Legal-for-trade weighing systems in open automation applications

Weighing technology plays an important role in many areas of logistics and in commerce. For example, legal-for-trade, automatic weighing instruments are often used when bulk materials are measured out or loaded. Measurement technology specialist HBM provides the components and systems for weighing technology, to allow scales to be flexibly integrated into filling or loading systems. Digital scale electronics are a typical example of this. The optional Anybus modules used to implement the different fieldbus interfaces simplify integration into a wide variety of automation architectures.

In Germany, the weighing instruments used in commercial applications must be verified. One of the reasons for this verification (statement of conformity) is as follows: “... to create conditions for accurate measurement in commercial transactions, in the interests of fair trading.” (www.eichamt.de) Wherever commerce involves goods that are weighed, there is a great temptation to manipulate the scales. When a ship is unloaded, for example, automatic weighing instruments quickly weigh goods to a value of several million euro. This is why Germany and the European Union apply a series of laws and regulations, designed to ensure that all the weighing instruments used in commerce are verified. The requirements of the European Measuring Instruments Directive MID 2004/22/EC are implemented in German law by the Weights and Measures Act.

The advantages of digital weighing technology

Nowadays, modern weighing instruments increasingly make use of digital load cells, such as the FIT® load cells (Fast Intelligent Transducer), from HBM. Here, the measured value is digitized directly in the load cell, and then transferred to a display and evaluation unit, the so-called scale electronics. The big advantage of digital data transfer is that there is practically no interference on the measuring leads.

A suitable way to prevent manipulation of the measured value during transmission is to encrypt it. The DIS2116 digital scale electronics from HBM, in conjunction with type C16i digital load cells, for example, allow legal-for-trade, manipulation-proof weighing instruments to be set up. The data are transferred from the load cells to the scale electronics in encrypted form. The Advanced Encryption Standard (AES) is used here, with a 256-bit key. Using this modern encryption algorithm also meets the requirements of WELMEC guideline 7.2. As manipulation is prevented during the transmission of data, the legal-for-trade capability of a scale is always ensured.

But there are further options available with digital signal transmission. These include, for example, transferring the data over a wireless transmission system. This automatically gives the transducers electrical isolation, whilst simultaneously providing greater physical separation between the transducers and the scale electronics. As digital load cells also transfer status and error messages in addition to the actual measured values, it is very easy to implement asset and facility management at the same time. Faulty transducers are easily identified and it is a simple matter to replace them, as required. This greatly reduces service and maintenance expenditure.

Communication with control systems

In industry, measuring instruments are often integrated into automated systems on the basis of a PLC, for example. So-called automatic weighing instruments frequently work without intervention from an operator. The measuring process starts and stops automatically. So as well as manipulation protection, it is particularly important for this type of scale application that the weighing technology can easily communicate with the controller over the usual interfaces and bus systems. This is firstly because it controls the functions of the weighing instrument, and secondly because it reads out the measured values and continues to process them.

HBM now uses Anybus technology from HMS Industrial Networks GmbH in their DIS2116 digital scale electronics. The complete fieldbus connection is housed in a small, optional module that is simply inserted into the scale electronics, at the back. The big advantage of the system is its ease of interchangeability. With scale electronics, it is possible to simply use an additional fieldbus without having to implement customized special solutions for the fieldbus connection.

HBM currently provides Anybus modules for CANopen, Profibus DP, Profinet IO and Modbus TCP (Ethernet). For example, there is minimal expenditure involved should a system integrator want to re-implement a weighing instrument that has already been set up in an environment with CANopen as the bus system, in an environment with Profibus DP. It merely requires a suitable Anybus module – virtually no changes have to be made to the scale setup or to the software.

Easily integrated digital weighing technology

Measurement technology specialist HBM is a competent system partner for weighing technology, providing all the requisite components for legal-for-trade, industrial applications, from load cell to scale electronics. Practically all industrial weighing applications can be implemented.

The extensive options of the DIS2116 scale electronic, particularly for communication, simplify the integration of the weighing technology into a primary control architecture, such as one based on a PLC. A wide variety of applications with different fieldbus systems can be quickly and flexibly implemented with optional Anybus modules.