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MINIATURE
FLUID POWER CONTROLS
MICROHYDRAULICS Fluid power systems are noted for their high power density — permitting high force and torque output from relatively small components when compared to electromechanical systems. Microhydraulics makes it feasible to obtain a significant amount of force from a minimal power source within a very restricted space envelope. Thus, it can provide a straightforward solution to problems that are often beyond the limits of traditional mechanical options. In many cases, these systems are ideally suited for wide-ranging applications like medical orthotic and prosthetic equipment, human-assist lifts, exoskeletons, hand tools, rescue robots, aircraft and missiles, race cars and oceanographic instrumentation. Engineers might be tempted to simply downsize typical commercial components when the need arises to control motion and force in very small powered systems. However, the reality is a bit more complex because scaling laws are not intuitive, according to researchers Jicheng Xia and William Durfee of the University of Minnesota. For example, they note that in a cylinder, force is proportional to area (L2) while weight is proportional to volume (L3). On the other hand, the thickness and weight of a cylinder wall required to contain a fixed pressure goes down with bore size. Thus, the final weight of a hydraulic system at small scale cannot be determined by proportionally scaling a large system. Also, the fundamentals associated with pressure-driven flow dictates that high pressures are required to permit high flow rates through micro-sized channels. In laminar-flow conditions, an order-of-magnitude decrease in the hydraulic diameter of a channel increases by two orders of magnitude the pressure difference required to maintain a constant average flow velocity. Another critical barrier for increased hydraulic power density at reasonable efficiency is the seals. Surface effects such as friction drag of seals and viscous drag of gaps become significant in small bores and that impacts overall efficiency. Too tight and friction dominates; too loose and the pressurized fluid will leak past the seal. Cost and power consumption are also important considerations. Fortunately, a number of manufacturers have designed or re-engineered hydraulic components to work on a “miniature” scale. 86
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As one example, Bieri Hydraulik, a unit of Hydac International based in Liebefeld, Switzerland, makes six standard versions of Type AKP micro-axial piston pumps designed with three or five pistons. For instance, the 5-piston Size AKP36 pump measures only 1.4 in. (36 mm) in diameter by 3.9 in. (99 mm) long. It features a displacement of 0.36 cm3/rev with 250 bar maximum pressure and 4,000 rpm max speed. The Size AKP103 measures 1.9 in. (50 mm) in diameter x 3.8 in. (98 mm) long. It has 3 pistons, displacement of 0.1 cm3/rev, 500 bar max rated pressure, and runs at speeds to 5,000 rpm. A 5-piston version offers displacement of 0.3 cm3/rev. The quiet-running units reportedly offer high volumetric efficiency even at minimum speed of 100 rpm. They are valve controlled on pressure and suction side, so are not suitable as motors. The small units are used in offshore and oil and gas applications, in metering systems, and general hydraulics systems with small displacements. Hydro Leduc, Azerailles, France offers a complete range of fixed and variable displacement micro-pumps; micro-motors (speeds from 350 to 6,500 rpm); check, pressure-relief, solenoid and pilot valves; and complete micro-power packs for operating in widely varied working environments. For example, its PB32 fixed displacement micro-pump has a body diameter of only 1.28 in. (32 mm) with displacement as small as 0.0007 in.3, maximum speed of 5,000 rpm continuous and 6,000 peak, and maximum pressure of 4,350 psi continuous and 5,075 psi peak. A slightly larger PB33 HP version has a 0.0027 in.3 displacement and a maximum continuous pressure rating of 13,050 psi and max peak of 14,500 psi (1,000 bar). The Lee Company., Westbrook, Conn., makes an array of miniature, highperformance fluid control components, including Lee Plug expansion plugs, solenoid valves, flow restrictors, safety screens, relief and check valves, and shuttle valves. The company’s
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