Wayne L. Brown, P.E
DeKalb Technical Institute
This paper is intended to provide faculty, who teach control system principals, with a conceptual approach to the design of a personal computer-based distributed control system simulator employing industry-grade components in lieu of prepackaged t rainers.
Until recently, personal computers were never considered to be employable as the host computer in distributed control system (DCS) schemes. These systems relied on mini computers, loosely called mainframes, and proprietary DCS units. Therefore, the use of DCS schemes were restricted to companies that could afford the large capital outlay
IMPLEMENTING THE PC-BASED DCS SCHEME
Designing a distributed control system typically means incorporating a DCS scheme into an existing process. This can be accomplished by identifying the components required to communicate with existing equipment such as smart controllers and trans ducers. Therefore, the basic and most important DCS components are the control system software, host computer, and communications network. Since the control system software establishes software and hardware platform requirements and is the heart of any di stributed control system, the most efficient design procedure begins with the selection of the control software or SCADA.
There are several SCADA software packages available, such as USDATA’s FactoryLink, Intellution’s Fix DMACS, and WonderWare’s InTouch. For efficient operation, the SCADA package selected should operate on a multitasking platform and possess the c apability of communicating on a LAN or WAN. Also, the package should include a substantial symbol library. This last requirement may appear minor. However, drawing symbols in the process’s functional diagram can be extremely time consuming.
The software platform requirement is important to the efficient and dynamic operation of the SCADA package. Multitasking operating systems have typically, in the past, been employed by the supervisory and data acquisition software developers. In fact, this has been the primary reason for the delay in using personal computers in manufacturing control system applications. Due to the limited speed and memory capacity, the personal computer was not capable of employing a multitasking operating system . With speeds of three hundred megahertz, RAM memory of one gigabyte, and hard disk drive storage of several gigabytes, current PCs are not limited to single tasking, single user applications. In fact, multitasking platforms such as OS/2, WindowsNT, Windo ws95, and Linux, which are true multitasking operating systems, are in common use today.
The capability of interfacing with existing smart controllers must not be overlooked when examining SCADA software. The smart controllers, typically programmable logic controllers, usually specify the network topology and protocol to be employed. Therefore, some of these packages require additional software drivers and hardware to communicate with the smart controllers.
In order to gain one of the major advantages of the DCS scheme which is just-in-time manufacturing, a high speed communications network must be employed. Therefore, the existing smart controllers must be network compatible. Many network compatibl e smart controllers are equipped with proprietary networks, referred to as data highways. To employ an existing controller with this type of network, in a distributed control system, a hardware interface must be installed in the host computer. This piece of hardware is typically known as the network interface card (NIC) and is the gateway to the communications network. Since there are several types of NICs, careful consideration must be given to the topology and protocol employed by the controller’s data highway in order to insure compatibility.
The host computer requirements are typically specified by the SCADA software developers. However, with typical speeds of two hundred megahertz, hard disk drive (HDD) storage capacity of four gigabytes, and RAM memory storage capacity of sixty-fou r megabytes, most of today’s off-the-shelf PCs are more than capable of meeting these requirements. With these advances in PC technology, the personal computer-based distributed control system scheme could become a cost efficient and valuable part of an e xisting process control system.
Even though performance, ease of use, and interoperability are key evaluation criteria for any SCADA software package, the following is intended to provide the manufacturing engineer with a concise list of SCADA software evaluation criteria.
This criterion is concerned with the transmission medium, which is constrained by the network topology and how efficiently the system’s components communicate with each other.
This component is defined by the efficiency with which the software provides the operator with system failure information and the ease at which the operator is permitted to bring the system back to maximum operation af ter system failure.
This refers to the software’s ability to maintain the system’s database.
This criterion is concerned with the variety of processes and data that can be controlled by the SCADA package.
The SCADA package’s ability to assist in the resolution of system failures is evaluated by this diagnostic utility.
This component is concerned with the levels of security provided by the software.
Monitoring of a given process in real-time and control of that process, within preset parameters, is evaluated by this criteria.
This is the category for detecting, annunciating, managing, and storing alarm conditions.
This is the portion of the SCADA package that evaluates the process data. Production and quality is greatly effected by this data.
The graphical user interface (GUI) is evaluated using this criterion.
The software’s ability to display trending plots using historical and current data is considered in this category.
The production of logs and reports using current real-time data and data retrieved from historical files is evaluated under this category.
Due to the advancements in computer technology and low cost, a personal computer-based distributed control system can be installed for a fraction of the cost required just a few years ago. However, prior to selecting any piece of DCS equipment, f irst examine the existing equipment, in particular the smart controllers, for network compatibility. Then, examine and select the SCADA software to be employed.
With a little knowledge of computers, there are numerous automation opportunities available to the manufacturing engineer.