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Perfect complement for each other

Free-radiating and guided radar sensors compared
Perfect complement for each other

Continuous level measurements of bulk solids have undergone a dramatic change over the last ten years. Today, radar technology is an increasingly popular option. Two main measuring principles compete with one another here: free-radiating radar sensors that measure the level contactlessly and guided radar sensors that transmit their radar signals along a cable. If the strengths of both systems are leveraged correctly, the two methods complement each other perfectly and can handle practically any application in the bulk solids industry.

Author Jürgen Skowaisa Product Management Radar, Vega

Until the turn of the millennium there was nothing really new happening in the field of bulk solids level measurement. To be sure, mechanical sounding systems had been modernised and automated, yet their weaknesses and limitations remained: mechan-ical wear, measurement outages during filling and a costly power supply system.
Non-contact measurement with ultrasound has since been improved thanks to more powerful and efficient components. However, this method is still constrained by physical limits: strong signal damping due to dust and noise as well as restrictions on use at higher temperatures and with larger measuring ranges.
Even the hitherto reliable and widely used capacitive measuring principle is meanwhile suffering from limitations to a growing degree. Flexible production calls for a measuring method that works efficiently regardless of the properties of the medium being measured. Yet that is exactly where this method is problematic. Every time the medium changes, the sensor has to be readjusted. In practice, this is hardly feasible.
The only thing that might help would be to place the whole vessel on a load cell – a clever solution in theory, but a costly one, and in the case of large concrete silos impossible.
Many users simply had to live with these limitations until radar technology using powerful new instruments made its entry into the bulk solids industry.
Perfect wave for measuring bulk solids
Two different radar techniques or principles of operation can be employed depending on the application: free-radiating radar and guided radar, also known as guided microwave or TDR (time domain reflectometry). With free-radiating radar, the measuring signal is emitted via an antenna whereas with guided radar, a radar pulse travels along a cable and is reflected by the surface of the measured medium.
The two methods share the tremendous advantages of radar technology in bulk solids applications: immunity to the effects of dust, air turbulence, filling noise and temperature or pressure variations and the ability to be used in virtually any medium. The main difference between them is the mechanical configuration in each case and in particular the required high-frequency electronics. While free-radiating radar operates at a frequency of 26 GHz and recently even 79 GHz, thus rendering complex electronic components unavoidable, guided radar gets by with short pulse transmission in the frequency range from 1 to 2 GHz. This strength is of course also reflected in the price of the sensor.
Investment costs compared
Due to the higher outlay for high-frequency electronics, guided radar has a clear cost advantage over non-contact radar measurement. This applies especially to small measuring ranges and simple applications where no large extraction forces occur and the mechanical configuration can be kept simple. With large measuring ranges, solid and more expensive cables and process connections are required, which increases the price of the sensor considerably.
Free-radiating radar sensors are designed for a specific measuring range. They can be used in both small and large silos. Very large measuring ranges can be achieved even with simple plastic antennas. The investment costs are thus independent of the silo dimensions and the media to be measured.
A matter of reflection
To be able to detect a level reliably with a radar sensor, a portion of the radar signal must be reflected on the product surface and picked up again on the receiver. In the case of electro-magnetic waves, reflection is caused by an abrupt change in the dielectric properties on the surface of the medium. The greater the change, the larger the reflected signal.
Since the signals of guided radar are propagated along the cable as a concentrated electric field, this measuring method is relatively independent of the consistency of the target material. The signals are reflected on the boundary surface of the medium.
When free-radiating radar instruments measure very fine, slanting media, the signals are deflected sideways as though with a mirror. Only a fraction of them make it back to the receiver as reflection signals. Since the reflective properties depend on the wavelength and thus on the transmission frequency, sensors that operate at a higher frequency have significant benefits. Extremely fine media can be measured better with high-frequency radar sensors in the 79 GHz range.
Universal across all boundaries
The dynamic range, i. e. the ratio between the largest and smallest detectable signals, is regarded as an important criterion when calculating a sensor’s application limits. In the case of radar sensors, the dynamic range determines which media can be measured. This represents a major difference between guided and free-radiating radar. The cable of a guided radar sensor acts as an antenna, conducting not only the reflection signals to the receiver electronics but also stray electromagnetic radiation from the environment. This generates background noise. Increasing the sensitivity of the electronic components would not bring about any improvement. A method known as ‘probe end tracking’ is used instead to measure materials with very poor reflective properties. Poorly reflecting media reflect only a small portion of the transmitted signal. The larger portion passes through the medium right up to the end of the cable and is reflected there. However, the other dielectric properties change the signal propagation speed in the medium. Since the actual distance to the end of the cable is known, the level can be determined by this apparent change in distance. Although this is an ingenious method in principle, it only works reliably if the media properties are known and constant over the entire height of the silo.
Free-radiating radar sensors only detect the emitted frequency ranges. Owing to the use of new, high-performance electronic components, ever smaller signals can be detected. Modern radar sensors now have the capability to measure even tiny polystyrene spheres or aerosils.
Focusing on the essentials
Until now, guided radar has been able to exploit its strengths – in particular its optimal signal focus along a cable – mainly in very slim silos or tanks with baffles or other internal installations. Free-radiating radar sensors have usually needed very large antenna systems to achieve an equivalent level of signal focusing. However, new technologies and higher frequencies are now creating distinct advantages. Whereas in the past a 240 mm parabolic antenna was necessary for good focusing in tight spaces, today’s new 79 GHz sensors require an antenna with a diameter of just 75 mm, which makes installation a lot easier.
Situation in practice
In practice, the two methods complement each other perfectly. There are indeed preferences for one method or the other depending on the application and the industry, even though from a purely technical point of view both work pretty well.
Building materials: In a rock crusher for processing large, rough boulders, non-contact measurement is clearly superior. Even small rocks cause extreme abrasion to the cables or rods of guided radar sensors due to their hardness and sharp edges. In tall cement or lime silos, the enormous forces generated during material discharge subject the cable to heavy loads. A free-radiating system is the better so-lution in such cases. However, the situation is different in smaller silos containing fine aggregates or cement, where guided radar is an interesting alternative.
Grains and foodstuffs: Many foodstuffs are stored in high, narrow silos. This is exactly the kind of application where the new free-radiating radar sensors offer significant advantages. Thanks to their excellent signal focusing, these high-frequency sensors can be used to detect the filling levels in extremely tall and slim silos absolutely reliably. Here, too, guided radar is a good alternative for smaller silos employed in production as well as buffer silos in logistics. These sensors can represent a cost-effective solution in many areas of the plant.
Chemicals: Due to the wide variety of material properties encountered in the chemical industry, an almost universal measuring technique such as radar is a very attractive option. Both radar methods are very well suited here, the choice depending mainly on the medium and the dimensions of the silo. In plastics production and processing, modern, free-radiating radar sensors with a high dynamic range have a genuine advantage. They can detect even the tiniest reflection signals and are therefore also ideal for media that cannot be measured directly with guided radar.
A good team in the bulk solids industry
If one compares the two radar techniques in practice, it is evident that there is a large overlap where both work equally well. While guided radar sensors are more likely to be used in smaller silos, free-radiating systems are more at home in very large containers. A blanket statement and recommendation cannot be made in many cases because the choice is heavily de-pendent on the particular medium, the local conditions and above all the preferences of the user. The application experience of the sensor supplier and the quality of the on-site consultation are crucial in making the right decision.
cpp-net.com/0115401

Secure level measurement with radar

Vegaflex 82 and Vegapuls 69

The guided radar sensor is available in a rod or cable version. It measures the level of bulk solids accurately, even in applications with large amounts of dust, condensation or build-up. If the product signal becomes too weak or instable, Vegaflex 82 switches automatically to the stable signal from the end of the probe and is thereby able to adapt to changing measuring conditions. And in case heavy build-up becomes a problem, dynam-ic false signal suppression steps in. The signal processing system adapts automatically to the process conditions and increases measurement certainty.
The Vegapuls 69 free-radiating radar sensor for bulk solids level measurement operates at a frequency of 79 GHz, allowing optimal signal focusing. Interfering reflections from internals are thus significantly reduced. Thanks to the latest technologies, even media with very poor reflective properties can be measured reliably. Vegapuls 69 is available in two versions: one with a lightweight plastic antenna and one with a lens antenna integrated in the flange. The antenna can be easily aligned with a precision swivel holder and a special smartphone app.
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