Field of the invention The invention relates to piezoelectric actuators which, for instance, may be used in positioning devices.

More precisely, the invention relates to piezoelectric actuators of the"Stick-Slip" type.

Prior Art Movements with a very high resolution in the nanometer or even sub-nanometer range are a frequent need in science and technology. Besides traditional actuators (voice coil, moving magnet and others), piezoelectric elements have become state of the art in the past 20. years, and especially stepping motors based on piezoelectric elements are frequently used and also partially available as commercial products.

Burleigh Instruments Inc. have developed the"Inchworm" [May74, Biz74], an actuator that is based on three piezoceramic elements, with two of them clamping alternatingly a cylindric rod and the third element linking the two others. With one element clamping and the link element extending a movement is performed. In this extended state, the clamping is changed from the first element to the second and the linking element is contracted after releasing the first clamp. By repeating the whole sequence a stepwise motion is performed. This popular stepping principle is used in numerous translational and rotational devices for SPM and other applications [Rap90, DLG+95, KK99] Other systems are based on a walking motion, which is caused by two, or more piezoelectric elements, which are actuated one after the other [Shu93, SM97, MDF+00, MRK+01, LSMCM+01]. Often the actuators for each foot are lifted and then moved forward.

Because of their simplicity inertial drives have become popular as well. Theoretically only one actuator is needed, which is driven with an asymmetric signal. The actuator is in contact with a body (slider) that is able to slide in the direction of the actuator deformation. The asymmetric vibration of an actuator causes the slider to slip, while the actuator changes its shape rapidly, and to stick while it reverts to its original shape slowly. Two principles can essentially be distinguished, impact drive and stick-slip.

Impact drive actuators [HW90, HYFK90, KH90, YH93, ZBCS95] consist of two bodies connected by a piezoelectric actuator, with one of the bodies (slider) being in contact to a base and held by a normal load generating a friction. The impulsive force caused by the rapid deformation of the actuator moves both bodies and overcomes the friction of the slider. The impact drive actuator moves one step. The following slow deformation in the opposite direction will only have the body, which is not in contact with the base moving. Repeating this sequence will cause a stepping motion (see figure 1, right).

Stick-Slip actuators allow for a considerable performance improvement, as the resolution here is not limited to one single step. The actuator is fixed on one side to a mass, on the other side to a guiding element having ideally a very small mass. This assembly (slider), which is in contact with a guide, will not move during a rapid deformation of the piezoelectric element, as due to the inertia of the slider there will be a slip motion between slider and guide, but it will move during a slow deformation. It is easy to understand that for this actuator the resolution depends only upon the input signal resolution and can thus reach the same resolution as obtained with a single piezoceramic element. It should be noted that the same principle can be applied as well, if the actuator with the guiding element are attached to a fixed base, and a slider is placed on the guiding element (s). The advantage to the aforementioned set-up as well as to the impact drive is that there are no supply wires in movement (see figure 1, left).

A common characteristic for all stick-slip actuators is the relatively simple structure and the small number of components, yet there are still limits especially for miniaturization. A particularity is, that the contact elements, which guide the slider (the object to be moved) are actuated, thus combining guiding and actuating functions in one and the same element, which might necessitate in certain cases several actuators for one degree of freedom.

In [HRG92, BK93, dWdWPS95, E096, BC98, MYO+99] and [MDS99], shearing mode actuators are used for displacement. Shearing mode actuators are piezoceramic elements that have a poling direction perpendicular to the electric field for operation. The actuators are fixed to a base and guiding elements such as, e. g., half spheres of a hard material, are attached to the actuator on the upper side, causing the slider to move, if an asymmetric signal is applied to the upper and lower electrodes. Another possibility are piezoelectric tubes used in a very similar manner as bimorph elements: at least two different zones in the material are activated in the opposite sense, and as one part contracts and the other extends, a bending deformation is obtained. A deformation similar to the monomorph effect may as well be obtained if just one zone is activated and the other remains neutral. The tubes, which are bent slowly in one direction and rapidly in the opposite direction will cause the desired stick-slip motion [Bes87, MK91, HRG92, BJB92, LRD+93, SA097]. The most basic realization of the principle is the use of a simple piezoceramic element in transversal mode in a flat or tube shape [Agr92, Yak97] or multilayer actuators [Hac98, CL99, KK01] to generate the motion. As with multiplying the number of steps a virtually unlimited range can be obtained, the small possible step size is often an advantage, as a smaller step will allow for a higher resolution.

Tube actuators and their electrode structure are manufactured individually, which means already a high material cost without counting assembly yet. Because of a particular poling procedure, shearing mode actuators can only be fabricated up to a very limited size, which means that only a small number of elements can be obtained with one fabrication step. They furthermore have to be operated at considerably

lower field strengths, as otherwise a re-poling in an unwanted poling direction will occur. Multilayer actuators are also available at a considerably higher cost than shearing mode elements or normal transversal mode elements. Using the latter is the most inexpensive option, although the system design might be considerably more complicated than for the others. For a high strain very thin and fragile actuators are needed, which are charged laterally, which means that the load has to be compensated in order to prevent breaking the brittle elements. If mounted vertically, a larger load might be supported, but the system gets more bulky and the assembly more complicated.

Summary of the Invention An object of the invention is the application of a piezoelectric element consisting of at least two active zones working in a push-pull principle to generate a displacement perpendicular to an applied electric field. If this displacement consists of an alternating fast and slow motion it can be used for inertial positioning principles.

Another object of the invention is the possibility to fabricate the aforementioned actuators out of a bulk material and the possible fabrication of multiple neighbouring push-pull actuators for a positioning in several degrees of freedom.

These and other objects are achieved with the piezoelectric actuator as defined in claim 1.

Further objects of this invention will become apparent in the following description.

Brief description of the figures Figure 1: Inertial drives: stick-slip (left) and impact drive (right).

Figure 2: Transversal actuator, deformation with applied voltage.

Figure 3: Functioning principle of a transversal positioning element with two active zones working in push-pull mode.

Figure 4: Functioning principles using two different voltages (left) or two different poling directions (right).

Figure 5: Functioning principle, the dotted line represents the deformation of the zones delimited by the electrodes if a voltage is applied.

Figure 6: Functioning principle (top view) for a 2dof actuation; in the case a) three analogical control channels are needed for one contact point, in the case b) two channels are sufficient, but must be switched for each mode, otherwise 4 channels are needed.

Figure 7: 1-dof slider using 3 rectangular actuators similar to figure 3 and v-shaped /half-sphere shaped contact elements (left, including side view) and 3 dof mobile platform with electrodes patterned on top and half spheres as contact elements fixed on the bottom (right).

Figure 8: Section of a push-pull element with multiple layers of piezoelectric.

Detailed description of the invention The displacement 6 of a transversal actuator as in figure 2 having the thickness t, the length 1 with the applied voltage V and without external forces (free strain), and neglecting hysteresis is At the same time, a small displacement in the direction of the electric field at occurs as well: 9, = d33V (2)

The piezoelectric constant d31 has a negative sign, which means that the element extends in the direction of the electric filed and shrinks in the perpendicular direction, if the applied electric field corresponds to the poling field.

If such an actuator is to be used in a positioning system, especially an inertial or "walking"drive, a mechanically robust actuator is desirable. As good piezoelectric materials (i. e. with a large d3, PZT) are usually very fragile, a certain thickness is needed, but a larger thickness on the other hand will decrease the deformation for a given voltage, as can easily be seen from formula 1. Furthermore, if a simple beam is used, off-plane vibrations will easily occur, if a force acts on its free end and it stretches or contracts rapidly.

To circumvent this, a longer beam can be subdivided in two complementary zones, which work in a push-pull mode by applying the electrodes accordingly (figure 3).

While one zone contracts, the other will expand, thus moving the electrodes area in the middle laterally forth and back, and this with the same displacement as in formula 1.

The opposite deformation in the two neighbouring regions can be obtained in at least two different ways: as mentioned before, equal positive and negative voltages are applied on the two electrodes, thus creating an electric field in the bulk material of the same field strength but with opposite direction. This will cause a transversal contraction in one zone of the bulk, and an extension in the other one (figure 4, left).

As this configuration considerably complicates the control electronics (opposite voltages have to be supplied to each contact point), it is possible as well to create these two regions by applying different voltages during the poling procedure to the previously patterned electrodes, and to use then the same voltage on both electrodes in order to obtain a similar push-pull movement (figure 4, right). It is even possible to apply a distinct electrode pattern for poling only and to replace it by a different pattern for operation.

The electrodes may be patterned by any conventional method, such as screen printing, lift-off, chemical attack of an existing electrode or other technologies. If the pattern is created upon fabrication of the piezoelectric actuator, it is possible to structure internal electrodes of multilayered actuators in the described manner as well. This will yield an n-fold deformation for the same applied voltage, as the deformation is proportional to the electric field.

A similar effect can also be obtained, if there are no beams cut out of the material, but structures are just patterned on the surface of the piezoelectric material or the individual layers in case of a multilayer actuator, with the electrodes delimiting active zones and the areas without electrode being passive. Figure 5 shows the simple case of a 1 degree-of-freedom actuator, where the two counteracting electrodes are not rectangular, as in the case of beams, but have the shape of half- circles. Figure 6 shows some possible configurations.

Several of these electrode pattern incorporated in the same PZT bulk material can be used to improve the guide for the slider and/or to increase the number of degrees of freedom. There is no need for an individual assembly of actuators.

If the circular electrode is further sectioned in circle segments, a movement in any direction can be generated, thus yielding the possibility to build 2 and 3 degree of freedom actuators (figure 6) by combining several similar actuators.

The actuators designed and fabricated in this manner can be used for a direct exploitation of the movement by attaching the necessary parts to the neutral zone (mechanical contact element in figure 3). If the contact element, as shown in figure 3, is designed accordingly, one or several actuators can be used in inertial-or walking drives, by fixing them to a base and applying the necessary voltage waveform pattern to the electrodes of the contact-"feet". If an asymmetric vibration (fast deformation in one direction alternating with a slow deformation in the other direction, see figure

1) is applied to the feet, a slider positioned on these feet will move in the direction of the deformation with a velocity proportional to the signal frequency (stick-slip, figure 7). It is also possible to have the slider positioned on several actuators and actuate them one by one in one direction and all together in the other direction with "walking"pattern. Furthermore, the same type of actuator consisting of active and passive zones in a piezoelectric material and a contact element can be used attached to a slider as a mobile platform as shown in figure 7 (left). This slider would move on a base thus allowing for very large ranges, if voltage pattern as described above are applied to it.

The effort for assembly operations can be decreased considerably, if a screen printing or similar procedure is not only used for the fabrication of the electrode pattern, but the necessary contact elements are fabricated in a similar manner. The contact points can be made of a glass or dielectric paste and screen printed as well. If, however, as in figure 8, a multi layer structure is fabricated, the pattern must be printed before sintering and the shrinkage must be taken into account.

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