Motion Control Takes Flight
| By: Kristin Lewotsky, Contributing Editor
Motion control isn’t just down on the production floor, it’s flying high in aerospace applications ranging from aircraft passenger door operations to stabilizing fin adjustments on cruise missiles. Instead of tolerating dust-filled mills, aerospace motion control systems must survive intense shock and vibration environments with wide temperature swings, providing high power density output in the smallest possible package, along with absolute reliability. It's a challenging market, but the rewards are many.
Industrial motion control components must be robust, but above all economical, easy to integrate, and compatible with volume manufacturing. The aerospace market requires a different business model and a different approach to engineering to meet a stringent set of performance requirements. “There are many considerations you encounter, especially on the military side, that you don't in industrial machine applications,” says Walter Smith, senior applications engineer, motor products at BEI Kimco Magnetics (Vista, California). Reliability on a factory floor and reliability at 30,000 ft. are two very different things. A component failure on a production line means downtime, lost revenues, and unhappy management. A component failure on an aircraft could mean loss of life.
“The amount of engineering done on the front end is probably far greater from the aerospace side than the industrial side,” says Nick Nagel, director of electronics R&D at MPC Products Corp. (Skokie, Illinois). “We use finite-element modeling extensively to look at all sorts of structural issues, whether it be extremely high loads, extremely high shock and vibration, all the way through extensive thermal analysis. The amount of nonrecurring engineering is very large relative to the overall program cost.”
Of course, even within the aerospace industry, reliability is a moving target. Regular usage applications like moving the flaps on a control surface impose different design requirements than fin adjusters on a missile that may fly for a matter of minutes?or sit in a silo for a decade. “When you launch the thing, you want it to work,” says Nagel. “We will have storage requirements of years. If it's a land-based silo it's not so bad but if it’s ship-based, you can have salt fog requirements that are pretty nasty.”
Light as a bird
First and foremost, an aircraft needs to get up in the air and stay there, so size and weight are key considerations for flight hardware. The problem boils down to getting the most power out of the smallest possible package, maximizing the power density and efficiency by making careful choices in everything from magnet material and gearing to the air gap between stator and rotor.
Of course, shrinking a motor can also be an exercise in sizing, literally. A motor that has to produce tens of watts of power continuously is going to be much larger in both size and thermal mass than a motor that only needs to produce that power for brief spurts. “If a motor is going to be used in an actuation application that happens for seconds, then you can really push the limits of what the machine can do,” says Nagel. For a thrust reverser assembly on the Airbus A380, MPC produced a motor that can generate 40 kW on a transient basis -- and weighs less than 10 lbs. “It's only a few inches in diameter and the few inches in length, so it's an extremely power-dense motor,” he says. “A servo motor rated at 40 kW in an industrial application would be rated at that on a continuous basis and it would be five times that size.”
High power density raises the prospect of cogging. That can be an issue in imaging applications, for example, motors that power the gimbals that stabilize sensor balls and targeting systems. Ensuring smooth motion means engineering the motor to minimize cogging effects. "It's how you skew the stator stack or the magnets on the rotor," says Bruce Du, engineer at BEI Kimco. "It depends on the configuration. Usually you can either use skewing techniques or use a larger air gap between the rotor and the stator." Another technique is to choose the right slot/pole combination. The trade off is performance -- the lower the winding factor, the lower the efficiency.
The degree to which cogging becomes an issue depends on the kind of motor, which in turn is driven by the application. BEI Kimco, for example, produces both servo motors for applications like image stabilization and brushless high power density motors called torquers for applications like moving cargo bins in the bays of cargo aircraft. Torquers are targeted primarily at applications involving high masses moving at low speeds, whereas servo motors tend to be used for high-accuracy, high-speed, point-to-point moves. For a torquer, power density tends to be much more important than cogging. For a servo motor, the opposite is typically true.
Aerospace applications can expose components to extreme temperatures and, in the case of marine applications, high humidity conditions like sea fog. Systems must survive extreme shock and vibration, whether in aviation or in space launch applications. If the components are intended for defense systems, they must meet one of many detailed military specifications, which can cover everything from temperature tolerance to ingress protection.
Electromagnetic compatibility requirements can be quite stringent over the frequency bands used for communications. The use of brushless motors can reduce some noise, and filtering can take care of others.
Aerospace systems are increasingly using commercial off-the-shelf (COTS) items in part, Nagel says, because of the increased reliability and availability of semiconductor components. That can simplify procurement and potentially reduce costs, but it brings challenges of its own. “It's a headache,” says Steven Rickenbrode, director of mechanical engineering at MPC Products. “Especially on the electronics parts, you often develop something with one set of components and six months later the components are obsolete and no one's producing them.”
A bigger issue can be that of radiation hardness. As transistors shrink, they become more vulnerable to radiation. “As you go to smaller and smaller transistor processes, single event upsets occur more,” says Nagel. “It's becoming more and more challenging to get radiation-tolerant devices, simply because the processes they manufacture them on are becoming obsolete.”
In order to ensure reliability and performance in the face of all of the factors discussed above, companies need to set up comprehensive test plans and evaluate performance throughout the design and manufacturing stages. In the case of software, it's not enough to write code that meets internal standards. Software for components used on commercial aircraft, for example, must be tested, validated, and documented consistently with FAA standards. “The documentation tests and costs associated with it are tremendous,” says Rickenbrode. “Writing the source code itself is probably 2% of the work -- the rest goes into validation, verification, and documentation.”
Some of the issues presented by the aerospace market go beyond technical challenges. Aerospace and military R&D programs can take years to go from design to production, requiring a substantial ongoing investment from vendors playing in this arena. The customer is typically closely involved in the design and development process, with critical design reviews and so on. Such close cooperation ultimately benefits the project but can eat up significant time and effort.
The lead time works both ways. The advantage of being designed into a military or aviation program is a steady stream of work during the manufacturing phase -- and beyond. Properly maintained, jets can fly for decades. Motion vendors need to be prepared to maintain their products as long as the platform exists. “Most of our contracts require us to support the program for the life of the aircraft,” says Rickenbrode. “We have a complete department dedicated to aftermarket services.”
Like the medical market, the aerospace market carries a particular set of benefits and challenges. For the right company with the right engineering organization, though, it can be a very lucrative, very satisfying sector, indeed.