Machinedesign 1235 Sandia National Laboratories 0 0

Design by Objective: Minimizing size and weight

May 1, 2001
The incredible shrinking system is ever lifting the bar for motion designers. Custom solutions, hybrid components, and new materials all enter the game driven by a variety of market influences

The incredible shrinking system is ever lifting the bar for motion designers. Custom solutions, hybrid components, and new materials all enter the game driven by a variety of market influences.

Ask a system designer what the primary trend driver is, and chances are they’ll say medical equipment. Everyone wants something smaller, from the doctor’s hand tool to the pumps patients must wear, explains one engineer.

The medical environment is changing — a lot is being done with home health care and moving analytical chemistry from large labs to smaller regional labs where space is more of a consideration. And, of course, new medical treatments involve small, motor-driven equipment. For instance, a new treatment for mad cow disease and surgical incisions is expected to cause a surge in the centrifuge market, says Somdi Baccam, project manager for universal motors, Groschopp, Sioux Center, Iowa. In this treatment, white blood cells are separated from the blood in a centrifuge and, along with clotting materials, are rubbed on sores or incisions to scab over the wound and accelerate healing.

Other motion markets like robotics, material handling, appliances, and aerospace are also demanding leaner, more efficient designs. It’s certainly worth noting, for example, that worldwide sales of semiconductor equipment nearly doubled in 2000, according to Semiconductor Equipment and Materials International.

One of the biggest challenges facing designers in these arenas is the tradeoff between power density, or trying to get the same amount of power from a smaller package, and cost, says Thomas Ouellette, vice president of sales and marketing, EADmotors, Eastern Air Devices Inc., Dover, N.H. The drive for maximum power density naturally entails a great deal of customization.

Motors are a prime target for design “diets,” but no system component is free from scrutiny. Designers are looking for ways to embed, shrink, lighten, or eliminate without compromising performance or breaking the bank.

Downsizing the system

Take a 1-cu in. robot and cut the size by 75%. Sound impossible? The mission was accomplished at Sandia National Laboratories, Albuquerque, yielding what are likely the smallest autonomous, untethered robots ever created. Be it industrial, medical, automotive, or other equipment, designers everywhere are re-evaluating system configurations to shrink the envelope on the next generation. But it’s not enough just to tool smaller parts. Fundamental design changes are often required.

The Sandia robot, for example, had all its power, sensors, computers, and controls on board. But, it simply wasn’t small enough.

To slim down the original design, the designers switched to unpackaged electronic parts and electronic components in die form. The device body was then produced with a rapid prototyping process with cavities for three watch batteries, electronics- embedded glass substrate, axles, tiny motors, switches, and other parts. Early models had small wheels that limited mobility, so they were replaced with tracks.

These robots, at 1/4 cu in. and less than an ounce, could be used to disable land mines, search for chemical plumes in pipes, and work together in surveillance swarms, communicating with one another. In like fashion, motion systems across the board are being re-evaluated with careful consideration of size and weight factors.

When paring down such a system, there are compromises in terms of both mechanics and electrical components, says Ouellette. Sometimes designers have to balance component size and cost with total system cost. They might, for example, use a smaller, less expensive motor and drive it harder, but then incur additional costs to fan-cool it.

With a dc motor, it’s important to consider rectified line voltage. Designers may go with a 24-Vdc motor and then find out they need to add large transformers to step down the voltage coming in from the power company. One solution is to use windings that work well off rectified line voltage so a much smaller transformer will work.

Smaller motors tend to run at higher speeds, so they still generate a fair amount of power, but less torque. To reduce size in the system, you can go to a smaller motor, but will need to effectively generate a gear reduction or mechanical reduction somewhere in the system.

“Really, going with high-energy magnets is the best way to preserve power density as you shrink a permanent magnet motor.” says Ouellette. Another way is to put more power in to the motor, allow it to run hotter, and verify it is just as reliable as when it ran cooler.

Another trend is the movement toward electrification of processes that were previously fluid powered. For example, in passenger cars and HVAC equipment, pneumatic or hydraulic controls are giving way to electric motors driving gearboxes and power screws, often using lightweight plastic gearing. As motors evolve, moving to wider power and torque bands, observes Zan Smith, senior staff engineer, Ticona, Summit, N.J., and member of the American Gear Manufacturers Assn. Board of Directors, there may be a reduction in the number of gearboxes altogether.

Sometimes it’s not how many gearboxes, but the type used. In switching from a parallel-shaft gearmotor to one with right-angle bevel gears, for example, designers trimmed several inches off the length of a boat dock lift. The gearmotor in the original design protruded about 10 inches. Concerns that the protrusion was a collision hazard prompted engineers to make the switch. Now, a direct drive system consisting of pulley mounted on a right-angle planetary gearmotor from Groschopp lifts boats out of the water in significantly less space.

Of course, size and weight reductions must not compromise other design considerations such as noise level. Planetary gearing is praised for deriving high torque from a small package. However, planetary gears are noisier than spur gears, which can preclude their use in machines like wearable medical devices. As a compromise, MicroMo Electronics Inc., Clearwater, Fla., offers a combination spur and planetary gearhead. The hybrid component is quiet, but has a planetary set-up on the output stage producing more torque than a simple spur gear would.

Everywhere you look integrated components are cropping up, combining functions and eliminating mechanical connections. EADmotors, for example, eliminates couplings, brackets, and bearings on a standard motor leadscrew system by integrating all of the above into a 1.3-in.-long hybrid stepping motor/linear actuator.

Shedding those extra pounds

The right system configuration is one key to a compact system, while another approach is to choose the smallest, most efficient motors, gearheads, and such on the market. Many manufacturers are striving to meet the demand for lightweight components.

Groschopp, for example, has implemented several weight-reducing modifications. In some cases they’ve gone from a steel motor core to a reduced- weight molded core. In another design change, the lamination stack length was shortened, at the expense of adding more windings. However, the omitted stack length weighed more than the additional windings do. They’ve also done away with aluminum fans, as plastic fans are more readily available. Baccam explains, “Two years ago we wouldn’t have been able to put a plastic fan on because of cost.”

The bottom line, of course, is central in design decisions. While planetary gear technology has been around for years, it was, until recently, only available at a high cost. With a smaller-package planetary gear reducer, you get essentially three times the stall torque as you do from a parallel shaft reducer. With prices dropping, new markets for this efficient component are opening up. For example, it’s now a feasible option to replace belts, pulleys, chains, and sprockets with smaller planetary systems. Planetary reducers that are not truly servo grade, but have very low backlash and high torque at a low cost, are evolving to fill these types of applications. With this sort of swap out, cost savings are also realized in reduced system maintenance.

“I think you’re going to see that exodus, that people don’t want a 15-pound parallel shaft reducer where they can get a 10-pound planetary,” says Greg Pennings, Groschopp applications engineer.

He also reports more talk of rareearth magnets in motors, but points out they really only offer a powerdensity advantage over ceramics for intermittent duty jobs. In continuous duty thermal applications, they are not any better, Pennings says.

Material world

The key to size and weight reduction is continued work in materials, says Ouellette. Higher energy magnets and readily available insulation materials, adhesives, and lubricants once considered exotic can withstand higher motor operating temperatures and contribute to greater power density.

Today, plastic is showing up in motor housings, and magnetic plastics form motor armatures in small dc motors. Smith predicts a move to injection- moldable armatures for permanent magnet motors, noting there will likely be future growth for plastics in motion systems. With the move to electronic control systems, for instance, more chips and connectors, which are largely plastic, will be used.

It’s common for designers to reject plastics for motion components because they didn’t work for them as a direct metal replacement, Smith says.

“If you’re looking at a one-to-one replacement situation, you’re probably making one of the biggest mistakes in conversion,” he explains. Many people who tried this years ago are now revisiting plastics, often because competitors have successfully applied plastic components in their motion designs, according to Smith.

Ticona engineers suggest that merely treating plastic like a lowstrength or low-stiffness metal is a sure way to get in trouble. To avoid this mistake, engineers must consider part design, material selection, tooling and design, and construction and processing. With performance systems, verification is also an often overlooked step.

There are special considerations with plastics, such as the combination of environmental temperature and operating temperature. Also, because plastic is not a direct gear material replacement for metal, you may be looking at different design approaches, which may or may not take up more space. A plastic transmission can be equal to or greater in power than a metal one and lighter in weight. Plus, the cost savings in this type of replacement is often amplified by lower production expenses. In one instance, heavier metal appliance transmission components exceeded weight limits for assembly personnel to lift, so special assembly equipment was necessary. Switching to Ticona plastic components allowed for manual assembly.

Size sensitive

With feedback becoming a more vital part of systems, designers are integrating more sensors into smaller, streamlined designs. For example, reports Pennings, more and more people ask that encoders be mounted on motor back shafts for closed-loop systems. Encoders themselves are shrinking — MicroMo offers an integrated encoder that adds only 2.4 mm to the motor length.

Custom sensors tailored for integration are often reserved for companies that can afford high volume production and the cost of custom tooling and housings. But, that doesn’t necessarily exclude others from space-saving sensor configurations.

Israel-based Netzer Precision Motion Sensors Ltd. offers encoders designed specifically for retrofits where space was not originally allotted for such a device. These sensors are available in kit form that can be mounted in the host systems for a profile as small as 3 mm. And, to minimize additional weight when incorporating a sensor, all the components in Netzer’s encoder are injection- molded plastic.

Elsewhere, Balluff Inc., Florence, Ky., has come up with a 3-mm inductive sensor providing a 1-mm sensing distance — a longer range than previously available in such a small package. The sensor manages this without an external amplifier. As such, Tom Rosenberg, Balluff marketing communications manager, says two-piece units are passé, and self-contained units are the trend in sensors.

As confirmation of the company’s believe that thin is in, Balluff is in the midst of acquiring a Swiss company that focuses solely on miniature sensors.

Team effort

Optimization is not without risks. One link breaks, and the whole chain is compromised.

With the pressure to develop compact, efficient machines, successful system design requires the participation of all component suppliers early on. Ouellette emphasizes that the best way for a designer to reduce size and weight is to involve the motor manufacturer in the design process as early as possible.

“Many people still don’t know a lot about motors and they tend to make invalid assumptions,” he says. As a result they might end up with a large and heavy motor, where a smaller one might have been accommodated had the motor manufacturer given early input.

One advantageous motor option for size and weight savings is a motor kit as opposed to a complete motor. With brushless, dc kits like those offered by EADmotors, Elinco Div., couplings and adapter plates can be eliminated from pumps and other components. And width can be taken out of conveyors and drums. The kits are about 30% cheaper than traditional motors, says Ouellette, not to mention the savings gained by reducing components such as outboard belt and chain drives. Because the kits are integrated into the system, maintenance can be difficult and the motor cannot simply be swapped out. Implementation requires close design collaboration early on between the kit provider and system designers, because the system bearing has to support the rotor load.

Likewise, when specing plastic gears, “you need the knowledge base and teamwork which is slowly evolving as an approach to systems design,” says Smith. This includes the motor manufacturer, quality controllers, and materials suppliers understanding the application requirements and covering all necessary design considerations.

Component makers are also teaming up with one another to offer custom solutions. In one such case, Groschopp is working on a motorgearhead combination for a hospital bed mover. Rather than having nurses physically pushing patients in beds, the new system will deploy a wheel and a gearmotor will drive it, so the nurse will need only steer the bed. The drive system must be exceptionally small. Groschopp is working to take length out of the center housing and eliminate some couplings. The company does not have the proper motor in its own line-up, so it’s working with another motor manufacturer to directly mount its gear reducer to the other company’s brushless dc motor.

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