Proportional and Servo Valve Technology

1 Proportional and Servo Valve Technology8 Fluid Power JournalMarch/April 2003 The ability to achieve automated stepless control ofpressure and flow rate in fluid power systems hasundergone major development in the past twenty-fiveyears. Electrohydraulic Servo valves were invented in the late1930s as a high tech, though high cost, solution to motioncontrol needs. The mid-1980s saw the practical introductionof Proportional valves as a viable and reasonably pricedalternative to Servo valves . This article will explore thetechnology used in these Proportional and Servo valves aswell as attempting to shed some light as to what type ofvalve may be most appropriate for a given true, stepless control of pressure or flow wasn t thatcritical to the operation of a machine, preset pressure orflow control valves could be achieved by having a bank ofpreset valves .

2 The appropriate Valve would be connectedinto the circuit via the actuation of a solenoid Valve . Forexample, three discrete pressures could be achieved byhaving two pilot relief valves connected in parallel to thevent port of a ventable pilot operated relief Valve . Thesetwo valves would be isolated from the pilot operated reliefvalve by two-way normally-closed solenoid valves . By indi-vidually actuating the 2-way valves , three different pressurescould be achieved. But what if stepless pressure control wasrequired?

3 What if the pressure increase or decrease neededto follow a specific rate? What if that rate change wasn tconstant? Then what could the machine designer do? Until the invention of Servo valves , if the need existed toachieve varying pressure to an actuator in an effort tocontrol force or torque, one needed to either have themachine operator turn an adjustment knob, stroke a lever,or a mechanical means needed to be designed to have amechanical input device or linkage vary the setting of thevalve. The same need held true if flow needed to be control of a Valve could be fairly control of the Valve , while possibly being moreconsistent and repeatable, might not offer much flexibilityfor different adjustment electrical machine control systems were not all thatwell developed until the introduction of the micro-processor in the early 1980s.

4 As most machines that hadelectrical automation used relay logic, the sequence ofoperations of the machine was not easily revised. Relays aredigital, or on-off, devices. The introduction of themicroprocessor, and hence, the PLC (programmable logiccontroller) opened the door to a great amount of controloptions to machine designers. The operation sequence of amachine was no longer hardwired in relay logic. WhileBoolean operations were possible in relay logic, it wasinconvenient to do so, as well as difficult, expensive, andtime consuming to accomplish.

5 The common introductionof PLCs and Proportional valves greatly expanded thecontrol options available to machine initial Proportional valves that appeared on themarket are what are now commonly termed open-loop valves . In contrast to mechanical feedback (MFB) servovalves, a feedback link does not exist between the coilassembly and the Valve spool. Since a feedback loopbetween the input command and the Valve output doesnot exist, the feedback loop is op en rather than closed. Inan effort to improve the performance of proportionalvalves, relative to the performance of Servo valves ,manufacturers added linear variable displacementtransducers (LVDTs) to Proportional valves in order to sensethe spool (or poppet) position.

6 The output signal from theLV DT w a s f e d b a c k to the amplifier card. A summingamplifier on the amplifier card calculated the differencebetween where the spool was supposed to be and where itactually was, and the output to the coil was changed in aneffort to position the spool to achieve the desired outputbased on the input. These enhanced Proportional valves aretermed closed loop Proportional valves . Since the meansof feedback is electrical, not mechanical, this gave rise to theterm electrical feedback (EFB).

7 How Proportional valves Operate:An electrical input from some source is wired to anamplifier card, which in turn, controls a coil on theproportional Valve . Since the electrical input from mostsources is generally much lower in power than the amountof current required to operate the coil, the input currentmust be amplified. This function is fulfilled by an amplifiercard. The amplifier may be mounted on the Valve , some-times termed OBE (on board electronics) or be remotefrom the Valve . The source of the input may come from several devices, including a potentiometercontrolled by the machine operator, frompreset potentiometers, a joystick, or from amplifier card drives the Valve coil witha current signal.

8 As the current flowsthrough the coil, electromotive force isdeveloped, causing the armature of thesolenoid assembly to move. The armature, inturn, inputs force to the Valve spool, in a flowcontrol, pressure reducing , or directionalcontrol Valve , or the poppet in a pressurerelief Valve . The spool or poppet is offset by aspring. Therefore, the force input by thesolenoid assembly is opposed by the force ofthe Proportional directional controlvalves, such as the Valve illustrated in Figure1, have two solenoid assemblies, onesolenoid being located at each end of thevalve.

9 Proportional directional control valvesprovide control of direction as well as offlow. This particular Valve includes an the most part, dual coil proportionalvalves are based on standard on-offdirectional valves . The major differencesbetween direct acting on-off (sometimestermed bang-bang) and direct actingproportional directional valves are:1) The centering springs in proportionaldirectional valves are stronger than thecentering springs used in on-off ) Proportional solenoids are engineered toproduce more force than do on-off ) Proportional valves always use DC ) While the bodies of on-off valves andproportional valves are almost always identical(the same body part is used for both versionsby most manufacturers), the spools inproportional valves :a) are tailored to the flow rate the Valve isdesigned to control .

10 B) are available in a range of flow rateswithin a Valve size;c) and have metering notches that providefor a variety of flow rate profiles vs. theelectrical command input to the ) Spools for directional controls areavailable 1:1 and 2:1 flow ratios between thetwo work ports to allow for control ofhydraulic motors and double rod cylindersand 2:1 area ratio Proportional directional valves haveonly one coil. These valves typically havefour, rather than three, position 2 shows a schematic for a 4/4 (4-way,4-position) single coil Proportional that in the de-energized condition(extreme left flow envelope), all ports areclosed.