Commercial kitchens often purchase products such as potato salad, mustard or honey in large bucket-shaped containers. After filling, a hermetically sealed lid is pressed on to protect the product. The correct fit of this lid guarantees that the contents do not escape from the container during transportation and that the product reaches the consumer in a hygienically perfect condition. The lids can be of different colors, transparent, printed or have reflective stickers - conditions that cause considerable difficulties for optical sensors. What's more, they measure at certain points and therefore do not always detect a crooked lid.

Complete hygienic solution: The fully encapsulated ultrasonic sensor 'UMB800' with mounting bracket is fully EHEDG-certified.

© Pepperl+Fuchs

Ultrasonic sensors are therefore generally used in such applications. They detect the object regardless of its optical properties: The sensor is aimed at the center of the container from above and measures the distance to it using the reflected sound waves. The correctly positioned lid defines the target size. If it is missing, this is detected by the increased distance - in this case to the surface of the product in the container. If several lids are inadvertently pressed on, a shorter distance is measured. The so-called sonic lobe of the sensor detects a larger area, within which it recognizes the shortest distance. This means that even lids that are positioned at an angle can be reliably identified. The diameter of the sound beam at the measuring point can be adapted to the application. All incorrectly sealed containers can be reliably sorted out.

Insensitive to interference

In general, ultrasonic sensors are not irritated by irregular contours or recesses. In addition, the presence of dust and vapors in the application environment has only an insignificant influence on the propagation of the sound waves. Their functional principle makes ultrasonic transducers insensitive to moisture, splash water and adhesions. However, in conventional devices, the sound-generating membrane facing the process consists of a composite of materials and only has limited resistance to common cleaning chemicals. How can this be remedied? The installation of a standard ultrasonic sensor in a stainless steel housing, which at first glance seems obvious, significantly reduces its performance: its range is reduced and the unusable blind area becomes much larger.

Because of these limitations, fully encapsulated sensors, which are basically ideal for hygienic applications, have hardly been offered to date. In the pharmaceutical and food industries, slightly modified standard sensors are usually used, which require considerable additional effort for cleaning. Their life expectancy is also usually significantly shorter in such environments. Sensors with other measuring principles can be considered as an alternative in principle, but they do not offer the advantages of ultrasound and can pose other problems in terms of hygiene.

FDA- and EHEDG-compliant full encapsulation

To avoid this dilemma, Pepperl+Fuchs has developed a production process that allows a stainless steel membrane to be attached to the sensor housing by laser welding. Thanks to this method, the ultrasonic sensors of the 'UMB800' series are completely hygienically encapsulated without limiting their performance. Despite their small housing with a diameter of 18 mm and a length of 55 mm, they have a large measuring range with a blind zone of 70 mm and a range of 800 mm. The sensor housing is gap-free and hermetically sealed. It is made entirely of V4A stainless steel and achieves a surface roughness of just 0.8 µm. This minimizes the risk of microbial contamination and fully meets the requirements for hygienic applications. The sensor is EHEDG-certified and can withstand steam jet treatment as well as aggressive cleaning agents. Its ECOLAB certification and IP68/IP69K protection classes ensure that all cleaning processes can be maintained unchanged if it is installed in an existing system.

Typical hygiene applications

Thanks to these properties, the sensors in the series are suitable for use in hygienic environments, but also in aggressive environments such as chemical or vulcanization environments, as well as wherever particularly robust cleaning methods are required. Here are a few examples.

Food packaging in thermoformed trays: Before thermoformed trays are sealed, the system checks whether the tray contains the desired product in the desired dimensions. The sensor detects the product regardless of shape, color and texture. It is also unaffected by the irregular contours of natural products (such as meat, fruit, mushrooms or vegetables).

Level measurement in storage containers: Storage containers for liquids and granulates are used, for example, for filling processes for foodstuffs (potato chips, muesli, pasta, coffee powder) and pharmaceuticals (tablets, capsules, granulates, solutions). The sensor ensures that there is always a sufficient fill level in the container. The compact sensors can also be integrated directly into the product contact zone in small storage containers.

Measurement in aggressive environments: Thanks to its hermetic encapsulation, the sensor is even suitable for handling aggressive media. Its outer skin made of V4A stainless steel is completely resistant to almost all chemicals. This also applies to aggressive vapors. For example, the sensors can be used in vulcanization processes, such as in tire production. Conventional ultrasonic transducers would be attacked by the vapors released there and would decompose over time.

Author:
Carsten Heim is Product Manager for ultrasonic sensors at Pepperl+Fuchs Mannheim.