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POWER TRANSMISSION AND MOTION CONTROL.

OPERATING BY WIRE

Intuitive Surgical Inc. of Mountain View, Calif., makes remote surgical systems for minimally invasive procedures. According to vice president of engineering Gary Guthart, the system uses two 8- and one 12-mm-wide holes in a patient for bringing to the operating site two manipulators and a camera. Seated at a console, the surgeon can then duplicate the look and feel of open surgery while minimizing the size of the entrance wound.

The system, described in an article in December 1999, was approved by the FDA last July for abdominal surgery. Clinical trials continue for thoracic and cardiac applications.

Central to the system are the two manipulators that the surgeon controls through a consort of wrist, hand, and finger moves. At the heart of each manipulator, in turn, are dc servo motors from Maxon Precision Motors Inc. of Burlingame, Calif.

The motors serve as both inputs and outputs, Guthart explained. The motors and encoders both receive instructions from the surgeon and exert forces back through the surgeon's hands.

The Maxon motors, which use rare earth magnets in their stators, had good power density and smooth backdriving, two important considerations in this application, Guthart said. The engineers at Intuitive Surgical had to watch for what he called torque ripple: "If you feel the brushes go over the commutator, that's bad," he explained.

Guthart called the surgeon's manipulators masters, to distinguish their dual role on the surgeon's side of the system from the motors used on the patient's side. Those, which he termed slave motors, also needed precision and the ability to backdrive. That's because an assistant surgeon has to be able to move the end effectors initially to position surgical tools--a manual step.

Low hysteresis at the instrument tips was important, but so was their capacity to be backdriven, he said. Cable drive between motor and instrument helped to meet those requirements.

The instruments themselves are fully autoclavable, Guthart said. That in itself was no revelation. Embedded in each instrument, however, is a non-volatile memory chip, he explained. The chip tells the control system what a particular tool is-- scissors or forceps, say. It also tracks instrument use so a tool will be removed from service before it can't cut any more.

PLUGGED IN

As manufacturers of building materials have pared down in-house engineering staff, Automation & Motion Ltd. of Bottesford in Nottinghamshire, England, has found itself called in by more companies to shepherd half-finished automation projects along to completion.

Automation projects frequently involve metal strip feeding, punching, or cutting, according to Andy Perkins, a controls engineer there. For example, a pinch roll feed project for a press, Perkins said, might combine the design and building of mechanical, electrical, and control system elements right on through to the Internet link for remote diagnostics.

In a servomotor for such a feed, it's a motor's peak torque that's playing the lead role, Perkins explained. Automation & Motion has built pinch roll feeders capable of propelling stock into a press at peak speeds of 150 meters per minute. Of course, most stock never reaches this speed, he said, because the cut length of a piece means that the pinch rollers are either accelerating or decelerating, at rates between 30 and 50 m[s.sup.2].

To get the necessary peak torque levels, Automation & Motion has selected servomotors from Control Techniques Dynamics Ltd. of Andover in Hampshire, England. According to the manufacturer, its largest Unimotor ac brushless servomotor, the 190, produces a peak torque of 220 Nm. Maximum current rating is 70 amps.

The motor maker recently integrated a connector into the 190's housing, and can provide a ready-made cable for the other side. According to the manufacturer, the connectors bear UL approvals, and environmental sealing ratings of IP67. The idea behind the integral connector is to eliminate a wiring step at the machine.

ORANGEADE

In classic entrepreneurial fashion, Ron Doll recognized a need and filled it. The president of Orange Peel LLC of Cedarburg, Wis., Doll noticed over 20 years in the power transmission industry that many shafting guards were afterthoughts. Indeed, industry practice has long been to call in the local sheet metal fabricator to measure a machine for guards after it has been nailed into place. Doll saw a better way.

Beginning a year and a half ago, Orange Peel developed five standard guards for close-coupled applications. Built out of orange HDPE, the guards can be trimmed to fit both in length and height along dimensional trim lines. Typical installation time is around an hour, Doll said.

Orange Peel recently teamed up with Milwaukee-based drive maker Falk Corp. The orange guards will now accompany many of Falk's new Drive One line, saving the GEM cost, inventory, engineering, and manufacturing.

Ultimately, the new product will offer models in all 15 sizes, but for the time being, he said, the company is focusing on four in the middle of the torque range, between 200,000 and 525,000 in.-lbs. A bucket conveyor on a grain elevator is a typical application for such a drive, Wheelock said.

According to Drive One product manager Ken Wheelock, the new product line will bring Falk's many drives into a common family, ending a run of application-specific designs that developed over the years along different evolutionary branches. The company is concentrating its efforts on the right-angle shaft-mounted drives right now, Wheelock said.

Falk's use of Orange Peel guards highlights the way in which many components for the new drives came together, Wheelock said. Much of the design developed concurrently, he said, and vendors were brought in at the start of the design cycle to raise manufacturing issues.

DOING THE LAUNDRY

As industrial washers grow in capacity, so does the size of the laundry bags feeding them, said James Thorpe, a product development engineer at Jensen UK Ltd. of Banbury. His company's automated laundry system now handles laundry bags weighing 90 kg, thanks partly to extensive material testing during system development.

A heavy-duty automated laundry, one that serves a number of hotels or hospitals, can keep manual labor to a minimum these days, Thorpe explained. From washing, to folding, to weighing, except for separating, everything today can be automated. That lets laundry operators run energy-intensive machines at night, but keeps the task of sorting in the hands of day labor.

During sorting, a machine counts the linen automatically while monitoring weight. From there, laundry runs on one of several belt conveyors out to open-mouthed bags suspended from an overhead dual-trolley monorail. A vertical conveyor lifts filled bags up to the laundry system roof, Thorpe said, where they accumulate. At night, the trolley "culls" bags from the accumulator, dumping their contents into top-loading, corkscrew-style tunnel washers.

Previous designs used steel-raced hall bearings to suspend the trolley cars from aluminum tracks. But loads in those days were smaller, Thorpe said. As bag fill weight increased over the years, track wear began accelerating.

Thorpe said he spent six months looking at different thermoplastic replacement wheels before deciding on a polyaryletherketone polymer from Victrex USA Inc. of West Chester, Pa.

Thorpe had several concerns. Like the steel bearings they were to replace, the glass-filled plastic wheels he tested wore down the aluminum rails. Other materials, such as nylon 6, absorbed moisture in the hot, humid test site. Flat spots due to creep settled into many sample wheels as they sat overnight saddled with suspended loads. The Victrex material qualified for the job by minimizing track wear, moisture absorption, and creep, Thorpe said.

STAY CLOSE TO THE BOAT

A manufacturer and user of underwater inspection systems, Imetrix Inc. of Cataumet, Mass., builds a remotely operated vehicle for hull surveying. The Lamp Ray, as the system is named, goes overboard to conduct inspections that ship-certifying bodies, such as the American Bureau of Shipping and Lloyd's Register, require to deem a vessel seaworthy.

Where the Lamp Ray differs from other free-swimming ROVs, explained Imetrix's chief engineer, Ed Slate, is in its capacity to crawl along the side of a hull, in currents as swift as five knots, and precisely map the location of thinning hull sections relative to design drawings.

The underwater vehicle uses two thrusters for holding its three wheels against a hull. Two wheels are powered, while the third merely idles. Powering the wheels presented a bit of a design challenge, Slate said, because the engineers wanted to turn the wheel and another thruster from a single motor.

The engineers first considered de-rating the thruster, so it could spin simultaneously with the drive wheel without overpowering it. As it happened, they didn't have to. The thrusters delivered a flow soft enough so they could continue turning without impeding the ROV as it roved over the ship.

The mechanical solution turned out to be a dual-shaft right-angle gearbox, which drives the wheel as it turns the propeller through a belt. When the gearbox began failing underwater, however, Imetrix engineers brought their reliability problems to Thomson Industries Inc. of Port Washington, N.Y Thomson, according to Slate, "works on customs all the time," and came back with a smaller unit in less than four weeks. Compared to the old unit, "it had the same, if not better, power handling," he said. "It has been very tolerant of the underwater environment," he added.

CROSSING THE WINDER

Electrical wire, fiber optic cable, hose, tubing, pipe: Name just about any product that's made in a continuous length and it's likely to be a candidate for reeling up on a winder.

Mike Spence, president of Tulsa Power Products LLC, said the company makes small rewinders used by distributors to divide up bulk cable. It makes large-capacity winders, too, used by OEMs to gather up product after manufacturing.

A winder needs a traverser to guide cable back and forth across the face of a spool. The Tulsa-based manufacturer uses a packaged Uhing linear actuator from Amacoil Inc. of Aston, Pa.

Spence said the Uhing actuator is often his engineers' first choice because the self-contained unit, unlike ball-screw traversers, doesn't need much adaptation to make it work on their designs. A chain and sprocket off the winder drive is usually all that's required to impart motion to the traverser, Spence said.

The linear actuator moves because of three or four bearings whose crowned inner races ride along a spinning shaft. Each bearing is cocked relative to the shaft. Changing the angle between bearing and shaft varies the speed at which the unit travels. At the end of a traverse, a stop flips a lever on the housing, reversing the angle of the bearings and sending the actuator in the opposite direction.

According to Amacoil's marketing manager, John Scavitto, operators can change the pitch of the actuator while a machine is running. The outboard bearings work in tandem to determine direction, Scavitto explained, while the inboard bearing, or bearings, take thrust load. A fourth bearing is added to increase thrust capacity without going to a larger size, he said.

To improve the actuator's capacity for overhanging loads, Uhing recently added an optional slide that is flexibly connected to the actuator.

Because the actuator needs no threads to move, it works well in dirty environments without the use of bellows, Scavitto said. Also, it is intrinsically protected against overload.

CRANE MOVE

A veteran crane operator will frequently develop a feel for slowing a traveling crane without swinging its suspended load. A computer-controlled crane won't do that, according to NASA aerospace engineer Robert Youngquist. "It has to be taught," he said.

Youngquist and his associates at the Kennedy Space Center in Florida wanted to give that feel to a jib hoist at the Orbiter Processing Facility. Less swing time would mean faster loading for the Shuttle's cargo bay.

In order to build their first-order mathematical model, the team had to consider the motions of the crane head, the cable, and the load, while neglecting cable friction, cable mass, and any variations in cable length. Furthermore, they assumed that the load acted as a point mass and that the angle of the cable would never vary by more than a few degrees from vertical.

There are many acceleration profiles that are capable of eliminating relative motion between crane head and load, Youngquist explained. In all of them, the crane head moves back and forth as the crane changes speed, so that when the crane returns to constant velocity both the crane head and the load align, move at similar speeds, and don't accelerate.

Youngquist suggested several generalizations for the model to strengthen it. Accounting for changes in cable length as the crane moves was one. Accounting for larger cable angles was another.

SMOOTH CONNECTIONS

A tool and die maker for well over 30 years, Randy Vasovich, the president of RTD Manufacturing Inc. in San Jose, Calif., now builds what he suggests is a simple machine for polishing fiber optic connectors. Vasovich's design, which he modestly proclaimed to be "similar to a drill press," relies on a hand-operated fixture to draw 12 connectors at a time down to a diamond-surfaced polishing wheel. The pressure of the tool against the work is set through a spring-loaded micrometer, he said.

Vasovich tried out close to a dozen motors in hopes of finding one that could turn the diamond wheel without tiring. A little dc brushless model from Oriental Motor USA Corp. of Torrance, Calif., eventually distinguished itself among the ranks.

Vasovich said the compact motor was three or four times more powerful than the other models he tried. The maker claims that motor size is down by 55 percent compared with controllable speed ac induction motors of the same power.

In operation, the machine can use as many as nine different grades of diamond polishing, depending on an end user's needs, Vasovich said. Polishing at any one grade usually lasts two minutes. Then the machine operator changes the diamond plate and resumes polishing.

PRECISION OPTICS

Dover Instrument Corp. of Westboro, Mass., makes air bearings for the precision motion industry. In one application, a Canadian ophthalmic equipment manufacturer asked Dover to build the curve generator for a five-cell lens-making system. According to Dover's vice president of engineering, Phil Greene, the generator cuts the inside, concave portion of the lens to produce the prescription.

In this application, a high-speed fly-cutting spindle, oriented horizontally, mounts to a rotary table, Greene said. An air bearing supports the table, whose axis is vertical. The table varies the angle of the cutting tool dynamically relative to the lens as the cut progresses.

At first, Dover used an iron-core brushless motor to drive the table. Motor cogging brought on torque variations and minor velocity changes, leading to cyclic aberrations.

A slotless, brushless motor from EAD Motors Inc. of Dover, N.H., replaced the iron-core unit. The new motor eliminated cogging and improved velocity control.

To give an idea of the precision involved, Greene said the air bearing table uses a 250,000-line encoder to produce 256 million discrete position stops in a complete rotation. Every encoder line is interpolated 1,024 times, Greene said.

"During the cut, we can control to a following, or tracking, error in the single-digit count," Greene said. That equals fractions of arc-seconds of tracking error, he explained. "Where the tool meets the glass, it amounts to 5 to 10 nanometers of motion," he added.

The EAD motor had no cogging error, Greene said. It had a "perfect torque-to-current relationship."

The reason, according to EAD, is that the slotless, brushless dc motors have no stator teeth. The rotor has no preferred position, the manufacturer said. Stators consist of high-density copper windings in toothless, laminated rings.

THE BUZZ ON GEAR HEADS

Ohio-based Cincinnati Machine has built a total of six giant five-axis milling machines for Boeing. The aerospace manufacturer used the heavy mills at its Decatur, Ala., plant to machine 18-foot-diameter rings that connect the stages of the Delta IV rocket. According to the Cincinnati Machine project engineer, Barrington McCullough, replacing two gearboxes in each machine with planetary gear heads from Stober of Maysville, Ky., eliminated a troubling "servo buzz."

Servo buzz, McCullough explained, happens when you try to close a servo loop around a mechanical element having any kind of lost motion. When you increase the gain, "the servo goes unstable," he said. "It sits there and buzzes. It's no good for positioning." Servo buzz leads to other problems, he said. Mechanical wear on the mill increases. Its capacity to produce a high-tolerance finish diminishes. The noise grows wearisome.

Meeting with a Stober application engineer, Clayton Webber, McCullough found that the catalog tolerance for lost motion on the Stober gear head, about 2 arc minutes, was half that of other manufacturers' gear heads. Even better was that Stober stamped the measure of lost motion on the side of each gearbox, McCullough said. "Stober takes each box and manually checks it with x amount of torque. Then they stamp it and fill it with grease," he said.

The universal mill uses a 100-hp motor to move the carrier, McCullough added. It was the two axes in the head, what the industry labels a and c, that suffered from servo buzz. The other three axes--x, y, and z--represent longitudinal, transverse, and vertical movement of the head.
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Title Annotation:Intuitive Surgical Inc.
Comment:POWER TRANSMISSION AND MOTION CONTROL.(Intuitive Surgical Inc.)
Author:Sharke, Paul
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Mar 1, 2001
Words:2871
Previous Article:CALIBRATION AND CLONING.
Next Article:THE COSTS OF CUTTING MATH AND SCIENCE.
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