Abstract:
A cylinder-piston assembly includes a cylinder barrel having a first end and a second end. An end cap is coupled to the cylinder barrel at the first end having a cylindrical opening. A piston slidably disposed in the cylinder barrel has a piston rod connected to the piston. The piston rod passes through the cylindrical opening of the end cap. A spring is disposed circumferentially around the piston rod. A load cell disposed in the end cap is adapted to generate a signal indicative of the cylinder displacement based on a spring force exerted on the load cell during a movement of the piston in the cylinder barrel, wherein the load cell is pressure balanced to minimize effect of forces other than the spring force exerted on the load cell.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a cylinder-piston assembly. More specifically, the present disclosure relates to determine a cylinder displacement of the cylinder-piston assembly. 
       BACKGROUND 
       [0002]    Various types of machines, such as a wheel loader, an excavator, a dozer, and the like are used for a number of industrial applications such as construction, forestry, agriculture, mining and excavation. Generally, such machines include an implement through which a particular operation is carried out. For example, an excavator may include a boom attached to a frame of the excavator. The boom supports an arm connected to a bucket. The excavator may include individual hydraulic/pneumatic means to actuate the boom, the arm and the bucket. Hydraulic/pneumatic means may be a cylinder-piston assembly adapted to provide required motive force to the corresponding parts. An operator may control operation of the bucket by controlling a position of the cylinder-piston assemblies associated with various parts. 
         [0003]    Currently, various kinds of sensors, such as linear displacement transducers (LDT), magnetostrictive sensors, electromagnetic sensors, can be used as position sensing devices. However, devices to measure absolute cylinder displacement in harsh environments with a high degree of reliability is presently complex and expensive. 
         [0004]    D.E. Publication Number 10,2005,057,914 describes a hydraulic or pneumatic actuator. The actuator has a mechanical drift element movable inside a housing under hydraulic/pneumatic force. A spring biases the mechanical drift element in a relaxed position. A force sensor arranged between the spring and the mechanical drift element detects the force experienced by the spring. However, as the force sensor is arranged between the spring and the mechanical drift element, the force sensor is susceptible to various other forces as well and may not provide an accurate estimate of the forces acting on the spring and hence, the cylinder displacement. 
       SUMMARY 
       [0005]    In an aspect of the present disclosure, a cylinder-piston assembly includes a cylinder barrel having a first end and a second end. An end cap is coupled to the cylinder barrel at the first end. The end cap has a cylindrical opening. A piston is slidably disposed in the cylinder barrel. The piston has a piston rod connected to the piston. The piston rod passes through the cylindrical opening of the end cap. A spring is disposed circumferentially around the piston rod. A load cell disposed in the end cap is adapted to generate a signal indicative of the cylinder displacement based on a spring force exerted on the load cell during a movement of the piston in the cylinder barrel. The load cell is pressure balanced to minimize effect of forces other than the spring force exerted on the load cell. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0006]      FIG. 1  schematically illustrates a cylinder-piston assembly including a spring and a load cell, in accordance with an embodiment of the present disclosure; and 
           [0007]      FIG. 2  schematically illustrates the cylinder-piston assembly of  FIG. 1 , wherein the spring is a tapered spring, in accordance with another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Wherever possible, the same reference numbers will he used throughout the drawings to refer to same or like parts. Construction machines such as an excavator include an implement such as a bucket to manipulate a ground surface. Generally, the excavator controls the bucket through a hydraulic/pneumatic actuator. The hydraulic/pneumatic actuator may be a cylinder-piston assembly.  FIG. 1  illustrates a cylinder-piston assembly  10  in accordance with an embodiment of the present disclosure. The cylinder-piston assembly  10  may be used to control movement of an implement of a machine (not shown). The cylinder-piston assembly  10  may also be used to control movement of various other parts of the machine as well. The machine may be used in various industries and application areas such as construction, forestry, agriculture, mining, excavation etc. 
         [0009]    As shown in  FIG. 1 , the cylinder-piston assembly  10  includes a cylinder barrel  12 . The cylinder barrel  12  is a hollow cylindrical shaped body having a thickness ‘T’. The cylinder barrel  12  may be made up of any metallic material suitable to the scope of the present disclosure. A piston  14  is located in the cylinder barrel  12  which can slideably reciprocate inside the cylinder barrel  12  between a first end  16  and a second end  18 . The piston  14  may slide inside the cylinder barrel  12  due to a hydraulic or a pneumatic force. The cylinder barrel  12  has a first port  20  and a second port  21  to provide supply and withdrawal of hydraulic/pneumatic fluid to the cylinder barrel  12  respectively. The cylinder barrel  12  may also include stoppers (not shown) on the first and second ends  16 ,  18  to limit the sliding movement of the piston  14  inside the cylinder barrel  12  and avoid contacting the first and second ends  16 ,  18 . The piston  14  may slide in the cylinder barrel  12  between the first and second ends  16 ,  18  based on the supply/withdrawal of hydraulic/pneumatic fluid as per the requirement of a particular application. 
         [0010]    The piston  14  may be made up of a suitable metallic material such as a cast aluminum alloy, suitable to the scope of the present disclosure. The piston  14  is connected to a piston rod  22  that extends outwards from the cylinder barrel  12  from the first end  16 . The piston  14  may be connected to the piston rod  22  by a mechanical connection such as welding, brazing, adhesive means or a mechanical fastener. The piston  14  and the piston rod  22  may also be casted as a single piece. 
         [0011]    As shown in  FIG. 1 , an end cap  24  is attached to the first end  16  of the cylinder barrel  12 . The end cap  24  may be attached to the first end  16  of the cylinder barrel  12  by any mechanical joining process such as welding, brazing, mechanical fasteners etc. A seal (not shown) may be attached between the first end  16  of the cylinder barrel  12  and the end cap  24 . The seal may prevent leakage of any hydraulic fluid around the first end  16  along the piston rod  22 . The seal may be attached to either of the piston rod  22  or the end cap  24 . The end cap  24  is a metal part having a cylindrical opening to allow the piston rod  22  to pass through. The piston rod  22  has a first end  26  connected to the piston  14  and a second end  28  which may have means to be connected to an implement of a machine or any other part of the machine according to the need of the application. As illustrated, the piston rod  22  is partially enclosed inside the cylinder barrel  12  and partially extends outside the cylinder barrel  12 . 
         [0012]    A spring  30  is circumferentially located on the piston rod  22  inside the cylinder barrel  12 . The piston rod  22  acts as a guide to the spring  30 . The spring  30  may he any type of a spring such as tapered spring, disc spring etc. in accordance with the present disclosure. In an embodiment, the spring  30  may be an elastomer spring as well. The spring  30  is adapted to be compressed or extended as the piston  14  and the piston rod  22  slide inside the cylinder barrel  12 . The spring  30  may have a first end  32  attached to the first end  26  of the piston rod  22  and a second end  34  attached to aloud cell  36  attached to the end cap  24 . 
         [0013]    The load cell  36  is typically a transducer used to create an electrical signal having a magnitude directly proportional to a force being measured. The load cell  36  may be a hydraulic load cell, a pneumatic load cell, or a strain gauge load cell. The load cell  36  may be of any shape such as a cylindrical shape, a circular shape, a fiat plate shape etc. in accordance with the present disclosure. In an embodiment, the load cell  36  is of a hollow cylindrical shape. The load cell  36  is attached to the end cap  24 . The end cap  24  may have an opening  38  concentric to the cylindrical opening in the end cap  24 . The load cell  36  may be accommodated in the opening  38  such that some part of the load cell  36  is in contact with the second end  34  of the spring  30 . The load cell  36  may also be arranged in the end cap  24  in any other suitable way in accordance with the present disclosure. 
         [0014]    As the spring  30  compresses or extends, the spring  30  applies a spring force F on the load cell  36 . As the pressurized hydraulic fluid flows around the load cell  36  in the end cap  24 , the load cell  36  experiences force due to the pressurized hydraulic fluid from all sides. Thus, the load cell  36  is in a pressure balanced condition with respect to the hydraulic forces acting on the load cell  36  as the hydraulic forces tend to cancel each other out. Further, as the spring  30  is also applying the spring force F on the load cell  36 , the load cell  36  generates a signal indicative of the forces exerted on the load cell  36 . The spring force F constitutes a major part of this signal along with minimal contribution from other forces. 
         [0015]    As shown in FIG,  1 , the piston  14  and piston rod  22  are shown in two positions, where a first position is represented by solid lines and a second position is represented by dotted lines. The first and second positions are spaced apart by a displacement ‘D’. Generally, a cylinder displacement refers to the relative position of the piston  14  with respect to the cylinder barrel  12 , which may be defined by knowing the displacement ‘D’. For the sake of explanation, let us assume that the spring  30  is in a natural state i.e. a state of no compression or extension corresponding to the first position of the piston  14  and the piston rod  22 . The spring  30  experiences no force in the natural state. As the piston  14  and the piston rod  22  are displaced by a displacement ‘D’ and come to the second position, the spring  30  also extends by a length ‘D’ as the spring  30  is connected to the first end  26  of the piston rod  22 . The spring  30  experiences a force F which is a function of the displacement length ‘D’, such that F=f(D). The function f(D) defines a relationship between the displacement D and the force F. in an embodiment, the function f(D) may define a linear relationship between the displacement D and the force F. The function f(D) may define a non-linear relationship as well between the displacement D and the force F suitable to the scope of the present disclosure. 
         [0016]    The spring  30  exerts a pressure corresponding to the spring force on the load cell  36  as the spring  30  is attached to the load cell  36  at the second end  34 . In another embodiment, the second end  34  of the spring  30  may be attached to a plate (not shown) which may further be attached to the load cell  36 . The spring  30  may apply force on the plate and the plate may exert a uniform pressure on the load cell  36 . The load cell  36  generates a signal indicative of the force F exerted by the spring  30  which may be processed to calculate a corresponding force signal. 
         [0017]    A controller  40  is attached to the load cell  36  which receives the signal corresponding to the pressure exerted by the spring  30  on the load cell  36 . The controller  40  may be an Electronic Control Unit (ECU) of the machine. The controller  40  may have means to convert the signal received from the load cell  36  to a value corresponding to the force F. The controller  40  may have the function f(D) stored in memory so that the controller  40  can calculate the value of the force F based on the value of the displacement D. The signal may be an electrical signal in form of an electrical current/voltage. The controller  40  may have a look up table so as to convert the electrical current/voltage into the value of the force. The look up table may also contain information relating to the functional relationship f(D) between the force F and the displacement D. The look up table may relate value of displacement D to the current/voltage value produced by the load cell  36 . The controller  40  may have any such means to derive the value of the force from the signal generated by the load cell  36 . 
         [0018]    The controller  40  may further have means to calculate the cylinder displacement D. As the spring force F is a function of displacement D, by knowing the value of force F as well as the relationship f(D) between the spring force F and the displacement D, cylinder displacement can be calculated. 
         [0019]    Although, the spring  30  is shown in an extended state when the piston  14  and the piston rod  22  are in the second position, it should be contemplated that the second position of the piston  14  and the piston rod  22  can also be such that the spring  30  is compressed. In an embodiment, the spring  30  may be pre-stressed corresponding to the first position of the piston  14  and the piston rod  22  so that the spring  30  extends when the piston  14  and the piston rod  22  move from the first position to the second position. In such as case, the spring  30  will exert pressure on the load cell  36  in the natural state. Further, as the piston  14  and the piston rod  22  move from the first position towards the second position, the spring  30  will exert lesser pressure on the load cell  36  compared to the pressure exerted at the natural state. A difference between the two pressure values will provide the pressure applied on the load cell  36  due to the displacement of the piston  14  and the piston rod  22 . Thus, the force applied by the spring  30  on the load cell and the corresponding cylinder displacement can be calculated. 
         [0020]      FIG. 2  illustrates another embodiment of the present disclosure. In this embodiment, the spring  30  is a tapered spring. The diameter of the spring  30  increases from the first end  32  towards the second end  34 . Therefore, when the spring  30  is in a fully compressed state, the spring  30  would collapse on itself such as to minimize the space taken by the spring  30 . 
       INDUSTRIAL APPLICATION 
       [0021]    Construction or earth-moving machines may use various software algorithms embodied in a processor to control the machine autonomously or semi-autonomously. Such software algorithms may control a machine such as an excavator to automatically excavate a surface or a motor grader to automatically impart a desired grade to a surface etc. Implements of such machines are provided with commands so as to control the machine autonomously. The implement of the machine is controlled by a cylinder-piston assembly actuated by hydraulic/pneumatic means. With the autonomous control software in place, it becomes vital to know cylinder displacement exactly so as to provide accurate control commands to the machine. 
         [0022]    The present disclosure provides a solution determining the cylinder displacement. The cylinder-piston assembly  10  includes the piston  14  and the piston rod  22  which reciprocate inside the cylinder barrel  12 . Hydraulic/pneumatic fluid may enter/exit through the ports  20 ,  21  to make the piston  14  and the piston rod  22  slide between the first and second ends  16 ,  18  of the cylinder barrel  12 . For example, when the hydraulic/pneumatic fluid is supplied through the port  20  and/or the hydraulic/pneumatic fluid is withdrawn from the port  21 , the piston  14  experiences a force due to motion of the fluid and slides from the first end  16  towards the second end  18  of the cylinder barrel  12 . Alternatively, when the hydraulic/pneumatic fluid is supplied through the second port  21  and/or withdrawn from the first port  20 , the piston  14  slides from the second end  18  towards the first end  16  of the cylinder barrel  12 . 
         [0023]    The cylinder-piston assembly  10  includes the spring  30  circumferentially attached to the piston rod  22 . The first end  32  of the spring  30  is attached to the first end  26  of the piston rod  22  and the second end  34  of the spring  30  is attached to the load cell  36  attached to the end cap  24 . As the piston  14  slides inside the cylinder barrel  12 , the spring  30  exerts force on the load cell  36 . The load cell  36  generates the signal corresponding to the spring force and the displacement of the piston  14  relative to the cylinder barrel  12  can be calculated knowing the spring force F and the functional relationship f(D) between the spring force F and the displacement D. 
         [0024]    Also, as only the spring  30  and the load cell  36  are the additional components required, the cylinder-piston assembly  10  is fairly simple in construction and does not take extensive time in service and maintenance. Further, the load cell  36  is installed in the pressure balanced condition, therefore allowing the spring  30  being used to be of a low spring force sensitivity. This minimizes the working load on the cylinder-piston assembly  10  as the spring  30  poses no restrictions in functioning of the cylinder-piston assembly  10 . 
         [0025]    While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof.