasonic Sensors Become a Standard Product

Industrial ultrasonic sensors for production automation have been available for about 30 years. These large, expensive devices were originally deployed only in special application areas, but today can be found in almost every area of factory automation. As ultrasonic sensors evolved, they became significantly more compact, powerful and less expensive – and this development is far from complete. They promise to open up a whole new range of applications.


Sensory tasks

A sensor is a technical component that can record specific physical or chemical properties in qualitative or quantitative terms. In automation technology, sensors use signals to detect the status of a machine or plant, which also serve as input variables for the control unit. Sensors are characterized primarily by their physical functional principles. Inductive sensors react to changes in an electromagnetic field, whereas photoelectric sensors react to changes in light waves. Ultrasonic sensors operate in transmitting medium (gas, fluid, solid material) using acoustic waves. Ultrasonic sensors are used predominantly in factory automation to measure the running time or amplitude of sonic pulses in the air. With the widely-used time-of-flight measurement procedure, the sensor emits a package of ultrasonic pulses and measures the time taken until an echo is received (Figure 1). A single ultrasonic converter that functions as a transmitter and receiver is normally used here. The distance from the sensor to the object generating the echo is then calculated using the formula . (c: sonic speed, t: measured running time). In addition, there are also transmitter and receiver sensors that incorporate individual converters located either within the same housing or connected separately to allow the integration of both standard sensors and thru-beam sensors.



Fig. 1: With time-of-flight measurement, the sensor emits a package of ultrasonic pulses and measures the time until the echo is received. This is used to calculate the distance from the object.

Ultrasonic converters

The main component of an ultrasonic sensor is the converter. Today, durable solids converters are used in most applications and essentially consist of a combination of piezo ceramics for generating a mechanical vibration and a matching acoustic layer. A matching layer is required to adapt the extremely different acoustic impedances of piezo ceramics and air to one another. To achieve the best coupling, the acoustic impedance of the matching layer must be . (with ZK: acoustic impedance of piezo ceramic, ZL: acoustic impedance of air, Z = ρ * cM with ρ: density of the medium, cM: sonic speed of the medium). Without such measures, only a small fraction of the acoustic energy would be released into the air (during transmission) or picked up (in receiver mode), which would drastically decrease the sensing ranges. In addition to ensuring optimized adaptation to the acoustic conditions, a high degree of mechanical stability, high resistance to chemicals, a wide temperature range, good acoustic isolation from the sensor enclosure and last but not least, a low price are extremely important. Expertise relating to theoretical physical penetration and practical manufacturing methods is one important element contributing to the success of Pepperl+Fuchs sensors. The most recent development is a converter covered by a stainless steel membrane that enables the production of completely hermetically sealed ultrasonic sensors for distance measurement.


Function

A burst or a single pulse achieves the required resonance frequency in transmitter mode by applying voltages of up to several hundred volts to electrically stimulate the ultrasonic converter described. The sensor then switches to receiver mode, where the converter operates as a microphone. The receiver signal with a magnitude of a few millivolts is amplified, demodulated and transmitted to a threshold detector. The distance to the object is then calculated from the duration of the sonic pulse. The fact that the same converter is used for transmitters and receivers means that there is a blind area directly in front of the sensor where detection is impossible. Different hardware and software measures can drastically reduce the size of this blind area and increase immunity to interference.

As sonic speed in air with (with c0: sonic speed at0 °C(331.5 m/s), T: temperature in Kelvin, T0: absolute temperature at0 °C(273.15 K) is highly temperature-dependent, an operating temperature of100 K yields a change of approximately 18%. The sensor measures the temperature and compensates accordingly in order to suppress the interference more effectively. This enables a measuring precision of greater than 0,02 of the final value to be achieved across the entire temperature range.

The ultrasonic frequencies used are in the range from 40 kHz to almost 1 MHz, whereby because of the extreme rise in attenuation at high frequencies, only short ranges are possible. The detection ranges of the sensors extend from 100 mm and 10 m. Ultrasonic sensors are relatively slow, in particular over long measurement distances, because the propagation speed of airborne sound is approximately 1 million times slower than light. At an object distance of 10 m, the sonic run time is about 60 ms., which is more than adequate for many applications.

In addition to the key function of the ultrasonic converters mentioned, the hardware design and above all the processing of signals by the microcontroller inside the sensor are decisive performance characteristics. Instead of installing simple 8-bit controllers that offer only the most rudimentary functions, today powerful 32-bit controllers that can reproduce complex algorithms in real time, require less installation space, and reduce costs, are installed as standard. Examples include an adjustable sound cone width and the possibility of producing perfect measuring results under difficult conditions by measuring echo amplitudes (in addition to run times). The possibilities extend beyond the current scope and are far from being exploited to the full.


Characteristics and application advantages

Compared to photoelectric sensors, ultrasonic sensors are far more resistant to dirt and humidity. Minor damage to the surface of the converter is not critical due to the holistic nature of the device. Needless to say, the color or degree of transparency of the detected objects has no influence whatsoever. The durability of ultrasonic sensors is comparable with that of inductive sensors, but the sensing range is 100 times greater. There are also extensive similarities between the designs. Cylindrical ultrasonic sensors as small as size M12 and block-shaped models enclosed in typical proximity switch or photoelectric sensor enclosures are standard today. Furthermore, versions are available that are adapted to special requirements, e.g. measuring fill levels.

Sensors with digital switching outputs or an analog 4-20 mA interface are commonplace. However there are also sensors that run on 253 VAC with relay output. Most sensors can be taught-in or parameterized via an interface and therefore adapted to the respective application. The new IO-Link industrial standard was implemented recently.



Fig. 2: Ultrasonic sensor overview

Applications

Applications for ultrasonic sensors can be found in almost every sector involving automated processes: in the print industry for monitoring the level of ink in containers or presence checking, in mobile plant and agricultural machinery for measuring distances, positions and filling levels, in material handling, in the packaging industry, and in assembly and handling technology. Ultrasonic double sheet sensors used in printing machines, scanners, ATMs and similar machines are becoming increasingly popular as they prevent several sheets from being drawn in simultaneously. They do not measure run times, but amplitude damping through the objects. This technology was discussed in detail in atp edition 5/2010.


Fig. 3: Ultrasonic sensors are used in many sectors, such as the print industry, in mobile plant, in material handling, in the packaging industry or in assembly and handling technology.

Summary

Modern ultrasonic sensors offer an attractive alternative to other sensor types in countless application areas, especially where these sensors reach their physical limits or only allow the implementation of an automated system. The recent strong tendency towards miniaturization means that the sensors can be integrated into almost any machine. The dramatic increase in the performance of microcontrollers to generate complex algorithms has enabled the implementation of extremely demanding applications, which would have been inconceivable a few years ago. The implementation of the intelligent IO-Link parameterization and process interface makes the handling of sensors and integration into the machine control system even easier.

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