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.

TECHNICAL FIELD

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

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.

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.

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

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.

DETAILED DESCRIPTION

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. 1illustrates a cylinder-piston assembly10in accordance with an embodiment of the present disclosure. The cylinder-piston assembly10may be used to control movement of an implement of a machine (not shown). The cylinder-piston assembly10may 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.

As shown inFIG. 1, the cylinder-piston assembly10includes a cylinder barrel12. The cylinder barrel12is a hollow cylindrical shaped body having a thickness ‘T’. The cylinder barrel12may be made up of any metallic material suitable to the scope of the present disclosure. A piston14is located in the cylinder barrel12which can slideably reciprocate inside the cylinder barrel12between a first end16and a second end18. The piston14may slide inside the cylinder barrel12due to a hydraulic or a pneumatic force. The cylinder barrel12has a first port20and a second port21to provide supply and withdrawal of hydraulic/pneumatic fluid to the cylinder barrel12respectively. The cylinder barrel12may also include stoppers (not shown) on the first and second ends16,18to limit the sliding movement of the piston14inside the cylinder barrel12and avoid contacting the first and second ends16,18. The piston14may slide in the cylinder barrel12between the first and second ends16,18based on the supply/withdrawal of hydraulic/pneumatic fluid as per the requirement of a particular application.

The piston14may be made up of a suitable metallic material such as a cast aluminum alloy, suitable to the scope of the present disclosure. The piston14is connected to a piston rod22that extends outwards from the cylinder barrel12from the first end16. The piston14may be connected to the piston rod22by a mechanical connection such as welding, brazing, adhesive means or a mechanical fastener. The piston14and the piston rod22may also be casted as a single piece.

As shown inFIG. 1, an end cap24is attached to the first end16of the cylinder barrel12. The end cap24may be attached to the first end16of the cylinder barrel12by any mechanical joining process such as welding, brazing, mechanical fasteners etc. A seal (not shown) may be attached between the first end16of the cylinder barrel12and the end cap24. The seal may prevent leakage of any hydraulic fluid around the first end16along the piston rod22. The seal may be attached to either of the piston rod22or the end cap24. The end cap24is a metal part having a cylindrical opening to allow the piston rod22to pass through. The piston rod22has a first end26connected to the piston14and a second end28which 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 rod22is partially enclosed inside the cylinder barrel12and partially extends outside the cylinder barrel12.

A spring30is circumferentially located on the piston rod22inside the cylinder barrel12. The piston rod22acts as a guide to the spring30. The spring30may he any type of a spring such as tapered spring, disc spring etc. in accordance with the present disclosure. In an embodiment, the spring30may be an elastomer spring as well. The spring30is adapted to be compressed or extended as the piston14and the piston rod22slide inside the cylinder barrel12. The spring30may have a first end32attached to the first end26of the piston rod22and a second end34attached to aloud cell36attached to the end cap24.

The load cell36is typically a transducer used to create an electrical signal having a magnitude directly proportional to a force being measured. The load cell36may be a hydraulic load cell, a pneumatic load cell, or a strain gauge load cell. The load cell36may 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 cell36is of a hollow cylindrical shape. The load cell36is attached to the end cap24. The end cap24may have an opening38concentric to the cylindrical opening in the end cap24. The load cell36may be accommodated in the opening38such that some part of the load cell36is in contact with the second end34of the spring30. The load cell36may also be arranged in the end cap24in any other suitable way in accordance with the present disclosure.

As the spring30compresses or extends, the spring30applies a spring force F on the load cell36. As the pressurized hydraulic fluid flows around the load cell36in the end cap24, the load cell36experiences force due to the pressurized hydraulic fluid from all sides. Thus, the load cell36is in a pressure balanced condition with respect to the hydraulic forces acting on the load cell36as the hydraulic forces tend to cancel each other out. Further, as the spring30is also applying the spring force F on the load cell36, the load cell36generates a signal indicative of the forces exerted on the load cell36. The spring force F constitutes a major part of this signal along with minimal contribution from other forces.

As shown in FIG,1, the piston14and piston rod22are 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 piston14with respect to the cylinder barrel12, which may be defined by knowing the displacement ‘D’. For the sake of explanation, let us assume that the spring30is in a natural state i.e. a state of no compression or extension corresponding to the first position of the piston14and the piston rod22. The spring30experiences no force in the natural state. As the piston14and the piston rod22are displaced by a displacement ‘D’ and come to the second position, the spring30also extends by a length ‘D’ as the spring30is connected to the first end26of the piston rod22. The spring30experiences 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.

The spring30exerts a pressure corresponding to the spring force on the load cell36as the spring30is attached to the load cell36at the second end34. In another embodiment, the second end34of the spring30may be attached to a plate (not shown) which may further be attached to the load cell36. The spring30may apply force on the plate and the plate may exert a uniform pressure on the load cell36. The load cell36generates a signal indicative of the force F exerted by the spring30which may be processed to calculate a corresponding force signal.

A controller40is attached to the load cell36which receives the signal corresponding to the pressure exerted by the spring30on the load cell36. The controller40may be an Electronic Control Unit (ECU) of the machine. The controller40may have means to convert the signal received from the load cell36to a value corresponding to the force F. The controller40may have the function f(D) stored in memory so that the controller40can 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 controller40may 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 cell36. The controller40may have any such means to derive the value of the force from the signal generated by the load cell36.

The controller40may 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.

Although, the spring30is shown in an extended state when the piston14and the piston rod22are in the second position, it should be contemplated that the second position of the piston14and the piston rod22can also be such that the spring30is compressed. In an embodiment, the spring30may be pre-stressed corresponding to the first position of the piston14and the piston rod22so that the spring30extends when the piston14and the piston rod22move from the first position to the second position. In such as case, the spring30will exert pressure on the load cell36in the natural state. Further, as the piston14and the piston rod22move from the first position towards the second position, the spring30will exert lesser pressure on the load cell36compared to the pressure exerted at the natural state. A difference between the two pressure values will provide the pressure applied on the load cell36due to the displacement of the piston14and the piston rod22. Thus, the force applied by the spring30on the load cell and the corresponding cylinder displacement can be calculated.

FIG. 2illustrates another embodiment of the present disclosure. In this embodiment, the spring30is a tapered spring. The diameter of the spring30increases from the first end32towards the second end34. Therefore, when the spring30is in a fully compressed state, the spring30would collapse on itself such as to minimize the space taken by the spring30.

INDUSTRIAL APPLICATION

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.

The present disclosure provides a solution determining the cylinder displacement. The cylinder-piston assembly10includes the piston14and the piston rod22which reciprocate inside the cylinder barrel12. Hydraulic/pneumatic fluid may enter/exit through the ports20,21to make the piston14and the piston rod22slide between the first and second ends16,18of the cylinder barrel12. For example, when the hydraulic/pneumatic fluid is supplied through the port20and/or the hydraulic/pneumatic fluid is withdrawn from the port21, the piston14experiences a force due to motion of the fluid and slides from the first end16towards the second end18of the cylinder barrel12. Alternatively, when the hydraulic/pneumatic fluid is supplied through the second port21and/or withdrawn from the first port20, the piston14slides from the second end18towards the first end16of the cylinder barrel12.

The cylinder-piston assembly10includes the spring30circumferentially attached to the piston rod22. The first end32of the spring30is attached to the first end26of the piston rod22and the second end34of the spring30is attached to the load cell36attached to the end cap24. As the piston14slides inside the cylinder barrel12, the spring30exerts force on the load cell36. The load cell36generates the signal corresponding to the spring force and the displacement of the piston14relative to the cylinder barrel12can be calculated knowing the spring force F and the functional relationship f(D) between the spring force F and the displacement D.

Also, as only the spring30and the load cell36are the additional components required, the cylinder-piston assembly10is fairly simple in construction and does not take extensive time in service and maintenance. Further, the load cell36is installed in the pressure balanced condition, therefore allowing the spring30being used to be of a low spring force sensitivity. This minimizes the working load on the cylinder-piston assembly10as the spring30poses no restrictions in functioning of the cylinder-piston assembly10.