Abstract:
A method of operating a vibratory testing apparatus is disclosed including providing a table frame with at least one vibrator attached thereto and attaching the at least one vibrator to a solenoid valve, wherein an input of the solenoid valve is connected to a pneumatic air supply and an output of the valve is connected to the vibrator. The solenoid valve is connected to a controller and a first control signal is sent from the controller to the solenoid valve for opening the valve and allowing a first burst of air to the vibrator thereby causing the vibrator to vibrate the table frame at a first amplitude. Then a second control signal is sent from the controller to the solenoid valve for opening the valve and allowing a second burst of air to the vibrator thereby causing the vibrator to vibrate the table frame at a second amplitude.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention is related generally to vibratory testing equipment and, more particularly, to a method for improved vibratory testing and a system for implementing that method. 
       BACKGROUND OF THE INVENTION 
       [0002]    Few products are sold by their manufacturer without some type of testing being conducted. Such testing may be as simple as manually ascertaining whether certain parts are securely affixed—or as complex as “stress testing.” In stress testing (or “stress screening” as it is sometimes called), products exhibiting “infant mortality” fail outright during the test. Or as the result of such testing, a product may display evidence of early failure in the operating environment. 
         [0003]    One of the most common methods of stress testing involves testing a product by subjecting it to vibrations of the type, which might be encountered in actual product use. For example, U.S. Pat. No. 2,438,756 (Larsen) explains that the apparatus described therein is used to vibration-test electrical apparatus for airplanes, ships and the like. The unit described in U.S. Pat. No. 3,748,896 (Barrows) is said to be used for testing parts of a motor vehicle. And vibration testing is often conducted in conjunction with testing using another regimen, e.g., temperature. 
         [0004]    One type of vibration testing is known as repetitive shock testing. Such testing 
         [0005]    generally accomplished by utilizing a testing apparatus consisting of a table frame that is vibrated by a number of vibrators which impart vibration through impacts occurring in each vibrator. These vibrators are generally pneumatically powered. During the testing process a uniform vibration response is desirable because it ensures that all components being tested are exposed to approximately equal vibration levels over the entire table frame. 
         [0006]    Many different vibrator designs have been developed for use in vibratory testing systems. The primary focus of these designs to date have been to create a vibrator that imparts vibration onto a table frame and thus onto the object to be tested. These designs vary the physical design of the vibrator in order to create a vibrator that is capable of free running when connected to a supply of pressurized air. For example, U.S. Pat. Nos. 5,154,567 to Baker et al., 5,365,788 to Hobbs, and 5,493,944 to Felkins et al. all utilize varied channels and/or cut-outs on the piston within the chamber to create a vibrator that is capable of tree running once connected to an air source. In all of these designs the strength of the impacts and the frequency of the impacts generally increases as the pressure of the air supply is increased. In addition, some vibrator designs, such as &#39;788 patent to Hobbs, allow the vibrator to randomly vary the strength of the impacts through the mechanical design of the piston itself. 
         [0007]    The performance of a vibrator is usually shown as a power spectral density (PSD) which can be depicted as a chart showing g 2 /Hz over a determined number of different frequencies (Hz).  FIGS. 17 and 18  show two such examples.  FIG. 17  shows the PSD of a typical, impactor free running at 30 Hz. As can be seen, the chart shows numerous peaks at the harmonies of 30 Hz; this is commonly known as the “picket fencing” of the PSD. As understood by those of skill in the art, these peaks are not desirable since they represent frequencies at which the product is not properly tested. As shown in  FIG. 18 , by modulating the air pressure into that typical vibrator the peaks of the “picket fence” are reduced and widened. 
         [0008]    Another measurement of performance of vibrators is the acceleration imparted by each impact. As described above, in many typical vibrators as die pressure of the air supply is increased the amplitude of the acceleration and frequency of the impacts increase together. The effect is seen in  FIGS. 8-10  which show the strength and number of impacts overtime at high, medium aid low pressures respectively. 
       OBJECTS OF THE INVENTION 
       [0009]    It is an object of some embodiments of the invention to provide an improved method and apparatus for controlling the vibrators of a vibratory testing system that overcomes some of the problems and shortcomings of the prior art, including those referred to above. 
         [0010]    Another object of some embodiments of the invention is to provide an improved method and apparatus for testing a product. 
         [0011]    Another object of some embodiments of the invention is to provide an improved method and apparatus for running a vibratory testing apparatus that allows for control of the amplitude of tire acceleration, and/or frequency of the impacts within the vibrators. 
         [0012]    Another object of some embodiments of the invention is to provide an improved method and apparatus for running a vibratory testing apparatus that efficiently utilizes pressurized air. 
         [0013]    How these and other objects are accomplished will become apparent born the following descriptions and the drawings. 
       SUMMARY OF THE INVENTION 
       [0014]    In a first embodiment of the present invention a method of operating a vibratory testing apparatus is disclosed. The method includes providing a table frame with at least one vibrator attached thereto and attaching the at least one vibrator to a solenoid valve, wherein an input of the solenoid valve is connected to a pneumatic air supply and an output of the valve is connected to the vibrator. The solenoid, valve is connected to a controller and a first control signal is sent from the controller to the solenoid valve for opening the valve and allowing a first burst of air from the pneumatic air supply to the vibrator thereby causing the vibrator to vibrate the table frame at a first amplitude. Then a second control signal is sent from the controller to the solenoid valve for opening the valve and allowing a second burst of air from the pneumatic air supply to the vibrator thereby causing the vibrator to vibrate the table frame at a second amplitude. 
         [0015]    In some embodiments the solenoid valve is a two-way valve, while in other embodiments the solenoid valve is a four-way valve. In preferred embodiments the at least one vibrator includes a sealed body having first and second ends and defining a cavity therein, the cavity having first and second ends. A first air passage is defined in the sealed body near the first end of the body and is configured to allow gas to flow to and from the first end of the cavity. A second air passage is defined in the sealed body near the second end of the body and is configured to allow gas to flow to and from the second end of the cavity. A piston is sealed within the cavity and moveable between the first and second ends. 
         [0016]    In another embodiment, the solenoid valve is a four-way solenoid valve including an in port, two layout ports and an exhaust port. In such an embodiment the step of connecting the at least one vibrator to a solenoid valve includes connecting the first air passage to a first in/out port and connecting the second air passage to a second in/out port. In a preferred version of that embodiment the first burst of air causes the piston to impact the vibrator at the second end of the cavity and the second burst of air causes the piston to impact the vibrator at the first end of the cavity. In some versions the impact of the piston on the second end of the cavity includes a primary impact and at least one secondary impact, in highly preferred versions the impact of the piston on the first end of the cavity includes a primary impact and at least one secondary impact. 
         [0017]    However, in other embodiments of the invention, the first air burst may cause the piston to impact at the second end and then return via gravity and/or rebounding to its starting point. The second burst will then cause the piston to impact again at the second end of the vibrator at an amplitude independent of the first impact. Each impact may still include a primary and secondary impacts before returning to the starting position. 
         [0018]    In a further embodiment the controller varies the amount of time between the first burst of air and the second burst of air, thereby varying the amount of times per time period that the piston impacts either of the ends of the cavity. In a preferred version of that embodiment the controller randomly varies the number of impacts per time period. 
         [0019]    In yet another embodiment, the method includes the step of attaching an accelerometer to the table frame and to the controller, whereby the controller receives data from the accelerometer and varies the amplitude of the impacts by the piston based upon data from the accelerometer. The controller can then independently vary the frequency of the impacts and the amplitude of the impacts. 
         [0020]    In still further embodiments of the present invention a vibrator system for use with a vibratory testing table is disclosed. The vibrator system includes at least one vibrator having a sealed body with first and second ends and defining a cavity therein, the cavity having first and second ends. A first air passage is defined in the sealed body near the first end of the body and is configured to above gas to flow to and from the first end of the cavity. A second air passage is defined in the sealed body near the second end of the body and is configured to allow gas to flow to and from the second end of the cavity. A piston is sealed within the cavity and moveable between the first and second ends. A four-way solenoid valve having an in port, two in/outposts and an exhaust port and connects the first air passage of the vibrator to a first in/out port and the second air passage to a second in/out port. A pneumatic air supply is connected to the in port of the four-way solenoid valve and a controller is connected to the four-way solenoid valve, the controller capable of controlling the four-way solenoid valve to allow air from the pneumatic air supply to the vibrator in bursts. One burst moving the piston to impact with the body of the vibrator at one of the ends of the cavity and a subsequent burst moving the piston to impact with the body of the vibrator at the opposite end of the cavity, whereby the frequency of the impacts is controlled via the controller. In preferred versions the controller also is capable of controlling the amplitude of the impacts. 
         [0021]    In another embodiment the vibrator system includes a plurality of vibrators. In a preferred version the system also includes a plurality of four-way solenoid valves, each four-way solenoid valve connected to one of the plurality of vibrators and connected to the controller. 
         [0022]    In further embodiments the vibrator system, includes a regulator connected to the system between the pneumatic air supply and the four-way solenoid valves, the regulator controlled by the controller. Also a plurality of two-way solenoid valves may be attached. Each two-way solenoid valve is connected to the system between the pneumatic air supply and a four-way solenoid valve and connected to the controller and each two-way solenoid valve includes an in port connected to the pneumatic air supply and an out port connected to the in port of the four-way solenoid valve. 
         [0023]    In yet another embodiment of the invention a method of controlling the movement of a piston of a vibrator as part of a vibratory testing apparatus is disclosed. The method includes providing a vibratory testing apparatus including at least one vibrator. The vibrator includes a piston enclosed within a cavity of the vibrator and is attached, to a provided power system for powering the vibrator. A controller is provided and attached to the power system, the controller capable of controlling the power system. The controller activates a first burst of power from the power system to the vibrator, whereby the piston is moved within the cavity to impact within the vibrator at a first amplitude. Then the controller activates a second burst of power from the power system to the vibrator, whereby the piston is moved within the cavity to impact the vibrator at a second amplitude, wherein the second amplitude is independent of the first amplitude. Therefore, the controller controls the frequency of impacts by controlling the amount of time between sending the first and second control signals. 
         [0024]    The systems described herein can be powered via any methods or manners known in the art such as electric, hydraulic or air power. However, in preferred versions the power system is a pneumatic air system. It is preferred that the pneumatic air system includes a pneumatic air supply connected to a four-way solenoid valve, wherein the solenoid valve is connected to the at least one vibrator and to the controller. It is also preferred that the vibratory testing system includes a plurality of vibrators each connected to the pneumatic air supply via the four-way solenoid valve. 
         [0025]    In still further versions of such embodiments, the pneumatic air system includes a plurality of four-way solenoid valves and each four-way valve is connected to a corresponding vibrator and the controller. In other embodiments the pneumatic air system includes a plurality of two-way solenoid valves and each two-way solenoid valve is connected to the pneumatic sir supply, a corresponding four-way solenoid valve and the controller, 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic view of a vibratory testing system according to an embodiment of the present invention; 
           [0027]      FIG. 2  is a sectional view of a vibrator of the system of  FIG. 1  in a first position; 
           [0028]      FIG. 3  is a sectional view of a vibrator of the system of  FIG. 1  in a second position; 
           [0029]      FIG. 4  is a perspective view of the vibrator of  FIGS. 2 and 3 ; 
           [0030]      FIG. 5  is a schematic view of the operation of the vibrator in a first position; 
           [0031]      FIG. 6  is a schematic view of the operation of the vibrator in a second position; 
           [0032]      FIG. 7  is a schematic view of a vibratory testing system according to an alternative embodiment of the present invention; 
           [0033]      FIG. 8  is an acceleration chart of a prior art system operating at high pressure; 
           [0034]      FIG. 9  is an acceleration chart of a prior art system operating at medium pressure; 
           [0035]      FIG. 10  is an acceleration chart of a prior art system operating at low pressure; 
           [0036]      FIG. 11  is a PSD of a prior art system operating at 65 Hz; 
           [0037]      FIG. 12  is a PSD of a prior art system operating at 45 Hz; 
           [0038]      FIG. 13  is a PSD chart of the result of the system of  FIG. 1  operated at 10 Hz with a 50 grms setpoint; 
           [0039]      FIG. 14  is a PSD chart of the result of the system of  FIG. 1  operated at 10 Hz with a 50 grms setpoint; 
           [0040]      FIG. 15  is a PSD chart of the result of the system of  FIG. 1  operated at 5 Hz with a 50 grms setpoint; 
           [0041]      FIG. 16  is a PSD chart of the result of the system of  FIG. 1  operated at 5 Hz with a 25 grms setpoint; 
           [0042]      FIG. 17  is a PSD chart of a prior art system free-running at 30 Hz; 
           [0043]      FIG. 18  is a PSD chart of a prior art system modulating the air pressure to the vibrator; 
           [0044]      FIG. 19  is a PSD chart of the system of  FIG. 1  operating at random frequencies; 
           [0045]      FIG. 20  is an acceleration chart of the result of the system, of  FIG. 1  showing single impacts in two directions; and 
           [0046]      FIG. 21  is an acceleration chart of the result of the system, of  FIG. 1  showing multiple impacts in two directions. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0047]    Referring now to  FIG. 1 , a schematic drawing of a preferred embodiment of a vibratory testing apparatus  10  is shown. In this embodiment the vibratory testing apparatus  10  is powered via compressed air from a pneumatic air supply  12 . The vibratory testing apparatus includes a table frame  14  having a top side  16  and a bottom side  18 . A plurality of vibrators  20  are attached to the bottom side  18  of the table frame  14  and serve to impart vibration energy to the table frame  14 . 
         [0048]    The vibrators  20  are powered from the pneumatic air supply  12 . The air supply  12  is first routed into a regulator  22  that regulates the air from the air supply  12  to a known pressure. From the regulator  22  the air supply  12  is fed to a plurality of electronically controlled two-way solenoid valves  24 . As is understood in the art, all connections between the air supply  12  and the vibrators  20  are made through tubing capable of transporting the pressurized air front the air supply  12 . In the schematic drawings these are represented by dotted lines between the connected parts. In this preferred embodiment there is one two-way solenoid valve  24  associated with each vibrator  20 . From each two-way solenoid valve  24 , air is supplied to a corresponding four-way solenoid valve  26 . As shown in  FIGS. 5 and 6 , the airflow from the two-way solenoid valve  24  is connected, to the input  28  of the four-way solenoid valve  26 . The four-way solenoid valve  26  also includes a first in/out port  30 , a second in/out port  32  and an exhaust port  34 . The first  30  and second  32  in/out ports are in turn connected to a vibrator  20 . 
         [0049]    Referring again to  FIG. 1 , each of the two-way valves  24  are connected to a first solenoid driver  25  and each of four-way valves  26  are connected to a second solenoid driver  27 . The solenoid drivers  25 ,  27  are in turn connected to a controller  23 . As is understood in the art, the solenoid drivers control the electrical signals that cause the solenoid valves  24 ,  26  to open and close. Furthermore, as would be understood by one of skill in the art the first  25  and second  27  solenoid drivers could be incorporated into a single driver. In the schematic view of  FIG. 1 , all electrical connections between the various parts of the vibratory testing apparatus  10  are shown as solid lines. The controller  23  sends activation signals to the solenoid drivers  25 ,  27 , thereby causing the valves to switch between positions. 
         [0050]    Referring now to  FIGS. 2 and 3 , cut-away drawings of a preferred embodiment of a vibrator  20  is shown. The vibrator  20  includes a sleeve  36  giving the vibrator  20  opposite first  38  and second  40  ends. An impact block  42  is disposed at each of the first  38  and second  40  ends of the sleeve  36 . The sleeve  36  defines an interior cavity  45  of the vibrator. Each of the impact blocks  42  includes a passage  43  that allows air to flow from outside of the vibrator  20  into the interior cavity  45 . A piston  46  is disposed slidably within the interior cavity  45  of the sleeve  36 . It is preferred that the sleeve  36  and the piston  46  are fitted to limit air passage between them without the use of o-rings or the like. 
         [0051]    At the impact block  42  of the second end  40  of the vibrator  20 , the vibrator  20  is attached to a mounting block  58 . The mounting block  58  includes a vibrator side  60  attached to the vibrator  20  and a mounting side  62  for attachment to the table frame  14 . Generally attachment to the table frame  14  is accomplished via bolts or the like (not shown) which pass through a mounting sleeve  64  of the mounting block  58  as is known in the art. The mounting block  58  further includes a second port  66  which is aligned in communication with the passage  43  of the impact block  42  at the second end  40  of the vibrator  20 . Therefore, this second port  66  allows passage into the interior cavity  45  of the vibrator  20 . 
         [0052]    As can be best seen in  FIG. 4 , a set of rods  48  extend from the vibrator side  60  of the mounting block  58  and extend past the first end  38  of the sleeve  36 . An endcap  50  is fit over the rods  48  and is secured to the vibrator  20  via a nut  52  and washer  54  arrangement. The endcap  50  further includes a first port  56  that is aligned in communication with the passage  43  of the impact block  42  at the first end  38  of the vibrator  20 . Therefore, this first port  56  ( FIGS. 2 and 3 ) allows passage into the interior cavity  45  of the vibrator  20 . 
         [0053]    Referring now to  FIGS. 5 and 6 , the operation of the vibrators  20  is shown schematically. Air flows from the pneumatic air supply  12  and into each individual two-way valve  24 . The two-way valve  24  serves to supply pressure to the in port  28  of the four-way valve  26 . It is preferred that pressure be supplied via either of two methods. In a first method the two-way valve  24  is almost directly connected to the four-way valve  26 . Thus, when the two-way valve  24  is put into an open position air flows directly into the four-way valve  26  and the amount of pressure supplied to the four-way valve  26  is close to a direct ratio of the amount of air passed through the two-way valve  24  each time it is opened or cycled. In a second method the two-way valve  24  and the four-way valve  26  are separated by a length of line between them allowing a build-up of pressurized air between the valves. In this second method the purpose of the length of line is to remove any substantial effect on the pressure in the line when the four-way valve  26  cycles. Thus, the size or length of this line is based upon the volume of the air within the line; since minimizing pressure change in the line would be accomplished by each cycling of the four-way valve not substantially (preferably less than a +/−5% change in pressure) effecting the pressure in the line. As a result, when the two-way valve  24  is opened, air flows into the line rather than directly into the four-way valve  26  and the timing of the operation of the four-way valve  26  is independent of the operation of the two-way valve  24 . However, intermediate lengths or volumes of line can be utilized that range between these two methods. For example, a volume of line that allows +/−30% change has been utilized. 
         [0054]    The first in/out port  30  of the four-way valve  26  is connected to the first port  56  of the vibrator  20 . The second in/out port  32  is connected to the second port  66  of the vibrator  20 . In  FIG. 5  the four-way valve  26  is shown to a first position. In this first position, air from the two-way valve  24  flows through the in port  28  of the four-way valve  26  and into the vibrator  20  through the first in/out port  30 . The air enters the vibrator  20  through the first port  56  via the associated passage  43  and into the cavity  45 . This flow of air is triggered by the controller  23  which causes a first control signal to the second solenoid driver  27  to open the connection between the in port  28  and the first in/out port  30  of the four-way valve  26 . This flow of air causes the piston  46  to move within the cavity  45  of the sleeve  36  from its starting position at or near the first end  38  of the vibrator  20  toward the second end  40  of the vibrator  20 . 
         [0055]    Simultaneously, the second in/out port  32  of the four-way valve  26  is connected to the exhaust port  34  of the four-way valve  26 . Therefore, as the piston  46  moves within the cavity  45  the air within the cavity  45  opposite the first end  38  is expelled. The piston  46  then continues its movement and impacts upon the impact block  42  at the second end  40  of the vibrator  20 . In this preferred embodiment, the piston  46  rebounds from this primary impact partially back toward the first end  38  of the vibrator  20 . The remaining pressure from the four-way valve  26  into the cavity  45  at the first end  38  of the vibrator  20  then causes the piston  46  to have a secondary impact with the impact block  42  at the second end  40  of the vibrator  20 . There may be a series of secondary impacts based upon the pressure utilised. 
         [0056]    Referring now to  FIG. 6 , the controller  23  then sends a second control signal to the second solenoid driver  27  which causes the four-way valve  26  to switch to a second position and connect the in port  28  to the second in/out port  32 . In this second position, air from the two-way valve  24  flows through the in port  28  of the four-way valve  26  and into the vibrator  20  through the second in/out port  32 . The air enters the vibrator  20  through the second port  66  through the associated passage  43  and into the cavity  45 . This flow of air causes the piston  46  to move within the cavity  45  of the sleeve  36  from its starting position at or near the second end  40  of the vibrator  20  toward the first end  38  of the vibrator  20 . 
         [0057]    Simultaneously, the first in/out port  30  of the four-way valve  26  is connected to the exhaust port  34  of the four-way valve  26 . Therefore, as the piston  46  moves within the cavity  45  the air within die cavity  45  opposite the second end  40  is expelled. The piston  46  then continues its movement and impacts upon the impact block  42  at the first end  38  of the vibrator  20 . In this preferred embodiment, the piston  46  rebounds from this primary impact partially back toward the second end  40  of the vibrator  20 . The continued pressure from the four-way valve  26  into the cavity  45  at the second end  40  of the vibrator  20  then causes the piston  46  to have a secondary impact with the impact block  42  at the first end  38  of the vibrator  20 . There may be a series of secondary impacts based upon the pressure utilized. In most operations because of the speed at which the switching occurs, the secondary impacts would continue to occur until the controller  23  sends another control signal to the valve  26  to switch back to the first position. 
         [0058]    Through the use of this method of operation both the frequency of the impacts and the amplitude of the impacts can be controlled. The frequency of the impacts is controlled by the controller  23  which sends control signals to the second solenoid driver  27  thus controlling each opening and closing of the four-way valve  26 . Each primary impact is the result of the controller  23  signaling the four-way valve  26  to switch positions and each switch is equal to one primary impact. The amplitude of the impacts is controlled via the amount of pressure supplied to the four-way valve  26  for each impact. The controller  23  sends control signals to the first solenoid driver  25  causing the two-way valve  24  to open and close to regulate the amount of pressurized air coming into the four-way valve  26 . During the operation of the vibration testing apparatus  10  an accelerometer  68  may be positioned on the table frame  14  and connected to the controller  23 . As is known in the art the controller  23  can then utilize data from the accelerometer  68  to determine how to control the vibrators during the continuing operation. This is accomplished by conditioning the signal from the accelerometer (reference number  31  in  FIG. 1  represents this signal conditioning) to produce a process variable. This process variable is then compared by the controller to the setpoint which has been entered into the controller. The controller then varies the amplitude of the impacts based upon if the process variable is higher or lower than the setpoint. 
         [0059]    Referring now to  FIG. 7  an alternative embodiment of a vibratory testing apparatus  10  is shown. In this embodiment a single pilot controlled regulator  70  is utilized to supply air to the four-way valves  26 . This pilot controlled regulator would preferably be in addition to the primary regulator  22  of the pneumatic air supply  12 . The pilot controlled regulator system  70  is attached to the controller  23  which can thereby regulate the pressure being sent to the four-way valves  26 . Otherwise the operation of the embodiment is the same as described above. 
         [0060]    The operation of either of these embodiments allows for improved performance over prior vibratory testing systems. As discussed in the background section, typical vibration systems currently utilized in the held impart energy based upon vibrators that are free-running. This means that the vibrators run on a generally constant air feed and that the frequency and amplitude of the impacts within the vibrator are directly tied to each other. A decrease in the air supply pressure results in a decrease of both the vibrator operating frequency and the amplitude of the impact.  FIGS. 8-10 , show the operation of prior art vibrators in the Time Domain. The Time Domain graphs show that as the air supply pressure is decreased both the amplitude of the impact measured in acceleration (g) and the operating frequency measured in (Hz) decrease. This is typical of the prior art vibrator.  FIGS. 11 and 12  show the prior art vibrator are directly tied to each other. A decrease in the air supply pressure results in a decrease of both the vibrator operating frequency and the amplitude of the impact.  FIGS. 8-10 , show the operation of prior art vibrators in the time Domain. The Time Domain graphs show that as the air supply pressure is decreased both the amplitude of the impact measured in acceleration (g) and the operating frequency measured in (Hz) decrease. This is typical of the prior art vibrator.  FIGS. 11 and 12  show the prior art vibrator operated at 65 Hz and 44 Hz to get acceleration levels of 50 grms and 25 grms respectfully. The first large spike is the vibrator operating frequency and show a decrease from 65 Hz to 44 Hz to lower the acceleration level from 50 grms to 25 grms. Note that the vibrator can not operate at 44 Hz to get an acceleration level of 50 grms or 65 to get an acceleration level of 25 grms. 
         [0061]    In comparison,  FIGS. 13 and 14  show that the preferred embodiment vibrators described above can be operated at a set frequency (10 Hz) but at different acceleration levels (grms). The grms levels shown are 50 and 25 respectfully. The only way that the acceleration level can change while operating the vibrator at 10 Hz frequency is that the amplitude of the impacts has to change. The controller can decrease the air supply pressure therefore the amplitude of the impacts can decrease while operating the vibrator at a constant frequency of 10 Hz.  FIGS. 13 and 14  show that the amplitude of the impact is not dependent on the operating frequency as with the prior art. With the prior art, the vibrator operating frequency would have significantly decreased from a 50 grms acceleration level to a 25 grms acceleration level. The solenoid controlled vibrators described above operating at a set 10 Hz frequency is not limited to 50 grms or 25 grms but it is just as easily controlled to 1, 2 3, 4 or 50 grms or any value in between, above or below. This is accomplished by the controller adjusting only the amplitude of the impact while maintaining the 10 Hz operating frequency. Conversely the operating frequency can also be adjusted independently of the grms. This is shown in  FIGS. 15  (50 grms) and  16  (25 grms) while operating at a frequency of 5 Hz. 
         [0062]    While  FIGS. 13-16  are useful for showing the flexibility of the described vibrators, they are not representative of how the described vibratory system would be utilized.  FIG. 19  shows a PSD chart for the operation of the described system utilizing random frequencies of operation. Compare this chart to the prior art PSD of  FIGS. 17 and 18 .  FIG. 17  is a PSD of the prior art vibrator operating at approximately 30 Hz with a constant air pressure. This shows the typical picket fencing on the PSD that is present with prior art vibrator operation.  FIG. 18  shows the prior art vibrator with the air supply pressure being modulated. With the air supply pressure increasing and decreasing the vibrator&#39;s operating frequency and amplitude of the impacts also increases and decreases. This tends to reduce peaks and broaden the spikes on the PSD.  FIG. 18  shows the improvement over a vibrator operating at a constant air supply pressure but there are still significant spikes to about 250 Hz. With the amplitude and frequency dependent on the air supply pressure to the vibrator, the high operating frequencies tend to dominate the peaks because the greatest amplitude impacts occur at these frequencies. This leads to the usefulness of the current described system where the amplitude of the impact is not dependent on the vibrator operating frequency. The controller does not have to control the operating frequency to a single 10 Hz or 5 Hz operation. The controller can randomly select the operating frequency of the vibrator and maintaining constant amplitude of the impact at any and all operating frequencies.  FIG. 19  is the solenoid controlled vibrator operating with random frequencies and shows that the picket fencing caused by the vibrator impacts is substantially reduced. 
         [0063]    Another advantage of the described systems is shown in  FIGS. 20 and 21 .  FIG. 20  shows a Time Domain of the described system where only a primary impact is shown. A primary impact in a first direction  72  is followed by a primary impact  74  in a second direction as air is fed to the vibrator  20  as described above. In comparison,  FIG. 21  shows a Time Domain of the described system wherein rebound impacts or secondary impacts are shown. A primary impact in a first direction  72  is followed first by a secondary impact in first direction  76  before the system powers the vibrator in the opposite direction resulting in a primary impact in a second direction  74 . This gains the benefit of not only imparting different strength impacts but also allows for more efficient use of the air by allowing more than one impact from a single burst of air. 
         [0064]    While the principles of the invention have been shown and described in connection with specific embodiments, it is to be understood that such embodiments are by way of example and are not limiting.