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Proportional and Servo Valve Technology - Moog …

The ability to achieve automated stepless control of digital, or on-off, devices. The introduction of the pressure and flow rate in fluid power systems has microprocessor, and hence, the PLC (programmable logic undergone major development in the past twenty-five controller) opened the door to a great amount of control years. Electrohydraulic Servo valves were invented in the late options to machine designers. The operation sequence of a 1930s as a high tech, though high cost, solution to motion machine was no longer hardwired in relay logic. While control needs. The mid-1980s saw the practical introduction Boolean operations were possible in relay logic, it was of Proportional valves as a viable and reasonably priced inconvenient to do so, as well as difficult, expensive, and alternative to Servo valves .

Proportional and Servo Valve Technology 8• Fl uid Power Journal March/April 2003 The ability to achieve automated stepless control of pressure and flow rate in …

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Transcription of Proportional and Servo Valve Technology - Moog …

1 The ability to achieve automated stepless control of digital, or on-off, devices. The introduction of the pressure and flow rate in fluid power systems has microprocessor, and hence, the PLC (programmable logic undergone major development in the past twenty-five controller) opened the door to a great amount of control years. Electrohydraulic Servo valves were invented in the late options to machine designers. The operation sequence of a 1930s as a high tech, though high cost, solution to motion machine was no longer hardwired in relay logic. While control needs. The mid-1980s saw the practical introduction Boolean operations were possible in relay logic, it was of Proportional valves as a viable and reasonably priced inconvenient to do so, as well as difficult, expensive, and alternative to Servo valves .

2 This article will explore the time consuming to accomplish. The common introduction Technology used in these Proportional and Servo valves as of PLCs and Proportional valves greatly expanded the well as attempting to shed some light as to what type of control options available to machine designers. Valve may be most appropriate for a given application. The initial Proportional valves that appeared on the If true, stepless control of pressure or flow wasn't that market are what are now commonly termed open-loop . critical to the operation of a machine, preset pressure or valves .

3 In contrast to mechanical feedback (MFB) Servo flow control valves could be achieved by having a bank of valves , a feedback link does not exist between the coil preset valves . The appropriate Valve would be connected assembly and the Valve spool. Since a feedback loop into the circuit via the actuation of a solenoid Valve . For between the input command and the Valve output does example, three discrete pressures could be achieved by not exist, the feedback loop is open rather than closed. In having two pilot relief valves connected in parallel to the an effort to improve the performance of Proportional vent port of a ventable pilot operated relief Valve .

4 These valves , relative to the performance of Servo valves , two valves would be isolated from the pilot operated relief manufacturers added linear variable displacement Valve by two-way normally-closed solenoid valves . By indi- transducers (LVDTs) to Proportional valves in order to sense vidually actuating the 2-way valves , three different pressures the spool (or poppet) position. The output signal from the could be achieved. But what if stepless pressure control was LVDT was fed back to the amplifier card. A summing required? What if the pressure increase or decrease needed amplifier on the amplifier card calculated the difference to follow a specific rate?

5 What if that rate change wasn't between where the spool was supposed to be and where it constant? Then what could the machine designer do? actually was, and the output to the coil was changed in an Until the invention of Servo valves , if the need existed to effort to position the spool to achieve the desired output achieve varying pressure to an actuator in an effort to based on the input. These enhanced Proportional valves are control force or torque, one needed to either have the termed closed loop Proportional valves . Since the means machine operator turn an adjustment knob, stroke a lever, of feedback is electrical, not mechanical, this gave rise to the or a mechanical means needed to be designed to have a term electrical feedback (EFB).

6 Mechanical input device or linkage vary the setting of the Valve . The same need held true if flow needed to be varied. How Proportional valves Operate: Human control of a Valve could be fairly inconsistent. An electrical input from some source is wired to an Mechanical control of the Valve , while possibly being more amplifier card, which in turn, controls a coil on the consistent and repeatable, might not offer much flexibility Proportional Valve . Since the electrical input from most for different adjustment rates. sources is generally much lower in power than the amount Most electrical machine control systems were not all that of current required to operate the coil, the input current well developed until the introduction of the micro- must be amplified.

7 This function is fulfilled by an amplifier processor in the early 1980s. As most machines that had card. The amplifier may be mounted on the Valve , some- electrical automation used relay logic, the sequence of times termed OBE (on board electronics) or be remote operations of the machine was not easily revised. Relays are from the Valve . The source of the input may come from Proportional and Servo Valve Technology 8 Fluid Power Journal March/April 2003. several devices, including a potentiometer armature can remain partially shifted indefinitely controlled by the machine operator, from without an increase in amp draw.

8 By virtue of preset potentiometers, a joystick, or from a being able to partially shift the armature, the PLC. spool or poppet may be partially shifted as The amplifier card drives the Valve coil with well, resulting in a partial output from the a current signal. As the current flows Figure 1a - Courtesy of Denison Hydraulics Valve . through the coil, electromotive force is The simplest means of varying the current Some Proportional directional valves have developed, causing the armature of the used to drive a Proportional coil would be to only one coil. These valves typically have solenoid assembly to move.

9 The armature, in locate a rheostat (adjustable resistor). four, rather than three, position envelopes. turn, inputs force to the Valve spool, in a flow between a DC power supply and the coil. Figure 2 shows a schematic for a 4/4 (4-way, control, pressure reducing, or directional The problem with this solution is that any 4-position) single coil Proportional Valve . control Valve , or the poppet in a pressure current that isn't directed to the coil will be Note that in the de-energized condition relief Valve . The spool or poppet is offset by a changed into heat. This is analogous to using (extreme left flow envelope), all ports are spring.

10 Therefore, the force input by the a fixed displacement pump at less than its closed. In order to shift the spool to the solenoid assembly is opposed by the force of rated flow to supply an actuator. Just as the center position, the spool must travel the spring. excess pump flow is directed to tank over a through a condition in which flow from the Many Proportional directional control relief Valve at full pressure drop, generating pressure port will connect to one of the valves , such as the Valve illustrated in Figure heat, excess amperage will dump over the working ports, as the opposite working port 1, have two solenoid assemblies, one third leg of the rheostat, generating heat as is connected to tank.


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