應用於高加速壽命測試之電磁鎚與氣壓鎚的特性比較與其對測試結果之影響研究

Features of the Newly Developed Electrodynamic Hammer and the General Pneumatic Hammer as Used in Highly Accelerated Life Test Systems

莊祐誠(You-Cheng Jhuang)

指導教授 ： 陳永樹

元智大學/工程學院/機械工程學系/碩士(2012年)

https://doi.org/10.6838/YZU.2012.00354

摘要

高加速壽命試驗（Highly Accelerated Life Test-HALT）主要使產品曝露於嚴苛的環境，以較短的時間引發潛在的問題，以供產品的改良與設計。目前市面上HALT機台，大多以氣壓鎚來產生連續衝擊，因此本研究主要比較氣壓鎚與新型的電磁鎚用於HALT系統有何差異?氣動鎚利用高壓空氣在推動內部活塞，使其撞擊緩衝材來產生衝擊波。然而氣壓鎚在作動過程中會產生類似振動的諧和信號（活塞的往復運動所造成的）和衝擊（發生於諧和信號的端點）。電磁鎚的作動原理與電磁式振動機相當類似，而其外觀與氣壓鎚相同。比較單根氣壓鎚與電磁鎚的時域信號後，氣動鎚的往復衝擊的頻率和加速度的幅度，隨著輸入壓力變化而同時改變，且其各脈衝之間的再現性不高；而電磁鎚可以獨立控制的往復衝擊的頻率與強度。此外，氣壓鎚的輸出強度又受限於輸入壓力，因此其輸出強度會比電磁鎚來的低。雖然電磁鎚的輸出強度比氣壓鎚來的大，但是它的往復衝擊的頻率並不像氣壓鎚那麼高。從頻率響應頻譜發現，氣壓鎚活塞往復運動會使能量集中於較低頻率的範圍；而衝擊的響應會使能量集中於較高頻的範圍，這意味著待測物會同時受到低頻能量與高頻能量的影響。為了解兩種不同衝擊鎚對於量測結果有何不同，分別使用懸臂樑(一維)和印刷電路板(二維)進行測試。從待測物的頻譜可以清楚看到，除了有待測物本身自然頻率外，還可以看到有活塞的往復運動頻率。然而以電磁鎚系統激發待測物時，在待測物的頻譜上僅有其本身之自然頻率。
氣壓鎚系統利用輸入壓力的提高，來增強其環境應力，在增壓的過程中，往復衝擊的頻率也隨之改變，如果往復頻率與待測物的自然頻率相同時，待測物即發生共振，甚至造成破壞。這使得在測試過程中，很難判定待測物是因為受到輸入的衝擊波本身強度太大而造成失效，抑或因為共振而造成待測物失效？這是使用傳統氣壓鎚於高加速壽命測試必須注意之要點。

關鍵字

可靠度 ； 高加速壽命測試 ； 衝擊 ； 氣壓鎚 ； 電磁鎚

並列摘要

Highly accelerated life test (HALT) is the method that exposes the products to the harsh environment by combining both vibration and thermal loadings simultaneously. It has the advantages of triggering the potential failure of the products at a shorter time period for their quality design improvement. The pneumatic hammers are used to generate repetitive shock in the current HALT machines. This study investigated the differences in the HALT system which was driven by the original pneumatic and the newly designed electrodynamic (ED) hammers as the force driving source.
The cylindrical pneumatic hammer was driven by the compressed air in pushing its inside piston to hit against the anvil in generating impacts. Thus, both harmonic signals (due to the piston’s reciprocating movement) and impacts pulse (happened at peaks of the harmonic signal due to hit on anvil) were generated. Similar in principle to those electrodynamic shakers as used in vibration tests, the ED hammer has a much smaller cylindrical construction which is identical to that of the pneumatic hammers’. It generates only impacts in general. Comparing the acceleration results both for pneumatic and ED hammers themselves only(exclude the table), it was noticed that the pneumatic hammer had both the reciprocating impact frequencies and the magnitudes of acceleration increased simultaneously following the raise of the supply pressure. Also the repeatability of its shock pulses was not quite stable. Whereas, the ED hammer could control the reciprocating impact frequency and the force independently. Besides, the pneumatic hammer had limited impact strength due to the constraint from the input pressure of the compressed air while ED hammer can easily generate high magnitude impacts by controlling the energized time to the electromagnetic coils but its reciprocating impact frequencies were not as high as those of pneumatic hammers’.
From the frequency response spectrum of those accelerations time history of the pneumatic hammer testing, it was found that the reciprocating piston motion will cause higher acceleration response amplitudes in the lower frequency ranges while impact will generate higher acceleration amplitudes in the higher frequency range. It means that products will be subject to vibration accelerations resulting from piston’s reciprocating movement and shock (from impacts on anvil) at the same time. However, Ed hammer has impact accelerations only.
In order to understand the influence on the test specimen response in using these two different hammers, a cantilever beam (1-D) and a printed circuit board-PCB (2-D) were tested separately. The time history of the response was then transformed into shock response spectrum (SRS) that peak response accelerations could be seen clearly at both the natural frequency of the test specimen itself and at the reciprocating frequency of the impacting piston’s. However, only the former was observed for the testing with the ED hammer.
The reciprocating impact frequencies of the pneumatic hammer system vary with the supply pressure increases. When the reciprocating impact frequency happened to be the same as the natural frequencies of the test specimen, resonance will occur. This often causes misjudgment in the HALT test for products’ failure. In other words, if the increasing step accelerations in HALT test caused failure of the test specimen, it really hard to judge this failure was resulted from resonance or the levels of magnitudes of the impact pulses. This is what needed to be cautious when checking the failure with pneumatic hammers in HALT.

並列關鍵字

Electrodynamic hammer ； Reliability ； Highly accelerated life test ； shock ； Pneumatic hammer

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