Frequency Mapping of FOLFSS using Electro-Dynamic Shaker with Vibration Analysis and Stability studies with Impact-Shock Analysis SH Gowtham Gudimella 1, V Ramesh Kumar 2, YST Raju 3 1 Scientist-B, GCS-Division, CVRDE (DRDO), Chennai, India harigowtham.g@gmail.com 2 Scientist-E, GCS-Division, CVRDE (DRDO), Chennai, India rameshkumar.v@cvrde.drdo.in 3 Scientist-F, GCS-Division, CVRDE (DRDO), Chennai, India raju.yst@cvrde.drdo.in Abstract Flap Operated Loaders Fire Safety Switch (FOLFSS) is designed and developed for the safety purpose of crew in an Armored Fighting Vehicle (AFV). Its acceptance test conforms to undergo vibration, impact and shock tests which need to be verified in the particular frequency range. Along with these verifications critical frequencies in x, y and z directions and also system sensitive functional frequencies are of major concern to understand behavior of the system. For finding these frequencies the input needs to be simultaneously applied in such a way that actual system response can be found which happens under actual performance or calibration tests. Only standalone test in any one (x or y or z) direction gives the response data using electro-dynamic shaker. So, this paper aims at finding the critical and system sensitive functional frequencies of FOLFSS using the vibration test data obtained using an electro-dynamic shaker. These frequencies in turn will be corroborated with combined response function data of the system. Stability studies are done with Impact-Shock analysis. Key Words: FOLFSS, Vibrations, Impact, Shock, Electro-dynamic shaker, Response analysis 1. Introduction Functionality and performance are the key indicators of any system to evaluate its efficiency or effectiveness. So, while designing a system/device care needs to be taken to ensure it delivers its specified output in the particular frequency range. These devices after production needs to be tested for pre and post functionality of tests like temperature, vibration, shock and impact test. And vibration test reveals the characteristics of the device along with resonances if any. Any system component, subassembly or assembly also needs to be under the vibration bounds specified in data sheets as specified in Reference [1]. FOLFSS- Flap Operated Loaders Fire Safety Switch is a device which is fitted in an AFV. This consists of cam mechanism whose turning movement from the flap determines the loaders safety. This device will be used to ensure automatic safety of loader in an AFV. FOLFSS has intricate mechanical sub assemblies and electronic limit switch mounted structurally in the system. So, functional performance of FOLFSS along with critical frequencies in standalone x or y or z directions needs to be analyzed for effective design considerations in the design stage itself.
First FOLFSS will be tested for functional performance by ensuring the working of limit switch and movement/engagement of mechanical components. This will be verified by pressing the upper knob of the system. Next, system will be subjected to required performance vibration test. After confirming the no-resonance state system will be tested again for functional performance to see that there is no deviation in its functional performance. Vibration test conditions for the FOLFSS unit are as follows: Frequency-60 to 400 Hz RMS Acceleration-4g (m/s2) Band width-100 Hz Sweep Rate- 1 octave per minute Duration-30 minutes in all three mutually perpendicular directions (x, y, z) This vibration test is performed on an electro-dynamic shaker setup at Bharat Electronics Limited (BEL), Chennai. This paper deals mainly with the analysis pertaining to the vibration data obtained using above test setup and deducing useful inferences related to frequencies of operation, which will be used for design. 2. Experimental Set Up and Procedure Experimental setup used for the vibration test is an electro-dynamic shaker (made Unholtz-Dickie Corp). This shaker unit consists of an armature and field coil which will be excited using a computer control unit (VWin) [2] by giving required voltage to excite the base of the shaker. Depending upon the direction in which data needs to be obtained FOLFSS can be aligned or fitted to the shaker unit. A layout of the experimental set up is shown in figure 1. Figure 1. Electro-dynamic Shaker unit along with VWin Vibration controller connected to Computer
Input is given to shaker through an amplifier unit by computer using an Auxiliary drive and connector panel. The output is collected through charge amplifier and connector panel by computer for post processing acceleration. With the given vibration test specifications 60 to 400 Hz frequency, and one octave per minute sweep rate, we can obtain the number of sweeps for 30 minutes. From Reference [3], Octave band consists of bands 31.5, 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz. So, for 60 to 400 Hz it comes out to be 2.75 minutes. So number of sweeps required is 30/2.75=10.91~11 sweeps. Here with respect to control value acceleration rms i.e., 4g required voltage will be given to the shaker as input. Entering the required vibration test specifications in the control unit display of computer starts the test and average of all the 11 sweeps will be obtained. In x-direction FOLFSS is fitted to the test bed base rigidly and axial motion is given. In y-direction FOLFSS is aligned in planar perpendicular direction to the previous x-direction. In z-direction perpendicular motion is given to shaker by fitting the FOLFSS rigidly to the test bed. Suitable fixtures are used to avoid any jerks during experiment. 3. Results and Analysis 3.1 Resonance and Vibration bounds Experimental run in all three x, y and z directions for 1.5 hrs gives three graphs showing acceleration vs. frequency. And the graphs are almost straight lines with very small deviations at some frequencies with ordinate as 4g. This doesn t give any idea about the required resonances or critical frequencies. So, using the VWin controller resonance check is done in the computer itself, and it is found that there are no resonances in any direction. Also, vibration bound specifications from Reference [1] mentions that heavy lines reproduced as shown in figure 2 are bounds for a maximum acceleration of 10g, within frequency range 5-500 Hz and an upper limit for the displacement of 0.3 inch. From the test data of all directions (x, y & z) it is found that maximum displacement is 0.5 mm which is equal to 0.0197 inch and this is less than the upper limit. So, FOLFSS conforms to meet the no-resonance and vibration bounds conditions. With this inference we can see the FOLFSS meets its functional requirements in the particular frequency range. Figure 2. Specifications of Vibration Bounds
3.2 Critical Frequencies in three mutually perpendicular x, y and z directions With the test data obtained for x, y and z directions a magnified plot has been plotted using MATLAB as shown in figure 3. This test data gives reference (g), control (g), drive (V-pk) and 'HInv' (V/g). 'HInv' is inverse of response function i.e., the ratio of input to the output in the frequency domain. From the figure we can see the crests and troughs in the mid frequency as well as high frequency region. In x-direction direction control acceleration value is in line with the reference accelerationrms except in range of 254 Hz to 263Hz and above 290 Hz. In y direction control acceleration is inline except in the range of 162 Hz to 212 Hz and above 375Hz. In z direction control acceleration is inline except in the range of 223 Hz to 253 Hz. Taking a critical deviation value of ±0.1g for x-direction; ±0.2g for y and z directions from reference we can obtain the critical frequencies. These are being shown in the figure 3. Now considering x-direction an attenuation of 3.886 g (-0.114g) has been observed at 299 Hz. And this wont affects the system performance, because the FOLFSS is aligned and fitted in the tank in the x-direction only. So, this frequency 299 Hz is a critical frequency in x-direction but will not affect the device unless it is being in very low level operation in the field. Figure 3. Response acceleration Vs frequency in x, y and z directions And another important observation is that at the frequency 180.3 Hz amplification of 4.201 g (+0.201g) has been observed in y- direction. This direction being the planar perpendicular direction to x-direction similar characteristics are found in this frequency region. With a similar deduction as above
for x-direction this frequency 180.3 Hz is a critical frequency in y-direction in amplification zone of concern. In z-direction it has been observed that at frequencies 232 Hz (4.225g), 233 Hz (3.899g), 235 Hz (3.774g), 236 Hz (4.258g), and 239 Hz (4.219g) an oscillatory behavior occurred. In x-direction FOLFSS s critical frequency 299 Hz is only attenuating which is of not a major concern because of its alignment in the same direction as the tank is moving. Where as in y-direction sharp turn s in mutually planar perpendicular directions cause the system to excite at frequency 180.3 Hz and if this matches with tanks operating frequency this excites vibration in wall of tank in axial direction which will be a problem. And in high frequency y-direction at around 387 Hz this frequency turns out to excite the mounting bracket to which FOLFSS is attached into oscillations in axial direction. In z-direction as the excitation of FOLFSS is in perpendicular plane to the motion of tank in xy-plane in mid frequency region itself in zone of 232-239 Hz frequency turns out to excite the tanks wall to which FOLFSS is attached into oscillations. 3.3 System Sensitive Functional Frequencies of FOLFSS In the field of actual operation of FOLFSS in AFV, it is being subjected to combined x, y and z directions vibration. So, combined system RMS averaged acceleration has been plotted in figure 4. To find the system sensitive functional frequencies standard deviation of the RMS averaged values is calculated. And around 6 frequencies have been obtained for which the RMS averaged value is upward deviation from 4g which are greater than value of 0.05g. Whole range of frequencies in the form of histogram has been drawn as shown in figure 5 to see the modal-weight age. From the figure 5-histogram data again frequencies are calculated by taking into the modal weightage of the frequencies in the ranges of 160-180 Hz as highest contribution with count of 81. And frequencies in the ranges of 231-250 Hz with count of 40 as well as 372-396 Hz with count of 32. Among these three zones 180, 232,233, 236, 239 and 240 Hz are found to be critical system sensitive functional frequencies. These frequencies are very important in the consideration of functional operation of the FOLFSS unit. Figure 4. Combined RMS averaged acceleration in x, y and z directions
Figure 5. Sensitive functional frequency distribution in the 60-400 Hz frequency range 3.4 Corroboration of System Sensitive Functional Frequencies of FOLFSS with response analysis From the vibration test data Voltage-peak and 'HInv' are obtained. From these values response 'H' has been found by taking inverse of Hinv. And response 'H' has been plotted w.r.t. frequency, RMS averaged combined response has also been plotted in figure 6. It can be observed that in x-direction, response is very low which confirms that x-direction doesn t affect the position of FOLFSS. And in x-direction, at 299 Hz a critical attenuation-amplification is being observed which confirms 299 Hz as critical system sensitive frequency. In the same way in y- direction 180 Hz and 387Hz also confirms as a system sensitive functional frequencies. And in z- direction the response between 232-239 Hz confirms this zone as critical system sensitive functional frequency zone. Thus z-direction is effected in particular frequency zone rather than a single/dual discrete frequencies like x or y-directions. Due to this reason that FOLFSS is being fitted in the AFV in the plane of xz (or yz when it takes turn direction changes this way). Combined rms-response confirms frequencies 180, 232, 233, 236, 239 and 240 Hz (170 to 250 Hz range from figure. 6) as system sensitive functional frequencies. From figure 6, it can be observed at 180 Hz near peak frequency is being observed from acceleration response. Along with these two frequencies 232, 233, 236, 239 and 240 forms the important system sensitive functional frequencies which corroborate the system sensitive frequencies obtained.
Figure 6. 'H'-System Response Vs Frequency This frequency region 170-250 Hz forms an operating range of interest for the designers to design the device in an effective manner. And in particular 232-240 Hz frequency zone is being observed as important excitation zone which can affect the FOLFSS structural operation characteristics. All the results have been tabulated in table 1 to understand the behavior of system. It can be observed that critical frequencies are matching with the final system sensitive functional frequencies. This approach helped us to obtain the combined system characteristics which are very difficult unless device as a part of tank undergo trails in field area. In table 1 it can be observed that underlined values represent the importance of particular frequencies in X, Y, Z and combined-system directions. Understanding of the corroboration from the exhaustive data and converting it into information one needs for designing future systems is being given importance in this work.
Table 1. Frequencies of Interest - FOLFSS Direction of Operation Critical Frequencies (Hz) X 299 Y 180 System Sensitive Functional Frequencies (Hz) Z Combined-System 232, 233, 235, 236, 239 180, 232, 233, 236, 239, 240 180, 232, 233, 236, 239, 240 3.5 Impact Studies Unlike vibration studies impact studies are limited to moderately short duration with moderately high intense moderate number of discrete repeated pulses with half-sine wave. Duration: 10ms, Control acceleration: 10g, Auto pulses: 10 Converting the time domain into frequency domain resultant control acceleration is 0.2361g. From the figure 7, it is observed that impact between 60-400 Hz followed a continuous attenuating combined response and system gets stabilized in high-frequency zone except in 295-302 Hz zone. This zone of 295-302 Hz can be corroborated from the response graph as shown in figure 8.Impact zone is observed to be less intense attenuating. And continuous impact in frequency zone as above leads to structural discontinuities which expect a further strengthening.
Figure 7. Impact control acceleration Vs frequency in x, y and z directions Figure 8: Impact Response acceleration Vs frequency in x, y and z directions
3.6 Shock Studies International Journal of Advances in Science and Technology, Shock is an excitation. Vibration is the mechanical response to an excitation. Shock is the force impact on a surface area which is form of stresses, while vibration is the movement either horizontal or vertical caused by a force which is a displacement. Shock studies are extremely short duration with very high intense short number of discrete pulses with half-sine wave. Duration: 2.5ms, Control acceleration: 100g, Auto pulses: 3 Converting the time domain into frequency domain resultant control acceleration is 0.4289g. From the figure 9, it is observed that impact between 60-400 Hz followed an attenuating sinusoidal combined response and system reaches 0.2905g in high-frequency zone. Unlike impact response, corroboration inference cannot be obtained within in 60-400Hz for shock response graph as shown in figure 10. Figure 9. Shock control acceleration Vs frequency in x, y and z directions
Figure 10. Shock Response acceleration Vs frequency in x, y and z directions It can be observed that resultant impact control acceleration of 0.2361g is near to attenuating sinusoidal combined shock response 0.2905g at high frequency. Thus the system is considered to be stable w.r.t. impact and shock response as the system response is attenuated at high frequencies. Post production functional performance is evaluated and FOLFSS is found to be satisfactorily working. 4. Conclusion All independent critical frequencies in three mutually perpendicular x, y and z directions are found. System sensitive functional frequencies are being obtained using combined RMS averaged, standard deviation and modal analysis which are being corroborated by doing combined response function analysis. It is observed that 170-250 Hz frequency range is of interest for designers for effective FOLFSS design. And also 232-240 Hz as important excitation zone which can affect the FOLFSS structural operation characteristics. Impact and shock response studies show the stability of FOLFSS in the overall frequency range.
5. Acknowledgement The authors wish to express thanks for the support from BEL, Chennai, India. The post production testing support of BEL by providing fixtures and experimental items is excellent. 6. References [1] Thomson W.T and Dahleh M.D., Theory of Vibration with Applications pp. 478-479, (Pearson Education), USA, 2007. [2] Users Manual of Electro-dynamic Shaker unit along with VWin Vibration controller. [3] Bies D.A and Hansen C.H., Engineering Noise Control-Theory and practice pp. 43 Chap. 1 (Spon Press) London and New York, 2003 Authors Profile SH Gowtham Gudimella received his B. Tech degree in Mechanical from VNIT-Nagpur and M.E. from IISc-Bangalore. He has brief experience in TCS-Chennai and BHEL- Hyderabad. He is currently working as Scientist-B in GCS Division of CVRDE-DRDO, Chennai-India. He is a life member of Acoustical Society of India (ASI). He was a recipient of CV RAMAN Gold Medal by Vishwabharthi Academy. He was a recipient of PRATIBHA Award for his performance in CEEP by Government of A.P-India. He was awarded the HS PAUL Award by Acoustical Society of India (ASI) for best paper in NSI- 2009. His research interests are of design, vibrations, acoustics, FSI, mathematical modeling and hydraulics. V Ramesh Kumar received his B.Tech degree in Mechanical Engineering from Guindy College of Engineering. He has received his Masters degree from IIT Madras in manufacturing stream. He is currently working as Scientist-E in GCS Division of CVRDE- DRDO, Chennai-India. His areas of interests are new product design, Modern Manufacturing & Design Methodologies and All Electric Drives-Azimuth/Elevation. YST Raju has received his Bachelor s Degree in Mechanical Engineering from Andhra University, Visakhapatnam in 1986 and Masters Degree in Fluid Mechanics and Thermal Sciences from IIT Kanpur in the year 1988. He joined DRDO as Junior Scientific asst at NSTL in May 1982 and joined CVRDE-DRDO, Chennai-India as Scientist-B during the year 1988. He is currently working as Group Head of IFCS division in CVRDE. He has diversified experience and multidisciplinary know how in the field related to technical issues of Hydraulic, mechanical and electronics. Apart from that he is key person involved in ARJUN's prototype evaluation, induction, user interaction and product support.