IDAN Shock Mount Isolation Vibration Study November 1, The operation of shock and vibration isolation base plate

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1 dr. Istvan Koller RTD USA BME Laboratory. Background In 998, Real Tie Devices USA, Inc. introduced a novel packaging concept for ebedded PC/04 odules to build Intelligent Data Acquisition Nodes. This syste, called IDAN for short, has been very successful due to its rugged design that aintains PC/04's inherent odularity. The syste is designed to withstand harsh environents, shock and vibration. To assure that the design standards were et, the RTD USA BME Laboratory was tasked with evaluating the vibration resistance of the shock ount isolation plate. Here are the results of the study. 2. Introduction The IDAN shock and vibration isolation base plate is ideal for ounting IDAN systes in aircraft, shipboard or vehicular applications. The syste can be installed in any attitude yielding superior high frequency vibration isolation along with low resonant aplification which increases equipent reliability and extends coponent life. The IDAN-Base-SM includes a flanged base plate for ounting, 4 shock ount isolators, and an IDAN botto plate which can be assebled onto any standard IDAN syste. We easured the efficiency of the insulator vs. frequency and the ass of the IDAN and perfored an extended vibration test using sweep sine wave and rando noise excitation. The IDAN syste was in continuous operation throughout the tests without failure. 3. The operation of shock and vibration isolation base plate The siplified construction of a shock ounted IDAN is shown in Figure. IDAN fraes Shockount Syste Platfor Figure. Siplified shock ounted IDAN Page of 3

2 The shockount syste is actually a spring with loss. Therefore, the concentrated paraeter echanical odel of the syste is according to Figure 2. = total oving ass of IDAN r l r l = loss resistance of shockount spring v I v s v p c Mechanical excitation generator Infinite ass c = copliance of the shockount v p = platfor velocity v I = IDAN velocity v s = shockount velocity Figure 2. Concentrated paraeter echanical odel of the syste The echanical ipedance is the force divided by velocity on a echanical eleent. The following analogy between the echanical and electronic systes can be used: Mechanical paraeter [unit] Force [N] Velocity [/s] Mass [kg] Copliance [/N] Mechanical Resistance [Ns/] Electrical Paraeter Voltage Current Inductance Capacitance Resistance Based on this, the electrical equivalent of the echanical setup is: Page 2 of 3

3 c v s v I v p r l Figure 3. Electrical equivalent of the echanical setup. Therefore, we can write the velocity of the IDAN using the current dividing rule: v I = v p + rl sc + rl + s sc We can then define the transfer function between the IDAN velocity and the platfor velocity: where F( ω) = v v I p s + ω = s s + D + ω ω ω = rl c, ω 0 =, D=r c l c The absolute value of F( ω) can be seen in Figure 4. Page 3 of 3

4 F( ω) db +20dB/D a = 20lg D ω 0 ω log ω -40dB/D -20dB/D Figure 4. The absolute value of the transfer function F( ω ). The transfer function in Figure 4 corresponds to the easured transfer function. 4. Measuring the transfer function of the shock and vibration isolation base plate Figure 5 shows the easureent setup of the isolation base plate transfer function. + v I v p Acceleroeters Rando Noise Volteter 3g 6g 0g Power Aplifier Shaker Measuring aplifier In Syste Analyzer Measuring aplifier In White noise signal out Figure 5. The easureent setup of the isolation base plate transfer function Page 4 of 3

5 We have easured the isolation transfer function in two IDANs. IDAN Syste is a typical, relatively coplex IDAN and IDAN Syste 2 is a iniu syste. IDAN Syste IDAN Syste 2 CMM686GX233 cpumodule CMT03 HDD utilitymodule (with Sandisk drive) DM6420 Data Acquisition datamodule ECAN527 CAN bus utilitymodule CM202 Ethernet utilitymodule EPWR04 Power Supply utilitymodule CMM686GX233 cpumodule EPWR04 Power Supply utilitymodule As discussed previously, the transfer function is: F( ω) = v v The transfer function expresses the ratio between IDAN and the platfor velocity. We can easure the acceleration: and since and where s is the coplex frequency, so F( ω) = v v F( )= a ' ω a Page 5 of 3 I p I p d a(t) = v(t) dt a( ω) = v( ω)*s I p * s a I ' F( ω) * s = a = We used Maxiu Length Sequences (MLS) sequences to excite the syste. The MLS series are pseudo rando periodic series with periodic dirac autocorrelation function. Because of this feature, MLS series generate unifor distribution rando nubers approxiating white noise. It can be shown that the ipulse response of a syste is the cross-correlation function of the syste response and the excitation MLS sequence. We used this ethod to easure the transfer functions. The bandwidth of excitation was approxiately 2 khz. The excitation voltage of the shaker in this bandwidth was white. This eans that the excitation was a constant voltage p

6 excitation vs. frequency. The shaker excited the syste in constant velocity ode, because of the constant voltage excitation, up to 50 Hz and in constant acceleration ode fro 50 Hz up to 2 khz (see Figure 9). We easured the transfer function at 3 g, 6 g, and 0 g platfor RMS accelerations. There were no significant differences between the easured transfer functions at various excitation levels. Figures 6 and 7 show the easured transfer function of the isolation base plate of IDAN Syste and IDAN Syste 2 respectively. Figure 6. Measured transfer function of the isolation base plate with IDAN Syste Page 6 of 3

7 Figure 7. Measured transfer function of the isolation base plate with IDAN Syste 2 The figures show that above the resonance frequency the isolation suppresses vibration efficiently. The resonance frequency depends on the ass of IDAN, which can be seen coparing the higher ass IDAN in Figure 6 with the lower ass IDAN in Figure 7. The transfer function was easured several ties before and after the large aplitude, extended tie vibration test. There were no significant changes in the transfer function. Using these diagras, we can calculate the paraeters of the concentrated echanical equivalent network of the isolation. First let's see consider IDAN Syste. The ass of the oving part is: = 2.66 kg because ω 0 = c The resonant frequency fro the transfer function is: ω 0 40Hz Therefore, the copliance of the shock ount is: Page 7 of 3

8 c = = ω kg 40 2π s s µ = 596. * 0 = 59. kg N Because the attenuation at the resonance frequency is: a = 20lg D and fro Figure 6: a = 0dB The loss factor (D) can be derived fro the attenuation at resonance frequency: Because the loss factor: a D = 0 = 0 = The echanical resistance of the insulator: D=r l c The second cutoff frequency: r = D l c 266. kg = 03. = *0 N ω = = =34 Hz r l c Ns * 0 N This fits the transfer function in Figure 6. Ns In the sae anner, we can calculate these equations in the case of inial IDAN Syste 2. The ass of IDAN Syste 2 is: =.65 kg The resonant frequency fro the transfer function is: ω 0 = 5Hz Therefore, the copliance of the shock ount is: c = = ω 0.65kg 5 2π s s µ = 59. * 0 = 59. kg N Page 8 of 3

9 Because the attenuation at the resonance frequency is: a = 20lg D and fro Figure 7: a = 0dB The loss factor (D) can be derived fro the attenuation at resonance frequency: Because the loss factor is: a D = 0 = 0 = The echanical resistance of the insulator is: D=r l c r = D The second cutoff frequency is: l c 65. kg = 03. = *0 N ω = = = 204 Hz r l c Ns * 0 N This fits the transfer function in Figure 7. Ns Based on the siple odel in Figure 2 we were able to explain the easured transfer function. 5. The vibration test of IDAN with shock and vibration isolation base plate The easureent setup can be seen in Figure 8. Page 9 of 3

10 Tie function sapled by the datamodule and its spectra IDAN WinCE consol Acceleroeter Working IDAN Rando Noise Volteter Signal generator Sweep Sine or rando signal Power Aplifier Shaker Measuring aplifier Spectru Analyzer Figure 8. IDAN vibration test setup The acceleration of the IDAN platfor in the case of constant voltage excitation vs. frequency can be seen in Figure 9. We can divide the function into two parts. One part is the constant velocity and the second one is the constant acceleration. The constant velocity part can be characterized by a +20 db/d line because: a( ω) = v( ω)*s The constant acceleration part ends at 2kHz, which was assured by a low-pass filter in series with the rando noise generator. Page 0 of 3

11 Relative Acceleration in db Figure 9. The IDAN platfor acceleration with constant voltage excitation vs. frequency 5..The vibration test of IDAN with rando noise We used MLS sequences to excite the syste which eans, that the platfor acceleration spectra is according to Figure 9 because the MLS voltage spectra is constant vs. frequency. The RMS value of the platfor acceleration was increased in three test cycles fro 3 g to 6 g and finally to 0 g. All tests lasted one half an hour. During rando excitation, the IDAN systes were continuously operating without failure. 5.2.The vibration test of IDAN with sweep sine This test cycle used a constant aplitude sweep sine wave to excite the syste which eans, that the platfor acceleration aplitude was according to Figure 9. The constant aplitude acceleration part was 6 g. The constant velocity part was 0.2 /s. Figure 0 shows the acceleration, velocity and aplitude of the IDAN platfor. The sweep start frequency was 5 Hz and the stop frequencies were 200 Hz and 2 khz in two test cycles. The sweep speed was 30 seconds in both cases. After a sweep cycle, the next cycle started autoatically. The total test tie was 30 inutes. During the test, the IDAN systes were continuously operating without failure. Page of 3

12 Acceleration /s 2 db +20dB/D 6g 0.6g Velocity /s db 5Hz 50Hz 0.2 /s -20dB/D 2kHz log ω 4.8 c/s Aplitude db 5Hz 0.64c 50Hz -20dB/D 2kHz log ω dB/D 3.8µ 5Hz 50Hz 2kHz log ω Figure 0. Acceleration, velocity and aplitude of the IDAN platfor under test. 6. Suarization of the IDAN vibration test results Page 2 of 3

13 6.. The shock ount Resonance frequency: Hz depending on the nuber of IDAN fraes Aplifying at resonance frequency: less than 0 db Vibration suppression above resonance frequency: -40 db/d up to 200 Hz and -20 db/d above 200 Hz 6.2. Sweep sine vibration test The IDANs under test were continuously operating without failure before, during and after the test. Frequency range: 5 Hz Hz Constant platfor velocity excitation frequency range: 5 Hz - 50 Hz Constant velocity value: 0.2 /s Constant platfor acceleration excitation frequency range: 50 Hz - 2 khz Constant acceleration RMS value: 6 g Sweep tie: 30 seconds Total sweep cycles: 60 Total easureent tie: 30 inutes 6.3.Rando noise vibration test The IDANs under test were continuously operating without failure before, during and after the test. Frequency range: 5 Hz Hz Spectra of platfor acceleration: +20 db/d up to 50Hz, constant fro 50 Hz to 2 khz RMS vale of platfor acceleration: 0 g Tie of test: 30 inutes Page 3 of 3