Made-to-measure Closed-Loop Control

Pneumatic control circuits are highly cost-efficient and are thus an effective way of supporting process optimisation.

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Pneumatic control circuits are highly cost-efficient and are thus an effective way of supporting process optimisation. These benefits are being recognised by more and more operators of water and wastewater treatment plants. But what degree of position control is required for which application?

There are basically three levels of closed-loop control when it comes to automating water and wastewater treatment plants, with the typical approach involving the integration of an external positioner such as the CMSX from Festo.

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The benefits of pneumatic control circuits are being recognised more commonly by water operators.
External Positioners CMSX.

Level 1: Economical controller for maintaining position

The first level in the automation of water and wastewater treatment plants is the use of conventional external positioners like the economical CMSX. The position of the process valve can be flexibly determined via an analogue signal.

In closed-loop mode, the CMSX continuously compares the setpoint signal with the actual position of the quarter turn actuator. Any deviations can thus be immediately detected and corrected. In the standard variant of the positioner, its micro controller continuously displays the current position of the process valve.

This information is also available externally via an analogue feedback signal. The closed-loop controller doesn't require any compressed air at standstill.

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The regulated linear actuator DFPI, integrating all the components required for closed-loop control in the housing

Level 2: Positioner function in the valve terminal concept

The number of fixed-bed filters ranges from two for small, local water treatment plants up to 48 or more in water treatment plants for large cities.

Any automation concept can thus be decentralised and flexible as well as combine pneumatic and electrical components. The decentralised automation platform CPX with the pneumatic valve terminal MPA and the controller CPX-CEC has proven especially suitable.

It controls the pneumatically actuated process valves locally, processes the electrical field signals from the end-position switches, sensor boxes and measuring devices, controls the filtration circuit and establishes the connection to the process management level.

Each fixed-bed filter is assigned its own control cabinet in which the decentralised automation platform is integrated together with a control panel.

Level 3: Linear actuator with closed-loop control

The highest level of integrating closed-loop control tasks in the automation of water and wastewater treatment plants is integrating the closed-loop control function into the actuator itself.

This is because external attachments for closed-loop control frequently cause problems: lever systems often need to be adjusted, aggressive environmental conditions lead to corrosion and dust deposits cause wear at moving parts. Furthermore, externally mounted attachments are susceptible to damage during installation and operation.

By contrast, the actuators from Festo's DFPI series don't suffer from these problems. All the system components required for closed-loop control are integrated in the housing. The cylinder, displacement encoder, valve block and positioner are safely accommodated in the housing and offer further advantages compared with modular systems such as compact dimensions and robustness for use in outdoor systems.

Conversion from electrics to pneumatics

An impressive example of an application of the DFPI is the treatment and storage of rainwater runoff in sewer systems. All in all, there are approximately 45,000 rainwater basins in operation in Germany alone. In harsh ambient conditions such as these, it has proven advantageous to convert the process control for a rainwater basin to the pneumatic DFPI. It reliably protects the position control system from corrosion and moisture in its housing. Another advantage is that compressed air is easy to store. This means that energy reserves are always available in the event of a power failure.

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