The Benefits of Personal Computer-BasedControl Systems
PCs have become more useful as part of open architecture control systems.
The purpose and scope of this papers is to explore the benefits of using PC for industrial control. The standard Personal Computer (PC) has begun to be used for direct machine control in a number of industrial applications. The plants which have taken this step have done so to take advantage of the reduced cost and increased flexibility of the PC. Other plants which have considered the use of PC's may not be familiar with the benefits of this type of control system.
This paper explores the benefits which must be considered when evaluating the use of Personal Computers instead of Programmable Logic Controllers for the direct control of industrial equipment.
From the perspective of an industrial control application, the exact hardware configuration of a controller is not particularly important. Figures 1 and 2 show diagrams of typical hardware solutions. All controllers share the common characteristics that they use a microprocessor as the heart of the system. Around the processor, there are memory devices (read-only and/or read-write) and I/O devices. Frequently, the controller will include hardware which communicates with remotely located I/O through a network.
In many industrial applications, it is necessary for the hardware to withstand extreme environmental conditions. Hardware platforms which meet these requirements are readily available as both PLCs and PCs
Clearly, both custom solutions (PLCs) and standard solutions (PCs) have the requisite hardware to perform industrial control. The system software is what makes a particular machine an industrial controller, and what usually distinguishes a PC from a PLC.
All industrial controller software shares common characteristics:
user-written control code
real-time execution of that control code
direct communication with industrial I/O
System software with all of these basic characteristics is currently available for standard PC's from several vendors.
The hardware which is used to build PC's continues to improve at a very rapid pace. A new generation of personal computer hardware becomes available every six to nine months. By contrast, a new generation of PLC hardware becomes available every two to three years.
With each succeeding hardware generation:
The PCs become faster. The Pentium systems widely available today outperform even the fastest PLCs by margins of 20:1 or more.
The PCs become cheaper. High performance PCs are readily available for less than $2000. Low cost PCs are well under $1000.
The PCs get more memory. Systems with 16MBytes of memory are commonplace. Systems with 64 Mbytes are readily available.
The PCs support more peripheral devices. CD-ROM drives, sound cards, voice recognition, optical scanning, high capacity tape drives, printers and a multitude of varieties, specialized I/O (e.g. digitizing oscilloscopes), and other items are widely available and inexpensive.
In addition to the improvements with each generation, PC hardware offers benefits which survive across many generations. For example:
Add-in hardware interfaces (ISA, PCMCIA, SCSI, etc.) are standardized.
Parallel and serial ports are standardized.
Networking facilities are standardized.
Video interfaces are standardized.
Input devices (mouse, keyboard, etc.) are standardized.
The popularity of PC's in commercial applications has fostered extensive activity aimed at making them easier to install, easier to understand, and easier to use. In particular, Microsoft WindowsT has greatly enhanced the accessibility of computers to non-technical people.
The fruits of this activity can be made available to users of PC based control systems. Since the underlying system software can be designed to support Windows on the same PC, that level of productivity and ease of use are available to all users of the control system (programmer, operator, technician, supervisor).
Ease of use becomes enhanced productivity for the control system designer. Too often control engineers spend development time fighting the design and development tools for their control system, rather than figuring out how to make more or better product.
Specifically, the availability of Windows improves productivity in the following ways:
The graphical user interface (GUI) which Windows presents allows all parts of the control system to appear to similar to all users. By taking advantage of this commonality, PC based control systems can reduce the extent of training required for control engineers, maintenance technicians, and machine operators.
Advanced features such as cut/copy/paste, undo/redo, find/replace, drag-and-drop, etc., are common in Windows applications. These features reduce the amount of time required to do many common tasks while programming and maintaining the control system
Documentation of the control system is improved by the ready availability of numerous word processing, spreadsheet, and database programs.
Development tools can be easily attached to a network, allowing the results of various engineering activities to be shared among teams of engineers.
Third Party Tools
The ability to support Windows with a PC-based system provides the control system designer with a host of exciting capabilities using third party software packages. Consider the following possibilities...
It is necessary to print a shift-end report at the end of each production day. Rather than having to purchase special equipment and create custom software to handle the task, it is possible to:
Use Microsoft Excel, Lotus 1-2-3, or any similar spreadsheet product with DDE to directly extract the desired information from the control system.
Set up the spreadsheet to perform any necessary calculations on the raw information, create appropriate graphs/charts, and fill in any textual data.
Set up a macro to run the spreadsheet and automatically print the desired report on any printer that is supported by Windows.
In a complex application, machine maintenance must be supported by engineering drawings and photographs of the machine, together with extensive documentation of the control system design, itself.
To support this system, the following pieces are put together:
The drawings, photographs, etc. are digitized and stored on a CD-ROM.
The control system is programmed to maintain internal diagnostic information which reflects the state of the machine and the expected activity. The creation of this information is simplified by the Windows user interface available for designing and debugging the control system.
A spreadsheet uses DDE to extract diagnostic information from the control system. This information is used in an index to lookup possible topics on the CD-ROM.
The contents from the CD-ROM are displayed to the operator using an easy hypertext tool similar to the standard Windows on-line help system.
If desired, sophisticated off-line diagnostic tools (an expert system) can be applied to the extracted information, providing in-depth analysis of the state of the machine, possible causes for a malfunction, and recommended remedies.
The machine operator and/or maintenance technician now has fingertip access to highly detailed information about the current state of the machine, its intended operation, and the probable cause(s) of a malfunction. This information allows any necessary repairs to be made very quickly.
In many cases, it will be possible for the operator to repair the machine immediately, without having to wait for a technician to diagnose the problem.
Most major vendors of industrial I/O devices provide a connection between their I/O devices and a standard PC. Usually, this connection is in the form of a card that goes in the backplane of the PC and attaches to the vendor's specific I/O network. In addition, many vendors supply I/O devices which sit directly in the PC backplane. Backplane resident I/O devices cover a broad range of capabilities -- from discrete inputs and outputs to multi-axis motion control.
With properly designed system software, PC based control systems can take advantage of this fact to provide their users with profound possibilities. In particular, an input is now just an input. It is no longer important which vendor the input was manufactured by or where it is located.
Better yet, most PC's have multiple slots available for add-in cards. This means that the system software can allow the user to use inputs and outputs from more than one vendor in a single control system.
At the same time, it is very important that the system software not limit access to the special features that are available on some I/O families. For example, some I/O families provide device diagnostics. If these functions are not available to the control system, then the I/O devices lose a large portion of their value.
In a vendor independent environment, I/O is a commodity item. It can be purchased on the basis of price, features, or other factors -- without regard to compatibility with the control system. This fact opens many possibilities:
Specialty equipment manufacturers (OEM's) are frequently asked to build their product around a different I/O family. The I/O vendor independence offered by PC based control systems means that the design and programming of their equipment's control system does not need to be started over from scratch. It is a simple matter of specifying compatible I/O devices from the new vendor's catalog.
End users can choose the I/O device that best meets the needs of their applications. The best RTD inputs from one vendor can be used in the same control system as the best 24 volt DC I/O from another vendor and the best 4-20mA devices from yet another vendor. Likewise, the best rack mounted I/O can be combined with the best distributed I/O.
Control system designers can develop controls without worrying about the particular foibles of one vendor's I/O system.
In an industrial environment, it is imperative that the control system be reliable. It must provide consistent, error free control of the process. Reliability needs to be examined with respect to two considerations:
Physical reliability - how well can the hardware stand up to the harsh physical environment of the manufacturing floor?
Software reliability - how well can the underlying system software stand up to long term use in a time-critical environment?
On the physical reliability front, standard, commercial grade PC's are not normally designed to tolerate the shock, vibration, temperature, and electrical noise frequently found on the manufacturing floor. There are, however, many vendors which provide PC's built for this environment. They use the same construction techniques used by the PLC makers to provide robust products:
Four point supports for add-in cards.
High strength metal for the enclosures.
Sealed front panels for moisture penetration resistance.
Electronic components rated for high temperature operation.
Shock and vibration tested electronic assemblies
These construction practices result in 'industrial' PC's with lifetime failure rates that are similar to PLC's.
Another aspect of hardware reliability is the cost and difficulty of replacing a failed unit. The PC is available world wide, on short notice, from many vendors. In a pinch, a commercial grade PC from the local electronic superstore can even stand-in for an industrial grade unit. PLC's are available only through the particular vendor's distribution system, forcing users to maintain a costly local inventory of spare controllers.
When considering software reliability, there are two important aspects which must be considered. These are:
How well does the software stand up to long term use? How well is the control system protected from external upsets?
How easy is the software to use? How well can control system data be made available for external uses?
Availability: In order for a control system to be useful, it must be running ('available'). If the system software is properly designed, the control system will be stable and robust. The user's control code will be well protected from other activity that might be happening.
Modern control systems (both PLC's and PC's) are based on 'real-time operating systems' (RTOS). These operating systems manage the allocation of CPU time, system memory, interrupts, and other resources among the processes running on the machine.
The real-time operating systems used by control systems are all basically similar, with a structure like the one shown in the figure 3. Some vendors use operating systems developed in-house, others use commercial operating systems. When evaluating a the software reliability of a vendor's control system, it is important to look at the track record of the operating system which forms the basis for system reliability. Steeplechase Software uses the iRMX operating system from Intel. With nearly two million copies in the field, it is a well-proven, reliable operating system.
Accessibility: Control systems are not islands anymore. In fact, some of the information that they use for controlling equipment is equally important for running the manufacturing enterprise itself. Further, if the control system is hard to use, hard to understand, or hard to get information to and from, then it will not adequately satisfy the needs of the users.
PC-based control system software provides substantially more accessibility than traditional PLC based control systems. The Windows environment is not only for programming. It is also available during operation. This means that:
Standard Windows data exchange methods can be readily applied to move information between the control system and the rest of the enterprise.
The programming environment is directly available on the controller itself. It is no longer necessary to have a separate computer for programming purposes.
A multitude of off-the-shelf Windows applications can be used to analyze or manipulate the control system data while the control system is running. Recipe management, SPC/SQC, material accounting, etc. all become easy.
Industrial control systems are not purchased in a vacuum; there is a specific job for them to do. Using a PC-based control system gets the total job done quicker, more efficiently, and with less integration effort than conventional control systems.
PC-based control systems are available which integrate many commonly used portions of the overall control solution. In particular:
Real-time control - the ability to execute user-written control strategies, correctly, over a long period of time.
Operator Interface - the ability to include a graphical operator interface on the same PC as the basic control system. Modern operator interfaces include the ability to display graphic images on the screen and to change their size, position, color, etc. based on the operation of the control system.
Programming - the ability to create and debug the control strategies to be used. This also includes the ability for people at all levels of the organization to easily understand programs created by another person.
Documentation - the ability to quickly and easily extract information from the control system. This information covers both the design and the operation of the control system.
Communications - the ability to communicate via whatever networking hardware is available with other control systems, dedicated operator interfaces, machines in other buildings or around the world. No special effort is required on the part of the user to make this happen.
For more information, contact Dave Gee, vice-president, Engineering,
Steeplechase Software Inc. at Tel: 313/995-3348; Fax: 313/995-7218