Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1998-09-09
2001-09-25
Enad, Elvin (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06294859
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an electrostrictive or piezoelectric actuator device including a solid state actuator element that has a variable length upon application of an electrical voltage, and including a transmission mechanism that amplifies the displacement or stroke movements of the actuator element, wherein the transmission mechanism comprises rigid frame members connected to each other by elastically flexible joints.
BACKGROUND INFORMATION
Actuator devices of the above mentioned general type typically include a solid state electrostrictive or piezoelectric element as an actuator, which becomes elongated in a lengthwise direction due to strain induced in the electrostrictive or piezoelectric material upon application of an electrical voltage thereto. The present specification applies to both electrostrictive and piezo-electric actuator elements, which will generally be referred to as electrically strainable solid state actuator elements, which comprise corresponding electrically strainable solid state materials.
Since the electrically induced strain in electrostrictive and piezoelectric materials is rather small in relation to the voltage that is applied to the material, the resulting strain must be amplified or multiplied to provide a sufficient stroke or displacement range to be useful as an actuator in most mechanical or physical applications. For this reason, it is known to provide a laminated stack of layers of electrically strainable solid state material to form a solid state actuator element, so that the total stroke or displacement of the stack element is a series addition of the electrically induced strains of all of the respective solid state material layers. However, even the multiplied displacement achieved by such an electrostrictive or piezoelectric stack element is inadequate for many applications.
Therefore, attempts have been made to provide a mechanical linkage or transmission mechanism, and particularly a step-up transmission mechanism, for amplifying the stroke displacement provided by the solid state actuator element. U. S. Pat. No. 4,937,489 (Hattori et al.) discloses an electrostrictive actuator including at least one electrostrictive element and a transmission mechanism coupled thereto for amplifying the initial displacement of the electrostrictive element. The transmission mechanism consists of a solid and relatively massive metal plate having a groove or cut-out for receiving the actuator element therein, and a plurality of slits so as to form a substantially rectangular frame made of rigid frame members connected to each other by integral, flexible hinge joints formed by thinner slitted or notched areas of the metal plate material. The result is a linkage frame effectively made up of six rigid bodies and six hinges. An elongation of the actuator element causes a deformation of this linkage frame such that a small change in length of the electrostrictive member is kinematically converted into a multiply amplified output stroke provided by one of the linkage bodies of the actuator device.
In view of the rather high tension loads effective on the elastic hinge joints, these hinge joints must have a rather large tensile strength and stiffness, and therefore must have a correspondingly large cross-sectional area. This simultaneously causes the disadvantage that the overall bending stiffness of the bending hinge, and also the outer fiber strain resulting in the material of the hinge due to the bending movement, increase sharply with increasing cross-sectional area of the hinge joint. As a result, the hinge joints, which are appropriately dimensioned for the prevailing loads, cause an elastic return force and bending resistance that acts against the electrically induced length variation of the actuator element and therefore noticeably reduces the effective stroke of the actuator device.
SUMMARY OF THE INVENTION
In view of the above, it is an object of the invention to provide an actuator device of the above mentioned general type, embodied with a compact structure that has a high strength and stiffness in the lengthwise loading direction, yet simultaneously achieves a high energetic efficiency and a long operating lifetime. Particularly, it is an object of the invention to provide a structural arrangement of a transmission mechanism for amplifying the stroke of an electrically strainable solid state actuator element, wherein a sufficient tensile strength is achieved, while reducing the bending stiffness and the resulting outer fiber strain in the hinged joints of the transmission mechanism, and reducing the resistive force of the hinge joints acting counter to the actuating force of the actuating element over a large stroke range, i.e. a large bending range of the joints. The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages, as apparent from the present description.
The above objects have been achieved in an electrostrictive or piezoelectric actuator device according to the invention, including an electrically strainable solid state actuator element and a transmission mechanism adapted to amplify the stroke motion of the actuator element. The transmission mechanism comprises a linkage frame of rigid frame members connected to each other by elastically flexible joints. More specifically, each flexible joint is made up of a plurality of adjacently arranged elastic hinge members having respective parallel extending hinge bending axes. Furthermore, the frame members include unitary frame members and divided frame members that are respectively made up of at least two separate, individual lever arms or link rods. The hinge members of each respective flexible joint are all connected on one side to a single unitary frame member that integrally connects the respective hinge members to each other, and are connected on the other side individually and respectively to the individual link rods that form the respective divided frame member. The individual link rods of a given divided frame member are preferably, but not necessarily, parallel to each other and of equal length.
Thus, according to the invention, the individual flexible joints are divided respectively into a plurality of hinge members, and at least one of the frame members connected to each flexible joint is divided into a respective plurality of individual link rods. Otherwise, the structural parameters of the transmission mechanism, for example especially the total cross-sectional area, and the total tensile strength and stiffness of the flexible joints, can be the same as an arrangement using single flexible joints and only single link rods, because these parameters are directly determined from the load characteristics. Nonetheless, due to the inventive arrangement, the bending stiffness of the flexible joints is reduced significantly, namely proportionally to the square of the respectively selected degree of division of the flexible joints into individual hinge members. As a result of this reduction of the joint stiffness, the elastic return force or counter force exerted by the flexible joints on the transmission mechanism countering the deformation force of the actuator element is also significantly reduced, and the useful stroke of the actuator device is substantially increased. As a further beneficial effect, the outer fiber strain of the material forming the hinge members, relative to the bending angle, is similarly reduced, because the thickness of each individual hinge member is reduced. For this reason, the overall operating life time of the actuating device is considerably improved.
A preferred embodiment of the invention uses a two-fold division of the flexible joints and the divided frame members. Namely, each flexible joint includes two parallel hinge members, and each divided frame member includes two individual link rods. In such an embodiment, the bending stiffness of the flexible joints is already reduced by 75%, and the outer fiber strain in the hinge members is reduced by 50%, in comparison to
EADS Deutschland GmbH
Enad Elvin
Fasse W. F.
Fasse W. G.
Medley Peter
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