Control technology as a breakwater

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Cocktail glass in motion
Cocktail glass in motion
Researchers have discovered how sloshing movements can be actively suppressed during the highly dynamic transportation of liquids.

In highly automated industrial processes, machines, materials and goods are often moved very quickly. It is important that these movements are carried out precisely and safely. Handling liquids in open containers therefore poses a particular challenge, as uncontrolled sloshing movements must also be avoided. This comes into play, for example, when hazardous liquids or molten metals are being transported and spillage poses a significant safety risk. The use of control technology is also relevant in medical technology or pharmaceutical production, where open containers often have to be accelerated or decelerated very quickly.

Liquid waves meet control technology

A team in the research field of control engineering and process automation (Institute of Mechanics and Mechatronics) recently developed several new control methods that make it possible to move open liquid containers highly dynamically without simultaneously causing the liquid to slosh. The methods allow the container to move in any spatial direction and at the same time make maximum use of the transport system’s freedom of movement to prevent the liquid from moving. For example, the researchers were able to demonstrate that when using a multi-axis industrial robot for transportation, the liquid remains virtually motionless despite the highly dynamic movement of the container. The team was also able to show that even violent sloshing movements can be stopped in a controlled manner within a very short time. They investigated this in an industrial transport system based on magnetic levitation.

During their investigations, the team discovered a dynamic storage condition for the liquid container that can completely eliminate the first - and usually dominant - sloshing mode of the liquid surface. By selecting a special system input, the liquid-filled container also acquires the important property of differential flatness, which significantly simplifies the design of a highly dynamic control system. By specifying a so-called differentially flat output, even complex spatial movements of the tank can be easily determined with guaranteed suppression of the liquid movement. The computational effort for this is relatively low," explains Stefan Jakubek.

The problem of the container shape

The extent to which a liquid sloshes depends largely on the shape of the respective container. This is because it determines the boundary conditions which, in combination with the partial differential equations of fluid dynamics, determine the movement of the liquid surface. As this relationship is very complex, it has so far only been possible to specify control methods for containers with relatively simple shapes.

This problem has now been solved for the first time in cooperation with the Measurement Technology and Actuators research department. This is where the expertise for finite element (FE) methods for multi-field problems is bundled. "A control engineering model of the dynamics of the liquid surface in containers of any shape can now be derived automatically by determining the resulting fluid reaction forces using an FE model of the liquid dynamics," says Florian Toth. The model and its properties make it possible to directly calculate the controls for any spatial movements of the container. The multidisciplinary team was able to demonstrate how effective this approach is using the example of a filled cocktail glass, which is particularly prone to unstable fluid movements due to its shape.

For research, these results and the "proof of concept" represent significant progress in the control of liquid movements in industrial applications. At the same time, they open up new possibilities for more precise and efficient handling of liquid containers of various shapes.

The following video illustrates the new control methods and shows their outstanding performance: