Abstract:
A system for positioning an object includes a base and a platform for detachably retaining the object. A first linear actuator is pivotably coupled to a first pivot axis of the platform and a first pivot axis of the base. A second and a third linear actuator are pivotably coupled to a second pivot axis of the base and the first pivot axis of the platform. A fourth linear actuator is pivotably coupled to the second pivot axis of the base and a second pivot axis of the platform. The first, second, third and fourth linear actuators being selectably adjustable in length to position the platform at a select position about a predetermined arc of travel.

Description:
This application claims priority to U.S. provisional application 61/154,354, filed Feb. 20, 2009, the contents of which are hereby incorporated by reference. 
    
    
     FIELD 
     The present invention relates generally to photometric test and measurement equipment, and in particular to a goniometric positioning system for use in conjunction with photometric test and measurement equipment. 
     BACKGROUND 
     Goniometric multi-axis positioners (generally called “goniometers” and “goniophotometers”) have been available for a number of years in the lighting industry. Goniometers are used to accurately and precisely position and orient a test object at a plurality of positions in order to evaluate the object&#39;s photometric properties, for example the spatial luminous intensity distribution of a light emitting or light reflecting object. Goniometers are typically described as having either a “Type A” or “Type B” configuration. An example Type A goniometer  10  is shown in  FIG. 1 , while a Type B goniometer  100  is shown in  FIG. 2 . 
     With reference to  FIG. 1 , Type A goniometer  10  is a common configuration used in the transportation lighting industry. Goniometer  10  includes a test platform  12  attached to an inner frame member  14  and is rotatable with respect to the inner frame member about an axis of rotation “X 1 .” Inner frame member  14  is attached to an outer frame member  16  and is rotatable with respect to the outer frame member about an axis of rotation “Y 1 .” Thus, the “left-right” rotational axis X 1  is nested within the tilt or “up-down” axis Y 1 . This basic configuration is widely used to test automobile, aircraft and other transportation lighting devices. 
     With reference to  FIG. 2 , Type B goniometer  100  includes a platform  102  attached to a horizontal member  104 . Horizontal member  104  is rotatably attached to a frame member  106 . Platform  102 , horizontal member  104  and frame member  106  are all rotatable together about an axis of rotation “X 2 .” Platform  102  and horizontal member  104  are also further rotatable together about a tilt axis “Y 2 .” As can be seen from  FIG. 2 , Type B goniometer  100  is configured such that rotational axis X 2  is located beneath tilt axis Y 2 . Accordingly, the entire frame  106  of the goniometer rotates for the right-left motion. This type of goniometer is commonly used for testing of displays and commercial lighting fixtures. 
     Some variations of the basic goniometer design exist. For example, some goniometer systems have been built in a “half frame” configuration  200 , shown generally in  FIG. 3 . In the half-frame configuration a platform  202  is fixed to an inner frame  204 , the inner frame being cantilevered from an outer frame  206 . Platform  202  is rotatable about a rotational axis X 3 . In addition, inner frame  204  and platform  202  are rotatable together about a tilt axis Y 3 . A test object (not shown for clarity) may also be adjusted to a desired height H 3  by fixturing or tooling equipment that is either incorporated into platform  202  or is detachably coupled to the platform. 
     The open-end cantilever goniometer  200  of  FIG. 3  has some advantages over the closed-box frame designs of  FIGS. 1 and 2  due to the lack of an outer frame  206  member at an unsupported end  208  of inner frame  204 . As can be appreciated by comparing  FIG. 3  with  FIGS. 1 and 2 , an outer frame  206  member proximate end  208  could interfere with the movement of inner frame  204  in situations where a large test object is attached to platform  202 . However, given that many vehicle lighting devices have a left-hand and a right-hand configuration, there is still the potential for interference in some testing scenarios. For example, while no test object-to-outer frame  206  interference may be experienced at the unsupported end  208  of inner frame  204 , interference between the test object and the outer frame may still occur on the opposing, supported side of the inner frame. The nature of the half-frame goniometer design also requires a relatively large, heavy structure and massive bearing assemblies to minimize positional error with regard to platform  202  due to deflection of the cantilevered inner frame  204 . In some cases this drawback lends an advantage to the box closed-frame designs of goniometers  10 ,  100  due to their inherently balanced weight distribution. 
     A third configuration of goniometer, known as a “sector gear positioner”  300 , is shown in  FIG. 4 . This positioner is a reapplication of a type of positioner used for antennae and artillery aiming devices. A platform  302  is affixed to a large sector gear  304  and is rotatable about a rotational axis X 4 . The sector gear  304  is coupled to a gear drive  306  that moves the sector gear and platform together to predetermined positions about a tilt axis Y 4  having a range of motion θ 4 . A test object (not shown for clarity) may also be adjusted to a desired height H 4  by fixturing or tooling equipment that is either incorporated into platform  302  or is detachably coupled to the platform. 
     A disadvantage of sector gear positioner  300  is that the range θ 4  of up-down motion of platform  302  is limited to a tilt angle of about ±30 degrees from a horizontal orientation due to the sector gear  304  interfering with a light emission path of a test object mounted to the platform at tilt angle extremes. For most transportation lighting it is necessary to run some tests with the light emission of the test object oriented to about a 90-degree “up” position. This is particularly true with respect to forward lighting, such as headlamps for automobiles, as well as aerospace lighting. The “down” direction, i.e., the light emission of the test object oriented to about 180-degrees from the “up” position, is not as much of an issue because all goniometers are limited in this direction due to the mounting requirements of most test objects. 
     As can be appreciated from the foregoing discussion, current goniometers suffer from significant limitations with regard to the size and shape of objects that can be tested, due to the potential for interference between the test object and the structure of the goniometer. This interference limits the range of motion of the goniometer, in turn limiting the amount of photometric data that can be gathered. Current goniometers also typically consume a significant amount of laboratory space that could otherwise be used for other purposes. Furthermore, available goniometers are typically extremely heavy, making them expensive to transport and requiring significant foundational support at their point of installation. There is a need for a goniometer that addresses these shortcomings. 
     SUMMARY 
     A goniometric positioning system is disclosed according to an embodiment of the present invention. The system employs a set of linear actuators configured as a four-bar linkage to achieve the desired goniometer test article positioning characteristics. 
     One aspect of the invention is a system for positioning an object for photometric testing. The system includes a base, and a platform for detachably retaining the object. A first linear actuator is pivotably coupled to a first pivot axis of the platform and a first pivot axis of the base. A second and a third linear actuator are pivotably coupled to a second pivot axis of the base and the first pivot axis of the platform. A fourth linear actuator is pivotably coupled to the second pivot axis of the base and a second pivot axis of the platform. The first, second, third and fourth linear actuators are selectably adjustable in length to position the platform at a select position about a predetermined arc of travel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the inventive embodiments will become apparent to those skilled in the art to which the embodiments relate from reading the specification and claims with reference to the accompanying drawings, in which: 
         FIG. 1  is a prior art Type A goniometer; 
         FIG. 2  is a prior art Type B goniometer; 
         FIG. 3  is a prior art half-frame Type A goniometer; 
         FIG. 4  is a prior art sector gear goniometer; 
         FIG. 5  is a rear-quarter view of a goniometer positioning system according to an embodiment of the present invention; 
         FIG. 6  is a front view of the goniometer system of  FIG. 5 ; 
         FIG. 7  is a side view of the goniometer system of  FIG. 5 ; 
         FIG. 8  is an illustration of examples of multiple positions of a goniometer system according to an embodiment of the present invention; 
         FIGS. 9A ,  9 B,  9 C and  9 D are perspective, side, end and top views respectively of the goniometer system of  FIG. 8  according to an embodiment of the present invention; 
         FIG. 10  is a block diagram of the general arrangement of a goniometer control system according to an embodiment of the present invention; 
         FIG. 11  shows the dimensional parameters associated with computations for positioning linear actuators of the goniometer system of  FIG. 10 ; and 
         FIG. 12  shows a calibration system usable in conjunction with a goniometer system. 
     
    
    
     DETAILED DESCRIPTION 
     A goniometric positioning system  400  is shown in  FIGS. 5 through 10  according to an embodiment of the present invention. Goniometer  400  comprises a platform  402  that is movably supported by adjustable-length members such as a set of linear actuators  404 ,  406 ,  408  and  410 , each being pivotably coupled to and extending between the platform and a fixed base  412 . Linear actuator  404  is pivotably coupled between a pivot axis PA 1  at base  412  and a pivot axis PA 2  at platform  402 . Linear actuators  406 ,  410  are pivotably coupled between a pivot axis PA 3  of base  412  and pivot axis PA 2 . Linear actuator  408  is pivotably coupled between pivot axis PA 3  of base  412  and a pivot axis PA 4  of platform  402 . 
     Linear actuators  404 ,  406 ,  408 ,  410  may be any type of device now known or later invented that applies force in a linear manner. Example types of linear actuators include, without limitation, rotary-to-linear motion converters such as electro-mechanical actuators, segmented spindle actuators and moving coil actuators. Other types of linear actuators may directly generate linear force, such as hydraulic actuators, piezoelectric actuators, linear motors and wax motors. 
     With reference to  FIGS. 9A through 9D , if linear actuators  404 ,  406 ,  408 ,  410  are provided as electro-mechanical actuators they may each comprise an electric motor  411  such as, without limitation, a dc brush, dc brushless, stepper and induction motor. The motor is coupled to a rotary-to-linear motion converter  413 . The rotary-to-linear motion converter may be, without limitation, a lead screw or ball screw. The electric motor may be directly coupled to the rotary-to-linear converter. Alternatively, a gear reduction may be interposed between the electric motor and the converter. 
       FIGS. 10 and 11  show the general arrangement of a goniometer control system according to an embodiment of the present invention. A control  414  (which may be integral to system  400  or an external component coupled to the system) may include a microprocessor or other computing means and may operate in accordance with a set of predetermined instructions, such as a computer program, to resolve appropriate positions for platform  402  throughout a predetermined arc range of motion θ 5 . Once an appropriate platform  402  position is determined control  414  computes, using the programmed instructions, the appropriate extension positions L 1  for linear actuator  408 , L 2  for linear actuators  406 ,  410  and L 3  for linear actuator  404  to achieve the desired position. Control  414  then operates linear actuators  404 ,  406 ,  408  and  410 , via a driver  416 , to provide electrical, hydraulic or other signals (represented by the solid arrows in  FIG. 10 ) to move each actuator to their appropriate linear positions. The appropriate extension positions may be determined in an open-loop fashion, such by control  414  issuing a predetermined number of electrical output pulses via driver  416 , the pulses being provided to a stepper motor  411  of each of linear actuators  404 ,  406 ,  408  and  410 . Alternatively, the positions of linear actuators  404 ,  406 ,  408  and  410  may be controlled by control  414  in a closed-loop fashion using feedback elements  418  coupled to the linear actuators, the feedback elements each providing position feedback signals to control  414  for the linear actuator with which they are associated. Such feedback signals are generally represented by the broken line  420  in  FIG. 10 . 
     With reference again to  FIG. 8 , in operation linear actuators  404 ,  406 ,  408  and  410  may be operated either individually, all together or in sub-groups to accurately and precisely position platform  402  to a number of predetermined positions and orientations about circular arc θ 5 , the linear actuators functioning together as a four-bar linkage.  FIG. 8  shows platform  402  positioned at three discrete points of arc θ 5 , the platform being accurately and precisely positionable at any position about arc θ 5  within the limits of travel of L 1  for linear actuator  408 , L 2  for linear actuators  406 ,  410  and L 3  for linear actuator  404 . Control  414  may be programmed to manually and/or automatically move platform  402  to the predetermined positions and orientations. Alternatively, control  414  may be configured to receive automatic and/or manual control signals from an external source (not shown), such as from an operator of the system or a computing device. 
     With reference to  FIGS. 10 and 11 , linear actuators  404 ,  406 ,  408  and  410  may each be extended or retracted to a determinable length to achieve a particular or select position of platform  402  about arc θ 5  in accordance with Equations 1, 2 and 3, below. The L 1 , L 2 , and L 3  lengths are a function of adjustable parameters, θ 5  and H. The remaining parameters are fixed and are defined by the chosen geometry of system  400 .
 
 L   1 =SQRT(( B +( R *COS(θ 5 +( A  TAN(−( C/ 2)/( H+A )))))) 2 +( D +( R *SIN(θ 5 +( A  TAN(−( C/ 2)/( H+A ))))) 2 )  Equation 1
 
 L   2 =SQRT( B +( R *COS(θ 5 +( A  TAN(( C/ 2)/( H+A )))))) 2 +( D +( R *SIN(θ 5 +( A  TAN(( C/ 2)/( H+A ))))) 2 )  Equation 2
 
 L   3 =SQRT((( R *COS(θ 5 +( A  TAN(( C/ 2)/( H+A )))))) 2 +(( D+E )+( R *SIN(θ 5 +( A  TAN(( C/ 2)/( H+A ))))) 2 )  Equation 3
 
     where:
         L 1 =length of actuator  408     L 2 =length of actuators  406 ,  410     L 3 =length of actuator  404     A=vertical distance between surface of platform  402  and a plane formed by pivot axes PA 2 , PA 4      B=Horizontal distance between pivot axes PA 1 , PA 3      C=Horizontal distance between pivot axes PA 2 , PA 4      D=vertical distance from pivot axis PA 3  to H-V (theoretical center of rotation)   E=vertical distance from pivot axis PA 1  to pivot axis PA 3      R=SQRT (H 2 +(C/2) 2 )   θ 5 =select up-down tilt angle of platform  402     H=commanded height adjustment. For photometric testing H is generally specified so as to position the theoretical center of light for the item under test at the intersection of the X 5  and Y 5  axes (H-V).
 
These computations may be performed by control  414  and/or an external computer or similar device coupled to the control.
       

     Control  414  may be implemented in any conventional form of analog or digital (e.g., a microprocessor or a computer) closed-loop servo controller having operational aspects including, but not limited to, force, velocity and directional controls for driver  416  and/or linear actuators  404 ,  406 ,  408  and  410 . Control  414  may further include a predetermined set of logical instructions, such as a computer program, to define the various operational aspects of the control. Control  414  may also receive, via an input  422  ( FIG. 10 ) instructions from an external device, such as photometric measurement equipment and/or calibration equipment. 
     The aforementioned position feedback elements provide information to control  414  relating to the positions of linear actuators  404 ,  406 ,  408  and  410 . The feedback elements may be any conventional type of feedback element now known or later invented that is compatible with the architecture chosen for control  414 , such as an absolute or relative position encoder. In other embodiments the feedback elements may be an arrangement of electromechanical or solid state limit switches or proximity-sensing elements located at predetermined positions. In some embodiments of linear actuators  404 ,  406 ,  408  and  410  utilizing a stepper or brushless DC motor a limit switch or proximity sensor at known or calibrated positions of linear actuators  404 ,  406 ,  408  and  410  may serve as index points for a predetermined set of instructions used by controller  414  to count the number of commutation pulses required to reach a predetermined position of the linear actuators. In addition to position information, the feedback elements may provide control  414  with information relating to the velocity of linear actuators  404 ,  406 ,  408  and  410  when they are moving. 
     In some embodiments of the present invention the aforementioned logical instructions (which may reside in control  414  and/or an external control, such as a computer terminal) may include a command to position platform  402  at a position which will be a function of “height” (which defines the radius of the arc of travel of the platform), an up/down angle about axis of rotation Y 5 , and a right/left angle about axis of rotation X 5 . The right/left angle of platform  402  may be directly set and/or measured in any conventional manner. The height and up/down angle position of platform  402  may be computed using an algorithm wherein the up/down angle, “ƒu/d,” is a mathematical function of the extension lengths of actuators  404 ,  406 ,  408  and  410  (actuators  406  and  410  being generally the same length) at each commanded up/down position. In other words, the extension lengths of actuators  404 ,  406 ,  408  and  410  are a function of the commanded up/down angle and height. 
     Alternatively, system  400  may be commanded to move through arc θ 5  ( FIG. 8 ) at a prescribed speed while photometry equipment observing a test object (not shown) attached to platform  402  “scans on the fly” while measuring light emissions from the test object. In this embodiment the speed or “feed rate” of each linear actuator  404 ,  406 ,  408  and  410  is controlled in a predetermined manner. This motion requires the linear actuator  404 ,  406 ,  408  and  410  speeds to vary during the path of motion, and in some cases may require the direction of at least some of the linear actuators to reverse during the move. The movement of platform  402  may be controlled internally by control  414 , or externally such as photometric measurement equipment and/or calibration equipment. 
     In one embodiment of the present invention it is desirable to maintain a “closed loop” form of position control of platform  402 . In addition to the aforementioned position feedback elements  420  providing a positional communication back to control  414  regarding the status of linear actuators  404 ,  406 ,  408  and  410 , a second set of encoders may be attached to each of the three length axes of the actuators to confirm in a precise manner whether the actuator is actually in the commanded position. If a difference in position greater than a predetermined tolerance is detected, then control  414  will act to readjust linear actuators  404 ,  406 ,  408  and  410  to achieve the commanded position. Control  414  may further include an output  424  providing data in any desired analog and/or digital format. The output data may include, without limitation, tilt and rotation angles for platform  402 . 
     In some embodiments of the present invention platform  402  is rotatable to accommodate various lighting test requirements. Preferably, platform  402  is rotatable about axis of rotation X 5 , which is oriented generally orthogonal to a plane “F” defined by pivot axes PA 2 , PA 4  of the platform ( FIG. 11 ). 
     With reference to  FIG. 12 , in another embodiment of the present invention a target  500  consisting of a board  502  having concentric circles  504  of a contrasting color thereon may be used to calibrate system  400 . Target  500  may be placed on a wall or on a stand perpendicular to the up/down tilt axis Y 5  of goniometer system  400  with its center at the “0,0” center of the rotational axis X 5  of the goniometer. A pair of laser emitters  506  may be mounted on the base of platform  402  so that, when energized, laser beams  508  emitted by the emitters travel about arc θ 5  ( FIG. 8 ) corresponding to the commanded positions of platform  402 . This will provide a user with visual confirmation that system  400  is in the proper, commanded position. Alternatively, the proper up/down position of platform  402  may be verified using a bubble protractor. For precise confirmation of positions, one may also utilize a theodolite. 
     A comparison with prior art goniometer designs shows a number of advantages of the present invention. Firstly, the rotational interference between the edges of large items to be tested and the side frame members of the prior art box frame ( FIGS. 1 ,  2 ) and half-frame ( FIG. 3 ) configurations is completely eliminated. Thus, virtually any size object can be tested, so long as the object is within the load limitations of linear actuators  404 ,  406 ,  408  and  410  and so long as the object fits within the test room. 
     In many test facilities there is a limitation of the facility space available for installation of the goniometer positioner. Both the Type A ( FIG. 1 ) and Type B ( FIG. 2 ) prior art positioners require considerable space outside of the optical working area for the mechanism driving the motion of the device. This can force laboratory layouts that require excessive space. In some cases the size of the frame may be too large for the space intended. Often the size of the equipment causes extraordinary difficulties in shipping and in installation. Sometimes special doors may be required or it may even be necessary to remove a wall to move the system into the its final installed position. The present invention is compact, overcoming the drawbacks of prior art positioning systems. 
     System weight is also important, for several reasons. Firstly, shipping costs are always a concern and the weight and physical size of the goniometer will directly impact these costs. It is not unusual for the weight of a prior art goniometer system to exceed a thousand pounds. This limits the test facilities to those that can accommodate large, expensive goniometer installations. For example, an end user must be particularly concerned about the allowed load rating for the floor of the laboratory. This can be a significant issue for end users who desire to locate the goniometer in an upper-floor location where a thick concrete foundation is not usually available. In contrast, some configurations of the present invention are designed to weigh about 350 pounds, a substantial improvement over prior art systems. 
     Lastly, with regard to shipping, the present invention may be partly disassembled so that the components can be hand-carried to the testing site if necessary and then reassembled in place. Consequently, complex rigging equipment and large doors are not required to install the present invention. 
     While this invention has been shown and described with respect to a detailed embodiment thereof, it will be understood by those skilled in the art that changes in form and detail thereof may be made without departing from the scope of the claims of the invention.