The present invention relates to a piezoelectric actuator having a piezoelectric bending (or flexural) transducer.  The present invention further relates to a valve that is actuatable by such an actuator. 
When using such actuators, the piezoelectric bending transducer is fixedly clamped either only at one end or at both ends.  Thus, a valve that is able to be used as a pneumatic or hydraulic valve is known, for example, the reference German Patent Application No.  DE 36 08 550 purportedly concerns a piezoelectric bending transducer fixed at its connection end, which is used for connecting to an electrical voltage source, inside a valve housing, in a narrow side of the housing, while the opposite, unattached end of the bending transducer, which bears a sealing element for closing a valve channel, is projected into the inside of the housing. 
In contrast, the reference U.S. Pat.  No.  5,630,440 purportedly concerns a piezoelectrically actuated valve having a bending transducer that is clamped at both ends in the housing and consequently is fixed at both ends.  By fixing both ends of the bending transducer, only a minimal and consequently insufficient deflection path, bending or flexural lift may be effected.  In comparison to a configuration where only one end of the bending transducer is fixed, this may achieve a comparably high restoring or working force. 
Embodiments according to the present invention are directed to providing a piezoelectric actuator that functions as reliably and effectively as possible during normal operation.  Further embodiments according to the present invention are directed to providing a valve, such as a pneumatic valve, that may be operable in an especially reliable manner using such an actuator. 
Another exemplary embodiment of the present invention is directed to providing that the first end of the bending transducer, which may be plate-like and/or rectangular, may be supported in a rotatable fashion, while the opposite, second end may be guided so as to be movable in the longitudinal transducer direction.  The reversibility of the piezoelectric effect may allow this arrangement to be usable as a sensor as well. 
The transverse guidance of the one transducer side or of the one transducer end may render possible a longitudinal movement of the bending transducer when transitioning from the rest state to the energized state.  At the same time, rotatably retaining the opposite transducer end (or the opposite transducer side) may prevent a counter-torque, which, otherwise, may inevitably occur as a result of the normal excitation of the bending transducer when this transducer end is securely clamped and consequently immovably fixed and which may cause the transducer material to break. 
In another exemplary embodiment of the present invention, the piezoelectric bending transducer may be guided, at least at the rotatably secured transducer end or may also be at both ends of the transducer, in a cylindrical holding element and securely joined thereto.  In this context, the holding element provided at the rotationally fixed or retained fixing end of the bending transducer may be embedded in a suitably round or circular receiving groove that wraps around the cylindrical retaining (or holding) element, for example, around more than half of the cross-sectional area, i.e., more than 180xc2x0.  This receiving groove, or a housing groove, may extend over the entire length of the cylindrical holding element.  The appropriate receiving (or housing) groove may also wrap around the holding element only locally, for example, only in the center and/or outer region.  The rotatably retained transducer end may be designed as a connection end that may be connected to an electrical voltage source for activating the actuator or transducer. 
Analogously, in another exemplary embodiment of the present invention, with respect to the deflection direction of the bending transducer, the holding element provided at the transversely displaceable end of the bending transducer may be inserted over its entire axial length or only partially in an appropriate, for example, approximately U-shaped receiving (or housing) groove. 
According to exemplary embodiments of the present invention, a valve having such a piezoelectric actuator may reliably close the valve channel to be blocked and, thus may be deactivated, even under extreme operating conditions, for example, under high pressure and high flow velocities of a medium flow to be controlled, by increasing the force, while at the same time, achieving a greatest possible deflection path or bending range.  This increased pressure force or pressing force with which the sealing element may be pressed or pushed by the piezoelectric bending transducer of the actuator against the valve seat of the corresponding valve channel may be generated by the bending transducer itself, while accompanied by a simultaneously sufficient deflection path, since according to the present invention, the bending transducer is held at both ends inside of the valve housing, yet is only stationary at one transducer end, while the other transducer end is practically float-mounted and is, consequently, only supported in the manner of a counter support.  As a result, on the whole, the operational performance of the actuator as such and in its use for actuating a valve may be improved, and the operational reliability of the valve may be enhanced. 
In this context, exemplary embodiments of the present invention may be based on the knowledge that in the case of a given supply voltage or operating voltage and, therefore, of a constant flexural torque of the piezoelectric transducer, the force generated by the transducer may increase with decreasing lever arm.  Thus, if the bending transducer is fixed on both sides, the deflection force, which is exerted by the bending transducer as a result of a deflection of the bending transducer, and which acts as a pressing force exerted by the sealing element against the valve seat of the corresponding valve channel, may be significantly greater than that of a bending transducer clamped on one-side and having a free-moving unattached end.  If in this context, a transducer end is guided in an axially movable manner, the deflection path may be at the same time greater than in the case of a bending transducer fixed on both sides.  Moreover, in comparison with clamping this transducer end to immovably fix it, securing the fixed transducer end in a rotatable manner may prevent a corresponding counter-torque and, thus, may prevent the bending transducer from rupturing at this fixed end. 
In exemplary embodiments of the present invention, the bending transducer may be designed to be bent in the off-circuit state.  For the valve control, this deflection or curvature may be concave in the direction of the valve channel.  In other words, to control the valve, this deflection or curvature may be orientated toward the channel that is to be deactivated and closed in the rest state.  As a result, the valve channel is able to be reliably closed in a particularly simple and effective manner already in the initial state or the rest state.  Two such actuators having two appropriate bending transducers held in such a manner and arranged to run essentially in a parallel direction with respect to one another may also be provided inside of the valve housing. 
The bending characteristic of the bending transducer in the off-circuit or de-energized rest state may be achieved in that the bending transducer itself is prestressed or biased in the manner of a flat spring.  Such prestressing may be achieved in turn using a suitable manufacturing method.  In this context, a laminated bending transducer having a layer construction including at least one electrically conductive carrier layer and one piezoceramic layer is used as a baseline, heating and subsequent cooling of the layer construction resulting in layers prestressed to different extents due to different, material-dependent contraction properties.  The prestressing of the layers to different extents then results in a deflection of the flat bending transducer about a neutral axis (or a bending axis) that runs perpendicularly to the bending transducer""s longitudinal sides and parallelly to its narrow sides. 
According to exemplary embodiments of the present invention, an actuator, whose curved and prestressed piezoelectric bending transducer may be float-mounted at one transducer end and pivoted at the opposite transducer end, may be used to achieve a long deflection path or large bending lift and a particularly high restoring force or working force, while simultaneously having a long service life and reliable operational performance.  Thus, in the case of a valve piezoelectrically actuated by such an actuator, the sealing effect of a valve channel that is closed or is to be closed may be more effective such that the operational reliability of the valve may be significantly increased.