Abstract:
Apparatus, and an accompanying method for use therein, that utilizes working and stopping servo-controlled hydraulic pistons wherein the stopping piston acts as a controlled mechanical stop for the working piston. Both pistons are spaced apart along and coaxially arranged around a common shaft, with each piston moving in a separate cylinder. The working piston is securely attached to the shaft, while the shaft moves through a central, longitudinal bore of the stopping piston. The stopping piston effectively “floats” in its cylinder and produces a greater force than the working piston. A radially extending stop element, situated on the shaft, has a surface configured to abuttingly engage with a complementary surface on the stopping piston such that the stopping piston, once appropriately positioned, controllably stops continued movement of the working piston in a very short time and over a very short distance with little strain induced in the apparatus.

Description:
CLAIM TO PRIORITY  
       [0001]    This application claims priority of our United States provisional patent application entitled “SERVO-CONTROLLED INTEGRAL STOP FOR SERVO-CONTROLLED PISTON IN HYDRAULIC SYSTEMS”, filed May 22, 2002 and assigned serial No. 60/382,637; which is incorporated by reference herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to apparatus, and an accompanying method for use therein, that utilizes first (working) and second (stopping) servo-controlled hydraulic pistons wherein the second piston acts as a controlled mechanical stop for the first piston. Advantageously, the apparatus can controllably stop the first piston, traveling at relatively high speeds, in a very short time and over a very short distance while advantageously inducing very little, if any, elastic strain into the apparatus.  
           [0004]    2. Description of the Prior Art  
           [0005]    Metallic materials play an indispensable role as an essential component of an enormous number of different products. Such materials are produced  
           [0006]    typically in large ingots or other shapes and are controllably deformed by, e.g., rolling, forging or extruding into readily useable and conventional sheet, plate, coil or wire form for subsequent machining or forming. These deformations typically occur on a repeated incremental basis, such as through a multi-stand rolling mill where the material is repeatedly passed through successive pairs of rolls. Each pass incrementally compresses, i.e., deforms, the material into thinner stock. Typically, each pair of rolls is spaced equal distance from the next pair but has a smaller inter-roll spacing (“roll bite”) than the next pair. Hence, as the material becomes thinner it travels at a faster rate through successive roll pairs and this decreases the time occurring between each compression. Extrusion, forging and braking operations also typically involve incremental deformations until the material is properly sized.  
           [0007]    In production environments, small incremental deformations are typically produced at high rates. However, correctly configuring a mill, forge or brake to properly deform production stock and impart a desired amount of strain to the material along with other physical/metallurgical characteristics can be a tedious, time-consuming and expensive process—particularly since such a machine needs to be taken out of productive use for an extended time to properly adjust its operational parameters. Consequently, to avoid the need for costly downtime, thermodynamic material testing systems are employed to simulate rolling, extruding, braking and forging processes on relatively small metallic specimens. Resulting simulation data is then used to properly set various operating parameters of production equipment and, by doing so, minimize its non-productive downtime. Illustrative simulators of this type include the “Gleeble” and “Hydrawedge” systems manufactured by Dynamic Systems Inc. (DSI) of Poestenkill, N.Y., which is the present assignee hereof (with “Gleeble” and “Hydrawedge” being registered trademarks owned by DSI).  
           [0008]    Systems which deform metallic materials, particularly including material testing systems, often utilize linear motion of a piston/anvil combination produced by servo-controlled hydraulic systems, and particularly those which accelerate and stop pistons at very high speeds. Such movement is necessary to impart a desired amount of deformation to the material, situated, e.g., between a pair of anvils, at a desired strain rate and over as much of a resulting deformation as possible. In these environments, linear piston systems moving with velocities up to 10 meters per second are frequently used, with velocities of 1 to 2 meters per second being quite common.  
           [0009]    In particular, in such testing systems, a fundamental problem arises in that a piston, while traveling at such a high relative velocity, must often be stopped in a manner, essentially immediately, through which its velocity does not decrease even over a small distance, else the strain rate imparted to the specimen will decrease over a stopping distance of the piston. Further, those systems typically utilize mechanical mechanisms of one sort or another to stop the piston which, while the piston is being stopped, disadvantageously introduce some strain into various structural components of the system itself. This added strain, by effectively compressing a frame of the system, tends to slightly elongate the stopping distance and thus adversely impact the resulting deformation of the specimen.  
           [0010]    Another area in which high-speed deformation is becoming increasingly important is sheet metal processing. Here, a need to reduce production costs requires that press brakes used to deform metallic material, i.e., bend metal sheets, operate at increasingly high speeds. Conventional bending machines have a set of shaped dies in which the material is held and then formed or bent. The dies are mounted in rather large, heavy beam structures. Usually, one beam is mounted rigidly, while another is mounted on linear sliding ways. Traditional brakes rely on producing linear motion for the ways through a large flywheel and suitable connecting/pivoting arms mounted between one beam and the flywheel. Relatively modern brakes control the motion of the die/beam using hydraulic servo-controlled piston/cylinder systems. A precision with which the material can be bent depends upon how quickly the die can be stopped at a bottom of its stroke (travel). As the speed of the die increases, its stopping distance becomes increasingly arbitrary. Given this, the stroke often has to be disadvantageously run at a decreased speed to consistently stop the die at a precise location. Additionally, the metal being deformed often provides a variable load to the die. This variable load causes the control system to compensate while the dies are being stopped at a desired position, but again generally necessitates that reduced speeds are used to obtain precise bends in pieces then being processed. Such decreased speeds disadvantageously reduce material throughput. Hence, to consistently deform material at relatively high speeds and increase throughput, the stroke has to be accurately controlled both in terms of its velocity, throughout the stroke length, as well as its stopping distance.  
           [0011]    In situations, be it in material testing systems or in production equipment, where material is being deformed at high-speeds, mechanical stops are often used to stop a high-speed anvil, ram or die at a precise position. Unfortunately, a position of such a mechanical stop has to be changed each time the desired amount of travel is changed.  
           [0012]    Therefore, apparatus is needed to stop motion of, e.g., an anvil, a ram or die in an exact position even at very high speeds in order to provide consistent results. Such a stop should impart very little, if any, strain in the apparatus so that the stopping position remains the same regardless of the changes in a load then being deformed. This entails that an end of a high-speed stroke must be precisely controlled as well as being easily and rapidly changeable.  
           [0013]    U.S. Pat. No. 5,092,179 (issued to H. S. Ferguson on Mar. 3, 1992) describes one such thermodynamic material testing system. As shown in FIG. 5 thereof, a stroke of piston  509  and shafts  540  and  545  are stopped by stop disc  543 . A position of specimen material  570 , being deformed, is advanced by hydraulic cylinder  590 , piston  592 , wedge combination  585 / 582 , shaft  575 , load cell  574 , plate  568 , anvil base  565  and anvil  560 ′. Each time specimen  570  is advanced by the wedge combination toward the left, anvil  560  is retracted and then rapidly advanced to the right, thus deforming specimen  570  until stop plate  543  hits cross-stop  550 . A drawback inherent in this system is that, during each hit, an amount of strain occurs elastically in an entire wedge assembly that supports a load on the right side of the specimen (load cell side). This elastic strain allows anvil  560  to move in the direction of deformation, thus decreasing the amount of deformation in the specimen and slightly compromising a final thickness of the specimen after each deformation. Once the system has been used to deform a particular specimen at a certain temperature, a computer-controlled deformation schedule (deformation program) that controls the system can be modified to accommodate for expected loss (increased material thickness) in deformation that would result from the elastic strain. However, doing so is a passive correction and never exact. For multiple deformations involving 3 or more hits to the specimen, appropriate modifications to the program become time-consuming and tedious to determine. Further, each hit becomes less precise as the number of hits increases. Therefore, a stopping mechanism is readily desired, for use in a thermodynamic material testing system, that imparts very little, if any, strain back into any structural component of the system itself during each hit.  
           [0014]    Thus, the need still exists in the art for a stopping mechanism for use with a servo-controlled hydraulic system, such as that used in a material testing system, in which a piston can be stopped from a very high speed at an exact location without over-travel and without substantial reduction in its speed right up to the moment of its stopping. The system should be capable of repetitive hits with each final stop at a predetermined position regardless of the speed of the piston. The stopping system should produce very little, if any, strain.  
           [0015]    Further evidence for the need for such a stopping mechanism can be seen from the following. A modern high-speed servo valve can be closed, from 80 percent of its maximum opening, typically in 0.003 seconds. If a piston controlled by that valve is moving at 1 meter per second—which often occurs in production equipment and material testing systems, then the piston will travel approximately 1.5 mm during stopping. This distance is clearly unacceptable where, in testing systems and high-speed press brakes, distances controlled to less than 0.05 mm are desired. Linkages, shafts and wedges of existing stopping mechanisms can have strains, under expected operating loads, of 0.3 or 0.4 mm. While these reduced strains are considerably better than that which results from use of no stopping mechanism at all, a stopping mechanism that produces far less strain in the mechanism itself is still needed.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention advantageously overcomes the deficiencies associated with high-speed use of servo-controlled hydraulic systems known in the art where very rapid stopping is required with essentially little, if any, strain occurring in the stopping mechanism. Through the invention, a first high-speed (working) piston is stopped by an adjustable mechanical stop, formed of a second (stopping) piston coaxially situated to the working piston.  
           [0017]    Advantageously, the present invention permits the stopping position of the working piston to be rapidly changed. Furthermore, the invention, by virtue of its stopping characteristics and inducing minimal resulting strain in the stopping mechanism, permits the servo system to repetitively and rapidly actuate the working piston, many times, with nearly ideal stopping positions each time regardless of the speed of that piston. High-speed stopping occurs over extremely short stopping distances and is essentially immediate.  
           [0018]    In accordance with the teachings of the invention, the working and stopping pistons are controlled by separate servo-control hydraulic systems and are both coaxially located on a common piston shaft for the working piston, with these pistons being longitudinally spaced apart on the shaft. Both pistons controllably move within separate corresponding piston cylinders. The stopping piston slides on the piston shaft with the shaft extending through a central longitudinally-oriented bore on the stopping piston. To stop further movement of the working piston, the stopping piston abuttingly engages, via complementary surfaces, with a radially extending circular stopping element on the shaft, e.g., a shoulder extending outward from and concentric with the shaft. Preferably and to provide positive stopping action, the stopping piston is sized and operated with sufficient hydraulic pressure to provide higher forces than the working piston. Illustratively, the working piston may provide a maximum force of 40 tons and the stopping piston a maximum force of 80 tons or more.  
           [0019]    In operation, the stopping piston is programmably moved, through appropriate computer-control of its servo-control hydraulic system, to a desired stopping position for the working piston. The working piston is retracted (before, coincident with or after the stopping piston is moved) and, once the stopping piston is properly position, then extended at a high speed. The working piston stops its extension whenever a surface of the stopping element on the piston shaft abuttingly engages a complementary surface situated on an upper side of the stopping piston. To change the stopping position, the stopping piston is simply moved, again through appropriate control over the servo-controlled hydraulic system, and the process is repeated, and so forth for multiple hits.  
           [0020]    The stopping mechanism is comprised of only the stopping piston and hydraulic oil used to position that piston. There are no stopping linkages, wedges, shafts or other mechanical parts which would change dimension under a changing load. Accordingly, the amount of strain that occurs in the stopping mechanism is significantly reduced.  
           [0021]    In accordance with a feature of the invention, two stopping elements (i.e., upper and lower stopping elements) can be positioned on the piston shaft, with a corresponding stopping element situated on either side of the stopping piston. In this manner, and with complementary surfaces being formed on the upper and lower surfaces of that piston, the stopping piston can stop movement of the working piston in both its upward and downward (retraction and extension) directions, rather than just in a downward direction. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0023]    [0023]FIG. 1 graphically depicts typical curves  101  and  105  that represent a path of a piston during relatively low- and high-speed travel, respectively, such as would occur in a press brake or a thermodynamic material testing system, with, as shown, travel velocity being decreased at an end of piston travel to obtain correct travel distance and stopping position;  
         [0024]    [0024]FIG. 2 graphically depicts curve  201  corresponding to curve  101 , shown in FIG. 1, but with a significantly increased travel speed and appreciably reduced stopping distance, both possible through use of the present inventive mechanical stop;  
         [0025]    [0025]FIG. 3 depicts, in cut-away sectional view, first embodiment  300  of the present invention, here for stopping movement of working piston  301  in its downward direction (extension); and  
         [0026]    [0026]FIG. 4 depicts, also in cut-away sectional view, second embodiment  400  of the present invention, here for stopping movement of working piston  401  in both its upward and downward directions (extension and retraction). 
     
    
     DETAILED DESCRIPTION  
       [0027]    After considering the following description, those skilled in the art will clearly realize that the broad teachings of the invention can be readily utilized in conjunction with any one of a wide variety of applications that rely on high-speed motion of hydraulic pistons where such a piston must be reliably, consistently and accurately stopped in minimal distance without inducing undue strain in a stopping mechanism (and, through it, e.g., into structural components of a mechanical device inter-connected thereto). Such applications illustratively include high-speed press brakes and thermodynamic material testing systems which simulate, e.g., rolling mills, extruders and hammer forges.  
         [0028]    [0028]FIG. 1 graphically depicts typical curves  101  and  105  that represent a path of a piston (not specifically shown) during relatively low- and high-speed travel, respectively, as would typically occur in a press brake or a thermodynamic material testing system.  
         [0029]    As shown, curve  101  depicts a relatively slow travel path (in millimeters) of a servo-controlled hydraulic piston as a function of time (in milliseconds). Curved portions  102  and  103  occur as a result of time intervals that are required to start the piston in motion and to stop it, respectively. Start and stop times of a servo system are dependent upon a response of an hydraulic servo valve, inertia of the system and tuning of conventional PID (proportional, integral and differential) settings of a servo loop contained within the system. The PID settings often have to be adjusted as a maximum speed of the piston increases.  
         [0030]    Dashed lines  104  and  104   a  indicate possible initial positional deviation expected for the stopping position of the piston. Specifically, dashed line  104  represents possible positional under-shoot, while dashed line  104   a  represents possible positional over-shoot of the piston. Clearly, the under-shoot is of little consequence in a deformation process as long as the piston eventually corrects its position to a desired value. The precision to which the piston will reach the exact desired position depends upon a value of the P (proportional or system gain) setting of the PID terms. Usually, as the value of the P term increases, the closer will the final position of the piston be to the desired value. However, also, as the value of P increases, so too will a likelihood that the servo system will oscillate. Consequently, the final setting of the P term is usually a compromise. As such and on one hand, during relatively slow-speed travel, a final piston position can become extremely close to its desired value at an end of its programmed travel. On the other hand, during high-speed travel, the final piston position may be very close to its final programmed value but is rarely, if ever, exactly equal to it. In that regard, during high-speed travel, a relatively small amount of time and therefore distance is required for the piston to reach the desired position, but, owing to mechanical delays and other mechanical response characteristics of the hydraulic servo system, the piston will typically over-shoot its desired final position (as indicated by dashed portion  104   a ).  
         [0031]    Curve  105  depicts a relatively high-speed travel path of the servo-controlled piston as a function of time. Positional under-shoot  106  and positional over-shoot  106   a  are larger than those for the low-speed travel depicted by curve and the over-shoot may include some oscillation (also not shown but well known). If the piston is being used to move a ram or anvil to compressively deform a specimen, as would be the case in a thermodynamic material testing system, then positional over-shoot causes undesired deformation of the specimen and imparts excessive strain in the specimen. Should the piston move, e.g., a die on a hammer brake, then in bending sheet metal, the over-shoot would over-bend the metal.  
         [0032]    [0032]FIG. 2 graphically depicts curve  201 , corresponding to curve  101  shown in FIG. 1, but with a significantly increased travel speed and appreciably reduced stopping distance, both achievable through use of the present inventive mechanical stop.  
         [0033]    Here, FIG. 2 depicts a relatively high-speed travel path  201  (in millimeters) as a function of time (in milliseconds) for the piston as well as a rapid stopping position  202 , where a high-speed (first) piston hits a mechanical stop implemented by a second (stopping) piston as taught by the present invention. The servo control of the first piston is not required after reaching position  202 , because the second piston prevents the first piston from further travel. The PID setting of the servo system that controls movement of the first piston is far less important here, as that servo system is no longer in control of this piston once it hits the second piston. The second piston overpowers and, for all practical purposes, immediately stops further movement of the first piston, since the second piston provides much higher forces that the first piston.  
         [0034]    By virtue of providing what is effectively an immediate and abrupt stop (indicated by line  202 ), the present invention permits the first piston to advantageously operate at a considerably increased travel speed (as indicated by he nearly vertical section of curve  201 ) over that associated with a upwardly sloped central portion of curve  101  shown in FIG. 1.  
         [0035]    [0035]FIG. 3 depicts, for clarity in cut-away sectional view, first embodiment  300  of the present invention, here for stopping movement of working piston  301  in its downward direction (extension).  
         [0036]    Housing  340  contains two hydraulic cylinders  308  and  317  oriented in a tandem configuration. For simplicity, while the housing, for each of the embodiments shown in FIGS. 3 and 4, is formed of several structural pieces, the fasteners (which are readily apparent to those skilled in the art) used to secure these pieces together, along with all appropriate seals, have been intentionally omitted for simplicity.  
         [0037]    First (working) piston  301  controllably moves bi-directionally within cylinder  308  in the directions (extension and retraction) indicated by arrow  330 . This piston is integral to piston shaft  312  which extends from end locations  302  to  303 . Shaft  312  is formed of two pieces (not specifically labeled) for ease of assembly. These two pieces are assembled (specifically screwed) together, via a large screw and thread, with an alignment ring (not shown) at joint line  302   a . Threaded extension screw  304  emanating from shaft  312  and particularly end location  303  extends to a suitable, conventional linear displacement transducer (not shown) to measure a current position of piston  301 . This transducer is connected to a servo controller and programmer (conventional and also not shown). Inasmuch as both the servo controller and programmer are conventional and well-known in the art, neither will be discussed in any further detail hereinafter. A position signal from the transducer is applied, as a position feedback signal, to the servo controller which, in turn, controls servo valve  305 .  
         [0038]    Valve  305  directs pressurized hydraulic fluid (hydraulic oil) supplied by a suitable, conventional hydraulic pump (not shown), via conduits  306  and  307 , into one of cylinder regions  308   a  and  308   b  and out of the other. As depicted and within cylinder  308 , region  308   a  lies above working piston  301 ; while cylinder region  308   b  lies below it. The specific region that is to receive fluid (and correlatively the region that is to lose fluid), and hence the direction of the flow of the fluid through the conduits, is determined by the servo controller based on whether working piston  301  is to be extended (moved upward) or retracted (moved downward). The opening of the valve determines a rate at the fluid enters one cylinder region and leaves the other and hence a speed at which working piston  301  moves. The time during which the valve remains open, in conjunction with the size of the valve opening, determines the final position of the piston within the cylinder. Cylinder  308  is sealed at both ends by suitable seals and bearings  309  and  310  (identical seals and bearings, not specifically labeled, are also used at a distal end of cylinder  317 ). Piston  301  has suitable seals  311  to separate cylinder regions  308   a  and  308   b  from each other.  
         [0039]    Second (stopping) piston  313  has central longitudinal bore  350  through which this piston is coaxially mounted on and controllably moves (specifically slides), in a direction given by arrow  332 , along piston shaft  312 . This piston moves within cylinder  317  that contains cylinder regions  317   a  and  317   b . As depicted, regions  317   a  and  317   b  are respectively situated above and below stopping piston  313 . Bearing  314  and seals  315  and  315   a  separate cylinder regions  317   a  and  317   b  from each other.  
         [0040]    Cylinder regions  317   a  and  317   b  are connected to servo valve  320  via conduits  319  and  318 , respectively, to servo valve  320 . Servo valve  320  is also connected to a suitable hydraulic pump and a second (stop position) servo controller (not shown). As with cylinder elements  308   a  and  308   b , the specific region  317   a  or  317   b  that is to receive fluid (and correlatively the region that is to lose fluid), and hence the direction of the flow of the fluid through the conduits, is determined by the stop position servo controller based on whether stopping piston  313  is to be extended (moved upward) or retracted (moved downward). The opening of valve  320  determines a rate at the fluid enters one cylinder region and leaves the other and hence a speed at which stopping piston  313  moves. The time during which the valve remains open, in conjunction with the size of the valve opening, determines the final position of the piston within the cylinder.  
         [0041]    Shaft  324 , connected to stopping piston  313 , is itself connected via threaded extension screw  325  to a suitable (second) linear displacement transducer (not shown) that provides a feedback signal indicative of a current position of the stopping piston  313 . This position information is fed to the second (stop position) servo controller/programmer to control the current position of stopping piston  313 .  
         [0042]    A load for compression work, e.g., a ram or anvil, is attached to shaft  312  at its end  302 . Stopping piston  313  is not connected to any external work and is used only to stop piston shaft  312  at a predetermined desired position, and hence produces no output working force to, e.g., an anvil, ram or die or similar device. The stopping piston basically “floats” in cylinder  317 .  
         [0043]    Circularly-shaped coaxial, radially expanded section (element)  321  (here forming a shoulder), having lower face  322 , is rigidly, securely and integrally formed on shaft  312  as a stop element. This section is intentionally shaped to be too large to pass through central bore  350  of stopping piston  313  and hence must stop whenever shoulder  322  (here being a lower face) of element  321  abuts against upper face  323  of stopping piston  313 . Lower face  322  and upper face  323  are shaped in a complementary fashion to each other. Stopping piston  313  is usually larger in size than is piston  301  in order to over-power and precisely limit the travel of working piston  301  and shaft  312 . For example, when both cylinders  308  and  317  are equally pressurized with hydraulic fluid, working piston  301  may produce a maximum force of 40 tons, while stopping piston  313 , which stops the travel of working piston  301 , may produce a maximum force of 80 or more tons. Advantageously, the larger the maximum force capability of stopping piston  313 , the stiffer the stop will be and accordingly the more precise the stopping position of working piston  301  will be. Further, an amount of hydraulic oil that supports stopping piston  313  also contributes to stopping error due to compressibility of the oil. Hence, that amount should be minimized. But, as will be seen below, the stopping mechanism can be readily set to compensate for this error.  
         [0044]    Since both pistons  301  and  313  are individually controlled as to their positions, the position of stopping piston  313  may be changed at any time. If working piston  301  is retracted (moves upward in a distal direction from piston  313 ), then stopping piston  313  can move independently of the working piston. If the working piston is in abutting contact, along faces  322  and  323 , with the stopping piston, then moving stopping piston  313  upwardly will also move working piston  301  in the same manner and by the same amount. Conversely, attempting to move working piston  301  downward, while the stopping piston is being held in its current position, will not be possible inasmuch as stopping piston  313 , which is larger than the working piston and correlatively produces a larger force, cannot be over-powered by the working piston. In normal operation, working piston  301  would be retracted away from stopping piston  313  before the latter piston is repositioned. Thereafter, working piston  301  is extensibly driven downward until stopping element  321  runs directly up against stopping piston  313 , and faces  322  and  323  engage and abut against with each other, at which point any further downward movement of working piston  301  and shaft  312 , for all practical purposes, immediately halts.  
         [0045]    As shown, face (stopping surface)  322  of stopping element  321  advantageously has a sloped surface as its stopping surface. The slope increases a surface area (over a surface which is perpendicular to a longitudinal axis of piston shaft  312 ) which contacts face (stopping surface)  323 , the latter being identically sloped to match that of surface  322 . The actual slope, i.e., angular incline, of these stopping surfaces is predefined but not critical as long as it is sufficiently large, given impact forces involved, to provide enough contact area to adequately reduce resulting contact pressure during stopping to a level that will not damage either of the faces but nevertheless provide effective stopping action.  
         [0046]    As indicated, stopping piston  313  has no force output, since it has no piston shaft. The stopping piston may thus be considered to be “floating” in that it simply moves under control in cylinder regions  317   a  and  317   b , but does not provide any force output. Shaft  324  is only used, coupled via screw thread  325  to a linear position transducer (not shown), to provide a corresponding position feedback signal and thus to provide a modality through which current position of stopping piston  313  can be measured.  
         [0047]    In operation, servo valve  320  would be controlled, by its corresponding servo-controller and in a programmed fashion, to position stopping piston  313  to stop piston shaft  312  at a precise desired location. Servo valve  305 , operating under control of its corresponding servo-controller, would then programmably cause working piston  301  and piston rod  312  to travel downward at the desired velocity until faces  322  and  323  abut against each other. If working piston  301  is controlled to move at a relatively high velocity, e.g., one or more meters per second, then the impact forces at faces  322  and  323  may be large.  
         [0048]    Current, commercially available high-speed servo valves can operate in 3 to 6 milliseconds (i.e., change position by 80 percent, open or close). If piston shaft  312  were traveling at 1 meter per second, the distance of travel in just 3 milliseconds would be 3 millimeters. Some applications require that a working piston, e.g., working piston  301 , be stopped within a small fraction of a millimeter (often 0.1 mm or less), while it is traveling at 1 meter per second or more. The relatively slow operation of such high-speed servo valves, as compared to a requirement to precisely stop the piston traveling at such a high speed in a very short distance, dictates the need for the inventive adjustable stop. The inventive stop mechanism advantageously fills that need. Moreover, the position of the stop, i.e., that of stopping piston  313 , is completely adjustable using its corresponding stop position servo controller. The adjustment can be rapidly accomplished, using an appropriate high-speed servo control system, in order to permit high-speed operation of both pistons  301  and  313 , and particularly multiple stopped retraction-extension cycles of working piston  301  in a relatively short time (so as to permit, e.g., a ram or anvil, to which end  302  of piston shaft  312  would typically be connected, in a thermodynamic material testing system to impart a series of rapidly occurring hits onto a specimen then being deformed).  
         [0049]    [0049]FIG. 4 depicts, along in cut-away sectional view, second embodiment  400  of the present invention, here for stopping movement of working piston  401  in both its upward and downward directions (retraction and extension).  
         [0050]    To facilitate understanding, highly similar reference numerals, with only a change in their first digit, have been used in FIGS. 3 and 4 to designate similar, if not identical, corresponding elements in these two figures. Given the commonality between the structures shown, only the differences therebetween will be specifically discussed. To implement bi-directional stopping, two separate stopping elements have been formed in piston shaft  412 : element  421  situated above stopping piston  413  and element  426  situated below it. Both elements are formed as radially extended portions of piston shaft  412 , having a sufficiently enlarged radius over that portion of the shaft which passes through central bore  450  of stopping piston  413 , with sloping faces  422  and  428 , respectively. These faces abuttingly engage, when working piston  401  is moving in an downward or upward direction, with complementary shaped sloped upper and lower faces  423  and  427 , respectively, of stopping piston  413 . Similar to shaft  312 , shaft  412  is fabricated in sections and then appropriately screwed together using threaded sections.  
         [0051]    Hence, stopping piston  413  can stop working piston  401  and piston shaft  412  by abutting contact along either faces  422  and  423 , or faces  427  and  428 . This permits high-speed stopping of working piston  401  and shaft  412  to occur in either direction of travel.  
         [0052]    Some compression of the hydraulic oil in cylinder regions  417   a  and  417   b  (depending upon force and direction of travel of working piston  401 ) will occur when the force of working piston  401  is abruptly transferred to stopping piston  413  during a high-speed stop.  
         [0053]    In both embodiments  300  and  400  shown in FIGS. 3 and 4, under known force conditions, an amount of compression of the hydraulic oil and resulting strain introduced into the parts of the system under load can be corrected by programming an amount of offset in the opposite direction, to the direction of travel of working piston  301  or  401  into the position of stopping piston  313  or  413 , respectively, to offset or cancel this effect.  
         [0054]    Though the present invention has been described in terms of using a single stopping piston to bi-directionally stop the working piston, the inventive stopping mechanism is not so limited. In that regard, two separately controllable upper and lower stopping pistons (each moving in its own cylinder) and again tandemly arranged (and each with its own position feedback transducer) can be substituted for single stopping piston  413  (and its cylinder  417 ) depicted in FIG. 4. Here, an upper stopping element could be situated above, i.e., outward of, the upper stopping piston (similar to the location of stopping elements  321  or  421  shown in either FIG. 3 or  4  relative to the stopping piston  313  or  413 , respectively) and a lower stopping element situated below (again outward of) the lower stopping piston (similar to the location of stopping element  426  shown in FIG. 4 relative to stopping piston  413 ). Both stopping pistons would be situated on and moveable along the same piston shaft as the working piston. Such an arrangement permits the stops for both retraction and extension of the working piston to be separately set relative to each other. Alternatively, the stopping elements can be appropriately situated on the piston shaft inward (i.e., sandwiched between), rather than outward, of the two stopping pistons. Depending on a specific application, other orientations for the cylinders than those shown and described for the stopping and working pistons, such as being non-adjacent, could be used as well.  
         [0055]    Furthermore, for the embodiment shown in FIG. 3, a single stopping element could be located on the piston shaft and situated below, rather than above, the stopping piston in order to stop movement of the working piston in just its upward direction (retraction).  
         [0056]    In addition, while both embodiments utilize, for simplicity, a common piston shaft for both the working and stopping pistons, such a common shaft is not necessary. Other configurations can be used and, in fact, the shaft extending from the working piston towards the stopping piston can be different from that extending upward from the working piston. Moreover, the former shaft need not be a single shaft running through a central bore of the stopping piston but can instead be formed of one or more shafts, where each runs through a different bore in the stopping piston provided that one or more of these shafts has an appropriate stopping element situated on it.  
         [0057]    Although various preferred embodiments which incorporate the teachings of our present invention have been separately shown and described in considerable detail herein, those skilled in the art can readily devise many other different embodiments that still incorporate the inventive teachings.