Abstract:
A hardness tester having a frame and a rotatable turret movably supported on the frame is provided. A plurality of load cells are fixedly mountable on the turret, and a plurality of indenters are fixedly attachable to the load cells, respectively. A user interface selectively provides signals to a motor to move the turret into contact with a test specimen via one of the indenters to thereby apply a load on the test specimen. The indenters are fixed with respect to the turret and do not move in relation to the turret when the turret is brought down to bear on the test specimen. The load cells measure the load applied to the test specimen. A closed loop control system receives load measurement signals from the load cells and controls movement of the turret, preventing the motor from applying load in excess of a predetermined selectable load amount input by a user via the user interface. The invention preferably includes a plurality of indenter adapters, each attached to respective undersides of the load cells. Each indenter adapter includes a slot into which the indenter is fittable, and least one set screw for adjusting a horizontal location of the indenter.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to hardness testers, and more specifically to penetration hardness testers that can perform more than one type of penetration hardness test. 
     2. Description of Related Art 
     Penetration hardness testers are well-known in the art, and generally include a diamond or ball tipped penetrator and means to apply minor or major loads of predetermined magnitudes through the penetrator to a test specimen in successive load cycles. The hardness of the surface being tested produces results such as a Rockwell number or Brinell number. The hardness is related to the depth of penetration of the penetrator into the surface when a selectable value of compressive force is applied to the penetrator. Optical measurement of diagonal length of an indentation is performed for Vickers and Knoop tests, for example. 
     In prior art Rockwell type hardness testers, the force that is exerted on the penetrator is produced by gravity acting on weights, and this in turn is transferred by mechanical means to the penetrator. The depth of penetration is generally directly measured from the tool and generally displayed on a dial indicator, digital display or other display apparatus. Prior art apparatus requires gravity acting on weights, and the measurement of the tool movement through mechanical assemblies is subject to impreciseness as the tester is repeatedly utilized subjecting the apparatus to wear through repeated mechanical movement. 
     The use of deadweight testers and their mechanical impreciseness over time has led to the use of a load cell as part of the means to measure the application of force to the test specimen. An example of a system employing a load cell is found in U.S. Pat. No. 4,535,623 entitled Material Hardness Testing Apparatus by Paul Gilberto, a patent assigned to a predecessor of the assignee of the present application and now owned by the instant assignee. In the &#39;623 patent, a load cell is located adjacent the penetrator, and deadweights are avoided in conducting the hardness tests. A mechanical threaded advancing means is employed to apply the load to the test specimen, and the load on the load cell is related to the force on the test specimen. The mechanical action in the &#39;623 patent for applying force by the tester, by its very nature, will, over time cause impreciseness because of the relative movement of the threaded screw and its driven elements. Such inaccuracies can become significant in the measurement process as the underlying measurements are used as a basis for many determinations thereafter. 
     The use of feedback control closed loop systems can lessen the impreciseness which is attendant to materials hardness tests. U.S. Pat. No. 4,435,976 describes the use of a load cell to determine the forces applied during Brinell tests and employs a feedback loop to automatically compensate factors which affect the accuracy of the measurements, such factors being temperature and friction. The apparatus in the &#39;976 patent utilizes a mechanical bearing connected between the indenter and the load cell, which mechanical bearing, itself, can cause inaccuracies in the measurement process because of its repeated mechanical movement and the wearing of the bearing. 
     The indenter will penetrate to some depth or displacement in the test specimen. A measurement is made of the displacement, and in prior art penetration hardness testers, there are moving mechanical parts which move relative to each other located between the actual displacement and measured displacement. Such relative mechanical movement can contribute to sources of friction or lost (non-recoverable) displacement between the point of displacement measurement and the test specimens so as to impair the repeated accuracy of the hardness test. 
     All known bottom-referencing type hardness testing machines, both using load cell or deadweight style, employ an elevating screw to accommodate different specimen sizes. The mechanical forces employed in the elevating screw also can contribute to degradation of displacement measurement accuracy because of the possibility of additional deflection loss which can contribute to the inaccuracy of the displacement measurement. U.S. Pat. No. 5,616,857 to Merck et al. and assigned to the instant assignee (the teachings of which are incorporated herein by reference), for example, teaches the use of different sized platforms for mounting and supporting specimens in lieu of an elevating screw. 
     The use of load cells has increased the accuracy of hardness testers. However, load cells are made less accurate by increasing the amount of dead weight hanging therefrom. Weight and/or structure below the load cell creates dynamic forces that decrease the accuracy of the load cell readings. 
     Another drawback to conventional microhardness testers is that they cannot be easily reconfigured from being able to perform one type of hardness test to being able to perform another. This detraction can be particularly inconvenient when it is desired to perform more than one test on the same specimen. An interesting non-microhardness tester is taught in U.S. Pat. No. 5,177,999 to Tobolski et al., assigned to a predecessor in interest to the instant assignee and now owned by the instant assignee (the teachings of which are incorporated herein by reference). Tobolski et al. teach the provision of several indenters mounted on a rotatable turret. The turret is brought down to the specimen, and the spring-loaded indenter provides an indentation. If a second test is desired, the turret is rotated so that a different indenter is positioned over the specimen. A dead weight assembly provides the load in the Tobolski patent. Because the indenters are spring loaded, they move with respect to the turret. As a result, additional moving parts render the load applications and measurements thereof less accurate, in that there are greater opportunities for dead mechanical movement in the system. Moreover, one is limited in the number of different indenters one may use by the availability of mounting spots on the turret. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide an improved hardness tester which eliminates the inaccuracies of prior hardness testing apparatus. 
     Another object of the present invention is to provide such an improved apparatus which is easy to operate, substantially unchanging over time and location and produces reliable and accurate results. 
     Another object of this invention is to provide such an apparatus which advantageously employs current technology to provide improved results and may be readily adapted to provide additional test data. 
     Another object of the invention is to provide a penetration hardness tester that is capable of performing more than one type of hardness test. 
     Another object of the invention is to provide a penetration hardness tester that is capable of easily switching between being able to perform more than one type of test. 
     Another object of the invention is to provide a penetration hardness tester upon which it is easy to mount and replace different indenters. 
     Another object of the invention is to provide a penetration hardness tester which operates over a large ranges of forces. 
     Other objects, advantages, and features of this invention will become apparent from the description of the invention which is a hardness tester having a first frame and a rotatable turret movably supported on the first frame. A motor is movably supported on the first frame; the motor is connected to the turret and selectively moves the turret with respect to the first frame. A plurality of load cells are fixedly mountable on the turret, and a plurality of indenters are fixedly attachable to the load cells, respectively. A user interface is provided electrically connected to the load cells and the motor. The interface selectively provides signals to the motor to move the turret into contact with a test specimen via one of the indenters to thereby apply a load on the test specimen via the one of the indenters. The indenters are fixed with respect to the turret and do not move in relation to the turret when the turret is brought down to bear on the test specimen. The load cells measure the load applied to the test specimen. 
     In a preferred embodiment, the invention further includes a second frame, movably mounted on the first frame. The turret is rotatably mounted to the second frame. The motor is connected to the second frame, and the motor selectively moves the second frame with respect to the first frame. The invention preferably further includes a closed loop control system electrically connected to the motor and the load cells and the user interface. The closed loop control system receives load measurement signals from the load cells and controls movement of the second frame, preventing the motor from applying load in excess of a predetermined selectable load amount input by a user via the user interface. Preferably, the invention includes a plurality of indenter adapters, each attached to respective undersides of the load cells. Each indenter adapter includes a slot into which the indenter is fittable and at least one set screw for adjusting a horizontal location of the indenter. 
     The only weight below the load cell in the instant invention is the indenter and the indenter adapter. The removal of weight and/or structure below the load cell decreases the amount of dynamic forces that could be created and thus decreases the inaccuracy of the load cell readings. Also, by providing indenters that are fixed with respect to the turret, the invention has fewer moving parts and is more accurate, in that there are fewer opportunities for dead mechanical movement in the system. Finally, the inventive indenter adapters allow for different indenters to be mounted and removed from the turret quickly and easily. The indenter adapters also enable the user to make fine adjustments on the position of the indenter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the exterior of a preferred hardness tester according to the invention. 
     FIG. 2A is a front perspective view of the hardness tester of FIG. 1 with the cover removed. 
     FIG. 2B is side exploded perspective view of the some of the internal components of the hardness tester of FIG.  2 B. 
     FIG. 3A is a top plan view of the frame and motive components of a hardness tester according to the invention. 
     FIG. 3B is an exploded perspective view of the motive components depicted in FIG.  3 A. 
     FIG. 4A is an enlarged front plan view of the turret, load cell, and indenter adapter according to the invention. 
     FIG. 4B is an exploded partially transparent perspective view of the load cell and indenter adapter according to the invention. 
     FIG. 5 is a schematic of the closed loop feedback control system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will now given of the inventive hardness tester with reference to FIGS. 1-5. As shown in FIGS. 1-2B, the inventive tester  5  includes a base and a support frame  12 . Support frame  12  is fixed to the base and serves as the structural foundation of tester  5 . In FIG. 1, only rear support  13  is visible. Mounted on base  10  is a specimen stand  14 . Specimens are placed on stand  14  for hardness testing purposes. A control panel  16  and interactive display  20  enables the user to select and modify the various parameters of the testing process, including the amount of load to be delivered to the specimen, which indenter is to be used, focusing of objectives, and the like . The user can see the results of the test on user interface display  20 . A bracket  22  can be used to mount display  20 ; the bracket allows the display  20  to be tilted for sitting or standing user modes of operation. The lower interior elements of tester  5  are shielded from dust and otherwise protected by cover  18 , and the upper moving interior elements of tester  5  are protected by main cover  24 . 
     As shown in FIGS. 2A-B, support frame  12  includes support columns  30  and  32  which are connected by a brace  34 . The support frame also includes an upper support platform  36  and a lower support platform  37 . Moveably mounted on the support frame is a movable turret frame  40 . Frame  40  includes upper cross-support  41  and lower cross-support  41 ′. Upper and lower cross-supports  41  and  41 ′ include bores through which support columns  30  and  32  pass. Upper cross-support  41  also accommodates long screws  42  and  44  which provide the means for moving the turret frame  40  with respect to support frame  12 . Each of long screws  42  and  44  has a nut attached to it between upper cross-support  41  and upper support platform  36 . 
     The motive power for moving turret frame  40  vertically with respect to support frame  12  is supplied by motor  50  (see FIGS.  3 A-B). Several pulleys  52 ,  54 , and  56  are employed to transfer torque from motor  50 . Motor  50  is provided with shaft  57  and shaft extender  58 . A drive belt  60  is connected to all of pulleys  52 ,  54 , and  56  as well as shaft extender  58 . Pulleys  52  and  54  are attached to long screws  42  and  44 , respectively. Idle pulley  56  is provided to take up slack on drive belt  60 . When motor  50  is activated, shaft  57  rotates, causing drive belt  60  to rotate pulleys  52 ,  54 , and  56 . The rotation of pulleys  52  and  54  causes long screws  42  and  44  to rotate. Because nuts  46  and  48  are fixedly mounted to turret frame  40  and threaded around screws  42  and  44 , the rotation of screws  42  and  44  causes turret frame  40  to move along the screws, either up or down depending on the direction of the rotation of the screws. In this way, the motor  50  can move turret frame  40 , and thus turret  80 , closer to or further from a test specimen mounted on stand  14 . As shown in FIG. 2B, a scale  70  is provided fixed to the support frame  12 , and an optical sensor  72  is fixed to turret frame  40 . The position of the turret frame  40 , and thus the indenters on the turret, can be determined by sensor  72  and used as explained below. 
     The support columns  30  and  32 , screws  42  and  44 , platforms  36  and  37 , frame  40 , motor  50 , and the associated pulleys and nuts are collectively called the actuator. In one embodiment, the actuator is mounted on rear support  13  but is modularly removable; that is, the entire actuator assembly may be removed and mounted in a different system. 
     Attached to the bottom portion of lower cross-support  41 ′ is turret  80 , upon which are mounted one or more optical objectives  82 . An optical assembly  84  is provided in line with the working position of turret  80 , i.e., the position of the turret that is directly above a test specimen placed on stand  14 . A user of the microhardness tester may look into optical assembly  84  to view a sample on stand  14  in a magnified fashion so as to examine indentations made by the indenters of the hardness tester. Microscope illuminator  86  is connected to optical housing  89  via connector  88 . Illuminator  86  provides a light source to aid the user in examining the specimen. 
     Also attached to the turret  80  are load cells  90  and indenters  92  mounted thereon (see FIGS.  2 A and  4 ). A plurality of indenters  92  is preferably provided so that a plurality of indentation tests (e.g., Vickers, Knoop, etc.) may be performed by the same tester on the same specimen without removing or replacing an indenter from the turret. When it is desired to perform a first microhardness test (e.g., Vickers), the turret is rotated so that the corresponding indenter  92  is in the working position over the specimen. If a different microhardness test (e.g., Knoop) is subsequently to be performed on the specimen, the turret is again rotated so the second indenter  92  corresponding to the second test is in the working position over the specimen. In one embodiment of the invention, the user manually rotates the turret  80  to position the desired objective  82  or indenter  92  in the working position above the specimen. In another embodiment, the turret  80  is provided with a rotator (not shown) which is controllable via the control panel  16 ; the user selects the desired objective or indenter via entering the proper keystrokes on the control panel. 
     The precise horizontal positioning of the indenter  92  with respect to the specimen is enabled by the inventive indenter adapter shown in FIG.  4 . Load cell  90  is attached to turret  80  via attachment piece  91 . Attachment piece  91  may be threaded so that the load cell  90  screws into a threaded recess in the turret  80 . Alternatively, attachment piece  91  may be magnetic, spring loaded, or the like. Beneath load cell  90  is attached an indenter adapter  94 . Load cell  90  includes a slot  96  for receiving a mounting pin  98  protruding upwards from indenter adapter  94 . Indenter adapter  94  is provided with a slot  100  for receiving mounting pin  102  of indenter  92 . Pin  102  is not threadedly attached to slot  100 . Rather, three screws  104 ,  106 , and  107  are fitted into bores  108 ,  110 , and  111  respectively, and they secure pin  102  inside slot  100 . Pin  102  is machined about ½ mm smaller in diameter than slot  100 . Screw  104  is spring loaded and exerts force against pin  102  to keep it snugly within slot  100  and abutting against set screws  106  and  107 . Screws  106  and  107  are not spring biased but can be turned to adjust the position of pin  102  within slot  100 . Bores  108 ,  100 , and  111  are provided in indenter adapter  94  through which the set screws  106  and  107  and the spring screw  104  pass to contact pin  102  in slot  100 . A spring wave washer  112  and adapter clamp  114  are provided to maintain indenter  92  abutted against indenter adapter  94 . Screws  116  secure clamp  114  to adapter  94 . Screws  116  are made tight enough so that spring wave washer  112  can exert a retaining force against the flange  115  of indenter  92 , but are loose enough so that the horizontal position of indenter  92  may be adjusted by the rotation of set screws  106  and  107 . 
     The inventive hardness tester includes a closed loop feedback control system for enabling the proper amount of load to be supplied to the specimen. A central processing unit  120  (CPU) is provided, as shown schematically in FIG. 5, in the feedback loop. The closed loop system is responsive to loads applied to the indenter  92  and the test specimen by setting a desired load in the CPU  120  via control panel  16  and sensing the load applied at the load cell  90 . 
     Another aspect of the present invention is the ability to accurately control the rate of application of the load, and the control system of this invention provides means to determine the rate of application of the load system. 
     The control loop of the present invention is a proportional integral derivative gain control loop with real time stiffness compensation. This type system enhances sensing an error signal in the closed loop to intensify the sensitivity of the apparatus. The use of the optical sensor  72  and scale  70  provides indenter displacement information. Fiber optic and/or laser devices may also be effectively used to sense the position or location of the turret frame  40  and thus the position of indenter  92 . 
     The invention can also protect the indenter from accidental damage. For example, if the turret is moving is a non-test mode, e.g., when one is moving the turret into position to begin a test, any load sensed by the load cell will halt the movement of the turret, since the turret is not supposed to encounter any resistance in a non-test mode. This is accomplished by CPU  120  operating in the closed loop feedback control system. 
     Also, the dwell time of the indenter can be varied. A user may program the indenter to apply the desired load for a predetermined period of time. Such a command would be enterable via user interface display  20 , for example. 
     In accordance with an aspect of the present invention, by employing a closed loop system with motor  50 , pre-test positioning may be achieved much more quickly. By employing the central processing unit  120 , improved data gathering is realized including scale changing and other aspects of the testing procedures. Further, in addition to the rate of application of the load, the amount of penetration as well as other parameters in the measuring process may also be sensed and utilized to further define the mechanical properties of the materials being tested. The tester includes the ability to produce related pairs of information about the displacement and related load. The tester, therefore, can identify and cancel out any displacements not specifically the result of specimen deformation, as well as locate the surface of the specimen from the data pairs. As a result, the tester can determine the actual depth of indenter penetration into the specimen for any given load. 
     In the present invention, there is less mechanical movement between connected parts that can affect test results, and the elimination of such mechanical movement decreases the number of inaccuracies due to friction, dirt buildup, and repeated mechanical use. This is especially true with the load cell being directly coupled to the indenter without intervening moving parts, and this is further made advantageous by the direct mounting of a fixed non-moving indenter onto the turret. Thus, all moving mechanical parts are within the closed loop system; as a result, friction, dirt buildup, and mechanical wear no longer affect the force applied. This provides more reliable, repeatable testing over time and allows better comparison of data obtained from testers in different locations. 
     As a feature of this invention, “smart” indenters may be employed with this system in which calibration factors for different measurements can automatically be loaded into the system to automatically compensate for changes or differences which occur in the different indenters. 
     In operation, the invention is used as follows. A test specimen is placed on stand  14 , and it is desired to perform several different microhardness tests on the specimen. A user enters commands into control panels  16  and  20  so as to cause a specific type of test to be performed with a specific load. Other variables, such as indenter dwell time, may be programmed in this manner. The user selects which indenter he wants to use for the test. This can be done either manually or automatically, depending on the embodiment of the invention, as described above. The control panels  16  and  20  forward the commands as electric signals to CPU  120 , which in turn controls the actuation of motor  50 . Motor  50  is activated and causes turret frame  40  to move downwardly towards the specimen. As turret frame  40  moves, optical sensor  72  senses the position of the turret frame  40  by reading the scale  70  fixed to the support frame  12 . When the indenter  92  contacts the test specimen, load cell  90  senses that contact and relays that information to the CPU  120 . The CPU makes note of the no-load first contact position of the turret based on the position reading of optical sensor  72 . Thereafter, CPU  120  sends signals to motor  50  to continue to provide torque to long screws  42  and  44  and thereby continue to move turret  80  downwards. Because the indenter  92  is contacting the specimen, the continued application of motive power by motor  50  causes the indenter to exert a load on the specimen and thus leave an indentation in the specimen. The load cell  90  senses the load being applied and reports that load measurement back to the CPU. When the load sensed by the load cell  90  is equal to the predetermined load selected by the user, the CPU  120  disengages the motor  50  so that the specimen is not further loaded. The change in position from the point where the indenter  92  first contacted the specimen until the point where the desired amount of load is reached is equal to the depth of the indentation made in the specimen. This depth measurement is displayed on display  20  along with other pertinent information. If the user wishes to examine the actual indentation, the user rotates turret  80  so that optical objective  82  is in the working position above the indentation. The user activates microscopic illuminator  86  and peers into optical assembly  84 . Should another test be desired, the user rotates the turret  80  so that the second indenter is aligned over the specimen. 
     This invention has been described with reference to a preferred embodiment and other embodiments are considered within the scope of this invention as defined by the appended claims. For example, any number of different indenters and objectives may be provided on the turret. Also, although the drawings depict two set screws and one spring-loaded screw being used in the indenter adapter, any convenient number of screws may be employed in the indenter adapter to enable the user to position the indenter precisely in the horizontal plane. Further, the drawings depict a turret that is movable in a vertical direction with respect to the fixed support frame. However, the invention is not so limited; rather, the invention includes a hardness tester in which the movable turret frame moves horizontally with respect to the fixed support frame as well.