Foxboro A2 Whitepaper
Mar 07, 2011
Case Study: System Cost and Engineering Advantages over PLC’s
Author: Stephen Arnold, Neal Systems
Introduction
This whitepaper sets out to show the relative merits of using a Foxboro A2 system in favor of a PLC based system and will describe the system components and discuss the advantages of the Foxboro A2 system including the development software. A real-life project, in which the customer was initially considering both an AB PLC-based system and the Foxboro A2, has been taken as an example of the cost differences between a Foxboro A2 solution and a PLC solution.
The Foxboro A2 system is a scaleable system starting at a single loop controller and increasing to a several hundred loop distributed system. It can exist with or without a SCADA front end and has been designed from the ground up to reduce engineering effort during the development, installation and commissioning phases of a project.
In this whitepaper the engineered cost to develop the process control system showed a savings of 23% over the PLC engineered solution. Savings were generated in virtually every facet of the project: hardware, HMI configuration, process configuration, and start-up.
What factors need to be assessed when considering a new control system
Upfront cost, project cashflow, product risk, % uptime, product support and control accuracy are all-important factors when deciding upon which control system to purchase.
Upfront cost: The initial project cost of a system using Foxboro A2 is, in most cases, significantly lower than an equivalent PLC system. When all costs are taken into account, typical savings of 25% are often achieved.
Project cashflow is significantly eased with Foxboro A2 since the entire remote I/O installation can be performed with the purchase of only 20% (in terms of cost) of the I/O system. Customers are able to postpone purchasing the rest of the modules until shortly before installation is completed. This is because the I/O system is made up of a rack, inexpensive termination assemblies and I/O modules. The system is wired completely with just the rack and termination assemblies, something PLC’s don’t accommodate.
Risk: There are several types of risk involved. There is the risk that the system will not perform as required, the risk that operators will object to the change and not utilize the system to its maximum performance and the risk that future requirement changes will not be able to be implemented (flexibility of the system). The Foxboro A2 is an evolution from a previous family of Invensys products and as such there are thousands of systems installed across multiple industries around the world. One of the biggest areas of concern in trying a new system is the concern over configuration time and in particular communications. This concern can be removed with the use of Control Modules and transparent communications. Control modules are pre-built, pre-tested macros that represent pumps, motors valves etc. Since they are already proven, no design and no test need to go into their development. ProfiBus or Modbus is used to communicate with the remote I/O and with either, the control program simply addresses the rack, slot and channel numbers of the I/O and the system takes care of the rest. The savings here, in engineering time alone, are enormous.
Owing to the incredible ease of use of the system there has been virtually no resistance to change from operations personnel and the emphasis from Foxboro on ease of use, especially the use of standardized templates has reduced training requirements as well as the possibility of incorrect operation. The ability to move sections of code between networked controllers and rebuild in a couple of minutes gives Engineers unprecedented flexibility for future requirements. The typical PLC and associated third party HMI package require the creation of templates and scripting features. This takes engineering effort and increases the chance of not meeting the original intent of the feature. The standardized templates in the Foxboro A2 system remove this problem completely.
% uptime: Hot swap capability on I/O without the requirement to hook up laptop means mean time to replace a bad module is measured in minutes. Full use of the comprehensive Foxboro A2 diagnostics in conjunction with the Operations server alarm filtering allows for the prediction of a problem either with comms or hardware. The mean time between failures for the T640F controller is close to 100 years.
Support: The configuration package has context sensitive help such that the user can press F1 at any time and the help screen appropriate to the cursor position will appear. The user can jump straight from this help screen to the section in the manual dealing with the topic in more detail. More extensive help is achieved through the Foxboro technical support center, which is staffed with Engineers having a deep understanding of the system.
Accuracy: The analog inputs are available in up to 18 bit, there is an A to D on every analog channel and with available auto-tuning, adaptive gain and full isolation Foxboro provides a level of control accuracy and repeatability that is without competition at this price level. On a recent project, temperature control of +/- 0.05 deg C was achieved and maintained over a period of several days by running a system model in the remote I/O.
The Foxboro A2 system takes a single large database and allows the developer to literally break it apart and run different sections in different controllers. The communications are virtually invisible and unlike other systems they require absolutely no configuration. The Foxboro A2 uses a system of Function Block ‘caching’ whereby a mirror image of any block may be run in one or more multiple controllers. ......
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