FIELD OF THE INVENTION 
     The invention relates generally to seismic vibrators for imparting a force onto a baseplate in contact with earth materials, and more particularly to a seismic vibrator for vibrating masses with reduced likelihood of inducing stress, strain, or harmonics. 
     BACKGROUND OF THE INVENTION 
     Land vibrators are known which include a base plate in contact with the earth, a reaction mass, and a linear actuator for reciprocating the reaction mass relative to the base plate. 
     U.S. Pat. No. 3,745,885 discloses a conventional hydraulic vibrator with a cylindrical piston and cylinder. More particularly, the vibrator is comprised of a double acting drive piston slidably received within a cylinder formed in a reaction mass. When hydraulic fluid is introduced into the cylinder alternately on opposite faces of the piston, the reaction mass is forced into reciprocal motion. 
     Marine vibrators also are known which employ a reaction mass in the same manner as land vibrators, where a reciprocating force is imparted to a single baseplate in contact with the water. Other marine vibrators, such as that disclosed in U.S. Pat. No. 3,482,646, substitute the reaction mass of a land vibrator with a second baseplate substantially identical to the first. A flexible seal between the baseplates allows them to move with respect to each other without compromising the watertight integrity of the assembly. Each baseplate works against the mechanical and reactive impedance of the other one to form a pair of acoustic projector surfaces experiencing equal and opposite forces, and therefore moving in opposite directions. 
     Other known actuators, some of which are not applicable as seismic vibrators, are described in the following patent summaries: 
     U.S. Pat. No. 3,172,338 discloses a hydropneumatic actuator which acts as a tool and die manipulator, rather than a vibrator. The actuator includes three coaxial, concentric, and coextensive cylinders, with each cylinder having in slidable relation therewith a piston. Not all pistons have double ended rods. If equal pressure is applied to each side of the piston, therefore, an unbalanced force is generated, and hence an asymmetrical displacement of stroke occurs. 
     U.S. Pat. No. 4,143,736 discloses a seismic transducer for generating waves in an elastic medium. The transducer includes a reaction mass with parallel cylinders formed therein, and a piston member slidably received in each cylinder. Each piston includes a double ended rod, with one rod end connected to an energy coupling plate in contact with the elastic medium to be vibrated. A pressurized fluid supply, pressurized fluid storage, manifold and servo valve are used to introduce hydraulic fluid alternately to opposite sides of the pistons to induce a reciprocal motion into the reaction mass. 
     U.S. Pat. No. 4,178,838 discloses a seismic energy vibrator which includes a reaction mass with parallel cylinders into which pistons with a double ended rod are reciprocally received. One rod end of each piston is attached to an energy coupling plate in contact with the earth. When hydraulic fluid is alternately introduced to opposite sides of the pistons, the reaction mass is forced into reciprocal motion. One aspect of the disclosed invention is that hydraulic flow porting is simplified to provide only a single hydraulic flow passage for each piston rod, thereby improving structural integrity. 
     U.S. Pat. No. 4,386,889 discloses a positive displacement pump having a plurality of slidably reciprocating, concentric, annular pistons mounted between parallel walls of the pump. When the pistons are reciprocated in a predetermined and controlled sequence, the fluid is caused to flow through the pump. The purpose of the apparatus is to pump liquid, not to produce vibrations. 
     U.S. Pat. No. 4,424,012 discloses an in-line pump having a cylinder in which an annular piston is reciprocally driven to cause fluid to flow along a fluid carrying line. The piston rods are single ended, and thus produce asymmetrical forces on the up and down strokes. The pump is valved in such a way that an applied reciprocating force on the single-ended piston rods produces a unidirectional flow of oil in the inlet and outlet ports. In the present invention, a reciprocating flow of pressurized fluid produces a symmetrical force on the piston which is transmitted through double ended piston rods to vibrate external masses. 
     U.S. Pat. No. 4,608,675 discloses a toroidal piston in a toroidal enclosure with only one chamber formed between the base of the piston and the base of the enclosure. The piston is of sufficient size to act as the reaction mass. The rapid release of air into the chamber transmits an impulse into the ground as the baseplate of the enclosure is pushed against the inertia of the piston to produce seismic energy. No coherent vibrational energy, however, is produced. That is, no reciprocating vibrational force is produced. 
     U.S. Pat. No. 4,691,803 discloses a seismic energy generating system including a base plate in contact with the earth, a master cylinder filled with water that is mounted on the base plate, and a receiving cylinder within the master cylinder into which a projectile is fired to generate a hydraulic force that is coupled through the base plate to the earth. This device is another impulse generator, and does not produce a reciprocating vibrational force. 
     U.S. Pat. No. 4,939,983 discloses a fluid pressure operated positioning apparatus which includes a ring enclosure having coaxial inner and outer walls, and an annular piston surrounding the inner enclosure wall. Axially parallel piston rods are connected to the annular piston, and extend in a sealed manner through one or both enclosure end caps. The positioning apparatus is used to apply a continuous force between two objects, generate rotational motion, control a robotic manipulator arm, or position a workpiece or table for machining or grinding. No reciprocating vibrational force is produced. 
     U.S. Pat. No. 5,189,263 discloses a portable geophysical energy source including an earth-coupling element, a seismic energy source connected to the earth-coupling element, a lower water container placed between the seismic energy source and the earth coupling element to act as a hold-down mass, and an upper water container placed above the seismic energy source to act as a reactance mass. The hydraulic actuator used in the system is of a conventional circular piston in a cylinder design. 
     U.S. Pat. No. 5,360,951 discloses a seismic energy source which includes a first plate resting on the earth&#39;s surface, one or more piezoelectric transducers mounted on the first plate to convert electrical energy into mechanical vibrating energy, and a second plate resting on top of the transducers and anchored to the earth by an earth clamping mechanism. When electrical energy is imparted to the transducers, a mechanical vibratory motion is imparted into the earth through the first plate. 
     U.S. Pat. No. 5,410,946 discloses a die press with a dual stage hydraulic actuator having a single cylinder body with a pressure chamber in which a first piston and a second piston are independently slidably inserted. Each piston includes plural single ended piston rods which protrude through only one end of the cylinder, and which are equally spaced circumferentially. The piston rods of the first piston are surrounded by the piston rods of the second piston. The cylinder body has a center column which allows the first piston to separate first and second pressure chambers, and the second piston to separate second and third pressure chambers. The first and second pistons are advanced by introducing pressured oil into the first pressure chamber, and then the second pressure chamber. The pressurized oil in the second pressure chamber acts as a rigid fluid connector to transmit the movement of the first piston to the second piston. Asymmetric rather than symmetric forces are produced on either side of the pistons given the same fluid pressure. 
     U.S. Pat. No. 5,701,801 discloses a mechanically redundant actuator with structurally redundant members, which attach the actuator body between a stationary anchor point and a movable control surface of an aircraft. The actuator is comprised of a single circular piston within a cylinder. The piston rods are double ended. 
     None of the above prior art describes an actuator in which a reciprocating flow of fluid produces a symmetrical force on the piston which is transmitted through double ended piston rods in a manner to distribute a reciprocating vibrational force over a large surface area of the mass to be vibrated, thereby reducing likelihood of inducing strain, stress, or harmonics in the mass. 
     SUMMARY OF THE INVENTION 
     The present invention is a hydraulic actuator comprised of a toroidal enclosure, a toroidal piston disposed within the toroidal enclosure and having plural double ended piston rods slidably extending in a fluid sealed manner through end caps of the toroidal enclosure, with the toroidal piston defining upper and lower chambers of the toroidal enclosure into which hydraulic fluid is alternately introduced to force the piston into reciprocal motion to vibrate external masses. 
     In one aspect of the invention, same fluid pressures in the reciprocating fluid flow produces equal forces on the faces of the piston, and thus symmetrical up and down strokes of the piston. 
     In another aspect of the invention, the plural double ended piston rods distribute the force generated by the actuator over plural points of a surface area of the mass being vibrated to reduce the likelihood of stress, strain, or harmonics in the mass. 
     In still another aspect of the invention, the toroidal enclosure may act as a reaction mass or may be connected to a reaction mass. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate a preferred embodiment of the invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain the principles of the invention. 
     FIG. 1 is a functional block diagram of a hydraulic/electronic system of a seismic vibrator in accordance with the invention; 
     FIG. 2 is a perspective view of a toroidal hydraulic piston in accordance with the invention; 
     FIG. 3 is a perspective view of a toroidal enclosure for housing the toroidal piston of FIG. 2; 
     FIG. 4 is a cross-sectional view of the toroidal enclosure  50  along line  4 — 4  of FIG. 3; 
     FIG. 5 is a cross-sectional view of the toroidal enclosure  50  along line  5 — 5  of FIG. 3; 
     FIG. 6 is an illustration of the flexible conduits connecting the flow channels of the manifold  20  to the oil ports of the toroidal enclosure  50  of FIG. 3; 
     FIG. 7 is an illustration of the flow channels formed within the manifold  20  of FIG. 1; and 
     FIG. 8 is a cross-sectional view of the manifold  20  of FIG. 1 along line  8 — 8  of FIG.  7 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the descriptions which follow, like reference numbers refer to same elements. 
     Referring to FIG. 1, a functional block diagram of a seismic vibrator system  10  is shown which comprises a hydraulic pump  11  that supplies a hydraulic liquid such as oil under pressure along a hydraulic flow line  12 , and through a high pressure accumulator  13  to a hydraulic flow line  14  leading to a servo valve  15 . 
     A control unit  16  controls the operation of the servo valve  15  by issuing control signals on a conducting line  17 . The servo valve  15 , in response to the control unit  16 , channels the high pressure oil from accumulator  13  through one of flow lines  18  and  19  to a manifold  20 . The manifold  20  in turn channels oil flow between lower chamber ports  21  and flow line  19 , and between upper chamber ports  22  and flow line  18 . The lower chamber ports  21  communicate the oil to a lower chamber bounded by a lower surface of a toroidal piston slidably seated within a toroidal enclosure, and the upper chamber ports  22  communicate the oil to an upper chamber of the toroidal enclosure bounded by an upper surface of the toroidal piston. By application of high pressure oil first to one chamber, and then to the other chamber, the piston is caused to reciprocate within the toroidal enclosure. The force generated by the piston is the product of piston surface area and differential piston pressure. Conventional hydraulic vibrators may produce a maximum pressure differential of the order of 3,000 psi. 
     As the toroidal piston is forced to move either upward or downward within the toroidal enclosure by application of high pressure oil through the high pressure accumulator  13 , the servo valve  15 , and the manifold  20 , lower pressure oil is forced out of the toroidal enclosure and through manifold  20  to one of flow lines  18  and  19 . The low pressure oil received by the servo valve  15  from flow lines  18  and  19  is applied along a hydraulic flow line  23  to a low pressure accumulator  24 . The low pressure accumulator causes the oil to be returned by way of a hydraulic flow line  25  to the hydraulic pump  11 . 
     More particularly, upon command of the control unit  16 , servo valve  15  establishes a conduit to direct high pressure oil from the high pressure accumulator  13  to the lower chamber of the toroidal enclosure, and a conduit to displace lower pressure oil in the upper chamber into the low pressure accumulator  24 . As a result, the toroidal piston moves upward. In like manner, when control unit  16  commands the servo valve  15  to establish a conduit to direct high pressure oil from the high pressure accumulator  13  to the upper chamber of the toroidal enclosure, and a conduit to displace lower pressure oil in the lower chamber to the low pressure accumulator, the toroidal piston moves downward. By alternately pressurizing the upper and lower chambers of the toroidal enclosure, the toroidal piston is driven in a reciprocating motion. 
     The forces generated on either side of the toroidal piston to cause the reciprocating motion are equal forces produced by equal pressures in the upper and lower chambers of the toroidal enclosure. A symmetrical displacement on the up and down strokes of the toroidal piston thereby occurs. 
     The control unit  16  thus operates to cause the toroidal piston to reciprocate within the toroidal enclosure. The motion energy of the reciprocating piston is coupled to the land or water mass in which vibration energy is to be introduced. 
     The hydraulic pump  11 , high pressure accumulator  13 , control unit  16 , servo valve  15 , and low pressure accumulator  24  are commercially available products incorporated into conventional land vibrators which may be purchased from any one of the following vendors: Input/Output Incorporated, 11104 West Airport Boulevard, Stafford, Tex. 77477 (Model 362); Sercel Incorporated, 17155 Park Row, Houston, Tex. 77128(Model M26HD/623B); and Industrial Vehicle International, Incorporated, 6737 East 12th Street, Tulsa, Okla. 74112 (Model Hemi 60). 
     Referring to FIG. 2, a doughnut or toroidal shaped piston  30  is illustrated with eight parallel rods  31 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 , and  38  spaced around a circumference of the piston, and extending through and perpendicular to the toroidal ring of the piston  30 . Each of the rods  31 - 38  extends above and below the toroidal piston  30 . 
     FIG. 3 illustrates a toroidal enclosure  50  within which the toroidal piston  30  of FIG. 2 is slidably enclosed. The rods  31 - 38  of piston  30  extend slidably and in a fluid sealed manner through the end cap  51  of the toroidal enclosure  50  to impart the motion of the toroidal piston  30  to the mass to be vibrated. It is to be understood that the toroidal enclosure  50  may constitute the reaction mass of the vibrator  10 , or the toroidal enclosure  50  may be attached to the reaction mass. 
     FIG. 4 is a cross-sectional view of the hydraulic actuator of the present invention along line  4 — 4  of FIG. 3, with toroidal piston  30  slidably seated within toroidal enclosure  50 . Rod  31  is affixed to toroidal piston  30  with rod ends  31   a  and  31   b  slidably extending through endcaps  51  and  52  of the toroidal enclosure  50 . The packing and liquid sealing methods used to allow the rod  31  to slidably extend through boreholes  53  and  54  are conventional and well known. An upper oil port  55  leads to an upper hydraulic chamber  57  for applying oil under pressure to the upper surface of toroidal piston  30  to move the piston downward, and allow the oil in chamber  57  to flow out of the chamber  57  as the piston moves upward. A lower oil port  56  leads to a chamber  58  of toroidal enclosure  50  to apply oil under pressure to the lower surface of toroidal piston  30  to move the piston upward, and to allow the oil in chamber  58  to flow out from the chamber  58  when the toroidal piston  30  moves downward. Chambers  57  and  58  are circularly disposed above and below toroidal piston  30  without interruption within the toroidal enclosure. The reciprocating movement of the toroidal piston  30  is transmitted by way of rod  31  to a mass to be vibrated. 
     In the above description, it is to be understood that oil port  55  is one of upper chamber ports  22  of FIG. 1, and that oil port  56  is one of lower chamber ports  21  of FIG.  1 . 
     FIG. 5 is a cross-sectional view of the hydraulic actuator of the present invention along line  5 — 5  of FIG.  3 . When oil is applied under pressure to the upper oil port  55 , the toroidal piston  30  moves downward to force the oil in chamber  58  to flow out of the lower oil port  56 . In like manner, when oil under pressure enters the lower oil port  56 , the toroidal piston  30  is moved upward to force the oil in chamber  57  to flow out of the upper oil port  55 . By alternately applying high pressure oil to ports  55  and  56 , the toroidal piston  30  is forced into a reciprocating motion which is coupled to a mass by way of the rods  31 - 38 . 
     Referring to FIG. 6, the manifold  20  is shown positioned central to the toroidal enclosure  50  with conduits  70 - 77  interconnecting upper chamber ports  22  of FIG. 1 with first flow channels formed within the manifold  20 . Such flow channels lead to a first oil port  80  of FIG. 6 in the manifold  20 , which oil port is in liquid flow communication with flow line  18  of FIG.  1 . In like manner the conduits  90 - 97  of FIG. 6 connect the lower chamber ports  21  of FIG. 1 with second flow channels formed in the manifold  20  that lead to a second oil port  100  of FIG. 6 in the manifold  20 . The oil port  100  is in liquid flow communication with the flow line  19  of FIG.  1 . In the preferred embodiment, the oil ports  80  and  100  of FIG. 6 extend through the upper surface of the manifold  20 . 
     In operation, when the manifold  20  receives oil under pressure into one of oil ports  80  and  100 , oil of lower pressure exits from the other of the oil ports. By way of example, when oil under pressure is received at oil port  80 , the oil is channeled by manifold  20  to conduits  70 - 77  leading to upper chamber ports  22  of FIG. 1, and into chamber  57  of FIG. 4 to force the toroidal piston  30  downward. As a result, residual oil is forced out of the chamber  58  through the lower chamber ports  21  of FIG. 1, and applied by way of conduits  90 - 97  of FIG.  6  through manifold  20  and out oil port  100  to flow line  19  of FIG.  1 . In like manner, if oil under pressure is received into oil port  100  of manifold  20  as illustrated in FIG. 6, the oil is channeled by manifold  20  to conduits  90 - 97  leading to the lower chamber ports  21  of FIG. 1, and into the chamber  58  of FIG. 4 to force the toroidal piston  30  to move upward. As a result, the residual oil in chamber  57  is forced out through port  55  of upper chamber ports  22  of FIG. 1, and is channeled by manifold  20  to flow line  18  leading to servo valve  15 . 
     FIG. 7 illustrates the flow channels  70 ′- 77 ′ formed within manifold  20  which emanate radially from oil port  80  to respectively connect to the conduits  70 - 77  of FIG.  6 . The flow channels  90 ′- 97 ′ of FIG. 7 in like manner emanate from the oil port  100  of manifold  20  to connect respectively to the conduits  90 - 97  of FIG.  6 . 
     FIG. 8 is a cross-sectional view of the manifold  20  along line  8 — 8  of FIG. 7, and illustrates the relative position of the oil port  80  with respect to the oil port  100  of manifold  20 , and the location of flow channels  71 ′ and  75 ′ with respect to flow channels  91 ′ and  95 ′. 
     Although a preferred embodiment of the invention has been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the claims, and consequently it is intended that the claims be interpreted to cover such modifications, variations, and equivalents.
A hydraulic actuator comprised of a toroidal piston within a toroidal enclosure, with differential fluid pressure alternatively applied to upper and lower surfaces of the piston to cause a reciprocating motion, and with plural double ended piston rods extending in parallel above and below the piston, and slidably extending in fluid sealed relation through end caps of the toroidal enclosure to distribute the vibrational force produced by the reciprocating piston over plural points of a surface area of the mass to be vibrated, thereby reducing the likelihood of stress, strain, or harmonics in the mass.