Altan T. Metal Forming Handbook

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3.5.7 Architecture and hardware configuration

The control system is divided into modules according to functional

groupings. This applies to the circuit diagram, the arrangement of the

switch cabinet and structure of the PLC software. The functional mod-

ules do not necessarily correspond to the mechanical structural units –

in some cases similar functions such as all emergency stop circuits, all

protective grilles and the lubrication are grouped together.

Criteria used to determine the control structure include the logical and

functional coherence between different system elements and the mini-

mization of interfaces between the individual groups of functions. A well

designed architecture of the control system enhances clarity and trans-

parency of the electrical documentation and thus reduces the time spent

in design and start-up as well as in troubleshoooting during production.

3.5.8 Architecture of the PLC software

The PLC software is designed to correspond to the functional groupings

of the circuit diagram. Functional sequences, in particular those used

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Press control systems

Fig. 3.5.5 PLC European manufacturer

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

for workpiece handling and die changing, are programmed using

sequential programming methods. Where fast response times are called

for, the relevant PLC input signals are defined as interrupt inputs

which, as the name suggests, interrupt cyclically processed control soft-

ware functions and trigger a response program which responds to exter-

nal events in an extremely short time.

3.5.9 Future outlook

Programmable logic controllers are ideally suited to their respective

application as single-purpose systems. They are particularly well suited

for the processing of logic operations. The trend is towards equipping

systems with ever more extensive, cost-intensive processing capability.

Development of programming languages, on the other hand, has

slowed down almost to a halt. Programs are still being written in the

form of ladder diagrams and instruction lists, which are comparable to

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Fundamentals of press design

Fig. 3.5.6

PLC American

manufacturer

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

an assembler language. These originally application-oriented program-

ming languages are now being used for achieving visual display, data

processing, data management and communication functions.

The consequences of this trend are high software development and

maintenance costs, non-transparent programs – which not even pro-

gram design standardization can make any easier to understand – and

long lead times to put the press into operation. To permit the integra-

tion of this type function, the PLC has to be equipped with additional

expensive hardware modules.

In comparison, industrial computers, for example PCs, offer the ben-

efits of graphic user interfaces, data processing capability and facility

for object-oriented programming through the use of powerful pro-

gramming tools. The basic requirements imposed on a future-oriented

press control system, i. e. the ability to access as many functions and as

much information as possible directly at the machine, are only achiev-

able in the future through the use of greater intelligence and more com-

puting capacity. In addition, modern automation engineering calls for

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Press control systems

Fig. 3.5.7 Switch cabinet with components for main press drive

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

control systems with greater networking capability through standard

networks for the exchange of control, production and process data as

well as the capability for diagnosis and intervention in the control sys-

tem through public telecommunication networks. The PLC alone is not

able to cope with these demands, although its continued existence as a

subordinate, fast machine control concept is still justified.

Practically all the components required for realization of the above

functions are available in the industrial computer. Alongside its com-

puting performance, the industrial computer also profits from the

open-ended software world. Furthermore, despite the steady rise in per-

formance limits, the costs of the technology are falling in many cases.

In view of this trend, control structures featuring distributed intelli-

gence and higher-level industrial computers interlinked with fast com-

munication systems are certain to gain increasingly in popularity.

Added to this is the increased use of distributed input/output modules

and sensor/actuator communication systems with a view to reducing

the necessary installation and wiring input. Methods of programming

the control software will become more standardized and simplified in

the future, for example by the introduction of the international stan-

dard IEC 1131.

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Fundamentals of press design

Fig. 3.5.8 Components of a servo drive system

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

Other approaches exist in the form of programming through the use

of status graphs, application (“object”) oriented programming and user

language. In this case, reusable modules with any degree of complexity

can be generated and stored in a user-specific macro library. The pre-

pared objects in the library are in parametric form and can be grouped

with a minimum of complication to cover a large number of the press

or machine functions. All that is necessary to then join the interfaces

for data exchange.

Information technology is being integrated to an ever greater degree

into the plants or machines themselves.

As internationally stable “information highways” become available,

new forms of fault diagnostics and maintenance back-up will naturally

develop, allowing possible faults and operating errors to be ascertained

and mechanical faults to be traced using video technology or micro-

phones, so realizing a system of temporary or continuous on-line moni-

toring.

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Press control systems

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

3 Fundamentals of press design

3.6 Press safety and certification

3.6.1 Accident prevention

Due to their mode of operation and the energy input levels involved,

presses represent – compared to many other machines – a higher poten-

tial hazard to the operator. However, as a result of considerable effort,

notably the revision of the German accident prevention regulations for

presses VBG7n5.1 dated April 1, 1987, the number of serious accidents

occurring on or at presses has been substantially reduced. In addition to

complying with legislation on safety requirements based on the “state

of the art”, manufacturers must also have the capability to recognize

and implement sensible safety measures. The manufacturer’s own sphere

of responsibility in this field is a major portion of European safety legis-

lation. As part of the service package offered by manufacturers, the

operating instruction manual contains special safety provisions and

instructions.

A major goal is to make the press operator recognize the necessity for

safety measures. The unpredictable nature of human reaction in surprise

or emergency situations must be taken into account here. Highlighting

the accident risk is the first and most important step when training

machine personnel on safety matters. As long as the operator or mainte-

nance personnel are not convinced of the necessity for safety measures,

they will be inclined to find ways of bypassing them. Excessive or super-

fluous safety measures by the manufacturer are also certain to create

greater willingness on the part of operating staff to attempt to override

safety functions. Reasons that justify negligence with regard to safety

measures become quickly insignificant if an accident does actually occur.

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

3.6.2Legislation

Alongside legislation and regulations defining requirements imposed

on technical products – such as the German Pressure Vessel and Appli-

ance Safety Acts – legislation has also been adopted which regulates cas-

es of human injury caused by technical equipment. If the machine

manufacturer adheres to the technological standards defined by the so-

called characteristic requirements, it may be assumed that he will be

exempt from liability. An accident taking place despite these precau-

tions is defined as a “statutory accident”, in which case the companies

providing industrial accident insurance are responsible for settling any

financial claims arising as a result of the damage.

In Europe, these characteristic requirements are defined by the EC

Machine Directive, the EMC (electromagnetic compatibility) Directive

and the Directive Governing Simple Pressure Vessels. Product liability is

also treated on a standardized basis in Europe through the EC Directive

for Harmonization of Product Liability Legislation.

In the case of particularly dangerous machines, state agencies or

institutions acting on behalf of state agencies are commissioned to

check and certify equipment provided the technical requirements of

the equipment have been correctly implemented. Foreign trade in tech-

nical products is obstructed to a considerable degree by differences of

opinion on the best ways to achieve safety and on the targeted stan-

dards of safety in the individual countries. Machine manufacturers

operating on a global basis need to study the numerous different safety

requirements and standards in force in each different export market.

Thus, they must become familiar with the different safety philosophies,

and must use locally approved components.

3.6.3European safety requirements

Since January 1, 1995, new safety regulations have been in effect in the

EEC (European Economic Community) which are briefly outlined

below (Fig.3.6.1). EC legislation is based on a system of decrees and

directives. EC decrees apply directly in the respective member state,

while directives are taken up by the national governments, which have

to convert them within a reasonable period into national law with or

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Press safety and certification

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108

Fundamentals of press design

Fig. 3.6.1 Chart showing European safety legislation:

EC Machine Directives 89/392/EEC (ECMD) CE Mark

EC law governing technology:

Objectives:

– Harmonization of safety standards in Europe

– No compromises on safety in a member state

– Free trade in machinery

EC regulations EC directives

89/392/EEC (Machine Directive)

87/404/EEC (Simple Pressure Vessels)

89/655/EEC (Operating Resources)

Immediate validity National legislation

Appliance Safety Act (9. GSGV)

Product Liability Act

Standards

Harmonized standards Non-harmonized standards

Standardization order from the Other EN standards

commission, publication in the Official National standards

Journal of the EC

National lists of useful and important

standards for implementation of the

89/392/EEC

Type A standard

General principles

EN292 (General principles for design)

Type B standard (horizontal standard)

Definition of safety devices, stipulations of general relevance

with different machines

EN294 (Safety distances to prevent)

EN574 (Two-hand operated devices)

EN953 (Insulating safety devices)

Type C standard (vertical standard)

Stipulations relating to a particular machine category

EN692 (Mechanical presses)

EN693 (Hydraulic presses)

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

without their own additions. A series of directives was issued relating to

the characteristic requirements of technical products. For machine tool

manufacture, the directives listed in Table 3.6.1 together with the end

of the transitional period are of particular importance.

The EC Machine Directive (ECMD) 89/392/EEC establishes the major

health and safety requirements imposed on machines. Thus it is the

most important directive for the machine manufacturer and was con-

verted into national German law by the 9

decree relating to the Appli-

ance Safety Act. The product liability law, which has been in effect since

November 15, 1989, was derived from the EC Directive on Harmoniza-

tion of Product Liability Legislation. Both the EC directives and EC

decrees are based on harmonized EN standards formulated on behalf of

the Commission and published in the EC Official Bulletin after harmo-

nization. These standards allow the machine manufacturers to comply

with the relevant directives. The manufacturers can, but are not oblig-

ed to, comply with the standards. The manufacturer can have recourse

to the presumption that the requirements of the directive are fulfilled

by application of the harmonized standards.

In the standards, concrete solutions are put forward for achievement

of the prescribed safety standard: safety grilles on presses, for example,

are required to have a closing force of no more than 150 N. Otherwise, a

danger of crushing exists which can, however, also be avoided through

the use of a contact strip. In case of failure to comply with a harmonized

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Press safety and certification

Directive Content Valid without restriction from

89/392 ECMD 1/1/95

93/68 Amendment to ECMD (Identification) 1/1/95

93/44 Amendment to ECMD (Safety components) 1/1/97

91/368 Amendment to ECMD 1/1/95

87/404 Simple pressure vessels to 30 bar 1/1/93

89/336 Electromagnetic compatibility 1/1/96

73/23 Low voltage directive 1/1/97

86/188 Machine noise 1/1/90

89/655 Operating resources directive 1/1/97

Table 3.6.1: European standards

Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998

standard, the manufacturer can fulfill the health and safety standards

using alternative methods. In contrast, however, failure to comply with

the ECMD will result in prosecution of the manufacturer, withdrawal of

the product from the European market and decommissioning of the

hazardous press.

EN standards are classified under three categories, each with its own

function and purpose. General guidelines and safety targets are set

down in so-called A standards. B standards impose requirements on cer-

tain facilities, for example safety equipment used for different types of

machines. C standards define machine categories and the requirements

imposed on these. For a given machine category, the C standards lay

down the way in which risks are likely to occur and how they should be

evaluated and safeguarded against. Due to the generality with which

machines are grouped to create machine categories – the mechanical

press category, for example, ranges from the notching machine to the

large-panel transfer press – it can, in certain cases, be expedient to devi-

ate from the standard while still complying with the requirements of

the ECMD.

As documentary evidence of compliance with health and safety

requirements, the machine manufacturer issues the owner with a Cer-

tificate of Conformity (Fig.3.6.2)according to annex II A, and attaches

the CE mark to the machine. No machine has been commissioned in

the EEC without a Declaration of Conformity or CE mark since January

1, 1995.

The ECMD differentiates between independently working machines

and machines intended for installation as part of an overall production

line. A production line comprised of several components constitutes a

machine which has only one CE mark and only one declaration of

conformity. For all production line components which are incapable of

operating independently, the subcontractor issues a manufacturer’s

declaration in compliance with annex II B. Under certain circum-

stances, component machines of this type do not permit safeguarding

unless integrated in the overall line. An individual destacker without

the press, for instance, permits free access to hazardous movements and

is thus incapable of complying with the ECMD. The party responsible

for the overall line, whose function it is to issue the declaration of con-

formity, must therefore ensure that the manufacturer of the compo-

nents

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Metal Forming Handbook / Schuler (c) Springer-Verlag Berlin Heidelberg 1998