PIEZOELECTRIC SOLID STATE MOTOR STACK

Abstract: A piezoelectric solid state motor stack (200) having a plurality of piezoelectric disks (210, 208, 206, 202) interleaved with a plurality of electrodes (204), with at least one first piezoelectric disk (210) of a first thickness sandwiched between at least two second piezoelectric disks (208) of a second thickness, the second thickness being different from the first thickness. The structure includes at least two electrodes (204), each electrode being interleaved with at least two of the plurality of disks, such that each electrode is in contact with a surface of at least one of the plurality of disks. The electrodes are connected to and biased by a source of electrical potential to produce an axial displacement between opposite end surfaces of the stack. The stack may include four different disk thicknesses, with the thickest disks (202) sandwiching decreasingly thinner disks (206, 208, 210) and further electrodes. The order and thickness of the disks may be symmetric about a center axis perpendicular to the longitudinal axis of the stack.

Description: PIEZOELECTRIC SOLID STATE MOTOR STACK BACKGROUND OF THE INVENTION 1. Field of the Invention The field of the invention relates generally to solid state motor actuators. More particularly, the invention relates to a piezoelectric solid state motor stack structure. Still more particularly, the invention relates to a solid state motor stack structure having a wide operating temperature range. 2. Related Art For decades electroexpansive materials have been employed in stacked structures for producing actuation used for fuel injection and valve control in diesel engines, for example. Commercially manufactured solid state motor stacks, or actuators, are produced using piezoelectric disks interleaved with metal foil electrodes. Application of high voltage (e.g., 1000V DC) , low current power to alternately biased electrodes causes each of the piezoelectric disks to expand or axially distort. The additive deflection of the stacked disks is typically amplified by hydraulics to effectuate useful actuation. An example of a conventional electromechanical actuator having an active element of electroexpansive material is found in United States Patent No. 3,501,099 to Glendon M. Benson. Benson's 1970 patent is directed to both an actuation amplification structure and a method for manufacturing piezoelectric stacks. Sheets of ceramic material are rolled, compacted and punched into ceramic disks. After a cleaning process, the disks are stacked with alternate sets of continuous disk electrodes disposed between the ceramic disks. The stacks undergo a pressurized cool-welding process, followed by an elevated temperature and pressure bonding process after common electrodes are connected to the two electrode groups. The stacks are poled by application of a DC voltage and then encapsulated with a plastic insulative cover prior to final mounting within a transducer housing. An example of a conventional low voltage multilayer piezoelectric ceramic actuator with varying thickness layers is taught by Takahashi et al. See Meeting on Ferroelectric Materials and Their Applications (5th), 1985. p. 206-208. Takahashi et al. have developed a multilayer ceramic actuator used for impact matrix printer heads with an applied electric voltage of lower than 100V. Due to the low driving voltages, the Takahashi et al. multilayer actuators do not produce forces of large magnitude, and thus, the ceramic disks are prone to less failures. Takahashi et al. have studied such actuators using finite element method analysis in an attempt to reduce stresses induced at an adhesive layer between the ceramic actuator and a mass load to which the body is attached. The structure taught by Takahashi et al. includes two different thickness active layers and an inactive, non-stressing, layer disposed at one end of the multilayer actuator. The mass load and ceramic layers have rectangular surface areas. A surface of the mass load is attached to a surface of an inactive layer by an adhesive. Adjacent the other surface of the inactive layer is a first active layer with a first thickness. Stacked under the first active layer is a number of second active layers of a second thickness, the second thickness being smaller than the first thickness. Two sets of electrodes are alternately interleaved with the active layers for application of the driving voltage. The article reports that no mechanical rupture of the adhesive layer occurs as a result of such a structure. This result is further attributed to an edge hook on the mass load that overlaps down along a side edge of the inactive layer. The present invention overcomes the deficiencies of the conventional technology noted above. SUMMARY OF THE INVENTION The present invention is directed to a piezoelectric solid state motor stack having a plurality of piezoelectric disks interleaved with a plurality of electrodes, with at least one first piezoelectric disk of a first thickness sandwiched between at least two second piezoelectric disks of a second thickness, the second thickness being different from the first thickness. At least two electrodes are interleaved with at least two of the plurality of disks such that each electrode is in contact with a surface of at least one of the plurality of disks. The sandwiched disks have surface facets partially covered with a conductive layer. When the electrodes are connected to and biased by a source of electrical potential, an axial displacement is produced between opposite end surfaces of the stack. The invention may further include at least two disks of a third thickness, different from the first and second thicknesses, sandwiching between them the combination of the disks of the first and second thicknesses. Moreover, electrodes may be interleaved with the disks of the third thickness and the disks of the second thickness. The invention may still further include at least two disks of a fourth thickness, different from the first through third thicknesses, sandwiching between them the combination of the disks of the first through third thicknesses. Still further electrodes are interleaved with the disks of the fourth thickness and the disks of the third thickness. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood if reference is made to the accompanying drawings in which: FIG. 1 is a cross-sectional view of a housed piezoelectric stack in connection with the present invention; FIG. 2 is an expanded view of a piezoelectric stack in connection with the present invention; and FIG. 3 is a sectional view of the disk/electrode stack of FIG. 2 with electrode tabs connected to respective busses and lead wires. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Piezoelectric solid state motor stacks according to the present invention are high-force devices that have minimized stack failure due to disk breakage caused by stress. Piezoelectric solid state motor stacks according to the present invention can be used to improve engine performance, reduce emissions, and reduce engine noise. The utility of the present invention is not, however, limited to engine valve and fuel injector actuation. The invention may be used in brake or shock absorbing systems, for example. Moreover, the invention may be used in a wide variety of devices or systems requiring fluid or mechanical actuation, as well as shock or sound wave production. A particular advantage of the solid state motor stack of this invention is its ability to operate over a wide temperature range. Tests on prototype motor stacks have shown good results in operating environments of -40°C to 100°C. It should be understood that the present invention is directed to a piezoelectric solid state motor stack structure. However, the terms solid state motor stack and electroexpansive actuator, for example, are synonymous. Throughout this discussion, the piezoelectric solid state motor stacks will be commonly referred to as "stacks." FIG. 1 shows the j ezoelectric solid state stack motor 102 of the present invention in a housing 104. The preferred embodiment for the housing and encapsulation of the present piezoelectric solid state stack motor invention is shown in the concurrently filed, commonly assigned co-pending application Serial No. (Attorney Docket No. 1246.0120000/90-215), titled "Coating Surrounding A Piezoelectric Solid State Motor Stack", the disclosure of which is incorporated herein by reference. The housing is a steel case and is cylindrical in shape with a hollow cylindrical cavity for housing the solid state stack. Threads 106 attach the piezoelectric solid state motor housing to an engine head. Plateau 108 represents a hexagonal cross section, if viewed from the top of FIG. 1. This hexagonal structure is not shown in the figure, but is used for tightening and loosening of the piezoelectric solid state motor housing on the engine head. Throughports are bored in the top end of the housing to permit bus lead wires 114 to exit the housing. FIG. 2 is an expanded diagram of the disks forming the stack structure 200. Ceramic disks suitable for use in making a motor stack are commercially available. The piezoelectric solid state motor stack is bounded by non-polarized ceramic end caps 202. The stack also includes thick ceramic disks 206, medium thick ceramic disks 208, inner thin ceramic disks 210, bus bars 212, 212', and lead wires 214, 214'. Preferably, each facet surface of the thick, medium thick and thin disks is coated with a conductive coating, preferably aluminum. The application of the conductive layer may be accomplished by various methods as would become evident to those of skill in the art. The conductive layer is preferably applied to all of the surface area of the upper and lower facets of the disks. Uniformity of the conductive layer is desired. However, uniformity and full facet surface coverage may be limited by the method used to apply the conductive layer. The conductive layer may be applied by sputtering, spraying or mechanically rubbing atomic layers of the conductive material onto the facet surfaces of the disks. Chemical vapor deposition may also be used to apply the conductive layer. The conductive layer may also comprise copper, nickel or silver. The conductive layer functions to more evenly spread the electric field across the surface of the disk when a voltage is applied to an adjacent electrode in the final stack structure. The inner thin disks 210 are sandwiched by the medium thick disks 208. In a preferred embodiment, approximately 71 thin disks are used. The combination of the thin and medium thick disks are in turn sandwiched by the thick disks 206, and the combination of the thin, medium thick and thick disks are sandwiched by the end caps 202. The thick, medium thick and thin disks have thicknesses of approximately 1.016, 0.762 and 0.508 mm, respectively. The thickness of the end caps is preferably 2.500 mm. Each of the first through fourth disk thicknesses has a tolerance of about + 0.013 mm. FIG. 3 is a sectional view through line A-A in FIG. 2 showing the electrode assembly 300. A top electrode 304 has a tab 316 electrically connected to a bus bar (wire) 312, for example, by soldering or laser welding. A lead wire 314 is then electrically connected (again, by soldering or laser welding, for example) to the bus bar 312. A next adjacent electrode (not shown) has a tab 318 offset from tab 316 (by about 180° in the example shown) . Tab 318 is electrically connected to a second bus bar 312'. The bus bar 312' in turn is electrically connected to a second lead wire 314'. Assembly of the above structure will now be discussed. Preferably, an assembly fixture comprir ng a half-cylindrical tube with a metal plug located <-- • _. the bottom, assembly begins by inserting a first end- cap ceramic disk into the assembly fixture. (Note that the facet surfaces of the end caps are not coated with a conductive layer.) A first electrode is aligned in the assembly fixture on top of the first end cap ceramic disk. The tab of the first electrode is given a first orientation. The stacking of a thick disk is next, followed by another electrode. This next electrode is stacked on top of the thick disk and its tab is given a second orientation. In the presently preferred embodiment shown, the second orientation is 180° from the first orientation. The orientation, or circumferential displacement between successive electrode tabs, may vary as long as the « respective orientations are maintained for alternating electrodes during the stacking process. It is only necessary that the electrode tabs and bus bars be spaced apart sufficiently to prevent arc over between ' bus bars and to prevent localized disk heating generated by the applied electric fields. A medium thick disk is stacked on the second electrode. A third electrode is stacked on top of the medium thick disk. The tab of the third electrode is oriented oppositely to that of the second stacked electrode and thus similarly to that of the first electrode. A first thin disk is then stacked in the assembly fixture. A fourth electrode, again with an opposite orientation to the third stacked electrode, is then placed on the first thin disk. Successive thin disks are stacked with interleaved electrodes, such that every successive electrode is oriented oppositely to the immediately preceding electrode. When the last thin disk has been stacked, another medium thick disk, thick disk, and end cap are stacked with electrodes interleaved therebetween. Finally, a second end cap is stacked on top of the last electrode. The ceramic end caps 202 are not sandwiched by electrodes 204. Once assembled, the disk/electrode stack comprises two longitudinal rows of disk tabs offset from each other by up to 180°. In one embodiment, each tab of each row of tabs is bent and presoldered to facilitate connection to a bus bar. Each of the tabs is slotted so that the bus bar may be positioned in the slots. The tab length, as measured parallel to a line tangent to the circumference of the electrode, should be sufficient to permit free expansion and contraction of the stack during operation. Other bus bar-electrode combinations are contemplated. These are described in concurrently filed, commonly assigned co-pending applications, Serial No. (Attorney. Docket No. 1246.0090000/90-217), titled "Slotted Bus Bar For A Piezoelectric Solid State Motor", and Serial No. (Attorney Docket No. 1246.0070000/88-298), titled "Single-Piece Multiple Electrode Conductor", the disclosures of which are incorporated herein by reference. In the presently preferred embodiment, the electrode bus bar connection is accomplished as follows. Solder is applied to each of the tabs via a hand soldering gun, for example. The tabs may also be dipped in a solder bath. Two 22-gauge tin-copper wire bus bars are appropriately positioned along respective presoldered bent tab rows. The bus wires may be cleaned with isopropyl alcohol and treated with a liquid solder flux. Once positioned, the bus wires are soldered to the respective electrode tabs to form a bus structure. The final housing assembly will now be discussed. Poling, or alignment of the thick, medium thick, and thin ceramic disks' dipoles is necessary to achieve axial expansion of the piezoelectric ceramic disks upon the application of an electrical potential to the stack. This is preferably done once the stack is assembled in accordance with guidelines provided by the ceramic manufacturer. In the presently preferred embodiment, poling of the stack is accomplished at an elevated temperature (about 145°C) . A poling voltage signal is applied to the stack in three stages. During the first stage a 0-volt to 1200-volt charge is applied linearly during a two-minute interval. For the second stage the voltage is held at 1200-volts for 10 minutes. Finally, the voltage is linearly reduced to 0-volts during a five-minute period. The stack is then cooled to room temperature. The ceramic end caps are not polarized during the poling process because they are not sandwiched by electrodes. The bus bars 212, 212' are soldered to the lead wires 214, 214, respectively, and a layer of shrink- wrap tubing is applied to the lead wires as an insulator. While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. Thus the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims: WE CLAIM; 1. A piezoelectric solid state motor stack (200) having a plurality of piezoelectric disks interleaved with a plurality of electrodes, comprising: at least one first piezoelectric disk (210) of a first thickness sandwiched between at least two second 'piezoelectric disks (208) of a second thickness, said second thickness being different from said first thickness; and at least two electrodes (204) , each electrode being interleaved with at least two of the plurality of disks, such that each electrode is in contact with a surface of at least one of the plurality of disks; wherein when said electrodes are connected to and biased by a source of electrical potential, an axial displacement is produced between opposite end surfaces of the stack. 2. A piezoelectric solid state motor stack (200) according to claim 1, wherein the plurality of piezoelectric disks further comprises: at least two disks (206) of a third thickness, different from said first and second thicknesses, sandwiching the combination of said disks of said first and second thicknesses (210, 208). 3. A piezoelectric solid state motor stack (200) according to claim 2, further comprising: further electrodes (204) interleaved with said disks of said third thickness and said disks of said second thickness. 4. A piezoelectric solid state motor stack (200) according to claim 2, wherein the plurality of piezoelectric disks further comprises: at least two disks (202) of a fourth thickness, different from said first through third thicknesses, sandwiching the combination of said disks of said first through third thicknesses (210, 208, 206) . 5. A piezoelectric solid state motor stack (200) according to claim 4, further comprising: still further electrodes (204) interleaved with said disks of said fourth thickness and said disks of said third thickness. 6. A piezoelectric solid state motor stack (200) according to claim 5, wherein said electrodes comprise a first and second group connected to first and second parallel buses (212, 212'), respectively. 7. A piezoelectric solid state motor stack (200) according to claim 5, wherein said electrodes comprise a first and second group, each of said electrodes within said first and second group being connected in series. 8. A piezoelectric solid state motor stack (200) according to claim 5, wherein said sandwiched disks comprise surface facets partially covered with a conductive layer. 9. A piezoelectric solid state motor stack (200) according to claim 5, wherein said sandwiched disks comprise surface facets, said surface facets being substantially covered with a conductive layer; and said disks of said fourth thickness and act to electrically isolate the stack.

Inventors: DAM CHUONG Q, HESTER VIRGIL R, KELLEY KURTIS C, TOWE CAREY A, DAM, CHUONG, Q., HESTER, VIRGIL, R., KELLEY, KURTIS, C., TOWE, CAREY, A.
Applicants: CATERPILLAR INC, CATERPILLAR INC.
Application year: 1990
Main classification: H01L41
IPC codes: H01L41, H02N2