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Error free logic design

19 May 2015

Suzanne Gill reports on a software solution which replaces legacy paper-based design workflow – eliminating errors during the creation of automation logic and validating it according to IEC61131-3. 

Specification of software requirements for control systems, especially for PLC based designs, is a key requirement in many engineering industries. The traditional approach has been to produce text documents or logic diagrams created in drawing packages such as Microsoft Visio. However, as a manual process this is error-prone and requires a great deal of manual checking of tag names and logic functionality. 

UReason, a real-time reasoning solution provider, believes it can offer a better solution with UDesign, an industry agnostic software tool for designing, testing, simulating, and validating automation logic according to the IEC61131-3 standard.

UDesign was originally created for use in highly regulated industry sectors, with a focus on control systems, design and simulation. The animation and visualisation of logic created using the software is said to eliminate the possibility of tag name errors. Sections of logic, or the entire logical design, can be checked for correct operation, which helps ensure a more robust design can be provided to the software vendors for implementation with no design errors. 

The traditional process control design workflow scenario firstly sees process requirements defined and text design documents produced, followed by the creation of control and instrumentation design documents. Because this is a manual process, mistakes often occur and because this is bespoke it does not allow for reuse of the design.

Errors from input documents are often carried forward into PLC code generation and Factory Acceptance Testing (FAT) and commissioning, which results in costly rework of software and commissioning delays.

In contrast, because the control descriptions in UDesign use interactive logic diagrams, animated visualisation of logic is possible. Simulation capabilities also allow the logic to be tested to ensure it works, so it is possible to simulate a design and then validate it.

On one typical application example – a small scale pump control system comprising around 50 pages of logic – no software errors were found during the FAT. Traditionally, such a project would require correction of up to 30 errors.

Early design review
UDesign’s user interface also allows early review of the design by less control-oriented colleagues allowing for useful operational and maintenance input into the design before implementation.  Because the system enables errors to be designed out so early in the design process the FAT and commissioning phases should become less troublesome and can be completed in a more timely manner.

With its Windows-based user interface UDesign is intuitive in use. Reusable libraries of functions can be created for reuse throughout a project. In addition, most standard implementations and all I/O can be configured in an Excel spreadsheet. These features can offer high productivity increases.
For a control systems engineer who is used to drawing Function Block Diagrams and Sequential Function Charts in Visio or other systems, UDesign should pose no problems. The main consideration revolves around structuring the code and any libraries so they meet necessary standards for implementation.

A validated design
A validated control system design gives confidence that the final implementation will act in accordance with the process design intent. This is particularly important for systems with a high safety requirement or critical investment protection functions. UDesign can create configurations for the ‘process’ response as well as the control systems design. This enables designs to be validated logically, from a process perspective. UDesign can also be used to provide an operator training tool, testing process and control modifications before implementation on plant – a particular benefit for industries that have a continuous process where fault scenarios cannot be easily recreated. Simulation of the control logic and process response allows fault scenarios to be created and operator response tested.

UDesign’s ability to offer real-time debugging early in the design process, without the need to compile code offers the flexibility to modify just parts of the solution, if process requirements should change. Its typical benefits were demonstrated in the design of mechanical handling system, whose input design documents and control system specification had already been created using traditional methods. As a verification exercise this was remodelled in UDesign, which highlighted a number of errors, including some which would have caused serious problems in the operation of the equipment. Having analysed the system with UDesign it was possible to agree the correct design intent with the plant operators and ensure that this was passed to the software supplier before the detail code was produced.

UDesign’s simulation scaling feature, called  ‘ghosting’ can also offer particular benefits for creators of replicable control units such as power distribution networks where a design can be simulated across all similar units by automatically creating all process variables and control logic from a single design template.

Highly regulated industries, where the design process is lengthy, and approval of design is arduous, would gain particular benefits from this process. Systems which have a high value process, where downtime is costly or expensive equipment and machinery would be put at risk from control system failures would also benefit from the ability to minimise operational failures. Similarly, it can be used to carefully analyse the design of safety critical systems.
The solution also offers benefits throughout the lifecycle of the process control programme. If the design has been properly validated and tested for functionality future modifications can be quickly validated. Any changes can be trialled and validated before implementation to check that unintended operations are avoided. Also, as a plant is being constructed or once in operation, UDesign can be used to train new operators or refresh existing ones – a particularly valuable resource for training teams on to handle fault scenarios which cannot be recreated on the operational plant.

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