Abstract:
An ultrasonic electro-mechanical resonant system and instrument that provides improvements in the design and implementation of a feedback system. The disclosed configuration and orientation of coils enhance the motional or velocity feedback signals while minimizing the effects of transformer coupling. A two coil and a three coil approach is disclosed that takes advantage of non-homogeneous magnetic fields. An asymmetrical arrangement enables velocity signals to be coupled into the coils without requiring additional signal conditioning or capacitive elements.

Description:
[0001]    This application claims the benefit of priority to U.S. Provisional Application Ser. No. 61/319,922 entitled “ULTRASONIC SYSTEM CONTROLS, TOOL RECOGNITION MEANS AND FEEDBACK METHODS”, filed Apr. 1, 2010 by Richard H. Paschke, the entirety of which is incorporated by reference herein for all purposes. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to ultrasonic electro-mechanical resonant systems. More specifically, the present disclosure relates to improvements in the design and implementation of a feedback system employing the configuration and orientation of coils that enhance the effects of motional or velocity feedback signals and minimize the effects of transformer coupling. 
         [0004]    Additionally, the present disclosure relates to a detection system to determine the impedance or inductance of a tool placed in an ultrasonic handpiece. This system includes the ability to differentiate or select tool characteristics prior to and after activation. 
         [0005]    2. Background of Related Art 
         [0006]    In general, magnetostrictive, electro-mechanical resonant systems are driven by applying an AC signal to a coil that activates a magnetic or ferro-magnetic member, hereinafter referred to as a transducer, by creating a magnet field. The resultant magnetic field creates compressional or standing waves in the transducer, causing it and parts connected thereto to vibrate. The aggregate assembly of the transducer and connected parts, which are hereinafter referred to as tools, are typically supported at nodal points of the longitudinal motion to minimize any loss of motion or kinetic energy of the vibrating tool. The energy created in the vibrating tool may be applied in performing ultrasonic machining, welding, cleaning, dental calculus debridement, or other applications. It is desirable to operate the tool with maximum amplitude of vibration at the working end of the tool. This is achieved when the drive signal is at one of the frequencies of resonance of the transducer or tool. 
         [0007]    The desired frequency of resonance is affected by wear, tool geometry, operational temperature, and loading of the tool. To maximize the utility of the tool, the operational frequency of the system drive should be capable of varying in response to the dynamic tool resonance conditions. 
         [0008]    Currently produced ultrasonic systems that use some form of feedback to control the drive frequency are limited by a multitude of compromises. The following are some examples of these compromises. 
         [0009]    Systems that use the current and voltage characteristics in the drive circuits typically simulate a motional characteristic at a single operating point and use its value for all drive levels. 
         [0010]    Most feedback systems that employ a feedback coil near the free end of the transducer to detect the velocity or motion of the transducer use a few turns of reverse drive winding to minimize the transformer coupling effects. This poses several problems. Such reverse winding require the drive levels to be higher than otherwise required because the reverse winding subtracts from the total drive signal. Additionally, the feedback winding needs to be isolated from the end of the drive winding by adding a gap between the windings. Shortening the length of the drive winding limits the total length of the driving magnetic field. The number of turns on the reverse winding is critical because they affect the phase relationship between the drive and feedback signals. 
         [0011]    Systems that employ two symmetrical windings wound in reverse magnetic sense are position sensitive, which is in part due to the non-homogeneity of the drive field. These systems are also sensitive to nodal point positioning of transducers with systems using interchangeable tools. This configuration of feedback typically requires some form of post feedback signal conditioning which modifies the phase information, and/or requires the addition of a capacitor across the winding(s). 
         [0012]    Some ultrasonic systems provide the option of interchangeable tools. The performance of these tools can vary as a result of certain parameters of the tools. If the impedance varies, for example, corrections could be made to the performance with knowledge of the parametric value. In another instance, a system may have removable tools that provide fundamentally different operations such as ultrasonic vibration and induction heating. 
         [0013]    Currently available systems typically use voltage or current levels and frequency or phase information in conjunction with logic circuits that interpret the values. For example, Karnaugh maps or similar Boolean operations are used to select one operational mode or the other. Inherent limitations associated with these prior art systems are that these inputs are often ambiguous due to the effects of manufacturing variations in the tools and operational variations during tool loading. 
         [0014]    The present disclosure takes advantage of the non-homogeneous magnetic fields and has been successfully modeled and works without additional signal conditioning. It is the asymmetry of the two coil configuration that makes the difference, with the three coil configuration producing the best results. 
       SUMMARY 
       [0015]    In accordance with one embodiment of the present disclosure, a configuration of three coils, wherein the winding direction of the two distal coils are in the same direction, and a center coil that is wound in the opposite direction from the distal coils. In a preferred configuration, the sum of the windings of the two distal coils equals the number of windings of the center coil. The spacing between the two distal coils relative to the center coil may be symmetrical or asymmetrical. In some cases a small number of additional turns may be added to the distal coils to compensate for the effects of a non-homogeneous magnetic field as the position of the distal coils approach the ends of the drive coil. Another effect that results in inducing voltage into the feedback coils is the changing permeability of the vibrating transducer. A relatively symmetric dynamic stress pattern exists along the length of the transducer with the maximum effect located in the nodal region. This configuration minimizes the effects of transformer coupling between the drive and feedback coils. 
         [0016]    The effect of the pick-up configurations above are further enhanced by addition of a gap in the drive windings. The two sections of the winding are continuous and wound in the same direction including at least one gap at a distal position of the coil. Removing a number of turns in the drive winding in this manner produces a field correction that acts to depress the induced transformer voltages in the feedback coils. 
         [0017]    Another embodiment comprises an asymmetrical two coil configuration with the coils wound in opposite directions. In one configuration, the coils are on opposite sides of the transducers nodal region. The spacing between the coils is determined in part by the length of the transducer and the centerline of the spacing between the coils can be positioned about the nodal region to increase the motional feedback effect. The larger coil can be placed on either side of the nodal region with the preferred position being the one producing the greatest induced motional feedback voltage. This configuration also provides a minimization of the transformer coupling by means of the opposite sense of the windings and the asymmetry of the windings. The feedback signal is generated at least in part by the summation of the induced motional or velocity signals in the feedback coils. The asymmetry of the coils also enhances the motional feedback signal based on the non-homogeneity of the drive field. 
         [0018]    An aspect of this disclosure includes determining the impedance or inductance of a transducer placed in the handpiece. In one embodiment, a bridge circuit is configured to have one leg connected to the drive coil such that it has minimum affect on the operation of the handpiece, drive system or feedback. A suitable stable oscillator is placed across the bridge and an ancillary circuit is connected in quadrature to the oscillator. The output of the bridge is balanced or nulled with a standard tool in place. The nulling operation can be done once during alignment of the circuit, e.g., during testing, at predetermined intervals during normal operation, upon application of power to the drive circuit, or whenever the drive circuit is not activated depending on the application. The null value can then be stored or otherwise processed so that its value can be used as a comparison value when another transducer or tool is placed in the handpiece. The circuit parameters can also be adapted to allow nulling of the circuit when no tool is in the handpiece. In this way, the detection circuit can provide a means to limit power to the handpiece when no transducer is present. 
         [0019]    An ancillary circuit can be a simple operational amplifier or a series of amplifiers, signal conditioning circuits or buffer stages that produce the desired output characteristics. A linear voltage curve proportional to the value of the transducer or tool inductance is a non-limiting example. It is envisioned that the circuitry described is capable of tracking changes in the inductance from uH to mH ranges. 
         [0020]    Current ultrasonic systems have inherent limitations in their ability to capture and lock onto transducers due to manufacturing tolerances of transducer inductance. A transducer outside a specific range of inductance often results in the failure of the electronics to operate the tool, which is in part due to phase shifts outside the normal operating limits of the system. The proposed disclosure has the ability to detect the transducer inductance and apply a correction or change to the oscillator circuit&#39;s operational parameters, which allows the system to function normally. 
         [0021]    A further embodiment of this disclosure is a means to optimize transducer performance, control the operational power range of the tool, or alert the operator that the transducer has been damaged through handling. A common cause for the transducer to lose inductance is due to work hardening the magnetostrictive material. For example, dropping a transducer a height of 4 feet can result in a 3% loss of initial inductance. Even slight deformation of the transducer due to handling and re-straightening the transducer can result in a 10% to 15% loss in inductance. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    Various embodiments of the presently disclosed control, feedback and detection circuits are described herein with reference to the drawings wherein: 
           [0023]      FIG. 1  is a view of a three-coil feedback system connected to a dental scaler system in accordance with one embodiment of the present disclosure; 
           [0024]      FIG. 2  is a view of a two-coil feedback system in accordance with an embodiment of the present disclosure; 
           [0025]      FIG. 3  is a schematic diagram of a control circuit in accordance with an embodiment of the present disclosure; and 
           [0026]      FIG. 4  is a graph of a linear output of a detection circuit in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Particular embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings. In the following description, well know functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
         [0028]    Turning now to  FIG. 1 , disclosed is a dental scaler system  10  including a dental scaler device  14 , and a handpiece  1  that is operatively coupled to the dental scaler device  14  via a cable  13 . The dental scaler device  14  includes a power supply  9 , which may be either internal or external to the dental scaler device  14 , an oscillator circuit  8 , a control circuit  7 , and an output driver  6 . The power supply  9  provides one or more voltages to the scaler device  14 . The one or more voltages provide input to the scaler device  14  (e.g., indicator lights) and its oscillator circuit  8 , control circuit  7 , and output driver  6 , which in combination convert the DC voltage into high frequency signals for driving the handpiece  1  and provide power to process the feedback and control signals. 
         [0029]    The handpiece includes an energizing coil  2 , and feedback coils  3   a ,  3   b , and  3   c , which in combination provide a signal via feedback line  11  to control the oscillator circuit  8 . In the example shown, the sense of the drive coil  2  winding is counter clockwise. The feedback coils  3   a  and  3   b  are wound in a clockwise sense, and feedback coil winding  3   c  in a counterclockwise sense. When wound in this configuration, the coupled signal induced by transformer action of the handpiece with a transducer  4  inserted into the handpiece  1  is minimized and the signal induced by the motion of the activated transducer, also referred to as velocity feedback is predominant. Although the transducer  4  is shown as a laminated component, a solid transducer or a ferromagnetic transducer or other suitable construction may be utilized. Those skilled in the art will recognize that the sense of the feedback coils  3   a  and  3   b  may also be wound in a counterclockwise sense and the feedback coil  3   c  wound in a clockwise sense. 
         [0030]    In one embodiment, the number of windings in feedback coil  3   c  is slightly smaller than the sum of the windings for feedback coils  3   a  and  3   b  due to the smaller transformer coupling effect near the ends of the stack. In practice, the total length of the coils  3   a ,  3   b , and  3   c , including the spacing between the coils, should be approximately ⅔ the length of the drive coil. 
         [0031]    By placing feedback coil  3   c  in the nodal region  5   a  and feedback coils  3   a  and  3   b  near the loops of vibration  5   b  and  5   c , the combination of feedback coils  3   a ,  3   b , and  3   c , become less sensitive to the axial displacement of transducer  4 . In a preferred embodiment, the configuration provides placement of the feedback coils  3   a ,  3   b , spaced a distance from the ends of the drive coil  2  to minimize the effects of non-homogeneities of the magnetic field. 
         [0032]    Continuing with reference to  FIG. 1 , the oscillator circuit  8 , which in the case of a phase lock circuit, would begin sweeping its frequency either up or down from a nominal starting point until the transducer  4  begins to vibrate. As the vibrations build, the oscillator circuit  8  approaches the transducer  4  frequency of resonance, the feedback  11  is input to the oscillator circuit  8  via handpiece connector  13  and reinforces the vibration of the transducer  4  wherein the oscillator circuit  8  locks onto the operational frequency of the transducer  4 . The output of the oscillator circuit  8  is connected to control circuit  7 , which processes the signal and couples it to the output driver  6 . The signal of output driver  6  is connected via handpiece cable  13  to the handpiece  1  of the dental scaler system  10 , which provides the power to drive the transducer  4 . The common handpiece lead  12  for the drive coil  2  and feedback coils  3   a ,  3   b , and  3   c , the drive  15  and feedback  11  are connected via handpiece cable  13  to the oscillator circuit  8 , control circuit  7 , and output driver  6  of the dental scaler device  14  as shown. 
         [0033]    Turning now to  FIG. 2 , which shows an alternate handpiece configuration wherein feedback coils  3   d  and  3   e , are in an asymmetrical arrangement. Feedback coil  3   e  has a greater number of turns than feedback coil  3   d . In a preferred embodiment, the ratio of turns for feedback coils  3   d  and  3   e  is 1.25, and the total length of the coils including the spacing between coils is approximately ⅔ the length of drive coil  2 . This configuration is further differentiated from the configuration shown in  FIG. 1  by placement of the feedback coils  3   d  and  3   e  with reference to the nodal region  5   a  of transducer  4 . Feedback coils  3   d  and  3   e  are positioned to minimize the effects of transformer coupling from the drive coil  2 , with the spacing between the coils positioned in the nodal region  5   a , during operation. It is envisioned that the position of feedback coils  3   d  and  3   e  may be reversed such that the signal obtained for the motional component of the activated transducer  4  is maximized. Alternate handpiece configuration  20  is connected to dental scaling system  10  in the same manner as disclosed hereinabove. 
         [0034]    Referring now to  FIG. 3 , the control circuit  30  includes a detection circuit  31 , wherein the value of the inductance of an operational tool  32  is determined by comparing the output  26  of the tool  32  to a value obtained during calibration of a standard transducer  4 . For example, and without limitation, the inductance range for a typical dental scaler system  30  may be 260 to 340 uH for a 30 kHz system and 340 to 460 uH for a 25 kHz system. By definition, a standard transducer for the 30 kHz system would be 300 uH and 400 uH respectively for a 30 kHz and a 25 kHz system. The calibration or nulling operation may be performed with a standard transducer  4  placed in handpiece  1 , with the dental scaler system  10  with its ultrasonics either activated or not activated, as described in detail below. 
         [0035]    With continuing reference to  FIG. 3 , a typical detection circuit  31  is shown with a frequency source  18 , fixed bridge resistors  20 , and  22 , fixed bridge capacitor  21 , and adjustable bridge resistors  17  and  19 . Fixed bias resistors  23  and  24  are selected as required by the ancillary circuit  32  to achieve an acceptable null value at output  26 . It is envisioned that the capacitive reactance in the detection circuit  31  will exactly oppose the inductive reactance of the transducer  4  when the circuit is in balance (null position), allowing the value of the transducer  4  resistance R S  and inductance L S  (not explicitly shown) to be reliably determined. In general, the values of the fixed bridge components  20 ,  21 , and  22  are known, and the variable components  17  and  19  are adjusted until the bridge is in balance (nulled). The values of R S  and L S  can be determined based the value of the other components. For an example without limitation, L S  equals the product of the value of fixed resistor  22 , times the value of adjusted variable resistor  17 , times the value of fixed capacitor  21 . R S  equals the value of the product of the value of fixed resistor  22 , times the value of adjusted variable resistor  17 , divided by the adjusted value of variable resistor  19 , minus the value of fixed resistor  20 . Applying the above formulae, calculates the values for components  17 ,  19 ,  20 ,  21 ,  22  with component  18  at 30 kHz with a standard transducer  4  measuring 300 uH as respectively; 19.4 kilo Ohms, 28.9 kilo Ohms, 2.2 kilo Ohms, 0.093 micro Farads, and 3.3 kilo Ohms. 
         [0036]    In further detail, calibration may be performed when the ultrasonics tool  32  is not activated, e.g., the level of drive line  15  is at the same level as common line  11  and no current is flowing in drive coil  2 . A transducer  4  is placed in the handpiece  1 . The power to the dental scaler system  10  is on, but the ultrasonic circuits are not activated. The purpose of calibration is to provide a decision point for the control circuits. The drive coil  2  is connected to bridge circuit  31  via interface  16 . It is envisioned that the interface  16  may comprise a direct connect of both leads on drive coil  2 , a direct connection of a single lead and a capacitive coupling on the second lead of drive winding  2 , or a mechanical or solid state relay connection on one or both leads of drive coil  2 . A transducer  4  with a predetermined value of inductance, for example 290 uH, is placed into handpiece  1 . The null position  1  is achieved by varying adjustable resistors  17  and  19 , in combination until output  26 , is at zero volts as shown on curve  29 ,  FIG. 4  by the intersection of the horizontal line, shown at a zero null  33 , and the vertical inductance line  28 , shown for standard transducer  4 ,  FIG. 1 . At the point where curve  29  is generated by the range of measured inductance values. For an example, and without limitation curve  29  would be generated by measurement of inductance values of tool  32  ranging from 260 to 340 uH for 30 kHz operation. 
         [0000]    It is envisioned that for detection and measurement of high Q inductances of the transducer  4 , components  19  and  21  may be placed in series rather than in parallel as shown in detection circuit  31 . 
         [0037]    Now with reference to  FIG. 4 , the null condition is shown on control graph  40  as the intersection of the zero null voltage  33  (horizontal line) and the inductance line  28  representing the inductance value of a standard transducer  4  (vertical line). Replacing the standard transducer  4  with tool  32  whose inductance is in the range of 260 uH to 340 uH, for a 30 kHz system, thereby producing curve  29 . It is envisioned that other points on the curve  29  can also be used as a null point by changing the bias conditions in the ancillary circuit  25 . The output  26 , of the ancillary circuit  25  for any tool  32 , whose inductance deviates from the stand transducer  4  inductance of 300 uH, represents an error voltage. This error voltage is represented on output curve  26  as a point  34  whose inductance is lower than 300 uH and point  35 , whose inductance is greater than 300 uH. 
         [0038]    While output graph  40  shows a linear output curve  29 , a non-linear output curve may be substituted in the case where, for example, a non-linear output would be better suited to provide an expanded control range for control circuit  7 . It is well known in the art that a simple combination of bipolar transistors and operational amplifiers can be configured to convert linear signals into non-linear signals. 
         [0039]    From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. While several embodiments of the disclosure have been shown in the drawings, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.