Abstract:
A shaker system ( 10 ) that employs a mechanical amplifier ( 40 ) for increasing the shaking capacity of the shaker ( 14 ). The mechanical amplifier ( 40 ) includes a spring ( 54 ) positioned within a support column ( 42 ) where the spring ( 54 ) is attached to an interface ring ( 24 ) supporting the load ( 26 ) to be tested at one end and the ground at an opposite end. The spring ( 54 ) can take on different configurations, and in one embodiment is a sinusoidal spring ( 54 ) having half-circle sections ( 60 ) of a predetermined radius to satisfy the resonant frequency requirements for a particular load. The resonant frequency of the spring ( 54 ) causes the shaking to be amplified, meeting the testing requirements.

Description:
GOVERNMENT RIGHTS 
     This invention was made with Government support under NAS5-32954 awarded by the National Aeronautical and Astronautical Space Administration. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a mechanical amplifier for a vibrational testing device and, more particularly, to a series of mechanical amplifiers used in connection with a shaker, where the amplifiers employ a spring. 
     2. Discussion of the Related Art 
     Performance testing of various structures is important to insure that the structures meet load and force requirements for a specific purpose. Therefore, it is known to employ shakers to provide vibrational tests of various structures and components, such as spacecraft and electronic components. One particular shaker used for this purpose is the T4000 shaker system available from Uniholtz-Dickie. 
     In one known performance test, it is necessary to test the spacecraft structure to 8 g&#39;s in an axial configuration. The spacecraft structure, including mass simulators, weighs approximately 5800 pounds. The moving mass of the T4000 shaker system weighs approximately 1700 pounds. Therefore, the ability to accelerate 7500 pounds to 8 g&#39;s at 25 Hz is required for a particular vibrational test. This corresponds to approximately 0.25 inches peak-peak displacement and 60,000 pounds of force. However, the T4000 shaker system can only generate 40,000 lbs peak load, and thus is not able to provide the desired input. 
     Various solutions can be used to meet the necessary testing requirements in this example. These solutions include using a larger shaker system, combining two or more smaller shaker systems, or reverting to static or component level testing. Static or component level testing tends to be labor intensive, and thus is not a desireable alternative. Using a larger shaker system requires that the shaker system be purchased at a very significant cost. Additionally, combining two or more shakers requires suitable fixtures and the like for combining the shakers. The fixtures necessary to combine two or more shakers to meet the testing load capacity would be labor intensive, and also require significant development costs beyond the cost of the actual shaker systems. Also, using larger shaker systems, or combining two more shaker systems increases the space necessary to provide the test. 
     What is needed is a technique for increasing the vibration capacity of an existing vibrational shaker, without having to incur significant costs. It is therefore an object of the present invention to provide such a shaker system. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a shaker system is disclosed that employs one or more mechanical amplifiers for increasing the vibrational capacity of the shaker. The mechanical amplifier includes a spring positioned within a support column, where the spring is attached to an interface ring supporting the load to be tested at one end and the ground at an opposite end. The spring can take on different configurations, and in one embodiment is a sinusoidal spring having half-circle sections of a predetermined radius thickness and width which satisfies the resonant frequency requirements for a particular load. The resonant frequency of the spring causes the vibration to be amplified, meeting the testing requirements. 
     Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a shaker system employing a plurality of mechanical amplifiers, according to an embodiment of the present invention. 
     FIG. 2 is a perspective view of one of the mechanical amplifiers separated from the shaker system shown in FIG. 1; 
     FIG. 3 is a perspective view of one of the mechanical amplifiers separated from the shaker system shown in FIG. 1, and exposing the spring within the amplifier column; 
     FIG. 4 is a side view of the spring removed from the mechanical amplifier shown in FIG. 3; 
     FIG. 5 is a perspective view of a spacer on which the amplifier spring is mounted within the column shown in FIG. 3; and 
     FIG. 6 is a graph with amplification on the vertical axis and frequency ratio on the horizontal axis showing the amplification curve for various damping values. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a mechanical amplifier for a shaker system is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
     FIG. 1 is a perspective view of a shaker system  10  that includes a shaker  14  and a support assembly  16  mounted to the shaker  14  and to the floor. Appropriate control circuitry (not shown) causes the shaker  14  to vibrate on an axis  18  in conformance with the predetermined design criteria of the shaker system  10 . A head  20  and a head expander  22  are mounted to a top of the shaker  14 , and an interface ring  24  is mounted to the head expander  22 . A load, here a spacecraft  26 , is mounted on the interface ring  24  opposite to the head expander  22 . Operation of the shaker  14  causes it to vibrate which excites the spacecraft  26  in a desirable manner for vibrational testing. The shaker system  10 , as described so far, is consistent with the known T4000 shaker system. 
     In accordance with the teachings of the present invention, the shaker system  10  includes a plurality of mechanical amplifiers  40  positioned around the shaker  14  in a symmetrical manner. In this example, there are four amplifiers  40  symetrically positioned around the interface ring  24 . However, other numbers of amplifiers  40  can be used in other applications consistent with the discussions herein. Each mechanical amplifier  40  is secured to an extension  38  of the interface ring  24  at one end and to the floor at an opposite end. As will be discussed in detail below, the mechanical amplifiers  40  provide amplification of the vibration generated by the shaker  14 , so as to provide increased force for the test. 
     FIGS. 2 and 3 are perspective views of one of the mechanical amplifiers  40  removed from the shaker system  10 . The other amplifiers  40  are the same (within manufacturing tolerances). The mechanical amplifier  40  includes a column  42  connected to a base plate  44  that is mounted to the floor. The column  42  includes four walls  46  attached together to define an elongated square enclosure defining a space  48  therein. A spacer  50  is positioned in the space  48  and is mounted to the base plate  44 . A spring  54  is mounted to the spacer  50  and extends out of a top end  56  of the column  42 . The column  42  provides a stabilizing force to the spring  54  in the lateral direction. FIG. 4 shows a lengthwise view of the spring  54  and FIG. 5 shows a perspective view of the spacer  50  removed from the amplifier  40 . 
     The spring  54  includes a series of interconnected half-circle spring portions  60  extending between a spacer plate  62  and an end plate  64 . The end plate  64  is a block member that includes a friction member  66  secured to each side of the end plate  64 . The friction member  66  can be made of any suitable material, such as Deirin, that allows the end plate  64  to easily slide along the inner surface of the walls  46  within the column  42 . 
     The spacer  50  includes an elongated square block portion  70  attached to a support plate  72  by leg members  74 . An end  76  of the spacer  50  is secured to the base plate  44  by any suitable securing mechanism, such as bolts or the like. The spacer plate  62  is secured to the support plate  72  by any suitable securing device, such as bolts, so that the combination of the spacer  50  and the spring  54  define an elongated member positioned within the column  42 , where a portion of the end plate  64  extends above the end  56  of the column  42 . A top surface of the end plate  64  is secured to the extension  38  of the interface ring  24  by any suitable securing device, such as bolts or the like. 
     Upon operation of the shaker  14 , force is applied through the head  20 , the head expander  22  and the interface ring  24  to the end plate  64 . Downward force on the end plate  64  causes the spring  54  to compress in a spring resilient manner. A return action of the spring  54  applies upward force to the interface ring  24  which increases the energy shaker system  10  if the spring action is at (or near) the right frequency. 
     The spring  54  is designed to provide amplification for a particular resonant frequency. The length of the spring  54 , the number of spring portions  60 , the radius of the spring portions  60  and the resiliency or flexibility of the spring  54  are all specially designed for a particular system and frequency. The spring  54  needs to be both stiff and flexible to operate as desired. The spring  54  needs to be stiff so that it is able to support and provide the large forces necessary in the shaking test. The spring  54  needs to be flexible so that it compresses enough to provide the desired amplification. Each spring portion  60  adds up to give the desired stiffness. Each spring portion  60  displaces a certain amount so that the addition of all of the displacements provides the flexibility. The spring  54  is made of a high quality steel, for example, 15-5PH steel. The number of spring portions  60  is selected to provide the desired flexibility. The design of the spring  54  is selected to provide a standing mode resonance at the frequency of the shaker  14 . In one embodiment, the inner radius of the spring portions  60  is one inch, the thickness of the spring  54  is one inch and the width of the spring  54  is about five inches. 
     FIG. 6 is a graph with amplification on the vertical axis and frequency ratio ω/ω n  on the horizontal axis for four different damping values (ζ) 0.125, 0.1, 0.05 and 0.01. The amplification of the spring  54  is determined by the damping factor as 1/2 ζ. An amplification factor of two or more can be obtained when 0.8&lt;ω/ω n &lt;1.2 and ζ&lt;0.125 (12.5% critical damping). Also, there is more margin for error if the resonant frequency is above the target frequency. Using 25 Hz as a target frequency, in one embodiment, the spring  54  was designed with the following properties. K total =weight (2 pi freq) 2 /386.2 approximately≈480,000 lbs/in; total dynamic deflection (peak-peak)=2(gs) (386.1)/(2 pi freq) 2  approximately≈0.25 inches; and a minimum life of approximately 1,000 cycles. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.