Piezoelectric actuator

Details Piezoelectric actuator Piezoelectric actuator

2000-08-25

2001-09-18

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S366000

Reexamination Certificate

active

06291929

ABSTRACT:

PRIOR ART
BACKGROUND OF THE INVENTION
The invention is directed to improvements in piezoelectric actuators, in particular for actuating control valves or injection valves in internal combustion engines in motor vehicles, having a piezoelectric actuator body, in particular in the form of a multilayer laminate of layered plies of piezoelectric material and intervening metal or electrically conductive layers acting as electrodes, these electrode layers being contacted by electrically conductive common electrode leads, and the actuator body is surrounded by a metal module wall defining an interstice therebetween that contains the common electrode leads.
One such piezoelectric actuator disclosed in German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH.
As is well known, piezoelectric actuators can for instance be used for injection valves of a vehicle motor and in brake systems with anti-lock and traction control systems.
Such injection valves equipped with piezoelectric actuators have an injection nozzle controlled by a tappetlike closure device. An operative face toward the nozzle is disposed on the tappet and is acted upon by the pressure of the fuel supplied to the nozzle; the pressure forces seek to urge the tappet in the opening direction of the closure device. The tappet protrudes with a plungerlike end, whose cross section is larger than the aforementioned operative face, into a control chamber. The pressure effective there seeks to urge the tappet in the closing direction of the closure device. The control chamber communicates with the fuel supply, which is at a high pressure, via an inlet throttle and with a fuel return line that has only low pressure, via an outlet valve that is throttled as a rule or is combined with an outlet throttle. When the outlet valve is closed, a high pressure prevails in the control chamber, by which the tappet is moved in the closing direction of the closure device, counter to the pressure on its operative face toward the nozzle, or is kept in the closing position. Upon opening of the outlet valve, the pressure in the control chamber drops; the magnitude of the drop in pressure is determined by the size of the inlet throttle and by the throttle resistance of the opened outlet valve, or the outlet throttle combined with it. As a result, the pressure in the control chamber decreases when the outlet valve is opened, in such a way that the tappet is moved in the opening direction of the closure device, or held in the open position, by the pressure forces that are operative on its operative face toward the nozzle.
In comparison with electromagnetically actuated injection valves, piezoelectric actuators can switch faster. However, in the design of a piezoelectric actuator, it must be noted that internal losses in the piezoelectric body of the actuator cause lost heat, which has to be dissipated so that the actuator will not overheat. Since the ceramic materials of the piezoelectric ceramic have poorer heat conductivity, the dissipation inside the actuator body, which substantially comprises ceramic material, is unfavorable, especially in long actuators, whose length is greater than their width.
Because of the electrodes that are located in the active part of the actuator body, its heat conductivity crosswise to the electrode layers is higher by a factor of three to five than at right angles to this, since the piezoelectric ceramic material is a poor heat conductor. Naturally, this factor depends on the geometric conditions, such as the thickness of is the ceramic layers, the thickness of the electrode layers, and on the thickness of the non-continuous electrode from the edge of the actuator body. Since the convection, that is, the output of heat to the air, is poor, the greatest part of the heat must be dissipated via the end faces. Since the thermal conduction, as noted, is poor perpendicular to the electrode layers, the heat takes the following course, in simplified terms. It is guided by the internal electrodes of the active part of the actuator body to the outer electrodes. There, it is distributed relatively well and thus quickly reaches the vicinity of the face ends of the actuator body. Since the outer electrodes have no contact with the metal actuator base or the metal retaining plate in the region of the actuator head, the heat has to flow via the poorly conducting face ends of the actuator body. Since the outer electrodes are located on the outside, the heat also flows to the end face only in the outer regions of the actuator body.
Cooling the actuator with a liquid coolant, such as fuel, water, motor oil and the like, which is theoretically possible, is unfavorable, first because of the risk of a short circuit from the water component that is contained both in the fuel and in motor oil, and second because the actuator module is more expensive because of complicated seals, which must preclude the coolant used from escaping from the actuator module, especially when the actuator becomes heated.
OBJECT OF THE INVENTION
It is therefore the object of the invention to make a piezoelectric actuator possible in such a way that reliable cooling of the actuator body, which heats up during operation, is possible without using a liquid coolant, and that the piezoelectric actuator can be mounted simply and needs no special seals as in the case of liquid cooling.
SUMMARY OF THE INVENTION
To attain this object, a piezoelectric actuator according to the invention is characterized, in that the piezoelectric material has blind metal electrodes on both face ends, each of which electrodes is located in a passive piezoceramic cover layer between the respective outermost active piezoceramic layer and the movable retaining plate and actuator base.
The term “blind electrode” in this case means that these electrodes, introduced on the face ends of the actuator body to dissipate the actuator body heat, do not extend to the edge to any side. Thus they have no metal-to-metal contact with the outer electrodes, and in other words the electrical properties remain unchanged.
For heat dissipation, this has the following effect. Once the short edge spacing between the outer electrode and the blind electrode or electrodes has been overcome, the heat can easily penetrate from the outside inward, and can then take the last remainder of the dissipation path, utilizing the entire end face of the actuator, until reaching the metal actuator base and the movable metal retaining plate. The heat dissipation can thus be improved through the bottleneck at the face ends of the actuator body and can reduce the maximum thermal load on the actuator.
In a preferred embodiment, the piezoelectric actuator is designed in such a way that a plurality of parallel blind electrodes are located, spaced apart from one another, in each of the passive cover layers of the actuator body.
In a multi-layer actuator of this kind, the layer thickness of the piezoelectric material is 0.1 mm, for instance. In that case, fourteen blind electrodes, for instance, can be spaced apart by 0.1 mm in an upper and lower cover layer whose height is approximately 1.5 mm.
Here it is assumed that the layer structure in the cover layers conforms to that of the active region of the actuator body in terms of the layer thickness.
This kind of layer structure of the actuator body with an identical layer thickness is preferred from the standpoint of economical manufacture of the actuator body. However, it is not absolutely necessary for the invention. The layer thickness in the active region can differ from that in the cover regions. Furthermore, a greater or lesser number of blind electrodes can be provided in the lower cover layer compared to the upper cover layer.
The above and additional advantageous characteristics of the piezoelectric actuator according to the invention will be described in further detail below in conjunction with the drawings.


REFERENCES:
patent: 4633120 (1986-12-01), Sato et al.
patent: 5237239 (1993-08-01), Inioe et al.
patent: 5266862 (1993-11-01), Ohya
patent: 5438232 (1995-08-01), Inioe

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