Thursday, April 2, 2009
How a HMI connects to the PLC
What is OPC?
OPC is open connectivity via open standards. They fill a need in automation like printer drivers did for Windows. See the summary of current and emerging OPC Specifications and OPC Certification.
OPC is open connectivity in industrial automation and the enterprise systems that support industry. Interoperability is assured through the creation and maintenance of open standards specifications. There are currently seven standards specifications completed or in development.
Based on fundamental standards and technology of the general computing market, the OPC Foundation adapts and creates specifications that fill industry-specific needs. OPC will continue to create new standards as needs arise and to adapt existing standards to utilize new technology.
OPC is a series of standards specifications. The first standard (originally called simply the OPC Specification and now called the Data Access Specification) resulted from the collaboration of a number of leading worldwide automation suppliers working in cooperation with Microsoft. Originally based on Microsoft's OLE COM (component object model) and DCOM (distributed component object model) technologies, the specification defined a standard set of objects, interfaces and methods for use in process control and manufacturing automation applications to facilitate interoperability. The COM/DCOM technologies provided the framework for software products to be developed. There are now hundreds of OPC Data Access servers and clients.
Everyone's favorite analogy for needing the original Data Access Specification is printer drivers in DOS and then in Windows. Under DOS the developer of each application had to also write a printer driver for every printer. So AutoCAD wrote the AutoCAD application and the printer drivers. And WordPerfect wrote the WordPerfect application and the printer drivers. They had to write a separate printer driver for every printer they wanted to support: one for an Epson FX-80 and one for the H-P LaserJet, and on and on. In the industrial automation world, Intellution wrote their Human Machine Interface (HMI) software and a proprietary driver to each industrial device (including every PLC brand). Rockwell wrote their HMI and a proprietary driver to each industrial device (including every PLC brand, not just their own).
Windows solved the printer driver problem by incorporating printer support into the operating system. Now one printer driver served all the applications! And these were printer drivers that the printer manufacturer wrote (not the application developer). Windows provided the infrastructure to allow the industrial device driver's solution as well. Adding the OPC specification to Microsoft's OLE technology in Windows allowed standardization. Now the industrial devices' manufacturers could write the OPC DA Servers and the software (like HMIs) could become OPC Clients.
The resulting selfish benefit to the software suppliers was the ability to reduce their expenditures for connectivity and focus them on the core features of the software. For the users, the benefit was flexibility. They could now choose software suppliers based on features instead of "Do they have the driver to my unique device?" They don't have to create a custom interface that they must bear the full cost of creating and upgrading through operating system or device vendor changes. Users were also assured of better quality connectivity as the OPC DA Specification codified the connection mechanism and compliance testing. OPC interface products are built once and reused many times; hence, they undergo continuous quality control and improvement.
The user's project cycle is shorter using standardized software components. And their cost is lower. These benefits are real and tangible. Because the OPC standards are based in turn upon computer industry standards, technical reliability is assured.
The original specification standardized the acquisition of process data. It was quickly realized that communicating other types of data could benefit from standardization. Standards for Alarms & Events, Historical Data, and Batch data were launched.
The OPC server is the intermediate layer which helps the PLC and HMI talk to each other. It acts as interpreter.
Why do we need the OPC server?
It is because there are different HMI software and Different PLCs. If the HMI is manufactured to talk with a specific PLC directly, then it won’t be able to talk with any other (different make), so in order to be able to use any PLC with any HMI, the industry has defined the OPC standard. The HMI software manufacturer should present his specific OPC server software to the clients, and by using that, the clients can connect any PLC (Which supports Modbus) to that HMI .
* DCS - Distributed Control System
* HMI - Human Machine Interface
* SCADA - Supervisory Control and Data Acquisition
* PLC - Programmable Logic Controller
* DAQ-Data Acquisition