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Solenoids > Application Examples: Intravenous Equipment
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| Solenoids used in Intravenous Equipment |
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A manufacturer in the medical equipment industry needed assistance with a new product design. The problem to be solved was vital to the success of their new machine design, which was in the final stages of prototyping. Noise needed to be greatly reduced, and without a subsequent increase in solenoid temperature. In fact, a reduction in temperature was very desirable.
In their application, a bank of several solenoids were mounted on a valve manifold. These solenoids were used to open a spring-loaded valve which controlled the flow of various liquids into an intravenous unit. The various solenoid valves were actuated at different times to maintain a precise programmed rate of medical fluids and medicines. Early prototypes used the STA tubular solenoid, and after extensive testing for durability, they were approved. The STA Series provides long life while generating a minimum amount of wear.
This same testing in this application, however, unveiled several system deficiencies. The "clang" generated as the valve pins bottomed on both stops was amplified through the metal manifold, making an unacceptable noise level for hospital environments. Also, under worst-case operating conditions, transmitted heat off the solenoids warmed the medical fluids in the machine close to their maximum tolerance for IV introduction into the human body.
Two schemes for noise reduction were selected for evaluation. The first was an attempt to soft land the solenoid armature. Although possible, the electronics to accomplish this were difficult. Soft return from the energized position proved especially difficult to control. The second method involved incorporation of dampening bumpers into the valve to cushion the valve pin impacts. This method worked well experimentally, but resulted in a force problem. To assure that the valve fully opened, the valve pin had to bottom before or at the same time the solenoid plunger seated.
Introducing the bumpers added more variables in the tolerance specification, which had now reached ±0.022". This new tolerance dictated that the solenoid would have to pull in at full power and hold at reduced power with a hold-in air gap range of 0.000" to 0.045". This wider range proved too much for the design at the power level required to restrict heating.
Several ideas were developed to solve the problem. The most notable observation was that the built-in air gap cone was not being utilized. Since this cone represented about 0.021" of additional air gap added to the maximum of 0.045", it was eliminated. Quick analysis of the maximum pull stroke and force also indicated that pull-in force was not a problem; holding force was.
We suggested a change from 60° to 90° cones to further boost holding force. One last idea with promise was to reduce the tolerance on our plunger cross hole location. We knew that the STA offered much better repeatability for this dimension, with a bottomed plunger, than any of our previous tubular solenoid designs. By cutting ±0.005" from the standard tolerance, we could reduce the maximum holding gap from 0.045" to 0.035". (This is a significant amount for holding forces.)
Prototypes were rapidly built and compared to benchmark units. Changes such as lengthening the armature to offset the removal of the internal air gap cone, and tightening a few tolerances were evaluated for cost impact. The modifications proved to have minimal impact on selling price.
A complete tolerance study of the cross-hole location data suggested a slight adjustment from the calculated location would better center the statistical mean of this dimension. The change was made later when the design modifications were approved and production began.
After more testing, the customer was satisfied with the results. So much improvement was realized, that the hold-in voltage could actually be reduced slightly to further control heating.
One negative side effect was slower release times. This was the result of the closer air gap when the internal air gap cone was removed. The problem was quickly rectified by replacing the diode coil suppressor with a higher speed doide/zener diode combination.
In conclusion, Saia-Burgess' ability to accurately control plunger location and the versatility of cone angle changes allowed us to tailor a design to meet our customer's needs.
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