Abstract:
Apparatus and method for controlling the frequency of the current in the excitation coil of the handpiece of a dental magnetostrictive ultrasonic scaling unit, or similar transducer. A microprocessor continually samples a predetermined function of the current through the excitation coil, and adjusts the frequency for a function maximum, performing coarse and fine frequency adjustments. The function can be proportional to the current, its time-derivative, or combination thereof. A voltage-controlled oscillator is employed, controlled by pulse-width modulation from the microprocessor. The base frequency scan is performed each time the handpiece is energized by the practitioner, assuring automatic optimal frequency adjustment at all times and under all conditions. Apparatus according to the invention does not require transformers, sensing coils, or complex power- or impedance-sensing circuitry, and covers a wide range of resonant frequencies for different insert types. A configuration with multiple handpieces is supported.

Description:
The present application claims benefit of U.S. Provisional Patent Application No. 60/655,103 filed Feb. 23, 2005. 

   FIELD OF THE INVENTION 
   The present invention relates to controllers for ultrasonic transducers and, more particularly, to the control of the excitation frequency of a magnetostrictive ultrasonic transducer for dental use. 
   BACKGROUND OF THE INVENTION 
   The use of a magnetostrictive transducer for an ultrasonic dental device, such as a dental scaler, is well-known and standardized throughout the dental profession. Such devices are characterized by having a handpiece into which a removable insert with a working tip is placed. The handpiece contains an excitation coil which is electrically connected via a cable to a control unit that provides the excitation energy to the coil. The removable insert contains a stack of plates of magnetostrictive material which expands and contracts when subjected to a time-varying magnetic field. A suitable time-varying magnetic field is created by directing a time-varying electrical current through the excitation coil surrounding the inserted tip, and thereby vibrations are induced in the insert and carried to the tip. The vibrating tip is then used by the practitioner in dental work, a non-limiting example of which is to remove calculus from the surface of teeth. 
   Although the fundamental concept as described above is widely employed in the same basic form, there is considerable variation in the manner by which the excitation current is controlled, in particular the frequency of the time-varying excitation current. The removable insert has a resonant frequency related to the natural acoustic modes of vibration of the magnetostrictive stack contained therein, and it is desirable to excite vibrations within the insert at or near the resonant frequency. Doing so will optimize the vibrational energy in the insert, and will thus optimize the magnitude of the tip vibration for most efficient use in cleaning the teeth. 
   There are two common sizes of insert, having resonant frequencies of approximately 25 KHz and 30 KHz, respectively. It is thus desirable that the control unit be able to generate time-varying currents at or near these frequencies. A number of different device configurations have been developed to accommodate this requirement. 
   U.S. Pat. No. 5,151,085 to Sakurai, et al. (herein denoted as “Sakurai”) discloses an oscillator for driving an ultrasonic transducer, wherein the oscillator is controlled by feedback from a multi-winding transformer. The transducer of Sakurai, however, is not of the magnetostrictive variety, and does not feature an excitation coil. Instead, Sakurai relies on a rather complex arrangement of inductors, transformers, and amplifiers to detect and match the impedance of the transformer. The handpiece disclosed in Sakurai has no excitation coil; moreover, the controller disclosed in Sakurai is not compatible with magnetostrictive inserts. 
   Likewise, U.S. Pat. No. 5,180,363 to Idemoto, et al. (herein denoted as “Idemoto”) discloses a complex system built around an oscillator featuring impedance-matching transformers and a phase-locked loop for detecting phase mismatch in the feedback signal. As with Sakurai, Idemoto&#39;s handpiece lacks an excitation coil; the transducer disclosed in Idemoto is not of the magnetostrictive variety; moreover, Idemoto&#39;s controller is incompatible with magnetostrictive inserts. 
   U.S. Pat. No. 5,451,161 to Sharp (herein denoted as “Sharp &#39;161”) discloses a magnetostrictive insert with an excitation coil and a transformer for providing feedback to a transistor oscillator. In the oscillator of Sharp &#39;161, the transistor collector-emitter current flows through the primary winding of the transformer, and also through the excitation coil, which is in series with the transformer&#39;s primary. The current induced in the secondary winding of the transformer flows into the base of the transistor, thereby causing the oscillator to oscillate near the resonant frequency of the magnetostrictive insert. The oscillator frequency, however, is not precisely at the resonance point of the insert, because there are additional components involved in the feedback circuit which have energy storage effects. Thus, the oscillator frequency is the resonant frequency of the entire circuit, not that of just the magnetostrictive insert itself. Furthermore, the oscillator of Sharp &#39;161 has a limited range of operation, and normally can accommodate only inserts having a restricted range of resonant frequencies. Therefore, to allow the controller to be utilized with inserts having a resonant frequency of 25 KHz as well as inserts having a resonant frequency of 30 KHz, Sharp &#39;161 provides a switchable capacitance in the transformer&#39;s secondary circuit, to provide the oscillator with two frequency ranges. Thus, Sharp &#39;161 requires the practitioner to change the switch setting when changing from one type of insert to the other. 
   U.S. Pat. No. 5,730,594, also to Sharp (herein denoted as “Sharp &#39;594”), partially overcomes the limitations of Sharp &#39;161 by providing a phase-locked loop oscillator to provide automatic tuning. The transformer feedback of Sharp &#39;161 is not suitable for such an arrangement. In addition, Sharp &#39;594 mentions prior art use of a second coil in the handpiece, adjacent to the excitation coil. The second coil provides the feedback for automatic tuning. Besides the need for an additional coil in the handpiece, Sharp &#39;594 also exhibits some limitations in the automatic tuning of the excitation frequency, and therefore provides manual tuning capabilities to overcome those limitations. It is noted that U.S. Pat. No. 6,190,167, also to Sharp (herein denoted as “Sharp &#39;167”), is a continuation of Sharp &#39;594 and presents no additional material. 
   U.S. Pat. No. 6,241,520 to Gofman, et al. (herein denoted as “Gofman”), discloses a variation on an oscillator which includes the excitation coil as an integral part of the oscillation circuitry. The inductance of the excitation coil substantially determines the frequency of oscillation of the oscillator. Gofman also features ancillary coils and capacitors (“tank circuits”) in the oscillator circuit, so that there are other factors determining the frequency of the oscillation. Thus, as with Sharp &#39;161, as discussed previously, the frequency of oscillation is near, but not exactly at, the resonant frequency of the magnetostrictive insert. Furthermore, Gofman still requires several coils in addition to the excitation coil, thereby incurring additional circuitry complexity and bulk. 
   U.S. Pat. No. 6,503,081 to Feine (herein denoted as “Feine”) discloses the use of a microprocessor to set the frequency of oscillation, such that the power delivered to the excitation coil is maximized. Feine asserts that the microprocessor can be programmed to sense the power input to the excitation coils, perhaps with the use of auxiliary circuitry or components. Feine, however, does not describe how such programming is to be accomplished, nor specifically how to construct such auxiliary circuitry, nor what such auxiliary components might be. But Feine does suggest using voltage-current phase difference measurements or power response slope measurements to determine the maximum power transfer point, in order to set the oscillation frequency to the resonant frequency of the magnetostrictive insert. Although Feine thus suggests a means of reaching the resonant frequency, the requirement for additional power-measurement circuitry imposes further requirements and limitations. 
   There is thus a widely recognized need for, and it would be highly advantageous to have, a means of automatically adjusting the oscillation frequency of the excitation current of a magnetostrictive insert to be substantially at the resonant frequency thereof, in a simple and direct manner that does not require additional transformers, feedback coils, tank circuits, or complex circuitry. This goal is achieved by the present invention. 
   SUMMARY OF THE INVENTION 
   The present invention is of a method and apparatus for controlling the excitation frequency of current flowing through the excitation coil in which a magnetostrictive insert is placed. In the present application, a dental scaler apparatus is used as a non-limiting example of an application for such control method and apparatus. In this non-limiting example, the apparatus is used by a dental practitioner in the cleaning of a patient&#39;s teeth. The examples and drawings depicting a dental scaler are understood to be for illustrative purposes only, and do not limit the scope of the present invention, which encompasses other dental and comparable medical uses of ultrasonic devices. The term “magnetostrictive ultrasonic dental device” herein denotes any ultrasonic apparatus intended for dental or medical use which utilizes a magnetostrictive ultrasonic transducer. 
   It is an objective of the present invention that the frequency be set at an optimal value in a fully automatic manner that does not require any manual adjustment or settings by the practitioner. It is also an objective of the present invention that the frequency be automatically set at an optimal value for a variety of different insert resonant frequencies, over a range at least from 23.5 KHz to 32 KHz, similarly without requiring any settings to be made by the practitioner. 
   It is moreover an objective of the present invention that the frequency be continually adjusted for optimal performance, and that the frequency be optimally set each time the practitioner energizes the handpiece, such as by means of a foot-operated switch. In this manner, should the practitioner adjust the power to the handpiece, apply additional pressure to the tip, or change the insert, the control apparatus automatically and continually sets the frequency for optimal performance. 
   It is furthermore an objective of the present invention that the above operating characteristics be attained through relatively simple and inexpensive circuitry and components, preferably utilizing integrated circuitry to the greatest extent possible, and reducing the need for reactive components, such as coils and transformers. In keeping with this, it is an objective of the present invention that multiple handpieces, optionally containing inserts of different resonant frequencies, be accommodated without the need for additional complex circuitry. 
   Therefore, according to the present invention there is provided a control unit for setting the frequency of the excitation current flowing in an excitation coil of a magnetostrictive ultrasonic dental device, the control unit including: (a) a voltage-controlled oscillator for generating a variable frequency signal; (b) a driver for setting up and regulating the excitation current according to the variable frequency signal; (c) a current sensor in series with the excitation coil, to output a current-sense signal corresponding to the current flowing through the excitation coil; (d) a function block operative to receive the current-sense signal and output a function signal proportional to a predetermined function thereof; and (e) a microprocessor for receiving the function signal and for controlling the voltage-controlled oscillator according to the function signal; wherein the excitation coil is not part of the voltage-controlled oscillator and is not connected directly to the voltage controlled oscillator. 
   In addition, according to the present invention there is provided a method for controlling the frequency of excitation current flowing in the excitation coil of a magnetostrictive ultrasonic dental device, the method including: (a) providing a current sensor operative to sense the magnitude of the excitation current flowing in the excitation coil and output a current signal proportional to the magnitude; (b) providing a function block operative to output a function signal proportional to a predetermined function of the current signal; (c) providing a controllable frequency-generator means; (d) sensing and storing the value of the function signal; (e) increasing the frequency and sensing the value of the function signal; (f) repeating, if the value of the function signal increases, the increasing the frequency; (g) decreasing the frequency, if the value of the function signal decreases after increasing the frequency; (h) repeating, if the value of the function signal increases, the decreasing the frequency; and (i) locking the frequency, if the value of the function signal decreases after decreasing the frequency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
       FIG. 1  is a block diagram of a magnetostrictive ultrasonic dental scaler device according to an embodiment of the present invention. 
       FIG. 2  is a flowchart of a method for controlling a magnetostrictive ultrasonic dental scaler device according to an embodiment of the present invention. 
       FIG. 3  is a block diagram of a magnetostrictive ultrasonic dental scaler device having multiple handpieces according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The principles and operation of a magnetostrictive ultrasonic dental device control unit according to the present invention may be understood with reference to the drawings and the accompanying description. 
     FIG. 1  is a conceptual block diagram of a magnetostrictive dental scaler device according to an embodiment of the present invention. A control unit  101  controls the current through an excitation coil  105  in a separate handpiece  103 . A magnetostrictive insert  135  is placed within handpiece  103  within excitation coil  105 . (Insert  135  is shown schematically in  FIG. 1 . In practice, insert  135  is placed physically within the confines of excitation coil  105 , such that the tip of insert  135  is exposed and available for cleaning the surfaces of the patient&#39;s teeth.) 
   One end of coil  105  is connected to a driver  111  which sets up a current to flow therein, as follows: Driver  111  includes a voltage source  107 , and the other end of coil  105  connects to a return path  108 . (Driver  111  is conceptually shown having voltage source  107 , whereas in practice, the voltage source can be any suitable voltage point; in practice, driver  111  can be implemented solely with return path  108 . As exemplified herein, however, the driver includes the voltage source.) 
   The input to driver  111  is the output of a voltage-controlled oscillator (VCO)  131 , so that driver  111  regulates the flow of current through coil  105  in a time-varying manner at the frequency of voltage-controlled oscillator  131 . That is, driver  111  functions as a power amplifier for the output of voltage-controlled oscillator  131 . 
   The input to voltage-controlled oscillator  131  is the voltage across a capacitor  129 , which is charged through a resistor  127  by a Schmitt trigger  125  whose input is a pulse-width modulated signal  123  from microprocessor  109 . Thus, microprocessor  109  can alter the frequency output from voltage-controlled oscillator  131  by changing the duty cycle of pulse-width modulated signal  123 . In an embodiment of the present invention, microprocessor  109  sets the duty cycle of pulse-width modulated signal  123  in order to maximize a function of the current flowing through coil  105 . Data about the current is input as follows: 
   Driver  111  sets up and regulates the current flowing through coil  105  to ground through a current-sensing RC network  113 , such that the voltage drop across RC network  113  is proportional to the current flowing through coil  105 . This voltage drop represents a current-sense signal, which then goes to an input point  120  to a function block  118 . At an output point  122 , function block  118  outputs a signal which is proportional to a predetermined function ƒ of the current I which flows through coil  105 . In an embodiment of the present invention, the function ƒ(I) is denoted as ƒ A (I), which is proportional to the current I. In this embodiment, function block  118  is represented by a block  118 A, which is schematically shown as having a short circuit  151  between input point  120  and output point  122 . In another embodiment of the present invention, the predetermined function ƒ(I) is denoted as ƒ B (I), which is proportional to dI/dt, the time-derivative of the current I. In this embodiment, function block  118  is represented by a block  118 B, which is schematically shown as having an operational amplifier  153 , whose non-inverting input receives the signal from input point  120 , and whose inverting input receives feedback through a resistor  157  which charges a capacitor  155 . In this manner, the output of operational amplifier  153 , which goes to output point  122 , is proportional to dI/dt. 
   In an additional embodiment of the present invention, the predetermined function ƒ of the current I contains terms proportional to both the current I itself and the time derivative of the current dI/dt. This embodiment is very general, in that by varying the respective constants of proportionality, the predetermined function can be varied smoothly from being a function of the current I only, to being a function of the time derivative of the current dI/dt only. 
   The signal from output point  122  goes to the non-inverting input of an operational amplifier  117 , whose output goes into an analog input  110  of a microprocessor  109  through a diode  119  and a resistor  121 . Microprocessor  109  is equipped with an internal A/D converter which converts the analog input into a digital representation for further processing. Preferably diode  119  and resistor  121  form the feedback loop for operational amplifier  117 . In this manner, microprocessor  109  is able to continuously determine the peak value of the predetermined function ƒ of the current flowing through excitation coil  105 . 
   The initiating of ultrasonic vibration is triggered by the action of the practitioner, typically by pressing on a foot-operated switch  139 . Foot switch  139  is considered to be either in an “on” state or in an “off” state. In an embodiment of the present invention, the “on” state occurs when foot switch  139  is depressed, and the “off” state occurs when foot switch  139  is not depressed. In another embodiment of the present invention, the “off” state occurs when foot switch  139  is depressed, and the “on” state occurs when foot switch  139  is not depressed. When the practitioner depresses switch  139 , microprocessor  109  is signaled to enable driver  111  over a line  141  to allow time-varying current to flow through coil  105 . When switch  139  is not depressed, however, microprocessor disables driver  111 , also via line  141 . 
   It is noted that according to the present invention, the feedback which controls the frequency of oscillation is solely in connection with the sensed current passing through the excitation coil, and that the excitation coil is not part of the oscillator circuit and does not connect directly with the oscillator circuit. 
     FIG. 2  is a flowchart of a method according to the present invention for setting the frequency of the current flowing through the excitation coil. This method requires having a means of generating a controllable frequency for the excitation current and a means of sensing the magnitude of excitation current. Other required components are referenced to  FIG. 1  and the previous discussions. After a power-on operation  201 , the control unit is initialized in a step  203 . Then, at a decision point  205 , the foot switch (switch  139  in  FIG. 1 ) is checked. If the foot switch is not depressed, then the excitation coil driver (driver  111  in  FIG. 1 ) is disabled and decision point  205  is repeatedly checked, as shown. If the foot switch is depressed, then the excitation coil driver is enabled, and the frequency scan is begun in a step  211 . 
   The frequency scan starts with reading and storing the value of the function ƒ of the current in a step  213 . The value of the function ƒ of the current is stored in a data element  215 . (The mechanism for reading the value of the predetermined function ƒ of the current is described above and illustrated in  FIG. 1 .) Next, in a step  217 , the frequency of the oscillator (voltage-controlled oscillator  131  in  FIG. 1 ) is increased, and at a decision point  219  the value of the actual function ƒ of the current is compared to the stored value in data element  215 . If the function ƒ of the current is higher, then step  213  is repeated, which will once again increase the oscillator frequency. If, however, the function ƒ of the current is lower, then the new value of the function ƒ of the current is stored in a step  220 , and in a step  221 , the oscillator frequency is decreased. That is, if increasing the oscillator frequency leads to an increase in the value of the function ƒ of the current, the oscillator frequency is increased again. If, however, an increase in oscillator frequency leads to a decrease in the value of the function ƒ of the current, the oscillator frequency is decreased. 
   Similarly, at a decision point  223 , the value of the actual function ƒ of the current is compared to the stored value in data element  215 . If the function ƒ of the current is higher, then step  220  is repeated, which will once again decrease the oscillator frequency. If, however, the function ƒ of the current is lower, then in a step  225  the frequency is locked and stored in a data element  227 . That is, if decreasing the oscillator frequency leads to an increase in the value of the function ƒ of the current, the oscillator frequency is decreased again. If, however, a decrease in oscillator frequency leads to a decrease in the function ƒ of the current, the oscillator frequency is locked and stored. 
   It is noted that if the function ƒ of the current neither increases nor decreases in the check of decision points  219  and  223 , step  225  is performed to store and lock the frequency. Because the values of the function ƒ of the currents are digitized (such as by the A/D conversion of microprocessor  109  in  FIG. 1 ), there is a non-zero probability that there is no change in the function ƒ of the current. 
   In a step  229 , a predetermined time delay is imposed, after which the foot switch state is checked again in step  205 . In this manner, the frequency is continually adjusted to achieve maximum value of the function ƒ of the current through the excitation coil. According to an embodiment of the present invention, the scan can be performed at regular time intervals many times per second. This allows apparatus according to the present invention to continually update the frequency to take into account changing conditions. Moreover, if the practitioner interchanges tips during a procedure, a control unit according to the present invention will automatically find the optimum frequency regardless of the operating characteristics of the new insert. In an embodiment of the present invention, the frequency scanning method is held in abeyance when the foot switch is released. 
   It is noted that in the prior art of Feine, the frequency is scanned and adjusted before the foot switch is depressed, thus setting the frequency under a no-load condition, rather than during actual operating conditions as performed according to the present invention. 
   In an embodiment of the present invention, the frequency increase in step  217  is a “coarse” (or relatively large) frequency increase, whereas the frequency decrease in step  221  is a “fine” (or relatively small) frequency decrease. In this embodiment, the frequency is first scanned coarsely with increasing frequency, and then when the optimum operating point has been passed, the frequency is scanned finely with decreasing frequency until the optimum operating point is reached. In another embodiment, the frequency is first scanned coarsely with decreasing frequency, and then when the optimum operating point has been passed, the frequency is scanned finely with increasing frequency until the optimum operating point is reached. 
     FIG. 3  illustrates a configuration having an additional handpiece  303  containing an excitation coil  305  and an insert  335 . Instead of driver  111  being connected directly to excitation coil  105  as illustrated in  FIG. 1 , the output of driver  111  goes to a handpiece selector  351 , which connects driver  111  either to coil  105  or to coil  305 . Microprocessor  109  controls handpiece selector  351  to make the appropriate selection. More than two handpieces are also possible in a similar way. In this manner, a practitioner can have multiple handpieces with different tips installed for rapid deployment during a procedure. 
   While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.