Automatic weighing instruments for legal-for-trade applications: requirements and directives

Weighing technology plays an important role in many industrial applications, for example in bulk material logistics. Depending on the application, different types of automatic weighing instruments are used. Weighing instruments that are to be used in legal-for-trade applications need to comply with the European Measurement Instruments Directive (MID). The exact requirements and definitions for the different types of weighing instruments are specified by the International Organisation for Legal Metrology (OIML).

In principle, all modern electromechanical weighing instruments for industrial applications have a similar structure: The signal supplied by a force sensor that measures the weight force is conditioned in an amplifier for visualization on a display or subsequent processing in weighing electronics. In industrial applications, an additional control is used for controlling the weighing process - this setup is commonly called automatic weighing instrument (AWI). Strain gage-based (SG) load cells are generally used as sensors.

Different types of automatic weighing instruments

Depending on the task in legal-for-trade applications, different types of automatic weighing instruments are defined; the functions and weighing processes for each type are specified in the corresponding OIML directive.

The first type of weighing instrument is the "Automatic Gravimetric Filling Instrument". This type of weighing instrument is used for filling containers with a predetermined and virtually constant mass of product from bulk. The filling of bags is a typical application for such weighing instruments. For filling the containers as fast as possible, a special feeding device subdivided into coarse and fine flow is used in many cases. The "Automatic Gravimetric Filling Instrument" complies with OIML directive R61.

The second type of weighing instrument is the "Discontinuous Totalizing Automatic Weighing Instrument". It determines the total mass by weighing each discrete load in sequence and summing the weighing results. Unloading bulk materials from ships is a typical application for this type of weighing instrument. Details, for example taring, are specified in OIML directive R107.

The third and last type of weighing instrument is the "Automatic Catchweighing Instrument" in compliance with OIML directive R51. The weighing instrument weighs single loads that may vary with each weighing process. Such weighing instruments are used, for example, in wheel loaders which enable the quantity of bulk material emptied from a container to be determined through legal-for-trade weighing during loading.

Load cell, amplifier and scale electronics

Legal-for trade weighing instruments for industrial applications need to comply with the European Measurement Instruments Directive (MID 2004/22/EC) which has been transposed into national law in the German Weights and Measures Act (Eichgesetz).

Modern weighing instruments increasingly incorporate digital load cells such as HBM's FIT load cells (Fast Intelligent Transducer). The measured value is digitized directly in the load cell and transmitted to a display and analysis unit. Encryption of the measured value is an appropriate means of preventing manipulation of the measured value during transfer.

HBM's DIS2116 and DWS2103 digital scale electronics used in combination with digital load cells enable legal-for-trade, manipulation-proof weighing machines to be implemented. Data is sent from the load cells to the scale electronics in encrypted form.

However, analog load cells, too, enable legal-for-trade, automatic weighing instruments to be implemented. For this purpose, HBM offers the AED transducer electronics which digitizes the signal supplied by an analog load cell and then sends it to the scale electronics or a superordinate control system. In this case, too, securely encrypted transfer to the scale electronics ensures both protection against manipulation and legal-for-trade capability.

The transducer electronics offers many more advantages. A wide range of fieldbus interfaces that are optionally available allow easy integration of the weighing equipment into automation environments, for example, based on a PLC. CANopen, Profibus DP and DeviceNet are currently supported. In addition, serial RS232 or RS485 interfaces can be used. This allows for high flexibility in system integration and enables legal-for-trade weighing technology to be incorporated into existing applications.

In these applications, the transducer electronics can directly perform control tasks. This results in significantly increased processing speed in comparison to solutions involving communication at first via the scale electronics and subsequently via the PLC. In particular in filling machines this enables direct control of the feeding device and this accelerates the filling process.

To sum up: Competent system partner

HBM relies on expert knowledge in measurement technology and is a competent weighing technology system partner providing all components for legal-for-trade industrial applications - ranging from analog or digital load cells through amplifiers or transducer electronics and scale electronics. Many different types of analog and digital load cells enable virtually any type of industrial weighing application to be implemented. The wide range of options offered by AED transducer electronics and DWS2103/2116 scale electronics in particular in communications facilitate integration of the weighing equipment into superordinate, PLC-based control architectures. Open interfaces enable different applications to be quickly and flexibly implemented.

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