Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-11-07
2004-10-19
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S323160
Reexamination Certificate
active
06806620
ABSTRACT:
The invention relates to a piezoelectric drive, in particular for the generation of rotational and translational movements which can be carried out continuously or stepwise.
The inventive motor can be employed in automation systems, in robot technology, as a drive for microscope tables, for fine-positioning of various types of coordinate tables, in optical and laser systems, as well as in numerous other devices in which translational movements with high precision accuracy are required.
Piezoelectric motors or drives which are based on the utilisation of acoustic transducer travelling waves have been known for a longer period, with reference being made here for example to EP 0 475 752 and U.S. Pat. No. 5,596,241. Such motors, however, have the drawback that it is not possible to manufacture them as miniature drives, because the minimum length of the waveguide of these motors must be a multiple of 6&lgr; to 10&lgr;. In addition, the manufacture is complicated and expensive.
Linear piezoelectric motors which utilise standing acoustic waves are also known, e.g. from U.S. Pat. No. 5,453,653.
Such motors are relatively small and their manufacture is simple. A monolithic plate-shaped piezoelectric oscillator with a long and a short side and with a friction element which is arranged on one of its small surfaces is used as the drive element in such motors.
One of the large surfaces of the piezoelectric oscillator carries a first and a second electrode group. On the second one of the oscillator surfaces a continuous electrode is arranged. Each of the first and the second electrode group represents two equally sized diagonally arranged rectangular areas of the metallised piezoelectric ceramic surface. The source of the electric excitation of acoustic oscillations directs the voltage to the continuous electrode and to the first or second electrode group.
Due to the asymmetric configuration of each of the electrode groups with respect to the longitudinal axis of the oscillator the electric source voltage generates an asymmetric deformation in the oscillator plate. This results in the friction element performing a movement on a closed path. Depending on which electrode group the electric voltage is applied, the friction element moves in a forward direction or in the opposite direction. The moving friction element causes a movement of the pressed-on element. The operating frequency of the motor is in the vicinity of the resonance frequency of the second oscillation mode of the flexural oscillations of the oscillator along the oscillator length.
It is disadvantageous with such motors that an asymmetric deformation of the oscillator plate is required for the generation of acoustic oscillations. Such a motor has trajectories which differ substantially from points on the function face of the oscillator. This leads to a substantial difference in the tangential components of the oscillation velocities of these points. The latter causes instability of the movement velocity of the driven element, which is highly dependent on the real contact site of the surface of the driven element with the function face of the driven element.
Moreover, a great difference in the tangential components of the oscillation velocities causes different degrees of wear of the function face of the friction element. This renders the motor operation instable over a longer operating period.
With velocities above 0.1 m/s the non-uniformity of the movement velocity of the driven element of known motors reaches approx. 50%. With lower movement velocities, i.e. below 0.01 m/s, the inaccuracy amounts to 80% and more. Such a non-uniformity limits the application range of the motors and complicates the construction of electronic velocity stabilisers, in particular for the range of very small velocities.
Moreover, high exciter voltages are required for such motors.
The construction of a motor according to U.S. Pat. No. 5,453,653 comprises only one friction element on the surface of the piezoelectric oscillator. This renders the oscillator mechanically instable which, with high movement velocities of the driven element, reduces the positioning precision and leads to complicated constructions.
Moreover, the use of only one friction element limits the maximum possible force developed by the motor with only one piezoelectric oscillator. In known motors, this force amounts to approx. 10 N which is insufficient for many applications. The use of several oscillators combined to one packet in turn limits the positioning precision of the driven element.
It is therefore the object of the invention to specify a piezoelectric drive or motor, respectively, which comprises a uniform movement velocity of the driven element at high and low velocities, which has a higher operating stability over a longer operating period of the motor, requires a low excitation voltage, develops a high force, has a stable oscillator construction, and comprises a means or device for tracking the oscillator resonance frequency.
The solution of the object is achieved with a subject as is described in the valid main claim, with the dependent claims comprising at least suitable embodiments and developments.
According to the invention the first and the second electrode group represent two areas of identical configuration which are located on the two large opposite metallised surfaces of the plate-shaped piezoelectric transducer or oscillator (of the piezoelectric plate). Each of the two electrode groups forms at least one independent generator of non-connected acoustic standing waves which propagate along the long side of the piezoelectric oscillator or of the plate, respectively. The first electrode group forms a longitudinal wave generator and the second one a flexural wave generator of acoustic waves. The source of the electric excitation of acoustic oscillations comes from a basic generator which is electrically connected with signal inputs of a two-channel power amplifier. Each output of the two-channel power amplifier is electrically connected with the corresponding electrode groups.
Due to the fact that with the proposed motor the first electrode group forms at least one independent generator of standing acoustic longitudinal waves and the second one forms at least one independent generator of standing acoustic flexural waves and that these are designed in such a manner that no connection exists between them, i.e. that the waves produced by the generators do not influence each other, the waves propagating in the piezoelectric oscillator are pure longitudinal and pure flexural waves.
Such waves lead to pure elliptic trajectories of the oscillator body and have virtually an identical shape with hardly differing amplitudes in the areas of the defined maxima of standing flexural waves.
This enables a movement of the points in these areas on the function faces of the friction elements with virtually the same velocity. All of these elements together therefore enable a considerable increase in stability both with low and high movement velocities of the driven element.
The configuration of the electrode groups is realised in such a manner that the generators of acoustic longitudinal and flexural waves fill the entire volume of the piezoelectric plate. This reduces the exciter voltage in an advantageous manner.
With the proposed motor the first electrode group represents rectangular areas of the metallised surface of the plate-shaped oscillator. Here, the height is equal to the width. In between a unidirectionally polarised piezoelectric ceramic is disposed in a normal (vertical) direction relative to the electrodes. The electrodes are located at the sites of oscillation velocity nodes of the standing acoustic longitudinal wave which propagates in the oscillator. Such an electrode construction which forms a generator of acoustic longitudinal waves enables the generation of pure longitudinal waves in the oscillator plate.
The second electrode group also represents a rectangular area of the metallised surface of the plate-shaped piezoelectric oscillator. The height is again equ
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