Abstract:
The hydraulic actuator comprises: a hydraulic input cylinder having an input piston; a hydraulic output cylinder, which is hydraulically coupled to the input cylinder; and a pressure-limiting valve, which limits the output cylinder with respect to pressure in dependence on the usability of a force on the input piston. The method is a method for operating such a hydraulic actuator, wherein the drive actuator is deflected with deflections having a deflection duration at a deflection frequency for the duration of an acting or a non-acting phase of the hydraulic actuator, wherein the deflection duration defines a movement stiffness of the hydraulic actuator and the deflection frequency defines the resulting deflection speed of the hydraulic actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the National Stage of International Application No. PCT/EP2015/056371, filed Mar. 25, 2015, which claims the benefit of German Patent Application No. DE 102014214977.0, filed Jul. 30, 2014. The entire contents of these documents are hereby incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The present embodiments relate to a hydraulic actuator and to a method for operating a hydraulic actuator. 
         [0003]    Greater flexibility of conventional machine automation is desired (e.g., within the scope of “industry 4.0”). “Soft roboting” has gained importance in recent times. Manufacturing robots are intended to cooperate directly with humans during soft roboting. Actuators having a variable stiffness and a low mass are to be provided for this purpose. Models for actuators of this kind are muscles of the human body that, depending on the application, may prestress, increasing the stiffness. However, muscles of the human body are not prestressed in a normal state; therefore, in the event of a collision with a stiff object, the force on the body and the risk of injury is minimized. However, actuators that have been used to date have had a high stiffness. Sensors that identify obstacles and stop the actuator in serious cases or divert the actuator away from an obstacle are used in order to avoid collisions. 
       SUMMARY AND DESCRIPTION 
       [0004]    The scope of the present invention is defined solely by the appended claims and is not affected to any degree by the statements within this summary. 
         [0005]    The present embodiments may obviate one or more of the drawbacks or limitations in the related art. For example, an improved actuator (e.g., with variable stiffness)is provided. A method for operating an improved actuator of this kind is also provided. 
         [0006]    The hydraulic actuator according to one or more of the present embodiments includes a hydraulic drive cylinder (e.g., with a drive piston), a hydraulic output cylinder hydraulically coupled to the drive cylinder, and a pressure-limiting valve configured to limit the pressure on the output cylinder depending on an action time of a force on the drive cylinder and/or the drive piston. 
         [0007]    The pressure-limiting valve may be controlled by the hydraulic actuator based on the action time of the force on the drive piston. The pressure-limiting valve limiting the pressure on the output cylinder sets the stiffness of the hydraulic actuator based on this pressure-limiting operation. Therefore, the stiffness of the hydraulic actuator may be set based on the action time of a force on the drive piston. 
         [0008]    The hydraulic actuator according to one or more of the present embodiments includes a drive actuator coupled, in terms of movement, to the drive cylinder or to the drive piston. The hydraulic actuator may be electrically controllable. The hydraulic actuator may be a piezo actuator, an electrodynamic actuator, or an electromagnetic actuator. An actuator of this kind may be easily electrically controlled. 
         [0009]    In the case of the hydraulic actuator according to one or more of the present embodiments, the drive cylinder may be hydraulically coupled by a non-return valve and a first throttle to a prestressing volume located in a prestressing hydraulic cylinder (e.g., with a prestressing piston). The prestressing hydraulic cylinder or the prestressing piston actuates the pressure-limiting valve. In this way, the prestressing volume, which suitably actuates the pressure-limiting valve by the prestressing piston, may be hydraulically set in a simple manner by the non-return valve and the throttle based on the action time of a force on the drive piston. 
         [0010]    The prestressing volume is hydraulically connected to a reservoir by a second throttle in the hydraulic actuator. In this way, the prestressing volume may be fed from the reservoir or emptied into the reservoir given a corresponding action time. 
         [0011]    In an embodiment, the pressure-limiting valve is provided for limiting pressure in relation to the reservoir or for releasing pressure into the reservoir in the hydraulic actuator. In this way, when there is a low stiffness of the hydraulic actuator, the output cylinder may release pressure into the reservoir, or depending on the position of the pressure-limiting valve, maintain a high stiffness. 
         [0012]    An output piston may be guided in the output cylinder in the hydraulic actuator according to one or more of the present embodiments. In this way, the output piston functions as an operating element of the hydraulic actuator according to one or more of the present embodiments. As an alternative, the output cylinder may be formed with a folding bellows such that at least a portion of the output cylinder forms the operating element of the hydraulic actuator according to one or more of the present embodiments. 
         [0013]    In an embodiment, the output cylinder is coupled to the drive cylinder by a prestressed non-return valve. In this way, driving of the drive cylinder or of the drive piston may be passed on when there is a sufficient pressure difference between the drive cylinder and the output cylinder. However, depending on the pressure conditions, the drive may also be reset without having a direct consequential effect on the output cylinder (e.g., so that, as a result of periodic movement of the drive cylinder or of the drive piston, a large linear stroke is achieved by the output cylinder). 
         [0014]    A method according to one or more of the present embodiments for operating a hydraulic actuator including a hydraulic drive cylinder (e.g. with a drive piston and a drive actuator coupled in terms of movement to this drive cylinder or drive piston) is provided. In the method, the drive actuator is deflected with deflections having a deflection period at a deflection frequency for the duration of an operating or a non-operating phase of the hydraulic actuator. The deflection period defines a movement stiffness of the hydraulic actuator, and the deflection frequency defines the resulting deflection rate of the hydraulic actuator. 
         [0015]    In an embodiment, a drive actuator is electrically controllable (e.g., a piezo actuator, an electrodynamic actuator, or an electromagnetic actuator) in the disclosed method. 
         [0016]    In an embodiment, the hydraulic drive cylinder, the hydraulic output cylinder, and/or the prestressing hydraulic cylinder are/is each formed with a folding bellows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  schematically shows a hydraulic circuit diagram of a hydraulic actuator according to an embodiment. 
           [0018]      FIG. 2  schematically shows a graphical illustration of operating modes (a), (b) and (c) of the hydraulic actuator according to the embodiment depicted in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The hydraulic actuator  5  according to the embodiment illustrated in  FIG. 1  includes a piezo actuator  10  coupled in terms of movement to a drive piston  15  of a hydraulic drive cylinder  20 . 
         [0020]    The drive cylinder  20  includes a hydraulic drive volume  25  filled with hydraulic oil in a known manner. The drive volume  25  is hydraulically coupled to a hydraulic output cylinder  35  and is configured to open when there is a sufficiently high opening pressure. The non-return valve  30  is correspondingly prestressed. The output cylinder  35  includes an output volume  40  at the drive end that moves an output piston  45  located on the output side. 
         [0021]    The drive volume is also coupled to a reservoir  55 , such that the drive volume may feed the reservoir  55  via a non-return valve  50 . 
         [0022]    The drive volume  25  may also feed a prestressing volume  70  of a hydraulic prestressing cylinder  90 , which controls a pressure-limiting valve  80  via a prestressing piston  75 , via a non-return valve  60  and a throttle  65  arranged downstream of the non-return valve  60  in the passage direction. The prestressing volume  70  is coupled to the reservoir  55  via a second throttle  85 . The pressure-limiting valve  80  limits the pressure or releases the pressure on the output volume into or in relation to the reservoir  55 . 
         [0023]    The hydraulic actuator  5  according to the embodiment illustrated in  FIG. 1  is operated in the manner described below. The individual operating modes are characterized by driving of the piezo actuator  10 , as shown in the actuating travel/time graphs (a), (b) and (c) depicted in  FIG. 2 .  FIG. 2  is described in greater detail below. 
         [0024]    In a first operating mode, the hydraulic actuator is operated with a low system stiffness and actuated at an actuating rate v 1  (e.g., different from zero). 
         [0025]    The piezo actuator  10  is actuated as graphically shown by curve C 1  in  FIG. 2 . In (a), the piezo actuator  10  is quickly deflected (e.g., the actuating path s an  rises with a steep gradient hS over time t). The pressure in the drive volume  25  of the drive cylinder  20  increases such that the non-return valve  30  coupling the drive volume  25  to the output volume  40  opens, and the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70  opens. Because the deflection of the piezo actuator  10 , and therefore the increase in pressure in the drive volume  25 , are short in this first operating mode, virtually no hydraulic oil flows in the direction of the prestressing volume  70  through the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70 . This small flow of hydraulic oil again flows away into the reservoir  55  via the throttle  85  coupling the prestressing volume  70  to the reservoir. Therefore, virtually no pressure builds up in the prestressing volume  70 . Therefore, the hydraulic oil flows almost exclusively into the output volume  40  such that the output piston  45  is driven out with a resulting actuating path s ab  of the hydraulic actuator  5 . 
         [0026]    The deflection of the piezo actuator  10  is again suddenly reduced (e.g., depicted as steep negative gradient hA of the curve C 1  in  FIG. 2 ( a ) ). As a result, the non-return valve  30  coupling the drive volume  25  to the output volume  40  closes, and the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70  closes. Negative pressure is created by the reduced hydraulic oil in the drive volume  25 , and as a result, the non-return valve  50  coupling the drive volume  25  to the reservoir  55  opens and the missing hydraulic oil flows from the reservoir  55  into the drive volume  25 . 
         [0027]    If, in the first operating mode of this cycle (e.g., rapid deflection and resetting of the piezo actuator  10 ) is repeated, the output piston  45  is continuously deflected. If a counterforce acts on the output piston  45 , the pressure in the output volume  40  increases with this counterforce and the hydraulic cross section of the output cylinder  35 . Because the threshold in the pressure-limiting valve  80  is very low because of the missing pressure in the prestressing volume  25 , hydraulic oil already flows back from the output volume  40  into the reservoir  55  via the pressure-limiting valve  80  when there is a low counterforce on the output piston  45 . 
         [0028]    In a second operating mode, the hydraulic actuator  5  is operated with a high system stiffness and actuated at an actuating rate v 1  (e.g., different from zero). 
         [0029]    The piezo actuator  10  is actuated as graphically shown by curve C 2  in  FIG. 2 . In (b), the piezo actuator  10  is quickly deflected (e.g., the actuating path s an  rises with another steep gradient hS over time t). 
         [0030]    The pressure in the drive volume  25  increases and the non-return valve  30  coupling the drive volume  25  to the output volume  40  opens, and the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70  opens. Because the hydraulic oil flows away into the output volume  40 , the pressure in the drive volume  25  falls as in the first operating mode described above. 
         [0031]    In contrast to the first operating mode, deflection of the piezo actuator  10  is kept constant for a specific time (e.g., portion p of the curve C 2  in  FIG. 2 ( b ) ). Because the non-return valve  30  coupling the drive volume  25  to the output volume  40  has a defined opening pressure, the non-return valve closes when the pressure difference between the drive volume  25  and the output volume  40  is smaller than the opening pressure of the non-return valve  30 . Because the piezo actuator  10  is deflected, the remaining pressure is applied to the non-return valve  60  coupling the drive volume  25  to the prestressing volume. Because the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70  is not prestressed, hydraulic oil may flow via the non-return valve  60  and the throttle  65  arranged downstream of the non-return valve  60  until the pressure difference between the prestressing volume  70  and the drive volume  25  is greater. Although a small portion of hydraulic oil again flows back into the reservoir  55  via the throttle  85  coupling the prestressing volume  70  to the reservoir  55 , the pressure in the prestressing volume  70  increases. As a result, the opening threshold in the pressure-limiting valve  80  is increased. 
         [0032]    After a specific time, the piezo actuator  10  is suddenly reset to original actuating path s an  of the actuator (e.g., the steep negative gradient hA of curve C 2  in  FIG. 2 ( b ) ). As a result, hydraulic oil is sucked from the reservoir  55  into the drive volume  25 , as in the first operating mode described above. If the throttle  85  coupling the prestressing volume  60  to the reservoir  55  were not installed, hydraulic oil would be sucked out of the reservoir  55  and out of the prestressing volume  70 . 
         [0033]    The described cycle (e.g., deflection and resetting of the piezo actuator  10 ) is repeated. If a counterforce acts on the output piston  45 , the pressure in the output volume  40  increases again. However, the threshold in the pressure-limiting valve  80  is greater than in the above-described first operating mode because of the increased pressure, and as a result, a higher force may build up on the drive piston  15  and the flow of hydraulic oil out of the output volume  40  is reduced. The system stiffness of the hydraulic actuator  5  is increased in this way. The level of this stiffness is set by the drive profile of the piezo actuator  10 . 
         [0034]    In a third operating mode, the hydraulic actuator  5  is operated with a high system stiffness and is not actuated (e.g., actuated at an actuating rate of v 0 =0). 
         [0035]    The piezo actuator  10  is actuated as graphically shown by the curve C 3  according to  FIG. 2 . In (c), because of slow deflection (e.g., comparatively shallow gradient nS) of the piezo actuator  10 , the pressure in the drive volume  25  scarcely increases. As a result, only the non-return valve  60  coupling the drive volume  25  to the prestressing volume  70  opens, but the non-return valve  30  coupling the drive volume  25  to the output volume remains closed. As a result, no hydraulic oil is pumped into the output volume  40 . Hydraulic oil is pumped only into the prestressing volume  70 , resulting in the threshold of the pressure-limiting valve  80  and the system stiffness of the hydraulic actuator  5  increasing without the output piston  45  deflecting. 
         [0036]    After a specific time, the piezo actuator  10  is suddenly reset to original actuating path s an  of the actuator (e.g., the steep negative gradient hA of the curve C 3  in  FIG. 2 ( c ) ). 
         [0037]    Further exemplary embodiments not specifically illustrated and corresponding to the exemplary embodiment illustrated in  FIGS. 1 and 2 , an electrodynamic or an electromagnetic actuator is provided instead of a piezo actuator  10 . 
         [0038]    In further exemplary embodiments not specifically illustrated, the drive, output, and/or prestressing cylinders provided may be hydraulic cylinders in the form of folding bellows without pistons guided therein, instead of hydraulic cylinders with pistons guided therein. 
         [0039]    The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification. 
         [0040]    While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.