Sensors in chocolate paradise
Capacitive sensors are a fine thing in and of themselves: they can detect levels of liquids, powders and granular materials through nonmetallic container walls without the sensor coming into contact with the media. However, conventional capacitive sensors also have a number of disadvantages.
The sensitivity of the sensor must be adjusted in a time-consuming process so that the signal is triggered not by the container itself, but by the container wall together with the medium. And if you ever try to accurately detect the level of chocolate, ketchup, etc, using a conventional capacitive sensor, you will discover that these media do not drain without leaving residue, and deposits regularly lead to sensing errors, which make error-free measurement impossible.
This challenge was faced also by Swiss chocolate manufacturer Gysi when seeking to equip the agitators of various tempering machines for heat treatment of chocolate when retrofitting new sensors for level regulation. The previous level detection system based on measuring the pressure difference was getting up in years and repeatedly had to be checked and cleaned at regular intervals, which incurred considerable effort and downtime. Therefore, Gysi looked for a new solution.
First attempts with a built-in sensor from the machine manufacturer, however, did not look promising. “We couldn’t leave the application unattended,” said Ulrich Streit, who is responsible for the technology at Gysi. “The container kept overflowing. That was caused by sensing errors right in the temperature range from 45 to 28°C, in which the tempering machine operated. All it took was a temperature change of a few degrees to change the permittivity of the medium in use enough that the sensor no longer switched correctly.
“Then we looked for an alternative and decided on a sensor with SmartLevel technology from sensor specialist Balluff. In making this selection, we could completely rely on Balluff.
“Thomas Zumbrunn, the responsible sales engineer of the Balluff subsidiary, provided us with comprehensive consultation. Together we chose the right sensor based on the polarity of the medium to be measured — the chocolate — and then checked it in a test installation in real-world operation. It worked right away. Now we have a solution that works without any errors whatsoever, even in long-term operation.”
“The new SmartLevel sensors of the M18 design also operate according to the capacitive principle,” explained Zumbrunn. “But with strongly conductive media, they open up new application fields while being significantly easier to handle. Thus they are capable of independently compensating for container walls and deposits, which enables error-free measurement without elaborate readjustments.
“At the same time, they are also compatible with all sensors used for level measurement of media having a dipole character. This applies to immersion applications and level detection through nonmetallic walls with a maximum thickness of 10 mm. As a result, even detection of chocolate through the 3 mm-thick membrane of the plastic sleeve into which the sensor is screwed is an easy task.”
This is possible because SmartLevel sensors operate at an oscillator frequency significantly higher than conventional capacitive sensors. In addition, the patented electronic processor unit gathers more information than is usually the case with capacitive level measurement. It evaluates not only the capacitance, but also the conductivity value of the medium. Since compact media have high, thin films of the same medium, but only low conductivity values, the new sensors have no trouble distinguishing between thin deposits and the real level. This means that sensing errors with media that do not drain without leaving residue, such as chocolate, are largely prevented.
Gysi now has six machines retrofitted with the new sensors, and there are already plans to retrofit additional systems. “Since we do the conversion ourselves,” said Streit, “the expenses are low. To do it, the sensor is simply inserted into a sealed plastic sleeve and rotated into a separate metal container in the container wall such that it is flush with the wall.”
The SmartLevel sensor is easy to adjust with a potentiometer, which has to be done only once after the installation. Furthermore, it operates without needing any maintenance whatsoever. Now cleaning procedures for just the sensor are entirely unnecessary; they are taken care of as part of the regular maintenance cycle.
The sensor in the container wall detects the level of the chocolate directly through the end face of the plastic sleeve in the container wall. If the chocolate falls below a certain fill level, the sensor triggers and after 30 seconds liquid chocolate is refilled until the optimum fill level is reached.
Unlike conventional capacitive sensors, these fill-level indicators do not have to be readjusted, neither during operation nor when changing the recipe. Thus the switch point between white and dark chocolate, for example, differs by only 3 mm.
“It is somewhat more expensive than a standard capacitive sensor,” said Streit, “but when you find a perfect solution that works permanently, the price plays only a subordinate role. The device pays itself off within a very short time.”
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