New 'Soft DCS' Reaches the Market

PCSoft (Braintree, Mass.) set a goal to build an open distributed control system (DCS)—one that provides features and functions of a traditional DCS, utilizes a defacto-standard operating system, makes extensive use of open standards, and resides on a single hardware platform.WizDCS is the result of its efforts and indications are PCSoft has exceeded its goal.

07/01/1998


Key Words
 
  • Software

  • Process control & instrumentation

  • Soft control

PCSoft (Braintree, Mass.) set a goal to build an open distributed control system (DCS)—one that provides features and functions of a traditional DCS, utilizes a defacto-standard operating system, makes extensive use of open standards, and resides on a single hardware platform.

WizDCS is the result of its efforts and indications are PCSoft has exceeded its goal. The effort parallels other PCSoft developments of Microsoft products for industrial markets.

With Microsoft Windows NT and 32-bit application development experience from the WizCON SCADA product, the soft-control and IEC 1131-3 experience gleaned from the WizPLC product, and the host of knowledge gained in deploying OPC (OLE for process control) applications, PCSoft developers continued applying what they knew. WizDCS is a soft process controller operating on Windows NT with integration of advanced discrete and analog control algorithms, SCADA, data management, and networking.

Open standards used

WizDCS is an OPC client and server. As a client, WizDCS can connect to a variety of I/O devices and networks including Profibus DP, Interbus, and DeviceNet. WizDCS leverages OPC's I/O interface features such as:

  • Data integrity information, which allows users to interlock control strategies with I/O device integrity;

  • Point grouping, which provides efficient data transfers; and

  • Data reporting by exception.

As an OPC data server, WizDCS can exchange data with other control and monitoring applications.

PCSoft provided DCS functionality by extending the IEC 1131-3 function library to include elements necessary in developing continuous process control stratagies. Elements such as lead, lag, auto-tune PID, and deadtime are included in more than 50 algorithm blocks. Users are provided three ways to add to the library of function blocks. Blocks can be developed using Visual Basic (VB); making use of the existing function block library and then "compounding" into named objects; or using PCSoft's algorithm tool-kit. Advantages of using the optional tool-kit include: the new algorithms are developed in C++, making them efficient and fast; and once developed, the new module appears as a named object in the standard function block library.

WizDCS provides simple batch unit recipe and unit sequencing capabilities suitable for centrifuge, extruder, or autoclave-like applications. Working with a spreadsheet recipe interface , users create a named sheet for each recipe (see photo). The first column is reserved for named tags, remaining columns are used to identify recipe grades, and individual grade/tag values are entered into cells. WizDCS is designed to integrate with PID's (Phoenix, Ariz.) OpenBatch product for complex batch applications.

Fixed sequences and batch operations are developed using sequential function charts (SFCs) which can be animated and dynamically annotated for use as part of the operator interface (see photo). SFC debugging is simplified by placing the sequence in "debug" mode and setting a "breakpoint" within the sequence. When the breakpoint is reached, sequence execution is halted awaiting user advancement. The user can single step through the sequence or set another breakpoint further in the sequence.

WizDCS provides on-line editing of function block diagrams (FBDs) and SFCs. For example, to make an on-line change, the user calls the FBD diagram.

After the desired additions and changes are placed on the FBD diagram, a description of the change, order of change implementation, and status information is shown in the "current changes" dialogue box where tools are available to modify the implementation order, or discard or apply changes. Changes are "slipstreamed" into the control strategy and live data begins to appear in the new FBD modules.

Logging of analog data correlated with process events is an important requirement of DCS installations. WizDCS addresses data logging by providing datasets and a data logger mechanism. Datasets are groups of tags whose logging properties (i.e., time or event based, disk write frequency, disk usage warnings, etc.) are the same.

If datasets are pens on a chart recorder, the data logger is the chart-speed drive. Data logger control is provided using visual basic application (VBA) script files, tag value triggers, or time. Logged data can be exported directly to Microsoft Access, or placed into common file formats such as ODBC, CSV, and ASCII.

Security is provided for individual users who are assigned to one or more groups, with each group having defined privileges. WizDCS provides an administrator group plus four default security groups. Additional groups can be added to meet varying project requirements.

Performance 'not an issue'

The intensity of floating-point math requirements in process control applications often uses more CPU time than discrete applications. WizDCS addresses CPU usage by capitalizing on Windows NT multiprocessing and multithreading capabilities. As applications grow, additional CPUs can be added enabling Windows NT to automatically distribute computing requirements across the available processors. Each WizDCS document (FBDs, flow charts, scripts, etc.) runs in an independent thread. Separate priorities for debug and run modes, set during configuration, eliminate wasted processor time and produce better results than scan or time based execution systems.

Developers at PCSoft have done their homework. The WizDCS provides the functionality necessary for deploying process control applications on a single PC-based platform.






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