Waterproof Step Motors Make Waves in Industry
Lap Notes Waterproof Step Motors Make Waves in Industry LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. Richard HalsteadPresidentEmpire Magnetics, Inc. WATERPROOF STEP MOTORS Because of the preponderance of water and solvents, motion control components are often exposed to the risk of corrosion and failure. Waterproof motors are a key part of the solution to water-induced motor failure problems. Today’s waterproof motors are specifically designed to survive a minimum of 30 feet under the sea, a general industrial standard for the “waterproof” designation. These are not retrofitted standard step motors, but are truly waterproof motors, which operate very well above the waterline, too — in a wide variety of industrial and marine settings. The motors have been fitted with redundant shaft seals, O rings, hermetic cable feedthrough, pressure equalization and other waterproofing features. Together, these design features allow a step motor to answer a design engineer’s need for a motor that will not fail after even prolonged exposure to, or submersion in, most liquids. Why don’t ordinary step motors meet this need? Although these motors are the mainstays of industry and the laboratory, and provide reliable and accurate positioning for minimal cost, their design includes several weaknesses with respect to moisture. First, the coils of the standard stepper are wound with magnet wire, a solid copper wire that is coated with an enamel or varnish. While the coating process is good, it is not perfect, and the varnish always has some small pin holes in it. When this magnet wire is wound into a coil, it is unlikely that any two pin holes will line up and “short out.” Ordinarily, the spacing of the wires provided by the layers of exterior enamel prevent conduction from one pin hole to the next. However, if even a little water gets between the wires, it will promote conduction from pin hole to pin hole. In this situation, the current from the power supply will quickly destroy the motor windings. Although quality insulation and impregnation can prevent this problem, they add greatly to product cost. Step motors are constructed of magnetic iron, and corrode very easily when subjected to moisture. To optimize torque, the gap between the rotating and non-rotating teeth of the motor is held to about .002 in. When the motor is operating, these teeth get hot, and the temperature is typically high enough to turn any water in the motor into steam. Steam is corrosive, such that the magnetic iron in the teeth rusts faster than usual. Once the rust fills the .002 in. air gap, rotor and stator teeth begin to rub. This robs useful torque from the motor and breaks the iron oxide loose. If the rust is dry, it forms an abrasive powder, if it is wet, it forms an abrasive slurry. This oxide slurry gets into the motor bearings, and being abrasive, the bearing wear accelerates. As the bearings wear, the centering of the rotor becomes less accurate, causing the rotor to wobble as it turns and eventually hit the stator. The combination of oxide build-up on the magnetic teeth and the loss of bearing accuracy can cause a standard step motor to fail in a period as short as two weeks. While it is possible to substitute non-corrosive metals for the magnetic iron, the metal will be more expensive, it will cost more to machine, and motor performance will be significantly degraded. Attempts could be made to prevent moisture from entering a motor. However, standard step motors are not designed to facilitate seals. They are designed to be low in cost. As a result, proper sealing of the motor would require a complete re-design of motor parts, with an increase in manufacturing cost. A proper design approach for motors exposed to moisture incorporates shaft seals, O rings, cable feedthrough, and pressure equalization mentioned earlier with other features that combine to create a “real” waterproof motor. For instance, while standard motor housings are made of painted cold rolled steel and/or aluminum, a waterproof motor’s exterior is commonly made of stainless steel to resist corrosion. A waterproof motor is designed to accommodate O-ring seals, allowing the watertight sealing that is not practical in a standard motor. Further, the wiring that leads to the motor coils in a waterproof motor requires a hermetically-sealed feed-through device to prevent water from wicking into the motor via the cable conductors. Other seals are required. For instance, a threaded pipe plug at the rear of the motor is designed to allow the housing to be sealed after the motor connections have been made; this plug is also fitted with an O-ring seal. The motor shaft itself is provided with redundant shaft seals. Additional life-extending features include double insulation, coated laminations, and fittings for pressurization. Some of the above features may be found on a standard motor, but it is the combination of all of these features together that produces a waterproof motor with a superior service life. The existence of the new waterproof motors, and the design innovations they represent, do not simply extend motor life in moisture-filled applications — in many cases, the applications would not be possible without this type of motor. Recent Waterproof Step Motor Applications: Naval/military Aquaculture Undersea exploration Robotic submarines Salvage robots Undersea mining Oil exploration Sonar arrays Communications Automated undersea lights and cameras Automated hydrophone arrays Wave power generation Flood control systems Floating seafood factories Hydrofoil air cushion controls Antenna and gun pointing systems Food processing Cleaning, chopping, dispensing, packaging, vacuum sealing, inspecting and labeling Corn dog machines Automated ice cream cup fillers Meat slicers Pineapple coring machines Juice box filling and sealing machines Drug manufacturing: Pill pack sealing, packing, labeling, gene splicing microscope stages, media plate filling Medical supply manufacturing: Artificial blood vessels, rubber gloves, sterile bandages Automated machine tools: Mills, lathes, screw machines, grinding applications, cut-off tools Automated inspection: Water jets and ultrasound in water
Considerations for Step Motors in Space Applications
Lap Notes Considerations for Step Motors in Space Applications LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. Richard HalsteadPresidentEmpire Magnetics, Inc. The application of stepper motors in space is a design problem whose complexity is frequently underestimated. While most of the research in this field has focused on bearings and lubrication, there are a number of other issues to be considered. The environmental challenges of space-related applications are immense, and include vacuum (10-8 to 10-12 torr), deep space cryogenic (20 Kelvin), thermal shock (sun at +200°C to shade at -200°C in one to two seconds), launch vibration, ionized particles and radiation. In addition, there is a limited power budget, severe weight and size restrictions, and reliability and life requirements. Many engineers approach these problems by assuming that standard commercially made motors are the correct basis from which to construct a motor for space applications. Unfortunately this assumption is incorrect. Designers of commercial motors focus on reducing the cost of high volume production of standardized products. While there is certainly a drive towards 100% quality, the cost penalty for motor failure in the industrial/commercial world is relatively low and the reward for cutting costs is high. Therefore the commercial design goal is a prescription for disaster in space flight applications. Commercial Motors in Space: A Recipe for Failure Electrical Electrical failure in a motor is simple. Either the wires short out, or they break and the circuits open. To improve the reliability of the motor, insulation materials can be upgraded, and special attention must be paid to the lead wire connections between the motor and the electronics. A careful thermal analysis to assure sufficient cooling capacity of the system is also important for electrical reliability. Empire has fully reviewed redundant motors and redundant windings on each space program we have consulted on, and each time the user has ruled this approach impotent due to the weight penalty inherent in mechanically redundant motors. We have concluded the most pragmatic solution is having redundant electrical circuits in the same mechanical motor housing. Redundant windings are required to support redundant electrical circuits. Simply winding the coils by taking two pieces of wire in hand (bifilar) offers redundancy against bad solder joints, broken lead wires and failed electronics. This does little to protect against overheating and insulation failures, and the probability of both wires being damaged in the same thermal event is great. Two separately wound coils inserted with a phase separator are less prone to fail, but a thermal meltdown is still likely to destroy both windings. To compound matters, the slot areas of the motor were designed to have a single coil filling the entire slot. If one tries to install two coils into the slot, each coil can only contain half the normal wire amount. Since motor torque is a function of amps times the number of coil turns squared, it will take about four times as much current to achieve full torque in the motor when using a half coil. Since coil heating is a direct result of amps squared times the resistance of the coil, coil heating in a redundant winding is likely to be 4-16 times greater than a normal coil. Oversizing the motor to make up for torque loss increases the weight nearly as much as making the motor fully redundant. To date, all space flight designs have compromised by using bifilar windings, redundant electronics, redundant lead wires and slightly oversized motors, backed up by mechanical and thermal modeling and extensive testing. In addition to electrical shortcomings for space applications, the typical standard, commercial motor is manufactured from magnetic iron, which is stamped, coated, glued, stacked and assembled into the basic motor structure. These first few construction steps lead to an entire series of problems for motors in space applications, namely outgassing and mechanical. Outgassing Outgassing is a major reason why commercial motors are an inappropriate choice for a space application. In a vacuum environment, the lubricants typically found in the bearings of standard motors will vaporize and the organic materials will evaporate – a phenomenon known as “outgassing.” Some materials like petroleum based grease vaporize so quickly that they literally create clouds of vapor in a vacuum chamber. Other materials such as silicone lubricants vaporize more slowly, but result in serious application problems of their own. NASA has funded numerous studies on lubricants and much of the data can be found in NASA publication 1124 (This data is too extensive to be covered here). The insulation material and lamination cement or glue used in the stator of a commercial grade motor are selected for rapid application and low cost, with no concern for vacuum outgassing. During the stamping operation, lubricants are added to reduce tool wear. If not carefully removed, these trace materials will outgass into the vacuum causing a variety of problems. The metal coating process traps these same die lubricants, making it more difficult to remove them, and the coating itself may or may not be vacuum-compatible. The motor stator is manufactured by adding insulation materials around the laminated core, winding wire coils, adding the lead wires and impregnating the windings with varnish – a metal housing may or may not be used to enclose the stator core assembly. Adding to the range of problems, the varnish used to impregnate the windings is a hard material intended to maintain coil positions. This varnish is susceptible to outgassing and hides the electrical connections from inspection. Mechanical In addition to material outgassing, there are a number of mechanical issues related to the design and manufacture of standard commercial motors which are important to the space craft designer. The stamping operations used to provide low cost motor parts also introduce stress into the metals. The stresses will be relieved by the launch vibration and thermal cycling experienced in space, which cause the parts to change their mechanical shape. Motor failure
Using Purge and Pressurization to Prevent Explosions
Lap Notes Using Purge and Pressurization to Prevent Explosions LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. By Sean Clarke, Epsilon Technical Services Limited Introduction We have mentioned in previous articles that there are many types of protection concept. One of the simplest concepts to understand and apply to almost any type of apparatus is purging. Purge, pressurization and Ex p or EEx p are all terms used to refer to this concept. The concept works on the principle of keeping the flammable substance away from the source of ignition and ensuring the surface temperature of the purged enclosure is non-incendive. OVERVIEW The technique of pressurizing and purging enclosures of electrical apparatus is to prevent the ingress of a flammable atmosphere. Purging is a widely accepted protection concept for explosion protection. It is accepted world-wide (using European Standards, NFPA or IEC Standards) and is relatively straightforward to comprehend. Explosion protection is achieved by keeping the potentially explosive atmosphere away from any source of ignition (thermal or electrical). The potentially ignition capable apparatus is mounted inside an enclosure, the enclosure is then pressurized to a positive pressure relative to the atmospheric pressure (a positive pressure of 0.5mbar is all that is required). As long as this positive pressure is maintained, no gas (or even dust) will be able to enter the enclosure, hence the internal equipment can not be exposed to a potentially explosive gas. There is however a chance that an explosive gas mixture may have entered the enclosure prior to the positive pressure being achieved. To ensure that the enclosure is pressurized with a non-explosive gas (i.e. Air or Nitrogen) the enclosure is ‘purged’ to flush out the existing contents and ensure that all areas of the enclosure contain only the purging gas (purging of internal dusts have not yet been considered). It normally takes between 5 and 10 volume changes to ensure that the enclosure is ‘purged’. (In Europe the first edition purge standard defined five air changes as a minimum, in North America the minimum is defined as 10 air changes) It is a condition of certification for Zone 1 equipment that power can not be applied to the equipment until the ‘purging’ (a specified flow of purging gas for a specified time) has been completed. To ensure continuity in the effect of the purging, the maximum leakage rate for the enclosure is also specified. When the purging has been completed, power can still not be applied until the specified positive pressure (at least 0.5 mbar) has been achieved. In the event of a failure to complete the purging cycle (drop in flow or incomplete duration) or if the enclosure pressure drops below the specified positive pressure, power to the equipment shall be removed (for Zone 1) or an alarm indication shall be given (Zone 2). In the event either of these conditions, the entire purging cycle starts again with the full purge time duration. The control of the automatic purging and pressurization is normally by a ‘Purge Control Unit’ (PCU). The PCU is required to measure flow and pressure, and must fail-safe in all conditions. The enclosure that houses the equipment to be purged must have sufficient physical integrity to withstand impacts and overpressures. The enclosure should also be designed to facilitate the free flow of air. As enclosure integrity is required to a level of IP40 (no holes greater than 1mm), any non-metallic material must be tested for durability and longevity (against effects of heat and light etc.). External considerations, such as the surface temperature of the equipment or static from plastic parts, must be considered. To ensure incandescent particles can not be vented from the equipment, a spark arrestor must be fitted (or the vented gas must be ducted to a safe area). This technique is virtually unlimited particularly in physical size or power rating of the apparatus being protected. Purge control Units Purge control units must be able to measure and act on the following information: Pressure (pressurization) Flow (purging) Time (Purging) The operations performed by the PCU must be ‘failsafe’ by virtue of test and assessment with one fault (i.e. a valve failing), this is even more prevalent with the advent of the ATEX Directive. Failure modes of components must be considered; even relays can fail open or short-circuit (normally open relays can ‘arc-weld’ in to a closed position). Purging timers must always re-set to zero if they are interrupted during the purging cycle or after a purge failure. There are two basic types of control: Constant Flow (CF)- The air flow for the purging and pressurization stages are the same. The flow is left as a constant after it is set, and power is applied after a set period of time. Leakage Compensation (LC) After purging, the air flow is reduced to a figure just above the leakage level to maintain the pressurization. The PCU is required to switch from an initial high flow rate (often referred to as fast purge) to a much lower flow rate on completion of the purge time. CF systems are simpler to design, but are more expensive (in air or nitrogen) to run. There are other examples of hybrid systems (CF/LC) but in reality they are just variations on the two basic types. PCU’s are normally either pneumatic or electrical. If the PCU is mounted in the safe area only the operation will require verifying (unless it contains intrinsically safe outputs). PCU’s mounted in the potentially explosive atmosphere will require certifying both as safety systems and as potential ignition sources (although sources of ignition from pneumatic systems were not considered until the ATEX Directive). Each STATE of the system is defined in response to the inputs of the monitoring devices. The states are required to be unique. The logical conditions for the occupation of each state are required to be uniquely defined by BOOLEAN logical
Motor Designs To Survive Hostile Environments
Lab Notes Motor Designs To Survive Hostile Environments LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Richard HalsteadPresidentEmpire Magnetics, Inc. A new generation of step motors is allowing motion control applications to prosper where they were previously impossible or, at best, very difficult. The reason? Previous step motor designs were developed for the relatively “benign” conditions in a factory. New step motor designs — specially developed for motor-hostile environments — can reliably withstand the effects of radiation, underwater operation, vacuum operation, and extremely high and low temperatures, among other conditions. Accordingly, these motors are the key to a wide range of new motion control opportunities. VACUUM MOTORS 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. ne 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
Semiconductor Manufacturing Requirements Drive New Automation Technologies
Lab Notes Semiconductor Manufacturing Requirements Drive New Automation Technologies LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Richard HalsteadPresidentEmpire Magnetics, Inc. In addition to highly repetitive tasks, positioning equipment has long been used to provide very specialized functions and processes. A good example is the semiconductor manufacturing industry. According to a recent press report, at least forty new wafer fabrication plants are presently in the planning stages or actually being built. Since each plant represents a $2 billion to $6 billion investment, it is easy to estimate that there is some $200 billion going into this market in the next 3-5 years. And, since the process equipment represents the bulk of the investment, this is an important market for manufacturers of positioning and automation equipment. In semiconductor manufacturing, the immediate presence of operators has always been an unacceptable source of contamination. The newer facilities are getting cleaner by completely removing people from the process; engineers are also changing the equipment designs to eliminate that source of contamination. Initially, attempts by semiconductor manufacturing companies to get wafer fabrication processes under control were aimed at moving the process into an environmentally-controlled “clean room.” By filtering the air, and carefully controlling the materials allowed into the clean room, the amount of particulation was reduced. By 1970, a Class 1000 clean room was considered quite good. However, some of the equipment — and operators moving around in “bunny suits” –still created a large number of particles. To avoid this problem, it was necessary to build even cleaner areas inside the clean room. This was done with laminar flow hoods and other devices, in an attempt to reduce the number of particles that can come into contact with the silicon wafers. In 1973-1975, Class 100 areas within the clean room were considered good. About this time, the equipment in the area was carefully redesigned, removed and/or replaced, again minimizing the number of particles generated by the equipment. Equipment itself also began to be approved for clean room uses. The overall construction of a clean room, and all of its contents, became the focal point. By 1979, clean rooms rated at class 100, and which contained hoods with class 10 ratings, were becoming the standards of excellence. Since the classifications are based on the number of particles floating around in a cubic volume of air, it became more and more difficult to remove the last few particles contained in the air. The obvious answer was to remove the air from the system. Therefore, in the 1980s, equipment manufacturers began moving their semiconductor manufacturing processes into vacuum chambers. While the vacuum environment may be a part of specific processes, it also has the advantage of not supporting particle motion. Newer facilities move the wafers in and out of super-clean containment devices, as they are transported from one process to the next. Often this is a series of vacuum chambers, with transport systems in between. However, the chemical processes are still controlled by humans working in the process areas. It is likely that the future trend will be to make large sections of the process line, or even the entire process line, operate in a vacuum. The system will essentially be a long pipeline with many segments that can be sealed off from one another. Once the wafers have moved into a segment or section, that section is sealed, the appropriate process is accomplished and the wafers are then transported to the next section. By having large numbers of processes linked into a series of vacuum chambers, several advantages are gained. The lack of air in the system prevents transport of contaminating particles, and the time spent pumping down the process chambers to the required levels of vacuum can be eliminated. The process becomes more of a continuous flow, as opposed to a batch mode. The leading-edge technologies in semiconductor manufacturing are spin-offs of development work that was originally done to create the “beam lines” found at National Laboratories. The long vacuum chambers and equipment used to make these beam lines are patterns that will lead system designers to the long vacuum systems that will probably be the wafer fabrication lines of the future. In these beam lines and in the wafer fabrication process, it is necessary to have equipment that is fully automated and very precise. It must also operate in a vacuum and not generate hydrocarbons or make particles. There are few suppliers of such equipment, and few pieces of general use equipment fill all of these requirements. Our firm, Empire Magnetics, Inc. is a supplier of high precision vacuum-rated motors and Class 10 motors rated for clean room use. In a joint effort with Lawrence Berkeley National Laboratories, Empire Magnetics is developing new motors that are suitable for use in vacuum and space applications. Through these and other efforts, the National Labs are assisting U.S. manufacturers in applying new technologies, with the goal of maintaining a competitive edge in the worldwide market. This article first appeared in MOTION, published by Motion Corporation, P.O. Box 21730, Carson City, NV 89721-1730. Top Motion Control Solution Provider in 2023 Also named Top Cryogenic Company of 2022 “The specific construction of the magnetic circuit is one of the key design features of a successful cryogenic motor – at which Empire Magnetics takes a lead” Read more.. Empire Magnetics offers: Stepper Motors Servo Motors Gearboxes Resolvers Stand Alone Resolvers Brakes Rotor Nuts Turnkey Projects Repair Services
Radiation-Hardened Motors
Lap Notes Radiation-Hardened Motors LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. ORNL Develops Fuel Pin Dissolver with Empire’s Radiation-Resistant Motors Motion control in radiation-intensive environments poses a serious challenge to the design engineer. Conventional step and microstepping motors are susceptible to high-energy gamma radiation particles that will attack the motor materials. Usually, the organic compounds are most susceptible to breakdown by radiation, and as a result the lubricants, varnish, lamination bonding, and cable insulation in standard, commercially-available motors will all deteriorate over relatively short periods of time. A new generation of radiation-resistant motors has been developed to greatly expand the design possibilities in highly radioactive environments. One new motor design was recently a part of a dissolver system designed by Oak Ridge National Laboratory (ORNL). The dissolver system is used to reprocess spent nuclear fuel. A key step in the fuel recycling process is the separation of the spent fuel oxides from the stainless steel cladding by dissolving pieces or pellets in hot nitric acid. In this application, the motor provides the agitation necessary to speed the dissolving process. The environmental factors, in addition to the functional requirements of this application, presented a significant challenge to the motion control engineers. The motor had to provide sufficient power and torque to move the load. The system also needed to have positional as well as velocity control, and had to be totally reliable, even when the assembly would be subjected to massive doses of radiation, heat, and nitric acid fumes. Replacement of components is expensive, but certain failure modes would be environmental disasters. As such, anything that could be done to lengthen useful life was considered a good investment, while avoiding catastrophic failure was critical. After a considerable search, Oak Ridge was able to identify Empire Magnetics as the company that could provide radiation-resistant components closest to its requirements. Motors with 2 x 108 Rads TAD were already a part of Empire’s product offering, but ORNL wanted to know if the company would be willing to push the technology to achieve 1×109. An agreement was made to have Empire Magnetics provide motors on a “best effort” basis while ORNL would test them through the REACH program. Dr. Brad Weil would eventually oversee the testing program, and Dr. John Draper would cross-check the results. Extracts of their reports are shown as page inserts. Empire provided stepper motors and brushless servo motors for testing purposes. In the final analysis, all of the Empire-supplied equipment was still functional when the testing was concluded due to budgetary constraints. By the end of the testing, some of the motors had been subjected to total accumulated doses in excess of 1×109 rads, others had reached 5×108 rads, depending on their location in the hot cell. All of the Empire supplied motors were tested for a long enough period to indicate that they would have lasted much longer had the test been continued. Based on the results of the testing, Empire contracted to provide an assembly to drive the dissolver system. For this purpose, a stepper motor, cycloidal gearbox and resolver assembly were selected. The assembly included an 87:1 gear ratio in an in-line arrangement. This mechanical arrangement allowed the use of a brushless resolver driven directly by the output shaft of the gear assembly. In this way, the absolute position of the output shaft could be tracked, and it would allow the operator to know the position of the shaft immediately upon power-up … a very important feature to avoid accidentally dumping radioactive nitric acid solution. This same feature eliminated the mechanical errors of the motor and gearbox from the feedback loop. The radiation-resistant stepper motor was powered by a bipolar chopper drive and controlled by a PC-based local controller. The resolver feedback was supported by an electronics card built by Empire, and Empire provided the complete subsystem consisting of the radiation-resistant motor, gearbox and resolver in an integrated mechanical assembly. This assembly was sealed and protected against the nitric acid fumes. The electronics package, along with the software, were a part of the subsystem, allowing Empire to provide a completely functional, tested subsystem that met all of the specifications. The in-cell motor is controlled by a remote motor control station out-of-cell. The motor is driven by a bipolar chopping drive that is controlled by the motor control computer, which functions as a slave device and is intended to receive instructions from a host computer via a serial RS-232 communications link. The heart of the motor control computer is the indexer, which manages the basic motor motion profiles. Parameters such as distance, direction, speed and acceleration are set by forming command sequences of ASCII mnemonics. The command sequences define a motion profile and are programmed into the indexer via the serial communications link. The subassembly had been integrated into the final machine, and has been in operation since April 1991. Empire has not been asked to make any service calls in all that time. Here are excerpts from the original motor testing that was sponsored by the American Nuclear Society, from a paper by J.V. Draper, B.S. Weil and J.B. Chesser of the Robotics and Process Systems Division, ORNL: “Four AC servo motors were tested on the hypothesis that motor performance would decline with the dose received because of degradation of stator coil insulation or motor bearings. The motor assemblies for each unit included (1) the motor, (2) a resolver, (3) a roller chain sprocket to provide an inertial load on the motors, (4) a motor support stand, (5) a thermocouple, (6) a servo motor amplifier, and (7) a resolver interface. Three motors were placed in the irradiation cell and one motor was placed outside the cell, along with all of the amplifiers and resolver interfaces. The AC servo motors experienced 1.03 x 105 R/h and received a total accumulated dose of 1.03 x 109 R. During each operating cycle,
Motors in Radiation-Intensive Environments
Lap Notes Motors in Radiation-Intensive Environments LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Ruth Anne AbstonInstrumentation & Controls DivisionMeasurement & Controls Engineering SectionOak Ridge National Laboratory Motion control in radiation-intensive environments poses a serious challenge to the design engineer. Conventional step and microstepping motors are susceptible to high-energy gamma radiation particles that will attack non-metallic materials. As a result, lubricants, varnish, lamination bonding, and cable insulation will all deteriorate over time and finally crumble. A new generation of radiation-hardened step motors, however, has greatly expanded the design opportunities in highly radioactive environments. One new motor design recently assisted Oak Ridge National Laboratory (ORNL) in developing a significant new methodology for reprocessing spent nuclear fuel. An important step in the fuel recycling process is the separation of spent fuel oxides from their cladding by dissolving the oxides in nitric acid. Responsible for the development of nuclear fuel reprocessing technologies, ORNL — in collaboration with the Power Reactor and Nuclear Fuel Development Corporation of Japan (PRN) — recently developed a new concept for facilitating this separation process. Previous fuel pin dissolvers have been “batch” systems, which used metal baskets to drop the fuel pins into a series of acid solutions. The batch systems were relatively inefficient, since directing concentrated nitric acid at the hardest-to-dissolve fuel segments could not be effectively accomplished. The new concept devised by ORNL is a “multistage continuous rotary dissolver” that operates as a continuous, rather than batch-type, process. The new design provides a counter-current of nitric acid that is constantly forcing itself against the hardest-to-dissolve areas of the fuel pins, and with the highest acid concentrations. The drum in the new multistage dissolver resembles an Archimedes screw, consisting primarily of a horizontal auger enclosed by a cylindrical shell — eight stages are formed by the turns of the helical walls of the auger. Sheared nuclear fuel pins segments are fed into the first stage of the dissolver. After a period of agitation to assist the dissolution process, the drum is rotated 360° to transfer the undissolved fuel and cladding to the next stage. Another charge of fuel pin segments is added to the first stage, and the process continues. As the fuel pins are being processed from stage #1 to stage #8, the product stream, consisting of nitric acid and dissolved fuel oxides, flows counter-currently to the solids through the holes in the auger, which are drilled concentrically about the shaft of the drum. The prototype’s components were required to be able to withstand the extremely hostile environment in which the dissolver would have to operate: massive radiation and the continuous presence of nitric acid. The key component to the prototype’s success was the drive unit, which serves three primary functions: 1) provides controlled forward and reverse motion for the stage-wise transfer of fuel pin segments, 2) allows precise positioning of the drum for discharging and loading of pin segments, and 3) agitates the drum, using a rocking motion profile, to aid the dissolution process. The design requirements for the prototype dissolver motor specified that a torque had to be produced that was sufficient to accelerate the dissolver drum at 40 rpm/s to a speed of 10 rpm. Positioning accuracy within 1/3 of a degree was also required for loading pin segments. The presence of nitric acid required that a sealed motor be specified, in order to prevent damage from corrosive acid vapors. Finally, the motor and the associated in-cell components had to be radiation-hardened to withstand a total accumulated dosage of 108 rads gamma radiation. ORNL was able to find only one motor design that could withstand accumulated radiation dosages of 108 and which had all of the required performance specifications, a radiation-hardened hybrid permanent magnet step motor from Empire Magnetics, in Rohnert Park, Calif. Empire’s step motor was submitted to prolonged testing in a radiation environment, and was ultimately deemed suitable for use in the prototype dissolver when it successfully survived 109 rads gamma radiation for a prolonged period. The Empire motor was a zero backlash model that features an in-line 87 to 1 cycloidal gear reducer, a design feature that increases motor output torque without lessening motor life. A radiation-hardened brushless resolver is connected to the gearbox output shaft, the resolver providing angular positioning capability with a resolution of 4,096 steps per revolution of the dissolver drum. The motor, gearbox and resolver are mounted inside a lead box to enhance the existing radiation resistance. The in-cell motor is controlled by a remote motor control station out-of-cell. The motor is driven by a bipolar chopping drive that is controlled by the motor control computer, which functions as a slave device and is intended to receive instructions from a host computer via a serial RS-232 communications link. The heart of the motor control computer is the indexer, which manages the basic motor motion profiles. Parameters such as as distance, direction, speed, and acceleration are set by forming command sequences of ASCII mnemonics. These command sequences define a motion profile and are programmed into the indexer via the serial communications link. The motor control computer also performs certain I/O functions. A thermistor inside the motor housing provides an indication of motor temperature as well as over-temperature protection. The motor control computer monitors the motor temperature and initiates motor power cutoff if the motor temperature exceeds the high temperature setpoint. Feedback from the resolver is monitored and provides position information for use by the motor control system. This position information is also available to the operator. Numerous other system status reports are available to the operator, including on/off status and a signal to indicate that a move was successfully completed. These features enable the integration of the motor control functions with dissolution process system operation. The prototype multistage dissolver has been in uranium testing at ORNL since April 1991, and the testing will continue until the fall
Motors for Positioning in Vacuums
Lap Notes Motors for Positioning in Vacuums LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Rick HalsteadPresidentEmpire Magnetics, Inc. Operating a motor in a vacuum is often perceived as a design challenge. Among engineers, the problems — outgassing, contamination and temperature — are well known. What is less well known is that there are now well-developed solutions to each of these problems. Historically, the ability to move or position samples, products, mirrors and sensors in a vacuum area (some of the most common requirements) has been accomplished with drive mechanisms and motors located outside the vacuum chamber. In this type of control solution, the drive mechanism transmits its motion through the vacuum chamber wall using sealed couplings. There are a number of disadvantages to this traditional vacuum-motion control approach, however. One major problem is that putting the motors external to the vacuum chamber and using magnetic or mechanical feed-through mechanisms forces the engineer into the use of a limited number of design configurations. For instance, it is very difficult to implement an X-Y stage (in which one stage moves on top of the other) inside a vacuum chamber when external motors are being used, since the mechanical components used to transfer motor power greatly restrict the design possibilities. Further, the accuracy, repeatability, and resolution of the positioning system inside the vacuum chamber are compromised. On the other hand, placing the motor directly into the vacuum frees the engineer to use a larger number of physical arrangements, since the motor cables can be routed in any number of ways without inhibiting the activity in the chamber. By directly coupling the motor to the load, the accuracy and other system specifications can also be vastly improved. Finally, because mechanical feed-throughs for the vacuum system are often as expensive as a vacuum motor in its entirety, a system that features an internally-placed motor often costs less than the externally-mounted alternative. For all these reasons, more and more vacuum applications are utilizing specialized motors that have been designed for use in a vacuum, and which take into account the unique environmental conditions that are present. Standard motors are not an appropriate choice for a vacuum application. In general, standard motors will not survive in a vacuum of 10-4 Torr or lower. The primary reason is that the lubricants in the bearings will vaporize and the insulation materials of the motor and cable will evaporate, a phenomenon known as “outgassing.” Outgassing within a vacuum chamber is obviously very negative: in addition to destroying the motor, the vaporized materials condense on precision optical components and delicate mechanical devices, thus fouling the application. Some materials like petroleum-based grease vaporize so quickly that they literally create clouds of vapor in the vacuum chamber. Other materials vaporize more slowly, but can become an application nightmare. Silicone is one of these nightmares, since once a vacuum chamber has been contaminated with silicone, it is nearly impossible to clean it all out, and it will continue to spread to anything and everything that is placed into the chamber. Motor cooling in a vacuum is also a problem. Conventional motors are cooled primarily by convection into the air that normally surrounds them, and to a lesser degree by conduction through the mounting surface. When a motor is energized in a vacuum, convection cooling is not available, and heat is dissipated primarily by conduction from the motor to the mounting structure. As a result, when operating a motor in a vacuum, provisions for heat exchange and higher temperature operation must be made. It’s one thing for a motor to operate successfully in a vacuum, and another for the process itself to be successful with the motor present: consequently, leakage is another problem with motor operation in a vacuum. Even if a standard motor does not outgas contaminants in a vacuum, it is likely that the vacuum process will be hampered with the motor present. For example, a step motor that is not properly treated will “leak” air molecules long after the vacuum is applied. Leakage is the slow release of captured or clinging gas molecules from minute cracks in the motor laminations, windings, bearings and metal surfaces. The leakage occurs when the surfaces of these porous materials are not treated, resulting in unacceptably long pump-down time or an inadequate vacuum level. This leakage is a mechanical problem as opposed to the materials problem of outgassing. Finally, high voltage exposed conductors on motors in a vacuum can create corona effects. At certain vacuum levels, the rarified air will easily ionize, and current will flow between unprotected high voltage conductors (this principle was the basis of the well-known “electron tube”). The solutions to these vacuum chamber/motor design problems — outgassing, cooling, leakage, and corona effects — are all to be found in a properly-designed vacuum motor. Outgassing, for instance, in which motor materials such as bearing grease, paper slot liners, conformal coatings, winding insulation, and many kinds of cements or glues vaporize entirely or partially, can be prevented by the careful selection of appropriate materials. Teflon is a commonly-used material in vacuum applications, as it is quite stable, has good temperature characteristics, and is readily available at a reasonable price. Most metals are acceptable for use in a vacuum (exceptions include cadmium and zinc), but stainless steel is a particularly good material for vacuum/motor applications. In general, material outgassing rates are not significant between atmospheric pressure and 10-4 Torr. In this range, many commercial plastics are usable, but lubricants usually need to be selected carefully. If the vacuum is in the 10-7 Torr range, most natural materials must be eliminated, and only a limited number of plastics are usable. At this range, vacuum lubricants are essential. At 10-9 Torr, most plastics are excluded and dry lubricants must be used. Outgassing can also result from a lack of motor cleanness. Motor materials are subjected to a variety of contaminants
Motor Selection for Deep Sea Applications
Lap Notes Motor Selection for Deep Sea Applications LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Rick HalsteadPresidentEmpire Magnetics, Inc. Like few other hostile environments for industrial components, the deep sea is a forbidding place in which to launch a motion control application. Very cold temperatures, the corrosive effects of sea water and extremely high pressures (as much as 5,000 p.s.i. at 11,600 feet below the surface) combine to create an environment in which off-the-shelf components will quickly fail. Consider: in order to select a motor for deep sea operation, you must prevent water entry to the motor, assure that motor materials are resistant to corrosion, make allowances for material shrinkage (O-rings and other elastomeric materials compress in the depths), prevent water entry into electrical cables, account for power losses that occur over the 1,000+ ft. umbilical cable from the ship, and choose a filling oil whose thermal properties assure it is still a liquid at high pressures and low temperatures. Three current deep sea applications by Empire Magnetics make clear some of the problems and solutions in using motors undersea: * The Bedford Institute in Newfoundland, Canada is using waterproof stepper motors for research it is conducting in the Hudson River. The motors are required to operate at depths of 450 feet in very cold water, and are powered from aboard ship by means of an umbilical cable. Empire supplied a size 42 frame stepper motor with a stainless steel exterior, featuring an oil-filled motor and a piston pressure-compensated assembly. First specifying a depth of 200 feet, the initial motor design was used successfully at that depth. Now, the application requires operation at depths up to 450 feet, and the motor has been re-worked to meet their needs. The Bedford Institute tests its equipment in a pressure vessel that can be used to test items at up to 5,000 p.s.i. While the previous model operated up to 1,000 p.s.i., the new model’s modifications will allow operation at much greater pressures. * Another deep sea application, at Woods Hole Oceanographic Institute (WHOI), Woods Hole, MA, is even more challenging. Requiring a stepper motor and brake assembly that would be submerged to a depth of 2,000 meters, WHOI specified a motor design for a new, unmanned remote undersea vehicle. The entire vehicle is oil-filled and pressure-compensated, and since it operates by remote control, the drive electronics are also carried in the vehicle. To accomplish this goal, a stainless steel dry tank keeps the drive electronics at normal pressures, while the rest of the system is subject to the pressure generated 2,000 meters below the ocean surface. * The Scripps Institute in La Jolla, CA had a similar, but perhaps even more complicated problem. The Institute required motors to drive manned, undersea sleds, or ROVs. Among other complications, the motor assemblies had to provide a lot of power, but be very efficient, in order to maximize battery life. On the other hand, the voltage had to be low, so there was no safety hazard to the divers. The system had to be lightweight enough that the divers could carry the ROVs across the beach; be rugged enough to take the pounding of the surf when the units were being launched and retrieved; be corrosion-resistant and tolerant of sand, sea life and other foreign materials; and be cost-effective, very reliable, fault-tolerant and redundant. The Scripps project is still a work-in-progress, but their current solution has been to experiment with battery-powered brushed DC motors. This technology meets most of the above requirements except for the reliability. A little water in the brush and commutator area of these motors, and it’s up to the diver to swim home. Scripps has tried to fill the DC motors with oil, but the oil gets between the brushes and the commutator, where the insulation properties of the oil causes problems. Although it would be possible to use high voltage to break through the oil film, the high voltage is a safety hazard for the divers. Empire and Scripps are continuing to research the use of brushless motors, but the electronic control package is expensive, fragile, not waterproof and bulky. Development of custom electronics is currently out of the reach of the Scripps’ budget. As science and industry continue to expand their reach to the depths of the oceans, new and challenging requirements for remote controls and automation will continue to appear. If you would like more information about motors for deep sea applications, or information on other types of motors for hostile environments, contact Empire Magnetics, Inc., 5830 Commerce Blvd, Rohnert Park, CA 94928; (707) 584-2801. Top Motion Control Solution Provider in 2023 Also named Top Cryogenic Company of 2022 “The specific construction of the magnetic circuit is one of the key design features of a successful cryogenic motor – at which Empire Magnetics takes a lead” Read more.. Empire Magnetics offers: Stepper Motors Servo Motors Gearboxes Resolvers Stand Alone Resolvers Brakes Rotor Nuts Turnkey Projects Repair Services
Specialty Motors for Corrosive Environments
Lab Notes Specialty Motors for Corrosive Environments LAB NOTES is a newsletter we have developed to meet the needs of the research and lab-oriented design engineer who has specialized needs for motion control components. by Rick HalsteadPresidentEmpire Magnetics, Inc. In the motion control industry, hostile environments often require specially-designed motors to combat corrosion, and a small company in Rohnert Park, California has built an entire business around this industry need. Empire Magnetics, Inc., a manufacturer of stepping motors for the motion control industry, is using stainless steel alloys with varying nickel chromium content to manufacture models that will be used underwater and in other environments where ordinary motors are unsuitable. The company is using alloys ranging from 303 stainless for ordinary submerged applications, to 316 stainless for exposure to caustic or marine environments. In a recent application, engineers at Arnold Air Force Base were were assigned to identify and qualify motors suitable for use in a vacuum chamber at cryogenic temperatures. The specification called for small motors operating in a vacuum of 10-7 Torr at liquid hydrogen temperatures (24o Kelvin). Angular position feedback was a requirement to accommodate closed loop velocity and position controls. Two major design problems are inherent in this type of application. The first involves the vaporization of lubrication and insulation materials at low pressure. The second problem has to do with the behavior of materials at very low temperatures. The stress of large temperature changes, low temperature brittleness, and varying contraction rates of dissimilar materials work to degrade the structural integrity of motors manufactured from conventional materials. Empire Magnetics, among other suppliers, contracted to provide a motor for evaluation. A 57 mm diameter step motor was combined with a feedback resolver to meet the unique requirements of the application. Both devices were housed in an exotic nickel chromium steel alloy frame, selected for thermal stress resistance and dimensional stability. Resolver technology was selected for the feedback system due to the similarity of resolver components with those of the motor. When used with readily-available motion control electronics, this package would provide shaft positioning resolution of 25,000 increments per revolution of the motor shaft with a feedback resolution of up to 16,384 increments per revolution. To reduce outgassing at low temperature, insulation materials were made of selected polymers. Magnet and lead wire materials were carefully specified to avoid outgassing or fracture. Bonding agents normally used to build the motors were replaced with adhesives having a low coefficient of thermal expansion. Each of the metal components of the motor was examined in detail. AlNiCo (Aluminum Nickel Cobalt) magnets were selected in favor of rare earth combinations, since AlNiCo retains magnetic properties better at low temperatures. Stainless steel ball bearings, lubricated with dry film, were used for the same reason. All machined metal parts were stress-relieved. Sixteen different motors were submitted for testing by multiple vendors. The motor submitted by Empire Magnetics was one of only two operable devices. Finally, the successful functioning of the resolver made Empire Magnetics’ stainless steel unit the only complete assembly to operate as required by the original specification. An edited version of this article first appeared in the March 1991 edition of Nickel Magazine, 15 Toronto Street, Suite 402, Toronto, Ontario M5C 2E3, CANADA. Top Motion Control Solution Provider in 2023 Also named Top Cryogenic Company of 2022 “The specific construction of the magnetic circuit is one of the key design features of a successful cryogenic motor – at which Empire Magnetics takes a lead” Read more.. Empire Magnetics offers: Stepper Motors Servo Motors Gearboxes Resolvers Stand Alone Resolvers Brakes Rotor Nuts Turnkey Projects Repair Services