Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-03-19
2001-01-16
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S318000, C310S338000
Reexamination Certificate
active
06175180
ABSTRACT:
This invention broadly relates to a method for controlling a piezoelectric actuator by dynamically detecting its electromechanical characteristics as well as to a device, particularly a phacoemulsifier, including a piezoelectric actuator controlled by means of such method.
More particularly, this invention relates to a method for use in a device including a piezoelectric actuator, which enables to optimize the electric excitation characteristics of said actuator during its operation, thereby assuring the control of the amplitude of its mechanical oscillations and maximizing the efficiency of this device under variable load and environment conditions.
It is known that instruments provided with piezoelectric actuators are extremely popular in various medical-surgical fields. A not exhaustive list of such application fields also includes the ophthalmic surgery, in which said instruments are employed for cataract removal operations; the neurosurgery, for tumor removal operations, as well as odontoiatry.
Only by way of illustration and not by way of limitation, reference will be made in the following description to utilization of such instruments, known under the name of phacoemulsifier devices, in the field of ophthalmic surgery for cataract removal operations.
Cataract removal is one of the most common intervents in the field of ophthalmic surgery and, among the possible techniques for carrying out such intervents, phacoemulsification is becoming more and more popular.
The so-called phacoemulsification technique provides for introducing a needle through a small incision so as to reach the crystalline area and for crumbling the cataract into small pieces, by means of a mechanical action of the needle that oscillates at ultrasonic frequency and with amplitude of some tens micrometers. Such small pieces are subsequently removed from the eye of the patient by means of an aspirator system connected to the same oscillating needle.
The removed material volumes are concurrently replaced by a physiologic salt solution which is delivered through a conduct coaxial to the needle, generally formed by a silicone sleeve arranged outwardly of said needle, which also has the important function to prevent a direct contact between said oscillating needle and the eye tissues, such as the cornea or sclera, thereby preventing the local tissues from being overheated, which could cause necrosis effects thereof.
The conventional phacoemulsifier devices comprise an actuator member comprised of a stack of two or more piezoelectric material ceramics, which are arranged in series relation from a mechanical view point and in parallel relation from an electric view point. Said ceramic stack is mechanically coupled to a pair of metal masses, usually of titanium or steel, one of which, in turn, is coupled to said needle. These ceramic members are excited by a sinusoidal voltage and, by inverse piezoelectric effect, correspondingly modify their own geometries and generate oscillating mechanical strains that are coupled to said pair of metal masses, which, in turn, transmit an oscillating axial displacement to the needle point acting on the concerned cataract.
The system including said ceramics, the pair of metal masses and the needle acts as an oscillator which axially resonates at its own characteristic frequency, determined by the geometry and by the mass of the system. In particular, stationary waves are established in said system and the stationary waves so generated have a point of maximum displacement or “antinode” coincident with the apex of the point and a statical point or “node” coincident with the center point of the ceramic stack.
The circuitry generating the sinusoidal signal for excitation of the system should be adapted to control both the frequency and the amplitude of the excitation signal. In particular, the excitation frequency should always be coincident with the intrinsic mechanical frequency of the system, while the amplitude of the signal determines the power transferred to the system. As a general rule, the amplitude of the excitation signal is adjusted by the surgeon, who trims it based upon the hardness of the concerned cataract as well as on other operatory conditions.
In this respect, all conventional phacoemulsifier devices have various problems due to the difficulties encountered in efficiently controlling the frequency and the amplitude of the excitation signal.
In fact, one of the problems connected with such phacoemulsifier devices is due to the fact that the cataract exerts a load effect on the mechanical system comprising said oscillating needle: when the needle point contacts the cataract, not only the characteristic mechanical frequency of the system varies, but also the oscillation amplitude takes a decreasing behavior and consequently also the emulsifying effect decreases.
As a common practice, aiming at obtaining, during the contact phase between the needle point and the cataract, an oscillation amplitude sufficient to generate in any case the desired emulsifying effect, the excitation generating circuitry transfers a higher power to the involved mechanical system. However, since said circuitry generally does not detect the dynamic variations of the mechanical system, this entails an extremely large oscillation amplitude when the needle point is not in contact relation with the cataract.
Such excessive amplitude oscillation causes a number of drawbacks.
In the first place, cavitation phenomena occur and these phenomena cause, on one hand, the formation of small air bubbles and local boiling actions which noticeably limit the view field of the surgeon. thereby urging him to repeatedly discontinue the emulsifying action on the crystalline for sucking the bubbles themselves and, on the other hand, they reduce the life time of the components of the phacoemulsifier device involved in the cavitation.
In addition, in the case of oscillations of particularly large amplitude, the friction between the oscillating point and the coaxial silicone sleeve can cause such a local overheating as to damage the eye tissues in contact with the sleeve itself.
Lastly, when the oscillation of the mechanical system have excessive large amplitudes, they cause excessively high mechanical stresses on the ceramic components. This, in turn, can cause their breakage or their overheating to temperatures near the Curie depolarization point, thereby degrading their piezoelectric properties.
Various solutions have been studied to minimize the consequences of the mechanical load induced by the cataract on said piezoelectric oscillator and/or to limit the oscillations of the mechanical system when it does not interact with the cataract.
The first solution is a completely mechanical solution and it is based upon utilization of oscillating masses of noticeable dimensions, characterized by a high moment of inertia, so as to be less responsive to the mechanical load furnished by the considered cataract. However, such a solution does not completely solve the problem and besides that it adds further drawbacks. In fact, the increased weight and the large dimensions degrade the manual controllability characteristics of the phacoemulsifier device by the surgeon. In addition, a greater amount of power is needed to maintain these masses in oscillating conditions and, as a consequence of this, greater amounts of heat are generated.
The second solution is based upon measurement of the elongation of the oscillating system during the excitation, by insertion of a displacement sensor and/or an acceleration sensor in the system, such as one or more further piezoelectric ceramics that, upon being pressed by the oscillating masses, generate a signal proportional to the elongation itself, by direct piezoelectric effect. An analog or digital circuitry utilizes such signal in order to produce a feedback signal aimed at controlling the amplitude of the excitation oscillation, so as to stabilize the amplitude of the mechanical oscillation and so make it independent of the applied load. Anyway, also this solution has some drawbacks.
Angelini Giov. Battista
Regini Gualtiero
Dougherty Thomas M.
Optikon 2000 S.p.A.
Samuels , Gauthier & Stevens, LLP
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