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Design, Analysis and Optimization of Vibration Interface for an Avionic Equipment
1K. Hema Latha, 2G.Bhasker, 3V.Sandhya Reddy, 4H. Krishna Murty Dora
1,2,4Mechanical Engineering Department, M.J.C.E.T, Banjara Hills, Hyderabad-34, Telangana, INDIA.
3Mechanical Engineering Department, C.B.I.T, Gandipet, Hyderabad-75, Telangana, INDIA.
Abstract— Vibration analysis is the cornerstone of any preventative maintenance program. A proven method of Predictive maintenance is real time testing which can be done without impacting the operation by performing the analysis during normal production time. The first step is to understand the problem from its outward symptoms - this means observations and measurements to quantify the symptoms and then analyze to interpret the data.
Vibration is the most important modes of failure in avionic equipment. The avionic equipment fitted into the aircraft hast to withstand high vibrations. To ensure the performance of Avionic equipment under actual environmental conditions (vibrations, shock, humidity dust and temperatures), it is necessary to subject the equipment to vibration test by simulating the vibration conditions. As the vibration equipment cannot be placed directly on the vibration machine’s shaker table, a mechanical structure called Interface (fixture) is placed between the equipment and the machine.
The work aims at designing the interface that supports the unit on the vibration testing machine while taking in to consideration that its natural frequency does not occur in the excitation range of the unit. These interfaces are generally used for supporting electronic equipments (which are used in missiles, fighter aircrafts.) while the equipment is subjected to various tests like vibration tests, acceleration tests and shock tests.
Keywords: Avionic equipment, vibration analysis, environmental conditions, simulation interface, vibration machine, shaker, natural frequency.
I. INTRODUCTION
Electronic products, such as commercial off-the-shelf (COTS) products used by the military, have many physical dimensions and material properties that cannot be tightly controlled, yet are very critical to life. As a result, understanding the product is a critical element in vibration testing. Vibration has always been very complex, often oversimplified by applications of empirical formulas to predict capabilities. Empirical methods used in the 1970's often served to define a conservative limit for design of electronic card systems.
These methods fail to account for the complex configurations of electronic systems; and as a result, often result in expensive failures. Often these failures are a result of local effects - stresses caused by structural discontinuities such as supports, card cutouts, or
adjacent components. Fatigue failure of any structural system has wide variations in life capabilities. The life capability of electronics is further complicated by the expected variations in geometric features and material properties of electronic parts.
The most common use of vibration test in developing reliable electronic products has been a combination of testing and evaluation with empirical relationships. But, empirical relationships provide guidelines, not a real numerical definition of life at the point of failure. The empirical formula approach served well during a time when all companies involved in electronics could not afford the computer power and expertise necessary to evaluate electronics at the point of failure. No equation is capable of covering configuration variations and modal response combinations for modern electronics.
But today, desktop computer power and computer software programs make the capability to evaluate vibration life available to all. The detailed analysis performed by the computer may take several hours, but provides detailed information on the life capabilities at component level and modes dominating failure of each.
With this level of understanding, all parts of the development process considering vibration become more efficient. This is especially true for testing. Testing programs, which tend to be very expensive, become far more efficient because they are now understood with extensive detail. Some think of vibration ESS as black magic, mystically causing flaws to turn into failures. But flaws become failures by application of stress cycles.
Understanding the system at stress level one may understand the process. Random vibration commonly is used in product testing, with various resonances excited simultaneously, much as they are in service. Whenever a failure takes place during highly accelerated life testing (HALT), environmental stress screening (ESS), highly accelerated stress screening (HASS), or other testing, we need to identify the root cause of that failure. When vibration is understood at the root cause level, design changes can be implemented with the greatest probability of success and at the lowest cost..
II. VIBRATIONS
A. Vibration and its sources
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road. Vibration is occasionally desirable. For example the motion of a tuning fork, the reed in a wood wind instrument or harmonica or the cone of a loud speaker is desirable vibration, necessary for the correct functioning of the various devices. More often, vibration is undesirable, wasting energy and creates unwanted sound/noise. For example, the vibrational motions of engines, electric motors or any mechanical device in operation are typically unwanted. Such vibrations can be caused by imbalances in the rotating parts, uneven friction, the meshing of gear teeth, etc. Careful designs usually minimizes unwanted vibrations.
Figure1:One of the possible modes of vibration of an idealized drum
Vibration Sources:
Mechanical vibrations can have many different sources:
Mechanical hammers and presses.
Unbalanced rotating machine parts
Reciprocating machine parts, like pistons, connecting rods shaping machines
Ball bearings
Reactive forces of gas pressure in the internal combustion engines, pumps, etc.
Single-phase motors and generators (electro-dynamic forces)
Earthquakes
In household products such as blenders and washing machines,
In ships and submarine the vibrations due to the engines and due to buffeting by water.
In airplane, missiles and rockets the vibration occur at the surfaces of the wings and tail.It is due to jet and rocket engines and due to aerodynamic buffeting. These vibrations may set up in consequence of eddying air in the slipstream from surfaces at the front of the machine, these eddies exert alternating forces which can be dangerous if their frequency lies in the region of the
natural frequency. This effect is known as buffeting.
Most of the vibrations in a missile during subsonic flight are due to sound field developed by rocket engines. This is due to the extreme turbulence of the jet exhaust downstream from the rocket engine.
B. Random Vibration Units:
Random vibration environments in the electronic industry normally deal in terms of the power spectral density (n), which is measure in gravity units, which is dimensionless. It is given by the ratio of acceleration to acceleration due to gravity.
G = a/g
Random vibration can also be expressed in terms of velocity spectral density, and in terms of displacement spectral density.
Table I. RANDOM VIBRATION UNITS
Quantity Units
Power Spectral
Density=(acc)2/Frequency
(m/sec2)2/Hz
Velocity Pectral
Density=(vel)2/Frequency
(m/sec)2/Hz
Displacement Spectral
Density=(disp)2/frequency
(m)2/Hz
C. Basic Failure Modes In Random Vibration:
There are four basic failure modes that must be considered and controlled in order to produce a reliable electronic system. These failures are the following conditions:
High acceleration levels
High stress levels
Large displacement amplitudes
Electrical signals out of tolerance
Considering high acceleration levels, there are a number of electronic components, such as relays and crystals oscillators that does not function electrically when their internal resonances are exited.
Considering high stress levels catastrophic failures can occur in major structural elements. This cn be avoided by increasing the stiffness of the structure to raise the resonant frequency. This reduces the dynamic displacements and stresses, so the fatigue life is improved.
Large displacements amplitudes result in collision between adjacent PCB,s when they are too close together. This result in broken and cracked components, circuits traces, and solder joints.
Considering electrical signals out of tolerance, this can be caused by relative motion in cables and harshness, high temperatures relative motion with in capacitors, resonance within crystal oscillator, a slipping potentiometer, relative motion in a transform\er core, and excessive motion in a tube filament.
III. VIBRATION TESTING
Vibration testing is accomplished by introducing a forcing function into a structure usually with some type of shaker. Generally, one or more points on the structure are controlled to a specified vibration level. Two typical types of vibration test performed are random and sine test. Sine test are performed to survey the structural response of the device under test (DUT). Vibration testing is the process of applying a controlled amount of vibration to a test specimen, usually for the purposes of establishing reliability or testing to destruction. In practice the test article is securely mounted on a shaker table or actuator, which may be operated by electro- dynamic or hydraulic force; typically hydraulic force is used at very low frequencies because of the large displacements involved, and electro-dynamic force is used where higher frequencies are involved.
Some type of signal source is necessary to drive the amplifier, and an accelerometer is needed to measure the vibration response of the test article. Accelerometers are referred to as "Integrated", meaning they have a built-in amplifier and need a current source for power, or
"Charge" type, requiring an external charge-converter to make a usable signal from their output. If the test article is large, then the response may vary across its surface and multiple accelerometers may be used and the outputs averaged.
The overall response curve is usually VERY non- uniform due to the response curves of the amplifier and shaker, and mechanical resonances of the shaker, test article, and mounting fixtures. To cure this, a controller is used to servo the actual measured response to the desired response curve. Controllers may be rack- mounted analog instruments or digital computer-based products.
Two methodologies are commonly used: Swept-sine and Random testing. In the Swept-sine approach the frequency is swept back and forth with amplitudes corresponding to the desired test levels. In Random testing the frequency spectrum of a noise source is shaped to represent the environment that the article will operate in. An additional test approach is Classical shock testing where the article is subjected to one or more high level shock pulses; this is similar to a one time drop-test that might occur in shipping. In all three approaches the test level can be increased until destruction occurs, thus establishing the safety margins.
IV. PURPOSE OF AN INTERFACE
The shaker head on a vibration machine usually has some form of a hole pattern that permits the installation of machine screws. These holes can often be use to mount small electric components for vibration testing.
Large electronic boxes require some sort of mechanical adapter that will permit the shaker head to transfer the vibration motion to the electronic box. This adapter is commonly known as vibration test interface. The vibration test interface thus acts as interface between the
unit and the machine. It is an extension of the armature in the form of very rigid structure that can transfer the required force at the required frequency. Vibration test interface are available in such a large variety of sizes and shapes that, it is difficult to give general statements, which can be useful for a particular design.
Many, approach interface design from the viewpoint of static strength and stiffness. The interface and load weight are estimate and multiplied by g level of the test.
This yields the force transmitted. This force is quite modest in terms of static strengths, so that the designer then proceeds to clamp the specimen to the interface and the interface to the shaker table with a few bolts and clamping sections, which are entirely adequate to cope with these low static forces. This approach fails to account for the dynamic condition that occurs at high frequencies encountered in most vibration tests. An optimum interface would have its lowest natural frequency about 50% higher, than the highest required forced frequency, in order to avoid fixture resonance during the test.
Introducing a highly damped interface structure can reduce severe interface resonance. This can be in the form of laminated structures where energy is dissipated at several interfaces. Introducing a highly damped casting such as zirconium, magnesium often used for structures that require high damping and stiffness with light weight. The reason for why it is desirable to keep the natural frequency of n interface at least 50% higher than the highest forcing frequency is that, a resonance can magnify acceleration forces. If an improperly designed vibration interface is used to support a sensitive electronic component to receive 100g if the interface has transmissibility of 20 at its resonance.
A. Basic Interface consideration
Sharp changes in the cross section of any interface should be avoided since it results in a reduction of the effective spring rate without a proportional reduction in the mass and furthermore it results in a lower natural frequency for the interface.
It is important to always consider the stiffness to weight ratio of the interface to make maximum use of the interface mass and understanding the characteristics of the basic frequency equation. To know what factors affects the resonant frequency. If an interface fails, it is either because of a faulty analysis or because an analysis substantially correct throughout was followed by an interface design that did not overcome the problem and difficulties clearly shown by the analysis. The natural frequency of a body can be obtained by the formula F=(1/2π)(k/m)1/2 Where K= stiffness in N/m
M= mass of the vibrating body in kg But K=load/deflection=W/δ
F=(1/2π) (g/δ) Where,
δ=deflection in meters,g=acceleration due to gravity in m/sec2, f=frequency in Hertz.
Considering that the interface is simply supported and uniformly loaded
I=1/12(b*d3) Δ=5/384(WL3/EI)
Where, I=Moment of Inertia, m4, E=Young‟s Modulus, N/m2, W=Weight(kg).
b,d = breadth and length of plate considered
Calculating „I‟ and „δ‟ for a simple block is easy but it is very complex for a block with a number of holes.
B. Good Interface Design:
The test specimen and the test specification should be well understood. Preliminary design was analyzed and kept at lowest natural frequency, 50% higher than the frequency of force applied.
A good Interface design avoids sharp changes in the cross section. The stiffness to mass ratio suits optimum design and avoids bolted interface assemblies except where it may be required for stiffness and damping. The interface is small as possible, has simple design and is design symmetry and has dynamic similarity. The effective length of the bolt threads engagement when calculating the effective bolt spring rate was considered.
V. PROBLEM
Avionics are electronic equipments used in aircraft and they demand a great deal of the mechanical design skill for achieving sound and reliable products. Mechanical engineering activity has thus become a crucial contribution in reducing the size and weight and in increasing the life of the equipment.
The use of the avionics in the strategic or military applications is rapidly increasing and these equipments are becoming more and more sophisticated. Acceleration levels in airplanes, missiles, automobiles and ships have increased as their speeds have increased. These factors result in a sharp increase in the failure rates of lea wires, solder joints connectors, screws, cables, castings and other structural members. The main causes of failure of the equipment in the field are environmental parameters.
The word environments in the field are environmental parameters (like vibration, shock, dust and temperature that affect the behavior of a structure.
Hence data defining environmental conditions are collected and analyzed to assist in obtaining various ultimate objectives like:
The design of the equipment has to withstand the rigorous severe environmental conditions. The specification of laboratory tests designed to evaluate experimentally the capability of equipment to withstand severe conditions of vibration and shock.
The object of the test is to simulate the aircraft working conditions, where the designed avionic equipment is supposed to work. The working conditions of the aircraft as per military standards range from 20 to 2000Hz frequency levels. The equipment under test cannot be mounted directly on the vibration machine due to the difference in the pitch of the holes of the unit and the vibration machine. So, an interface (vibration fixture) enables to couple the unit with the vibration machine. If the natural frequency of the interface lies in the test range, resonance occurs resulting in amplification of the input, which is undesirable.
The methodology involved is listed out as follows:
Geometric modeling of the product i.e., designing of the vibration fixture.
Creation of properties depending on selection of suitable material for the fixture-Analysis to find the natural frequency of the vibration fixture.
Meshing the model.
Checking the model for its integrity.
Giving a run for the analysis for a given environment i.e, Vibration testing for Resonance search.
VI. VIBRATION FIXTURE RESONANCES
Vibration fixtures, at times, have resonant frequencies that exist in the frequency range needed for conducting vibration tests. These resonant frequencies can cause significant problems when running vibration tests over ranges which include these resonances. A test engineer will attempt to control the shaker system through a feedback accelerometer which controls the level of vibration input into the test specimen. However, the control accelerometer can only control the level of vibration at that point. It cannot change the resonant behavior of the vibration fixture. Often times, vibration tests, are performed to qualify or verify the acceptability of specific hardware for certain environments.
Figure 2: Typical fixture components.
Figure2. Shows a typical schematic of the elements of a fixture. Shaker systems are used to generate forces or accelerations to replicate some known operating condition or to generate an input spectrum that envelops
the actual environment. Usually, a specification mandates the particular input that is needed to satisfy the requirement. The interface between the shaker and the test article is the test fixture. As defined in this article, the test fixture includes the shaker armature, expander head (or slip plate) as well as the attachment fixture (i.e., .the item designed to accommodate the test specimen).
So, when we refer to the fixture, we mean everything between the shaker‟s driver coil and the test specimen.
(Just because we can‟t see the armature, doesn‟t mean that it‟s not part of the fixture setup). The ultimate goal would be to make the fixture infinitely stiff and mass less. This essentially implies that the fixture would be resonance free, especially over the test frequency range of interest. This further implies that the surface to which the test article is attached moves as a rigid body with uniform displacement at the test article interface. This can be extremely difficult, if not impossible, to achieve, especially for larger shaker systems. These systems will generally have some resonant behavior due to the mass/stiffness characteristics of the armature, expander head (or slip plate) and attachment fixture to accommodate the test article.
VII. DESIGN AND ANALYSIS CONCEPTS
Figure 3: Process cycle
The steps involved in Finite Element Analysis can be listed out as :
Geometric modeling of the product
Creation of properties depending o the material of product
Meshing the model
Checking the model for its integrity
Defining boundary conditions
Giving a run for the analysis for a given environment.
Table II .Comparisions of results
Description Thickness
30 mm
Thicknes s 32mm
Thickness 36mm
Thicknes s 38mm
Thicknes s 42mm
Square Plate 450x450mm 1422.3 1517.1 1706.8 1801.6 1991.2
Square Plate 450x450mm with 30x30 chamfer 1521.2 622.7 1825.5 19269 2129.7 Square plate 450x450 mm with80x80 chamfer 20381.1 2174 2445.8 2381.6 2853.4 Hexagon Plate 450 mm flat to flat 2726.6 2908.4 3271.9 3544.5 3817.2
Circular Plate450mm diameter 3849.0 4106.0 4619.2 4851.3 5389.1
Table II shows the results comparison of various fixture models considered. Table shows the results obtained for Natural frequency of the various fixture models, considering Aluminum alloy material, for various shapes and sizes. All the plates are modeled with same pattern of holes and modal analysis is done for achieving the above results. Details of hole pattern on all the plates are:
M10 hole at centre
4 M10 holes on 4”PCD
8 M10 holes on 8”PCD
8 M10 holes on12”PCD
Figure4 (a , b): Variation of frequency with thickness of plate
The results are shown in the following figures:
Figure 5 a:Modal Analysis of square plate 450 x 450 mm
Figure 5b: Modal Analysis of square plate 450 x 450 mm with 30x 30 mm chamfer
Figure 5c: Modal Analysis of square plate 450 x 450 mm with 80x 80 mm chamfer
Figure 5d:Modal Analysis of Hexagonal plate 450 mm flat to flat
Figure 5e:Circular plate of diameter 450 mm
VII. CONCLUSIONS
Modal analysis is done for various thickness of the fixture considering Aluminium Alloy material (IS:
736-74, Grade 4345) whose composition is Copper: 3.8% to 6.0%, Magnesium:
0.2%to0.8%,Silicon 0.5%to1.2%, Manganese:0.3%
to 1.2%,Aluminumremaining,
Titanium/chromium0.3% optionally used, Young‟s modulus: 0.69*10e11N/m2: Density: 2770kg/m3.
Modal analysis carried out using ANSYS v10. and the results are tabulated.
It is observed that as the thickness increases, natural frequency is increasing(size point of view)
It is also observed that as the sides are nearing to the circle, natural frequency is increasing (shape point of view).
ACKNOWLEDGMENT
The work was carried out at SLRDC, M/s. Hindustan Aeronautics Limited, Hyderabad Division.
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