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Motion Control to the Extreme

Harsh environment motion applications fall into broad categories ranging from sealed waterproof and dust-proof environments to extreme conditions, such as operating motors in a vacuum while exposing them to radiation.

"Radiation and vacuum environments are somewhat similar because there is a need for a detailed material selection process," says Rick Halstead, president of Empire Magnetics. "With radiation, Gamma rays or energetic particles such as neutrons strike the material breaking up the long organic chains such as the Teflon insulation in the lead wires, organic varnish, polymer tape or glue in the motor assembly."

Motion Control to the Extreme
Motion control in radiation-intensive environments poses a serious challenge to the design engineer. Conventional step and microstepping motors are susceptible to high-energy particles or rays that break down motor materials. Usually, the organic compounds break down rapidly. As a result, the lubricants, varnish, lamination bonding and cable insulation in standard, commercially available motors will deteriorate in a relatively short period of time when exposed to radiation.

Halstead says the question is not if, but how fast the materials will break down in these conditions. In the past, the government published data on material breakdown in radiation applications, but now, due to security concerns, that information is not easily available.

If you plan to operate a motion system in a vacuum, "most people don't realize that organic
materials in the motor will begin
to boil away," says Halstead. "So the question is what materials should be used within the
assembly. Certain materials, even metals such as cadmium and zinc, will vaporize in a vacuum."

Empire Magnetics is currently working on motors for the supercollider in Europe and Lawrence Livermore Labs' National Ignition facility. These kinds of applications are challenging because the motors operate in both radiation and vacuum environments.

"The materials list is very small when you're looking for materials that are radiation-resistant and also won't outgas in a vacuum," says Halstead. Another part of this difficult application is how radiation changes the structure of your materials. For example, if you start with a particular material such as a polyamide, when the radiation breaks down the long chain molecules, you end with up with different length molecules, and all of those molecules are now different materials and no longer polyamide."

Halstead says the key is to understand the outgassing properties of the secondary breakdown products, but no one has done any studies on the spectral outgassing of these materials over time because there is no commercial reason to do so. The National Lab basically is selecting the best radiation materials available, with low outgassing properties, and is testing those materials in this particular application.

New radiation-resistant motors greatly expand the design possibilities in highly radioactive environments, and Empire Magnetics works with customers on radiation-resistant components and designs. Motors with 2x 0E8 Rads T.A.D. are part of Empire's product offering, but they also have experience with custom motors and testing designs subjected to total accumulated doses in excess of 1x10E9 Rads.

Linear Motors in Harsh Environments
"More engineers are looking to use linear motors to operate in harsh environments," says Jerame Chamberlain, sales manager with Nippon Pulse. "New linear motor designs offer technical advantages in these applications because many motors manufactured today don't have a way to protect themselves and need to be completely shrouded in these types of applications."

Chamberlain says the magnet track itself is exposed in many designs, so that if the environment has metal filings, for example, debris can become trapped inside the magnets and within the air gap. Another important issue is waterproofing. With many linear motor designs, it is hard to control and protect the coils from getting into the water. Even on U-shaped waterproof motors where the forcers are made out of epoxy, there is no way to reliably control the position of the coils within the epoxy.

Motion Control to the Extreme
Click here for larger image.
Nippon Pulse linear motors use a cylindrical design where the magnet track is a round rod down the center of the motor which can be protected by a stainless-steel tube. The design lends itself more readily to harsh environments because the magnets are protected.

With dust or metal particulates in a production area, the buildup of these materials can gradually affect the magnetic field and will close up the motor's air gap.

Other types of contaminants that aren't ferrous can also become trapped and cause problems, such as cotton fibers. With linear motors in application environments such as cotton fibers, its larger air gap will result in better performance versus a ballscrew or belt drive where the contaminant might eat away at the belt or damage the ballscrew.

"Typically, with the way a linear motor is built, if you draw a square with a hole in it, the rod goes down the middle of the motor," says Chamberlain. "The circle in the middle of our motors is a fiberglass tube that acts like a bobbin for a rotary motor and the coils are wrapped around it. In our design, we completely fill that box with an epoxy material so it becomes a solid part."

While there isn't a published IP rating for the motor, it has been used in applications where the motor is actually operating submerged underwater for several days or longer at a time. IP67 calls for a motor to be submerged for one to three hours under a meter of water.

 "We run our motors completely submerged and have current applications doing wafer slicing in de-ionized water where previously they had attempted to use a U-shaped motor," Chamberlain says. Even though it was promoted as a waterproof motor, because they couldn't control the coil position, the de-ionized water kept destroying the coils."

Nippon Pulse linear motors have been successfully used in applications such as nuclear reactor refueling. They have used de-ionized water in a nuclear reactor for the positioning of rods. In applications where the motor is submerged, a waterproofing version of the motor fills the center with epoxy to allow it to run underwater. In radioactive environments, the motor design has to be a custom solution because of the need to work with the manufacturer to review the materials in the motor. The standard lead wire, for example, must be replaced with a special wire that is more robust in the radiation environment.

Motion Control to the Extreme
Washdown Operation
Because of the need to wash down equipment in food processing plants, Thomson Industries Inc. offers linear units with a novel sealing system designed to withstand the pressures and chemicals present in that environment. A long piece of extrusion driven by two main drive systems, either a ballscrew or belt, incorporates both a thrust member and a guidance member. The guidance can be circulating ball technology, profile or round rail, but this particular application used a prism or polymer guide which is an engineered plastic.

Even though the system is sealed, when the carriage traverses along the extrusion, there is a small gap where applying a high-pressure wash to the part, liquid will still get inside. Contaminants are kept out, but no linear unit seals well enough to provide an IP65 or IP66 rating around the carriage area.

The strategy for this application is to make the linear system appropriate for washdown operation. A standard anodized aluminum extrusion is cut to the proper length, and then re-anodized which gives it another coating and coats the ends that have been machined. All stainless-steel hardware and a polymer bearing protect against any material or water that gets inside the unit. The bearings are an engineered plastic and work as a plain member between the bearing and the extrusion.

"If material gets inside the unit, the linear system wipes away or pushes the material along the edge of the extrusion," says Jim Marek, business unit director, global slides & tables, for Thomson.

Food Processing Gearbox
Motion Control to the Extreme
Another motion system designed for harsh environments by Thomson 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 of the gearhead.

"When it comes to gearing, the application can't accept any ingress into the mechanism itself," says Marek. "When designing motion systems for these kinds of environments, you are forced to live with the presence of contaminants and 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 handle the environment. On the gearhead side, contaminants can't be allowed to get into the unit because the carburized gears will rust."

The design strategy is to use stainless-steel (303) housings on the outside to keep it from rusting, and use cutlery or food-grade materials inside. Marek says the shafts are typically stainless steel (174 PH), which has similar corrosion-resistant properties to 303, but can be hardened and provides better strength properties.

The overall design strategy for the gearbox is to prevent ingress. Any joint must have O-ring seals and a labyrinth-designed seal originally engineered for outdoor equipment is placed on the input. This non-contacting seal provides nested seals, one mounted to the shaft and the other to the housing. One seal is on the stator, and the other is on the rotor. The intricate labyrinth circuit works like adjacent profiles, similar to a tongue-in-groove profile in that it is continuous, but does not come into contact with each other.

 
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