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| An Overview of Servoid Technology for the Design Engineer. |
WHAT IS A SERVOID? A Servoid is a linear, proportional, D.C. solenoid with an externally adjustable gain. It produces rectilinear motion which is linearly proportional to the electrical current input. Gain adjustability is only one of the features which distinguishes a Servoid from a conventional proportional solenoid.
Output displacement gain can be adjusted either at the factory, or after the Servoid is installed in the parent system, even while the system is operating. This makes Servoids particularly useful in systems requiring calibration at final system acceptance testing.
Another distinguishing feature of this device is that it has only one moving part: its armature. The armature weighs 5 grams nominal and is mounted in spring systems of 300 to 1000 pounds/inch rate. This results in a natural frequency between 400 and 1000 Hz.
The dynamic response of the unit is flat within 3 dB to 140 Hz.
The Servoid's unique spring/mass system tolerates high vibration and acceleration. The configuration of its metallurgically bonded spring cartridge results in excellent stability and repeatability characteristics.
Typically, Servoids will have working strokes of from .010 to .030 inch and are designed to consume from 4 to 12 watts of power. These design factors depend on the force and stroke required and the selected spring cartridge (spring rate).
Servoids have no dynamic seals or rubbing surfaces. Mechanical hysteresis and wear points are eliminated. Extreme temperature effects are near zero. Contaminants have no effect on the unit's operation. These features combine to provide exceptional durability.
A Servoid has an encapsulated dry coil which is electrically isolated. Wetted surfaces and static seals can be made compatible with most fluid media. The Servoid structure withstands common operating pressure ranges.
A Servoid generates no radio frequency noise. |
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| HOW DO SERVOIDS WORK? |
One of the important differences between a Servoid and other proportional solenoids is the Servoid's in-service, gain adjustment capability. That feature is the result of matching linear electromagnetic characteristics with a linear spring system.
In contrast, conventional proportional solenoids match non-linear electromagnetic characteristics with opposing non-linear springs to achieve linearity. The Servoid design eliminates this handicap. Servoid gain adjustment does not affect linearity.
The design usually contains a permanent magnet which establishes a constant field. In this configuration, linear movement begins with the first application of current to the coil.
Without the permanent magnet, the gain curve will be non-linear from lift-off to about 20% of the travel, which is acceptable for some applications.
In either case, the output stroke of the servoid is extremely repeatable. It can be used in either open or closed loop servo systems. |
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| HOW ARE SERVOIDS USED? |
Servoids are commonly used as direct acting valve drivers or as pilot actuators in multi-stage fluid control devices or systems in which pressure and/or flow must be altered as a function of some other variable which can be expressed as a D.C. milliamp signal.
In valve and regulator applications, Servoids can be hard coupled to a valving element or positioned over a fixed orifice to provide variable primary flow or pressure control. Such devices are commonly called single-stage and multi-stage Servoid valves or regulators.
When Servoid valves are combined with other functional devices such as differential pressure regulators or pressure/flow sources, they are later referred to as Servoid subsystems.
The Servoid may also be used as a linear force transducer where output force is proportional to input current. Here too, the gain is externally adjustable. |
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| Shortcut to Servoid Valve Product Page |
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