TechArchive

Original article date: May 1998

Developing a better alternative than IGES for translating data between CAD systems seems to have taken an age. Will STEP be outgrown before it is widely implemented? Alan Quinn and Jaime Peters interpret the current developments and applications.

Object Linking and Embedding (OLE) is now taken for granted by users of Microsoft operating systems. Files from any OLE application may be placed into any other OLE enabled application and edited in place. Because the file is always edited by its own native application there is never any problem translating the data between applications where formatting information or data itself may otherwise be lost.

Whilst users in the 2D world have enjoyed the advantages of OLE for many years the same cannot be said for users of 3D CAD/CAM/CAE software. Here information is routinely lost or misinterpreted in the transfer of data between applications and all the techniques for exchanging data inherently use a “lowest common denominator” approach to exchanging data. Once a model has been exported in a neutral format all high-level detail such as parametric dependencies is lost “stripped away” by the translator. Such detail is extremely important when working with solid models built with constrained or variational geometry. Even if the model is read back into the same system it is extremely difficult to modify or re-model it.

Currently data interchange between CAD systems is predominantly carried out via a version of the neutral file format known as the Initial Graphics Exchange Specification (IGES). Although commonly available there are many flavours of IGES and consequent inconsistencies in the interface can be the cause of problems when integrating systems from different manufacturers. There are also native translators to and from many major proprietary formats including CATIA format essential within the supply chain of the automotive industry.

STEP was supposed to provide a vendor-neutral alternative. What has been happening in this arena? In the aerospace industry it is known that Boeing Pratt & Whitney Rolls-Royce and GE Aircraft Engines regularly exchange engine data using an exchange process based on STEP. CAD suppliers Dassault Systemes (CATIA) EDS (Unigraphics) and Computervision (CADDS) provide standard translators and these have been integrated to provide a complete end-to-end validated transfer mechanism. The data exchanged is an integrated structure of solid model geometry product identification relationships between assemblies and components and administrative data.

It was as long ago as 1984 that work started on ISO10303 Industrial Automation Systems – Product data representation and exchange an international standard that set out resolve both these issues. The name STEP is an unofficial derivation from the STandard for the Exchange of Product Model Data.

The key difference between STEP and previous data interchange standards is the separation of what is to be exchanged from how it is to be exchanged. This has enabled several different levels of implementation to be defined which split basically into data exchange by file and shared data access through a standard interface. Formal specification of the exchange file format permits a greater degree of commonality and automation in the development of interfaces than previous standards.

Specific application protocols have already been developed for many industries and it is here that work continues with more CAD/CAM systems now delivering STEP translators as a standard feature. One of the areas currently benefiting from the development of STEP is oil and gas installation design.

PISTEP is the UK Process Industries STEP consortium and has been a key contributor to the development of STEP application protocols. Although the parts that support the Process Industries are not yet an International Standard this has not stopped early adopters from exploiting the valuable work that has already been done. PISTEP is starting to deliver benefits to its members who include some of the large oil exploration and recovery companies operating based in Northern Europe. When this is completed they will have a library of some 8 0 components which can immediately be used in designs by a range of CAD systems.

By having libraries of completely standard components it is easy to understand how the STEP concepts can be attractive to big industry but equally find an explanation for why it has taken some time for realistic component libraries to materialise.

The STEP data is stored in a data repository (sometimes referred to as a data warehouse) and is accessed in real time by a CAD application using what is called the Standard Data Access Interface (SDAI). The SDAI complements the data interchange formats and is described by the ISO standard. IBM Prism and Intergraph are amongst the companies who have already developed products that will deliver this technology.

Another derivative of STEP is also being used as part of the open standards developments for programmable logic controllers known as PLCOpen. Formed in 1992 in the Netherlands PLCOpen is an industry grouping of PLC manufacturers supporting IEC1131-3 an open programming standard for industrial controllers. IES1131-3 defines three levels of compliance for PLC programs. By implementing this standard on many program development environments users can move between different brands and types of controller and exchange applications.

Today its proponents view it as a way to simplify control system training and support as well as a way to improve factory-floor operations. Whatever the proprietary nature of the devices being programmed data interchange both during application development and at run-time is always consistent.

Although IEC 1131 does not yet address all issues of a common automation programming interface it is a necessary first step on the main road to open automation systems. A common programming interface will have a far-reaching impact. IEC 1131 product vendors mention reduced personnel-training requirements for programming and maintaining PLCs as well as for any open architecture platforms. This can significantly reduce total system cost. Additional benefits accruing to users include improved programming quality because of reusable code.

Done right IEC 1131 programming could also lead to decreased downtime. A common programming language with high-level languages should make troubleshooting and diagnostics simpler especially if all devices sport the same look and feel much like an “MS Windows” for the plant floor. A second phase will address comparable graphical programming which requires a greater level of effort to delineate interfaces and functional definitions among graphical elements for the three graphical languages.

Today’s IEC 1131-3-based systems cannot use each other’s code interchangeably. It is also generally accepted that programs developed for one PLC will not easily port to another because of the difference in scan cycles and I/O assignments. But nearly all important manufacturers of PLCs meanwhile offer an IEC 1131-3 compliant programming system or are planning to do so. More and more end users are requiring IEC 1131-3 compliant programming in their decision making process. This forces also a broad range of small and medium sized suppliers offering PLCs industrial PCs or motion controllers to fulfill this requirement.

Now an initiative called OLE for Design and Manufacture a logical extension of the OLE technology used in office applications to allow for example an Excel spreadsheet to exist as such in a Word document is being looked at for 3D CAD/CAM and CAE applications. It aims to deliver direct access to native CAD models without data translation using the proven concept of a “container” and a “server” where during linking one application (the server) generates its own 3D data within the graphics window of another (the container). In the Excel and Word example Excel is the server and Word the container.

“Products are now being released that include OLE for D&M interfaces and more will appear in 1998″ says Steve Hobbs development director at Delcam. “It is important to stress that STEP and OLE for D&M are complementary technologies and we want to work within the whole data exchange community. The DMAC interface provides a live connection so that entire models can be passed between applications without any translation. SolidWorks or SolidEdge models can go directly into PowerMill for machining of the parts with no loss of features.”

What is in it for the user? Probably the biggest single user benefit is the ability to share data between CAD CAM and CAE applications without the need for traditional data translation through IGES STEP or some other method and hence no loss of high-level data in the 3D model. A downstream OLE for D&M application might not display all this detail but it is still there available for in-place editing.

May 1998