Abstract:
Concepts presented herein relate to a portable device that includes a frame and a fixture for engaging a mechanical probe to be calibrated. The fixture can be a platform of hard material that receives pushing action of the mechanical probe. A displacement sensor senses position of the platform with respect to the frame. An actuator is coupled to the displacement sensor and a controller is coupled to the actuator. The controller operates the actuator to cause the platform to move to a position (as indicated by the displacement sensor), while the force required to cause the displacement is measured with a force sensor.

Description:
BACKGROUND 
       [0001]    The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0002]    Mechanical probes are used in a variety of different systems such as micro-manipulators, profilometers used in atomic force microscopes (AFMs), and hardness testers. The utility of such probes depends on the accuracy to which the force and displacement calibrations are known. These calibrations may depend on probing frequency, temperature, humidity, and other factors. Currently, multiple devices are used in the calibration of such probes. For example, the force calibration may be determined by balancing the probe against reference weights, while the displacement calibration may be determined using laser interferometry or step-height standards. 
       SUMMARY 
       [0003]    Concepts presented herein relate to a portable device that includes a frame and a fixture for engaging a mechanical probe to be calibrated. The fixture can be a platform of hard material that receives pushing action of the mechanical probe. A displacement sensor senses position of the platform with respect to the frame. An actuator is coupled to the displacement sensor and a controller is coupled to the actuator. The controller operates the actuator to cause the platform to move to a position (as indicated by the displacement sensor), while the force required to cause the displacement is measured with a force sensor. 
         [0004]    A method is provided for using the portable device to perform force calibration for a mechanical probe. The method includes holding the platform in a fixed position while the mechanical probe applies force to the platform. The force sensor of the device measures the force required to balance the action of the mechanical probe, thereby allowing a force calibration for the mechanical probe. The method is repeated for one or more positions of the platform, there by allowing force calibration as a function of probe position. 
         [0005]    Another method is provided for using the device to determine the displacement calibration for a mechanical probe. The method includes moving the platform to a series of positions while the mechanical probe applies a constant force to the platform such that contact between the mechanical platform and the probe is maintained. To minimize effects from electronic and thermal drift, motion of the platform can be cyclical while changing amplitude of the motion. 
         [0006]    Yet another method is provided for using the device to determine stiffness of a mechanical probe. This method assumes that the force and displacement sensors of the mechanical probe are well calibrated through the use of conventional methods or other means, including those described herein. The method includes holding the platform in a fixed position and using the mechanical probe to apply a series of forces. Since the device is statically rigid, any displacement sensed by the mechanical probe is due to compliance of the probe itself and/or its supporting frame. The stiffness of the probe can be determined as the relationship between force applied by the mechanical probe and displacement sensed by the mechanical probe. 
         [0007]    The Summary and Abstract are provided to introduce some concepts in a simplified form that are further described below in the Detailed Description. The Summary and Abstract are not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION 
         [0008]      FIG. 1  is a schematic view of an environment using a calibration device. 
           [0009]      FIG. 2  is a flow chart of a method for analyzing a system with a calibration device. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    A calibration device  10  for calibrating a system  11  having a mechanical probe  12  is illustrated in  FIG. 1 . Probe  12  is coupled to a controller  14  that actuates probe  12  and provides an output  15  indicative thereof. Output  15  can be a force and/or a displacement measurement of probe  12 . System  11  can be any type of system that utilizes a mechanical probe. These systems include micro-manipulators, atomic force microscopes and hardness testers. An exemplary hardness tester is described in U.S. Pat. No. 4,848,141, the contents of which are hereby incorporated by reference in their entirety. 
         [0011]    Device  10  can be provided in a suitable enclosure (illustrated in dashed lines) that includes one or more of the components described below. In one embodiment, the enclosure is portable to allow easy transfer from one probe to another probe. If desired, the enclosure can include one or more ports to transfer information related to force and/or displacement detection to a computing device. 
         [0012]    Device  10  includes a load-controlled, displacement sensing (LCDS) assembly  16  coupled to a frame  20  that is used to measure loads applied to the probe and/or provide a load to the probe  12 . As such, the LCDS assembly  16  is coupled to a fixture  22  and configured to control the fixture  22 . In one embodiment, fixture  22  includes a platform for engaging probe  12 . The platform can be made of a hard material so as to prevent substantial deformation thereof when engaged with probe  12 . 
         [0013]    The LCDS assembly  16  includes a permanent magnet  26  mounted in the frame  20 , a coil  28  and a displacement sensor  30 . A controller  32  controls current to the coil  28 . The controller  32  receives feedback signals from the displacement sensor  30 . Displacement refers to a change in position or to a position relative to a known position. The LCDS assembly  16  can also take other forms such as a piezoelectric assembly, rather than the electromagnetic assembly herein illustrated. The force sensor (which measures current to coil  28 ) and displacement sensor  30  are calibrated using traditional means. For example, the force sensor may be calibrated using reference weights, and the displacement sensor  30  may be calibrated using laser interferometry. 
         [0014]    The displacement sensor  30  is provided in order to provide feedback to the controller  32 , which in turn provides current to the coil  28 . The displacement sensor  30  senses the position of the fixture  22 . In the embodiment illustrated, the displacement sensor  30  comprises a capacitive sensor having a pair of fixed, stationary plates  36 A with a movable plate  36 B located therebetween. The movable plate  36 B is coupled to fixture  22 . Displacement of the fixture  22  is measured by the displacement sensor  30 , the output of which is connected to a DC displacement detector  40 . The detector  40  digitizes the DC displacement signal, which is provided to the controller  32 . 
         [0015]    The controller  32  controls a current source  42  that provides current to the coil  28 . Current can be provided to the coil  28  in order that the displacement sensor  30  be maintained substantially in a fixed position. Current can also be provided to move fixture  22  through a plurality of known positions. 
         [0016]    In addition, or alternatively, an oscillating load can also be provided. The oscillating load can be provided by superimposing an alternating current (AC) onto the drive current applied to the coil  28 . The frequency of the oscillating force applied is typically in the range of from 0.5 to 200 Hz; however, depending on the design of the LCDS assembly  16 , the concept can work from about 0.5 Hz to 1 MHz. The amplitude of the oscillating force may be in the range of from about 10 −10  to 1 Newton, although forces less than or greater than this range can also be provided. 
         [0017]    In  FIG. 1 , an AC signal generator  50  under control of the controller  32  injects or superimposes an AC signal into the output current signal of the current source  42 . An AC displacement detector  54  detects the resulting AC displacement. The detector  54  may be a lock-in amplifier, which is tuned to measure the amplitude of the AC displacement at the applied frequency together with the phase of the displacement signal relative to the applied signal. The amplitude and phase signals are digitized by the detector  54  and provided to separate inputs of the controller  32  for analysis or storage in a mass storage device  60 , along with the DC force and displacement, discussed above. 
         [0018]    The device  10  decouples the load sensitivity from the load capacity by using the LCDS assembly  16 . A suitable LCDS assembly  16  and capacitive displacement sensor  30  are available from the Nano Instruments Division of MTS Systems Corporation of Eden Prairie, Minn. Control of the load coil  28  and feedback from sensor  40  can be similar to that described in U.S. Pat. No. 6,679,124, the contents of which are hereby incorporated by reference in their entirety. 
         [0019]    In the embodiment illustrated, the displacement sensor  30  is a capacitive displacement sensor, as described above, wherein the fixture  22  is supported by very flexible leaf springs. Rather than depending on the deflection of a spring element in a conventional load cell in order to determine force, the device  10  is operated by using a feedback loop to maintain a known position of the fixture  22  by changing the current in the coil  28 . This results in static rigidity (i.e., there is little or no deflection of the load mechanism associated with large forces placed by probe  12 ). As discussed above, the known position of the fixture  22  can remain substantially unchanged when only static loading is applied. Alternatively, when an oscillatory force is applied, the known position varies in time while the average position can remain substantially unchanged. 
         [0020]    Device  10  can be utilized to calibrate devices that utilize a mechanical probe because it allows a simple calibration of both force and displacement in a single, portable unit. For example, measurements obtained by system  11 , such as output  15 , can be compared to measurements obtained by device  10 . A number of factors are considered in a design optimized for both displacement and force calibration. One factor is mutual independence of mechanisms for actuation, force measurement, and displacement measurement. Another factor is linearity of the leaf springs supporting fixture  22 . Yet another factor is the ability to move fixture  22  to more than one position. 
         [0021]      FIG. 2  is a flow diagram of a method  200  for analyzing and calibrating system  11  that utilizes mechanical probe  12 . At step  202 , method  200  can proceed to calibrate force for system  11  (and thus proceed to step  204 ), calibrate displacement for system  11  (and thus proceed to step  220 ), and/or determine stiffness of probe  12  (and thus proceed to step  230 ). Using device  10 , one or more of the force calibration, the displacement calibration, and the stiffness determination can be performed as desired. 
         [0022]    To calibrate force, method  200  proceeds from step  202  to step  204 , wherein a position of fixture  22  is set as indicated by displacement sensor  30 . At step  206 , a force is applied through mechanical probe  12  to fixture  22 . Action of probe  12  is balanced with device  10  by inducing a reaction force at step  208 . A force output for device  10  and system  11  is recorded at step  210 . An amount of current supplied to coil  28  is indicative of the force used to maintain fixture  22  in a known position. It can be decided at step  212  to change the force applied through probe  12  to fixture  22 . If so, method  200  returns to step  206  to apply the new force, balance the action at step  208  and record force output at step  210 . Otherwise, method  200  proceeds to step  214  to determine if a position should be changed. If a new position is desired, fixture  22  is controlled at a next known position, as indicated by the displacement sensor. 
         [0023]    Method  200  can perform measurements for a new position by returning to step  204  and repeating steps  206 ,  208  and  210 . This process can be repeated as desired for further known positions. Based on the measurements recorded in step  210 , the force calibration can be determined for system  11  at step  216 , for example by comparing outputs of system  11  and device  10 . Method  200  then decides whether to provide additional calibration at step  218 . If another calibration is desired, method  200  can then return to step  202  to provide further analysis of system  11 . 
         [0024]    To calibrate displacement for system  11 , method  200  proceeds from step  202  to step  220 . At step  220 , a position of fixture  22  is set as measured by displacement sensor  30 . At step  222 , a minimal force through probe  12  is applied to maintain engagement with fixture  22 . Device  10  is used to induce a reaction force to balance action of probe  12  at step  224 . The position output of device  10  and system  11  is recorded at step  226 . A determination is made to change the probe position at step  227 . Method  200  can return to step  220  to set the position. Displacement for system  11  can be calibrated at step  228  from measurements recorded during step  226 . For example, the displacement of fixture  22  (as known by displacement sensor  30 ) can be compared to displacement sensed by probe  12 . Method  200  can then return to step  202  from step  218  if desired. 
         [0025]    To determine stiffness of probe  12 , method  200  proceeds from step  202  to step  230 . At step  230 , a position of fixture  22  is set. At step  232 , a force is applied through probe  12  to fixture  22 . At step  234 , device  10  is used to balance the action of probe  12 . Force and position output of system  11  can then be recorded at step  236 . Step  238  determines whether additional forces should be applied at step  232  and step  240  determines whether additional positions for fixture  22  are needed for further measurements. At step  242 , stiffness of probe  12  is calculated as the relationship between the force applied by probe  12  and displacement sensed by probe  12 . This calculation may include accounting for displacement due to elastic or plastic deformation resulting from contact between the probe and the platform. Such displacement can be calculated independent of deformation in the probe itself. 
         [0026]    Given method  200 , device  10  provides a portable device in which force calibration, displacement calibration and determination of stiffness for a mechanical probe can be provided. Thus, multiple devices are not needed to provide an analysis of various systems of a mechanical probe. 
         [0027]    Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.