Motion Control to the ExtremeMotion Control to the Extreme
March 10, 2011
Harsh environmentmotion applications fall into broad categories ranging from sealed waterproofand dust-proof environments to extreme conditions, such as operating motors ina vacuum while exposing them to radiation.
"Radiation and vacuum environments are somewhat similar because there is aneed for a detailed material selection process," says Rick Halstead, presidentof Empire Magnetics. "With radiation, Gamma rays or energeticparticles such as neutrons strike the material breaking up the long organicchains such as the Teflon insulation in the lead wires, organic varnish,polymer tape or glue in the motor assembly."
Motion Control to the Extreme_A
Motion control in radiation-intensive environments poses a seriouschallenge to the design engineer. Conventional step and microstepping motorsare susceptible to high-energy particles or rays that break down motormaterials. Usually, the organic compounds break down rapidly. As a result, thelubricants, varnish, lamination bonding and cable insulation in standard,commercially available motors will deteriorate in a relatively short period oftime when exposed to radiation.
Halstead saysthe question is not if, but how fast the materials will break down in theseconditions. In the past, the government published data on material breakdown inradiation applications, but now, due to security concerns, thatinformation is not easily available.
If you plan to operate a motion system in a vacuum, "most people don'trealize that organic
materials in the motor will begin
to boil away," says Halstead. "So the question is what materials should be usedwithin the
assembly. Certain materials, even metals such as cadmium and zinc, willvaporize in a vacuum."
Empire Magnetics is currentlyworking on motors for the supercollider in Europe and Lawrence Livermore Labs'National Ignition facility. These kinds of applications are challenging becausethe motors operate in both radiation and vacuum environments.
"The materialslist is very small when you're looking for materials that areradiation-resistant and also won't outgas in a vacuum," says Halstead. Anotherpart of this difficult application is how radiation changesthe structure of your materials. For example, if you start with a particular material such asa polyamide, when the radiation breaks down the long chain molecules, you endwith up with different length molecules, and all of those molecules are nowdifferent materials and no longer polyamide."
Halstead says the key is to understand the outgassing properties of thesecondary breakdown products, but no one has done any studies on the spectraloutgassing of these materials over time because there is no commercial reason todo so. The National Lab basically is selecting the best radiation materialsavailable, with low outgassing properties, and is testing those materials inthis particular application.
New radiation-resistant motors greatly expand the design possibilities inhighly radioactive environments, and Empire Magnetics works with customers onradiation-resistant components and designs. Motors with 2x 0E8 Rads T.A.D. arepart of Empire's product offering, but they also have experience with custommotors and testing designs subjected to total accumulated doses in excess of1x10E9 Rads.
Linear Motors inHarsh Environments
"More engineers are looking to use linear motors tooperate in harsh environments," says Jerame Chamberlain, sales manager with Nippon Pulse. "New linear motor designs offertechnical advantages in these applications because many motors manufacturedtoday don't have a way to protect themselves and need to be completely shroudedin these types of applications."
Chamberlain says the magnet track itself is exposed in many designs, sothat if the environment has metal filings, for example, debris can becometrapped inside the magnets and within the air gap. Another important issue iswaterproofing. With many linear motor designs, it is hard to control andprotect the coils from getting into the water. Even on U-shaped waterproofmotors where the forcers are made out of epoxy, there is no way to reliablycontrol the position of the coils within the epoxy.
Motion Control to the Extreme_D small
Click here for larger image.
Nippon Pulse linear motors use a cylindrical design where the magnettrack is a round rod down the center of the motor which can be protected by astainless-steel tube. The design lends itself more readily to harshenvironments because the magnets are protected.
With dust or metal particulates in a production area, the buildup ofthese materials can gradually affect the magnetic field and will close up themotor's air gap.
Other types of contaminants that aren't ferrous can also become trappedand cause problems, such as cotton fibers. With linear motors in applicationenvironments such as cotton fibers, its larger air gap will result in betterperformance versus a ballscrew or belt drive where the contaminant might eataway at the belt or damage the ballscrew.
"Typically, with the way a linear motor is built, if you draw a squarewith a hole in it, the rod goes down the middle of the motor," saysChamberlain. "The circle in the middle of our motors is a fiberglass tube thatacts like a bobbin for a rotary motor and the coils are wrapped around it. Inour design, we completely fill that box with an epoxy material so it becomes asolid part."
While there isn't a published IP rating for the motor, it has been usedin applications where the motor is actually operating submerged underwater forseveral days or longer at a time. IP67 calls for a motor to be submerged forone to three hours under a meter of water.
"We run our motors completelysubmerged and have current applications doing wafer slicing in de-ionized waterwhere previously they had attempted to use a U-shaped motor," Chamberlain says.Even though it was promoted as a waterproof motor, because they couldn'tcontrol the coil position, the de-ionized water kept destroying the coils."
Nippon Pulse linear motors have been successfully used in applicationssuch as nuclear reactor refueling. They have used de-ionized water in a nuclearreactor for the positioning of rods. In applications where the motor issubmerged, a waterproofing version of the motor fills the center with epoxy toallow it to run underwater. In radioactive environments, the motor design hasto be a custom solution because of the need to work with the manufacturer toreview the materials in the motor. The standard lead wire, for example, must be replaced with a special wirethat is more robust in the radiation environment.
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Because of the need to wash down equipment in foodprocessing plants, Thomson Industries Inc. offerslinear units with a novel sealing system designed to withstand the pressuresand chemicals present in that environment. A long piece of extrusion driven bytwo main drive systems, either a ballscrew or belt, incorporates both a thrustmember and a guidance member. The guidance can be circulating ball technology,profile or round rail, but this particular application used a prism or polymerguide which is an engineered plastic.
Even though thesystem is sealed, when the carriage traverses along the extrusion, there is asmall gap where applying a high-pressure wash to the part, liquid will stillget inside. Contaminants are kept out, but no linear unit seals well enough toprovide an IP65 or IP66 rating around the carriage area.
The strategy for this application is to make the linear system appropriatefor washdown operation. A standard anodized aluminum extrusion is cut to theproper length, and then re-anodized which gives it another coating and coatsthe ends that have been machined. All stainless-steel hardware and a polymerbearing protect against any material or water that gets inside the unit. Thebearings are an engineered plastic and work as a plain member between thebearing and the extrusion.
"If material gets inside the unit, the linear system wipes away orpushes the material along the edge of the extrusion," says Jim Marek, businessunit director, global slides & tables, for Thomson.
Motion Control to the Extreme_B
Another motion system designed for harsh environments byThomson is its Micron AquaTrue gearbox. When used with a meat slicer,the gearbox is attached to a motor and the slicer is mounted to the output ofthe gearhead.
"When it comes to gearing, the application can't accept any ingress intothe mechanism itself," saysMarek. "When designing motion systems for thesekinds of environments, you are forced to live with the presence of contaminantsand have to devise ways to keep the larger pieces of debris out of the unit.You also have to apply a bearing system and internal components that can handlethe environment. On the gearhead side, contaminants can't be allowed to getinto the unit because the carburized gears will rust."
The design strategy is to use stainless-steel (303) housings on theoutside to keep it from rusting, and use cutlery or food-grade materialsinside. Marek says the shafts are typically stainless steel (174 PH), which hassimilar corrosion-resistant properties to 303, but can be hardened and providesbetter strength properties.
The overall design strategy for the gearbox is to prevent ingress. Anyjoint must have O-ring seals and a labyrinth-designed seal originallyengineered for outdoor equipment is placed on the input. This non-contactingseal provides nested seals, one mounted to the shaft and the other to thehousing. One seal is on the stator, and the other is on the rotor. The intricate labyrinth circuit works like adjacentprofiles, similar to a tongue-in-groove profile in that it is continuous, butdoes not come into contact with each other.
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