Moog Brief History of Servovalves.Pdf

MOOGINC.CONTROLS OIVISION,EASTAURORA,N.Y. 14052 A BRIEF HISTORY OF ELECTROHYDRAUllC SERVOMECHANISMS- R. H. MASKREY W. J. THAYER This article appeared in the ASME Journal of Dynamic Systems Measurement and Control, June 1978 Fluid power control, that is the trans- cessed more easily in the electronic of lo4 to 106, the servovalve is a very mission and control of energy by means medium than as pure mechanical or effective forward loop “amplifier” as well of a pressurized fluid, is an old and well fluid signals, while the delivery of power as an electrical to hydraulic transducer. It recognized discipline. The growth of fluid at high speeds could be accomplished is the development of the servovalve, power has accelerated with our desires best by the hydraulic servo. This mar- then, that has paced and will continue to control ever increasing quantities of riage of electronics and hydraulics into to pace the growth of electrohydraulics. power and mass with higher speeds and electrohydraulic servomechanisms cre- greater precision. More specifically, where ated both a solution to an existing class GENEALOGY precise motion control is desired and of control problems and a demand for a Electrohydraulic servomechanisms space and weight are limited, the con- whole new strain of components. The trace their roots back through a variety venience of high power-to-weight ratio evolution of these components is really of disciplines. The earliest contributor is makes hydraulic servomechanisms the a story of increasingly demanding appli- the fundamental work in hydraulics from ideal control elements. cations each of which caused the cre- the early Greek and Roman civilizations. ation of better, or more efficient, or more During the Industrial Revolution a num- The demand to achieve more accurate reliable, or faster devices. To satisfy this ber of eminently practical inventions led and faster control at high power levels, demand, new manufacturing methods especially in the areas of machine tools, to the development of a variety of hydro- had to be conceived and original testing mechanical devices which increased the primary flight controls, and automatic techniques developed. fire control produced an ideal marriage influence of hydraulics. A second branch of hydraulic servomechanisms with elec- The key element in this family of of the family tree is the field of process tronic signal processing. Information mechanisms is the electrohydraulic ser- controls, where pneumatic devices for could be transduced, generated, and pro- vovalve. With power gains on the order signal transmission were developed, re- fined and eventually used in power con- Ktesbios is credited with the invention of of the variable stroke hydraulic pump trol applications and in hydraulics. Mean- what must have been the first hydraulic and the accumulator. while, the advances in machining tech- servomechanism. Among his many in- During the 19th century, countless ventions was the water clock, which was nology were leading to the use of close feedback devices were invented, but only used to keep time over a long period, fitted parts, tighter dimensional controls those that had applicability to prime and better structural materials which cre- typically months. movers such as hydraulic governors ated an environment for high pressure Time was recorded by measuring the found acceptance. Hence, the technology hydraulic systems. The demand for high level of water in a vessel with the rate of servomechanisms was limited to pressures was accelerated during World of flow into the vessel held constant. mechanical and hydromechanical devices War I, especially by manufacturers of Ktesbios solved this problem of constant up through the start of the 20th century. naval equipment, and ultimately led to a flow by controlling the water level (hence Then came the application of electrical whole industry manufacturing high pres- pressure) in an upstream vessel which solutions to control problems, and an sure fluid power components. The spread- fed the timing vessel through a fixed acquiescence to a more theoretical treat- ing acceptability of this medium helped orifice. The water level was controlled by ment. Because of this diversification, the to fuel the demand for more sophisti- a float which opened and closed, propor- “Age of Enlightenment” came to servo- cated electrohydraulic control. tionally, a variable orifice feeding the up- mechanisms in the .1920’s, 30’s, and stream vessel. By any rigorous definition A third branch of the family tree grows 40’s with the involvement of people like this is a servomechanism which is still from the relatively recent field of electro- Kupfmuller, Nyquist, Oldenburger, Black, in use today, although not in water mechanical devices. Large motors and and Bode. clocks. In fact, the demise of the water solenoids used as prime movers, spawned The early 1900’s saw several signifi- clock was presaged in the 14th century a field of electromechanical servomech- cant developments, including the use of by the mechanical clock, and the float anisms, where miniature versions, (AC oil as the fluid medium, the use of elec- valve for hydraulic control appears to or DC servomotors and relays) were used tric actuation on directional control have gone with it for the next four as control devices. This provided three valves, and the development of pumping centuries. benefits: first, it led to miniaturization of elements. Mechanical feedback servos the motors (better magnets and materi- Float valves were next used in England appeared as vehicle power steering units als); second, it provided a reason for around 1750 for controlling liquid level in the late 1920’s even though they expanding the amount of knowledge in in domestic water supplies and in steam were not popularly applied to automo- the field of closed loop controls; and boilers. During this time, the 18th cen- biles until the late 1940’s. third, it led to the development of a family tury, designs for steam boilers prolif- During the period prior to World War of electromechanical transducers. The erated in England, in Russia, and eventu- II, several significant events occurred. In first development led directly to the ser- ally throughout the rest of Europe. This the process control field, pneumatics vovalve, while the latter two enhanced period, of course, was the Industrial were being used for computation, control the climate and created the demand for Revolution and it made its mark felt on and signal transmission. This work re- better closed loop control. hydraulic servos. As the boiler inventions grew, so did the control schemes. Many quired mechanical/pneumatic transduc- The latest contributor to the family is varieties of flow valves were patented, ers, and created a need for valves. Aska- the field of electronics. While the vacuum as well as a new device to control steam nia Regulator Company and Askania- tube amplifier was the device that made pressure. By 1800 these were called Werke, Germany, developed and patented the field of electrohydraulic control pos- regulators, a term that became more a valve using the jet pipe principle, in sible, it has been the advancement in common to all feedback devices, and which fluid pressure is converted into solid state electronics, allowing a signifi- some that were not. momentum of a jet. The jet is directed cant reduction in size and power con- between two receivers where the momen- The Industrial Revolution created in sumption, that has made it practical. In tum is recovered as a pressure or flow. its wake a flood of gadgets and inven- addition to the contribution to drive elec- Similarly, Foxboro developed the nozzle- tions that today are essential ingredients tronics, solid state technology has pro- flapper valve which uses the cylindrical of all electrohydraulic systems. Bramah’s vided new and innovative measuring de- curtain orifice area formed by a flat plate hydraulic press in 1796, for example, vices, novel transducers that open up moving toward a sharp edged nozzle. required the invention of a cup packing new fields of control, and expanded These two devices, Figs. 1 and 2, would by Henry Maudslay to make it work. This means of handling signal information so play important roles in the future devel- is the forerunner of our seal technology. that more complex control problems can opment of the servovalve. Those presses, consisting of a hand be solved. pump and a cylinder, gained popularity Siemens of Germany developed a dual- in the manufacturing industries of En- input valve that accepted mechanical EARLV HISTORY gland and doubtlessly led to the use of inputs through a spring and electrical The earliest contributors to fluid power steam driven pumps and the eventual inputs through a moving coil, permanent probably do not have their deeds re- transmission of power through hydraulic magnet motor. While relatively primitive corded, however the earliest meaningful lines. During the late 1800’s, many En- by today’s standards, this valve was used contributor to our subject must surely glish towns had central high pressure in a closed position loop, and became a be Ktesbios [l],’ an Alexandrian living hydraulic systems for power distribution. forerunner of valves used in aircraft during the period 285 to 247 B.C. Living Although this scheme was eclipsed by automatic flight controls. in a time and place well known for its the more efficient use of electric power ‘Numbers in brackets designate References at outstanding scientific achievements, distribution, it did lead to the invention end of paper. 2 HALF BRIDGE (SINGLE NOZZLE) FULL BRIDGE (DOUBLE NOZZLE) FIGURE 2 - Flapper-nozzle Valve FIGURE 1 - Jet Pipe Valve Several attempts at closed loop elec- flow. The spool was moved by a direct rmplemented feedback from the second trohydraulic control were made, of which actrng motor, usually a DC solenoid. Pro- stage by causrng the first-stage bushing the Trebel [2] patent rllustrated in Frg.

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