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| Solenoids in Fluid Analysis |
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A designer of fluid analyzing equipment required an actuator capable of pinching a tube. Some alternatives included actuators that operate only in ON/OFF modes. Fast, snap-acting devices could pinch the tube but had two drawbacks; they were noisy and their snap-acting motion could potentially damage the tubes containing the fluids, even causing breakage.
As a result, a slower, variable positioning device was needed. A variable positioning unit by its nature is also quieter, so both problems were potentially alleviated. The Saia-Burgess, Soft Shift, variable positioning actuator was chosen.
The Soft Shift actuator is a push or pull axial device that has the capability of moving at a slow, controlled rate. For instance, if a standard axial solenoid, without a spring, were held vertically so that the plunger was pulling up against gravity, and if the voltage to the coil was increased very slowly to try to slowly move the plunger, a critical voltage would be reached where the plunger would suddenly and rapidly move full stroke. A variable positioning actuator, however, is controllable so that the plunger can move slowly, whether against gravity or against a spring or other type of load.
The force-stroke curves for each type of solenoid, when compared, show why a variable positioning actuator is more controllable. The curves are essentially straight and linear while those of the standard axial solenoid increase non-linearity (exponentially). Therefore, the actuation force of this actuator maintains a nearly constant force on the plunger over the stroke while the conventional solenoid force increases by a factor of eight times or more over the stroke.
A conventional solenoid has pole pieces that are flat or conical-shaped, and the air gap between them diminishes as the plunger moves. The Soft Shift actuator has a flat pole for the plunger, but the base (stator) is a counter-bored cone into which the plunger can move freely. The air gap between the plunger and stator has a constant clearance. The shape of the stator pole is important as to its cross-sectional area. This area determines whether the force curves will be horizontal, increasing in force, or decreasing in force as the plunger moves.
Soft Shift Solenoids are typically designed to utilize the maximum feasible stroke
capability for each size. Also, the force curves are essentially horizontal. This permits use in applications where the primary concern of the application is quiet operation or where the load to be moved is sensitive to vibration or shock. These actuators also have cushion
washers internally to aid in quiet, shock-free operation. In addition, the voltage can be
applied slowly to take advantage of the low energizing capability. The de-energizing part of the cycle is also controllable by ramping the voltage down for slow dropout of the plunger.
Variable positioning actuators are a good choice for extended life applications because of the bearings. They also typically have a closed construction to prevent contamination.
These actuators typically have a high starting force at long strokes. Detented or friction
loads start to move at lower power which eliminates the otherwise overpowered situation
which adds to end-of-stroke acceleration and ending shock. Variable positioning actuators do not have the characteristic of high ending force of conventional solenoids and, therefore, are not as readily useful for "holding in" at reduced power. They can, however, be modified for a high ending force at the expense of increased noise and ending shock.
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