Abstract:
In a variable displacement type hydraulic pump/motor assembly including at least one internally movable component, such as a yoke, a swashplate, and the means for moving the swashplate, the improvement includes imparting on this component absolute location indicia markings and the addition of an optical/electronic sensor unit capable of reading such indicia markings fixedly located on a housing of the pump/motor assembly and including an optical read head extending into the housing interior and located at a spaced distance from the indicia markings for reading same, relative to a reference location, and producing a signal indicative of the absolute position of the movable component. The signal can then, in turn, be used for controlling the displacement of the pump/motor assembly. A method for determining the absolute component position is also set forth.

Description:
CROSS-REFERENCE TO RELATED CASES  
       [0001]     The present invention claims the benefit of the filing date of U.S. Provisional Patent Application Serial No. 60/686,084, filed May 31, 2005, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention pertains to a variable displacement type hydraulic pump/motor assembly having at least one internally movable component, such as a yoke or a swashplate, and means for moving the yoke/swashplate, wherein the improvement includes imparting thereon of absolute position location indicia markings, in the form of a bar-type pattern, and the addition of an optical/electronic sensor unit, including an optical read head, for reading such indicia markings and producing a signal indicative of the absolute position of this component, which can then be used for controlling the displacement of the pump/motor assembly.  
       BACKGROUND OF THE INVENTION  
       [0003]     While the present invention provides direct and/or indirect rotary position feedback by means of an optical sensing device, the current art includes the use of rotary potentiometers, such as LVDTs (Linear Variable Differential Transformers), rotary Hall effect sensors and mechanical feedback devices. Often, contact methods are employed for position feedback and cause wear of mechanical parts. In addition, it is difficult to design a pump/motor wherein a sensor can be so positioned as to read the position of the yoke/swashplate. Also, there are frequent problems in replacing and recalibrating replacement sensors.  
         [0004]     Some prior art sensors are affected negatively by temperature, especially high temperatures that, as the pump/motor housing or casing heats up, can either ruin the sensor or at least adversely affect its precision.  
         [0005]     The patent literature includes a large number of devices, sensors and control systems that pertain to variable displacement pump/motor assemblies and include, but are not limited to: U.S. Pat. No. 4,527,054 to White; U.S. Pat. No. 4,655,689 to Westveer et al.; U.S. Pat. No. 4,915,591 to Funke; U.S. Pat. No. 4,925,371 to Griesmar; U.S. Pat. No. 5,214,278 to Banda; U.S. Pat. No. 6,375,433 B1 to Du et al.; U.S. Pat. No. 6,623,247 B2 to Du; and U.S. Published patent application Ser. No. 2004/0115065 A1 to Du et al. However, none of these prior art devices pertains to an improved optical position sensing or to method for sensing the absolute position of a movable internal component in a variable displacement type hydraulic pump/motor assembly as set forth in the present invention.  
       SUMMARY OF THE INVENTION  
       [0006]     Accordingly, in order to overcome the deficiencies of the prior art devices, the present invention provides an improved optical position sensing device utilizing absolute location indicia markings, in the form of bar-type patterns, imparted on a movable internal component, such as a yoke, swashplate, or means for moving the swashplate of a variable displacement type of a hydraulic pump/motor assembly, wherein an optical sensor, fixedly located in the assembly housing and being located at a spaced gap from the indicia markings, is utilized for reading these indicia markings, relative to a fixed location, and produces an output signal indicative of the absolute position of the movable component, which signal can then be used for controlling the displacement of the pump/motor assembly.  
         [0007]     Specifically, in terms of structure, one embodiment of the present invention pertains to an improved optical position sensor for a variable displacement type hydraulic pump/motor assembly including an angularly movable yoke or an angularly movable swashplate, an optical/electronic sensor unit capable of reading indicia markings, for sensing the movement of at least one of the yoke, the swashplate and means for moving the swashplate, relative to a reference location in a stationary housing of the pump/motor assembly, the one of the yoke, the swashplate and the means for moving the swashplate, being provided with a plurality of absolute scale indicia markings, wherein the improvement comprises: a. the absolute scale indicia markings being in the form of a bar-type pattern and being located in a surface of one of the angularly movable yoke, the swashplate and the means for moving the swashplate; and b. the optical/electronic sensor unit being fixedly located on the stationary housing and including an optical read head sealingly extending into the interior of the housing and being located at a predetermined, spaced distance from the indicia markings, for sensing the indicia markings and producing an optical signal.  
         [0008]     One version thereof further includes means for converting the optical signal into an electrical signal indicative of the absolute position of the at least one of the yoke, the swashplate and the means for moving the swashplate. In a variation thereof, the electrical signal includes one of an analog voltage, proportional to the angular position of the at least one of the yoke and the swashplate, and a digital serial communication message containing the digital angle of at least one of the yoke and the swashplate. In another variation thereof, the electrical signal includes one of an analog voltage, proportional to the absolute longitudinal position of the means for moving the swashplate, and a digital serial communication containing the absolute longitudinal position of the means for moving the swashplate, the absolute longitudinal positions being indicative of the angular location of the swashplate.  
         [0009]     In another version thereof, the means for moving the swashplate takes the form of a setting piston capable of longitudinal movement, the setting piston being operatively pivotally interconnected with the swashplate.  
         [0010]     In a further version thereof, the distance is one of less than 1 mm. in width, less than 0.8 mm. in width and about 0.6 mm. in width. In a variation thereof, the distance is filled with a working fluid. In differing variations, the working fluid is translucent to light, with the light being visible light, while in another variation thereof, the visible light is strobed and confined to the red spectrum. In an additional variation, the light is IR light. In still another variation thereof, the working fluid is one of a liquid and a gas. In yet a differing version thereof, the liquid includes hydraulic fluid and the gas includes air, while in a variation thereof, there is an absence of a working fluid in the spaced distance.  
         [0011]     In an added version, the absolute scale indicia markings are provided on one of a ring member and disc attached to the yoke, while in a variation thereof, the indicia markings are integral with the one of the ring member and disc.  
         [0012]     In yet another version thereof, the absolute scale indicia markings are provided on  5  the swashplate, while in a variation thereof, the indicia markings are integral with the swashplate.  
         [0013]     In still an additional version, the indicia markings include a progressive, differing, binary number pattern of dark and light stripes, the binary number pattern corresponding with specific angular positions of the absolute scale indicia pattern on the noted surface. In a first variation thereof, the progressive, differing, binary pattern of dark and light stripes is anodized on one of a ring member and disc attached to one of the swashplate and yoke, with the pattern of stripes being in the form of an arched, radially-directed, bar-type pattern having an angular extent corresponding to at least the angular movement of the swashplate and yoke. In a second variation thereof, the progressive, differing, binary pattern of dark and light stripes is integral with the swashplate, the pattern of stripes being in the form of an arched, radially-directed, bar-type pattern having an angular extent substantially corresponding to the angular movement of the swashplate. In a further variation, the progressive, differing, binary pattern of dark and light stripes in integral with the means for moving the swashplate. In yet a differing variation, the means for moving the swashplate takes the form of a setting piston capable of longitudinal movement and operatively pivotally interconnected with the swashplate, the progressive, differing, binary pattern of stripes being integral with an outer peripheral surface of the setting piston and being in the form of a laterally-directed, peripherally curved, bar-type pattern having a longitudinal extent substantially corresponding to the maximum longitudinal movement of the setting piston, with longitudinal locations thereof substantially corresponding with angular positions of the swashplate.  
         [0014]     In a further version, the reference location substantially coincides with the location of the optical read head.  
         [0015]     In a variation of an earlier version, the electrical signal is utilized for providing positional feed back to a hydraulic controller for closed-loop control of one of the yoke, the swashplate and the means for moving the swashplate. In a further variation of this earlier version, the electrical signal is utilized for providing positional feedback for open-loop instrumental purposes.  
         [0016]     In a final version of this embodiment, the optical/electronic sensor unit includes a strobing LED lamp and is conducted, via fiber optic bundles, to the absolute scale indicia markings located in a surface of one of a ring member/disc attached to the yoke, the swashplate and the means for moving the swashplate. In a variation thereof, the spectrum of the light is red.  
         [0017]     Another embodiment of the present invention pertains to a method for sensing the absolute position of an angularly movable swashplate/yoke in a variable displacement type hydraulic pump/motor assembly, the assembly including: the angularly movable swashplate/yoke; means for moving the swashplate/yoke; an optical electronic sensor unit, capable of reading indicia markings, for sensing the movement of at least one of the swashplate/yoke and the means for moving the swashplate/yoke, relative to a reference location in a stationary housing containing the pump/motor assembly, the method including the steps of: a. providing one of the swashplate/yoke and the means for moving the swashplate/yoke with absolute indicia markings; b. forming the indicia markings in a bar-type pattern and locating same in a surface of one of the swashplate/yoke and the means for moving the swashplate; c. fixedly locating the optical/electronic sensor on the stationary housing; d. extending an optical read head, associated with the optical/electronic sensor, into the interior of the housing, at a predetermined, spaced distance from the indicia markings; and e. sensing the indicia markings and producing an optical signal indicative of the absolute position of at least one of the swashplate/yoke and the means for moving the swashplate/yoke.  
         [0018]     One version thereof further includes the step of converting the optical signal into an electrical signal indicative of the absolute position. In one variation thereof, the electrical signal includes the step of producing one of an analog voltage, proportional to the angular position of the swashplate/yoke, and a digital serial communication message containing the digital angle of one of the swashplate/yoke. In another variation thereof, the electrical signal includes the step of producing one of an analog voltage, proportional to the longitudinal position of the means for moving the swashplate/yoke, and a digital serial communication containing the longitudinal position of the means for moving the swashplate/yoke, the longitudinal position being indicative of the angular location of the swashplate/yoke.  
         [0019]     Another version thereof further includes the step of filling the spaced distance with a working fluid translucent to visible light. In one variation thereof the filling step is accomplished with visible light whose spectrum is solely red. Another variation thereof further includes the step of conducting the light of a strobing LED lamp, via fiber optic bundles, to the absolute scale indicia markings located in a surface of one of the swashplate/yoke and the means for moving the swashplate/yoke. A final variation thereof further includes the step of coinciding the reference location with that of the optical read head.  
         [0020]     A further embodiment of the present invention pertains to an optical position sensing device in combination with a variable displacement type hydraulic pump/motor assembly including one of an angularly movable yoke and an angularly movable swashplate, means for moving the swashplate, wherein the improvement comprises the addition of: a. absolute indicia markings, in the form of a bar-type pattern, on a surface of one of the yoke, the swashplate, and the means for moving the swashplate; and b. an optical/electronic sensor unit, capable of reading the indicia markings, fixedly located on a stationary housing of the pump/motor assembly and including an optical read head extending into the interior of the housing and being located at a predetermined, spaced distance from the indicia markings, for sensing the indicia markings, relative to a reference location in the stationary housing, and producing an optical signal indicative of the absolute position of at least one of the yoke, the swashplate, and the means for moving the swashplate.  
         [0021]     One version thereof further includes the addition of means for converting the optical signal into an electrical signal indicative of the absolute angular position of at least one of the yoke, the swashplate and the means for moving the swashplate. In a variation thereof the electrical signal includes one of an analog voltage, proportional to one of the angular position of at least one of the yoke and the swashplate and the absolute longitudinal position of the means for moving the swashplate, and a digital serial communication message containing one of the digital angle of at least one of the yoke and the swashplate and the absolute longitudinal position of the means for moving the swashplate, the absolute longitudinal position being indicative of the angular position of the swashplate. In another variation thereof the means for moving the swashplate takes the form of a setting piston, capable of longitudinal movement, pivotally connected with the swashplate.  
         [0022]     In a further version thereof, the absolute indicia markings include a progressive, differing, pattern of light and dark strips imparted upon on one of the yoke and the swashplate, the pattern of stripes being in the form of an arched, radially-directed, bar-type pattern having an angular extent substantially corresponding to the angular movements of the swashplate and yoke.  
         [0023]     In another version thereof, the means for moving the swashplate takes the form of a setting piston, capable of longitudinal movement and being operatively and pivotally connected with the swashplate, the recurring binary pattern of light and dark stripes being imparted on an outer peripheral surface of the setting piston and being in the form of a laterally-directed, peripherally curved, bar-type pattern having a longitudinal extent substantially corresponding to the maximum longitudinal movement of the setting piston, with longitudinal end locations thereof substantially corresponding with angular end positions of the swashplate.  
         [0024]     In a differing version thereof, the signal, indicative of the absolute position of at least one of the yoke, the swashplate and the means for moving the swashplate is utilized for controlling the displacement of the hydraulic pump/motor assembly.  
         [0025]     In an additional version thereof, the spaced distance is less than about 1 mm. and is filled with a working fluid translucent to visible light. In a variation thereof, the optical/electrical sensor unit includes a strobing LED lamp whose light is of the red spectrum and is conducted via fiber optic bundles to the absolute location scale indicia markings imparted on an outer surface of one of a ring member attached to the yoke, the swashplate and the means for moving the swashplate. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a perspective view of a portion of a variable displacement type hydraulic mechanism, such as a pump/motor assembly, partly cut-away, illustrating one embodiment of the optical position sensing device of the present invention, for determining the angular position of one internal component therewithin, herein shown at one extreme angular position thereof.  
         [0027]      FIG. 2  is a view, similar to that of  FIG. 1 , showing the one internal component at an intermediate angular position thereof.  
         [0028]      FIG. 3  is a view, again similar to that of  FIG. 1 , but showing the one internal component at another extreme angular position thereof.  
         [0029]      FIG. 4  is an enlargement of the generally circular area A of  FIG. 1 .  
         [0030]      FIG. 5  is frontal perspective view of a yoke, yoke ring and optical/electronic sensor unit, shown in the position of  FIG. 1 .  
         [0031]      FIG. 6  is a side perspective of the components of  FIG. 5 .  
         [0032]      FIG. 7  is an enlarged sectional view,. taken generally along line  7 - 7  of  FIG. 5   FIG. 8  is a top plan view of a yoke ring on the one internal component, utilized in  FIGS. 1-4 , illustrating the angularly-spaced, radially-directed, indicia markings thereon that are adapted to be read by an optical/electronic sensor.  
         [0033]      FIG. 9  is another perspective view of a portion of a variable displacement type hydraulic mechanism, such as a pump/motor assembly, partly cut-away, illustrating another embodiment of the optical position sensing device of the present invention, for determining the angular position of another internal component therewithin, herein shown at one extreme angular position thereof.  
         [0034]      FIG. 10  is a view, similar to that of  FIG. 9 , showing the another internal component at an intermediate angular position thereof.  
         [0035]      FIG. 11  is a view, again similar to that of  FIG. 9 , but showing the another internal component at another extreme angular position thereof.  
         [0036]      FIG. 12  is an enlargement of the generally circular area B of  FIG. 10 .  
         [0037]      FIG. 13  is a further perspective view of a portion of a variable displacement type hydraulic mechanism, such as a pump/motor assembly, partly cut-away, illustrating a further embodiment of the optical position sensing device of the present invention, for determining the linear position of a further internal component therewithin, herein shown at one extreme longitudinal position thereof.  
         [0038]      FIG. 14  is a view, similar to that of  FIG. 13 , showing the further internal component at an intermediate longitudinal position thereof.  
         [0039]      FIG. 15  is a view, again similar to that of  FIG. 13 , but showing the further internal component at another extreme longitudinal position thereof.  
         [0040]      FIG. 16  is an enlargement of the generally circular area C of  FIG. 13 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0041]     Referring now to the several drawings, illustrated in  FIGS. 1-8 , is one embodiment of the optical position sensing device, generally indicated at  20 , that finds utility in combination with a variable displacement type hydraulic pump/motor assembly  22 , here shown greatly simplified and in perspective as well as being partly cut-away, for both ease of illustration and understanding. Commercial pump/motor assemblies include, but are not limited to, type C-24 bent-axis pump assemblies having yokes, as well as type PV in-line pump assemblies having swashplates, both types being available from the Parker-Hannifin Corporation, of Cleveland, Ohio, U.S.A.  
         [0042]      FIGS. 1-4  illustrate a portion of a stationary pump/motor housing  24  that contains a movable yoke  26 . The yoke  26  is pivotable around a central axis  28  from the one extreme angular position thereof, shown in  FIG. 1 , via the intermediate position of  FIG. 2 , to the other extreme position, shown in  FIG. 3 . The yoke  26  is provided with a preferably integral ring/disc  30 , with an angular portion or arc  34  thereof, corresponding at least to the degree or extent of the angular movement of yoke  26 . Portion  34 , in turn, is provided with an array of absolute indicia markings  32 , in the form of an arc of spaced, radially-directed, individual, unique, bar-type markings representative of the absolute angular position of yoke  26 , relative to a reference location on stationary housing  24 . As is known in the art, an absolute scale marking allows the detection of an absolute position anywhere within the marked range which, in turn permits the detection of a position, relative to a known reference point or mark. U.S. Pat. No. 5,693,935 to Hassler Jr. et al., which is also assigned to the assignee of the present invention, sets forth a marking technique, applicable to piston rod surfaces that utilize a specific photo-sensing technology. The noted marking technique provides indicia markings in the form of a scale tube in the form of a series of rings and spacers that have differing light reflection characteristics. A rotary version of an optical position transducer DSP (Digital Signal Processor) control electronics and pattern sensor is positioned to read the markings and includes the required probe. U.S. Pat. No. 6,834,574 B2, also assigned to the assignee of the present invention, sets forth a cylinder with an optical position sensing device which, among other things, includes a piston rod indicia marking technique using a laser.  
         [0043]     In a preferred mode of the present invention, absolute scale indicia markings  32  are provided on the ring/disc member  30  that is, in turn either attached to or forms an integral unit with yoke  26 . Indicia markings  32  include a progressive, differing, binary number pattern of dark and light stripes  33   a ,  33   b , respectively, as best seen in  FIGS. 4 and 8 . This binary number pattern corresponds with specific angular positions of the absolute scale indicia markings imparted on the noted outer surface  36  of, for example, on the anodized outer surface  36  of an aluminum alloy material ring/disc member  30 .  
         [0044]     As best seen in  FIGS. 1-3 , an optical/electronic sensor unit  38  is fixedly located on stationary housing  24  and functions for sensing indicia markings  32 . As specifically illustrated in  FIGS. 4, 6  and  7 , sensor unit  38  includes an optical read head  40  that sealingly extends into housing  24  and is positioned so as to sense/read at least axial portions of radially directed indicia markings  32 . Read head  40  is located at a predetermined, spaced distance  44  from the outer end surface  36  of ring/disc  30 , with the spaced distance  44  being less than 1 mm. in width and preferably about 0.6 mm. in width. In addition, while there may be a total absence of working fluid, i.e., a vacuum, in spaced distance  44 , the presence of a working fluid, such as a gas or a liquid, e.g., air or a hydraulic fluid, is preferred. The working fluid, if utilized, needs to be translucent to light, such as visible light, and is preferably confined to the red spectrum. Depending upon the specific application, the types of materials being utilized, and the type of working fluid being used, etc., even IR light may be feasible.  FIG. 5  also illustrates a sensor unit connector  42  that serves for connecting sensor unit  38  to any desired type of control system (not shown).  
         [0045]     In operation, optical/electronic sensor unit  38  acquires high-speed photographic image data of absolute indicia markings  32 , processes the images and extracts an angular position measurement of ring/disc member  30  with respect optical read head  40  of optical/electronic sensor unit or transducer  38 . In terms of the output, unit  38  produces either an analog voltage proportional to the angular position of ring/disc  30 , or a digital serial communication message containing the digital angle that it transmits over a serial communication bus. It is important to keep in mind that the intended use of unit  38  is to provide positional feedback to a hydraulic controller for closed-loop control of the swashplate, to change the displacement of the pump/motor assembly  24 , or for open-loop instrumentation readings. The DSP processes the charge coupled device (CCD) image using a desired algorithm to determine a portion of a pattern embedded in indicia markings  32 . The determined portion of the pattern is indicative of the absolute position of read-head  40  relative to the position of yoke ring/disc  30 . This is accomplished by periodically strobing a red LED lamp whose light is conducted to ring/disc  30  through emitter fiber-optic bundles and bouncing the light off the deep anodized recurring pattern of dark  33   a  and light  33   b  strips, which make up indicia markings  32 . The red LED is used if the indicia ring is fabricated of anodized aluminum. It should be understood that other materials may require different frequency light. For example, the piston rod sensor, set forth in U.S. Pat. No. 6,834,574 B2, uses NIR light to see the laser etched markings on a chromed surface. Similar materials may also be used in carrying out the present invention. The resulting image is then transferred back up the fiber optic bundles of read head  40  into the CCD of sensor unit  38 . The signal or image from the sensor unit  38  is read into a DSP&#39;s memory in the form of an array of 132 10-bit linear pixel elements in one embodiment of the invention. It is important to understand the strobing of the LED since using a continuous light source may result in a “smeared” image as ring/disc  30  attains velocity. It should also be understood that flash intensity, length and focus are characteristics which have to be considered by those skilled in the art in the design of unit  38 .  
         [0046]     Turning now to  FIGS. 9-12  illustrated therein is another embodiment of the optical position sensing device, generally indicated at  120 , that finds utility in a variable displacement type hydraulic pump/ motor assembly  122 , here again shown greatly simplified and in perspective as well as being partly cut-away and fragmentary for ease of illustration and understanding. Parts similar to those of already described with reference to sensing device  20  utilize the same number augmented by prefix “1”.  
         [0047]      FIGS. 9-12  illustrate a portion of a stationary pump/motor housing  124  that contains a movable swashplate  46  that is pivotable about a predetermined axis (not shown) from one extreme angular position thereof, shown in  FIG. 9 , via intermediate positions—one of which is shown in  FIG. 10 , to the other extreme angular position, shown in  FIG. 11 , by at least one reciprocating setting piston  50  attached to a swashplate flange portion  48  via parallel, pivotable, coupling members  52 . Thus, it should be understood that the reciprocation of setting piston  50  produces the pivoting of swashplate  46  in the manner illustrated in  FIGS. 9-11 .  
         [0048]     Movable or pivotable swashplate  46  is provided, on an outer wall portion  136  and in an angular or arc portion  134  thereof, with a plurality of preferably integral absolute indicia markings  132 , in the form of spaced, radially directed or arched, individual, unique, bar-type markings, substantially similar to already previously described indicia markings  32 . Similarly, an optical/electronic sensor unit or transducer  138 , again substantially similar to previously described such unit  38 , is fixedly located on stationary housing  124 -and functions, at that location, for sensing indicia markings  132 . As best seen in  FIG. 12 , sensor unit.  138  also includes an optical read head  140 , similar to read head  40  that again sealingly extends into housing  124  and is positioned to sense/read at least axial portions of indicia markings  132 . Read head  140  is also located at a predetermined, spaced distance, not shown here but similar to previously-described spaced distance  44 , from the outer surface  136  of swashplate  46  and thus, optical position sensing device  120  is substantially similar in operation to previously-described optical position sensing device  20  and need not be described further.  
         [0049]     Continuing now with  FIGS. 13-16 , illustrated therein is a further embodiment of the optical position sensing device of the present invention, generally indicated at  220 , that finds utility in a variable displacement type hydraulic pump/motor assembly  222 , here again shown greatly simplified and in perspective as well as being partly cut-away and fragmentary for ease of illustration and understanding. Parts similar to those already described with reference to sensing devices  20  and  120  utilize the same number augmented by prefix “2”.  
         [0050]      FIGS. 13-16  again illustrate a portion of a stationary pump/motor housing  224  that contains a movable swashplate  246  that is pivotable about a predetermined axis (not shown) from one extreme angular position thereof, shown in  FIG. 13 , via intermediate positions, one of which is shown in  FIG. 14 , to the other extreme angular position, shown in  FIG. 15 , by at least one reciprocating setting piston  250  attached to a swashplate flange portion  248  via parallel coupling members  252 . Thus, it will be understood that the reciprocation of setting piston  250  produces the pivoting of swashplate  246  in the manner illustrated in  FIGS. 13-16 .  
         [0051]     Reciprocating setting piston  250  is provided, on an outer wall surface portion  236  and in an angular or arc portion  234  thereof, with a plurality of preferably integral absolute indicia markings  232 , in the form of spaced, laterally directed or arched, individual, unique, bar-type, indicia markings, very similar to already previously described indicia markings  32 . Similarly, an optical/electronic sensor unit or transducer  238 , again substantially similar to previously described such unit  38 , is fixedly located on stationary housing &#39; 224  and functions, at that location, for sensing indicia markings  232 . As best seen in  FIG. 16 , sensor unit  238  also includes an optical read head  240 , similar to read head  40  that again sealingly extends into housing  224  and is positioned to sense/read at least axial portions of indicia markings  232 . Read head  240  is again located at a predetermined, spaced distance, not shown here but similar to spaced distance  44  in sensing unit  20 , from the outer surface  236  of setting piston  250 . Thus, optical position sensing device  220  is very similar in operation and function to previously-described optical position sensing devices  20  and  120 . The main difference between device  120  and device  220  is that while the former directly senses/reads the rotary or pivotal movement of swashplate  146 , device  220  senses/reads the linear displacement of reciprocating setting piston  250  which, however, is translated into the rotary or pivoting movement of swashplate  246  via coupling members  252 . In other words, device  220  provides an indirect sensing/reading of the rotary/pivoting movement of swashplate  246 .  
         [0052]     Based on the previous discussion, it should be understood that the present invention pertains to several embodiments of optical position sensing units, such as  20 ,  120  and  220 , that function to determine the position of movable components with a hydraulic pump/motor assembly. Specifically, an optical sensor head is incorporated within the pump/motor assemblies to determine, either directly or indirectly, the exact position of one of the yokes or swashplates, etc. therewithin. As noted,.specific indicia markings  32 ,  132  and  232  are provided so that the optical sensor head can read/sense the noted indicia markings and send electrical signals to the pump/motor controls which, in turn, can monitor or move the internal componentry, such as yokes and/or swashplates, etc., to change the displacement of the pump/motor.  
         [0053]     The present invention provides much more accurate sensing than has been possible in the prior art in that the latter typically read the rotation of the pivot point of the yoke/swashplate. The present invention provides for optical reading, either directly or indirectly, of the radial extent of the movement of the yoke/swashplate, i.e., the yoke/swashplate is not marked at its pivot point, but rather is marked at distances either radially or axially away from the noted pivot point, thus improving accuracy.  
         [0054]     Typically, it is difficult to design a pump/motor assembly that can position the sensor so that it can read the angular displacement of the yoke/swashplate but the present invention overcomes this obstacle by designing the pump/motor housing so that it can readily incorporate the sensor in a position close to the component to be sensed. The noted housing is also designed for ease of maintenance in that sensor access is easy so that the sensor, or even the entire optical position sensing unit, can readily be replaced without undue inconvenience and does not require disassembly of the pump/motor. In addition, the replacement sensor or sensing unit does not need to be recalibrated since the marked components, e.g., the yoke ring, swashplate or swashplate setting piston, is the calibrated component.  
         [0055]     The present invention also presents a simplified design in that it utilizes but a minimum number of moving parts, with the sensing unit being incorporated within an existing part (housing) of the pump/motor assembly. Furthermore, the sensed portions, i.e., the indicia markings, are provided on existing parts of the pump/motor assembly, e.g., the yoke, swashplate or the swashplate setting piston. In addition, the spaced distance or gap between the sensor and the sensed component is not as critical as with other prior art sensors and the lack of physical contact therebetween essentially obviates frictional wear.  
         [0056]     Finally, it should be clear that although the previous description has referenced examples of specific sensed components, such as a yoke, swashplate or setting piston for the swashplate, other movable components within the confines of the pump/motor housing can also be sensed since the sensor or sensing unit can be positioned virtually anywhere on or within the pump/motor assembly.  
         [0057]     Thus, it is deemed that one of ordinary skill in the art will readily recognize that the several embodiments of the present invention fill remaining needs in this art and will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as described herein. It is therefore intended that the protection granted hereon be limited only by the scope of the appended claims and their equivalents.