In discrete applications, automation systems and quality control systems are often separate, producing a time lag between quality analysis and actual production. Wouldn't it be great to monitor quality information in real time and feed corrective instructions back to the control system in a closed loop before the production line makes a bin-full of scrap?
Process control has been doing just that for years. But on the discrete side, most quality processes have been offline. Explanations and excuses are many, but technology has now caught up and can offer many discrete applications the opportunity to actuate changes and close the loop without operator intervention, saving time, materials, and effort.
A current example of closed-loop, real-time, in-line quality control for discrete processes can be found in CNC (computer numerical control) machines that measure, monitor, and automatically compensate for tool wear without operator intervention.
Detect, prioritize, notify
"Much of feedback control is analogous to tool-cutting quality control compensation," observes John Gerry, Expertune CEO and president. "Valves wear, heat exchangers foul, and the PID loops continually try to hold setpoint. Performance will deteriorate over time, which is where you need to have an on-line performance monitor to detect, pinpoint, prioritize, and notify the appropriate people."
Expertune's PlantTriage and similar packages from other companies alert decision-makers—through enterprise-wide, performance monitoring and diagnostics—about available economic benefits, priorities, and required actions.
Discrete applications have been integrating the concept of in-line quality control, but typically in an open-loop architecture where a system automatically monitors/measures certain variables, stops the line, and alerts an operator to make changes before too much scrap is run. Such systems also might automatically segregate out-of-spec products based on measurements most often provided by high-speed data acquisition and logic linked with machine vision or other sensors.
In a closed-loop configuration, a logic device would use information analyzed in real time to send a message, actuate a change, and measure again in a self-correcting control/feedback loop.
Until recently, widely available tools didn't have real-time capabilities to allow closed-loop changes in discrete processes, suggests John Leppiaho, senior manager for Proficy software, GE Fanuc Automation. "Most customers don't know why equipment is down. The vision for automatic closed-loop correction is there, but we haven't seen it in practice."
The solution for many would be to better define product specifications and, as processes move away from a quality centerline, depending on the deviation, stop and fix the process. Doing so would improve overall quality and reduce waste, Leppiaho says. "Some product introductions can have 300 quality parameters; any ability to reduce new product cycle time is huge," he explains.
Today's software can offer tremendous advantages by using the S95-based data model, triggering Web reports and alarms from work-in-process issues and down time, allowing management by exception. Customers can create the specs in their data model so that if a process gets out of limits, an alert would be triggered. Such a setup can offer tremendous advantages. "This opportunity is like what HMI offered 10-15 years ago. Consider this HMI/SCADA software for the production level. Productivity gain potential at this level is huge," Leppiaho says.
Change priorities
"In the discrete world, commonly held views have been, 'Don't wreck the machine,' and 'Catch any bad product before it gets out the door,'" says Kevin J. Zaba, Rockwell Automation manager, process market development.
Now OEMs can improve machine performance by embedding the S88 phase state model into controllers, with equipment state (start, run, abort, recover) separate from control code, to improve machine performance, Zaba explains. Software can integrate machines and systems with mechanical, electrical, bill of materials, HMI/visualization, batch, historian, tracking, tracing, and other attributes. Such unification offers a sort of "Microsoft Office Suite for automation and design," providing big advantages by making code more visible, which is a better for closing the loop on discrete manufacturing within a machine or on a line.
Among SPC/SQC (statistical process control/statistical quality control) software vendors getting the message about the need for real-time is Datanet Quality Systems, maker of WinSPC software. That software has an "intelligent closed-loop feedback" feature that can "automatically flag and respond to user-defined violations." If it's flagged for operator attention, though, it's open-loop control. Self-correcting is closed loop. Some traditional SPC/SQC software vendors are just becoming familiar with the concept and real-time tools, which has left opportunities for automation software providers to offer those functions.
Two main changes will enable engineers to incorporate more closed-loop control into future systems, according to Todd Walter, data acquisition product manager, National Instruments (NI). "First, machines are becoming electronically controlled instead of mechanically controlled. Third-generation machines are emerging using servo motors and electronic gearing instead of mechanical gearing, camming, or a line-shaft. This trend provides the opportunity to dynamically adjust the machine without operator involvement.
"Second," Walter continues, "embedded technologies (field-programmable gate array and digital signal processors) are becoming more common in industrial control equipment. Programmable automation controllers (PACs) that incorporate these high-speed components enable engineers to achieve scan times in microseconds instead of milliseconds. This allows them to perform high-speed measurements and high-speed control." One application, he says, required closing a motion control loop at 200 kHz to achieve the quality and throughput for semiconductor production.
CNC machine tools, as mentioned, often measure and adjust for tool wear in real-time, or do so using statistically based analysis. For instance, a tool of a certain material working aluminum at known speeds for a certain number of hours would wear a predictable amount and could result in compensation without direct sensor input.
A brief check shows that Bosch Rexroth, GE Fanuc, Makino, Siemens, and Sunnen are among many vendors offering a closed-loop adjustment feature, either with direct measurements or statistical adjustments. That feature also is available in retrofits, for the vast installed base that cannot adjust for tool wear at present.
In any closed-loop process, measurement results are transmitted. Gaging products over the last decade offer finer resolution and greater stability through digital signal processing, explains Paul Sevin, vice president, Ovation Engineering Inc. Virtually all output results via RS-232 (or better). Better logic and software process the data transfer with no impact on part cycle time, Sevin says. "Benefits are elimination of scrap and reworkable parts due to manually miskeyed operator data (transposing digits such as 0.0021 instead of 0.0012, or inadvertent omission of the data sign '-' character when a correction is made). The result is more consistent process control, since only long-term process trends (such as tool wear or thermal effects) cause a corrective action to be taken," Sevin explains, rather than individual (and generally unrepeatable) work piece measurement values.
For machine tools, closed-loop control can be achieved with "high quality, repeatable, electronic based, gaging equipment coupled with accurate machine tools featuring relatively current (early 1990s or later) CNC controls," Sevin says. Open-loop, he continues, "may stop the customer from receiving a bad part, but does little to help reign-in costs associated with the production of scrap and rework materials. A closed-loop system, on the other hand, can not only keep bad product to a minimum, it can even generate additional savings through unexpected benefits, such as increased tool life (and decreased tooling budgets), greater machine utilization (through increased uptime and lower cycle times), and gains in worker productivity due to the elimination of manual tasks." Closing the loop could cost as little as $6,000, Sevin adds.
R. Andrew (Andy) Bedingfield, part owner of Soltus, a system integrator firm, sees many opportunities for improvements with closed-loop control, improved quality, material savings, waste reduction, and tighter coordination with enterprise systems, since closing the loop requires precise information flow. Vision systems, as they've fallen in price and increased ease of use, are widely used for closing the loop, especially among automotive manufacturers and leading suppliers.
Closed-loop control will aid the trend from continuous production lines to more cellular, customized manufacturing. The smaller the lot, Bedingfield explains, the more the bottom line depends on rapid set-up and production of on-spec product. He recommends tying in the control system earlier in the process, and slowing down line speed at beginning of a changeover to ensure a process is meeting specifications before ramping up speed. Quality measurements and closed-loop controls can drive that, Bedingfield says.
A closed-loop example of this approach includes compensating for extrusion-die wear with rising pressure that increase flow-speed. Such increases can extend the period between die changes, Bedingfield says, before pressure reaches unsafe limits and quality deteriorates.
If not embedded, how about a hand-held SPC?Mark T. HoskeIf quality control software cannot be embedded into your discrete manufacturing line yet, perhaps you'd like to move statistical process control/statistical quality control onto the plant floor in a hand-held computer.I&R Partners is among those offering such a tool, the DC-9005, SPC in a Pocket. The firm says, "The days of the dedicated SPC data collector are dead. With the PC getting smaller, faster, more powerful, and less costly, many of the SPC hardware specialists have either been acquired or have gone out of business." Manufacturers need portable SPC, though, and most notebook computers aren't reliable enough for roving audits, machine capability studies, incoming and final inspections, and large-part measurements, I&R says.DC-9005 has 802.11b Wi-Fi wireless networking, built-in barcode scanner, digital/RS-232 gage interfacing, SPC software factory installed, and a rugged IP54-rated enclosure. www.spcmadeeasy.com
For more information on companies mentioned in this article, visit www.controleng.com and the sites below.
www.expertune.com www.gefanuc.com www.ni.com (National Instruments)
www.ovationengineering.com www.rockwellautomation.com www.soltus.com www.winspc.com