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Engineering News 7491

DN Staff

August 26, 1996

24 Min Read
Engineering News

Pneumatics manufacturers clearing up
fieldbus confusion

Many pneumatic fieldbus systems are compatible with a wide variety of protocols

Park Ridge, IL--Fieldbus: For many engineers, the concept is fraught with uncertainty. More than five years after its introduction to the automation community, some machine builders are still unsure about its benefits. Others are intimidated by the raft of available protocols and the lack of a widely accepted standard.

"There's still a lot of confusion out there," notes Bill Moss, executive director of the Open DeviceNet Vendors Association (ODVA), an organization of 124 member companies dedicated to supporting DeviceNet, one of the many protocols available today. "It's hard for engineers to know which bus is best for their application."

The question of which bus to use has grown particularly challenging in the past three years. Scores of device manufacturers have devised their own fieldbus protocols. More than two dozen now exist, including DeviceNet, Smart Distributed System (SDS), Actuator Sensor Interface (ASI), Seriplex, INTERBUS-S, SERCOS, and CompactPCI, to name just a few.

Most of the protocols offer a combination of speed and information capacity. The fastest are targeted for on-off applications such as counting bottles on an assembly line or sensing the location of a metal with simple sensors like proximity switches or limit switches. Those that move greater volumes of information are aimed at process environments, such as chemical refineries or food-processing plants.

The fastest systems can send messages as quickly as 1 msec; the slower ones take up to 100 msec. Many of the protocols reside somewhere between those two extremes.

Wiring reductions. The good news, however, is that users of pneumatic systems are learning that some benefits of the technology are universal. Better yet, many manufacturers of pneumatic components are designing their systems for use with a variety of fieldbus protocols.

Users cite numerous advantages of fieldbus technology. Primary among those: the reduction of wiring. Installed in conjunction with specially designed pneumatic valve manifolds, fieldbuses can eliminate more than 25% of the wiring from large machines. The key is using fieldbus valve manifolds, which typically are pre-wired for connection to the fieldbus's "trunk cable." By configuring a system this way, engineers can eliminate the need to discretely wire each valve in the manifold block to the controller.

Engineers at Device Tech, Inc., Hampshire, IL, recently eliminated 30% of the wiring in an automated circuit breaker assembly machine by employing a DeviceNet-based fieldbus system. The machine, designed for Bussmann Circuit Components, St. Louis, MO, uses eight double-solenoid valves in each of its five manifolds, made by SMC Pneumatics, Indianapolis, IN. Because the machine employs 40 double-solenoid valves, the use of fieldbus technology was cost effective. "It's always a trade-off between reduced wiring and the added cost of special fieldbus components," explains Steve Schreiner, controls engineering manager for Device Tech, a designer and builder of custom assembly equipment. "But the wiring savings usually outweigh the extra cost of the fieldbus modules." Some users have also eliminated the need for cable trays to hold the wiring, as well as I/O terminations and junction boxes, Moss says.

Labor reductions. Moreover, the technology has enabled some machine builders to reduce labor. That's particularly true for engineers who design a machine in their plant, dis-assemble it for shipping, then re-assemble it at the job site. Using conventional pneumatic systems, each valve in a machine is wired to its own relay, and each relay needs to be re-wired during re-assembly. On big machines, which can use in excess of 200 pneumatic valves, re-wiring can be a time-consuming process. Pre-wired fieldbus modules, however, eliminate the vast majority of that re-wiring time.

Using Honeywell's SDS system, one machine designer reportedly cut wiring time from days to hours. The application, a bottle-handling machine, was so vastly simplified that the wiring process took only two hours. "The machine builder was stunned that they were able to wire all the controls without a schematic," notes Richard Dale, senior specialist for SDS at the Microswitch Division of Honeywell.

Schreiner says that fieldbus technology also reduces labor through the use of built-in diagnostics. Testing I/O is simpler, he says, because diagnostics capabilities en-able the system to answer such questions as whether the valves are on or off, the sensors are open, or there is a short somewhere in the system. "It's not only helpful in the construction of the machine, but in the running of it, as well,' he says. "Now, you get more up-time."

Acceptance inevitable. Equally important, much of the confusion around the fieldbus concept is being eased by component manufacturers. Most pneumatics suppliers support a wide variety of protocols. "More and more manufacturers offer their components with connectivity to various communication protocols," says Joachim Scholz, vice president of sales for Festo Corp., Hauppauge, NY. "It's a general trend, not only in pneumatics, but for anyone who builds electrical devices." Festo, Scholz says, supports 24 different protocols with its components.

Users say such broad support is critical. Though fieldbus manufacturers tout speed and information-handling capacity as their most important features, machine designers typically cite open architecture, availability of information, and vendor support as the keys to their decision-making process. ODVA, for example, publishes a 954-page technical specification and a list of vendors who support DeviceNet. SDS also promotes open architecture. "The importance of open architecture is that after all the components are hooked up, the system will work," notes Dale.

No matter the protocol, users agree that eventual acceptance of fieldbus technology is inevitable. Says Schreiner: "Whether you use DeviceNet or SDS or some other system, fieldbus is here to stay."

--Charles J. Murray, Senior Regional Editor

Protocol Glossary

Here's a brief sampling of a few of the many fieldbus protocols.

DeviceNet--One of the big U.S.-based open-architecture systems, it delivers data in packet sizes of only 8 bytes, making it well-suited for high-speed, discrete applications.

ASI--A simple system, used mostly in Europe, for networking low-level devices into a control and monitoring system.

SDS--Its performance lies somewhere between the extremes of speed and data handling capacity. Honeywell says "it is more powerful than bit-level on/off networks, yet faster and less complex than high-end fieldbuses."

PROFIBUS--One of the fastest fieldbus systems, PROFIBUS operates at about 12 Mbaud, compared to an industry average of about 1.5 Mbaud.

SERCOS--Primarily a high-speed motion bus, although it has I/O capabilities. More effectively handles multiple motion-control axes.


Machine tool extends jackscrews' reach

Cleveland, OH--Improved thread lead accuracies, surface finish, and greater wear resistance on large diameter lead screws enable engineers to simplify applica-tion designs.

Screws with lead accuracies of 0.004 inches/foot can now be manufactured on Nook Industries' 80-foot-long thread-rolling machine. Nook purchased the machine, one of four in the world, when demand for heavy-capacity actuators in-creased. "In-house capabilities improve lead and delivery times, and increase accuracies on large-diameter screws," says Larry Shindell, marketing manager for Nook Industries.

The machine is capable of threading 3- to 14-inch diameter screws in a single pass. "We can meet length requirements for almost any application," he says. The speed of the thread roller depends on the material being rolled, its diameter, and the thread pitch. "This machine improves screw production times and accuracy through the screw length," says Shindell.

One of the largest thread-roller machines in the world, it measures over 80 ft long and can roll bars up to lengths of 40 ft. The thread-roller was fully assembled at the manufacturer where Nook rolled 4-inch-diameter screws for a production application. Workers dis-assembled the 140,000-lb ma-chine into three components, the largest component weighing about 60,000 lbs. "It took three semi-trailers to transport the components from Wisconsin-based Tesker Manufacturing to Nook's new plant in Ohio," reports Shindell.

Because the machine had a successful "shake-down" at the manufacturer, technicians got the machine up andrunning in less than two weeks with no significantset-up problems. Major components of the machine include: base, heads, castings, spindle housing, worm gear reduction boxes, drive system, and electronics.

"Anti-backlash designs are effective for short jackscrew extensions," reports Nathan Knutson, director of antenna engineering for Norcross, GA-based Scientific-Atlanta. "But, they tend to bind up over long extension and retraction ranges because of thread lead errors."

Large satellite communication earth station antennas are used primarily as international gateways for voice and data communications. The Scientific-Atlanta 18-meter antenna requires precise positioning to within 0.01 degree in the elevation and azimuth axes.

Nook's 75-ton jackscrews provide precise positioning capability, Knutson reports, even in extreme Arctic environments. At the same time it provides the structural integrity needed for hurricane-force wind survival conditions.

Earlier antenna designs incorporated much more expensive anti-backlash configurations. "This thread-rolling technique will replace anti-backlash jackscrews for many applications," says Knutson. "Nook's design cuts costs by simplifying design, and reducing maintenance and adjustment requirements."


Guitars and CAD in accord at Gibson

Nashville, TN--Musicians who cradle Gibson Guitars' BluesHawk probably give little thought to the computers used to create it. However, Gibson designers now regard CAD as an important tool in the craft of designing new instruments and giving new life to old models.

Whether stroked, smashed, or simply slung over a back, guitars define the musician wielding them. Matthew Klein, a product engineer at Gibson, said venerable models such as the Les Paul are in demand among a small group of today's artists and collectors. Some devotees will pay tens of thousands of dollars for the privilege of owning and perhaps even playing a guitar made in the image of one made famous by a master. As the craftspeople who originated these instruments retire, Gibson's latest generation of designers use CAD to capture the accumulated skills of those who went before.

Most guitars are variations on a theme, so the process of designing a new model and recreating an old one are similar. An example of an existing guitar is traced on graph paper to record its contours. Klein said he still prefers manual drafting tools in the earlier stages because the full-size views are easier to manage in this format. Endpoints and radii are entered using AutoCAD. Then, designers are able to add detail and/or modify the geometry as required.

The company's Nashville operation concentrates on solid-body guitars, while a plant in Bozeman, MT, makes acoustic guitars, and another plant in Il-linois manufactures strings. According to Klein, CAD improves the ability of designers working in different locations to coordinate their efforts, resulting in greater accuracy and faster product-to-market cycles. With all parties using CAD, design data is shared as DXF files. Thus, designs created by one specialist can be incorporated into another designer's work. Furthermore, AutoCAD files can be sent to Gibson's machine shop for NC programming.


CAD helps bowlers improve their form

Hopkinsville, KY--To the uninitiated, designing a bowling ball doesn't seem like much of a challenge. In fact, the dynamics of a 16-pound ball rolling, spinning, and arcing its way down the lane at more than 20 mph confront the designer with a host of complex variables. Sources at bowling-ball producer Ebonite International Inc., however, claim to have simplified the task--and slashed product-turnaround time--through computer-aided design and simulation.

Much of the complexity comes from the fact that a typical bowling ball consists of an outer polyester or urethane shell molded around a dense inner core. The core gives each ball many of its unique features by altering its center of gravity, moment of inertia, and radius of gyration within limits set by the American Bowling Congress. Ebonite's Omega Lion ball, for example, utilizes a slightly ellipsoidal core and two opposite-positioned, high-density zirconium counterweights; the company's Turbo C features a single offset counterweight.

Further complicating the dynamics is the style of the individual user. While the most skilled American bowlers put a pronounced side-spin on the ball at release, others tend to favor an "end-over-end" release. Regardless of personal style, "the idea is to get the ball into the right position when friction takes over," explains Bert Shemwell, developmental chemist at Ebonite. At that point, he adds, the ball ideally will "break" into the pocket at an angle of six degrees.

Core design and behavior has traditionally been assessed largely through trial-and-error prototyping and testing. About three years ago, Ebonite began using DADS, a dynamics analysis program developed by CADSI (Gainesville, FL). Seated at Silicon Graphics Indigo workstations, company designers can "create" dozens of balls with varying core configurations, then simulate their velocities, trajectories, and breaking points on-screen with a confidence rate of 95%. Other variables accommodated by the program include surface smoothness and hard-ness, weight, composition, and lane conditions.

The computer tools have slashed new-product turnaround time from more than six months to just a few weeks, says Shemwell. "Not long ago, a company could introduce a good ball and keep the market for two years," he notes. "Now products come out so quickly that you need to have new designs ready all the time."


Fuel-level probe combats corrosion, cost

Dallas--Rochester Gauges Inc. has unveiled a new way to produce fuel-level sensors using conductive thermoplastics. The technology takes the form of a compact, vertical probe sensor.

In replacing bulky pivot-arm gauges made of metal, the rugged, noncorroding plastic probe reduces costs, improves durability, allows more flexibility in tank design, and interfaces more easily with computerized controls, according to Herb Ross, executive vice president and general manager of Rochester Gauges.

New conductive thermoplastic materials, developed by DuPont Engineering Plastics (Wilmington, DE), add to the accuracy and durability of the sensors. In addition, they eliminate the need for metallic conductive elements, which are subject to deterioration by galvanic corrosion. The probe design also uses nonconductive plastics for components that do not carry current.

The probe mounts on the top wall of the tank and extends down to the bottom. Its key structural components include the head, a support rod, a float, and a float stop at the bottom. The parts consist of DuPont Delrin(R) acetal resin or Zytel(R) nylon.

The vertical probe sensor functions as a three-wire, linear potentiometer or voltage divider. Two strips of the DuPont conductive plastic attach to opposite sides of the support rod along its length. In the potentiometer, one of the conductive strips functions as a resistor and the other as a wiper collector.

The float contains spring-loaded conductive thermoplastics parts. They function as electrical wiper contacts and are electrically linked to each other. As the float moves up and down according to fuel level, the spring-loaded contacts connect the collector strip to different points on the resistive strip.

The level-sensing circuit works as follows: Voltage is applied to one end of the resistor; the other end may be grounded. The wiper collector's output voltage is directly proportional to fuel level. The analog output signal may be signal-conditioned to drive an analog fuel-quality indicator, or it may be converted to digital output for interfacing with microprocessor-based fuel-management and instrumentation systems.

Resistance strips and electric contacts within the float consist of conductive moldable composite sheets (MCS) developed by DuPont. This material, pressed to produce the conductive strips, is based on thermoplastic polymers and proprietary conductive ingredients.


Plastic pump assembly outperforms aluminum

Dearborn, MI--Switching from diecast aluminum to plastic as the base material for an ABS pump and motor assembly, United Technologies Automotive (UTA) realized several benefits. Among them: elimination of five secondary operations, a saving of 20% in labor costs, improved watertight sealing, a 35% weight reduction, and inherent corrosion resistance.

UTA supplies the base assembly to ABS manufacturer Kelsey Hayes for current-model General Motors C/K light trucks. Eric Hancock, UTA engineering supervisor, believes the thermoplastic part is a first for ABS motor end caps.

In diecast aluminum, the part would have required machining of two faces and three diameters, plus a chromate coating to meet salt-spray corrosion specifica-tions. A staking operation also would have been need-ed to retain a ball bearing, which fits into one of the machined diameters.

In contrast, the thermoplastic part, made from Amodel(R) polyphthalamide (PPA) from Amoco Polymers, Alpharetta, GA, meets tolerance requirements without machining. The ball bearing is insert-molded into the part, eliminating staking.

Moreover, parts consolidation eliminated a grommet and two crimped wires. A stamped copper circuit is insert-molded into the part. It terminates externally, where lead wires are welded and protected by heat-shrink tubing.

The polymer selected for the end cap, AS-1566X HS, is a 65% glass- and mineral-reinforced resin. It has a high heat-deflection temperature (520F), and a high flexural modulus (2,7000 kpsi).


Roller coaster puts polyurethane wheels to the test

Primadonna, NV--When it comes to a demanding, high-load environment for wheels, not much can match a roller coaster. And among roller coasters, it's hard to beat the Desperado at Buffalo Bill's Resort & Casino.

Said to be the world's tallest roller coaster, the Desperado has a first drop that plunges 225 feet, the height of a 22-story building. Speeds can reach 80 mph, and gravity forces go up to 3g as the cars dip, twist, and turn on a thrilling two-and-a-half-minute ride.

Dynamic loading of the wheels produced by that combination of speed, acceleration, and tight curves provides a major test of the wheel design. But that's just the beginning. Air temperatures climb to 130F, and winds routinely deposit fine sand on the coaster's tubular steel tracks.

In this demanding environment, wheels that incorporate Hylene(R) PPDI polyurethane elastomer, supplied by Du-Pont Speciality Chemicals, Wilmington, DE, last an average of twice as long as those previously used, according to Arnold Young, Chief Engineer for the amusement park. The old wheels, made from poly-urethane based on NDI (naphthalene diisocyanate) woreout in as little as 150 hours, Young reports.

Longer wheel life means maintenance-cost savings add up on the ride's 15 coaches. "Doubling wheel life saves about $20,000 a season on wheel costs, not including the labor for wheel changes," Young explains.

The wheels, made from Uniroyal Chemical's Adi-prene(R) PPDI prepolymer, which contains Hylene, also virtually eliminate the risk of losing business when cars must be removed from service for unscheduled wheel re-placement. The PPDI wheels wear out by wearing down on a regular, predictable rate.

This wear mode differs markedly from that of wheels made from other poly-urethanes that failed when internal heat buildup caused deformation. In severe cases, chunks of material peeled away. Such failures require immediate removal of the car from service.

The Desperado's wheels measure 12 inches in diameter and three inches wide. The aluminum hub contains a 3/8-inch layer of the wear-resistant PPDI polyurethane.

Uremet Co., Santa Ana, CA, makes the wheels. To date, the firm has produced similar wheels for four major amusement parks. "In each case, our customers are get-ting one-and-a-half to two times the life of wheels pro-duced from competitive poly-urethanes," says Uremet President Mark Moore.


Solid modeling eases new material use

Phoenix, AZ--The Hybrid Power Modules (HPM) Operation of Motorola Inc.'s Semiconductor Products Sector recently designed a new motor-control power module, targeting zero-emissions automobiles, that is made of metal-matrix composites (MMC).

To accommodate the many changes anticipated with using this new material, yet still provide timely turn-around for each iteration, HPM worked from a core solid model on which all other operations were based.

James Fusaro, Motorola senior staff engineer and the package design team leader for the project, says this approach helped HPM complete design and testing for the power module in just four months--a great deal faster than if solid modeling had not been used.

After benchmarking several solid modeling systems, HPM selected I-DEAS Master Modeler from SDRC (Milford, OH). I-DEAS supported design for assembly (DFA) and design for manufacturing (DFM), two strategies HPM wanted to implement on this project..

And, says Fusaro, I-DEAS was easier to use than other programs HPM considered. "Solid modeling used to be thought of as a powerful but unusable tool," he says, "but I-DEAS' Dynamic Navigator changes that. It reduces commands and minimizes menu selections by guiding you through the geometry-creation process."

Core solid models containing all facets of the design were used to construct geometry files and finite element models. This information was transferred directly to external programs such as ANSYS finite element software from ANSYS Inc. and Amperes inductance programs from Integrated Engineering Software Inc. Solid models were also used as the main form of communication between suppliers and the stereolithography system used for rapid prototyping.

"To do something like this so quickly means everyone must work concurrently," says Package Design Engineer Guillermo Romero. "Solid models were an excellent tool for this because, by working from core models, everyone involved in the project could see in real-time what everyone else was doing."


Solar-power tower set for dual-use design

Barstow, CA--Stars generate fusion power: the radiation of the nearest can be collected as solar energy to operate the coffee maker. Events in this and distant galaxies emit gamma rays that may shed light on the origin of the universe. Now, a group of researchers led by the University of Chicago is teaming up with a consortium of public and private institutions to tackle issues both near and far at the same time.

Southern California Edison and the U.S. Department of Energy are the primary operators of the Solar Two experimental power plant, which went on-line in June. Out in the cloud-starved Mojave Desert, Solar Two spends its days directing sunlight reflecting off nearly 2,000 helio-stats--each 40 square meters--at a receiver located atop a 300-ft tower. The focus of all this attention reaches over 1,000F, which is hot enough to melt a nitrate salt mixture pumped up into the receiver. Molten salt is piped back down the tower into a storage tank, where it is available day or night to drive a steam turbine.

Solar Two happens to be the largest light-collection facility in the world. This makes it just the thing for collecting Cherenkov light that results from gamma-ray air showers. In effect, participants in the Solar Two Air Cherenkov Experiment (STACEE) project see Solar Two as a terrific large-area gamma-ray telescope.

The gamma rays that come from sources throughout the cosmos do not respect cycles of day and night on Earth. However, the brief flashes of blue Cherenkov light they produce when shooting through the atmosphere to parts unknown are best detected without the competing white light from the Sun. The light from these events is extremely faint and needs to be collected over a wide area and focused to be detected with current instrumentation. It just so happens, Solar Two does this all the time.

In order to be turned into a gamma-ray observatory, Solar Two's receiver has to be fitted with secondary reflectors: fresnel lenses or parabolic mirrors. These will direct a portion of the incoming light to a de-tector package, which includes a photo-multiplier tube camera. The instrumentation will be located 100 ft below the summit of the tower so it will not be fried in the daytime.

In initial proof-of-concept trials, STACEE participants have verified the performance of the heliostats' dual-axis tracking drives and measured the optical qualities of their mirrors. Rene Ong, the professor at U. Chicago who came up with the idea of using Solar Two as a gamma-ray observatory in the first place, said the STACEE collaboration plans to install a prototype detector making use of about 50 heliostats in August.


Complete 386 PC sits in a 'chip'

Palo Alto, CA--Measuring 2.20x3.00x0.450 inches, the SMX/386 OEModule is a complete PC/AT-compatible motherboard in a package the size of a large IC. The module's overall footprint is the same as a 51/4-inch disk drive: 2.344x3.145 inches. The small size and chip-like packaging suits it for designs that demand low power consumption, high reliability, and small size.

Running the show is a 33-MHz 386SX Intel or Advanced Micro Devices microprocessor. The PC module includes all standard PC I/O ports and pins plus 0.5 Mbyte of solid-state flash memory. The flash stores the BIOS and General Software's Mini-DOS operating system, which is compatible with MS-DOS interrupts and file systems, and has an MS-DOS-like command set. The 0.5-Mbyte flash even has 307 kbytes left over to emulate a floppy-disk drive for program or data storage.

The module lets designers eliminate multiple boards and cables, which can prove unreliable. And because the module is surface-mountable and has no moving parts, it's impervious to vibration.

"People choose the PC architecture for the software base--not processing power," said David L. Feldman, ZF president, when asked whether using a 386 microprocessor instead of a microcontroller was overkill. "It also improves time to market by letting designers do software development on standard desktop PCs with development tools they're already familiar with."

Specs include 5V operating voltage, 3.5W power dissipation, and 5-oz weight. The package is surface mountable, having no cables or connectors. Embedded applications for the product include: medical instruments, data loggers, security systems, test equipment, point-of-sale terminals, communications devices, and navigational instruments.

The 386 modules cost $595 each in 100-unit quantities and will be available by year's end. A development board is available that has a socket for a 2- to 15-Mbyte CompactFlashTM card from SanDisk, Sunnyvale, CA. Available now, the SBX/386 development board includes a cable set, embedded ROM-DOS from General Software, and utility software. Single units cost $595; the 100-unit price is $421. 486- and Pentium-based modules are in the works.


DEC Alpha zooms to 500 MHz

Hudson, MA--Peak execution rates of 2 BIPS--that's billions of instructions per second--mark the new 500- and 433-MHz versions of the Alpha 21164 RISC microprocessor from Digital Semiconductor, a business of Digital Equipment Corp.

The 500-MHz Alpha achieves a SPECint95 of 15.4 and SPECfp95 of 21.1, measures of integer and floating-point performance, respectively. DEC, which made these estimates, claims both benchmarks lead the microprocessor industry. (The 433-MHz version rates 13.3 SPECint95 and 15.9 SPECfp95.)

"Digital's 500-MHz Alpha 21164 should keep Alphain the performance lead, even as Intel rolls out its0.28-micron process for Pentium Pro in 1997," says Linley Gwennap, an analyst with San Jose-based Mi-croDesign Resources.

Digital Semiconductor designers say their design responds to three major market trends:

- Visual computing--demand is increasing for more performance in 3-D, video, and interactive multimedia applications.

- Processor-independent Windows NT is becoming the mainstream corporate operating system. Dataquest estimates that 10 million copies will be installed in 1997, and 50 million by 2000.

- The processor-independent WIN32 application programming interface is also going mainstream, driven by Microsoft for Windows 95 and NT, and by Intel for the Pentium Pro.

With its impressive floating-point performance, the new Alpha chips excel at visual computing. In fact, both can do MPEG 2 decoding, whereas the 200-MHz Pentium Pro cannot. Alpha runs 32-bit Windows native applications. And with Digital's new FX!32 software, Alpha chips can also run 16- and 32-bit x86 appli-cations as well as MS-DOS programs. However, those ap-plications will not run at na-tive speed.

The 433-MHz Alpha costs $1,492 and is shipping now. The 500-MHz version issampling now with production quantities slated forSeptember.

Digital has also been busy "value engineering" the lower-speed Alphas so OEMs can afford to put them in PCs. Look for 300- and 366-MHz Alpha PCs starting at $2,500 in 1997. These high-end PCs would suit such visual computing applications as image rendering, virtual reality, video conferencing, desktop video editing, and multi-media authoring.

--Julie Anne Schofield, Associate Editor

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