Mass flow controllers are compact, inconspicuous components that greatly influence industrial processes – for example, heat treatment. How can these components be individually adapted to different automation concepts?
The use of mass flow regulators (MFCs) is a state-of-the-art solution for the precise control of the gas atmosphere in hardening furnaces. However, such heat treatment plants are built very differently and have different gas supply requirements. Therefore, the components used must be adapted to the situation on site. For this purpose, it is also necessary that the mass flow regulators fit into the respective automation concepts.
The mass flow meters do not function independently when controlling the furnace atmosphere, as the required amounts of gas must be constantly regulated. The way the MFC is integrated into the respective system can vary widely. Possibilities range from data exchange via “classic” analog interfaces and digital network connections with all common fieldbus protocols, to plug-and-play MFC modules or complete control cabinets containing all components for controlling the furnace atmosphere.
Analog interface: a classic for simple applications
For smaller or simpler heat treatment plants where only a small amount of data is transferred, the classic analog interface can still be a good choice. Usually the usual 4 … 20 mA or 0 … 10 V signals are used, less often 0 … 5 V or 0 … 20 mA. Commissioning and maintenance are straightforward and the signals can be checked with simple tools. Analog devices are very easy to replace irrespective of the manufacturer as these signals are independent of the respective controller.
Only the actual value and setpoint of the controller as well as the error message in the form of a binary signal are transmitted via the analog interface. Since the introduction of digital electronics internally, the MFC with analog interface has also internally made much more information that can be read manually via the normally available service interface. However, it is a solution for maintenance or commissioning, not for day-to-day operations.
Digital interface: modern provider of diagnostic data
If, in addition to target and actual values, diagnostic data is also to be automatically transferred to the master controller, the mass flow controllers must communicate via digital interfaces, eg Profinet, EtherNet / IP, Profibus DP, Modbus TCP, EtherCAT, CANopen or RS485. Which bus is specifically used depends on the requirements of the overall system. In the selection, an important role is played by susceptibility to failure, cost of components, bus layout, availability of devices with this interface and safety aspects.
In principle, any information that is available internally can be transferred via digital interfaces. In practice, however, it is usually limited here to prevent too much data being transferred per cycle – and to limit the load on the bus by individual participants. Thus, in addition to the process data cyclically, i.e. every transmission cycle and several times per second, only the frequently required information is transmitted. The typical ones include, first of all, the temperature of the medium and the device as well as status messages from the device’s internal diagnostics. These are, for example, hardware errors, but also other operating states such as manual mode, auto-tuning or limit value detection. Furthermore, more comprehensive information is often provided, such as the degree of actuation of the actuators, i.e. how far the valve is open relative to the desired flow rate.
Less frequently used values are either transmitted less frequently or acyclically as the higher level controller polls them. This includes, for example, changes to control parameters, switching gas characteristics or user-specific values. In this way, however, it is also possible to read and visualize calibration and setting data from the MFC.
Basis for documentation, diagnosis and calculation
Digital interfaces thus open up great potential. By reading and storing continuous system data, the obligation to document quality assurance and the customer is automatically possible, especially since the respective device can also be clearly identified by its device and serial number. This guarantees traceability.
The possibilities that are “dormant” in the data provided by digital interfaces are also the basis of the current Industry 4.0 concepts. However, be careful: looking on your own, this information is in no way sufficient. Since the MFC only “knows” how far, for example, it needs to open the actuator for the required flow, this information alone is not sufficient for an in-depth analysis of the condition of the entire gas supply. Changed valve control can indicate very different reasons.
Detailed analysis is left to the operator’s discretion
The inlet pressure may be reduced as the gas cylinder may be empty. In the same way, the outlet pressure could also increase, although this would only be noticeable if the flow rate was high. If the values are lower than usual only at high flow, the inlet filter may also be clogged or there are other problems in the supply line. The significantly increased temperature at the control valve may also be responsible for the act. In order to draw further conclusions about the state of the entire system from changes in the degree of valve manipulation, further information is required, e.g. messages from pressure or temperature sensors or the level values of storage tanks. The operator is still responsible for the thorough analysis.
If you read the volume counters at the beginning and end of a batch process via the digital MFC interface, it is possible to record the consumption of a batch or a specific process step. This allows for a very precise calculation or settlement for the client. In addition, the total wear and tear can also allow conclusions to be drawn about the condition of the furnace, such as its tightness, or the condition of the processed products, such as reactivity, absorption of certain gases, or surface structure.
Subnets for large kilns
Sometimes there are also multi-tier bus systems with a very fast, complex bus for communication between parts of the system and a simpler bus for communication at the lowest level. The digitally controlled MFC can then be combined into such a subnetwork.
MFCs with a “simpler” interface are cheaper than those with a “high-end” interface, and the typical amount of data to be exchanged is relatively small. Such a subnet is connected to the higher level network through a gateway. In this way, local units can be inexpensively created, which are configured and installed as small “gas control” control cabinets or manifolds. They can be standardized and used as plug-and-play “teams” in larger systems. In principle, cabinets can always be the same, but can also be designed and individually adapted to the application.
These independent systems are then connected to the plant’s master PLC via, for example, an Ethernet connection point. They can also have their own logic, thanks to which adaptation to changed processes is possible without additional interference with the furnace control. Within this self-sufficient “black box” design optimization and further development are possible at any time without having to interfere with the furnace design itself. In addition, control cabinets offer built-in protection of components against environmental influences, mechanical damage and unauthorized tampering.
The “Made in Germany” expertise covers all applications
Bürkert Fluid Control Systems is one of the world’s leading manufacturers of measurement, control and regulation systems for liquids and gases. The solutions are used in many different industries and applications – from breweries and laboratories to medical, biological and space technologies. With a portfolio of over 30,000 products, the company, based in Ingelfingen in southern Germany, is the only supplier that covers all fluid control loop components, including measurement, control and regulation: from solenoid valves to process valves and analytical pneumatic actuators and sensors. Customer-specific system solutions and innovative products are constantly developed in five system houses in Germany, China and the USA as well as in four research and development centers.
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