Patent Document

BACKGROUND OF THE INVENTION 
     The present invention relates to industrial control systems providing motion control and in particular to a system of generating electronic cams used for programming motion control. 
     Mechanical cams may convert a rotary motion to a periodic linear motion, for example, through the use of a cam wheel having an irregular radius and attached to a rotating shaft. A follower riding along the perimeter of the cam wheel moves in and out according to the angle of the shaft and the radius of the cam wheel currently in contact with the follower. 
     Modern industrial control systems may implement cam motion by abstracting the cam as a set of points of a cam function or cam equation. The desired linear motion may then be implemented by an alternative technology such as a servo actuator without the need for a mechanical cam. By creating a virtual cam, the benefits of cam type motion are in the form of predetermined and well-defined velocities, acceleration, and jerk (the derivative of acceleration) and these characteristic may be easily modified via software modifications. 
     A variety of tools exist for the generation of virtual cam profiles for new designs. Normally the profiles from these tools are captured as a set of points in a cam profile table, for example, linking shaft rotation values with given linear motion values. These complex tools are very specific and are not flexible enough to meet the needs of all designs. Therefore most designers have additional tools and proprietary profiles that provide the ability to meet the needs of their design. Thus developing a successful cam profile can be extremely complex requiring expert assistance. 
     A common requirement in cam design is to replicate and modify a portion of an existing profile, when the original designer is not available, and the only information still in existence are a set of points used to define the input to output translation. As part of the replication it is often required to modify portions of the profile (such as distances or ratios) while maintaining the same behavior, i.e. maintaining the boundary conditions. Unfortunately, conventional tools and algorithms used to modify the design are incomplete and will often distort the original profile. Thus additional mathematical information used during the creation of the original profile needs to be available, in the form of re-engineering, reverse engineering or manual segmentation (splitting the profile into many segments in an attempt to control the overall profile). The result of this process is typically limiting (not all parts of all segments can be scaled within the given boundaries) or expensive (it takes significant time to regenerate the original design). 
     SUMMARY OF THE INVENTION 
     The present invention provides a cam development tool that can allow ready modification of existing virtual cams expressed in a variety of different forms, for example mathematical equations, cam tables, or discretely sampled relationships. This approach converts cams expressed in a multiple of different forms into a well-characterized form, termed a “base profile”. The well-characterized form is associated with pre-developed adjustment relationships which describe changes in the parameters of the well-characterized form necessary to implement changes in the cam performance. Cam modification instructions describing desired changes in cam performance may then be applied through the adjustment relationships to the well-characterized forms to output a modified cam profile. 
     Specifically, the invention provides a method or apparatus for generating virtual cam profiles that may receive a profile expressed in any of multiple forms describing a cam output variable and a cam input variable, where the cam output variable is a function of the earn input variable. The profile is converted into a base profile providing a functional expression relating the cam output to the cam input and having function parameters of a predetermined type, where the functional expression is associated with a set of adjustment relationships describing changes in the function parameters to produce changes in cam performance. Specifically, the base profile is a polynomial having polynomial coefficients in the adjustment relationships describes scaling of the polynomial coefficients to change cam performance in particular ways. Cain modification instructions are received indicating desired changes in cam performance and applied to the adjustment relationships to modify the function parameters so that a modified characterized profile can be output, the modified characterized profile describing earn inputs and cam outputs related by the functional expression with the modified function parameters. 
     It is thus a feature of at least one embodiment of the invention to provide a method of modifying existing cam profiles that can work with a variety of different types of cam profile expressions including earn tables, closed form expressions and even mechanical cams that can be physically measured. By preparing predefined adjustment relationships that work with a common expression form, and converting the different cam profile expressions into this common form, simple modification of cam performance can be obtained for each of these different types of cam profile expressions. 
     The base profile may be is a piecewise mathematical expression defined between multiple cam output values. 
     It is thus a feature of at least one embodiment of the invention to provide an arbitrarily complex base profile that can be assembled out of simple, well characterized functions susceptible to readily discoverable predefined adjustment relationships. 
     The multiple cam output values are points of zero acceleration in the cam profile. 
     It is thus a feature of at least one embodiment of the invention to provide a base profile that permits localized modifications to a segment without affecting other segments. 
     The base profile may be a piecewise polynomial spline. 
     It is thus a feature of at least one embodiment of the invention to provide a base profile using a well understood mathematical form. 
     The modification of the characterized profile may individually scale each segment and then piecewise assemble the segments after scaling to provide a modified characterized profile. 
     It is thus a feature of at least one embodiment of the invention to permit varied modifications of different portions of the cam profile. 
     The cam modification instructions may be for modifications of at least one of cam output position change for given cam input change and cam output velocity change for given cam input change. 
     It is thus a feature of at least one embodiment of the invention to provide for two commonly desired modifications of cam profiles: changing the profile excursion or maximum profile speed at various points. 
     The method may include the step of determining dynamic properties of the modified characterized profile. 
     It is thus a feature of at least one embodiment of the invention to provide well-characterized cam profile modifications readily analyzed and understood by the user. 
     The cam modification instructions may describe limitations on cam dynamics selected from the group consisting of acceleration and jerk. 
     It is thus a feature of at least one embodiment of the invention to permit modifications of cam profiles based on dynamic constraints in addition to specific changes in profile velocity or position. 
     The method may include the step of generating the characterized profile from a set of discrete points describing a cam profile. 
     It is thus a feature of at least one embodiment of the invention to permit generation of new cams from legacy cam designs that may be only represented by a set of data points. 
     The discrete data points may be converted to spline form by spline interpolation between the discrete points. 
     It is thus a feature of at least one embodiment of the invention to use well-understood techniques to provide a common expression of the cam profile for modification. 
     The modified characterized profile may be output to control an actuator according to a cam output signal being a function of a cam input signal sensed from a sensor applied to the modified characterized profile. 
     It is thus a feature of at least one embodiment of the invention to improve and simplify the process of generating virtual cam profiles for industrial control. 
     The spline interpolation may use a combination of linear segments and third order polynomials. 
     It is thus a feature of at least one embodiment of the invention to provide a computationally tractable interpolation process practical in an industrial control environment. 
     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified diagram of a conventional mechanical cam and its electronic profile that may be used for electronic cam operation; 
         FIG. 2  is a schematic block diagram of an industrial controller implementing an electronic cam using the profile of  FIG. 1 ; 
         FIG. 3  is a block diagram of an electronic computer implementing a program of the present invention for transfer of electronic cam profiles to the industrial controller of  FIG. 2 ; 
         FIG. 4  is a flowchart and tandem data representations showing the process of the present invention in preparing and modifying an electronic cam profile; and 
         FIG. 5  is a block diagram of a cam generation suite of which the program of  FIG. 4  may be a component. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , a mechanical cam  10  may provide for a cam wheel  12  rotating about an axis  14  on the shaft  16 , the shaft  16  defining a cam input (M) with respect to its angle θ. The cam wheel  12  may have a periphery  18  of varying radius with respect to axis  14  to move a follower  20  inward and outward along an x-axis defining a cam output (S). This example is but one of a number of possible cam designs which also include linear cams where the cam input is motion along an axis and the can output is motion along another axis. Similarly, the cam wheel may, for example, be a crank arm and lever combination or the like. 
     The functional relationship between the cam input and cam output may be expressed as a cam profile  22  providing a set of cam data points  24  approximating a cam profile function  26 . 
     Referring now to  FIG. 2 , an industrial control system  30  may include an electronic computer  32  providing one or more processors  34  communicating with electronic memory  36  and interface circuitry  38 . Electronic memory  36  may include a control program  40 , for example, expressed in the ladder logic or function block form as is understood in the art and may implement an electronic cam defined by a profile data structure  42 . The profile data structure  42  may store a set of cam data points  24  discussed above with respect to  FIG. 1  as a set of linked pairs of cam input values M i  and cam output value S i . Alternatively, profile data structure  42  may express the cam profile as a functional relationship, for example, as a spline or as an explicit equation describing the function of the cam profile function  26 . 
     The electronic computer  32  may connect through the interface circuitry  38  with an I/O module  44 , for example, receiving an angle value (θ) from an encoder  46  attached to a rotating shall  48  communicating with a machine. The angle value provides the cam input M i  which is used to index the profile data structure  42  to identify the corresponding cam output value S i  (x). This cam output value S i  may be provided, for example, through interface circuitry  38  to a servo motor drive  50  communicating with a servo motor  52 , for example, driving a rack  54  to provide a translation along an axis  56 . 
     Desirably, the relationship between M i  and S i  described by the profile data structure  42  produces not only the necessary functional relationship between an angle value (θ) and motion (x) along the axis  56  but also provides a desired dynamic characteristic of the motion characterized by peak or minimum velocities, peak or minimum accelerations, and peak or minimum jerk. 
     A variety of other electronic cam configurations are also contemplated by the present invention including those which translate between rotation and rotation, translation and rotation, and other motions. 
     The electronic computer  32  may also connect to a human machine interface  39 , for example, a graphics terminal providing a graphic display screen and a user input device such as a keyboard or the like. 
     Referring now to  FIG. 3 , the present invention provides a system  60  for generating cam profiles for use in the profile data structure  42  of  FIG. 2 . Generally, the system  60  may include electronic computer  62  providing for one or more processors  64  communicating with a memory  66  holding a stored program  68  as will be discussed in greater detail below. The computer  62  may provide for a human machine interface  70  including a graphic display terminal  72  and keyboard  74  or other input device such as a touchscreen or cursor control element. An interface  76  of the computer  62  may communicate, for example, by a network  80  or by transfer of discrete media  82  with the industrial control system  30  described above for transfer of information from the industrial control system  30  to the computer  62  in the form of a profile data structure  42  as will be discussed when the transfer of the desired new cam profile into profile data structure  42  is used in controlling the controller  30  as discussed above. 
     Referring now to  FIG. 4 , the program  68  may optionally receive the contents of a profile data structure  42  of a pre-existing cam as indicated by process block  84 . The profile data structure  42  will provide a base cam that can be modified by the program  68 . As noted above, the profile data structure  42  may hold any of an equation, a spline, or a set of cam data points  24  describing a cam profile function  26 . 
     The latter case of the profile data structure  42  holding cam data points  24  is particularly interesting in many real-world examples where the only description of a successful cam profile, for example, running on current equipment, is a set of discrete, linked cam data points  24  each associated with a cam input and cam output. This information may be input, as discussed in  FIG. 3 , by discrete media  82  or through a network  80  including a local network or the Internet. 
     At process block  86 , the contents of the profile data structure  42  in the form of cam data points  24  used by an existing control program or from a set of stored templates holding cam data points  24  for general use may be converted into a form of a spline  90 . This conversion process may include identifying each pair of the linked cam data points  24  to a spline type (generally linear or cubic) and then providing for a spline interpolation according to generally understood techniques. 
     A third order (cubic) spline generally has the form of:
 
 S=S   0   +C   1 ( M−M   p )+ C   2 ( M−M   p ) 2   +C   3 ( M−M   p ) 3   (1)
 
     where: 
     S is the value of the cam output (S 0  is the initial cam output value); 
     M is the value of the cam input; 
     M p  is the value of the cam input at the starting point of the spline segment or functional segment  92 , and 
     C 1 , C 2 , C 3  are the polynomial coefficients of a third order polynomial from order 1 to 3, 
     The spline  90 , which describes a position of the cam output value S i  (x) as a function of the cam input value M i  (θ), may be processed to determine and to display dynamic qualities of the cam so described including, for example: velocity, acceleration  91 , and jerk (only acceleration  91  is shown for clarity). 
     As indicated by process block  87 , the spline  90  may be broken into segments  92  between points where the acceleration  91  crosses through zero acceleration. Each of the segments  92 , as the spline  90  is so decomposed, will consist of multiple spline polynomials each identified by a range of cam input values M i  (θ) and corresponding cam output values S i  (x). The polynomials may be characterized by up to three coefficients for third order polynomial. 
     Dividing the spline  90  into the segments  92  allows modification of the segments  92  individually and their subsequent reassembly back into a spline  90 . 
     If the original base cam was expressed as a spline in the profile data structure  42 , process blocks  84  and  86  may be skipped and the spline described by the profile data structure  42  receive directly to be broken into segments  92 . 
     At process block  89 , each of the segments  92  has its characteristic parameters  93  extracted including: 
     Mn which is a range of the cam input of the segment; 
     Sn which is a range of the cam output of the segment; 
     Vn which is a change in velocity from the beginning of the segment to the end of the segment (e.g. starting velocity minus pending velocity); 
     An is a peak acceleration within the segment; and 
     Jn is a peak jerk within the segment. 
     At this point each of the segments  92  is fully captured in its characteristic parameters  93  over the entire segment and spline data  94  consisting of linked cam data points for cam input values M i  (θ) and cam output values S i  (x) together with the coefficients of the splines between those ranges. 
     A characterized segment  92  may be stored for future use or used immediately for the generation of a modified cam. In either case, a new cam profile may be generated by assembling segments  92  from the original base cam or chosen from a library of unrelated segments  92 . Typically, segments  92  in an existing cam profile function  26  having functioned well in the past will be employed. 
     Per process block  96 , each of the segments  92  may be modified by coefficients scaling according to modification values  98 . Typical modifications, for example, will increase the excursion distance of the cam output, or its velocity, and may make changes to the segment  92  subject to constraints, for example, a maximum acceleration or jerk. 
     When it is desirable to scale the segment  92  to create a new segment  92  the design technique supports the ability to constrain any three of the six parameters, listed below, at a time. Each combination of constrained parameters requires specific sealing equations. 
     Modification of segment  92  with respect to position can be done by a simple scaling of the values of the cam output values S i  (x) by a constant scaling factor (or analogous countervailing scaling of cam input value M i  (θ)). One or more of the segments  92  may be independently scaled in this way (e.g. with different scaling factors) and fit back together as will be described below. 
     Modification of the segment  92  with respect to velocity requires a more sophisticated coefficients scaling process. Referring to  FIG. 5 , a velocity scaling may desirably increase the final velocity of segment  92  from V A  to V B , for example, the former being the corresponding velocity in the spline  90  originally received at process block  84 . 
     A given profile of the segment  92  with a cubic spline will be fully described by the following six parameters where: 
     V0 is a start velocity of the segment; 
     V1 is an end velocity of the segment; 
     dS is a cam output position range of the segment; 
     dM is a cam input position range of the segment; 
     Amax is a peak acceleration within the segment; and 
     Jmax is a peak jerk within the segment. 
     So, for example, a new segment  92 ′ being a modified version of segment  92 , may be described with the values V0, V1, and dS. These values provide modification values  98  describing a desired modification of the segment  92  and may be used to derive the remaining of the six above described parameters as follows: 
     
       
         
           
             
               
                 
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     These modification values  98  (the values derived from them) may be used to determine the necessary coefficients scaling of each of the polynomials of the segment  92  as follows: 
                     C     1   ⁢           ⁢   new       =             V   ⁢           ⁢   1     -     V   ⁢           ⁢   0       Vn     *     C   1       +     V   ⁢           ⁢   0               (   7   )                 C     2   ⁢           ⁢   new       =           V   ⁢           ⁢   1     -     V   ⁢           ⁢   0       Vn     *     C   2     *     Mn   dM               (   8   )                 C     3   ⁢           ⁢   new       =           V   ⁢           ⁢   1     -     V   ⁢           ⁢   0       Vn     *     C   3     *       (     Mn   dM     )     2               (   9   )               
where C 1 , C 2 , and C 3  are the original coefficient values for the unmodified segment  92  and C 1new , C 2new , C 3new , V 0 , V 1  and dM are the modifications desired (or derived from those modifications) and Mn and Vn are the characteristic parameters  93  of the original segment  92 .
 
     For subsequent segments following  92 , the starting cam input and output values must be made to match the ending cam input and output values of the current segment. 
     When the segment is described not by a cubic spline but by a line, the velocity profile of the segment  92  will be fully described by three parameters: 
     V which is a constant velocity; 
     dM which is a cam input range; and 
     dS which is a cam output range. 
     The relationship between these parameters is as follows:
 
 dS=V*dM.   (10)
 
     Accordingly, a desired increase in velocity may simply scale the V term. Again the segment  92  must be offset at its leading boundary to match the cam output value of the trailing boundary of the preceding segment  92  by a simple offsetting process. 
     At process block  97 , each of the scaled segments  92  may then be reassembled. The scaling process preserves the zero acceleration values at the boundaries of the segments  92  allowing them to be simply spliced together to produce new spline  90  without acceleration velocity or position discontinuities. 
     At process block  100  the new spline  90 ′ may be evaluated at regular intervals and these points may then be enrolled in a new profile data structure  42  as cam data points  24 . 
     At process block  102 , the new profile data structure  42  may be executed in the controller for use in controlling a virtual cam in an industrial operation. 
     For any of the segments  92 , it may be desirable to constrain certain dynamic qualities of a segment  92 , for example, a peak acceleration, to less than a certain amount. In this case, the modification values selected from the six parameters characterizing a cubic spline segment  92  (described above) must include a value of peak acceleration, An, instead of the value of dS. 
     As before, the remaining values of dM, dS and J may be computed from the modification values as follows: 
     
       
         
           
             
               
                 
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     The change in the value of dS may be accommodated by a corresponding offsetting change in dS in the succeeding segment. So if dS in the current segment  92  must be lengthened, a shortened dS may be used in the modification of the subsequent segment. 
     Referring now to  FIG. 5 , the present invention may be incorporated into a cam design suite  104  executing as a program on the computer  62 . The cam design suite  104  may receive, for example, a data structure  42  as was described above, to generate a spline if necessary per process blocks  84 ,  86 ,  87 , and  89  of  FIG. 5  and then to accept a modification from the user at process blocks  96  and  97  to generate a new cam profile that may be output from the cam design suite  104  per process block  100 . As a further part of the cam design suite  104 , a procedural cam generation program  106  may allow cam calculation directly from polynomials specified by the user up to the ninth order. In addition, an inverse kinematics program  108  may allow cam calculation to be performed by inverse kinematics where the structure of a machine is defined and simulated in operation to extract the necessary cam data points  24 . 
     At each stage in this cam generation process, graphical representations of the earn and its dynamic qualities of velocity, acceleration, and jerk may be displayed together with peak values and starting and ending position values. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications are hereby incorporated herein by reference in their entireties.

Technology Category: 3