Medical Device & Diagnostic Industry
Magazine [3]
An MD&DI May 1997 Column
MOTION CONTROL
Improvements in motor technology help manufacturers meet the demands of developing state-of-the-art medical devices.
Motors have been the most popular means of turning electrical energy into mechanical work for more than 100 years. Since relevant standards were first promulgated in 1927 by the then-nascent National Electrical Manufacturers Association (NEMA), the majority of improvements in motor technology have been incremental. Beginning in the 1980s, however, product designers became increasingly dissatisfied with the status quo, and, under pressure from medical and other original equipment manufacturers (OEMs), motor suppliers have advanced the state of the art further in the last 15 years than in the previous two generations.
Improvements in motor capabilities have already helped device manufacturers bring such products as portable, disposable, and battery-operated instruments to market. Similarly, developments in manufacturing techniques and battery technology have enabled motor makers to produce actuators that are easier to integrate into a manufacturing process, more power efficient, longer-lived, less noisy, and smaller in size. While such improvements may now be taken for granted by many end-users, the technologies needed to realize them are often complex and involve far more than the substitution of one material or design feature for another. In addition, actuators for medical applications must frequently endure hostile environments, caustic fluids, radiation or steam, elevated temperatures, vacuums, and physical abuse. The expectation is that not only will they function flawlessly through all this, but they will also be cost-effective and fit into an envelope that is frequently defined even before the motor is selected. To meet these demands, motor manufacturers have drawn on numerous recent technological advances.
A working prototype of a brushless microelectromechanical systems (MEMS) gearmotor. The planetary gearhead is approximately 2 mm in diameter.
MATERIALS
In medical products where actuator reliability is critical, the motor is considered a precision component rather than a commodity device. For such applications, device manufacturers can specify motors with precious-metal brush systems, which afford smooth commutation and low electrical and audible noise; they also operate on start-up even after prolonged periods of inactivity. In less-demanding applications, motors with carbon-brush systems can be used. Usually less expensive than precious-metal systems, such motors are capable of sustaining high-current flows without damage. In addition, the recent inclusion of capacitor rings or resistance-capacitance networks in carbon-brush motors has lowered their electrical noise (creating less electromagnetic interference) through smoother commutation, as well as extended their expected lifetime by suppressing large electrical spikes.
Another recent advance has been the incorporation of advanced plastic and composite components in motor and gearhead systems to reduce costs, lower weight, and provide uniform products with short lead times. When used judiciously in gearmotor systems, plastic gearing can also help to reduce audible noise, an important consideration in the building of bedside or unobtrusive motorized devices. All plastics are not alike, however, so when specifying actuator components with plastic elements it is important to understand the environmental and load conditions under which the system will operate. Using hardened steels for shafts and cutting gears more precisely have also improved gearmotor capabilities by decreasing backlash and lengthening gear life.
Whether a manufacturer chooses a plastic- or metal-based actuator system, lubrication is an important consideration. The selection of the proper lubricating medium is essential in assembling a system that is reliable, quiet, appropriate in current consumption, and long-lived. Numerous synthetic lubricants and lubrication systems have become available in the last dozen years. Most of these systems were developed not by theoretical calculation, but through meticulous experimentation under a wide range of operating environments. Applications that involve hard vacuum, radiation, or extreme loads require particularly careful system selection. OEMs should consult with their motor supplier to ensure that the product they specify is the one that best meets their needs.
Like other motor elements, the choice of a magnet is important, and magnets with different properties cannot be interchanged arbitrarily for the sake of greater power output, increased oxidation resistance, easier availability, or lower cost. In the past, ceramic ferrite magnets held a large share of the medical motor market. Then, with increased demands for more power and a smaller package size, Alnico (an alloy based on aluminum, nickel, and cobalt) and samarium-cobalt magnets gained a larger market share, especially for high-power micromotors. Wide swings in the price of cobalt (from expensive to more expensive) together with its spotty availability, however, have led higher-end motor users to neodymium magnets. The possibility of using superconducting technology in next-generation magnet products has also been raised, but when this technology comes to fruition, it will most likely be used in windings for large integral horsepower motors, which have limited applications in medical devices. Switched reluctance motors offer another possible choice. These motors use no permanent magnets at all, but the cost of tooling for production is high and the motors usually require complex electrical drivers.
Another promising advance in motor materials is engineered ceramics, which are being used in specialized gearmotor and gearhead bearing systems. Compared with popular sintered bronze bearings and expensive stainless-steel ball bearings, sintered ceramic bearings provide up to 50% more load-bearing capability in precision gearing systems, yet add only pennies to the unit cost of a gearhead. Undoubtedly, these types of bearing systems will be used widely in the years ahead since they prevent premature gearhead failure without requiring the incorporation of steel ball bearings.
A brushless dc encoded servo motor for OEM applications.
DESIGN FEATURES
The advent of affordable computer numerical control production and assembly equipment has enabled motor manufacturers to hold tolerances in the micrometer range, maintain consistency from piece to piece, and customize products in small quantities. State-of-the-art machining, tooling, and quality systems also ensure that there will be minimal variability of a single product from lot to lot. The widespread adoption of ISO 9001 quality systems within the motor industry also provides a level of comfort about motor reliability for both the device manufacturer and the device end-user.
Many advanced motor design features are available, but perhaps the most important choice for medical device designers is between iron-core and coreless motors. In the classic iron-core motor, wire is wrapped around a piece of iron, and electricity (through the brush-and-commutator system) turns the wire and iron mass. In the case of coreless motors, a hollow armature, typically made of copper, is situated over a stationary magnet system. When energized, the coil--and only the coil--moves, thus accomplishing mechanical work through the shaft that is attached to it. The elimination of a rotating iron mass offers such advantages as fast dynamic response, high power efficiencies, and low current draw. These types of motors have become the rule rather than the exception in critical battery-operated or remotely situated devices where rapid cycling or long battery life is important. A typical coreless motor design is shown in Figure 1.
Figure 1. A typical coreless motor design.Figure 2. A typical servo-type brushless motor design.Figure 3. Brush-type and brushless motors with self-contained control modules.
Figure 2. A typical servo-type brushless motor design.
Another important motor design advance that has been gaining momentum since the 1980s is the brushless motor. As the name implies, a brushless system contains no brushes; the motor shaft is turned by electrical commutation. This effect is usually accomplished with Hall sensor elements or via sensorless methods in which the back electromagnetic field of the motor is monitored and a microprocessor is used to provide a commutation signal. Well suited for applications where very high rotational speeds (up to 100,000 rpm) are required, brushless motors also have the advantage of not being subject to brush wear--the major failure mode in traditional brush-type motor designs. Because bearing life rather than brush life is the limiting factor in the case of brushless motors, such systems can achieve lifetimes of 20,000 hours or more versus the 300 to 5000 hours that is typical of brush-type devices. The price for this longevity is the higher cost of ball bearings for brushless systems and the need for an electronic driver to accomplish commutation. Currently, brushless motors are being used in such applications as high-quality pumps, precision handpieces, centrifuges, and manufacturing equipment. An exploded view of a typical servo-type brushless motor appears in Figure 2.
When design engineers must choose between possible motor styles for a new device, motor suppliers can provide valuable assistance. Their knowledge of the available products as well as solutions in the pipeline can help OEMs cut time off the design schedule, thus speeding time to market and lowering product development costs. Such expertise can also be invaluable in avoiding the pitfalls of specifying an incorrect component and the classic and wasteful flaw of reinventing the wheel.
MANUFACTURING SYSTEM MOTORS
While the above discussion focused on motors for use in devices and instruments, medical OEMs can also benefit from advances in motors for the production systems used to manufacture medical devices. Until recently, most of the brush-type dc motors, brushless motors, and stepping motors used in manufacturing equipment were open-loop systems. That is, when power was applied to the motor it performed some turning, running, or incremental motion without reference to a fixed, or "home," position. Examples of such systems are myriad: drills, reciprocating saws, indexing devices, pumps, and so forth. Most of these are fractional-horsepower (<1 hp ~ <746 W) applications. As medical OEMs have extended their concerns about device costs to manufacturing expenses, such equipment has been identified as a source of recurring expenses (for ancillary equipment, accessories, machined parts, and the like). The impetus to decrease such costs is now the driver behind a move toward distributed control systems on the manufacturing floor.
Distributed control systems represent a radical departure from traditional manufacturing technology in that the entire actuator/ servo system (motor + gearhead + feedback device + microprocessor) is situated at the point where the work is done and is connected to a host system by as few as two or three wires. In contrast, in traditional schemes the control unit is a discrete station or cabinet that communicates with and controls the actuator components through a series of wires and cables. Eliminating these wires and cables reduces induced noise in the system and dramatically reduces wiring costs and complexity, sometimes by a factor of three or four.
Figure 3. Brush-type and brushless motors with self-contained control modules.
Two complete servo systems (one brush-type, one brushless) are shown in Figure 3. In this type of system, a high-performance motor or gearmotor with a feedback device is coupled with a self-contained microprocessor control module measuring only a few inches. The modules run as assignable nodes over a serial or net system and can be plugged in, switched around, or removed as needed as part of an open architecture system. The logic necessary to provide this flexibility is incorporated wholly in software. An added feature in the most advanced of these systems is the incorporation of a drive amplifier into the motion control module, which further minimizes costs, wiring, and integration complexities. Because these control devices communicate serially or over an open protocol, they can be used with any manufacturer's components, thus eliminating the problems sometimes caused by having to integrate a proprietary closed system into a manufacturing line.
FUTURE TRENDS
It is difficult, if not foolhardy, to predict specific technological advances. Nevertheless, based on input from OEM motor users and the many R&D programs under way in the motor and motion control industry, several current trends are likely to continue.
Motor Alternatives and Downgrades. Because moving parts are subject to failure, medical product designers are open to considering using other types of components to achieve the same results as motors. There are already several areas in which acceptable substitutes are available. In noncritical drug-delivery applications, the use of transdermal patches, osmotic systems, and implantables has eroded the market for motorized drug-delivery systems, and in monitors that record vital signs and critical incidents for subsequent analysis, tape drives are being replaced by microprocessor memory. Why turn tapes with motors when digital storage can be achieved directly?
There is also a growing pool of suppliers who can provide commodity-grade "motors by the pound" for those medical applications where a certain number of motor and gearing failures is acceptable. These products are most frequently used in disposable instruments where, if the unit fails on power-up, the user simply throws it away and switches to a replacement unit. Lower-quality motors can also be appropriate where the operating life of the motor is measured in minutes instead of hundreds or thousands of hours and where having operating specifications differ widely from unit to unit is acceptable.
MEMS. The miniaturization technology known as MEMS, which stands for microelectromechanical systems, has excited researchers and designers since its launch a dozen years ago as an initiative of the National Science Foundation and the Department of Defense. Based on semiconductor processing technology, MEMS promises micrometer-range sensors and actuators so small that they are difficult to see with the naked eye. Most national laboratories and many motor manufacturers have had development programs in place for several years to realize this promise. Unfortunately, many misconceptions exist as to how the technology might be commercialized. At present, the most popular adaptation is for automobile air-bag sensors.
Early advocates of MEMS assumed that with sufficient funds and research diligence a new generation of actuator products could be produced through lithography or micromachining that would be small (in the 50-nm1-mm range), inexpensive (<$1 apiece in quantities), lightweight, highly reproducible, and ubiquitous. While researchers have presented promising research in this area at many conferences and seminars, the reality is that the physics of the micro world are not necessarily the same as those of the macro world and the cost of producing these products is higher than that of traditional motor technologies. Other vexing problems are that very small motors are difficult to interface with other components, have very little usable power, and are tremendously inefficient. They are also very difficult to assemble and sometimes require very high voltages (on the order of 100300 V dc) to operate.
Nevertheless, working prototypes of hybrid MEMS motors are being produced today and some versions will probably be available to customers on a limited basis later this year. In the case of these mesoscopic motors, MEMS micromachining techniques are used to produce some of the motor and gearing parts, and more-traditional micromotor technologies are used to solve the assembly, lubrication, and power issues. A brushless dc motor with planetary gearhead in the 2-mm-diam range has many potential applications in the medical field, such as motorized catheters, minimally invasive surgical devices, implantable drug-delivery systems, and artificial organs. Research prototypes have been incorporated successfully in endoscopic surgical devices, micropumps, virtual reality operating theaters, telesurgery systems, unobtrusive prosthetic devices, intraocular microsurgery devices, microscopic diagnostic and analytical instruments, and microforceps.
Closed-Loop Systems. Another exciting trend is the utilization of large-scale servo systems in medical instrumentation. As described above, closed-loop servo control is usually thought of in terms of production machinery. In those systems, a feedback device such as an optical or magnetic encoder continually provides position and/or velocity information to a microprocessor, which in turn adjusts the torque and speed of a motor or gearmotor. In contrast, in a typical surgical device, the feedback that controls a motor may be a physician who is applying various degrees of physical pressure--to control the speed of a drill, for example. To provide greater consistency, or to allow a machine to perform a procedure inaccessible to human hands, it is desirable to integrate a feedback device into the instrument. Miniaturization now affords designers the opportunity to incorporate tachometers and encoders as small as 1/2 in. diam into motorized systems to achieve precise levels of control.
STANDARDS ISSUES
While it is beyond the scope of this article to address the many issues surrounding motor standards for the medical industry, it may be helpful to briefly discuss CE marking for products being exported to the European Union (EU). Except in certain instances (i.e., motors operating on supply voltages between 50 and 1000 V ac and between 75 and 1500 V dc) the CE marking of micromotors and gearheads is unnecessary, and probably inappropriate, unless they will be sold directly to end-users. There is a widespread belief that if a product designer specifies only components that hold CE marks, the assembly resulting therefrom will be CE compliant. This is not the case; it is the responsibility of the manufacturer, the importer into the EU, or the manufacturer's agent to ensure that the total assembly meets appropriate European regulations before the product is placed on the market.
Entities such as NEMA's Programmable Motion Control Group and the EU itself are now considering the question of the CE marking of individual components, with a pronouncement expected later this year. Meanwhile, current standards that apply specifically to motors are cited in the bibliography below, along with some books that describe specific motor types in detail.
CONCLUSION
With product differentiation playing an ever more important role in the marketing of motors and medical devices alike, advances in motor technology can benefit manufacturers as well as the end-users of their products. A variety of advanced motor materials and design features are already available, and the future promises further developments in such areas as micromotors and closed-loop microprocessor controls.
BIBLIOGRAPHY
Clifford M, Electric/Electronic Motor Data Handbook, Englewood Cliffs, NJ, Prentice-Hall, 1990.
Hanselman D, Brushless Permanent-Magnet Motor Design, New York, McGraw-Hill, 1994.
Kenjo T, and Nagamori S, Permanent-Magnet and Brushless DC Motors, Oxford, UK, Clarendon Press, 1985.
Kenjo T, and Sugawara A, Stepping Motors and Their Microprocessor Controls, Oxford, UK, Clarendon Press, 1994.
MG 1: Motors and Generators, Rosslyn, VA, National Electrical Manufacturers Association (NEMA), 1993.
MG 7-1993: Motion/Position Control Motors, Controls, and Feedback Devices, Rosslyn, VA, NEMA, 1993.
IEC 34-20-1: Control Motors, Geneva, International Electrotechnical Commission, 1997 (draft standard).