Eye on Size and Weight

July 1, 2002
When designing for space and weight critical applications, it’s a small world after all. From nanofuturistic to downright practical, you’ll want to consider these six little gems for your own creations.

We all know that ER is more than just a terrific television series about dramatic events in the emergency room. Yes, ER has its own special meaning to design engineers, often with the same panic-filled connotation, depending on just how far you have passed your project deadline. You know what I’m talking about here — fastER, cheapER, bettER, and all of their relatives. Oh, did I forget to mention yestERday? Relax, grab a hot cup of coffee, and read up on a few new methods to approach two of today’s most sought after design sisters — smallER and lightER. From shape memory alloys used in motors to ordinary pipe that morphs into a lightweight precision drive shaft, these recent innovations are sure to spark your creative enERgy.

A little muscle

Billed by market analysts as the first major change in electric motor technology in more than 170 years, the NanoMuscle actuator (motor + interface) is making waves throughout the motion industry. As one of the company’s key investors — Jacob Tal of Galil Motion Control — says, “The jury is still out, but this technology has earthshaking potential.” Unlike conventional motors that use electromagnetism as their source of motion, these motors — from NanoMuscle Inc., Antioch, Calif. — use shape memory alloys (SMA) to produce movement. The best news? They’re typically one-tenth to one-twentieth the size and weight — and a fraction of the cost — of traditional small motors and solenoids. For example, a NanoMuscle actuator weighing just 1.1 gm offers a load rating of 70 gm. The company also makes custom designs that offer longer stroke and higher force ratings than its standard actuators.

President and CEO Rod MacGregor says, “Right now, we’re doing what’s called market-driven R&D. If a customer comes to us and wants an actuator for a specific application, we’ll build it — within certain limits of course.”

Since these actuators do not require additional components — such as gearboxes and complex mechanical systems — to translate rotary to linear motion, they can be manufactured for a fraction of the cost of a comparably sized electromagnetic motor. Then there’s the noise issue. Electromagnetic motors produce electrical noise during operation and their gearboxes produce acoustic noise. NanoMuscle actuators produce neither.

Then and now

Commissioned by the U.S. Navy, SMA technology was developed by Raychem Corp. during the 1950s for military applications. However, Raychem’s use of SMA in motors never really took shape because of the unpredictable nature of the material. NanoMuscle believes it has found the most effective way to harness the power of SMA without sacrificing the durability and consistency of the wire structure. The SMA in a NanoMuscle actuator is made of nickel-titanium wire stretched out across a set of supports. An electrical current causes the wire (through heating) to snap back to its original shape, creating a force strong enough to lift 140 gm.

The macro-scale motion of a NanoMuscle actuator is produced by a large number of elements, each just a few nanometers across. These elements are assembled into thin wires around 50 μm in diameter. Several of these wires are then combined to produce a macro-scale linear motor with a stroke measured in mm. At the bottom of the hierarchy, each tiny element is actually a single crystal of NiTi specially treated to exhibit the shape memory effect. A microprocessor precisely regulates current to the SMA wire, controlling the speed of contraction. The actuator contracts when activated and requires a return force, such as a spring, to bring it back to its initial configuration. Life cycle is stated as more than 1,000,000 repetitions.

“Right now, we’re working on several new developments. Some involve NanoMuscles with higher forces — bigger, more powerful motors for use in automobile applications,” says MacGregor. “Another direction we’re going is making the units more self-contained, for example, a complete miniature system that could be used for mirror positioning in the photonics industry.”

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The motor’s contraction depends on the temperature of its SMA wires; these wires are electrically resistive and may be heated by applying suitable voltage. Once the wires reach a certain temperature, they start to contract, causing the motor itself to contract. Any displacement — from a small movement to a full contraction of the device — can be achieved with the appropriate application of power. “That’s where the tradeoff comes in,” says Tal. “The wire must be heated for the motion to occur. To make this happen faster, either a higher electrical current must be supplied, which can get impractical, or the wire must be made thinner, which means it can handle less of a load. The physics need to be worked with in order for the technology to provide faster movement.”

In its current state, the technology is ideal for use in any linear motion application that doesn’t need to move faster than approximately 300 msec. For example, in computer and electronics applications such as opening or closing a disc drive, in heating and cooling applications such as changing the angle of a vent, and in toy industry applications like controlling the movements of a doll’s eyes, the technology is a perfect fit. However, if movement needs to take place in the 10 to 15 msec range, such as in a speedy pick-and-place machine, then an electromagnetic motor is still the best bet — for now. MacGregor and Co. are working on a model that moves back and forth 40 times per second, by improving the system’s “packaging” and experimenting with various alloy combinations that stand up to extreme environments.

Mini machine vision camera

When adding components such as vision cameras to the ends of robotic arms, it is often critical to keep mass at a minimum. The CDC-100 from Cognex Corp., Natick, Mass., is reportedly the world’s smallest high resolution (1,280 x 1,024) digital CMOS camera designed specifically for machine vision applications. This little guy is roughly half the size and weight of conventional machine vision cameras. The CDC-100 merges high performance digital camera technology with low-cost CMOS image sensing, enabling cost-effective use of high resolution imaging in applications ranging from semiconductor wafer alignment to checking label positions on bottles to precisely guiding robots during material handling tasks.

The camera performs a direct 1:1 conversion of the digitized image, providing low noise images with no pixel jitter. This provides the machine vision system with better image data for making decisions, such as where an object is located and whether or not it is defective. A configurable region of interest (ROI) feature lets users optimize camera performance by selecting and acquiring a limited ROI within an image. (Think extreme closeup.) The camera is designed for use with the company’s new MVS- 8100D frame grabber, which features a full library of vision software tools such as technology for locating objects despite changes in angle, scale, or appearance.

Itty-bitty balls

A new family of balls made from silicon nitride has just been introduced by Thomson Precision Ball Co., Bristol, Conn., and is ideal for a variety of high precision medical and test applications including dental drills and gauges. They are also being used in automotive fuel injection and anti-lock braking systems, specialty bearings, and aerospace actuators.

When compared to steel, these silicon nitride balls offer designers a number of benefits including a 40% weight reduction, up to twice the material hardness for tight tolerances, a coefficient of thermal expansion 70% less than steel, and an operating temperature range up to 1,800°F.

Silicon nitride balls are also noncorrosive and anti-magnetic — a worthy option in low noise, high rigidity, and high load carrying applications. And, these balls can run dry in a vacuum environment up to 500°F without lubrication.

Drill, clip, and cut machine goes on diet

It’s not only humans shedding pounds as swimsuit season approaches — it’s the machines that help make the machines that take us to the beach. As an OEM of special equipment designed for automotive suppliers, the Metric Equipment Co., Lee Summit, Mo., decided it was time for the Automatic Drill, Clip, and Cut (ADCnC) Machine to slim down.

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This machine drills, cuts, and prepares rubber weather stripping for application to car doors and windows. The company wanted a motion control solution that would not only reduce wiring and labor costs, but also reduce scrap and inventory for its customers and provide tighter product fit and quality for auto manufacturers. With the help of Logic Control Sales, Kansas City, Metric selected GE Fanuc’s S2K Series digital servo amplifier to effectively reduce wiring by 50%, lower customer inventory from four components to one, and cut scrap rates in half.

The ADCnC machines, used in the finishing areas of rubber extrusion plants, are designed to uncoil a variety of extrusion profiles, drill holes into the extrusion, add a plastic clip for later attachment onto auto doors and windows, and cut the extrusion to desired length. The finished strips are sent to auto assembly plants where they are fed into a second machine for application. Increasingly burdened by the machine’s wiring and labor costs, Metric decided it was time to find a motion solution that could integrate with DeviceNet to reduce these costs. After finding hardware to integrate the machine’s valves and sensors onto the network, Metric tested the S2K Series servo amplifier with DeviceNet communications.

Able to convert a torque, velocity, or pulse signal from an external controller to the proper current required to drive the servomotor, the S2K offers a torque range from 5.7 to 322 lb-in. It also features frontpanel diagnostics, an auto-tuning function, and a compact design that minimizes valuable panel space in the ADCnC design. Robust power blocks eliminate motor stall conditions, and the fast servo response improves accuracy and machine cycle rate with a 122 μ servo loop update and multi-tasking realtime operating system.

The S2K also increased the machine’s production rates by allowing Metric to set different acceleration/ deceleration rates for quick starts and stops. Due to these adjustments, there was an immediate increase in accuracy and reduction in scrap. Another advantage of converting to the S2K and using DeviceNet is the elimination of 50% of the panel wiring. Before, Metric used a smart servo card, two termination boards, and a servo amp, which required both a ± 15 and a 5-Vdc power supply. With S2K, Metric now only needs the servo amplifier, and the only connections are the motor, encoder, and the four-wire network cable. Troubleshooting is easier, too, because operators can pull information from the drives for diagnostic purposes.

Split bearing solution saves weight

It’s just what every design engineer dreams of — an effective and inexpensive solution that can be used in any number of similar applications. The crafty engineers at Craft Bearing Co., Hampton, Va., recently used their standard split roller bearing to transform two standard 2.5 in. schedule 80 pipes into precision drive shafts for critical positioning equipment in the U.S. Navy Research Laboratory’s new Spacecraft Rendezvous and Docking Simulator.

Craft’s engineers designed this massive equipment to maneuver full-sized spacecraft in order to simulate the terminal docking phase during rendezvous in space. They needed a long, rigid lightweight drive shaft for precise control of the docking spacecraft. The engineers’ solution was elegant, yet simple — make the drive shaft from standard pipe rather than solid machined steel. This was made possible due to the use of a Craft split bearing, which required machining only in the area where the bearing was to be installed. The pipe’s rough 2.8-in. OD was easily turned to 2.7-in. for about six in. near the center of the 20-ft. pipe sections. Each end was bored to 2.3-in. ID to accept Trantorque keyless bushings.

Pipe sections were installed on either side of the girder-mounted, low-backlash, parallel shaft gear reducer. Short stub shafts at each end were fitted with drive sprockets. The standard Craft S1-212 split roller bearing was then installed around the shaft. Due to the high lateral stiffness and light weight of the pipe sections, only one bearing was required at the mid-span of each pipe drive shaft. When engineers compared the pipe drive shaft with a conventional 2-in. solid drive shaft, they found that the pipe version was 28% lighter, 70% stronger, and 140% more rigid.

The ability of the split roller bearing to be assembled around the shaft saved hours of machining, reduced installation time, and made the entire concept feasible.

Pendolino high-speed trains go polymeric

Worldwide rail supplier ALSTOM, United Kingdom — provider of 90% of the trains on the London Underground — has chosen Raychem high voltage (HV) polymeric insulation and cable assemblies, from Tyco Electronics, Harrisburg, Pa., for the new Pendolino highspeed advanced tilting trains it’s building for the UK’s West Coast Mainline service. Trains will begin passenger service later this year.

Tyco will supply polymeric pantograph feet, busbar insulators, flexible coil inter-car jumpers, intermediate roofline and downlead cable assemblies, and Bowthorpe surge arrestors for the 53 trains in production. Selection of the polymeric products was based on their history of reliability, cost savings, and opportunities for greater design flexibility. Components are one third of the weight, a quarter of the cross sectional area, and more tolerant to engineering stresses than their ceramic or brittle plastic counterparts. And, they are not susceptible to damage from vibration or stone impacts.

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