The problems inherent in operating a motor in a vacuum – outgassing, contamination and temperature – are well known to design engineers. Solutions are less well known.
Common requirements for vacuum operation include the ability to accurately move or position samples, products, mirrors and sensors. In the past, this has often been accomplished by trying to evade the effects of the vacuum, by locating drive mechanisms and motors outside the vacuum chamber. The drive mechanism transmits its motion through the vacuum chamber wall using sealed couplings and magnetic or mechanical feed-through mechanisms.
One major problem with this approach is that the engineer has a limited number of design configurations he can consider. It can be very difficult to implement an X-Y stage (in which one stage moves on top of the other) inside a vacuum area when motors are outside the chamber, since the mechanisms used to transfer motor power to the top stage must also accommodate the motion of the bottom stage. The accuracy, repeatability, and resolution of the positioning system located inside the vacuum chamber can be heavily compromised.
In contrast, placing the motor directly into the vacuum allows the engineer to consider a larger number of physical design arrangements without compromising the motion control activity inside the chamber. By directly coupling the motor to the load, the accuracy, repeatability and other system specifications can be vastly improved. Because mechanical feed-throughs are very expensive, a system that features an internally-placed motor often costs much less than the externally-mounted alternative.
Ordinary motors will not survive in a vacuum application of 10-4 Torr or lower. The reason is that the bearing lubricants will vaporize and the motor’s and cable’s insulation materials will evaporate in a phenomenon that is known as “outgassing,” resulting in destruction of the motor. Outgassing further results in vaporized materials condensing on any precision optical components or delicate mechanical devices that are present. Among other materials commonly found in a standard motor, bearing grease, paper slot liners, conformal coatings, winding insulation, and many kinds of adhesives vaporize in a vacuum.
Some materials are particularly inappropriate for a vacuum chamber, since they outgas so quickly that clouds of vapor are created. Other materials, such as silicone, are an even bigger problem: silicone is nearly impossible to remove by cleaning, and will continue to spread throughout the chamber, contaminating its contents.
Air leaking from the motor itself is another frequent problem with vacuum motor operation. A step motor that has not been adequately prepared will leak air molecules long after the vacuum is applied. Captured or clinging gas molecules slowly emanate from minute cracks in the motor laminations, windings, bearings and metal surfaces. If these porous materials are not treated, an unacceptably long pump-down time or inadequate vacuum level can result.
Motor cooling in vacuums is another problem. Standard motors are generally cooled by convection into the surrounding air, and less significantly, by heat conduction through the motor mounting surface. Convection cooling is not available in a vacuum. Heat is dissipated mainly by the least-efficient method: conduction through the mounting structure. As a result, vacuum operation can require special cooling devices and higher temperature operation.
Finally, corona effects can result from exposed high voltage conductors. At certain vacuum levels, the rarified air ionizes easily, and current can arc between unprotected high voltage conductors.
Solutions to all of these vacuum motor problems — outgassing, cooling, leakage, and corona effects — have been answered by new designs in vacuum motors. Outgassing, for example, can be prevented by the selection of materials with low vapor pressure. Teflon is one such stable material. Most metals are also suited to use in a vacuum (exceptions include cadmium and zinc). Stainless steel is a particularly good material for vacuum applications.
Material outgassing rates are generally not significant between atmospheric pressure and 10-4 Torr. In this range, many commercial plastics are usable, although lubricants usually need to be selected carefully. In vacuums approaching 10-7 Torr, most natural materials must be eliminated in a motor, few plastics are viable, and vacuum lubricants are mandatory. At 10-9 Torr, almost no plastics can be used, and dry lubricants are required.
Scrupulous motor cleaning limits outgassing problems. Trace materials from the motor manufacturing process are always present from a variety of sources: cutting oils are used on steel, lubrication is left on plastics when extruded from dies, and solvents are mixed with epoxies. Human hands leave an oil residue behind. Outgassing contamination is not critical in all applications, but if system components are not properly cleaned, outgassing of various contaminants will definitely result.
Manufacturers that specialize in motors for vacuum environments, such as Empire Magnetics, subject motors to a cleaning process that gets into crevices that vapor degreasing cannot access, accelerating molecular changes in materials that would otherwise outgas, making them inert. The sensitivity of vacuum applications usually requires the use of motors that have been constructed of non-volatile materials, vacuum baked, processed to extract contaminants, and then sealed.
Remedies for high motor temperatures include limiting power to the motor, and fabricating the motor with high temperature materials. A thermocouple or RTD (resistance thermometer device) can be installed in the motor to monitor temperature. The temperature information is used to modulate motor power, in order to keep the temperature within a safe operating range. When substantial amounts of power must be applied, cold plates or cooling jackets can be considered. For one example, Hughes Aircraft designed and installed a system that used the vapour from liquid nitrogen and which was controlled by a feedback system that monitored motor temperature. Even though the motor was heavily loaded for three months of continuous satellite testing, this system reliably held the temperature to a safe level.
Finally, to prevent the corona effect that can be generated by high voltage, exposed conductors must be insulated with appropriate coatings to prevent arcs.
Recent Step Motor Vacuum Applications:
- Satellite testing/positioning of satellite in vacuum chamber during assembly
- Observatory instrumentation/telescopes
- Alignment of laser beams for research
- Beam alignment during fusion research
- Semiconductor manufacturing
- Wide range of clean room applications for research and testing
- Aircraft component testing
- Superconductor research
- Ion deposition equipment
- Electron beam welders
- Electron microscopes
- Material testing facilities