Ensuring Quality Through Technology
Process control systems automate quality surveillance in modern continuous production lines for technical textiles, nonwovens and other textiles.
By Alfred Watzl
In the past, test samples usually were taken during individual production stages. The test results were available at a later date when it was no longer possible to influence running production. Conclusions as to necessary modifications of the production parameters could be drawn only by comparing shift reports.
However, direct monitoring of the production process lately has come to the fore, and individual process stages can be influenced directly. Production without on-line process monitoring would be unthinkable today.
As a result, ever more complex demands are made on measuring and testing technology, and on-line measuring systems are required.
On-line process monitoring allows the acquired data to be evaluated immediately so production parameters can be influenced directly. This results in uniform quality, high productivity and minimized raw material losses.
When looking at the production of nonwovens for technical textiles, there are four main processes:
• fiber opening/blending;
• web formation;
• web bonding; and
The first three stages are combined into a continuous process. Set-point entries for areal weight, production speed, drafting and web structure — machine direction/cross direction ratio or cloudiness, for example — and comparison with actual values provide the base for controlling nonwovens production. The individual stages are connected using a process control system (PCS). Such large data quantities can no longer be processed using conventional control techniques.
All current process data and fault messages are provided through interfaces, enabling statistics generation with regard to failure frequency of certain machine components for quality assurance and to do preventive maintenance work.
During the past few years, there have been enormous innovations in drive and control engineering for textile machinery construction. The trend has gone from simple independent machine control systems equipped with a minimum of operating and indicating elements to complex PCSs for machines or processing lines.
This development has been necessary to meet the increasing demands made on energy utilization, waste reduction, environmental protection, operating staff savings, fabric qualities according to ISO 9000 standards, and production outputs for ever smaller lots.
PCSs for controlling and monitoring production processes are an integral part of all machine control systems today. A programmable logic controller (PLC)-based machine control system in combination with computerized instrumentation and control machine technology has become the standard for textile machinery manufactured by Germany-based Fleissner GmbH, which has successfully used PCSs for years.
Staple-fiber processing lines, spunlace lines for nonwovens and technical textiles, open-width washing machines, raw wool scouring machines, carpet dyeing lines and similar complete lines from Fleissner are always provided with a PCS. Man-to-machine communication in small lines or individual machines mostly takes place via fully graphic operator terminals.
The following machine control systems are used in practical operation:
• conventional control systems including relay controls and mini-PLC;
• operator terminal with PLC; and
• industrial personal computer (PC) with PCS.
Continuous production lines today are delivered with a PCS and industrial PC because of the large amount of data that must be processed.
Modern flexible production lines require a large number of technological parameters and provide a lot of process information. This information can only be entered and represented by means of modern man-machine communication.
A clearly structured operating panel with simple mechanisms for operation, parameter assignment and listing is essential for high acceptance by machine operators.
Numerous ways of representation and an integrated alert processing system enable quick and extensive diagnostics, and thus serve to reduce machine standstills.
The degree of automation and the efficiency of modern textile machinery and man-made fiber lines are determined mainly by the control and instrumentation system used.
The fully automatic PCS developed by Fleissner is used to optimize economic
production efficiency and the quality of the fabricated products.
This PCS enables automatic calculation and optimization of all required machine parameters by specifying technological conditions.
A fully automatic PCS provides the conditions required for quality assurance systems to comply with ISO 9001:
• cost-efficient operation by optimizing energy consumption;
• optimum product quality by automatically establishing process parameters;
• quick and reproducible product changes by recipe management;
• easily retraceable line status representation with all required machine parameters on a full graphics color monitor;
• monitoring of limit values for actual process values and set-point entries;
• recording of measured values in trend form and storing for long periods (line optimization/quality control);
• password entry to avoid unauthorized access;
• possible connection to higher-order process control systems; and
• simple operation with an easy-to-understand user interface.
Complete Line Control Systems, Technical Concepts
The basic system comprises a PLC combined with an industrial PC in various
configurations. This industrial PC replaces conventional control system interfaces such as function
tables, control desks, display panels and equipment for recording and displaying data.
The number of operating elements is thus reduced to a few essential functional components. All control information to the line and status reports from the line pass through the programmable control(s) to the system computer.
The individual PLCs of complex line configurations are connected with the system computer by a data bus. Regardless of the scope of required control technology, the system uses standard hardware and software components.
The modular hardware component design optimizes circuit diagram preparation. A modular software system for PLC and visualization is also required for efficient line control system planning.
Basic control elements include the following:
• basic functions such as lamp test and others for general control functions;
• operating functions such as manual/automatic/inching and others;
• fault detection and evaluation for auxiliary equipment including emergency stops and voltage control;
• detection/monitoring of technological functions including infrared (IR) limit switches;
• set-point generation for transport drives;
• set-point generation for temperature control loops and software controller blocks;
• modules for IR exhaust air flaps position selection and detection;
• modules for linking visualization components in the operator terminal; and
• drive coupling through analog signals or bus systems.
Basic modular design concepts also are found in the structure of the visualization program. Based on a fixed framework structure, the following individual technological features are incorporated in the PCS system:
• menu for selecting the desired submenu;
• line/machine overview with essential machine parameters;
• system overview display of machine including production information, rates and downtimes;
• recipe management;
• log management;
• alert and message overviews;
• entry listings of parameter changes;
• trend displays;
• graphic overview displays of temperatures and pressures;
• entry screens for set points; and
• service screens for machine settings such as controller parameter assignment and entry of basic set points.
Recipe management usually is included when using a PCS. Product-specific recipes that can be selected by product names are used to store all relevant setting data for the process. When changing products, the complete data set of all set-point parameters can be transmitted to the PLC simply by selecting the respective recipe name, ensuring short changeover times for product changes and high reproducibility of technological data.
Individual operators are assigned passwords for different access authorization levels, allowing
them to change parameter values, operating modes, listing functions and other parameters. Functions
not enabled on the respective access level are not displayed in the screen menu, thus facilitating
operation and improving safety.
All PLCs can be provided with an optional serial interface that links them to other computers. All current process data, status and alert indications are provided through this interface.
Apart from hardware and software for quality control, startup and maintenance service also is essential to a quality assurance system. It is becoming increasingly common to connect the customer's PCS to the machinery manufacturer via a modem to ensure a quick solution by remote diagnosis in the event of a malfunction. Experienced electrical specialists therefore are able to attend to the customer throughout the process from order placement to machine delivery. This is a prerequisite for optimum service and safe, reliable production conditions.
Editors Note: Alfred Watzl is director, sales and marketing, Fleissner GmbH.