Abstract:
A shaker assembly for vibration testing includes first and second shakers, where the first shaker includes a piezo-electric material for generating vibration. A support structure permits a test object to be supported for vibration of the test object by both shakers. An input permits an external vibration controller to control vibration of the shakers.

Description:
This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present work relates generally to vibration testing and, more particularly, to generation of wide frequency band vibration forces. 
     BACKGROUND OF THE INVENTION 
     Conventional shakers used for vibration testing are limited in their ability to effectively apply a wide range of vibration frequencies to test objects of different masses. For effective vibration testing, the shaker must produce an output vibration force that is suitably proportional to the mass of the test object. As an example, conventional hydraulic shakers are generally not capable of generating vibration at relatively high frequencies (e.g., greater than about 300 Hz). On the other hand, conventional electro dynamic shakers are technically capable of generating sufficiently high output force at relatively high frequencies, but with attendant high electrical power consumption and associated expense. Conventional piezo-electric (PE) actuators are capable of producing sufficiently high output force at relatively high vibration frequencies, but have limited ability to generate that same level of force at relatively low vibration frequencies (e.g., less than about 200 Hz). 
     It is desirable in view of the foregoing to provide for application of high output vibration forces across a range of vibration frequencies that is wider than permitted by conventional technology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  diagrammatically illustrates an apparatus for vibration testing according to exemplary embodiments of the present work. 
         FIG. 2  diagrammatically illustrates an apparatus for vibration testing according to further exemplary embodiments of the present work. 
         FIG. 3  diagrammatically illustrates an apparatus for vibration testing according to still further exemplary embodiments of the present work. 
         FIG. 4  diagrammatically illustrates the shaker assembly of  FIGS. 1-3  in more detail according to exemplary embodiments of the present work. 
         FIG. 5  illustrates structural mounting and connection details of the shaker assembly of  FIGS. 1-4  according to exemplary embodiments of the present work. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present work mount the unit under test (test object) to a test platform connected to a PE shaker and mount the test platform with the PE shaker to the head of a further shaker, thus providing combined excitation to the unit under test. The further shaker, for example a hydraulic or electro dynamic shaker, generates controlled vibration for a relatively lower frequency range (e.g., 5-400 Hz), and the PE shaker generates controlled vibration for a relatively higher frequency range (e.g., 400-2,000 Hz). The further shaker serves as a reaction mass to the PE shaker. In some embodiments, a single accelerometer is used to control both shakers. In various embodiments, the shakers operate in various frequency ranges depending on shaker and test specifications. 
       FIG. 1  diagrammatically illustrates an apparatus for vibration testing according to exemplary embodiments of the present work.  FIG. 1  shows a shaker assembly that includes at  12  an arrangement of conventional PE vibration actuators  15  that collectively constitute a first shaker, also referred to herein as a PE shaker or a high frequency shaker. The PE shaker is mounted on a second shaker  11 , for example, a conventional hydraulic shaker or a conventional electro dynamic shaker. The second shaker  11  (also referred to herein as a low frequency shaker or LF shaker) provides high output force vibration in a relatively lower frequency range (5-400 Hz in some embodiments), and the first shaker  12  provides high output force vibration in a relatively higher frequency range (400-2,000 Hz in some embodiments). 
     A test object  13  is mounted on the PE shaker  12 . A control accelerometer  14  provided on the test object  13  provides a feedback control signal  102  indicative of vibration of the test object. In some embodiments, the accelerometer is provided on the PE shaker  12 . The feedback control signal  102  from the accelerometer  14  is provided to a control system, for use in controlling vibration of the shakers  11  and  12 . In the embodiments illustrated by  FIG. 1 , the control system includes conventional SISO (single input, single output) vibration controllers  16  and  17 , which are responsive to the signal  102  for respectively controlling the vibration produced by shakers  12  and  11 . The various vibration controllers described herein are also referred to by “VCS” (vibration control system). The SISO controllers  16  and  17  receive further control input from respective control computers  18  and  19 . The control computers  18  and  19  are programmed with specifications of the desired vibration test operations to be implemented by the respective shakers  12  and  11 , and operate, according to conventional techniques, to provide the respective SISO controllers  16  and  17  with control information indicative of the desired vibration test operations. As shown in  FIG. 1 , output control signals produced by the SISO controllers  16  and  17  are amplified by conventional amplifiers, and the resulting amplified control signals  100  and  101  are respectively provided to the shakers  11  and  12 . 
     As indicated above, in some embodiments, the LF shaker  11  may be activated to vibrate the test object  13  (along with the PE shaker) in a range of, for example, 5-400 Hz, and the PE shaker  12  may be activated to produce vibration in a range of, for example, 400-2,000 Hz. Advantageously, a test object of, for example, 5 lbs. or more, may be vibrated with sufficient force, across the entire 5-2,000 Hz frequency range of this example, with the test object mounted on the single shaker assembly of  FIG. 1 . In various embodiments, the PE and LF shakers are cooperable to vibrate objects of various weights across various frequency ranges. 
     In some embodiments, the LF shaker  11  provides vibration along a first axis from 15-200 Hz, and the PE shaker  12  provides vibration along a second axis from 200-2,000 Hz, with the first axis generally perpendicular to the second axis. For example, the first axis may be horizontal and the second axis may be vertical. In some embodiments, the respective shakers have parallel vibration axes. Some embodiments provide multiple pairs of these parallel vibration axes, with the parallel axis pairs arranged orthogonally relative to one another to create multi-axis vibration 
       FIG. 2  diagrammatically illustrates another apparatus for vibration testing according to further exemplary embodiments of the present work. The apparatus of  FIG. 2  is similar to that of  FIG. 1 , except a further control accelerometer  21  is provided on the LF shaker  11  to provide a further feedback control signal  22  indicative of vibration of the shaker LF  11 . The SISO controller  17  receives this further feedback control signal  22  instead of the feedback control signal  102 . The separate feedback control signals  22  and  102  are useful, for example, in embodiments wherein the shakers  11  and  12  are operated simultaneously to subject the test object  13  to a resultant combination of vibration forces produced by both shakers. 
       FIG. 3  diagrammatically illustrates another apparatus for vibration testing according to further exemplary embodiments of the present work. The apparatus of  FIG. 3  is similar to that of  FIG. 1 , except the SISO controllers  16  and  17 , and the control computers  18  and  19 , are replaced by a single MIMO (multiple input, multiple output) controller  31  and an associated control computer  32 . The MIMO controller  32  is responsive to the feedback control signal  102  for implementing conventional MIMO control operations to produce control signals for both shakers  11  and  12 . The control computer  32  is programmed with specifications of the desired vibration test operations to be implemented by both shakers  11  and  12 , and operates, according to conventional techniques, to provide the MIMO controller  31  with control information indicative of the desired vibration test operations. 
       FIG. 4  illustrates the shaker assembly of  FIGS. 1-3  in more detail according to exemplary embodiments of the present work. In the example shaker assembly  41  of  FIG. 4 , the PE shaker  12  is mounted to a mounting structure  43  that is adapted to mount the target object  37  to the PE shaker  12 . Another mounting structure  42  is mounted to the PE shaker  12  opposite the mounting structure  43 . The mounting structure  42  mounts the PE shaker  12  to the LF shaker  11 . In addition to providing low frequency vibration, the LF shaker  11  provides a reaction mass for the PE shaker  12 . 
       FIG. 5  illustrates a still more detailed example of the shaker assembly  41  of  FIG. 4  according to exemplary embodiments of the present work. The mounting structures  42  and  43  are rigid plates in the example of  FIG. 5 . The plates  42  and  43  have provided therein respective patterns of holes that provide PE actuator mounting sites. The PE actuators  15  (see also  FIG. 1 ) are interposed between and bolted (or otherwise suitably fastened) to the mounting plates  42  and  43  with the holes in the mounting plates  42  and  43  suitably aligned. The mounting plate  42  includes additional mounting holes for cooperation with mounting holes provided in the LF shaker  11  (not shown in  FIG. 5 ), so the PE shaker  12  may be mounted on the LF shaker  11  using bolts or any other suitable type of attachment 
     In some embodiments, the LF shaker  11  is The Cube™, a hydraulic shaker commercially available from Team Corporation. 
     Although the PE and LF shakers operate in substantially non-overlapping frequency ranges in the embodiments detailed above, in other embodiments, the PE and LF shakers operate in overlapping frequency ranges. 
     Although exemplary embodiments of the present work are described above in detail, this does not limit the scope of the work, which can be practiced in a variety of embodiments.