Modal Testing Excitation Guidelines
Contents
1. Current mode is typically used with sine and swept sine test signals and rarely or never used with burst signals Voltage mode amplification is the preferable choice for burst random and sine chirp excitation which are very widely used in modal testing with single or multiple shakers When using burst random excitation the response of the system needs to decay to zero before the end of the sample interval of the FFT analyzer time capture to minimize leakage When the amplifier is set up as a voltage amplifier then the back EMF effect the electromotive force caused by the vibration motion driving the shaker armature coil through the shaker s magnetic field provides resistance to the armature and helps cause the system response to decay more quickly This may seem to be inappropriate because it seems that the shaker system is then supplying damping to the measurement But it s not an issue as long as the force is measured for the entire measurement Then the correct input output relationship is mea sured Note that the force needs to be measured and not the electri cal parameters of the amplifier to make the correct measurement Linear Versus Switching Amplifiers Historically most shaker power amplifiers were linear type Class A or Class B Linear amplifiers have been superseded by the more efficient designs though they remain popular for their simplicity and continue to be available from most shaker manufacturers Linear amplifie2. In fact larger force levels tend to overdrive the structure exciting nonlinear characteristics of the structure and providing poorer overall measurements than with lower level force tests For this reason again on larger structures it is often desirable to use multiple shakers at lower force levels to more evenly distribute force than a few single shakers operating at high level forces Through Hole Armature Design Conventional vibration test ing uses a shaker with a traditional mounting platform or table design the test article is directly attached to the top surface of the armature with some base excitation applied usually monitored by controlling some prescribed acceleration The device under test DUT is normally subjected to some operating environment generic spectrum or some excessive environment to determine if the equipment is suitable for the intended service In the early days of modal testing with shaker excitation smaller shakers were used to apply some low level excitation to be able to measure a frequency response function Usually the shaker was attached with a long rod commonly referred to as a stinger or quill to impart force to the structure The purpose of the stinger was to dynamically decouple the shaker from the structure Because these traditional shakers were typically used for base excitation the armature attachment configuration was not optimal Usually some type of left right thread arrangement was made o
3. Lateral shaker stand for mounting the force gage or impedance head and attaching the shaker Alignment in these situations is much more difficult requiring that the shaker or test article be moved so that the fixed threaded hole places the stinger exactly in the correct position The main point is that the shaker must be aligned so that the stinger can be very easily threaded into the force gage or impedance head with no difficulty or binding If the excitation point on the structure requires suspending the shaker an appropriate fixture needs to be employed Figure 9 shows a typical shaker mounting installation used to laterally excite an automobile for a modal test of a body in white car frame The stand allows for coarse adjustment of the shaker s vertical and longitudinal positions A set of four turnbuckles used to hang the shaker to the stand allows for fine adjustment of the shaker position and alignment angle to the structure driving point Depending on the size or height of the test article the shaker stand shown in Figure 9 may be too small and some special fixture to hold the shaker needs to be used It is not uncommon for the special test fixture to be validated first and checked for its natural modes of vibration which may interfere during the actual test of the article In a suspended configuration at very low frequencies below 5 10 Hz the inertia provided by the shaker body may not be sufficient and the shaker may exceed
4. R C Gatzwiller K B Brown D L Important Aspects of Precise Driving Point FRF Measurements Using a Mechanical Impedance Head Sensor Proceedings of the XVI IMAC Conference pp 795 799 Santa Barbara CA February 2 5 1998 2 Registration Authority Tutorials IEEE Standard 1451 4 http stan dards ieee org develop regauth tut 3 The Modal Shop Modal Shaker Setup http www modalshop com video asp 4 Zimmerman R D Exciter Stinger Quixote Measurement Dynamics Inc Document 59006 November 1985 5 Cloutier D Avitabile P Bono R W Peres M A Shaker Stinger Effects On Measured Frequency Response Functions Proceedings of the XXVII IMAC Conference pp 197 203 Orlando FL February 9 12 2009 6 Olsen N L Using and Understanding Electrodynamic Shakers in Modal Applications Proceedings of the IV IMAC Conference pp 1160 1167 Los Angeles CA February 3 6 1986 SV The authors can be reached at mperes modalshop com and rbono modalshop com www SandV com
5. 100 lbf to be measured typically more than enough force range for most modal application scenarios Also available are impedance head transducers with TEDS transducer electronic data sheet capability as described by the IEEE 1451 4 standard The built in memory available on TEDS transducers stores sensor calibration information and specifications allowing plug and play functionality when the sensor is connected to the data acquisition system This simplifies system setup and minimizes chances of human error due to enter ing incorrect sensitivity values when setting up the channels Sensor Mounting Considerations A very important consider ation when mounting force transducers is recognizing that the typical force transducers used are uni directional This means two things first that force transducers are designed to accurately measure force on only one of its two mounting faces for example labeled top and base as seen on Figure 2 This is due to the fact that the force transducer itself has mass and stiffness They are designed and calibrated to read force accurately on one of its mounting faces so they need to be installed accordingly Note that for this model force transducer the top of the unit is the designed sensing surface and should be mounted directly to the test article Some impedance head transducers have an indica tion of exactly which side to mount to the structure In any case it is always a good ide
6. D analog to digital converters with built in signal conditioning ICP or IEPE inputs and built in source channels signal generator to drive the shaker system and provide mechanical excitation to the structure Advanced modal software incorporates robust geometry driven data acquisition wizards analysis methods and algorithms What was extremely difficult or almost an art many years ago has been greatly simplified for easier and faster modal testing In addition to all the recent advancements on the analysis side new methods and algorithms for modal parameter extractions the challenges of acquiring good data for experimental modal analysis are still very real The old adage garbage in garbage out becomes more present than ever if care and attention are not paid to some of the basic practical aspects behind test setups And as another old saying goes a chain is no stronger than its weakest link So in a forced input modal test system special attention must be paid to the excitation setup to ensure good quality data and representative results especially considering that it is the reference measure ment used in the processing of FRFs Accordingly it is critical to understand the practical aspects of shaker setup for modal testing measurements with respect to the force sensor the modal exciter and the power amplifier Obtaining Valid Measurements Sensor Selection Piezoelectric force sensors and piezoelectric impedance heads are
7. Modal Testing Excitation Guidelines Marco A Peres and Richard W Bono The Modal Shop Inc Cincinnati Ohio Electrodynamic shakers or exciters are commonly used in experimental modal analysis The practical aspects regarding the setup of the shakers stingers and transducers are often the source of test difficulties and avoidable measurement errors This article reviews the basics of shakers as beneficial to modal testing and common problems associated with setup issues and resulting measurement errors These include shaker alignment sensor considerations stinger selection amplifiers reciprocity assumptions and other test related circumstances A system setup for modal testing includes several measure ment and test components around the structure under test itself Typically one or more electrodynamic shakers also called modal exciters are employed to provide a known excitation input force to the structure Dynamic transducers are used to measure the input excitation force and the resulting vibration responses Once data are acquired the resulting frequency response functions FRFs obtained by the data acquisition system are stored for post process ing calculations data reduction curve fitting and mode extraction Figure 1 offers a simple representation of the main instrumentation components found in a modal test setup Multichannel dynamic signal analyzers DSAs are required to acquire data Many of today s DSAs have 24 bit A
8. a to refer to the transducer s user manual for identifying which mounting surface is intended to measure the force accurately Secondly because the sensor measures force in only one direc tion a stinger is used to reduce any possible side loads that may be transmitted Note that this piezoelectric force sensor is mounted to a thin rod style stinger that is stiff in the axial direction and flexible in the lateral direction This is detailed later in this article Another important consideration is that the force transducer should always be mounted directly to the test structure between it and the stinger and shaker assembly If the force gage is mounted on the exciter side as shown in the illustration in Figure 3 then the dynamics of the stinger become part of the measured function This is generally only an issue when using a conventional shaker for modal applications modal shakers as we will see later have www SandV com Figure 3 Typical stinger setup showing proper force transducer location rela tive to test article threaded stinger rod and exciter Left configuration shows correct way of mounting force sensor next to structure force gage divorces the stinger shaker from the structure as opposed to mounting the sensor on shaker exciter side where the stinger becomes part of test structure Figure 4 Impedance head mounted in skewed orientation to test struc ture a through hole armature design and would not a
9. during shaker excitation is less that the preload then the piano wire is an excel lent way to transmit force and conduct a modal test eliminating the effects of lateral stiffness in conventional stingers If the applied load during shaker excitation is more than the preload it will cause the wire to buckle and the shaker won t be able to pass the force to the structure This is analogous to an AC signal riding on a DC bias or offset with the equivalent of a mechanical clipping occurring when the AC signal is greater than the DC offset An alternative to the piano wire is a thin rod stinger design see Figures 2 and 4 which also utilizes the through hole armature design available on modal shakers Since this design is a stiff rod rather than a wire it does transmit some amount of force laterally However this style of stinger does not need to be pretensioned greatly simplifying setup As a result it is more commonly used as an acceptable compromise of performance and ease of use The effect of the stinger assembly s lateral stiffness on the overall system is very dependent on the stiffness of the structure being tested If the structure itself is very stiff then this is often not a serious concern However when the structure is very flimsy or has a significant amount of rotational effect at the attachment point of SOUND amp VIBRATION NOVEMBER 2011 11 Figure 10 Example of 31 N 7 Ibf mini shaker with integrated 100 W Cla
10. engineer needs to move the shaker around and try different excitation points This is very common in large channel count modal tests Shaker Quantity and Force Requirements The question on how many shakers are required by a certain modal test is often hard to answer Often test systems are limited by the total number of output sources in the data acquisition system or shakers available SOUND amp VIBRATION NOVEMBER 2011 9 in the test lab for modal testing Usually two to four shakers are sufficient for most tests particularly when testing larger structures like automobiles or aircraft Generally tests with more than five shakers are rare Ultimately there need to be enough shakers act ing as reference locations that are positioned so that the modes of interest of the structure are adequately excited and observed and good frequency response measurements are obtained This includes having multiple shaker reference locations to resolve repeated roots and or closely spaced modes The excitation levels for modal testing are usually reasonably low There is no need to provide large force levels for conducting a modal test especially if appropriate response transducers ac celerometers are selected with good sensitivity and resolution as well as high quality high resolution 24 bit technology is fairly standard in today s commercial offerings data acquisition systems The level only needs to be sufficient enough to make good measure ments
11. its stroke limits way before it exceeds its force capability To minimize this issue often heavy metal block masses are attached bolted to the base of the shaker trunnion to enhance performance providing more double or triple inertia to push against the structure Stingers Theory of Operation As mentioned earlier a stinger is always used on the interface between the shaker and the structure The primary reason for the use of an exciter stinger is to prevent lateral constraint forces and moments By design an exciter applies axial force to the test article with high fidelity Its armature is designed to not have the freedom to move in a lateral direction perpendicular to the force axis The test article on the other hand may have lateral motion at the forcing point This may be due to the geometry of the test article or due to a lateral mode of vibration This is especially true if the test article has a soft suspension If one were to connect the exciter directly to the forcing point the exciter will constrain the article s tendency to move laterally This resistance even if it is only a small effect can cause two www SandV com problems The first is that the force transducer will have a lateral force and moment that will not be measured accurately since it senses properly only along its principal axis The second is that the article feels the combined effect of the intended axial force and the unintended lateral force and
12. llow the force transducer to be mounted incorrectly The force transducer is usually mounted using a threaded adhe sive mounting base Figure 4 firmly attached to the test structure using dental cement two part quick epoxy or a cyanoacrylate type adhesive Super Glue or Loctite Often a piece of foil adhesive tape is first applied to protect the surface of the test article with the mounting pad bonded to the tape Dental cement is ideal because it is extremely stiff providing rigid attachment within the frequency range of typical modal testing If the test structure can be drilled and tapped with an appropriate thread directly attaching the force transducer to the structure is the best solution Electrodynamic Shakers Principle of Operation A shaker or exciter is an electrodynamic transducer consisting of a voice coil attached to a moving armature and a magnet structure with a small gap in which the voice coil moves see Figures 5 and 6 The magnet structure is designed to provide a strong magnetic field across the gap so when the cur rent flows in the voice coil it will experience a force dependent on the strength of the current and the magnetic field Small shakers www SandV com Figure 6 Electromagnetic force equation of conductor immersed in magnetic field F electromagnetic force L length of conductor I current vector B magnetic field vector with sine peak force capability below 500 N or 100 lbf or mos
13. moment As a result the test article would be excited with forces that are not measured at all These effects will show up as errors in the force or frequency response measurements An exciter stinger has a lateral bending stiffness that is much smaller than its axial compression or tension stiffness This means that when the exciter s armature is stationary a small lateral movement of the test article causes a small lateral force at the exciter while a small movement in the axial direction causes a much larger axial force In other words axial forces through the stinger are accompanied by little relative axial motion but lateral forces are accompanied by much larger relative lateral motion The lateral force and moment generated by lateral motion of the test article are therefore reduced An additional advantage of using a stinger is that a flexible stinger is more forgiving with positioning and aligning the exciter at the forcing point Without a stinger you may need to have the mounting centers of the exciter and force transducer within 0 5 mm 0 02 inch or closer to get a good bolted connection This is difficult to do if you have to move the entire exciter and its sup port A stinger can tolerate a misalignment of nearly 10 times this amount especially if the stinger is long This reduces your setup time Furthermore the use of a coupling nut makes attachment and removal easy compared with other connection methods Another adva
14. n a bead of tempo rary adhesive such as hot glue around the edge of the trunnion to secure the shaker during testing This will help to avoid creep during testing which Figure 8 Exploded view of chuck and collet stinger attachment on modal shaker with through hole armature design A force sensor or impedance head B stinger C chuck top piece D collet E chuck bottom piece F armature G modal exciter could cause further misalign ment and measurement errors One way to align the shak er during setup is to use the stinger In setting up a shaker test typically the stinger is slid into the shaker s through hole armature with the force transducer or impedance head attached to the end of the stinger With the shaker collet loosened the stinger can be extended in and out of the armature to obtain the desired length Once this is done the force gage or impedance head mounting pad can be affixed to the structure as explained previously If the alignment is correct the shaker stinger will easily un thread from the force transducer or impedance head and also thread right back in without any binding or difficulty whatsoever This should be accomplished without the stinger putting side load onto the shaker armature sliding easily within the chuck and collet assembly which assures that the shaker and stinger are properly aligned At times there may be a threaded mating hole in the structure www SandV com Figure 9
15. ntage of a stinger is the isolation of the test article from the exciter If a catastrophe should occur either by failure of the test suspension or by a transient voltage into the power am plifier a large force would be created at the connection between exciter and test article The stinger acts as a mechanical fuse as the weakest link absorbing the damage As a result the inexpensive stinger is sacrificed to save the much more expensive exciter and test article Piano Wire Stinger Of course the shaker s dynamic subsystem will never be perfectly decoupled and there will practically always be some slight cross axis force input to the structure As discussed earlier the intent of the stinger design is to be very stiff in the axial direction and extremely compliant to lateral loads to minimize this situation Piano wire stingers are an excellent way to circumvent the problems with lateral stiffness associated with conventional stingers The piano wire is pretensioned with a load that is greater than the alternating load to be applied a preload of three to four times the range is considered reasonable The piano wire is fed through the core of the through hole shaker armature so it is critical to have a modal shaker that is designed to accommodate this A simple preload can be applied with weights or an elastic tie down strap Figure 9 With the preload applied the collet is used to clamp the tensioned piano wire As long as the applied load
16. r pair is selected Voltage Current Mode Amplifiers As the shaker armature and coil move through a magnetic field during normal operation a voltage is induced in the circuit called back electromotive force or simply back EMF The voltage associated with the back EMF is proportional to the shaking velocity and it opposes the current coming from the amplifier The back EMF functions as an electro dynamic damping term in the system Most power amplifiers operate in voltage mode that is the output voltage is proportional to the input voltage waveform with some gain set by the user In addition to voltage mode some amplifiers can also operate in current mode where the amplifier s output voltage is adjusted to maintain the required current on the output to follow the input signal regardless of back EMF gener ated in the system Current mode operation allows measurement of free decay damping of the structure by turning the excitation signal off With current amplifiers the armature of the shaker coil is allowed to freely float after the excitation is terminated which is highly desir able for sine dwell or normal mode tuning normal mode testing 12 SOUND amp VIBRATION NOVEMBER 2011 Current mode is also preferable for studying nonlinearities which is often the case in some aerospace structures It also minimizes potential force dropouts at resonances that can compromise signal to noise ratio of the force excitation measurements
17. r some type of collar was designed to enable an easier attachment to the shaker It was a rather difficult arrangement no matter how the connection was made given a threaded interface on both at tachment ends In addition there had to be some thought given to shaker position and actual length of stinger needed If a different length stinger was needed then the shaker needed to be reoriented and realigned or different stinger lengths were used for the modal test Overall the setup of the traditional shaker for a modal test was very difficult and cumbersome Due to all these problems specific design configurations bet ter suited for modal testing applications were developed In the late 1980s ideas from the University of Cincinnati s Structural Dynamics Research Laboratory gave rise to the through hole ar mature with a chuck and collet design like gripping a drill bit on a hand drill that enabled very easy adjustment and attachment of the shaker to the modal test article as shown in Figure 7 A long stinger can slide into the shaker s through hole armature threaded to the force transducer attached to the test article be properly aligned and then clamped down with the chuck and collet at the appropriate length These components are shown as an exploded view in Figure 8 and a video demonstrating actual installment is available on the Internet This design also easily accommodates stingers of differ ent lengths if needed and the a
18. rrangement is so simple that it is difficult to imagine having to set up the test without this important feature Through hole armature design with a chuck and collet stinger attachment makes test setup so much easier that the term modal shaker usually refers specifically to a shaker that comes with a through hole armature as opposed to a traditional platform table shaker style used for general vibration testing Shaker Mounting and Alignment Proper force excitation requires the thrust axis of the modal shaker to be aligned with the force sensor or impedance head mounted on the structure 10 SOUND amp VIBRATION NOVEMBER 2011 under test Failure to do so may result in unmeasured forces transmitted to the structure due to the side loading of the sensor and possible mechani cal or electrical shaker damage due to forcing and rubbing of the armature coil Alignment issues cause difficulty in any modal test Care must be taken to provide the best alignment possible to attain the best pos sible measurements Modal shakers can be bolted to the floor or any suitable base by using the holes located in the base of the shaker trunnion By loosening the trunnion body the modal shaker s angular posi tion can be adjusted by rotating it in the trunnion base Ergo nomic handles are included in some shakers Figure 7 and 10 to facilitate the task of tighten ing loosening the trunnion and rotating the shaker Often it is helpful to ru
19. rs efficiencies are usually in the 50 to 75 range Switching amplifiers Class D are the most common type used in new designs for power amplifiers Theoretical power efficiency of Class D amplifiers is 100 That is to say all of the power supplied to itis delivered to the load none is turned to heat Real life practi cal efficiencies well over 90 are common allowing the design to be extremely power efficient lightweight and compact Figure 10 is a good example where amplifiers are now being integrated to some shaker designs Due to its high efficiency and low heat dissipation fans and large heat sinks are very small or simply not needed as opposed to the large ones always present on traditional linear amplifiers Conclusions This article presents some practical guidelines and experience based insight to effectively perform a modal test The review was presented without the use of any detailed mathematical relation ships Attention to excitation test setup details is critical for the acquisition of quality frequency response function measurements which are fundamental for the modal extraction analysis and consistent results Acknowledgements The authors would like to acknowledge Professor Peter Avi tabile from University of Massachusetts Lowell and Professor David Brown from University of Cincinnati for their invaluable support discussions and contributions to the aspects and topics reviewed here References 1 Merkel
20. ss D amplifier bottom the stinger then these lateral loads can become very important and a source of large measurement error In addition these rotational effects generally become more important at higher frequencies so it is always difficult to determine that actual impact on the overall results One easy way to determine the stinger lateral and rotational effects is to make several test runs with the length of the stinger varying by 10 and observe the change in the measured drive point frequency response Reference 5 provides a good overview and comparison on stinger types and effects on measured FRFs Shaker Amplifiers A power amplifier is always needed to provide the necessary energy to drive the shaker Many considerations come into play when selecting the right power amplifier for the shaker As elec trodynamic shakers are usually low impedance devices one must ensure the amplifier selected can indeed drive the shaker to its desired performance Compatibility between the shaker and ampli fier is fundamental along with other requirements such as broad frequency range low frequency response power rating power efficiency low harmonic distortion safety features interlocks etc Using a power amplifier made or recommended by the shaker manufacturer is the safe choice it typically guarantees the perfor mance characteristics of the shaker system sine or random force capability which can only be stated once a shaker and amplifie
21. t modal shakers in the market today typically use high strength permanent magnets Larger shakers use electromagnets instead field coils An alternating electric current in the voice coil causes the shaker to move forward and backwards in the magnetic field causing the armature and the test article to vibrate accordingly to a certain input signal Several different magnet systems have been used in electrody namic exciters They typically consist of a cast magnet of a special magnetic alloy or a ceramic material In general the greater the mag netic flux the greater the efficiency of the shaker Al Ni Co magnets an alloy of aluminum nickel and cobalt became available in the early 1930s and have been used in electrodynamic shakers until the 1980s In the mid 80s rare earth magnets became available and almost all modern shaker designs benefit from neodymium magnets based on an alloy of the rare earth metal neodymium iron and boron These magnets are about four times stronger than Al Ni Co magnets for a given size which offers a great benefit for shaker performance and usability Modern shakers can deliver forces more efficiently and can be constructed much lighter than previ ously Shaker manufacturers have been able to achieve up to a 67 reduction in weight enabling truly one man handling for the exciter In summary new lightweight shakers are easier to handle and fixture during installation especially when the test
22. the two most common transducers used for measuring input forces An impedance head is nothing more than a transducer that measures both force and resulting driving point response in one device Today an impedance head is typically an accelerometer and force transducer built together but it was origi nally based on a velocity transducer and force transducer This is where the name impedance head comes from and has lingered on today even though velocity is not normally measured This is a critical measurement in experimental modal analysis and it is recommended that impedance heads be used in most cases A com bination of a separate force transducer and accelerometer mounted next to each other is often used instead but the convenience and accuracy of measuring the driving point excitation with a single transducer and validating reciprocity between input locations is best obtained with an impedance head A force transducer with sensitivities in the range of 11 to 22 Based on SAE paper 2011 01 1652 presented at the 2011 Noise and Vibra tion Conference Grand Rapids MI May 2011 Copyright 2011 SAE International 8 SOUND amp VIBRATION NOVEMBER 2011 FFT Analyzer Structure Sines swept sing i r E anom Shaker oe Peien Amplifier Periodic random chap Figure 1 Typical modal test setup Figure 2 Force transducer shown installed on modal stinger mV N 50 to 100 mV lbf allows forces up to 445 N
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