Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-11-23
2002-03-19
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S332000
Reexamination Certificate
active
06359374
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to electrical relays. More specifically, the present invention relates to such relays having a piezoelectric film.
BACKGROUND OF THE INVENTION
Relays are used to switch a wide range of electrical circuits by using solid state or mechanical switches that permit and prevent electrical components from receiving power supply. Relays may perform satisfactorily in some applications, but often suffer some unfavorable performance characteristics, such as being unreliable, slow, large, costly, and/or power consuming. These unfavorable performance characteristics produce unreliable circuits and prevent the use of one specific relay in many different applications.
It is recognized that a relay may perform satisfactorily in a variety of circuits, applications and environments if certain beneficial features are achieved. Many of these features include high power load capability, low power switch actuation, minimization of arcing, cheap and simple fabrication, and small size. For example, relays with high current-carrying capability are needed for circuits in various satellite and cellular communications equipment. Furthermore, smaller, lower power relays are desirable for the ever shrinking size of circuitry in consumer electronics and other commercial and non-commercial applications. Although a variety of conventional relay designs attempt to achieve these and other beneficial attributes, conventional relays still suffer some undesirable characteristics.
For example, solid-state switches based upon semiconductor technology such as GaAs field effect transistors (FETs) may be utilized to construct an electrical relay. Such a relay is small in dimension and provides fast switching at low operating voltages. However, a solid-state relay can only switch circuits carrying small currents (e.g., in the micro-amp to milliamp range). Furthermore, due to the semiconductor nature of these FET devices (i.e., the p-n junctions), a solid-state relay generally possesses high on-resistance, which generally means that the impedance of power flow through the relay is relatively high.
Relay devices based on mechanical actuators are also known, including those driven by electrostatic, magnetic, thermal, hybrid, and bulk piezoelectric actuators. Because these switches are operated by mechanically connecting and disconnecting two metal contacts, mechanical relays often benefit from lower on-resistance than solid-state relays.
Electrostatic relays are actuated by applying an electrostatic field between two electrodes to move a thin, flexible film. These relays can be micromachined to very small sizes, and can produce relatively sizable displacements at relatively high frequencies to ensure an open circuit in an off condition. However, due to thin metal electrode layers that are generally used for the metal contacts, the current carried over the contacts is limited to the milliamp range. Furthermore, high voltage is typically required for electrostatic relay operation (e.g., 50-100 volts). Magnetic relays, like electrostatic relays, also provide large displacements between the metal contacts. Unlike electrostatic relays, however, magnetic relays can carry relatively large currents (milliamp to amp range) over the electrical contacts, and can produce very strong contact force between the circuit contact elements. Nonetheless, magnetic relays generally suffer from a number of disadvantages, including magnetic operation, short lifetime and large size. Thermal buckling relays also provide high force actuation and large displacement, but typically require high power consumption. Moreover, switching speeds are often slow in these actuators, and the thermal power required to drive thermal actuators of these thermal buckling relays can cause deleterious effects on circuit performance.
A hybrid drive relay is another type of mechanical relay, utilizing a combination of actuating devices. Conventional hybrid drive relays include hybrid thermal/electrostatic and piezoelectric/electrostatic actuators that combine the attributes of higher force actuation (e.g., thermal or piezoelectric) with higher displacement actuation (e.g., electrostatic actuation). The thermal/electrostatic drive, however, still requires thermal power for actuation which can cause deleterious effects on circuit performance. The piezoelectric/electrostatic drive uses the natural expansion and/or contraction of a piezoelectric material under an applied electric field in combination with an electrostatic drive to actuate one or more contact elements.
One such hybrid piezoelectric/electrostatic relay is described in U.S. Pat. No. 5,666,258, to Gevatter et al. Gevatter et al. discloses a hybrid micromechanical relay including both a piezoelectric actuator and an electrostatic actuator, where the two actuators operate in tandem to supply a relatively strong contact force between contact elements. The hybrid piezoelectric/electrostatic relay, however, utilizes the piezoelectric element merely to assist the electrostatic actuation by providing higher force. The thickness and geometry of the piezoelectric layer is not optimized such that the piezoelectric is the primary driver. Furthermore, this hybrid drive requires multiple processing steps on separate wafers for each driver element and also requires wafer bonding for final assembly. This method complicates device fabrication compared with monolithic micromachined structures.
Piezoelectric materials also may be exclusively utilized as actuating means in bulk piezoelectric relays, another type of mechanical relay. An example of a bulk piezoelectric relay is described in U.S. Pat. No. 4,595,855, to Farrall. Farrall discloses a piezoelectric relay formed by adhesive bonding assembly steps using a ceramic piezoelectric material. Like other types of mechanical relays, a piezoelectric bulk ceramic relay suffers from some weaknesses. The bulk ceramic relay is relatively large in dimension, and would likely be used as a discrete component added separately to a circuit to be switched, or machined into a bimorph or array structure. The relay can also require high operating voltages for actuation, in the range of several hundred volts.
Generally, therefore, an unsatisfied need exists in the industry for a low cost and easily fabricated relay with a high force output that allows for thick metal contacts for switching of high currents, fast switching speeds, long lifetime and relatively low operating voltages.
SUMMARY OF THE INVENTION
These and other advantages are provided, according to the present invention, by providing a miniature electrical relay having a thin film piezoelectric actuating element as the primary driver. A miniature electrical relay in accordance with the present invention can take numerous forms to provide desired results. For example, the present invention can be embodied in the form of a cantilever relay or a bridge relay. Such miniature relay advantages include cheap and simple fabrication, low operating power, high force output, high current switching capability, and integration with small circuits.
In accordance with the present invention, a piezoelectric actuating element is deposited on a movable contact of a miniature relay. The piezoelectric actuating element includes a piezoelectric material, preferably a piezoelectric film with a thickness in the range of 1-10 &mgr;m, sandwiched in between two metal electrode layers that function as piezoelectric electrodes. The piezoelectric actuating element can be actuated by applying an electric field across the metal electrode layers, such as by selectively connecting the metal electrode layers to a positive terminal and a negative terminal, respectively, of one or more power sources. The metal used to construct the metal electrode layers should be platinum, silver or some other metal that may withstand processing conditions of the piezoelectric material, which may be in the range of 500-700° C. in an oxidizing atmosphere.
The piezoelectric actuating element is affixed to a movable circuit c
Dausch David E.
McGuire Gary E.
Alston & Bird LLP
Budd Mark O.
MCNC
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