PC-based control has become one of the fastest-growing automation solutions for the factory floor. Control engineers are quickly dropping their proprietary PLC boxes and replacing them with PC-based control systems to gain the benefits of open-systems architecture. Similar to the shift away from proprietary Wang and Sperry-Univac mainframe systems to PCs in the office, the shift on the factory floor is equally dramatic. The drivers of this migration are pure business benefits: cost-effective integration of off-the-shelf hardware and software solutions; lower overall system costs; control system design cycles cut in half; enterprise-wide connectivity; and easier-to-use systems.
PC-based control combines the functions of several separate platforms into a single PC. This includes the runtime control engine, the programming and monitoring tools, the graphical operator interface, the data collection, alarming, and storage traditionally performed by the SCADA node, real-time simulation, and Windows DDE, DLL, or COM/DCOM data servers.
Beyond the obvious cost savings that result from eliminating an expensive proprietary PLC and several PCs, there are a number of technical factors that should be evaluated when moving to a PC-based control system:
- Hard real-time control;
- Eliminating the multiple database trap;
- Mix & match, commodity I/O;
- Off-the-shelf hardware and software components; and
- Advanced, easier-to-use Windows-based programming tools.
There are five fundamental rules of PC-based control that must be met to replace a PLC:
- Must provide deterministic operation. Control must be treated as the highest priority and ensure a predictable, repeatable response.
- Must survive a Windows crash. A machine control program must survive a General Protection Fault in Windows, or the "Blue Screen of Death" in Windows NT, and continue to operate in a safe manner.
- Must be isolated from poorly behaved Windows applications and drivers. The control system cannot be adversely affected by unstable Windows NT applications or drivers.
- Must survive a hard disk crash. It is essential that deterministic control not be interrupted in the event that a hard disk crashes or is removed. If the hard drive in the PC fails, the real-time control must continue to execute.
- Must be based on a proven real-time engine. The control engine must have a proven track record in mission-critical applications.
Hard real-time control is the technology that is required to meet these rules. As opposed to "Soft Logic," which runs a PLC emulator under Windows NT, hard real-time control couples the core of a PLC - a real-time operating system - with Windows NT on the same PC. Because Windows NT is completely isolated from the PLC core, Windows NT can "blue screen" or the hard drive can fail, and the control system keeps running.
One of the key benefits of Windows PC-based control software is the elimination of the multiple databases required to support a PLC system. Because the software incorporates the functions of the PLC, the programming PC, and the operator interface and provides real-time data access to other software applications running in Windows, a single common database can be used across all of the tools (see Figure 1).
Each element of the software can access the same database with a single set of tag names. I/O points are configured and given tag names once, and the same tag name is used throughout all of the software applications. The most important benefit of using the single tag in PC-based control systems is the elimination of human error.
Another key advantage of PC-based control is its ability to support multiple PLC I/O types. With an I/O scanner card, the PC can connect to and scan most industrial I/O. Examples of typical I/O families include Allen-Bradley's Remote I/O, Genius I/O, and Modicon Remote I/O. Open device-level buses include Interbus-S, DeviceNet, Smart Distributed System, and Profibus. A PC-based controller can mix several different I/O families in the same PC by simply installing scanner cards for each I/O family.
An even more powerful feature of PC-based control is the ability to switch I/O families, comparable to changing printers in Windows. A system designed for a particular I/O family can be changed to use a different I/O family through a simple drop-down box.
Manufacturers make a significant investment in developing their applications and processes, and recognize that this investment must be protected. To "future-proof" their investment, they are interested in using platforms that will support their investment into the future. The research and development dollars alone spent in the $200-billion PC industry exceed the total revenue of the $4-billion PLC industry.
One of the most significant advances introduced with PC-based control is graphical flowchart programming. Flowcharts offer a simple, intuitive graphical description of a process that is used and understood by nearly everyone on the factory floor. A common language allows engineers, operators, and electricians to easily interpret flowcharts that describe the step-by-step process to program, operate, or troubleshoot a machine.
Flowchart blocks describe the motions as simple function calls. These can be placed inside the flowchart logic (Figure 2) and can describe in written English how the motion is programmed. Commands such as "Jog," "Axis Symmetrical Move," and "Axis Relative Move" can be dropped in place. Configuring the motion blocks becomes as simple as filling out parameters in a dialog box such as axis, position, acceleration/deceleration rate, and velocity. With Steeplechase Software's patent-pending flowchart-enhanced ladder logic, the same motion blocks defined for flowchart programming can also be dropped directly into ladder-logic rungs.
Control system design time can be reduced by 50 to 70 percent using PC-based control and flowchart programming, compared with traditional PLC design using relay ladder logic. Flowchart programming also reduces machine downtime in two ways: by displaying the machine status on the PC screen and prompting the operator through repair steps, and by displaying the underlying machine steps with graphical troubleshooting flowcharts.
PC-based control with flowchart programming also provides the ideal tools for continuous process improvement. With a graphical, step-by-step sequencing of the machine's operation, it becomes easier to read, understand, and improve the operation of the machine.
As PCs have become more widely accepted in the manufacturing environment, a number of advances have taken place to add motion control capabilities to the PC. Smart servo systems using high-speed PID algorithms are built right into PC interface cards. These cards enable on-board digital signal processing chips to provide servo loop closures and path planning in the microsecond range. This capability makes servo systems an excellent fit for high-precision motion applications.
The benefits of hard real-time PC-based control can be extended to the motion control solution. Execution of the motion programs run under the hard real-time core is isolated from Windows NT. All five rules of PC-based control cited above can be extended to motion control.
The common tag database can be expanded to include the motion control parameters. By integrating the motion database with the rest of the PC-based control database, the motion parameters and data are directly accessible from the common programming environment, MMI, and other Windows applications either through DDE/DLL/COM/DCOM interfaces or over the network to remote PC applications. All of the benefits of a common tag database can now be expanded to include motion control on the PC.
There are a number of applications where PC control can be applied for simple single-axis and multiaxis motion control. These include gantry material handling, X-Y-Z overhead crane control, electronic component pick-and-place controls, and single-axis feed to length applications where material is fed from a roll into a press to be formed and cut. Other applications include transfer lines, material position, test stands, articulating profile motion, stepper applications, and point-to-point positioning applications.
This work was done at Steeplechase Software Inc., 1330 Eisenhower Place, Ann Arbor, MI 48108; (734) 975-8100; fax: (734) 975-8123. For more information contact Mike Messick, the author of this article, which was adapted from the 1999 Proceedings of the National Manufacturing Week Conference. Copyright © 1999 Reed Exhibition Companies.