Since the emergence of the electrohydraulic servovalve during the 1950s, engineers have searched for a way to achieve servo performance without high costs.
Now, that way may be at hand. By reducing part-count, eliminating conventional manufacturing steps, and simplifying assembly, Sauer-Danfoss staff engineer Wayne Anderson has developed a differential-pressure control valve with the potential to bring servo performance to lower-cost applications.
"This could have as much impact as the electrohydraulic servovalve originally did in the 1950s," Anderson predicts. "It gives aerospace-type performance to applications that wouldn't otherwise have it."
Anderson says that the new valve could cut the cost of pilot and boost-stage electrohydraulic valves by more than 50%. The new pilot stage, used with his patented pressure and flow-control boost stages, does not require electrical, pressure, or mechanical feedback due to the inherent differential pressure feedback of the new valve. Ultimately, he believes the new design could be used for direct drive of hydraulic pumps in applications involving construction machinery, agricultural equipment, mining vehicles, and industrial systems, such as machine tools.
New pressure control pilot: by applying electrical current to the left pole piece, Anderson's "teeter-totter" tilts as shown, resulting in higher pressure on one side.
Magnetic detraction. A key to the new design is its elimination of the permanent magnet that is typically used in electrohydraulic servovalves. By replacing the magnet with a pair of coils and non-contacting pole pieces, Anderson was able to transform the internal configuration of the valve, substituting a simple flapper assembly for an intricate one, thus doing away with many of the tight tolerances and difficult assembly steps required to build a conventional unit.
Still, eliminating the permanent magnet from the valve was not as obvious as it might first seem. To accomplish that, Anderson needed to find a different means to move the valve's flapper. The flapper, which is responsible for creating the pressure differences that characterize pressure control valves, has traditionally been moved by a magnetic circuit. In the new valve, Anderson set out to change that circuit, replacing its permanent magnet with the coils and pole pieces.
He devised an assembly that uses a simple dowel-pin pivot, instead of the more traditional torsion pivot. "This is basically a free-floating teeter-totter with no other forces on it," Anderson explains. "Because its movement is basically unimpeded, I didn't need a permanent magnet."
Anderson says that the elimination of the permanent magnet freed him up to make changes that would have otherwise been impossible. Prime among those was the development of a new nozzle plate that connects to the dowel-pin pivot. The nozzle plate, on which two co-planar nozzles are formed, is said to be far easier to manufacture than conventional nozzle plates. The reason: In the past, nozzles were typically pressed into blind internal areas of the valve, whereas the new design allows both nozzles to be combined into a single part. Because the new design uses a single-step, co-planar process, it leads itself to greater dimensional precision, Anderson says.
"During the past ten years, I have always wanted the faces of the nozzles to be co-planar and combined as one part, but the magnetic circuits never would have fit," Anderson says. "When I realized I could do away with the permanent magnet, that changed everything."
Snappy flapper. Anderson says that the magnet and nozzle plate alterations were key because they enabled him to make the valve's single most important change: combining the unit's armature and flapper, which are responsible for controlling the valve's differential pressure. Instead of employing three tightly-toleranced parts, the new armature-flapper is a simple, single piece, he says.
The resulting flapper-armature part is said not only to be simpler, but better for the servovalve's performance. Because servovalves typically have miniscule strokes measuring in the thousandths of an inch, integrated parts are said to be more likely to produce the required precision than complicated assemblies. "In the past, we had to weld three parts together, and we had to make sure we didn't overheat or twist them, because it could hurt the valve's performance," Anderson says. "Those assemblies could be very tough to put together."
Despite the simplicity of the new design, its performance is much like that of a conventional servovalve. During operation, the input current to one coil creates a pull on the armature, resulting in a differential pressure at the nozzle that is proportional to the applied current.
The new valve is expected to offer dramatic cost reductions when it reaches the market later this year. Anderson says it will fit in a variety of applications that didn't previously use servovalves, including winches, man-lifts, mills, lathes, and lens grinding machines, as well as steering systems and bucket control assemblies for construction vehicles.
"In the field of electrohydraulics, it's going to open up a lot of new avenues for servo technology."
|Contact Wayne Anderson, Sauer-Danfoss Inc., 3500 Annapolis Lane North, Plymouth, MN 55447; Tel: (763) 509-2039; Fax: (612) 559-5769, E-mail: [email protected] ; or Enter 508.|