Speed measurement of mixing heads in magnetic stirrers

A magnetic field sensor with a dual Hall probe detects the speed and rotation direction of the mixing head in the magnetic stirrers of stainless steel components manufacturer Armaturenwerk Hötensleben GmbH

Magnetic coupling of the mixing head ensures the integrity of the vessel and the product

Permanent magnets in the drive unit build a magnetic coupling with magnets in the mixing head. This is so strong that the mixing head can be driven through the vessel wall at 490 revolutions per minute. The vessel plate must be welded into the bottom beforehand in order to mount the stirrer. Thanks to the magnetic coupling of the drive and the mixing head, the integrity of the vessel is ensured throughout in spite of the mixing process. This therefore excludes the possibility of any sterilization problems and contamination, which can occur with conventional stirrers with a shaft bushing. Only the mixing head and the ceramic bearings are in contact with the medium. They are designed and manufactured so that they can be cleaned easily without any residue. 

Sensor detects speed and rotation direction 

Turck developed a sensor with a separate signal processor. The actual sensor head can therefore be manufactured with a very compact design in order to integrate it at the upper edge of the mixing head support. The sensor head is provided with two active faces, so-called Hall probes, which measure the behavior of the magnetic field by an internal signal offset over time. This therefore makes it possible to not only measure the speed but also the rotation direction. The sensor is assigned parameters for a set rotation direction. A malfunction is present if it later detects an inverted rotation direction. The LED on the processing unit is then red and the controller also indicates a warning, depending on the user setting. 

Process safety through speed and rotation direction monitoring

Not all customers require additional information such as the measurement of speed and rotation direction. The measurement of these two variables increases the safety of the production process in specialist sectors and hazardous plant areas. The pharmaceutical and biotech industry in particular is subject to stringent requirements with regard to the error-free recording, storing and documentation of data for every process step. An incorrect speed can quickly lead to a reduced product yield and significant financial losses. An undetected incorrect rotation direction of the mixing head endangers the magnetic stirrer itself and in explosion hazardous areas could even cause explosions due to electrostatic charge as a result of friction.

One sensor for all magnetic stirrers

AWH manufactures VPureMix magnetic stirrers in nine sizes, which are fitted with a different number of magnets on the drive and mixing head. If the customer orders a mixer, AWH configures the sensor via IO-Link. The interface for IO-Link is nevertheless kept inaccessible for the end customer. “The fact that we can cover all sizes with just one sensor is really useful. The software for setting the sensors is also really easy to use. A drop-down list enables me to select the relevant stirrer size. All other parameters are applied automatically via the stored data sets. Everything is done in up to three clicks,” AWH product manager Anja Hauffe explains. “Even the mounting of the sensor impressed me. The separate sensor unit cannot be damaged, and unlike those of other manufacturers, the processing unit is not made from plastic but stainless steel.”