Abstract:
An actuator device for producing a linear movement, has a hydraulic actuator which includes a first piston element for actuating the actuator and a second piston element for producing the linear movement. The piston elements are assigned respective fluidically coupled working chambers, the volumes of which can be changed by movement of the respective piston element. A piezoelectric actuator is provided for exerting a force on the first piston element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is based on and hereby claims priority to International Application No. PCT/EP2013/058621 filed on Apr. 25, 2013 and German Application No. 10 2012 206 834.1 filed on Apr. 25, 2012, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]    The invention relates to an actuator device for producing a linear movement and to a method for setting a position of a linearly movable element. 
         [0003]    For certain adjustment tasks, there is a need for actuators by which linear adjustments that are as precise as possible can be performed. Ideally, such actuators are as small as possible, operated electrically, resistant to environmental conditions, especially contamination, and as wear resistant as possible. 
         [0004]    It is known that such adjustment tasks can be performed manually with some effort and that electromagnetic linear actuators, spindle drives or pneumatic/hydraulic actuators can be provided. The use of piezoelectric or magnetoresistive actuators is also known. For relatively large deflections, piezoelectric actuators which operate on the caterpillar or inchworm principle are also known. For this purpose, three actuators are generally required, namely a clamping actuator element at the front and rear and an actuator element for forward motion. 
         [0005]    Known actuators are generally complex and are not resistant to contamination and wear. 
         [0006]    In addition to the actuator types described above, the related art also includes a linear drive which manages with just a mass, a clamping device and a single actuator element. A piezoelectric actuator is positioned between the mass and the clamping device and is supplied with a rising voltage in such a way that the deflection force resulting therefrom accelerates the mass but the force remains below the static friction force of the clamping device on which the actuator is supported. When the maximum deflection of the piezoelectric element is reached, the control voltage is reduced so quickly that the resulting contraction of the piezoelectric element leads to a negative acceleration force which, although it reduces the movement of the mass, is so great that the static friction of the clamping device is exceeded and therefore the clamping device is taken along in the direction of movement and, in the next deflection phase, forms the new point of support, now displaced in the direction of movement. If the sequence is reversed instead of the control profile comprising “slow deflection, rapid contraction”, the direction of movement of said linear motor is also reversed. 
         [0007]    One disadvantage of this method is the dependence of the deflection on the magnitude of the current static friction of the clamping element. Particularly if the static friction changes due to the ingress of oil or dirt, the desired precision or indeed the entire functioning ability of the adjusting device may be lost. Moreover, as with all mechanical systems in which two surfaces rub against one another, the system is prone to wear over the long term and hence to changes in system properties. 
         [0008]    JP 2000 314402 A discloses an actuator device for producing a linear movement, having a hydraulic actuator, which comprises a first piston element for actuating the actuator and a second piston element for producing the linear movement, which piston elements are assigned respective fluidically coupled working chambers, the volumes of which can be changed by movement of the respective piston element. In this case, a piezoelectric actuator is provided for exerting a force on the first piston element. Moreover, the respective working chambers are connected by two oppositely oriented check valves. JP 2001 012402 A discloses an actuator device having two piston elements, which are assigned respective fluidically coupled working chambers. A respective volume of the working chambers can be changed by movement of the respective piston element. A magnet element for exerting a force on one of the piston elements is furthermore provided. 
         [0009]    DE 10 2008 046 562 A1 discloses a hydraulic linear drive having a piston, which is mounted slidably in a cylinder, wherein the cylinder has an inlet, via which a fluid can be introduced into the cylinder by a pumping device. The cylinder furthermore has an outlet, via which fluid can be removed from the cylinder. Provision is made here for a piezoelectric and/or magnetostrictive actuator to be provided to drive the pumping device. 
         [0010]    An actuator having an electrostrictive body can be taken as known from JP S62 28507 A. A rod can be moved by the electrostrictive body. 
         [0011]    Finally, EP 1 190 829 A2 discloses an actuator device for producing a linear movement, having a piston which is arranged in a cylinder. The piston can be moved by subjecting the piston to a pressure by a working fluid. Here, the working fluid is put under pressure by a driving force of a piezoelectric element in order to drive the piston. 
       SUMMARY  
       [0012]    One potential object is to provide an actuator device and a method of the type stated at the outset which allow the production of well-defined linear movements in a particularly simple, low-cost, precise and wear resistant manner. 
         [0013]    The inventors propose an actuating device of this kind for producing a linear movement comprises a hydraulic actuator having a first piston element for actuating the actuator and a second piston element for producing the linear movement, which piston elements are assigned respective fluidically coupled working chambers, the volumes of which can be changed by movement of the respective piston element. A piezoelectric actuator is furthermore provided for exerting a force on the first piston element. Coupling piezoelectric control and hydraulic transmission allows particularly precise production of linear movements. In particular, the two piston elements can be configured in such a way that the force which the piezoelectric element produces can be transmitted with a predetermined ratio. The advance of the second piston element as a reaction to the piezoelectric actuation is also then subject to such a ratio, and therefore relatively large forward movements of the piezoelectric element can be converted into relatively small piston movements, for example, leading to particularly precise positioning of the second piston element and of elements connected thereto. As a result, the actuator device described is suitable especially for high precision adjusting operations and the like. 
         [0014]    The respective working chambers are connected to one another by two oppositely oriented check valves. To permit controlled transfer of the working fluid between the working chambers. According to the proposal, the two check valves have different opening forces. 
         [0015]    It is furthermore expedient if the working chamber of the first piston element is coupled to a reservoir for a working fluid of the hydraulic actuator via a restrictor element. By the device described, the following possibility for producing linear movements which go beyond even the range of movement of the piezoelectric element itself is obtained. In a first phase of movement, the piezoelectric element is controlled in such a way that it produces a rapid movement. The rapid movement is transmitted to the first piston and produces a pressure in the working fluid which is sufficient to open the valve between the working chambers in the direction of the second working chamber. Fluid then flows into the second working chamber and there moves the second piston element, producing the desired forward motion. 
         [0016]    In a second phase of movement, the piezoelectric element is moved slowly counter to the direction in which it was moved in the first phase of movement. Although the first piston element is thereby likewise pulled back in the opposite direction, the slow movement means that the check valve oriented from the second to the first working chamber does not open. On the contrary, the resulting change in volume in the first working chamber is compensated for by the reservoir through the restrictor element. The piezoelectric element and the first piston element are thus reset to the initial position thereof, while the second piston element remains in its position. 
         [0017]    By repeating the two phases alternately, any desired further forward movement of the second piston element exceeding the actual range of movement of the piezoelectric actuator can be achieved. This allows particularly precise positioning of the second piston element over long linear paths of movement and simultaneously entails the advantage that no energy has to be expended to maintain a predetermined position of the second piston element. Moreover, in contrast to the actuators described at the outset, based on the inchworm principle, the sequence of motion is independent of the static friction of the participating components and is therefore not impaired by contamination, oil ingress or the like. 
         [0018]    In this case, the working chambers can expediently be formed by respective cylinders, in which the pistons are movably accommodated. One alternative to this is the formation of the working chambers by respective bellows, which are coupled to the piston elements. In this embodiment, all the essential moving parts of the actuator device are encapsulated and thus protected from contamination, resulting in operation of the device in a manner which is more reliable and subject to less wear over the long term. 
         [0019]    The inventors furthermore propose a method for setting a position of a linearly movable element, in which a force is exerted on a first piston element by a piezoelectric actuator, wherein the volume of a working chamber assigned to the first piston element is changed by the exertion of the force, and the force applied is transmitted to a second piston element, the associated working chamber of which is fluidically coupled to the first working chamber. 
         [0020]    As already described in respect of the device, a movement of a linearly movable element coupled to the second piston element exceeding the range of movement of the piezoelectric actuator itself is made possible in this way. As already explained above, to achieve this a movement in a predetermined direction is carried out by moving the piezoelectric actuator so quickly in the predetermined direction in a first phase of movement that the pressure produced in the first working chamber by the movement overcomes the closing force of a check valve fluidically connecting the working chambers in the direction of movement. The movement of the piezoelectric actuator is thus transmitted to the second piston element in the first phase of movement. 
         [0021]    In a second phase of movement, the piezoelectric actuator is moved so slowly counter to the predetermined direction that the pressure produced in the first working chamber by the movement does not overcome the closing force of a check valve fluidically connecting the working chambers counter to the direction of movement. In contrast to the first phase of movement, the two working chambers are not fluidically connected here, and the movement of the piezoelectric actuator is thus not transmitted to the second piston element. As already explained, it is possible in this way to produce a movement which goes beyond the actual range of movement of the piezoelectric actuator, it being possible for this to be further amplified or, alternatively, reduced by the ratio between the two piston elements. 
         [0022]    In order to compensate for the change in volume in the first working chamber during the second phase of movement, working fluid is fed to the first working chamber from a reservoir or discharged from said chamber to the reservoir via a restrictor element during the second phase of movement. 
         [0023]    The first and second phases of movement are preferably repeated alternately until the second piston element is in a predetermined setpoint position. In the manner described, it is thus possible to achieve linear movements of any length which are limited only by the range of movement of the second piston element but not by the maximum deflection of the piezoelectric actuator. Overall, the method allows particularly precise production of linear movements with as little wear as possible and with a particularly compact mechanical construction of the device on which it is based. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0024]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0025]      FIG. 1  shows a schematic representation of an illustrative embodiment of a device having hydraulic cylinders as hydraulic actuators; 
           [0026]      FIG. 2  shows a hydraulic equivalent circuit diagram of the device according to  FIG. 1 ; 
           [0027]      FIG. 3  shows a schematic representation of an alternative illustrative embodiment of a device having bellows as hydraulic actuators; and 
           [0028]      FIG. 4  shows a hydraulic equivalent circuit diagram of the device according to  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0030]    A device, denoted overall by  10 , for producing a linear movement, e.g. for adjustment tasks, comprises a hydraulic actuator  12 , a first piston  14 , which is accommodated movably in an associated accommodation space  16 , and a second piston  18 , which is supported movably in a second accommodation space  20 . Accommodation space  16  is connected to the second accommodation space  20  by a first line  22  containing a one-way valve  24 , which opens in the direction of the second accommodation space  20 . A second line  26  containing a second one way valve  28 , which opens in the direction of the first accommodation space  16 , connects the second accommodation space  20  to the first accommodation space  16 . The first accommodation space  16  is furthermore connected to a reservoir  34  for operating fluid by a line  30 , which includes a restrictor element  32 . 
         [0031]    To actuate the first piston  14 , a piezoelectric actuator  36  is provided. A force exerted on the piston rod  38  of the first piston  14  by the piezoelectric actuator  36  is transmitted hydraulically by the first piston  14  to the second piston  18 , the piston rod  40  of which is coupled to the element to be moved. 
         [0032]    In order to bring about a movement in the direction of arrow  42 , the piezoelectric actuator  36  is controlled in accordance with the force/time diagram  44 . In a first phase of movement, the piezoelectric actuator  36  is moved quickly in the direction of arrow  46 . The movement is transmitted to the first piston  14 , while the pressure thereby produced in the first accommodation space  16  exceeds the closing force of valve  24 , with the result that operating fluid crosses from the first accommodation space  16  to the second accommodation space  20  and there exerts a force on the second piston  18 , with the result that the latter moves in the direction of arrow  42 . In a second phase of movement, the piezoelectric element  36  is pulled back in the direction of arrow  48 , but this takes place significantly more slowly than the first phase of movement. As a result, the piston  14  likewise moves in the direction of arrow  48  but the slow movement ensures that the closing force of valve  28  is not exceeded. Thus, no operating fluid crosses from the second accommodation space  20  into the first accommodation space  16 , and the second piston  18  maintains its position. The change in volume of the first accommodation space  16  due to the movement of the first piston  14  is compensated for by operating fluid which flows out of the reservoir  34 , through line  30  and restrictor  32 , into the first accommodation space  16 . 
         [0033]    Through alternating repetition of the two phases of movement, piston  18  can be pushed further forward than is predetermined by the range of movement of the piezoelectric actuator  36 . Here, the ratio between pistons  14  and  18  allows positioning of the second piston  18  with particularly high precision, while it is advantageous that no force has to be expended to hold the piston  18  in position. 
         [0034]    If piston  18  is to be pulled back in the opposite direction, i.e. in the direction of arrow  15 , the two phases of movement are reversed, as illustrated by the force/time diagram  52 . First of all, therefore, the piezoelectric actuator  36  is moved quickly in the direction of arrow  48 , so that the resulting change in pressure in the first accommodation space  16  exceeds the closing force of one-way valve  28  and liquid can flow out of the first accommodation space  20  into the first accommodation space  16 , causing piston  18  to move in the direction of arrow  50 . In the second phase of movement, the piezoelectric actuator  36  is moved slowly in the direction of arrow  46 , thereby ensuring once again that the closing force of one way valve  24  is not exceeded, with the result that the second piston  18  maintains its position and hydraulic fluid flows back out of the first accommodation space  16 , via the restrictor  32  and line  30 , into the reservoir  34 . Here too, a further movement of piston  18  can be produced by alternate repetition of the two phases of movement.  FIGS. 3 and 4  show an alternative embodiment of the device  10 , in which, instead of hydraulic cylinders, use is made of bellows  54 ,  56 , which are connected to the piston elements  14 ,  18  and thus form the accommodation spaces  16 ,  20 . The reservoir  34  is also formed by a bellows  58 . In other respects, the device  10  according to  FIGS. 3 and 4  functions in the same way as the device  10  according to  FIGS. 1 and 2 . The use of bellows  54 ,  56 ,  58  makes it possible to construct the entire device  10  in a substantially encapsulated way, ensuring that it has minimum susceptibility to contamination and wear. A device  10  of this kind can therefore be operated for a particularly long time with constant precision. 
         [0035]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide  v.  DIRECTV,  69 USPQ2d 1865 (Fed. Cir. 2004).