Abstract:
A breakaway unit sits atop a base plate of a coordinate measuring machine via a breakaway coupling system. A crash detection system including at least one crash detection sensor mounted on the breakaway unit and a crash detection controller stops the machine when vertical movement of the breakaway unit exceeds a threshold. Should collision occur, the breakaway coupling system allows the breakaway unit to separate from the base plate, preventing damage to instruments mounted thereon. The kinematic coupling system preferably comprises tooling balls engaging respective vee cones, vee grooves, and/or flats.

Description:
BACKGROUND AND SUMMARY  
       [0001]     Many multi-axis, multi-sensor coordinate measurement machines typically use crash-detection or crash-prevention mechanisms to avoid potential damage to probes and other sensing devices. Most of these mechanisms employ a release mount of the probe and/or sensors when a predetermined amount of force is applied to the probe and/or sensor. Because this force may include an impact element, the probe and/or sensors can be thrown out of alignment, requiring realignment and recalibration of the probe/sensors and the Z-axis at significant cost in time and funds. To reduce the amount of realignment and recalibration required after a collision, others have pursued various arrangements.  
         [0002]     Consider, for example, U.S. Pat. No. 6,852,002 to Stewart et al., assigned to Flow International Corporation and entitled, “Apparatus and Methods for Z-Axis Control and Collision Detection and Recovery for Waterjet Cutting Systems.” The cutting system includes a linear rail, a slide member coupleable to a cutting head and slidably coupled to the linear rail, an actuator having coupled to the slide member and fixed to the linear rail, a position sensor, and a controller. The actuator provides an adjustable support force that supports the weight of the cutting head, allowing the cutting head to be controllably positioned at a desired height above the workpiece. Stewart et al. use a first mount member coupleable to a controllably positionable mounting surface of the cutting system, a second mount member coupleable to the cutting head and disengageably coupled to the first mount member, and a sensing circuit having a plurality of first conductive elements disposed on the first mount member and a plurality of second conductive elements disposed on the second mount member. If the cutting head collides with an obstruction, the second mount member disengages from the first mount member to prevent breakage of the cutting head. After a collision, the second mount member is re-engaged with the first mount member without recalibration. Re-engagement of the second and first mount members can be performed automatically by a biasing member. While this is a step in the right direction, the arrangement can result in movement of the tool out of its aligned position. When the tool is reconnected, the degree to which it returns to its original alignment, and its repeatability, is not as high as a high precision metrological instrument requires.  
         [0003]     Also consider U.S. Pat. No. 5,867,916 to Matzkovits, assigned to Carl-Zeiss-Stiftung and entitled, “Coordinate Measuring Machine with Collision Protection.” This system is a coordinate measuring machine with a measuring arm on which a collision protector is provided. The collision protector can be deflected transversely of the longitudinal axis of the measuring arm when the measuring sensor system collides with an object. To operate the coordinate measuring machine with different measuring sensor systems and machining units, the coordinate measuring machine includes an identification unit that automatically identifies the measuring sensor system or machining unit. A securing unit is connected to the identification unit and allows adjustment of the torque required to deflect the collision protector in response to identification of the measuring sensor system or machining unit by the identification unit. While this prevents damage to the sensing unit, the collision protector is a breakaway portion of the measuring arm. When a collision induces movement of the collision protector, the arrangement does not guarantee precise realignment when the collision protector returns to its original position.  
         [0004]     U.S. Pat. No. 5,210,399 to Maag et al., assigned to Carl-Zeiss-Stiftung, and entitled, “Optical Probe Head with Mounting Means Providing a Free Recalibration of the Sensing Head after a Collision,” keeps all position-sensitive components rigidly fixed using a design similar to that of Stewart et al. An optical probe head has a front optic and an annular enclosure surrounding the front optic. The enclosure contains the illuminating device of the probe head. The front optic is rigidly attached to the optical probe head and the enclosure having the illuminating device and surrounding the front optic is attached to the optical probe head so as to be radially yieldable, such as with bearings related to the pin and ball arrangement of Stewart et al. In the case of a collision, only the enclosure having the illuminating optics is deflected, the imaging optics remaining undisturbed. In this way, Maag et al. state that a follow-up calibration of the probe head after a collision is no longer required.  
         [0005]     DE19622987 to Mettendorf et al., assigned to Mycrona, and entitled, “Collision Protection Appliance for Sensors on Coordinate Measurement Machine.” The appliance has a laminar clearance sensor (2) on the lower end of its measurement sensor (1). The sensor can be a capacitive device with its beam lobe directed both radially and axially. The beam lobe of the capacitive sensor can be directed radially and the axial protection against collision can be provided by a ring suspended from a mechanical switch.  
         [0006]     Embodiments solve this dilemma of realignment and calibration of the vertical axis and the primary measurement sensor, as well as secondary and tertiary measurement sensors, if present, by removing all collision-related release from the instrument tower. A mounting plate to which a fixturing device, such as a rotary module, can be attached rests on a base plate via a kinematic mount arrangement. The kinematic mount of embodiments allows the mounting plate to break away from the base plate in the event of a collision, yet provides enough resistance that ordinary operative fluctuations in moment and orientation of the mounting plate resulting from motion of the fixturing device do not initiate breakaway. Additionally, the kinematic mount allows the mounting plate to be replaced in the kinematic mount to within microns of its original position after a collision, eliminating the need for recalibration of the instrumentation. If, however, calibration is required, a simple, quick calibration can be performed using a removable artifact.  
         [0007]     Additionally, embodiments employ a crash detection system, preferably mounted on the mounting plate. The crash detection system of embodiments uses sensors, such as proximity sensors, and a controller to monitor the state of the mounting plate and, when the mounting plate breaks away from the base plate, stops the machine in which the breakaway/crash detection system is used. By making the breakaway/crash detection system part of the portion of the device that holds an object to be inspected, the sensors are isolated from shift due to a collision and thus do not need to be recalibrated after a collision/breakaway. Instead, the plate can simply be replaced on the base plate with no calibration, and the inspection can be restarted or resumed. If calibration is required, a very quick procedure can be employed involving a reference artifact placed on the plate. Thus, embodiments eliminate the need for realignment and calibration of sensors after a collision.  
         [0008]     The rotary module, or other fixturing devices, to which the breakaway/crash detection system is fixed, breaks away from the solid horizontal axis of motion in embodiments upon collision. The break away design allows the optical system, probes, sensors, and any other position-sensitive components to be rigidly mounted to the vertical axis of the machine and minimizes or eliminates the release and re-align problem. This preserves the accuracy and repeatability of the optics, sensors, probes, and axis of motion in the event of a collision. Additionally, by placing the breakaway design low on the horizontal axis, the possibility of any small errors accumulating during re-alignment is reduced, particularly in accordance with vertical axis. Such small errors are amplified as the focal or working distance of the sensors is increased. The break away design of embodiments thus overcomes the alignment and calibration issues present in the prior art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is an exploded schematic view of a breakaway system with crash detection on a stage of a precision measurement apparatus according to embodiments. 
     
    
     DESCRIPTION  
       [0010]     This description sets forth an exemplary embodiment with reference to the accompanying Figures. This exemplary embodiment is not limiting, and variations are encompassed by embodiments.  
         [0011]     As mentioned above, embodiments reduce or eliminate the need to realign and recalibrate the vertical axis and primary and, when present, additional measurement sensors. As seen, for example, in  FIG. 1 , embodiments of a breakaway and crash detection system  1  can include a stage  2  that supports a mounting or fixturing plate  10  on which a fixturing device  11 , such as a rotary module, can be arranged to hold and/or manipulate an object to be inspected. While the fixturing device  11  used with embodiments is shown in the FIG. as a rotary module, embodiments can be used with other such fixturing devices as appropriate.  
         [0012]     The breakaway system includes a kinematic mounting arrangement, preferably including three tooling balls  12 . As is known in the art, tooling balls are high-precision hardened steel balls. In embodiments, the tooling balls  12  are attached to one of the mounting or fixturing plate  10  and the stage  2 . As seen in  FIG. 1 , the tooling balls  12  are preferably mounted in the fixturing plate via posts that can include, for example, threads. The tooling balls  12  rest in features  14 ,  15 ,  16  mounted or formed on the stage  2  to form the kinematic mount, which supports the fixturing plate  10  via the tooling balls  12 . Since the stage  2  of embodiments can be made from materials not suitable for repeatable repositioning of the fixturing plate  10 , embodiments provide hardened pads in which the respective features  14 ,  15 ,  16  receiving the balls  12  are formed. The pads can be made from hardened, ground steel, for example, or any other suitable material.  
         [0013]     At least two of the pads preferably include features that engage their respective tooling balls  12  and prevent motion of the firing plate in at least one direction to constrain the fixturing plate  10  against translation in the plane of the stage  2 . For example, embodiments employ a vee cone  14  that prevents motion of one tooling ball  12  in the plane of the stage  2 , a vee groove  15  that prevents motion of its tooling ball  12  along a specific axis in the plane of the stage  2 , and a flat  16  that prevents rotation of the fixturing plate  10  about the axis defined by the other two tooling balls  12 . However, the arrangement allows, and induces, vertical motion of the plate  10 —motion perpendicular to the plane of the stage  2 —should the plate  10  collide with something or should something else collide with the plate.  
         [0014]     Thus, in embodiments, the three tooling balls  12  preferably engage with a vee cone  14 , a vee groove  15 , and a flat  16 , respectively. The tooling balls  12  and vee cones  14 , vee grooves  15 , and flats  16  are typically made from hardened, ground steel to preserve their dimensional accuracy and geometry. Such a system firmly holds the first, fixturing plate  10  to the second plate or stage  2  in a particular alignment even after repeated reseating of the balls  12  in their respective features  14 ,  15 ,  16 , which is how the alignment of embodiments is preserved.  
         [0015]     To prevent unintentional vertical translation of the plate  10  beyond what gravity provides, a biasing or preload arrangement  20  can be included. In embodiments, the biasing arrangement  20  includes a plurality of adjustable preload devices  21 , preferably mounted near the tooling balls  12 . An example of a preload device  21  suited for use with embodiments includes a housing  22  that supports one or more springs  23 , the springs  23  being connected to a wall  24  of the housing at one respective end and a pull bar  25  at the other respective end. The pull bar  25  in turn retains an end of a cable  26  that extends over a bushing  27  and down through the housing  22 , through the fixturing plate  10 , and toward the stage  2  to which it is connected. Embodiments employ a loop of cable  26  that has left and right legs, the ends attached to the springs  23 , and the extremity of the loop being hooked about an attachment point  28  in the stage  2 . A screw  29  extending through the preload device housing  22  and into the pull bar  25  allows adjustment of a preload induced by the device.  
         [0016]     As seen in  FIG. 1 , embodiments can include three substantially equally spaced such adjustable preload devices  21  on the first, fixturing plate  10 . Once installed and adjusted, the preload devices  21  bias the fixturing plate  10  and stage  2  together so that more force is required to induce vertical motion of the fixturing plate  10  that would result in breakaway of the fixturing plate  10  from the stage  2 . This prevents unintentional breakaway should a sudden motion or high-mass rotation of the fixturing device  11  cause the fixturing plate  10  to jump. A larger mass or higher center of gravity may require a higher preload and a smaller mass or lower center of gravity a lower preload. When used with a rotary module as the fixturing device  11 , for example, the mass of the module moves through an arc and may require a higher fixturing preload than other types of fixturing devices. The adjustable preload devices  21  provide the ability to accommodate these requirements and can also aid in returning the unit to its original mounting should breakaway occur.  
         [0017]     As seen in the accompanying FIG., the breakaway system of embodiments is preferably placed low on the horizontal axis of the machine to reduce the accumulation of small errors accruing during re-alignment, particularly with respect to the vertical axis. Such small errors are generally amplified as the focal distance or working distance of the sensor is increased. In operation, the fixturing plate  10  breaks away from the solid horizontal axis of motion in embodiments when a collision occurs. The breakaway arrangement of embodiments substantially eliminates the release and re-align problem of prior art devices with respect to the optical system, probes, sensors, and any other position-sensitive components rigidly mounted to the vertical axis. This preserves the accuracy and repeatability of the optics, sensors, probes and axis of motion.  
         [0018]     Embodiments preferably further include a crash detection system  30  that comprises at least one proximity sensor  31  capable of sensing small variations in vertical movement, preferably as little as 0.0005″ (12 μm). The at least one sensor  31  is connected to a proximity controller  32  that stops motion in the horizontal direction in a small amount of travel, preferably as little as 0.002″ of travel, if the sensor  31  senses movement of the fixturing plate  10 . In particular, embodiments preferably include three displacement sensors very near or adjacent the tooling balls  12  on the fixturing plate  10  and connected to the proximity controller  32 , as seen in  FIG. 1 . Embodiments contemplate the use of various types of position/displacement sensors. For example, proximity, reed, laser, capacitance, and/or force sensors can all be applied. In addition, any other type of position/displacement sensor could be employed as long as it meets the requirements of the system  30 . The proximity controller(s)  32  can be mounted on a support  33  attached to the fixturing plate  10 , though a remote controller  32  could also be employed. As shown, the controller mount  33  includes a plate  34  attached to the fixturing plate  10  with screws  35  or the like, but the mount  33  could instead be formed as part of the fixturing plate  10 .  
         [0019]     In embodiments, the breakaway/crash detection unit can include a mechanical stiffener  40 , such as the rail shown in  FIG. 1 , to prevent axial twist of the breakaway unit during operation of the fixturing device  11 , such as during rotation of a primary rotary under maximum loading conditions. The fixturing plate  10  is preferably kept to a minimum thickness to prevent loss of vertical measurement capability in such an arrangement. Including the stiffener  40  affords a high measurement volume to stiffness ratio, which reduces errors that can accumulate in the five axes of motion of the unit. The high ratio also reduces the effect of compounding errors in the final measurement results.  
         [0020]     It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.