Patent Publication Number: US-7210541-B2

Title: Transducerized rotary tool

Description:
CROSS-REFERENCE TO RELATED APPLICATONS 
   This application is a CIP of U.S. application Ser. No. 10/654,504, filed on Sep. 3, 2003, now U.S. Pat. No. 7,090,030, and the full disclosure of which is hereby incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to the field of automatic drivers for fasteners. More specifically, the present invention relates-to an apparatus for driving fasteners that is automatic and controllable. Yet more specifically, the present invention relates to a device for driving fasteners, where the apparatus delivers a specified torque. Yet even more specifically, the present invention relates to an automatic apparatus where the torque delivered is controllable from about 1 in-lb up to about 50 in-lb. 
   2. Description of Related Art 
   Many prior art devices exist that are capable of driving fasteners apertures, such as threaded bolt holes and the like. These tools typically require the user to activate a switch or a trigger to activate the device. Further, some prior art devices rely on power sources such as compressed air to drive the associated motor, which can limit the applicability of a device since producing compressed air requires space for a compressor and is generally impractical. Other devices that employ electrical motors produce an output whose speed and torque can vary and is not precisely controllable or not controllable at all. However many instances where it is required to employ a rotary tool, the ability to control the speed and torque is important. Some fasteners require that they be installed to a specified torque, and it is important that how much the fastener has been torqued be easily verified by the operator of the device. 
   Some of these devices include means to measure the rotational force, or torque, exerted by the particular device. These means range from monitoring the current consumed by the device, pressure sensors applied to working parts of the device, and included various sensors within the device. Examples of prior art devices useful for driving fasteners can be found in U.S. Pat. Nos. 4,487,270, 4,887,499, 6,424,799, 4,571,696, and 4,502,549. 
   Therefore, there exists a need for an apparatus and a method for securing fasteners that is reliable, accurate, and can precisely torque a fastener to a specified torque. An additional need exists for a tool to be durable, hand held, and provide an indication the preciseness of the directly torqued value. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention involves a rotary tool comprising a motor capable of providing a rotational force connected to a chuck assembly. Included with the present invention is a variable voltage device that is responsive to a magnetic field. The motor can be selectively controlled by operation of the variable voltage device—where the control includes on off switching as well as motor speed control. The tool of the present invention includes a push to start function, that is by urging the tool against the object being rotated, the rotary tool includes means to begin operation of the tool based on the urging force. The rotational velocity and/or amount of force delivered by the tool can vary based on the amount of forced applied during the urging. Optionally, the variable voltage device can be a Hall effect sensor, either linear or digital. 
   The present invention can further include a field device provided on the chuck assembly, where the field device is capable of emitting a magnetic field. Positioning the field device by selective movement of the chuck assembly controllably drives the motor. This is done since positioning the field device manipulates the magnitude of the magnetic field provided to the variable voltage device from the field device. The magnitude of the magnetic field proportionally relates to the proximity of the variable voltage device in relation to the field device. 
   The rotary tool of the present invention can further include a lever assembly having a field device formed thereon. The field device within the lever is also capable of emitting a magnetic field. Positioning the field device within the lever by selective movement of the lever assembly can controllably drive the motor. Positioning the field device manipulates the magnitude of the magnetic field applied to the variable voltage device from the field device within the lever. The magnitude of the magnetic field within the lever field device proportionally relates to how close the variable voltage device is in relation to the field device. Optionally, a handheld pistol grip assembly can be employed in lieu of the lever assembly. 
   Preferably included with the rotary tool of the present invention is a torque transducer capable of measuring the value of the torque generated by the chuck assembly. Optionally included with the transducer is at least one strain gauge in cooperative engagement with the torque transducer. The at least one strain gauge transmits data representing the torque generated by the chuck assembly. This data monitored by the strain gage is usable to terminate operation of the driver when the torque generated by the chuck assembly reaches a predetermined amount. 
   Also optionally included with the rotary tool of the present invention is at least one selector switch programmably capable of selectively reversing the polarity of the electrical power supplied to the driver. Additional selector switches can be included that are also programmable. The additional selector switches can be capable of selectively operating the driver in a different control mode. 
   Optionally, the present invention can comprise a system to drive fasteners comprising a rotary tool combinable with a controller assembly. Here the rotary tool includes a motor capable of providing a rotational force, a chuck assembly operatively connectable to the motor, and a variable voltage device responsive to a magnetic field. The motor is in operative communication with the variable voltage device. The controller assembly should be capable of providing control instructions to the rotary tool where the control instructions comprise maximum torque magnitude, speed, among other operational variables. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1A  depicts one embodiment of the present invention. 
       FIG. 1B  illustrates an exploded view of one embodiment of the present invention. 
       FIGS. 2A–2E  provide a partial cut-away version of embodiments of the present invention. 
       FIG. 2F  provides a cutaway view of an embodiment of the present invention. 
       FIG. 2G  illustrates a frontal view of an embodiment of the present invention. 
       FIG. 2H  illustrates a side view of a tranducerized element. 
       FIGS. 3A and 3B  depict a cutaway view of an embodiment of the present invention. 
       FIGS. 4A and 4B  depict a cutaway view of an embodiment of the present invention. 
       FIG. 5  presents an embodiment of the present invention combined with a controller. 
       FIG. 6  provides an exploded view of a gear box in combination with a motor. 
       FIGS. 7A and 7B  provide side and perspective views of embodiments of a tool grip. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention considers a rotary tool system comprising a rotary tool combined with a controller system. With reference to the drawings herein, one embodiment of the rotary tool  10  of the present invention is shown in perspective view in  FIG. 1A  and an exploded view in  FIG. 1B . The rotary tool  10  is capable of driving fasteners, such as bolts, nuts, screws, self-threading screws, etc. Further, the rotary tool  10  is capable of repeatably applying fasteners to a precise specifiable torque. In the embodiment of the invention as shown in  FIG. 1B , a motor  36  is included with the invention capable of initiating a force used to torque the fasteners. Preferably the motor is a brushless DC motor operating at 48V to 60V. The motor  36  employs a stator (not shown), a rotor (not shown), and a commutation module (not shown). The stator is comprised of a series of windings that surround the rotor. Magnets (not shown) are secured to the outer radius of the rotor and current is applied to the windings situated just counterclockwise of the magnets. The current within the stator creates an electromagnetic field that repels the magnets causing rotation of the rotor. The commutation module is attached to the rotor and has an indicator from which the angular location of the magnets is determined. By tracking the location of the magnets, the series of windings just counterclockwise of the magnets, at any given point in time, are energized which perpetuates rotation of the rotor. 
   In the embodiment of  FIGS. 1A and 1B  a gear box  38  is shown disposed adjacent the motor  36  is operative connected to the motor  36 . The gear box  38  contains a series of gears  39  configured into a gear train or system in mechanical cooperation with the motor  36 . The gears  39  are arranged to receive the output rotational force delivered by the motor  36  and convert that force into a specified torque at the output shaft  40  connected to the gear box  38 . Preferably the gear train is comprised of at least two gear stages, where each stage converts the rotational torque and speed produced by the motor  36 . It is also preferred that the gear box  38  function to increase the torque delivered by the motor  36  with a corresponding decrease in the rotation speed of the motor  36 . The preferred range of torque to be output at the gear box  38  ranges from about 1 in-lb to about 50 in-lb. 
   To maximize torque/velocity conversion while minimizing space, the preferred gear system is a planetary gear system comprising sun and planet gears.  FIG. 6  provides an embodiment of a motor  36  combined with a gear box  38 , where the gear box  38  is shown in an exploded view. In this preferred system the first stage sun gear  86  is attached to the motor  36  and engages a series of preferably three planetary gears  88 . The planetary gears  88  are all attached to a planet carrier  91 , from which extends a second sun gear  93  into a second planetary gear stage  95 . The output shaft of the second gear stage is the output shaft  40 . Preferably the gearbox  38  is sealed, this eliminates gear maintenance and protects the gears from foreign matter such as dirt. It is also preferred that the lubricant used exhibit high-pressure lubricity, and low viscosity in order to minimize the amount of lubricant used, which in turn reduces viscous shear. 
   Needle rollers  89  can be included between the annulus between the inner diameter of each planet gear (of each stage) and the outer diameter of the spindle  93  it rides on. The use of needle rollers  89  in this location of the gearbox  38  significantly reduces friction and wear. The needle rollers  89  also hold lubrication very well. The quantity of needle rollers  89  for use with each gear depends on the size of the individual gear and the gear box, it is believed that determining this quantity is within the scope of those skilled in the art. 
   To minimize contact between gear stages an axle bearing  90  is disposed into a conical cavity between the planets on the centerline of each planet carrier ( 91  and  97 ). When the mating sun gear ( 86  and  93 ) from the previous stage (or the motor  36 ) is inserted between the planet gear ( 88  and  94 ), its face comes to rest against the axle bearing  90 . Preferably the axle bearing is comprised of a hardened metal ball. This ball could be made from any number of hardenable materials. This configuration produces very little friction since the axle bearing  90  and the sun gears ( 86  and  93 ) are in tangential contact. When these two stages are rotating with respect to each other, the material surface velocities at the point of contact is very low and can generate almost no moment arm. The conventional way of doing this is to place thin thrust washers between stages at the full diameter of the planet carrier. This is very inefficient considering the large contact area and surface speeds. 
   In order to adequately handle axial and radial loads on the output shaft  40  of the gearbox  38  as well as limit axial and radial play, a combination of two bearings is used. The bearing on the outboard most end of the gearbox is a conventional radial bearing. This bearing is meant to carry any side loads placed on the output shaft  40  as well as a small amount of axial load. The inboard bearing is an angular contact bearing. This bearings primary function is to carry the axial loads, which are transmitted down the output shaft as well as a small amount of radial load. The load coupling of these two bearings is accomplished by a small spacer of a precisely held thickness, which is sandwiched between the inner races of both bearings. These bearings, in combination, produce a very free spinning, durable and accurate mechanism. Optimal performance was obtained by gluing the axle bearing  90  in place with a cyanoacrylate glue in addition to other tolerance adjustments. 
   Enhanced performance and efficiency has been realized by some of the design improvements to the gear box  38 , for example, the splined output shaft  40  was strengthened to carry more torsional load. The gearbox output shaft retainer ring (not shown) was improved to carry more axial load without breaking free. Heat treatment was added to surfaces on the planet carriers that come into contact with rotating planet gears. High-carbon steel alloy axles were included with the planet carriers to improve fatigue properties also the thickness of rear gearbox end cap was adjusted to minimize axial gear clearances. 
   Optionally the rotary tool  10  can be tranducerized to provide a real-time monitoring of the magnitude of the torque exerted onto a fastener by the rotary tool  10 . Preferably the torque monitoring system include a flexure  25  secured to the gear box  38  on the end of the gear box  38  opposite to where it is connected to the motor  36 . At least one strain gauge  85  can be included within the flexure  25  that senses the torque supplied by the motor  36  and transmits that sensed torque information to the tool controller  80 . Preferably four strain gages  85  are included with the flexure  25 . The flexure  25  is connected on its other end to the nose cap  26 . As can be seen in  FIG. 1 , the nose cap  26  includes slots  27  on its outer surface that mate with tabs  17  formed on the front end of the body  12  of the rotary tool  10 . As the motor  36  supplies torque to the fastener, the motor  36  in turn transmits an identical torque value to nose cap  26 . Since the present invention mounts the motor  36  to the flexure  25 , the flexure  25  experiences the torque supplied by the motor  36 . Thus by positioning a at least one strain gage  85  on the flexure  25 , the torque output of the motor  36  can be measured by the at least one strain gage  85 . As the tool communicates with a tool controller  80 , the torque output of the at least one strain gage  85  connects to the tool controller  80  as well. When the output torque of the motor  36  reaches a pre-selected torque, the tool controller  80  is programmable to immediately deactivate power to the rotary tool  10 , thus ensuring that the fastener being secured by the rotary tool  10  is not over tightened. 
   The at least one strain gage  85  is calibrated as an assembly using what is know as a dead weight calibrator. Weights, which are certified and traceable to NIHST, are used to generate a static moment by placing them on an arm at a specific distance. The calibration does not occur until the at least one strain gage  85  is combined within the rotary tool  10 . This is done in order to take into account frictional losses in the tool. Preferably, the at least one strain gage  85  can be a standard encapsulated strain gage that is modulus compensated for use on aluminum flexures. The signal produced by the detection of strain in the at least one strain gage  85  is carried to the controller  80  analog via the flex circuit  44  and the tool cable  82 . The flex circuit  33  attaches directly to the flex circuit therefore eliminating wiring in the rotary tool  10 . When the preferable configuration of four strain gages  85  is used, the four strain gages are attached to each other in a wheatstone bridge configuration using fine polyester varnished wire. The four dual element strain gages  85  are located 90° from each other on the flexure  26 . The use of four strain gages  85  is employed in order to minimize bending cross talk and improve accuracy. 
   A chuck assembly  28  is provided with the embodiment of the present invention of  FIGS. 1A and 1B . The chuck assembly  28  is connectable to the output shaft  40 , preferably through corresponding spline grooves formed on the outer surface of the shaft  40  and an aperture (not shown) formed axially within the shaft  29  of the chuck assembly  28 . As will be explained in further detail below, the length of the aperture should be long enough to allow the shaft  29  to slide back and forth along a portion of the length of the output shaft  40 . A socket  31  is provided on one end of the chuck assembly  28 , the socket  31  shown is suitable for receiving a fitting (not shown) specifically sized to fit the particular fastener being driven by the rotary tool  10 . Further, a sleeve  33  is provided that when tugged axially retracts a retaining ball within the socket  31  thereby enabling adding or removing the particular fitting for use with the rotary tool  10 . Also disposed on the chuck assembly  28  is a collar  35  slidable along the shaft  29 . The collar  35  includes threads  32  on the outer surface adjacent the nut  30  formed to fit threads (not shown) in the nose cap  26 . A ring magnet  34  is disposed on the end of the shaft  29  opposite the socket  31 . A snap ring (not shown) is included on the shaft  29  that retains the collar  35  on the shaft between the sleeve  33  and the snap ring. Thus while the collar  35  remains on the shaft  29 , it must be free to slide along the shaft  29  between the sleeve  33  and the snap ring. Accordingly when the chuck assembly  28  is screwed to the nose cap  26 , the shaft  29  can be slideably disposed in and out of the collar  35  a certain distance while still being retained within the chuck assembly  28 . 
   It should be pointed out that the rotary tool of the present disclosure is useful not only for driving and securing fasteners, but can also be useful as a drill motor, a sander, a buffer, a saw, and any other application where a rotary driving force is used. Moreover, the novel application of the push to start feature disclosed herein is applicable with all functions for which the present device can be used. 
   Optionally, illumination light emitting diodes (LEDS)  58  can be disposed on the forward end of the rotary tool  10 . Preferably four illumination LEDS  58  can be included that reside in ports  60  formed on the nose cap  26 . The illumination LEDS  58  should emit white light to provide illumination for the operator so the rotary tool  10  can be used in dark spaces. Also optionally provided are indicator LEDs  62  of various colors. Illumination of an indicator LED  62  of a certain color can provide operational information pertinent to the rotary tool  10 . For example, one of the indicator LEDS  62  can be designed to emit a green light when it has been determined that a fastener has been torqued to a correct torque value. Similarly, if too much torque has been applied to a fastener a red indicator LED  62  can be activated and if too little torque has been applied a yellow indicator LED  62  can be lit. The colors of the illumination LEDS  62  is merely illustrative and not meant to constrict the scope of the invention as any color light can be chosen to represent a particular torque condition. 
   Referring now to  FIGS. 3 and 4 , other electrical circuitry that can be included with the present invention include variable voltage devices (VVD) such as a Hall effect sensor. As is well known, the output voltage of the VVD depends on the magnetic flux density applied to the VVD. Thus, the output voltage of a VVD can be increased by subjecting the VVD to a magnetic field. Likewise, the output voltage of the VVD can be eliminated by removing the magnetic field. Accordingly a switching mechanism can be produced by combining a field device that produces a magnetic field, such as a magnet, with a VVD. A simple application of this phenomenon involves creating a voltage source by positioning a magnet (either permanent or electro) close to a Hall effect sensor. With regard to the present invention, the preferred field device is a permanent magnet, and the preferred VVD is a Hall effect sensor. 
   In  FIGS. 3A and 3B  one example of such a switching device can be seen. As can be seen from  FIG. 3A , the chuck assembly VVD  73  is disposed on the flexure  25 . As previously pointed out, the shaft  29  is slideable within the collar  35  and is thus axially moveable with respect to the rest of the rotary tool  10 . Absent a force urging the shaft  29  inward toward the rotary tool  10 , it is pushed outward by a spring  42  and is in its extended position as seen in  FIG. 3A . When the shaft  29  is in the extended position, the magnetic field emitted by the field device  34  has little or no effect on the chuck assembly VVD  73  and the chuck assembly VVD  73  will emit no voltage. In contrast, when the shaft  29  is pushed inward into a retracted position, the field device  34  should be sufficiently proximate to the chuck assembly VVD  73  that it will emit voltage. It is preferred that when the shaft  29  is fully retracted that the interaction between the field device  34  and the chuck assembly VVD  73  be such that the chuck assembly VVD  73  emit its maximum voltage. The voltage emitted from the chuck assembly VVD  73  should be used to drive the motor  36 . Therefore, the motor  36  can be activated or deactivated by retracting and extending the shaft  29 . It should also be pointed out that like all VVDS the chuck assembly VVD  73  will begin to emit a higher voltage in response to an increase in the strength of the magnetic field applied to it by the field device  34 . Thus the closer the field device  34  is to the chuck assembly VVD  73 , the more voltage the chuck assembly VVD  73  will emit, and in turn the faster the motor  36  will operate. Accordingly, one of the many advantages of the present invention is the ability to initiate operation of the motor  36  by slowly retracting the shaft  29 , and to operate the motor  36  at variable speeds depending on how far inward the shaft  29  is retracted. This introduces a novel approach to the operation of such devices. 
   Alternatively, the motor  36  of the rotary tool  10  can be variably driven by manipulation of the lever  20 . Referring now to  FIGS. 4A and 4B , an alternative embodiment of the invention is disclosed. Here a lever field device  76 , preferably a permanent magnet, is disposed within the body of the lever  20 . The lever  20  is hingedly attached to the rotary tool  10  on one of its ends via pins  54  inserted into ports of the end cap  18 . A corresponding lever VVD  78  is preferably positioned within a groove  47  formed on the outer surface of a wiring shell  46 . Similar to the chuck assembly  28 , a spring  21  is included to urge the free end of the lever  20  outward away from the body of the rotary tool  10 . When an external force is applied to the lever  21 , such as by an operator, urging the lever  21  toward the body of the rotary tool  10 , the lever field device  76  should begin to approach the proximity of the lever VVD  78 . Also similar to the operation of the chuck assembly VVD  73 , the lever VVD  78  will begin to emit voltage to the motor  36  as the lever field device  76  approaches it. Thus the motor  36  can be manipulated by depressing the lever  21  in much the same manner as it is manipulated by retracting the shaft  29 . Optionally, the lever  21  can be replaced by a pistol grip assembly  61 , where the pistol grip assembly  61  comprises a handle  65 , a base  69 , and trigger  72 . The handle  65  provides a grip for the users hand. The base  69  is secured to the handle  65  and securable to the body  12  of the rotary tool  10 . The trigger  72  can be hingedly attached to the base  69  and include a trigger field device  74  disposed thereon such that when the trigger  72  is depressed the trigger field device  74  is moved towards the body  12 . The pistol grip assembly  61  should be secured to the body  12  such that the trigger field device  74  will be proximate to the lever VVD  78  when the trigger  72  is depressed. Thus the rotary tool  10  can be actuated by depressing the trigger  72 . 
   Two or more selector buttons ( 14  and  16 ) can optionally be provided with the present invention to enhance the flexibility of the rotary tool  10  functions. Each selector button ( 14  and  16 ) can contain a field device, such as a permanent magnet within. When assembled, the selector buttons ( 14  and  16 ) should be aligned with selector button VVDS ( 70  and  71 ) disposed within the groove  47 . Springs  15  should be included with each selector button ( 14  and  16 ) to urge the buttons outward from the body  12  of the rotary tool  10  absent a force pushing the buttons inward. By programming the associated controller  80 , actuation of the selector buttons ( 14  and  16 ) inward can vary the function of the rotary tool  10 . For example, the controller  80  can be programmed such that inwardly pressing the first selector button  14  will toggle the polarity of the voltage delivered to the motor  36  thereby reversing the rotational direction of the chuck assembly  28 . Additional options include the requirement that the buttons ( 14  and  16 ) be depressed twice, similar to the operation of a mouse of a personal computer, before the requested function occur. The selector buttons ( 14  and  16 ) can be programmed to initiate or control any number of external devices or process either directly or indirectly related to the operation of the tool. More commonly the selector buttons ( 14  and  16 ) can be used to control the direction of rotation of the tool as well as changing preprogrammed tool set points or parameter sets. It is believed that the programming of the associated controller  80  can be accomplished by those skilled in the art without undue experimentation. 
   While standard wiring or circuit boards could be used, it is preferred that the circuitry of the rotary tool be included on a flex circuit  44 . The flex circuit  33  can provide a way to conduct power to drive the motor  36  and provide wiring to conduct control commands as well. As is well known, the flex circuit  44  can be comprised of a flexible resin like material, as such the flex circuit  44  can be tailored to fit within the present invention while consuming a minimum amount of space within the rotary tool  10 . Further, the illumination LEDS  58 , the indication LEDS  62 , and lever and selector button VVDS ( 70 ,  71 , and  78 ) can be situated directly on the flex circuit  44 . Design of an appropriate flex circuit  44  for use with the present invention is well within the capabilities of those skilled in the art. 
   A memory chip should be included with the rotary tool  10  preferably included with the flex circuit  44 . During final assembly and calibration of the tool, the memory chip is programmed at least with identification, calibration, and operating conditions desired by the rotary tool  10 . The information can include the model number of the specific rotary tool  10 , serial number, date of manufacture, date of calibration, maximum speed and maximum torque that the rotary tool  10  can attain, the calibration value, the motor angle counter per tool output revolution (this describes the gear ratio), and other useful operating parameters. Operation of the system requires constant real-time communication with a tool controller  80 . Programmed within the tool controller  80  are the operating parameters for the specific rotary tool  10  being used. During use the tool controller  80  interrogates the memory chip within the specific rotary tool  10  to ensure that the specific tool is capable of performing the intended task. If the tool is capable of performing the task at hand, the controller will allow the specific rotary tool  10  to be operated; otherwise the controller  80  will not activate the tool. This interrogation happens upon power up or when the specific rotary tool  10  is first connected to the controller  80 . The controller can be programmed with a lap top computer using a graphic user interface under the Windows operating system. 
   Once the rotary tool  10  has been assembled, including the addition of the programmed memory chip, the rotary tool  10  can be connected to the controller  80  via a cable  82  and the interrogation step is initiated. As noted above, as soon as the controller  80  determines that the rotary tool  10  is adequate to carry out the programmed function it can then provide power to the rotary tool  10 . Upon being powered up, the rotary tool  10  is ready for use. As is well known, the rotary tool  10  is used by inserting a fitting into the socket  31 , then coupling the fitting with the fastener that is to be driven. The rotary tool  10  can be activated in either a push to start mode, or by depressing the lever  20 . 
   Activation by the push to start mode includes the step of first inserting the fastener where it is to be fastened. For example, if the fastener is a threaded screw, in the push to start mode the screw will be inserted into the hole (threaded or unthreaded) where it is to be secured. Then a force can be applied by the operator to the rear end of the rotary tool  10  that in turn pinches the screw between the fitting and the hole. As long as this force applied by the operator exceeds the spring constant of the spring  42 , the shaft  29  will be retracted within the collar  35 . As previously noted when the shaft is retracted within the collar  36 , the field device  34  is located proximate to the chuck assembly VVD  73 —as is illustrated in  FIG. 3B . As previously noted, when the field device  34  approaches the chuck assembly VVD  73 , voltage is emitted from the chuck assembly VVD  73  that in turn begins to drive the motor  36 . Driving the motor  36  produces rotation of the chuck assembly  28  via the gear box  38  and output shaft  42 . Rotation of the chuck assembly  28  can be used to drive the fastener into securing engagement with the associated hole by the transfer of rotational force from the chuck assembly  28  to the fastener. 
   Alternatively, the rotary tool  10  can be operated by depressing the lever  20  up against the body  12  of the rotary tool  10 . In the embodiment of the invention in  FIGS. 4A and 4B  a lever field device  76  is shown disposed within the lever  20 . As the lever  20  is depressed towards the body, the lever field device  76  approaches the lever VVD  78 . In the same manner as the push to start mode, the lever VVD  78  begins to emit a voltage whose magnitude is in relation to the strength of the magnetic field applied to it by the lever field device  76 . The voltage emitted by the lever VVD  78  can then be applied to driver the motor  36  where the magnitude of the voltage emitted by the lever VVD  78  directly corresponds to the rotational speed of the motor  36 . 
   The push to start and throttle lever can either be used individually or in combination with each other. There are however instances where they are useful in combination. One can be used as an interlock for the other. It can be configured so that the throttle lever has to be fully depressed before the push to start can be activated. This configuration prevents operation of the tool before the operator has a good grip on it. Conversely it can be configured so that the push to start has to be fully depressed before the throttle can be activated. This configuration prevents the rotation of the tool before sufficient axial load is applied to the fastener as in the case of a self tapping screw. In the case of automated operation in a fixture, the push to start can be used as a form of presence detection. 
   During the time the rotary tool  10  is driving the fastener (either by the push to start mode or by depressing the lever  20 ), the magnitude of the torque delivered to the fastener by the rotary tool  10  is measured by the at least one strain gage  85  disposed within the flexure  25 . The strain gage bridge produces an analog output that is continuously monitored during tool operation. The strain gages should be arranged in such a fashion as to be only sensitive to torsion along the axis of the flexure  25 . Each strain gage  85  has two elements that are oriented 90 degrees to each other and 45 degrees to the axis of the flexure  25 . There are four gages arrayed around the circumference of the flexure in 90° intervals. Under torsion the strain gages  85  will unbalance the Wheatstone bridge therefore producing an output. Under bending, compression, or tension the loads will cancel therefore maintaining a balanced bridge and producing little or no output. The torque value measured by the at least one strain gage  85  is uploaded to the controller  80  as the controller  80  interrogates data from the rotary tool  10 . Thus, a real time measurement of the torque applied to the fastener can be obtained by the controller  80  through its constant monitoring of the at least one strain gage  85 . Further, the controller  80  can be programmed to instantaneously deactivate the rotary tool  10  when the torque measured by the at least one strain gage  85  matches the shut off torque stored in the controller  80 . More specifically, when the torque as measured by the strain gate  85  controller  80  combination reaches the preselected torque, the controller  80  immediately and actively stops rotation of the tool, thus ensuring that the fastener being secured by the tool is not over tightened. The braking or stopping of the tool is accomplished through the use of plug reversing and dynamic braking. Plug reversing involves applying full reverse power to the motor  36  until the strain gage  85  and controller  80  senses zero torque. Dynamic braking takes advantage of the fact that a motor  36  is also a generator. By shorting the power leads of the motor  36  to each other, the effect is to force the motor  36  to resist its own rotation in proportion to its rotational velocity. Therefore, one of the many advantages realized by the present invention is the ability to precisely tighten fasteners exactly to a desired torque without the danger of over or undertightening a fastener. This advantage is due in part to the real time monitoring of torque and the instantaneous response of the controller  80  actively deactivating the rotary tool  10 . 
   The controller can be programmed with a target torque and speed. Optionally the controller can be set to run the rotary tool  10  at two different speeds. The first speed would be relatively high and would run until a selected torque, which is not the target torque, is reached. The second, or downshift speed, would run slower and then stop at the target torque. For example if the target torque is 20 in-lbs the controller may be set as follows: Initial speed of 1000 rpm until a down shift torque of 12 in-lbs is reached. Then a down shift speed of 250 rpm until the target torque is reached. Additionally, angle measurement and control can be implemented. Angle control can either be substituted for torque or used in combination with torque. An AND relationship can be established with torque and angle. By setting a torque target of 20 in-lbs and an angle target of 60°, both targets have to be met or exceeded in order to count as a successfully fastened joint. The angle count is started at a threshold torque of perhaps 10 to 20 percent of the target torque. In this case that would be 2 to 4 in-lbs. Other parameters can be set to form upper and lower torque and angle limits around the targets. For example with a 20 in-lb target the limits may include a torque low limit of 18 in-lbs and a high limit of 22 in-lbs with an angle low limit of 50° with an angle high limit of 70°. These limits are used to form a window around the target for the purposes of establishing the criteria for a properly torqued fastener. If the angle is to low before achieving the target torque then the fastener has likely cross threaded. If the angle is to high then the fastener has likely stripped, broken or was not present. 
   In a preferred embodiment, the dimensions of the present invention enable it to be used by an operator with a single hand thus being a hand held device. Accordingly the dimensions of the rotary tool  10  should be in the range of from 7–9 inches in length and from about 1–2 inches in diameter. 
   EXAMPLE 
   In an exemplary embodiment of the present invention the motor  36  is coupled to a gear box  38  comprised of two gear stages, where the two stages provide a conversion of speed to torque. To maximize torque/velocity conversion while minimizing space, the preferred gear system is a planetary gear system. In this system the first stage sun gear is attached to the motor output shaft and engages a series of three planetary gears. The planetary gears are all attached to a planet carrier, from which extends a second sun gear into the next planetary gear stage. The output shaft of the second gear stage, which has a spline gear formed thereon, mates with the output drive. It is preferred that the gearboxes be in a sealed oil gearbox. Sealing the gearbox eliminates gear maintenance, helps keep the gears clean, and protects the gears from foreign matter. The light oil in lieu of a more viscous lubricant, such as grease, greatly enhances the efficiency of torque transmission. The preferred lubrication for this configuration provides a balance of good high-pressure lubricity, low viscosity as compared to conventional power tool greases, and enough tackiness to require only 1 milliliter of oil therefore greatly reducing viscous shear. 
   With regard to the field device  34  disposed on the shaft  29 , in the preferred embodiment the field device  34  is a ring magnet that is plastic injection molded using permanent magnet particles suspended in Nylon. This configuration provides relatively high field density combined with low cost. Further, the ring magnet should be radially magnetized, the outer diameter of the ring magnet is magnetized as a north pole and the inner diameter is oppositely polarized as entirely all south pole. However, the inner ring could be magnetized as all north pole and the outer diameter could be magnetized as all south pole. This is done so that the output of the Hall sensor within the chuck assembly VVD  73  stays consistent regardless of the rotational orientation of the shaft  29 . It is preferred that the Hall output vary as a result of axial movement only. If the ring magnet were magnetized with alternating poles on the outside diameter, the chuck assembly  28  would stop rotating as the poles reversed. All the gears are made from medium-carbon steel selected because of its hardness and heat-treating properties. Medium-carbon steel is also used in the planet carriers. The gear axles are made from a high-carbon steel that is a high strength gear material with excellent bending fatigue properties. 
   Some of the advantages realized by the present invention include a high degree of reliability and durability. The operating limit of many fastening tools before failure is about 500,000 cycles, in fact tools that are capable of operating up to 1,000,000 cycles without failure are considered very durable. In contrast the present invention has been found to operate in excess of 5,000,000 cycles without failure, which greatly exceeds the durability expectations of such a tool. Further, the present invention is also capable of this high number of cycles when subjected to high duty cycle applications. That is when an operating process is being repeated very quickly with many cycles per hour. Additionally, the performance of a gear box  38  produced in accordance with the specifications of this application is superior to many other gear boxes used for similar applications. For example, similar type gear boxes generally have a maximum operation rotational speed at up to 7000–8000 revolutions per minute (rpm), whereas the gear box  38  of the present invention is capable of rotational speeds up to 50,000 rpm. 
   The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. For example, the push to start feature can be physically disabled. Also, all four torque capacities can optionally be available in fixture mount configurations. A different front end cap is supplied with the tool to allow for easier and more reliable mounting of the tool in fixtured applications. Instead of a tapered end cap with headlights, a threaded end cap with a shoulder is provided including two different styles of mounting flanges. The fixture mounted configuration allows for the minimization of center to center mounting distances. In effect the tools can be mounted on 1.125″ centers 1.125″ is the diameter of the tool. This is important when fasteners are located very close to each other. This is of primary concern in automated applications where there is no human interaction or when multiple tools are mounted in combination with each other in a hand operated power head. Further, the variable voltage device can be any device that responds to some external stimulus, such as voltage, current, pressure, or magnetic, or that switches at a threshold of stimulus. The variable voltage device can be selected from items such as a linear response device, or a digital response device. 
   These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.