Electricity: conductors and insulators – Boxes and housings – Hermetic sealed envelope type
Reexamination Certificate
1998-12-18
2001-11-06
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Boxes and housings
Hermetic sealed envelope type
C257S790000
Reexamination Certificate
active
06313401
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to actuators and sensors used in precision spacecraft structures and, more particularly, to techniques for minimizing the effects of temperature changes on such structures. It is well known that certain materials exhibit a piezoelectric or electrostrictive effect and may be used as actuators, wherein an applied electric field causes mechanical strain or deformation along a selected axis of the material. An inverse effect allows strain to be sensed as a generated electrical signal. Devices of this type are especially useful in space applications, such as for vibration suppression, vibration isolation, active damping, health monitoring and shape control. Actuators or sensors can be conveniently incorporated into “host” structural members as implants or as patches applied externally.
A well recognized difficulty in any precision application of ceramic piezoelectric or electrostrictive sensors and actuators is that the actuator/sensor material may have a non-zero coefficient of thermal expansion (CTE) that does not match the CTE of the host structural element. This mismatch of CTEs can cause unwanted bending or other strain in the host structure, and can cause errors in sensed strains in the structure. U.S. Pat. No. 5,305,507 issued to George R. Dvorsky et al., entitled “Method for Encapsulating a Ceramic Device for Embedding in Composite Structures,” discloses a technique for encapsulating an actuator/sensor using a piezoelectric ceramic material, such as lead zirconate titanate (PZT) or lead magnesium niobate (PMN) (ceramic). Basically, the technique disclosed in the Dvorsky et al. patent is to design the encapsulating material to provide strain relief to offset the brittleness of the ceramic actuator and to provide electrical isolation for the ceramic actuator and leads. The effect of encapsulation, however, resulted in an actuator/sensor that has a net positive CTE. For applications requiring high precision, thermal distortion of structures must be limited or highly controlled. Temperature changes produce expansion or contraction when non-zero CTEs are present, and mismatching of CTEs can cause unwanted structural bending.
The present invention focuses on minimizing the net encapsulant CTE or matching the net encapsulant CTE to that of the host structure. An additional desire is to simultaneously maximize the actuation strength using a suitable figure of merit. The net result is a thermally stable sensor/actuator package.
In general, negative CTE materials are accompanied by high modulii. A practitioner skilled in the art recognizes that the steps taken to minimize the CTE reduce the actuation strength. That is, alignment of the high modulus fibers in the direction of desired low CTE results in increased stiffness. Maximization of actuation strength competes against this objective by requiring decreased stiffness. An important objective of this invention is to optimize both characteristics: CTE and actuation strength.
SUMMARY OF THE INVENTION
The present invention resides in a thermally stable actuator/sensor package for use in a host structural member. Briefly, and in general terms, the invention in one of its disclosed embodiments may be defined as a doubly encapsulated actuator/sensor package, comprising a ceramic piezoelectric or electrostrictive actuator/sensor having wire leads connected to electrodes of the actuator sensor; a primary encapsulating material surrounding the actuator/sensor except for the wire leads, which extend through the primary encapsulating material; and a secondary encapsulating material surrounding the primary encapsulating material and comprising multiple plies of a fiber-reinforced composite material applied to the primary encapsulating material at selected ply angles. The fiber-reinforced composite material and the ply angles are selected to provide a desired net coefficient of thermal expansion and a desired stiffness with respect to a selected axis.
More specifically, the primary and secondary encapsulating materials and the secondary encapsulating material ply angle are selected to provide a net coefficient of thermal expansion of approximately zero, or a coefficient of thermal expansion matching a specific structural material in which the package is to be adhered to or imbedded within. The selected fiber-reinforced composite material ply angle also results in a composite structure of sufficiently low stiffness to provide desirably high actuator force and sensor signal strength.
In the presently preferred embodiment of the invention, the actuator/sensor is of lead zirconate titanate (ceramic) material. The related method equally applies to the previously mentioned electrostrictive ceramics, such as lead magnesium niobate (PMN). The primary encapsulating material is a non-conductive fiber-reinforced composite material and a resin system; and the secondary encapsulating material is a fiber-reinforced composite material. In one specific form of the invention, the ply angle is in the range of approximately thirty to forty degrees with respect an axis in which strain occurs in the actuator/sensor package when in use.
The same principles used for secondary encapsulation may be used in a singly encapsulated embodiment of the invention. In this embodiment, an actuator/sensor is encapsulated in a fiber-reinforced composite material. The encapsulating material and the ply angle at which the material is applied are selected to provide a desired coefficient of thermal expansion and a desired stiffness with respect to a selected direction of strain.
The invention may also be defined as a method of making an encapsulated ceramic actuator/sensor package having a desired coefficient of thermal expansion and a desired stiffness. For double encapsulation, the method comprises the steps of encapsulating a ceramic actuator/sensor in a primary encapsulating material, except for a pair of contact leads extending through the encapsulating material; selecting a fiber-reinforced composite material and a fiber-reinforced composite material ply angle to provide a desired net coefficient of thermal expansion and a desired stiffness with respect to a selected direction of strain; and encapsulating the actuator/sensor and the primary encapsulating material in the fiber-reinforced composite material to form a composite package with the desired coefficient of thermal expansion and the desired stiffness with respect to the selected direction of strain. For single encapsulation, primary encapsulation uses a selected fiber-reinforced composite material and a selected ply angle to provide the desired coefficient of thermal expansion and the desired stiffness with respect to the selected direction of actuation.
More specifically, each of the encapsulating steps includes applying a fiber-reinforced composite material, and then curing material to complete the encapsulation. Further, the step of applying the fiber-reinforced composite material when encapsulating with the secondary encapsulating material includes applying the fiber-reinforced composite material as generally parallel plies, at the selected ply angle with respect to the selected actuation direction.
The invention may also be defined as a method of optimizing the performance of an encapsulated actuator/sensor package, comprising the steps of minimizing the net encapsulant coefficient of thermal expansion (CTE), or matching the CTE to that of a host structure, and simultaneously maximizing the net encapsulant actuation strength, by optimal selection of the encapsulation material and the encapsulation material ply angle.
It will be appreciated from the foregoing summary that the present invention provides an actuator/sensor package that is thermally stable and is, therefore, suitable for use in a variety of precision space applications. Use of the encapsulated actuator/sensor package of the invention permits precision structures to be designed with host structural members that are less stiff and are, therefore, of lower weight, volume and cost than similar mem
Doherty Kathleen M.
Felts Christopher W.
Hill Lisa R.
Lavinsky Craig A.
Triller Michael J.
Heal Noel F.
Ngo Hung V
Reichard Dean A.
TRW Inc.
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