Abstract:
A system for positioning a test head for compliantly docking the test head with a prober, handler, or other peripheral for automatically testing electronic components. The system includes a plurality of backdrivable linear actuators. Each actuator has a first end mechanically coupled to the test head and a second end mechanically coupled to a support for holding the test head, for example, a manipulator. In a first mode, a control system applies inputs to the actuators for variably extending the actuators to establish a desired position of the test head relative to the support. The desired position is generally a centered position of the test head within a compliance range; however, it may also be a non-centered position that tends to align the test head with the peripheral. In a second mode, the control system stops varying the input to the actuator. The actuator tends to maintain its position, but complies with external forces applied to the test head. In the second mode, the actuators can be driven in compliance with external forces both forward and backward, and provide only slight resistance to movement in both directions. The disclosed system for positioning a test head is particularly useful for providing compliant docking with extremely heavy test heads.

Description:
This invention relates generally to precisely positioning heavy objects, and more particularly to precisely positioning a test head of an electronic automatic test system for docking the test head with a prober, handler, or other peripheral for testing electronic devices. 
     BACKGROUND OF THE INVENTION 
     Manufacturers of semiconductor chips and assemblies use automatic test equipment (“ATE”) to verify the performance of devices before the devices are shipped to customers. ATE systems typically include a “test head” and a “tester body.” The test head houses portions of the test system that are preferably located as close as possible to the device under test, and connects to the tester body via one or more cables. For testing electronic devices, the test head connects or “docks” with a peripheral. The peripheral feeds a series of devices to the ATE system for testing, and the ATE system tests the devices. 
     Constraints affecting semiconductor test processes often make it impractical to move the peripheral to the test head. In most manufacturing facilities, therefore, the peripheral that feeds the chips remains stationary, and the test head is moved into position for docking with the peripheral. 
     A device called a “manipulator” moves the test head to the peripheral.  FIG. 1  shows an example of a manipulator  100  for supporting a test head  110 . A manipulator of this type is disclosed in U.S. patent application Ser. No. 09/615,292, entitled “AUTOMATIC TEST MANIPULATOR WITH SUPPORT INTERNAL TO TEST HEAD,” which is hereby incorporated by reference. This manipulator is expected to be used with the Tiger™ test system, which is currently being developed by Teradyne, Inc., of Boston, Mass. 
     As shown in  FIG. 1 , a manipulator  100  supports a test head  110  from a region internal to the test head  110 . The manipulator  100  rotates the test head upon a twist gear  114 , and swings the test head at the end of a horizontal member  116  upon a swing bearing  122 . The manipulator  100  raises and lowers the test head on linear bearings  124 . A base  120  supports the manipulator  100 , and preferably includes outriggers  126  to prevent the manipulator from tipping. 
     The manipulator  100  preferably includes actuators such as motors (not shown) on the twist gear  114 , linear bearings  124 , and swing bearing  122 . The actuators move the test head to the peripheral, and orient the test head for docking. The test head is then docked with the peripheral by finely adjusting the position and orientation of the test head. 
     Manipulators commonly provide a range of “compliance” that allows a test head to be rotated about one or more axes as the test head and peripheral are being docked. The range is “compliant” because the test head literally complies with forces applied to the test head, which during docking tend to cause the mating surface of the test head to become coplanar with the mating surface of the peripheral. Without compliance, the manipulator would have to finely adjust the test head to a coplanar orientation with respect to the peripheral by precisely controlling the manipulator&#39;s actuators. For an example of a manipulator that automatically controls its actuators to achieve precise docking, see U.S. Pat. No. 5,949,002, entitled “Manipulator for Automatic Test Equipment with Active Compliance.” 
     Near the peripheral, the test head must be moved with great care. Both the test head and the peripheral include fragile electronic assemblies that can be damaged by collisions between the test head and the peripheral. Generally, the test head includes alignment pins for entering alignment bushings within the peripheral. During docking, the alignment pins must be made to enter the alignment bushings without bending or breaking them. 
       FIG. 2  is an exploded view of the test head  110  of  FIG. 1 , which illustrates how compliant movement of the test head  110  used in the Tiger™ test system can be achieved. As shown in  FIG. 2 , the test head  110  is composed of two portions  110   a  and  110   b,  which fasten to either side of a C-shaped stiffener  112 . A central blade  210  extends from the manipulator  100  into the C-shaped stiffener  112 , where it is held in place by a spherical bearing  228 . The spherical bearing  228  mechanically couples the central blade  210  to the test head  110  via a transition insert  224 , and allows the test head  110  to be rotated with respect to the central blade  210 . The transition insert  224  has a fixed position relative to the test head (although it can be moved back and forth on rails to center the test head by rotating the lead screw  226 ). The transition insert  224  has a clearance area that completely surrounds the central blade  210 . The clearance area allows the test head  110  to be rotated over a limited angular range about the spherical bearing  228 . More specifically, the test head  110  can be rotated over approximately 5 degrees in each of the conventional directions of theta, tumble, and twist. These directions are illustrated, respectively, with the arrows  230 ,  232 , and  236 . 
     It is generally desirable that a test head be oriented toward the center of its compliance range about each axis for which compliance is provided, during the time when the test head is moved into position for docking. Centering each axis of a test head within its compliance range ensures that the test head always has some range of rotation available for providing compliant docking. 
     In the past, manipulators have used springs to bias each axis toward the center of its compliance range.  FIG. 3  illustrates this approach for spring-biasing a conventional fork-arm manipulator. Fork arms  316   a  and  316   b  of a manipulator support a test head  310 . Within each of the fork arms, springs  320   a  and  320   b  bias a shaft  318  that holds the test head toward the center of a compliance range along the length of the fork arms. 
     When the manipulator roughly aligns the test head  310  and presses the test head against the peripheral, the springs  320   a  and  320   b  adjust (one compresses, the other expands) in compliance with the applied force to allow the mating surfaces of the test head and peripheral to come together. When the test head is undocked from the peripheral, the springs  320   a  and  320   b  restore the orientation of the test head  310  to the center of the compliance range. 
     In the Tiger™ test system, the need to provide compliant docking of the test head has given rise to new challenges. The Tiger™ test head is extremely heavy, weighing approximately 1,270 kilograms (2,800 lbs.). We have recognized that a slight misalignment of the center of gravity of the test head from the location of the spherical bearing induces a large turning moment of the test head. In addition, cables attached to the test head tend to shift position as the test head is moved and rotated, and thus tend to offset the balance of the test head. If the test head were biased in its compliance range with springs, the springs would need to be extremely stiff to resist the large turning moment of the test head and the offsetting forces from the cables. However, we have recognized that stiff springs cause movement of the test head to become stiff, and thus non-compliant. Another solution is needed to satisfy the demands of gentle, compliant docking. 
     SUMMARY OF THE INVENTION 
     With the foregoing background in mind, it is an object of the invention to compliantly dock a heavy test head with a peripheral, without being impaired by large turning moments or offsetting forces. 
     To achieve the foregoing object, as well as other objectives and advantages, a system for positioning a test head includes a plurality of linear actuators. Each actuator has a first end mechanically coupled to the test head and a second end mechanically coupled to a support for the test head, for example, a manipulator. In a first mode, a control system applies inputs to the actuators for variably controlling the extension of each actuator, and thus for controlling a desired position of the test head relative to the support. The desired position may be, for example, a centered position within a compliance range of the test head. It may also be a non-centered position, which tends to align the test head with the peripheral. In a second mode, the control system stops varying the input to the actuators. The actuators tend to maintain their extensions, but comply with external forces applied to the test head. The actuators can be driven in compliance with the external forces both forward and backward, and provide only slight resistance to movement in both directions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which 
         FIG. 1  is a perspective view of a manipulator for holding a test head using an internal gimbal, in accordance with the prior art; 
         FIG. 2  is an exploded view of the test head  110  of  FIG. 1 ; 
         FIG. 3  is a simplified, schematic view of the fork arms of a conventional manipulator, which shows how compliance has been achieved in conventional manipulators using springs; 
         FIG. 4  is a top view of a test head actuation system according to the invention, as used with the manipulator and test head of  FIGS. 1 and 2 ; 
         FIG. 5  is an isometric view of the test head actuation system of  FIG. 4 ; 
         FIG. 6  is a side view of the test head actuation system of  FIGS. 4 and 5 ; 
         FIG. 7  is a back view of the test head actuation system of  FIGS. 4-6 , which shows the relative orientations of the actuators used to position the test head; and 
         FIG. 8  is a flowchart showing steps involved in docking a test head to a peripheral according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Structure 
       FIGS. 4-7  illustrate from different perspectives an actuation system  400  for positioning a test head  110  according to the invention, as used with the internal gimbal manipulator  100  of  FIGS. 1 and 2 . Referring to the top view shown in  FIG. 4 , the actuation system  400  includes a plurality of linear actuators, for example, pneumatic cylinders  414 ,  416 ,  418 , and  420 . The pneumatic cylinders are mechanically coupled to a support plate  410  at respective first ends and to a cylinder mounting bracket  412  at respective second ends. The support plate  410  is fixedly mounted to the twist gear  114  of the manipulator  100 , and the cylinder mounting bracket  412  is fixedly mounted to the test head  110 . The central blade  210  extending from the manipulator  100  supports weight of the test head  110 . Actuating the cylinders  414 - 420 , for example by varying their pneumatic pressure, has the effect of changing the relative position of the test head  110  with respect to the manipulator  100 , within the compliance range of the test head  110 . 
     Each of the pneumatic cylinders  414 - 420  has a body  422  that is rotatably attached to the twist support plate  410  using, for example, a clevis  430  and a clevis pin. A shaft  424  extends a variable length from the body  422  of each cylinder, and is terminated in a tie rod  426 . Each tie rod is rotatably attached to the cylinder mounting bracket  412  using a clevis  428  and clevis pin. Other types of mechanical connections that allow for rotational movement can also be used. 
     The pneumatic cylinders are commercially available from Festo AG &amp; Company, of Esslingen, Germany, and have 40 mm bore and approximately 90 mm (3.5″) stroke. The pneumatic cylinders  414 - 420  preferably have two pneumatic inputs each. Applying positive pressure at the first pneumatic input with respect to the second pneumatic input tends to extend the shaft  424  of the cylinder from the body  422 . Applying positive pressure at the second pneumatic input with respect to the first pneumatic input tends to retract the shaft  424  into the body  422  of the cylinder. The pneumatic cylinders are preferably operated at approximately 80 p.s.i. Each cylinder preferably includes a brake (not shown) for locking the extension of the cylinder in a fixed position. 
     The inputs to each pneumatic cylinder are preferably driven from a source of compressed air or other fluid via a valve manifold  436 . The valve manifold includes different segments for controllably applying pressure to each of the different pneumatic cylinders  414 - 420 . Each segment can preferably assume at least three valve configurations. In the first configuration, the segment applies positive pressure to the first pneumatic input with respect to the second pneumatic input of the respective cylinder. This generally entails conducting compressed air to the first pneumatic input and conducting exhaust air to the second pneumatic input, and results in the shaft  424  extending further from the body  424  of the cylinder. In the second configuration, the connections to the first and second pneumatic inputs of the respective cylinder are reversed, so that the manifold pressurizes the second input with respect to the first input. In this mode, the shaft  424  tends to retract into the body  422  of the cylinder. In the third configuration, the valve manifold blocks pressure to both pneumatic inputs of the respective cylinder, so that the first and second pneumatic inputs are neither compressed nor exhausted. In this mode, the respective pneumatic cylinder maintains its previously established condition indefinitely. 
     By controllably applying pressure, the pneumatic cylinders  414 - 420  can be actuated to center the test head  110  within its compliance range, or to orient the test head at any desired angle within its compliance range. Once the desired orientation is established, the condition of the cylinders can be held constant by controlling the manifold to block pressure to the cylinders (i.e., the third mode). 
     With pressure blocked, the pneumatic cylinders act like air springs, their shafts extending and retracting in compliance with external forces applied to the test head. The pneumatic cylinders do not necessarily provide “soft” compliance, however, i.e., they can not necessarily be moved easily. Because they hold only a relatively small volume of air, the applied force required to extend or retract the cylinder shafts by a given amount is relatively high. The preferred embodiment addresses this problem through the use of pneumatic accumulators  432 . In particular, a different pneumatic accumulator  432  is provided in series with each pneumatic input of each cylinder  414 - 420 . As there are a total of four pneumatic cylinders  414 - 420 , the actuation system  400  includes a total of eight pneumatic accumulators  432 . Each accumulator  432  has the effect of increasing the volume of the respective portion (either extending or retracting) of each cylinder. With the volume increased, the force required to move each shaft is reduced in proportion to the volume of the accumulator. As the accumulators can be made arbitrarily large, the force required to achieve compliant motion of the test head can be made arbitrarily small. 
       FIG. 7  illustrates a back view of the actuation system, i.e., from the vantage point of the manipulator. The twist support plate  410  has been omitted from the figure to provide a clearer view of the arrangement of the pneumatic cylinders  414 - 420 . This arrangement of pneumatic cylinders  414 - 420  has been carefully designed to simplify control of the movement of the test head  110  relative to the manipulator  100 . 
     In particular, first and second pneumatic cylinders  414  and  416  have longitudinal axes that are aligned substantially in parallel. Third and fourth pneumatic cylinders  418  and  420  have longitudinal axes that are aligned so that they cross each other within the perimeter of the test head  110 . Ideally, the longitudinal axes of the third and fourth pneumatic cylinders cross at a point in space that precisely intersects a central axis of the test head, i.e., an axis that extends through the spherical bearing  228  at the center of the test head  110  and through the center of the twist gear  114 . 
     This configuration of cylinders has significant properties. For example, if the first and second pneumatic cylinders  414  and  416  are actuated equally in opposite directions (one extended, the other retracted), the test head is caused to rotate in the twist direction and does not substantially rotate in the theta or tumble directions. If the third and fourth cylinders  418  and  420  are actuated by equal amounts in the same direction, the test head rotates in the tumble direction, but does not substantially rotate in the theta or twist directions. In addition, if the third and fourth cylinders are actuated by equal amounts in opposite directions, the test head is caused to rotate in the theta direction, but does not substantially rotate in the tumble or twist directions. Therefore, the configuration of actuators illustrated and described above promotes substantially independent control over the test head&#39;s movement in each of the conventional directions of theta, tumble, and twist. 
     Operation 
     The actuation system  400  preferably operates under the direction of a controller  440 . The controller  440  preferably includes a hand-held console that allows an operator to specify input for moving the test head to a desired position and orientation. In response to user input, the controller  440  applies a control signal to each segment of the valve manifold  436  for controllably pressurizing the pneumatic cylinders  414 - 420 . A sensor  442  is provided with each cylinder for measuring the extension of the shaft  424  from the body  422  of the respective cylinder. For each cylinder, the sensor  442  measures the cylinder&#39;s extension and reports the result to the controller  440 . In a closed loop fashion, the controller  440  modulates the respective segment of the valve manifold  436  to controllably guide the measured extension of the cylinder reported by the sensor  442  toward the location indicated by the user input. 
     The controller  440  preferably includes a command for centering the cylinders within their respective compliance ranges. In response to this command, the controller  440  drives each of the cylinders  414 - 420  to a position that the respective sensor  442  indicates to be the center of range. 
     The sensor  442  can be implemented in numerous ways, the specific form of which is not critical to the invention. Pneumatic cylinders are available with built-in magnetic sensors  442  that provide electrical signals indicative of their extension. In addition, numerous forms of optical encoders can be used for measuring and reporting the extension of the cylinder. In the preferred embodiment, however, a simple “string pot” is used for measuring the extension of each cylinder. As known to those skilled in the art, a string pot is an electromechanical device that includes a spring-loaded string wrapped around a potentiometer, and generates an electrical output signal proportional to the extension of the spring. Preferably, the body of the string pot is fastened to the body  422  of the respective cylinder, and the end of the string is fastened to the respective shaft  424 . Extension and retraction of the shaft  424  causes the output signal from the string pot to vary proportionally. 
     Operators of automatic test equipment are accustomed to controlling the position of a test head in the directions of theta, tumble, and twist. The controller  440  allows the test head  110  to be controlled in this way. In particular, the controller  440  monitors the difference between the positions of the first and second cylinders  414  and  416  to indicate a twist rotation, and forces a predetermined difference between these positions to establish a desired twist rotation. Similarly, the controller  440  monitors the difference between the positions of the second and third cylinders  418  and  420  to indicate a theta rotation, and forces a difference between them to establish a desired theta rotation. In addition, the controller  440  monitors the sum of the positions of the third and fourth cylinders  418  and  420  to indicate a tumble rotation, and forces a sum between them to establish a desired tumble rotation. In this manner, the controller  440  is able to control the rotation of the test head  110  in each of the conventional directions of theta, tumble, and twist. 
     We have recognized that pneumatic cylinders are not easily controlled with great precision. The difficulty in control arises primarily from the tendency of internal seals within the cylinders to stick when held in a constant position, and to quickly break free and over-extend as pressure is increased. The controller  440  accounts for this characteristic of pneumatic cylinders by permitting the feedback of the controller  440  to be satisfied as long as the extension of the cylinders falls within an allowable range of the target value. Providing an allowable range of acceptable values allows the cylinders to quickly settle close to the ideal, desired positions, without repetitively overshooting the desired position. 
     Operating Modes 
     The actuation system  400  has at least three modes of operation: (1) positioning mode; (2) compliant mode; and (3) locked mode. In positioning mode, the controller  440  continually adjusts the extensions of the pneumatic cylinders toward desired extensions, in response to user input and position feedback signals from the sensors  442 . In this mode, a user can control the position and orientation of the test head, for example, from a hand-held control pod, to move the test head toward the peripheral for docking. A user can execute a command for centering the test head within its compliance range, or can adjust the orientation of the test head to account for misalignments between the test head and peripheral, caused for example by sagging of the manipulator under the substantial weight of the test head. 
     In compliant mode, the controller  440  causes the cylinders to hold their last updated conditions. Compliant mode is preferably established by activating the valve manifold to block the flow of air pressure to both inputs of each cylinder (i.e., the third configuration of the valve manifold  436 , described above). In this mode, the cylinders tend to maintain the positions that were previously established in positioning mode. However, the closed cylinders act as air springs, allowing compliant movement either forward or backward under the influence of applied, external forces. Owing to the increased air volume supplied by the pneumatic accumulators  436 , compliance of the test head is sufficiently “soft” to allow the test head to be moved with relative ease. 
     In locking mode, brakes are applied to the pneumatic cylinders  414 - 420 . The cylinders cannot be positioned in this mode and they cannot be moved in compliance. The actuation system  400  preferably assumes locking mode after the test head and peripheral are docked, once the desired position of the test head is established and no further movement is desired. 
     The transition between positioning and compliant modes preferably occurs automatically. For example, in the preferred embodiment, a sensor  442  is included with each alignment pin to indicate when an alignment pin begins to be inserted into an alignment bushing. When the test head and peripheral are completely separated, the actuation system assumes positioning mode and remains in this mode until one of these sensors  442  is tripped. When the test head is brought to the peripheral and trips a sensor  442 , the controller  440  switches the actuation system to compliant mode. In compliant mode, the operator adjusts the position of the test head to align the other alignment pins with their respective alignment bushings, and to make the test head coplanar with the peripheral. Once this occurs, docking can be completed and testing can begin. 
     Alternatives 
     Having described one embodiment, numerous alternative embodiments or variations can be made. 
     As described above the linear actuators  414 - 418  are implemented with pneumatic cylinders. This is merely an example, however. Other types of linear actuators can be used, for example, hydraulic actuators, electronic linear motors, fast rack and pinion motors, or any other type of linear, backdrivable actuator. If hydraulic cylinders are used, the system could be equipped with extendable bladders or bellows. The bladder or bellows could be blocked during positioning mode to allow for stiff movement. In compliant mode, the bladder or bellows could be pumped to match the pressure in the hydraulic cylinders, and then opened into the hydraulic cylinders. The elasticity of the bladder or bellows would provide the elasticity needed for compliant movement. If electronic motors are used, the input current to the motors could be held constant upon the transition from positioning mode to compliant mode. The electronic motors would tend to maintain their positions, and could be driven backward or forward in compliance with applied forces. 
     The actuation system  400  is described above in connection with a test head that is supported with an internal gimbal. However, the principles of using linear actuators in positioning and compliant modes are applicable to other types of test heads, for example, those that are supported using fork arms. Therefore, the invention should not be construed as being limited only to configurations in which a test head is supported internally. 
     Moreover, the invention described above could be combined with the prior art of U.S. Pat. No. 5,949,002 (“Manipulator for Automatic Test Equipment with Active Compliance”). According to this variation, the particular arrangement of actuators disclosed herein could be used to actively apply forces to the test head to balance external forces acting upon the test head, to facilitate docking. 
     In addition, the actuation system  400  is pictured with certain elements on the top, bottom, right, and left. It should be understood that these orientations are provided as conventions and can be varied. For example, the actuation system  400  can be turned upside down or transposed right-to-left and still operate substantially as described above. Therefore, the actuation system  400  should not be construed as being limited to any particular orientation. 
     Each of these alternatives and variations, as well as others, has been contemplated by the inventor and is intended to fall within the scope of the instant invention. It should be understood, therefore, that the foregoing description is by way of example, and the invention should be limited only by the spirit and scope of the appended claims.