SERVO VALVES KEY TO AIRCRAFT CARRIER APPLICATION By Matthew McCall, Regional Sales Manager, Moog Inc.
An F/A-18F Super Hornet
ervo and proportional valves are most commonly flow-control, spool-type valves that meter fluid from a high-pressure source—typically a pump or accumulator system—to a rotary or linear actuator of some sort. From the actuator, the fluid then passes back through the valve, returning to the low-pressure tank or reservoir, making it a “meter-in/meter-out” valve. They control position, velocity, or pressure and/or force through a closed-loop electronic control system. These valves are extremely responsive dynamically, reaching commanded setpoints in milliseconds of receiving a command (generally ~1ms to <100ms). What makes a servo valve different than a proportional valve? A servo utilizes a spool-inbushing design, allowing for extremely precise fluid metering, with minimal leakage due to a controlled, tight diametric fit between the spool and its matched bushing, measured in millionths of an inch. With proportional valves, the spool fits neatly into the body itself, allowing for higher flows but with less precision and higher leakage. 18
AUGUST 2020
Fitment between the body and spool is typically measured in the ten-thousandths of an inch range. Both valve types use various methods to drive and position the spool, including mechanical feedback systems using no electronics and electrical feedback systems using an LVDT to measure the spool position. The three main spool driving systems are the nozzle/flapper hydraulic pilot design, jet-pipe hydraulic pilot design, and direct drive design, which does not use a hydraulic pilot but rather drives the spool with a mechanical linkage affixed to a linear or rotary motor or solenoid. Each design has its place in the military world: servo for extreme accuracy and precision and proportional for higher flow applications requiring less precision. Where might you find a servo or proportional valve in a military or defense application? Look for jobs requiring the highest levels of response, reliability, precision, and accuracy. Controlling actuation systems on supersonic fighter jets and launch vehicles? Check. Stabilizing gun turrets
(Photo courtesy of U.S. Navy. or positioning RADAR arrays Photo by Mass Communication and communications antenSpecialist 2nd Class Ryan nas? Check. Testing the next Seelbach/Released) generation of advanced materials in a classified lab? Check. Thrust vector control on rockets and fin control on missile systems? Check. Catching an aircraft on the deck of the USS Gerald R. Ford? Check. The USS Gerald R. Ford (CVN-78) is the latest and greatest aircraft carrier in the U.S. Navy’s fleet. Commissioned in July 2017, it is the largest and most advanced aircraft carrier in the world at roughly 1,100 feet in length, nearly 260 feet in beam, 25 stories tall from keel to tower, and displacing roughly 100,000 tons. The twin nuclear reactors power the ship to over 35 knots, carrying up to 90 aircraft, with a crew of around 6,000 sailors. It is a well-equipped, well-armed, floating city. Some of the major technological advancements over the previous Nimitz-class carriers include an electromagnetic aircraft launch system and an advanced arresting gear system WWW.FLUIDPOWERJOURNAL.COM • WWW.IFPS.ORG
