By Daniel P. O'Sullivan
Data acquisition has never been a more critical element in computerized systems as it is today, and the criticality of this real-time, on-line access can only become greater as systems develop to handle more sophisticated levels of process and manufacturing automation.
In simplified form, plant engineers and managers have `traditionally' accessed shop floor process data through utilization of a host control system into which all "field" instrumentation and automated manufacturing cells are fed and, generally, attached by often complicated, lengthy analog wiring.
Yet, for over 12 years, the technology has existed to dramatically simplify the way in which field equipment can be coupled to the host controller. This is now generically known as fieldbus technology. It eliminates the bulk of costly analog wiring utilizing, essentially, a single twisted pair system onto which field equipment may, literally, be hooked for greatly enhanced, digital data retrieval.
Since 1985, confusion and uncertainty has, however, flawed the growth of systems applications. The continuing lack of a firmly international established standard is at the heart of the problem. Yet the fieldbus concept offers substantial savings over "traditional" analog processing/manufacturing systems which, world-wide, cost producers in excess of $60 billion annually in plant maintenance—a level of spending that is three times greater than capital investment.
It is reasonably argued that as much as 40% of the cost of installing traditional systems can be saved when a fieldbus alternative is chosen. The dramatic reduction is achieved, principally, through the massively reduced length of overall wiring required. And, benefits will continue to be enhanced, well after plant commissioning. For example, the twisted pair enables field device inter-communication that will facilitate both down- and up-loading of data between device(s) and the host.
In more recent times, the plain fact is that process, manufacturing and building automation product vendors and systems end-users have been fast to recognize the benefits of digital field communications in the open systems world. Accordingly, an array of impressive, often standardized solutions have gotten into the marketplace, many of them under significant pressure for installation, though questions such as "Which is the most appropriate for my company?" right down to "What is fieldbus?" continue to be voiced.
Globally there is said to be hundreds of thousands of "nodes" now in use, spanning many industrial sectors. The ratification last year of EN 50170—the European standard for digital field communications—undoubtedly helped the implementation. However, because of the complexity of component choice now offered by a still-growing number of international product vendors, it is becoming clear that industry-wide standardization will not be likely. This aside, vendors' products often conform to the requirements of more than one fieldbus technology protocol and, as such, offer end-users far more freedom of choice in terms of add-on buying patterns.
As may be expected, much information on the advantages of fieldbus systems is available, the majority of technologies having umbrella organizations only too happy to answer questions, an excellent starting point being the World Wide Web. Check out:
Interbus at Mercedes and Porsche
Sindelfingen, Germany: The impressive body panel transfer press operated here by Mercedes-Benz AG utilizes Interbus technology, chosen, according to Dipl. Ing. Gunthart Rosenthal, Team Leader in the Press Line Planning Department (PWP), because of the high levels of acceptance associated with the protocol.
There were various reasons why Interbus was used in the vacuum transfer system. The reduced switch cabinet requirement made it more economical, and considerable financial savings were expected in the cabling and installation work. The press manufacturer, Mueller Weingarten (based in Weingarten and Esslingen), has been using Interbus for seven years. It offered the technology, in this case, as a result of cost advantages during design and planning, production, installation and commissioning of the line.
The cabling distances of about 40 m between the switch cabinet and the plant meant that tons of copper cables weren't needed. The benefits expected from the control system were principally concerned with easier operation of the field level in case of malfunctions, including having diagnostic functions implemented in all system components.
With a total press force of 73,000 kN the Sindelfingen plant is, since early 1995, used for pressing automobile side panels, roofs and hoods. The field level comprises some 7,000 sensors and actuators; it is controlled by five of the total of seven networked PLC systems, Interbus replicating an essential neural pathway that connects field level with the central controllers.
The body panel transfer press was ordered in 1992 and work started on installing the line components in late summer 1993, the line becoming operational in phased stages following completion of the mechanical press components. It differs from many other body presses in that it uses a vacuum transfer system. Workpieces are chiefly transported using two-axis suction carrier frames; the movements of these frames are firmly linked to those of the press ram by cam disks. Grippers are only used at a few points in order to stabilize the workpieces. The Mercedes-Benz unit can be divided into three sectors: the blank conveyor system; the press with its six transfer pressing stages; and the finished part stacking system.
The transfer pressing stages operate in synchronism with one another. "With a press of this scale, it is very important that the entire mechanical system should run smoothly and without any shocks or jolts. This has to be assured by adapting all units accordingly," explains Rosenthal, who has responsibility for planning the electrical control systems on press lines.
Supplied by U.S. vendor Phoenix Contact (Harrisburg, PA; also the originator/founder, incidentally, of Interbus-S) SIMATIC S5-155U programmable logic controllers lay at the heart of the press line's central controller systems. Unlike the traditional method of running control system connections through the I/O level of a PLC, they are, in this case, achieved using Interbus.
In addition to binary I/O modules, the line also has analog modules and PT 100 modules to which temperature sensors are connected. Analog Interbus modules are also used on the output end for activating servo valves and transmitting rated values to other field devices. Some 7,000 sensors and actuators are connected to five of the total of seven PLC systems by about 500 Interbus I/O modules with six Interbus master interface cards.
Interbus also links the solenoid valves on the hydraulic units to the central PLCs and is also used for activating the solenoid coils, polling the valve limit positions and transmit-ting the initiator messages. A control cabinet fitted with bus terminals and IP 20 I/O modules is set up next to each hydraulic unit as a physical connection. The situation is the same with banks of pneumatic valves: however, integrated Interbus interfaces mean there is no need for traditional connection techniques. A remote bus cable is directly connected to the integrated bus coupler via plug connectors, and the control box is dispensed with.
In another European application, Phoenix Contact is involved with Porsche in the production of the Boxster.
The following equipment has become consistent and recurring parameters in the AEG A250 controllers (in this case, 15) as bus masters, banks of Festo pneumatic valves, ABB robot controllers, Bosch welding controllers and Interbus, the high-speed sensor and actuator bus for networking peripherals. Interbus is used for grouping together the I/O nodes in all the body shop systems for the new roadster. Fourteen switch cabinets, containing contactors and logic control systems for body shop drives and conveyors, have been set up with Interbus modules, the system built in the Zuffenhausen plant by Inpro GmbH of Neckarsulm with Drauz Ingenieurbetrieb GmbH of Heilbronn as planning consultants. AD&P