TECHNOLOGY FOCUS: POWER TRANSMISSION AND MOTION CONTROL

This section was edited by Associate Editor Alan S. Brown.

Harmonic Goes Linear

A linear drive that brings the advantages of harmonic drives to linear motion can achieve high accuracy in long lengths that cause problems in screw-type drives, and it also works without gearheads or brakes, according to a manufacturer that has introduced one.

The manufacturer, Animatics Corp. of Santa Clara, Calif., says the design has fewer parts, requires less maintenance, and never needs lubrication.

To understand how it works, first consider the workings of a conventional harmonic drive. It consists of three pieces. The first is a wave generator, an elliptical steel disk that usually transmits power from a motor or servo. It fits inside a flex spline, a steel cup with gear teeth on the outside of its thin walls that deform to take on the wave generator’s elliptical shape. This elliptical assembly fits inside a rigid, toothed circular spline so that 30 percent of the teeth are engaged at all times.

Reduction plan: Pulleys act like harmonic drive gears to eliminate the need for a gearhead in this new linear drive.

So far, it sounds like just another collection of gears. But the flex spline has fewer teeth than the circular spline. This means that every time the flex spline makes a full rotation forward, the circular spline moves backward by the number of gears equal to the missing teeth. So if the flex spline has 200 teeth and the circular spline 202 teeth, then it would move backward two teeth for every rotation, a 100:1 reduction ratio in only a few inches of space.

So how does this apply to linear drives? Most conventional linear drives combine a motor and gearhead with pulleys to pull a linear stage back and forth. Animatics’ harmonic linear drive uses a recirculating belt that folds back upon itself as it winds over several pulleys with different diameters. The different diameters of the pulleys work like the mismatched gear teeth in a harmonic drive to force the belt to travel at different rates in different areas.

“By adding a few $5 pulleys, we’ve eliminated a $300 to $500 gearhead,” Animatics CEO Robert Bigler said. He also noted that since the belt is always in tension, it comes to a complete stop without backlash. “Gearheads have to have some play, because if you try to preload them, you’ll have problems with the bearings,” he added.

Bigler claims that the harmonic linear drive performs about as well as highly accurate but more expensive ball screw drives, especially in medium to long lengths where screws begin to vibrate.

The new drives come in standard stroke lengths of 100 to 3,200 millimeters and equivalent pitch ratios of 2.5 mm per revolution to 12.5 mm per revolution. The company’s HLD60 model generates up to 450 newtons of thrust with an average moment loading of up to 150 newton-meters.

Cheap Chip Can Drive

How do you boost electrical efficiency of the brushless motor in a dishwasher, refrigerator compressor, or other appliance? One way is to add a power inverter to match motor speed with load. This is just what appliance manufacturers have done with high-end white goods. Now

Mitsubishi Electric Europe B.V. claims it has an inverter that is economical enough for average household appliances. Mitsubishi’s Semiconductor European Business Group claims its new single-chip inverter, M81500FP, is the world’s smallest intelligent power module for the 90-watt range. It takes only three passive components—a ceramic bootstrap capacitor, a shunt resistor, and a second ceramic capacitor at the voltage supply pin—to operate the inverter.

At 17.5 millimeters by 11.93 millimeters, the entire printed circuit is designed for surface mounting, a low-cost automated process that bonds chips directly onto circuit boards without additional packaging. It is then run off the appliance’s microcontroller or digital signal processor.

The inverter, which is rated for 500 volts and 1 amp, not only drives the motor but protects against under-voltage, interlock, short circuit, and over-temperature while it also provides an open drain fault output. In case of short circuit, it can turn off the device within 15 microseconds.

The thinking behind all this engineering is to make an inverter inexpensive enough to use with just about any appliance.

The inverter reduces energy consumption, and enables motors and pumps to run smoother and with less noise. According to Van Trung Nguyen, general manager of Power Semiconductors Europe, the design “opens up new marketing and sales opportunities for the manufacturers.”

In addition to the inverter itself, Mitsubishi is offering an evaluation board (EVBM81500FP) that contains a microcontroller so engineers can familiarize themselves with the technology.

Foiled Again 

Foil bearings are making explosive progress, according to Hooshang Heshmat, an ASME Fellow and founder of Mohawk Innovative Technology Inc. in Albany, N.Y.

Foil bearings run on a thin film of air that enables them to achieve very high rotational speeds. How high? According to Heshmat, he once ran a 50-pound-thrust turbine engine so fast that it exceeded the ultimate yield strength of the shaft, which cracked and exploded into thousands of pieces. Nor was it any ordinary shaft: It was made of 440C, a high-carbon chromium bearing steel used in the Space Shuttle’s main engine.

The key to foil bearing performance lies in its foil. The shaft rests on a spring-loaded foil journal lining coated with solid lubricant. Once it starts moving, it rides on the foil until it moves fast enough to generate the air pressure needed to push the foil away. At this point, the shaft is riding on a film of air. Foil bearings use no liquid lubricants.

Airborne: Foil bearings enable shafts to rotate at extremely high speeds on a blanket of air. Recent innovations have increased their stability and operating temperatures.

The technology was invented 50 years ago. In the early days, foil bearings could support maximum loads of only about 10 pounds per square inch at room temperature. Today, Mohawk has bearings that carry loads up to 100 pounds per square inch and operate at temperatures as high as 1,700°F. The smallest foil bearings are 4 mm in diameter and achieve 1 million revolutions per minute. The largest bearings are 235 mm diameter and clock in at hypersonic speeds of more than 600 meters per second.

Mohawk’s latest bearings take advantage of two key innovations. The first has to do with the bearing’s ability to absorb shocks without failing. This involves a delicate balance of stiffness and damping. Heshmat likens it to tossing a child into the air. “As the baby starts coming down, you match his speed with your hands to slow him down gradually before catching him,” he explained. “We’ve designed the foil and springs around the shaft to achieve multilevel stiffness and damping to do just that.”

Heshmat’s team has also developed an innovative foil coating. It fulfills two important functions. First, it acts as a solid lubricant to smooth the startup of the bearing before it goes airborne. This prevents wear. The same coating also acts as a thermal barrier, enabling the foils to survive for tens of thousands of hours at temperatures up to 1,700°F.

The military funded much of Mohawk’s research for use in missiles, rockets, and jet engines.

Lubrication-free bearings could make possible turbochargers for diesel engines. As today’s diesel turbochargers age, oil leaks into the engine and comes out as particulates. Environmental regulations limit particulate emissions, such as the black soot that diesels emit as they warm up to operating temperatures. While foil bearings could double the cost of turbochargers, they would allow diesel manufacturers to cut back on the equipment now needed to capture particulate emissions.

Wiggle Room

Here’s a different take on the piezoelectric actuator, the device that takes advantage of the deformation of certain ceramics when an electric field is applied to them. The piezoelectric actuator changes shape and can move something.

A few years ago, David Henderson got the idea of using the shape-changing phenomenon to cause a nut to vibrate and drive a screw. What he came up with is a linear motor that can move about 100 times its own mass and can be manufactured in sizes down to about 1.5 centimeter thick, and perhaps even smaller.

Henderson received a patent for the idea in 2005 and has founded a company, New Scale Technologies Inc., which says it can make as many as 100,000 motors a month at its factory in Victor, N.Y. He is both co-CEO and chief technology officer. The other co-CEO, Ted Franceschi, is chief development officer.

Tiny driver: Less than 2 mm across, the Squiggle motor can drive 100 times its own mass.

The device consists of a screw inside a nut, with both parts made of stainless steel. The nut is encased in four piezoelectric plates. A voltage is applied to the plates at the frequency of the nut’s first bending resonant frequency, Henderson said. Depending on the size of the nut, that could be anywhere from about 40 to 200 kilohertz. The plates operate in pairs, and the current is applied separately to each pair with a 90-degree phase shift.

The result is an alternating deformation of the plates that causes the nut to vibrate in a circular motion. Henderson compared the action to the movement of the hips of a person spinning a Hula Hoop. Tangential forces from the threads of the nut work on the screw, which will begin to rotate and can move a load.

The company calls it the Squiggle motor. According to Henderson, the system requires an axial load to work properly. When there is no load on the screw, it may move, or it may not, depending on thread clearance and the mass of the screw. The company makes the motors in which the nut is as small as 1.5 x 1.5 x 6 mm. Henderson said it may be possible to manufacture them 33 percent smaller.

One of the applications for which New Scale advertises the motors is for fine control of lenses in tiny cameras, like the ones in cellular phones. New Scale recently expanded a two-year-old license with Tamron Co., a maker of optical equipment in Japan. Tamron is now licensed to make Squiggle motors for its own products at factories in Japan and elsewhere in Asia. It may also serve as a contract manufacturer for New Scale.

The company suggests a range of uses for Squiggle motors, from toys to electronic locks or microfluidic controls in drug pumps.

There are several options for the electronics needed to drive the motors, Henderson said. Among them, he said, are benchtop electronics in housings, separate PCBs using discrete electronic components that are ready for integration in a customer’s product, or a motor drive circuit on a single application-specific integrated circuit.

New Scale has partnered with austriamicrosystems AG, a manufacturer of integrated circuits in Unterpremstaetten, a suburb of Graz, Austria, to develop a chip that will be able to drive two Squiggle motors simultaneously. The collaboration will also develop a position sensor.

Austriamicrosystems earlier this year invested $6 million to buy a 25 percent stake in New Scale. Austriamicrosystems was formed in 1981 as a joint venture of American Microsystems Inc. and VOEST Alpine AG. It is headquartered in a 12th-century castle, Schloss Premstaetten, that VOEST purchased in 1981.

New Scale makes a range of Squiggle motors in standard sizes and offers to customize, as well. The smallest stock motor that New Scale makes, Model SQL-1.5-6, has a nut that is 1.5 x 1.5 x 6 mm. Its mass is 0.15 grams and it is rated to generate up to 0.2 newton, or 20 grams force. The stock model has a maximum travel of 6 mm and a travel speed of 5 mm per second. Its resolution is listed at 0.5 micrometer.

According to Henderson, a Squiggle motor is applicable “anyplace you want to replace a solenoid or stepper motor and want to go smaller.” They are especially advantageous at widths, or motor diameters, under 6 mm, he said.

Super Induction

British superconductor developer Zenergy Power plc and German machine manufacturer Bültmann GmbH have begun selling an industrial-scale, superconductor-based induction heater for metal processing. The technology, which they say is twice as efficient as conventional induction heaters, won the prestigious 100,000-euro Hermes Prize at this year’s Hannover Fair in Germany.

Metal fabricators use induction heaters to soften aluminum, brass, bronze, and copper in order to shape or extrude such parts as heat exchanger tubes, pipes, window frames, computer components, and automotive profiles.

The induction heaters themselves are electromagnets driven by high-frequency alternating currents. This induces eddy currents in the workpiece. The resistance of the eddy currents to changes in direction induced by the alternating current causes the metal to heat up. Induction heaters typically have energy efficiencies of 35 to 45 percent, and can consume an astonishing 1 to 5 percent of an industrial country’s total energy production, according to Zenergy.

One reason that induction heater efficiency is so low is hysteresis, the tendency of the magnetic cores to resist the rapid cycling of the magnetic fields. Superconductors provide a solution to that problem. They have zero electrical resistance, so there is no hysteresis, no matter how fast the current alternates to switch the polarity of the magnetic field.

This greatly reduces the amount of energy needed to power the induction coil. Superconductor induction heaters achieve efficiencies of 90 percent. The technology produces highly homogeneous and precisely controllable temperature gradients in the workpiece, with no risk of damage from local overheating. Superconducting induction heaters start up quickly. Their only real downside is expense, but Zenergy claims that companies can pay off the purchase price within five years based on energy cost savings alone.

So far, Zenergy and Bültmann have sold two commercial units. The first one, in September 2007, went to aluminum fabricator Weser Alu GmbH, which used it to replace an existing copper-based aluminum heater at its manufacturing plant in Minden, Germany. Based on preliminary trials, Weser Alu expected to decrease energy use to 160 kilowatt-hours, from 280 kilowatt-hours, while reducing heating times and improving temperature distribution.

The second unit builds on lessons learned in the first heater, the developers said. They said it has been sold to a manufacturer that makes automotive, electrical, air conditioning, and refrigeration components. The manufacturer will use the induction heater in a newly commissioned facility to extrude copper and copper alloy billets.