Patent Publication Number: US-2016231148-A1

Title: Floating optical sensor mount

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/444,478 filed Feb. 18, 2011, which is hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a piston-cylinder assembly having a sensor for detecting the position of a piston rod relative to a cylinder housing, and more particularly to a mounting arrangement for the sensor relative to the cylinder housing. 
     BACKGROUND OF THE INVENTION 
     Piston-cylinder assemblies are used in various actuator applications throughout industry, such as in construction equipment. Often it is advantageous for an operator to be aware of the specific position of a piston rod in a fluid pressure-operated cylinder since the working member being actuated is generally physically connected to the end of the piston rod. 
     U.S. Pat. No. 7,047,865, for example, discloses a known actuator with a cylinder housing and a rod that is axially movable relative to the cylinder housing. The actuator also has sensor for absolute position sensing in which light from a light source illuminates a pattern on the rod. The sensor is attached to the cylinder housing and detects light reflected from an adjacent portion of the pattern. The position of the rod relative to the housing is determined from the known position of the sensor relative to the housing and the portion of the pattern detected by the sensor. 
     In contrast to absolute-position sensors, relative-position sensor systems also are known, but they require periodic “zeroing” to maintain accurate position information. In the event of a power failure, for example, the rod generally must be retracted to a “home” position before being extended again. Depending on the circumstances when the hydraulic cylinder lost power, this might not be practical or even possible. An absolute position sensor always knows the position of the rod, even after a power failure. Knowing absolute position also can be used to more accurately control the actuator extension/retraction. 
     SUMMARY OF THE INVENTION 
     While absolute position sensors for piston-cylinder type actuators provide some advantages over relative position sensors, existing methods of mounting the sensors on piston-cylinders generally do not compensate for lateral rod deflection due to the rod&#39;s weight or other forces acting transverse to the axial or longitudinal dimension of the rod. This failure to account for lateral deflection results in poor sensor signal data, particularly for long rods, or large diameter rods. 
     The present invention provides a unique mounting apparatus for an absolute position sensor. Accordingly, the present invention provides a piston-cylinder actuator that includes a unique mount for an absolute-position sensor. The mount, made from a bearing material, provides a flexible connection between the sensor mount and the cylinder housing. This flexible connection allows the piston rod to deflect naturally, under its own weight or under other laterally-directed forces, while maintaining the distance and perpendicularity between the sensor and the marked rod surface, within acceptable limits. The sensor mount is made from a bearing material that will allow it to float directly on the rod surface without scuffing or otherwise damaging the rod surface, particularly the markings or other indicia on that surface. Due to the flexible connection between the sensor mount and the cylinder housing, the proper distance between the sensor and the rod surface can be maintained at all times. 
     More particularly, the present invention provides a piston-cylinder actuator that includes a cylinder assembly with a cylinder housing. The cylinder housing has a longitudinal axis. The actuator also includes a piston assembly that is generally movable relative to the cylinder assembly along the longitudinal axis. The piston assembly includes a piston rod having indicia extending longitudinally on the rod. The actuator further includes a sensor capable of reading the indicia on the piston rod, and a mount for supporting the sensor relative to the piston rod. The sensor mount is flexibly connected to the cylinder housing to allow the sensor mount to move relative to the cylinder housing to allow the sensor mount to remain in registration with the rod, while maintaining the sensor at a substantially constant position along the longitudinal axis relative to the cylinder housing. 
     An exemplary embodiment of the actuator provided by the invention further includes one or more of the following features. An exemplary cylinder assembly includes a seal gland mounted in the cylinder housing. The seal gland has a central aperture for receipt of the piston rod. The seal gland is separate from the sensor mount. In an exemplary actuator, clearance between the sensor mount and the piston rod is less than clearance between the seal gland and the piston rod. 
     An exemplary sensor mount is connected to the cylinder housing with a retaining wire that forms a wire lock. The retaining wire provides a flexible connection between the seal gland and the sensor mount. The flexible connection between the sensor mount and the cylinder housing is provided by ensuring sufficient clearances between the retaining wire, the seal gland, and the sensor mount. 
     An exemplary sensor mount includes a bearing surface for interfacing with the piston rod and an aperture transverse the bearing surface to provide the sensor with access to the indicia. The mount is made of a bearing material. The mount includes a collar that defines a passage therethrough for receipt of the piston rod. The mount includes a collar that extends around the piston rod. An exemplary sensor is an optical sensor. 
     The present invention also provides a piston-cylinder actuator including a cylinder assembly with a cylinder housing that defines a cylindrical volume, the cylinder housing having a longitudinal axis. The actuator also includes a piston assembly generally movable relative to the cylinder assembly along the longitudinal axis. The piston assembly includes a piston rod having indicia extending longitudinally on a surface of the rod. The actuator further includes a sensor capable of detecting the indicia on the piston rod, and means for supporting the sensor relative to the piston rod. The support means is flexibly connected to the cylinder housing to pivot relative to the cylinder housing while maintaining the sensor at a substantially constant position along the longitudinal axis relative to the cylinder housing. The support means includes a sensor mount made of a bearing material. 
     Finally, the present invention provides a method of providing a flexible connection for a sensor mounted on a piston-cylinder actuator. The method includes the steps of: a) separating an optical sensor mount assembly from a seal gland assembly; b) providing less clearance between the sensor mount and a rod than the clearance between a seal gland and the rod; c) providing a wire lock connection between the seal gland and sensor mount; and d) building in sufficient clearances between the wire lock, seal gland, and sensor mount to allow flex between the seal gland and sensor mount. 
     Further features of the invention will become apparent from the following detailed description when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary piston-cylinder actuator provided in accordance with the invention. 
         FIG. 2  is a longitudinal cross-sectional view of the actuator of  FIG. 1  as seen along lines  2 - 2 . 
         FIG. 3  is an enlarged view of an aperture-end portion of the actuator shown in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings and initially to  FIGS. 1 and 2 , an exemplary embodiment of an actuator provided by the invention is shown generally at  20 . The actuator  20  includes a cylinder assembly  22 , a piston assembly  24  that is movable relative to the cylinder assembly  22 , and a sensor assembly  26  with a sensor  30  for detecting the absolute position of the piston assembly  24  relative to the cylinder assembly  22 . The piston assembly  24  includes an elongated piston rod  32  that has indicia markings (not shown) provided on at least a so longitudinal portion of its peripheral surface  34 . These markings include patterns that vary along the length of the rod  32 , enabling the sensor  30  to identify the position of the rod  32  from a portion of the pattern adjacent the sensor  30 . Thus the indicia can be formed in the surface  34  of the rod  32 , embedded in the rod  32 , etched, carved, formed, or printed on the rod  32 , or otherwise provided by other devices that function to identify the position of the indicia relative to the rod  32 . The sensor  30  is supported by supporting means in the form of a sensor mount  36  that is made of a bearing material that floats on the piston rod  32  as the rod  32  moves, allowing the sensor mount  32  to remain in registration with the rod  32  without damaging the markings on the rod  32 . The sensor mount  36  is flexibly coupled to the cylinder assembly  22  to accommodate transaxial displacement of the rod  32  relative to a longitudinal axis  38  of the cylinder assembly  22  while maintaining a substantially constant position along the axis  38 . The flexible connection enables the sensor mount  36  to maintain a proper relationship between the sensor  30  and the surface  34  of the piston rod  32 , even when the piston rod  32  is deflected from alignment with the longitudinal axis  38  of the cylinder assembly  22  due to the weight of the rod  32  or other laterally-acting forces on the rod  32 , sometimes referred to as side load conditions. 
     Although the illustrated piston-cylinder actuator  20  is designed for axial movement of the piston assembly  24 , the sensor mount  36  provided by the present invention is applicable to other types of actuators, such as actuators with piston assemblies that rotate relative to the cylinder assembly. 
     Turning to further details of the various components, the cylinder assembly  22  includes a cylindrical housing  40 , and the piston assembly  24  is movable relative to the cylinder housing  40  back and forth along the longitudinal axis  38 . The illustrated cylinder housing  40  has two opposed ends, one end  42  being closed by a cylinder coupling  44 , while the other end  46  is adapted to receive a seal gland  50 . 
     The seal gland  50  has an outside surface  52  that mates and sealingly interacts with a corresponding inside surface  54  of the cylinder housing  40 . The seal gland  50  also has an inner annular surface  56  that defines a central axial aperture that receives the piston rod  32 . The aperture in the seal gland  50  allows reciprocating passage or movement of the piston rod  32 . The inner peripheral surface  56  of the seal gland  50  has a series of spaced circumferential recesses,  60 - 64 , which receive peripheral seals  66 - 70 , respectively. The peripheral seals  66 - 70  provide a leak-tight seal between the peripheral surface  34  of the piston rod  32  and the inner peripheral surface  56  of the seal gland  50 . The diameter of the seal gland aperture and the diameter of the piston rod  32  are dimensioned to permit the smooth passage of the piston rod  32  while preventing pressurized working fluid from migrating outside the pressurized volume of the cylinder housing  40 . The seal gland  50  generally is flush with the apertured end  46  of the cylinder housing  40 . 
     The piston assembly  24  includes the piston rod  32 , which extends through the aperture in the seal gland  50 , and a piston head  72  that is closely received within the cylinder housing  40 . The piston head  72  sealingly divides the cylinder housing  40  into two chambers  74  and  76 . The piston assembly  24  also can be referred to as a plunger assembly. The piston rod  32  is attached to the piston head  72  at one end, and an opposing end of the piston rod includes a rod coupling  78  for connecting the actuator  20  to an object to be moved. 
     The piston assembly  24  generally is movable along an axis of the piston rod  32 , which in  FIG. 2  is coextensive with the longitudinal axis  38  of the cylinder housing  40 . The piston assembly  24  and the cylinder housing  40  can reciprocate relative to each other depending upon which cylinder chamber  74  or  76  is pressurized. Pressure is supplied to the cylinder housing  40  by any desired external pressure source (not shown) to a first cylinder port  80  coupled to the chamber  74 , and exhausted via a second cylinder port  82  coupled to the cylinder chamber  76 , and vice versa, depending on the desired direction of movement. The piston rod  32  cooperates with the central longitudinal aperture in the seal gland  50  to close the apertured end  46  of cylinder housing  40 . 
     The piston rod  32  and the associated indicia on the rod  32  move relative to both the cylinder housing  40  and the sensor  30 , which reads the indicia adjacent the sensor  30  to determine the position of the rod  32  relative to the cylinder housing  40 . The sensor  30  preferably is an absolute-position sensor. An exemplary absolute-position sensor is an optical sensor, such as the Intellinder™ sensor from Parker Hannifan Corp. of Cleveland, Ohio U.S., although other non-optical type sensors also may benefit from the sensor mount  36  provided by the invention. The optical sensor  30  typically includes a light source (such as a light-emitting diode, generally referred to as an LED) and a light sensor (generally a charge coupled device or CCD) that are mounted on a circuit board (sometimes referred to as a printed circuit board or PCB). Light guiding elements guide the light from the light source to the surface  34  of the rod  32 , and reflected light from the rod  32  to the light sensor. 
     Maintaining a specific distance between the sensor  30  and the surface of the rod  32  is important to ensure effective sensing accuracy. If the distance is too great or too small, or if the sensor  30  does not face squarely (perpendicularly) on the surface of the rod  32 , the position-determining portion of the pattern visible to the sensor  30  will be out of focus. Testing has shown that a transverse or side load induced by the weight of the rod  32  itself or other laterally-acting forces on the rod  32 , deflects the rod  32  from the axis  38  of the cylinder housing  40 , forming an angle between the axis of the rod  32  and the axis of the cylinder  40 . The greater the distance between the sensor  30  and the cylinder housing  40 , the more exaggerated the problem becomes. This problem is more evident in actuators with larger diameter and longer piston rods. 
     The means for supporting the sensor  30 , such as the illustrated sensor mount  36 , includes a bearing platform  100  that is tethered to the cylinder housing  40  with a flexible coupling that accommodates deflection of the rod  32  from alignment of its longitudinal axis with the longitudinal axis  38  of the cylinder housing  40 . The bearing platform  100  is made of a bearing material. The sensor  30  is mounted directly to the bearing material, rather than being mounted on a support structure that is in turn coupled to a separate bearing material. The flexible coupling allows the sensor mount  36  to pivot or rotate about an axis transverse the longitudinal axis  38  of the cylinder housing  40  to accommodate the transaxial displacement or bending of the rod  32 , while remaining in contact with the rod  32  and in a substantially constant position along that axis  38 . 
     The flexible coupling is provided by a flexible retaining wire  102  and corresponding grooves  104  and  106  in the mount  36  and the inner surface of the cylinder housing  40 , respectively, that form a wire lock. The sensor mount  36  is thus separate from the seal gland  50 . The retaining wire  102  prevents movement between the mount  36  and the cylinder housing  40  along the axis  38 , but sufficient clearance between the wire lock, seal gland  50  and sensor mount  36  exists to allow the sensor mount  36  to flex relative to the cylinder housing  40 . In other words, the wire lock permits some rotation of the sensor mount  36  about an axis transverse the longitudinal axis  38 . The wire lock thus allows the sensor mount  36  to pivot relative to the cylinder housing  40 , thereby maintaining a consistent orientation and spacing between the sensor  30  and the surface  34  of the rod  32 . In minimizing or preventing movement along the longitudinal axis  38  of the cylinder housing  40 , the wire lock also maintains a substantially constant axial position of the sensor  30  relative to the cylinder housing  40 . Other types of flexible coupling also would work, such as a gimbal mount or other arrangement. 
     The illustrated sensor mount  36  is in the shape of a collar that extends completely around the piston rod  32 . The central aperture in the sensor mount  36  receives and is substantially filled by the piston rod  32 . The axis of the central aperture in the mount  36  preferably is coextensive with the longitudinal axis of the piston rod  32  at all times. The sensor mount  36  further includes a transverse passage or bore  110  that intersects the central aperture, preferably perpendicular to the axis of the aperture, which axis is aligned with the longitudinal axis of the rod  32  when assembled. The mount  36  preferably includes one or more wiper seals adjacent the sensor bore  110  to clear debris from the rod surface  34 . In the illustrated embodiment, an inner peripheral surface of the mount  36  has a circumferential recess  112  that receives a peripheral seal or wiper  114  that prevents debris on the surface  34  of the rod  32  that might interfere with the sensor&#39;s detection of the pattern on the piston rod  32  from entering the aperture. The opposing side of the sensor bore  110  is protected by the seals  66 - 70  in the seal gland  50 . The sensor bore  110  is thus interposed between the outer wiper  114  and the peripheral seals  66 - 70  of the seal gland  50 . 
     Accordingly, a method of providing a flexible connection for an absolute-position sensor  30  in a piston-cylinder actuator  20  comprises the following steps: 
     a) separating the optical sensor mount  36  from the seal gland  50 ; b) providing less clearance between the sensor mount  36  and the piston rod  32  than the clearance between the seal gland  50  and the rod  32 ; c) providing a wire lock connection between the seal gland  50  and the sensor mount  36 ; and d) building in sufficient clearances between the wire lock, seal gland  50 , and sensor mount  36  to allow flex between the seal gland  50  and sensor mount  36 . 
     While the sensor mount  36  has been shown and described as part of a complete system, the actuator  20 , the sensor mount  36  also could be provided as part of a kit for retrofitting existing actuators. Additionally, although the invention is described with reference to a piston-cylinder actuator, the sensor can be used with hydraulic actuators, pneumatic actuators, rotary actuators, or any other device that requires position sensing. Moreover, the invention is not limited to optical sensors, and the principles of the invention may be applicable to other types of sensors that need to be precisely positioned relative to the moving object being detected, such as a magnetic-type sensor that needs to have a read head closely spaced relative to surface with magnetically-readable indicia. 
     In summary, the present invention provides an actuator design with a unique sensor mount  36  made from a bearing material and a flexible connection between the sensor mount  36  and the cylinder housing  40 . This flexible connection allows the rod  32  to deflect naturally, under its own weight or under other laterally-directed forces, while maintaining the distance and perpendicularity between the sensor  30  and the marked rod surface, within acceptable limits. This flexible connection is attained by a) separating the sensor mount  36  from the seal gland  50 ; b) providing less clearance between the sensor mount  36  and the rod  32  than between the seal gland  50  and the rod  21 ; c) providing a wire lock connection between the seal gland  50  and the sensor mount  36 ; and d) building in sufficient clearances between the retaining wire  102 , the seal gland  50 , and the sensor mount  36  to allow ‘flex’ or limited relative non-axial movement between the seal gland  50  and the sensor mount  36 . The sensor mount  36  is made from a bearing material that will allow it to float directly on the rod surface  34  without scuffing or otherwise damaging the rod surface  34 , particularly the markings or other indicia on that surface. Due to the flexible connection between the sensor mount  36  and the cylinder housing  40 , the proper distance between the sensor  30  and the rod surface  34  can be maintained at all times. The concepts in (a)-(d) above, in conjunction with the sensor mount  36 , provide a unique solution to the lateral deflection problem. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.