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Process automation with integrated weighing and batching systems

Special publicationfrom atp 43 (2001) Number 5

Published by:Wolfgang Stiehl und Andreas Kaszkin

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1. Demands on weighing scales in the industrialprocess

Weighing and proportioning systems are assuming animportant role in many industrial processes, with thewidest possible variety of weighing tasks to be perfor-med. Both programmable controllers (PLCs) and pro-cess control systems (PCSs) are used for automatingthe production process.

There are integrated system solutions available to-day for the automation hardware of the PLC and pro-cess control systems. These solutions break down thetechnological barriers between programmable con-trollers and process control systems. Breaking downthese barriers opens up a new perspective on sharingweighing functionality between the weighing electro-nics and the PLC or PCS.

There are different types of weighing systems thatwork together with automation systems, dependingon the task. The weighing systems differ externally inmechanical design. The design of the weighing sy-stem forms the basis for fulfilling its function. Since al-most all modern weighing systems form a single unitwith microprocessor-based electronics, the electro-nics and the software can be regarded as a componentpart of the weighing system. Production automationmakes the following design demands on weighing systems:

• Flexibility in typical weighing functions

• Simple expandability of the weighing system

• Ability to adapt to the automation task

• Integrated communications concept

Weighing systems that meet these requirements canbe regarded as an integral component of the automa-tion system. In this sense, the weighing system is anintelligent automation object comprising

• Sensors

• Closed-loop control

• Actuators

It also performs its tasks in accordance with the specifications of the control system.

1.1 Flexibility in typical weighing functions

When considering typical weighing functions, theweighing systems used in the production process canbe divided into two groups:

• Weighing systems for continuous weighing

• Weighing systems for discontinuous weighing

In the case of continuous weighing systems, weigh-ing and proportioning is an ongoing process, e.g.weighing of material on a conveyor belt (belt scales),proportioning of an uninterrupted flow of materialfrom a weighing container (loss in weight).Discontinuous weighing systems are concerned withweighing portions in a process step, e.g. in producingmixtures according to a recipe (mixture scales), whenfilling sacks (bagging scales), or when checking pro-duct quantities (check weighers).The differences in the tasks also determine the diffe-rences in the type of typical weighing functions ofevery type of scales. This is explained using the exam-ple of mixture scales and differential proportioningscales.

Process automation with integrated weighing and batching systems Wolfgang Stiehl and Andreas Kaszkin, Siemens AG

Process automation with integrated weighing and batching systems

Weighing and dosing is an important industrial subprocess which influences quality of production and pro-ducts importantly. It is embedded in modern production structures which are integrated into business pro-cesses. The base of an optimum total process lies in the vertical consistency, from the management level tothe process instruments, and in the horizontal consistency over all process steps (from the receipt of raw materials to the delivery of end products).Aside from the accuracy in weighing and dosing, the conceptual integration of weighing systems into modernautomation system contributes increasingly to the lasting success of a business.

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Batching scales

In the case of batching scales, it is necessary to con-trol the proportioning operation as well as weight determination. During proportioning, the weighing elec-tronics compare the actual product quantity alreadyproportioned with the setpoint, and switches the pro-portioning signals for course feed and fine feed. A to-lerance check is made at the end of the proportioningoperation.

Fig. 1: Timing diagram of a proportioning cycle on batching scales

During a proportioning operation, through-flow moni-toring checks that a minimum material flow is guaran-teed. If required, proportioning aids such as vibratorsand aerators can be activated automatically.

Along with the typical proportioning functions ofbatching scales described here, special demands aremade of the weighing electronics in individual cases,such as setpoint correction during a proportioningoperation. This can become necessary due to chan-ging product properties, or new specifications from theprocess chain.

Loss in weight scales

With loss in weight scales, the electronics have an-other task in addition to measuring the weight – calcu-lating the mass flow from the mass curve over timeand controlling the mass flow in accordance with thespecified setpoint. The proportioning container is fil-led with the product, and the proportioning equipmentis completely weighed while the product flow exits theproportioning equipment in free fall. The weighing el-ectronics measure the weight of the station at fixed in-tervals (fractions of seconds).The weight loss per timeunit is calculated from the weight difference betweenthe two measured values, taking account of the timedifference. The follow-on controller uses the calcula-ted actual mass flow and compares it with the setpointmass flow.In the case of deviations, the control signals are output

to the proportioning equipment in order to achieve thedesired discharge.

In a concrete industrial application, different typesof disturbance influences are at work in the systemsuch as shocks or vibrations that corrupt the measure-ments of the weight. There are a variety of strategiesfor handling such disturbances. In addition, problemssuch as the refilling of the weighing container have tobe managed. The quantity of material stored in theproportioning station is continuously reduced by thedischarged mass flow. When the minimum weight isreached, the weighing container must be refilled. Dur-ing refilling, the proportioning equipment continues todischarge mass. Since control of the mass flow is notpossible during this time, material discharge is carriedout retaining the operating parameters prior to filling.

To maintain the desired mass flow, there is a varietyof strategies available, similar to those used duringthe gravimetric phase to deal with disturbance. It isimportant to gain control of the changes in productproperties in the weighing container using closed-loop control methods.

Fig. 2: Working principle of differential proportioning scales

Loss in weight scales that are simultaneously acomponent part of an automation system offer manymethods of optimizing the process sequence in con-junction with the automation components. Detailedstudy atp 9/98 [3] of integrating weighing technologyinto process control systems, and practical experiencewith filling and batch scales show that it makes senseto accommodate part of the scales functionality inspecial electronics. If these special electronics, thatcan be used primarily for signal preprocessing, pro-portioning monitoring, and possibly closed- loop con-trol, are implemented as a module in an automationsystem, all the benefits can be optimally combined.

By directly embedding the weighing electronics inthe automation system, and thus creating optimal in-teraction with the CPU, the functions can be shared ac-cording to requirements. Faults that could occur as aresult of violating the sampling theorem are preventedby high measurement resolution, real-time signal pro-cessing, and filtering in the weighing electronics.

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Freely programmable, modular weighing systems, as-sembled using standardized components of the automation system, provide good a basis for expan-ding the weighing functions in an automation system.

Thanks to the programmability and scalability of theautomation system, the different types of weighingfunctions can be implemented using just a few typesof weighing modules. The weighing module meets thestandard requirements, and adaptation to the specificsector takes place in the automation system, makinglogistics and maintenance easier and more efficient.Practice shows that this approach is extremely promis-ing. It enables plant operators to implement an opti-mal solution tailored to their requirements. Such astandardized and integrated solution offers significantbenefits in all phases of the plant life cycle: sequencesfor ordering, configuring, programming, startup, oper-ation, maintenance, and service are simplified.

1.2 Simple expandability of the weighing system

With batching scales, bagging scales, filling scales,loss in weight scales, and weighfeeders, extensiveclosed-loop and open-loop control functions must beimplemented in addition to the weighing functionality.

Multi-component scales for producing mixtures, forexample, can be put together from standard automa-tion system components (e.g. weighing modules, ana-log input/output modules, binary input/output mod-ules). The weighing modules are not only simpleweight transducers. They also handle the function ofsingle-component scales. Two digital outputs directon the weighing module are sufficient because in thecase of multi-component scales, they can be connec-ted to the proportioning equipment of a specific silovia digital output modules of the automation system.Thanks to scalable design, the number of storage silosis not limited by a specific hardware configuration ofthe weighing electronics. The recipe sequence controlof the higherlevel control system transfers to theweighing module the product-specific proportioningparameters for individual components such as set-point, coarse flow and fine flow cutoff value, tolerancelimits, settling time, and monitoring limit values.

Thanks to the sharing of tasks between the weighingelectronics and the PLC CPU, the control functions areundependent from the weighing and proportioningfunction when the PLC program is expanded to suitthe task. This reduces the probability of programmingerrors. The overhead for interface software for externalweighing electronics is reduced, possible runtime pro-blems in the case of system expansions are preven-ted, and the commissioning time is reduced. In addition, this imposes limits on the volume of infor-mation to be exchanged, and system efficiency is retained.

Fig. 3: Multi-component scales based on SIMATIC S7 in the distributed auto-

mation concept

This task sharing has benefits with time-critical opera-tions in particular, as the example of filling and bag-ging scales will show later. The weighing modulesperform the assigned task fully autonomously. Thismeans that emergency operation without a PLC CPU isalso possible. Since the controller is freely pro-grammable, the interlocks and logic operations can bedefined using the signal and scales states available inthe data of the PLC CPU.

1.3 Integration into the automation environment

As integral components of automation solutions [2],scales face additional demands to those of scales func-tionality (accuracy, resolution).

These demands relate to cost of ownership in partic-ular. Scales that on the one hand perform weighingtasks in an automation system, and on the other hand,work optimally together with this automation systemin all respects, must appear to the automation systemto have all the properties of an integral part of that sys-tem.What properties does an automation system expect?

Standardized hardware and software

Standardized and tested hardware and software formthe platform for the automation system. Optimizedconfiguring tools reduce the engineering overhead.High quality of the automation components forms thebasis for system reliability.

Alarm concept, diagnostics

During operation, it is extremely important not only toquickly detect and diagnose actual faults, but also todetect changes in plant characteristics and thus pre-vent possible faults in the future.

The diagnostics capability of an automation compo-nent encompasses system and process diagnostics.

System diagnostics includes acquisition, signaling,

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and evaluation of statuses within an automation orprocess control system.

Process diagnostics involves acquisition, signaling,and evaluation of process statuses. Process diagnos-tics analyses process-oriented statuses and sequen-ces and tests them for, e.g. reliability. The aim of diagnostics functions is to achieve more ef-ficient startup and operation. By using load cells withan exceptionally long service life and weighing modu-les with an MTBF of well over 100 years in conjunctionwith modern service methods it is possible to achieveexcellent availability.

1.4 Integrated communications concept

The integrated communications concept of an auto-mation system is also used by the integrated weighingmodule. It is incorporated into the structure of the or-ganization's communications levels.

Fig. 4: Weighing electronics in the communications network of the automa-

tion system

To an increasing extent, scales integrated into an auto-mation system can utilize the full performance power ofthe system, for remote loading of new software, for ex-ample. This can refer both to improvements in weigh-ing functionality as well as simple troubleshooting.

The necessary replacement of a module in theevent of a fault can be carried out in the shortest pos-sible time because the replacement of tools integratedinto the automation system is supported. These caninclude configuring tools, integrated detection routi-nes for new hardware, and automatic parameteriza-tion. A start has already been made in using the Inter-net for these purposes. Diagnostics functions providethe greatest benefit when statuses from the produc-tion process and the automation system are acquired,analyzed, and visualized, or actions are automaticallytriggered throughout the system and according tostandardized principles. The benefits of process di-agnostics can be shown clearly using the example of

filling scales from Siemens – SIWAREX A/AWS inte-grated into the automation system.The SIWAREX A module automatically controls fast fil-ling of the product with a cutoff accuracy of less than 1 ms. It calculates the statistical data from a largenumber of proportioning operations. Now it is easilypossible to analyze the data in an automation systemin order, for example, to determine a slow deteriora-tion in the proportioning quality, and to initiate a checkof the filling system at the right time, before the dete-rioration in proportioning quality necessitates actualstopping of the process.

2. Solution approaches for sharing theweighing functions

Sharing of weighing functions in the automation sys-tem has been subject to constant change in recentyears. The reasons for this are found in the search forefficiency in solving weighing tasks in the automationenvironment. The performance power of the hardwarecomponents is no longer of itself the decisive factor inchoosing a certain solution architecture. A modernweighing solution has to meet the following scales-re-lated requirements:

• High level of operating safety

• Simple operation

• Extremely good reproducibility

• High accuracy

It also has to meet the following requirements with re-gard to the automation characteristics:

• Integration (hardware/software)

• Flexibility

• Standardization

User-oriented implementation results in the follo-wing three aspects:

• Accuracy and reproducibility requirements demandthe use of special high-quality functional units forsignal acquisition, signal adaptation, A/D conver-sion, pre-processing, and open-loop and closed-loop control functions. The task demands that theweighing signals have a resolution of up to one mil-lion digitization steps. Control of material flows inproportioning and filling using the binary signalsfrom the scales must be carried out with a time resolution of down to one millisecond.

• In addition, different functions are necessary for thesolution of the overall task, depending on the appli-cation. The entire added-value chain of the produc-tion system must therefore be considered. The ex-amples given here are the automatic filling of silosor the removal of the end product. A system is re-quired here that allows the necessary functions tobe implemented simply.

• It is also necessary to integrate weighing systemsas fully as possible into the overall automation sys-tem. This not only encompasses the communicati-

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on system, but also requires the functional integrationand engineering of all automation functions with stan-dard tools.

These aspects result in the following solution thatmeets all requirements with minimum overhead:

• Weighing function modules possessing the requi-red hardware and firmware as standard, to meethigh accuracy requirements and time-critical tasks.These function modules have all the features of thestandard automation system and are therefore fullycompatible.

• Use of standard automation systems allows the im-plementation of application-specific tasks. This en-ables not only the use of the standards for enginee-ring, visualization, and archiving, etc., that are usedanyway, but at the same time total integration of theentire automation system without additional over-head. Here, sector-specific and application-specificsolutions can be implemented with particular flexi-bility. Special weighing and process engineeringmethods or recipes can be software-protectedagainst access by third parties (know-how-protected).

• This concept thus turns weighing technology intoan automation object integrated into the overall au-tomation system. Thanks to the total compatibilitymentioned above, the standard automation func-tions and the weighing functions now form a ho-mogenous unit from the user's point of view, thusmeeting demands for integration, ease of use, andflexibility based on existing standards.

Of course, in this solution the components used canbe arranged either in centralized or distributed confi-gurations. The advantage of centralized configurationis the time-optimal interplay of the controller CPU andthe weighing processor. With distributed configura-tion, that is, where components are integrated into thescales, the weighing system simply becomes an auton-omous field device connected to the automation tech-nology via the open PROFIBUS system.

3. The benefits of integrating scales intothe automation system, as shown by auto-matic filling scales (AWI) OIML - Automatic Weighing Instrument

Filling scales usually operate within the environmentof an automation solution. The product must be sup-plied, the containers to be filled must be available atthe right time, and further transport of the filled quan-tities must be controlled. The earlier solution of thistask provides for automatic filling scales (automaticscales for weighing) connected to a PLC (programma-ble controller). The scales operate independently andthe volume of data to be exchanged with the PLC mustbe kept low.

Fig. 5 gives a schematic representation of this solu-tion approach.

Fig. 5: Interfaces for weighing data in conventionally connect

Fig. 6: Filling scales based on SIWAREX AWS from Siemens

A new solution provides for the integration of theweighing electronics into the automation system. Theweighing electronics are a component part of the au-tomation system. The binary input/output modules areused for system control. The scales and other systemsections are parameterized (e.g. proportioning speeds)using the operator panel. If required, a printer can beconnected direct to the weighing electronics or to theoperator panel, or to the control system via a standardinterface module. Non-time-critical tasks can be defi-ned flexibly in the PLC central controller using thestandard languages of IEC 61131, for example. An example of just such an integrated weighing system is the combination of SIWAREX (weighing module) +SIMATIC (PLC) from Siemens.

Integration of the weighing electronics into the PLCopens up new, previously unavailable possibilities forthe plant planner and operator. These are essentiallythe following:

• No additional software for connecting external scales

• Optimal coordination of the system modules

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• Application-dependent scalability of the system

• Cost optimization through requirement-oriented system expansion

• Integrated documentation

• Integrated function and operator philosophy

• Solution of control tasks within one system

• Customized adaptations thanks to free programma-bility

• Innovations are system-wide

• Independence in selecting resources

Fig. 7: Automation concept with integral weighing electronics

The benefits of the integrated concept become moremarked in multi-scales systems. Functions like humanmachine interface, signaling and logging, plant con-trol, scales interlocks, and diagnostics, are availableonce for all scales on a single hardware platform anddo not need to be set up in the weighing electronics ofevery set of scales. This distribution of functions al-lows efficient dimensioning of the overall system.

4. Summary

The advantages and disadvantages of integratingweighing electronics into automation systems dealtwith in this article are summarized in Table 1. From thepoint of view of the planners, maintenance engineers,and operators, the benefits of the integrated solutionoutweigh the disadvantages (cf. Column 1 in Table 1).

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Benefits:

Disadvantages:

Table 1: Advantage of integrating weighing electronicsinto the automation system.

Weighing electronics inte-grated into the automati-on system

Integration or improvedconnections of PLC and weighing electronics

Uniform hardware basis

Uniform interfaces andprotocols

Standardized tools forconfiguring and mainten-ance

Integrated and automateddocumentation

Simple expansion facili-ties thanks to openinterfaces and sharedhardware platform

Improved spare partsmanagement thanks touniform hardware basis

Simplified planning andstartup thanks to standardized interfaces

External weighing electro-nics connected to auto-mation system

Weighing electronicsrestricted to automationsystem manufacturer

Weighing electronics notconnected to the automa-tion system

Wide range of differentproducts

Individual solutions for alltypes of scales possible

Additional vendor-specificspecial functions

Different hardware plat-forms

Different interfaces andprotocols

Interface adaptation re-sults in increased plan-ning and startup overhead

Different tools for plan-ning, startup, and mainte-nance

Some of the wide varietyof functions not required

High overhead for docu-mentation

Expansion possibilitiesfrequently limited by in-terface bottlenecks

Wide variety of types andmanufacturers generateshigh overhead for spareparts management

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