Abstract:
A method of controlling a woodworking machine having a movable cutting tool. The method monitors a signal for a change indicative of a dangerous condition between the cutting tool and a person, senses movement of the cutting tool, and performs an action to mitigate the dangerous condition when the signal change and movement of the cutting tool are both detected. A woodworking machine including a detection system adapted to detect a dangerous condition between a person and a working portion of the machine and then to perform some action to mitigate the dangerous condition is also disclosed. A motion detection system is adapted to detect motion of the working portion and to disable the reaction system when the working portion is not moving.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/661,766, filed Mar. 22, 2010, issuing as U.S. Pat. No. 8,051,759 on Nov. 8, 2011, which in turn is a continuation of U.S. patent application Ser. No. 11/810,196, filed Jun. 4, 2007, issuing as U.S. Pat. No. 7,681,479 on Mar. 23, 2010, which in turn is a continuation of U.S. patent application Ser. No. 09/929,234, filed Aug. 13, 2001, issued as U.S. Pat. No. 7,225,712 on Jun. 5, 2007, which claims the benefit of and priority from the following U.S. Provisional Patent Applications: Ser. No. 60/225,056, filed Aug. 14, 2000, Ser. No. 60/225,057, filed Aug. 14, 2000, Ser. No. 60/225,058, filed Aug. 14, 2000, Ser. No. 60/225,059, filed Aug. 14, 2000, Ser. No. 60/225,089, filed Aug. 14, 2000, Ser. No. 60/225,094, filed Aug. 14, 2000, Ser. No. 60/225,169, filed Aug. 14, 2000, Ser. No. 60/225,170, filed Aug. 14, 2000, Ser. No. 60/225,200, filed Aug. 14, 2000, Ser. No. 60/225,201, filed Aug. 14, 2000, Ser. No. 60/225,206, filed Aug. 14, 2000, Ser. No. 60/225,210, filed Aug. 14, 2000, Ser. No. 60/225,211, filed Aug. 14, 2000, and Ser. No. 60/225,212, filed Aug. 14, 2000. The identified patents and applications are all incorporated by reference in their entireties. 
     
    
     FIELD  
       [0002]    The present invention relates to safety systems, and more particularly to a high-speed safety system for use on power equipment. 
       BACKGROUND  
       [0003]    Beginning with the industrial revolution and continuing to the present, mechanized equipment has allowed workers to produce goods with greater speed and less effort than possible with manually-powered tools. Unfortunately, the power and high operating speeds of mechanized equipment creates a risk for those operating such machinery. Each year thousands of people are maimed or killed by accidents involving power equipment. 
         [0004]    As might be expected, many systems have been developed to minimize the risk of injury when using power equipment. Probably the most common safety feature is a guard that physically blocks an operator from making contact with dangerous components of machinery, such as belts, shafts or blades. In many cases, guards are effective to reduce the risk of injury, however, there are many instances where the nature of the operations to be performed precludes using a guard that completely blocks access to hazardous machine parts. 
         [0005]    Various systems have been proposed to prevent accidental injury where guards cannot effectively be employed. For instance, U.S. Pat. Nos. 941,726, 2,978,084, 3,011,610, 3,047,116, 4,195,722 and 4,321,841, the disclosures of which are incorporated herein by reference, all disclose safety systems for use with power presses. These systems utilize cables attached to the wrists of the operator that either pull back a user&#39;s hands from the work zone upon operation or prevent operation until the user&#39;s hands are outside the danger zone. U.S. Pat. Nos. 3,953,770, 4,075,961, 4,470,046, 4,532,501 and 5,212,621, the disclosures of which are incorporated herein by reference, disclose radio-frequency safety systems which utilize radio-frequency signals to detect the presence of a user&#39;s hand in a dangerous area of the machine and thereupon prevent or interrupt operation of the machine. 
         [0006]    U.S. Pat. Nos. 4,959,909, 5,025,175, 5,122,091, 5,198,702, 5,201,684, 5,272,946, and 5,510,685 disclose safety systems for use with meat-skinning equipment, and are incorporated herein by reference. These systems interrupt or reverse power to the motor, or disengage a clutch, upon contact with a user&#39;s hand by any dangerous portion of the machine. Typically, contact between the user and the machine is detected by monitoring for electrical contact between a fine wire mesh in a glove worn by the user and some metal component in the dangerous area of the machine. 
         [0007]    U.S. Pat. Nos. 3,785,230 and 4,026,177, the disclosures of which are herein incorporated by reference, disclose a safety system for use on circular saws to stop the blade when a user&#39;s hand approaches the blade. The system uses the blade as an antenna in an electromagnetic proximity detector to detect the approach of a user&#39;s hand prior to actual contact with the blade. Upon detection of a user&#39;s hand, the system engages a brake using a standard solenoid. U.S. Pat. No. 4,117,752, which is herein incorporated by reference, discloses a similar braking system for use with a band saw, where the brake is triggered by actual contact between the user&#39;s hand and the blade. 
         [0008]    It is often necessary for an equipment operator to touch the blade or other cutting device of power equipment when the blade or device is not moving (e.g., to adjust the blade, perform equipment maintenance, etc.). Thus, it would be desirable to disable the safety system when the blade is not moving since there is no danger to the user from contact with the blade. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic block diagram of a machine with a fast-acting safety system according to the present invention. 
           [0010]      FIG. 2  is a schematic diagram of an exemplary safety system in the context of a machine having a circular blade. 
           [0011]      FIG. 3  is a partial cross-section view of an exemplary magnetic sensor assembly according to the present invention, where the arbor is not in cross-sectional view. 
           [0012]      FIG. 4  is a schematic diagram of an exemplary circuit according to the present invention for use with a magnetic sensor assembly. 
           [0013]      FIG. 5  is a schematic view of an exemplary EMF sensor assembly according to the present invention. 
           [0014]      FIG. 6  is a partial cross-section view of an exemplary optical sensor assembly according to the present invention, where the arbor is not in cross-sectional view. 
           [0015]      FIG. 7  is a side elevation of an alternative optical sensor assembly according to the present invention. 
           [0016]      FIG. 8  is a cross-section view of the alternative optical sensor assembly of  FIG. 7 , taken generally along the line  8 - 8 . 
           [0017]      FIG. 9  is a schematic diagram of an exemplary circuit according to the present invention for use with an optical sensor assembly. 
           [0018]      FIG. 10  is a partial cross-section view of an exemplary electrical sensor assembly according to the present invention, where the arbor is not in cross-sectional view. 
           [0019]      FIG. 11  is a schematic side elevation of an alternative electrical sensor assembly according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    A machine is shown schematically in  FIG. 1  and indicated generally at  10 . Machine  10  may be any of a variety of different machines adapted for cutting workpieces, such as wood, including a table saw, miter saw (chop saw), radial arm saw, circular saw, band saw, jointer, planer, etc. Machine  10  includes an operative structure  12  having a cutting tool  14  and a motor assembly  16  adapted to drive the cutting tool. Machine  10  also includes a safety system  18  configured to minimize the potential of a serious injury to a person using machine  10 . Safety system  18  is adapted to detect the occurrence of one or more dangerous conditions during use of machine  10 . If such a dangerous condition is detected, safety system  18  is adapted to engage operative structure  12  to limit any injury to the user caused by the dangerous condition. 
         [0021]    Machine  10  also includes a suitable power source  20  to provide power to operative structure  12  and safety system  18 . Power source  20  may be an external power source such as line current, or an internal power source such as a battery. Alternatively, power source  20  may include a combination of both external and internal power sources. Furthermore, power source  20  may include two or more separate power sources, each adapted to power different portions of machine  10 . 
         [0022]    It will be appreciated that operative structure  12  may take any one of many different forms, depending on the type of machine  10 . For example, operative structure  12  may include a stationary housing configured to support motor assembly  16  in driving engagement with cutting tool  14 . Alternatively, operative structure  12  may include a movable structure configured to carry cutting tool  14  between multiple operating positions. As a further alternative, operative structure  12  may include one or more transport mechanisms adapted to convey a workpiece toward and/or away from cutting tool  14 . 
         [0023]    Motor assembly  16  includes one or more motors adapted to drive cutting tool  14 . The motors may be either directly or indirectly coupled to the cutting tool, and may also be adapted to drive workpiece transport mechanisms. Cutting tool  14  typically includes one or more blades or other suitable cutting implements that are adapted to cut or remove portions from the workpieces. The particular form of cutting tool  14  will vary depending upon the various embodiments of machine  10 . For example, in table saws, miter saws, circular saws and radial arm saws, cutting tool  14  will typically include one or more circular rotating blades having a plurality of teeth disposed along the perimetrical edge of the blade. For a jointer or planer, the cutting tool typically includes a plurality of radially spaced-apart blades. For a band saw, the cutting tool includes an elongate, circuitous tooth-edged band. 
         [0024]    Safety system  18  includes a detection subsystem  22 , a reaction subsystem  24  and a control subsystem  26 . Control subsystem  26  may be adapted to receive inputs from a variety of sources including detection subsystem  22 , reaction subsystem  24 , operative structure  12  and motor assembly  16 . The control subsystem may also include one or more sensors adapted to monitor selected parameters of machine  10 . In addition, control subsystem  26  typically includes one or more instruments operable by a user to control the machine. The control subsystem is configured to control machine  10  in response to the inputs it receives. 
         [0025]    Detection subsystem  22  is configured to detect one or more dangerous, or triggering, conditions during use of machine  10 . For example, the detection subsystem may be configured to detect that a portion of the user&#39;s body is dangerously close to, or in contact with, a portion of cutting tool  14 . As another example, the detection subsystem may be configured to detect the rapid movement of a workpiece due to kickback by the cutting tool, as is described in U.S. Provisional Patent Application Serial No.  60 / 182 , 866 , the disclosure of which is herein incorporated by reference. In some embodiments, detection subsystem  22  may inform control subsystem  26  of the dangerous condition, which then activates reaction subsystem  24 . In other embodiments, the detection subsystem may be adapted to activate the reaction subsystem directly. 
         [0026]    Once activated in response to a dangerous condition, reaction subsystem  24  is configured to engage operative structure  12  quickly to prevent serious injury to the user. It will be appreciated that the particular action to be taken by reaction subsystem  24  will vary depending on the type of machine  10  and/or the dangerous condition that is detected. For example, reaction subsystem  24  may be configured to do one or more of the following: stop the movement of cutting tool  14 , disconnect motor assembly  16  from power source  20 , place a barrier between the cutting tool and the user, or retract the cutting tool from its operating position, etc. The reaction subsystem may be configured to take a combination of steps to protect the user from serious injury. Placement of a barrier between the cutting tool and teeth is described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,206, entitled “Cutting Tool Safety System,” filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is herein incorporated by reference. Retraction of the cutting tool from its operating position is described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,089, entitled “Retraction System For Use In Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is herein incorporated by reference. 
         [0027]    The configuration of reaction subsystem  24  typically will vary depending on which action(s) are taken. In the exemplary embodiment depicted in  FIG. 1 , reaction subsystem  24  is configured to stop the movement of cutting tool  14  and includes a brake mechanism  28 , a biasing mechanism  30 , a restraining mechanism  32 , and a release mechanism  34 . Brake mechanism  28  is adapted to engage operative structure  12  under the urging of biasing mechanism  30 . During normal operation of machine  10 , restraining mechanism  32  holds the brake mechanism out of engagement with the operative structure. However, upon receipt of an activation signal by reaction subsystem  24 , the brake mechanism is released from the restraining mechanism by release mechanism  34 , whereupon, the brake mechanism quickly engages at least a portion of the operative structure to bring the cutting tool to a stop. 
         [0028]    It will be appreciated by those of skill in the art that the exemplary embodiment depicted in  FIG. 1  and described above may be implemented in a variety of ways depending on the type and configuration of operative structure  12 . Turning attention to  FIG. 2 , one example of the many possible implementations of safety system  18  is shown. System  18  is configured to engage an operative structure having a cutting tool in the form of a circular blade  40  mounted on a rotating shaft or arbor  42 . Blade  40  includes a plurality of cutting teeth (not shown) disposed around the outer edge of the blade. As described in more detail below, braking mechanism  28  is adapted to engage the teeth of blade  40  and stop the rotation of the blade. U.S. Provisional Patent Application Ser. No. 60/225,210, entitled “Translation Stop For Use In Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is herein incorporated by reference, describes other systems for stopping the movement of the cutting tool. U.S. Provisional Patent Application Ser. No. 60/225,058, entitled “Table Saw With Improved Safety System,” filed Aug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser. No. 60/225,057, entitled “Miter Saw With Improved Safety System,” filed Aug. 14, 2000 by SD3, LLC, the disclosures of which are herein incorporated by reference, describe safety system  18  in the context of particular types of machines  10 . 
         [0029]    In the exemplary implementation, detection subsystem  22  is adapted to detect the dangerous condition of the user coming into contact with blade  40 . The detection subsystem includes a sensor assembly, such as contact detection plates  44  and  46 , capacitively coupled to blade  40  to detect any contact between the user&#39;s body and the blade. Typically, the blade, or some larger portion of cutting tool  14  is electrically isolated from the remainder of machine  10 . Alternatively, detection subsystem  22  may include a different sensor assembly configured to detect contact in other ways, such as optically, resistively, etc. In any event, the detection subsystem is adapted to transmit a signal to control subsystem  26  when contact between the user and the blade is detected. Various exemplary embodiments and implementations of detection subsystem  22  are described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,200, entitled “Contact Detection System For Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser. No. 60/225,211, entitled “Apparatus And Method For Detecting Dangerous Conditions In Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, the disclosures of which are herein incorporated by reference. 
         [0030]    Control subsystem  26  includes one or more instruments  48  that are operable by a user to control the motion of blade  40 . Instruments  48  may include start/stop switches, speed controls, direction controls, etc. Control subsystem  26  also includes a logic controller  50  connected to receive the user&#39;s inputs via instruments  48 . Logic controller  50  is also connected to receive a contact detection signal from detection subsystem  22 . Further, the logic controller may be configured to receive inputs from other sources (not shown) such as blade motion sensors, workpiece sensors, etc. In any event, the logic controller is configured to control operative structure  12  in response to the user&#39;s inputs through instruments  48 . However, upon receipt of a contact detection signal from detection subsystem  22 , the logic controller overrides the control inputs from the user and activates reaction subsystem  24  to stop the motion of the blade. Various exemplary embodiments and implementations of control subsystem  26  are described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,059, entitled “Logic Control For Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC, the disclosure of which is herein incorporated by reference. 
         [0031]    In the exemplary implementation, brake mechanism  28  includes a pawl  60  mounted adjacent the edge of blade  40  and selectively moveable to engage and grip the teeth of the blade. Pawl  60  may be constructed of any suitable material adapted to engage and stop the blade. As one example, the pawl may be constructed of a relatively high strength thermoplastic material such as polycarbonate, ultrahigh molecular weight polyethylene (UHMW) or Acrylonitrile Butadiene Styrene (ABS), etc., or a metal such as aluminum, etc. It will be appreciated that the construction of pawl  60  will vary depending on the configuration of blade  40 . In any event, the pawl is urged into the blade by a biasing mechanism in the form of a spring  66 . In the illustrative embodiment shown in  FIG. 2 , pawl  60  is pivoted into the teeth of blade  40 . It should be understood that sliding or rotary movement of pawl  60  may also be used. The spring is adapted to urge pawl  60  into the teeth of the blade with sufficient force to grip the blade and quickly bring it to a stop. 
         [0032]    The pawl is held away from the edge of the blade by a restraining mechanism in the form of a fusible member  70 . The fusible member is constructed of a suitable material adapted to restrain the pawl against the bias of spring  66 , and also adapted to melt under a determined electrical current density. Examples of suitable materials for fusible member  70  include NiChrome wire, stainless steel wire, etc. The fusible member is connected between the pawl and a contact mount  72 . Preferably, fusible member  70  holds the pawl relatively close to the edge of the blade to reduce the distance the pawl must travel to engage the blade. Positioning the pawl relatively close to the edge of the blade reduces the time required for the pawl to engage and stop the blade. Typically, the pawl is held approximately 1/32-inch to ¼-inch from the edge of the blade by fusible member  70 , however other pawl-to-blade spacings may also be used within the scope of the invention. 
         [0033]    Pawl  60  is released from its unactuated, or cocked, position to engage blade  40  by a release mechanism in the form of a firing subsystem  76 . The firing subsystem is coupled to contact mount  72 , and is configured to melt fusible member  70  by passing a surge of electrical current through the fusible member. Firing subsystem  76  is coupled to logic controller  50  and activated by a signal from the logic controller. When the logic controller receives a contact detection signal from detection subsystem  22 , the logic controller sends an activation signal to firing subsystem  76 , which melts fusible member  70 , thereby releasing the pawl to stop the blade. Various exemplary embodiments and implementations of reaction subsystem  24  are described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,056, entitled “Firing Subsystem For Use In Fast Acting Safety System,” filed Aug. 14, 2000 by SD3, LLC, U.S. Provisional Patent Application Ser. No. 60/225,170, entitled “Spring-Biased Brake Mechanism for Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser. No. 60/225,169, entitled “Brake Mechanism For Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, the disclosures of which are herein incorporated by reference. 
         [0034]    It will be appreciated that activation of the brake mechanism will require the replacement of one or more portions of safety system  18 . For example, pawl  60  and fusible member  70  typically must be replaced before the safety system is ready to be used again. Thus, it may be desirable to construct one or more portions of safety system  18  in a cartridge that can be easily replaced. For example, in the exemplary implementation depicted in  FIG. 2 , safety system  18  includes a replaceable cartridge  80  having a housing  82 . Pawl  60 , spring  66 , fusible member  70  and contact mount  72  are all mounted within housing  82 . Alternatively, other portions of safety system  18  may be mounted within the housing. In any event, after the reaction system has been activated, the safety system can be reset by replacing cartridge  80 . The portions of safety system  18  not mounted within the cartridge may be replaced separately or reused as appropriate. Various exemplary embodiments and implementations of a safety system using a replaceable cartridge are described in more detail in U.S. Provisional Patent Application Ser. No. 60/225,201, entitled “Replaceable Brake Mechanism For Power Equipment,” filed Aug. 14, 2000 by SD3, LLC, and U.S. Provisional Patent Application Ser. No. 60/225,212, entitled “Brake Positioning System,” filed Aug. 14, 2000 by SD3, LLC, the disclosures of which are herein incorporated by reference. 
         [0035]    While one particular implementation of safety system  18  has been described, it will be appreciated that many variations and modifications are possible within the scope of the invention. Many such variations and modifications are described in U.S. Provisional Patent Application Ser. No. 60/182,866, filed Feb. 16, 2000 and U.S. Provisional Patent Application Ser. No. 60/157,340, filed Oct. 1, 1999, the disclosures of which are herein incorporated by reference. 
         [0036]    As mentioned above, safety system  18  may include a sensor or sensor assembly for detecting motion of the blade or cutting tool. The sensor assembly typically is coupled to send a signal to logic controller  50  indicating whether the blade is in motion. The logic controller may be configured to respond differently to the detection of a dangerous condition based on whether the blade is moving. For example, it is often necessary for a user of machine  10  to touch blade  40  when preparing the machine for use, and when installing or removing the blade. Usually, the user would disconnect all power from machine  10  while performing such operations. However, in the event that the user neglects to disconnect the machine from power source  20  before touching the blade, logic controller  50  would receive a contact detection signal from detection subsystem  22 . If safety system  18  includes a blade motion sensor, then logic controller  50  may be configured not to actuate firing subsystem  76  when the blade is not moving. Instead, the logic controller may be configured to take one or more other actions such as disabling motor assembly  16 , sounding an alarm, displaying an error, etc. Alternatively, the logic controller may be configured to take no action if contact is detected while the blade is not moving. 
         [0037]    In addition to detecting whether the blade is moving, safety system  18  may also be configured to determine the speed at which the blade is moving. This allows the logic controller to distinguish between rapid blade movement which could cause injury to the user, and slow blade movement which generally would not cause injury to the user. Thus, for example, a user could move the blade by hand without actuating firing subsystem  76 . In some embodiments, the blade motion sensor may be configured to determine relative blade speed. In alternative embodiments, logic controller  50  may be configured to analyze the signal from the blade motion sensor to determine relative blade speed. 
         [0038]    It will be appreciated that the speed at which a blade is considered likely to cause injury will vary depending on the type of machine  10  and blade  40 . For example, a 14-inch carbide tooth blade on a table saw will cause serious injury at a lower speed than a 5⅜-inch plywood blade on a cordless trim saw. Thus, an embodiment of safety system  18  for use on the table saw may be configured to actuate the firing subsystem only at blade speeds above approximately 10, 25, 60 , or 90 rpm, while an alternative embodiment of safety system  18  for use on the trim saw may be configured to actuate the firing subsystem only at blade speeds above approximately 40, 100, or 240 rpm. 
         [0039]    Alternatively or additionally, the logic controller may be configured to interpret blade motion as being dangerous only when detected during or soon after motor assembly  16  was in operation. In other words, the blade motion detection would only be active while the blade was being moved by the motor assembly and during a relatively brief period afterward while the blade was coasting to a stop. Any blade motion detected at other times would be ignored. 
         [0040]    Safety system  18  may include any of a wide variety of sensor assemblies to detect blade movement. Furthermore, each sensor assembly may be adapted as necessary depending on the particular type of blade  40  and/or the configuration of machine  10 . While several exemplary sensor assemblies are described herein, it will be understood that all methods and mechanisms suitable for automatically detecting the motion of a blade are within the scope of the invention. 
         [0041]    One exemplary embodiment of safety system  18  includes a magnetic sensor assembly  1000  configured to detect movement of the blade. It will be appreciated that the blade movement may be detected by monitoring the blade or any other portion of the safety system that moves with the blade, including the arbor, bearings, motor assembly, arbor pulley, etc. In the exemplary implementation depicted in  FIG. 3 , magnetic sensor assembly  1000  includes a Hall effect sensor  1001  and one or more magnets  1002 . A coil could also be used to detect magnetic field fluctuations from rotation. The magnets are mounted on arbor  42 . Sensor  1001  is mounted and configured to detect blade motion by detecting the movement of the magnets on the arbor. Sensor  1001  may be any suitable Hall effect sensor such as, for example, the sensor available from Micronas Intermetall of San Jose, Calif., under the part no. HAL 114. 
         [0042]    Hall effect sensor  1001  may be mounted adjacent the arbor by any suitable method. In the exemplary implementation, the sensor is mounted in a recessed region  272  of an insulating tube  268 . The insulating tube also supports charge plates  44  and  46 , as is described in more detail in U.S. Provisional Application Ser. No. 60/225,211, entitled “Apparatus and Method for Detecting Dangerous Conditions in Power Equipment,” filed Aug. 14, 2000, by SD3, LLC. The recessed region is disposed at least partially over a hole  273  in charge plate  44 . Alternatively the recessed region may be disposed over a hole  273  in charge plate  46 . In any event, magnet  1002  is disposed on arbor  42  to pass beneath or adjacent hole  273  as the arbor rotates within the insulating tube. Hole  273  allows sensor  1001  to detect the field created by magnet  1002  as it passes. Sensor  1001  includes one or more connector leads  1003  connectable to receive power from, and transmit signals to, logic controller  50 . 
         [0043]    Magnets  1002  may be mounted on the arbor in any suitable fashion. Typically, the magnets are mounted so as not to extend above the surface of the arbor. For example, the magnets may be press-fit and/or glued in a recess formed on the arbor. Alternatively, one or more of the magnets may be mounted to extend above the surface of the arbor. The size and number of magnets  1002  may be varied to control the signal produced by sensor  1001 . In alternative embodiments, magnets  1002  may be mounted at other locations such as an end of arbor  42 , on blade  40 , etc. 
         [0044]    Sensor  1001  may be connected to send signals to logic controller  50  via any suitable circuitry. For example,  FIG. 4  illustrates one exemplary rotation sense circuit  177  adapted to couple the signals from sensor  1001  to logic controller  50 . Those of skill in the art will appreciate that circuit  177  may be modified as needed for a particular application. 
         [0045]    Another example of a suitable method for detecting blade motion is through electromagnetic field (EMF) measurements. As is known to those of skill in the art, when power to an electric motor is shut off, the motor will produce EMF pulses on the input power cables as the motor spins down. Thus, where blade  40  is driven by an electric motor assembly  16 , the blade may be assumed to be in motion whenever an EMF pulse is detected on the power supply cables, as well as whenever power is being supplied to the motor assembly. 
         [0046]    Thus, in another exemplary embodiment depicted in  FIG. 5 , safety system  18  includes an EMF sensor assembly  1005  configured to detect motion of blade  40 . Sensor assembly  1005  includes an EMF detection circuit  1006  disposed in the power supply path between motor assembly  16  and power source  20 . Circuit  1006  is adapted to monitor power cables  1007  which extend between the power source and the motor assembly, and to detect the presence of EMF pulses on the cables. Alternatively, circuit  1006  may be disposed at any other location suitable for detecting EMF pulses from motor assembly  16 . Circuit  1006  may be any circuit or mechanism adapted to detect EMF pulses, such as are known to those of skill in the art. Circuit  1006  is also coupled to logic controller  50 , and adapted to convey a signal to the logic controller indicating the presence and/or absence of EMF pulses on cables  1007 . Optionally, circuit  1006  and/or logic controller  50  may be adapted to analyze the detected EMF emissions, and evaluate the speed of blade  40 . In such case, the logic controller may be configured not to actuate firing subsystem  76  when the speed of the blade is unlikely to cause serious injury to the user. 
         [0047]    In another exemplary embodiment, safety system  18  includes an optical sensor assembly adapted to optically detect movement of blade  40 . Safety system  18  may be configured to optically detect blade motion in a variety of ways. For example, a rotary optical encoder may be coupled to the arbor to detect rotation of the arbor. Any rotary encoder may be used, such as those available from Omron Electronics Inc., of Schaumburg, Ill. Alternatively, other optical sensor assemblies may be used as described below. 
         [0048]    Typically, the optical sensor assembly will be at least partially enclosed to prevent saw dust or other debris from interfering with the detection. One exemplary implementation of an optical sensor assembly is indicated generally at  1010  in  FIG. 6 . Sensor assembly  1010  includes an optical detector  1011  adapted to detect light from an optical source  1012 . Alternatively, plural optical sources and/or plural optical detectors may be used. It will be appreciated that any of a variety of different optical sources may be used which are known to those of skill in the art, including an incandescent or fluorescent bulb, light emitting diode (LED), laser diode, etc. Similarly, any of a variety of different optical detectors may be used which are known to those of skill in the art, including a photodiode, phototransistor, etc. 
         [0049]    In any event, the optical source is arranged so that the signal received at the optical detector when the blade is moving is different than the signal received when the blade is stationary. For example, the source and detector may be arranged so that a signal is received only when the blade is moving, or only when the blade is stationary. Alternatively, source  1012  and detector  1011  may be arranged so that the amount of emitted light that reaches the detector varies when the blade is in motion. 
         [0050]    The implementation depicted in  FIG. 6  uses this latter arrangement. Sensor assembly  1010  includes an LED  1012  mounted in insulating tube  268  to emit light through hole  273  in charge plate  44  or  46 . The light reflects off arbor  42  and is detected by a photodiode  1011  which is also mounted in insulating tube  268  adjacent hole  273 . The arbor includes one or more reduced-reflection regions  1013  adapted to reduce the amount of light reflected to photodiode  1011 . Regions  1013  may be formed by coating the arbor with a light-absorbing coating, roughening the arbor to cause random scattering of the light, etc. In any event, the reduced reflecting regions create a varying signal at the photodiode when the arbor is rotating. In contrast, a constant signal is produced at the photodiode when the arbor is stationary. 
         [0051]    The minimal clearance between arbor  42  and charge plates  44 ,  46  tends to maintain the space between the arbor and the photodiode/LED relatively free of debris which could block the signal. Alternatively, the insulating tube assembly may be sealed in a protective housing (not shown). 
         [0052]    In another alternative implementation depicted in  FIGS. 7 and 8 , optical sensor assembly  1010  includes a barrier member  1014  mounted on the arbor and disposed between photodiode  1011  and LED  1012 . Alternatively, the barrier member may be mounted on any other portion of cutting tool  14  or motor assembly  16  adapted to move with the blade. Barrier member  1014  includes one or more light-transmitting regions or holes  1015 , which may take any desired shape or size. The photodiode and LED are mounted in a support member  1016  attached to an arbor block  250 , and disposed on either side of barrier member  1014 . The photodiode is aligned so that emitted light will pass through holes  1015 . Likewise, the LED is aligned to detect the light which passes through the holes. Thus, as arbor  42  rotates, light from the LED is alternately blocked and transmitted by the barrier member, thereby creating a varying signal at the photodiode. 
         [0053]    Photodiode  1011  and LED  1012  may be connected to any suitable driving circuitry such as are known to those of skill in the art.  FIG. 9  shows one exemplary circuitry for producing an optical signal at LED  1012  and detecting the signal at photodiode  1011 . The particular values of the circuit components and voltage supplies may be selected as desired for a specific application. In any event, the photodiode is coupled to transmit a signal to logic controller  50  to indicate whether blade  40  is moving. 
         [0054]    In another exemplary embodiment, safety system  18  includes an electrical sensor assembly adapted to electrically detect movement of blade  40 . There are numerous methods and mechanisms for electrically detecting blade movement within the scope of the invention. The particular method and/or mechanism selected will typically depend on the specific type and configuration of machine  10 . For example, where charge plate  46  is configured to capacitively detect a signal induced in the blade, any incidental eccentricity in the blade or the blade rotation will cause the capacitance between the blade and charge plate  46  to vary as the blade rotates. As a result, charge plate  46  will detect a varying signal amplitude when the blade is rotating. Thus, a single sensor may be configured to detect both contact with the user and rotation of the blade. Preferably, the incidental variation fluctuation is insufficient in magnitude and/or rate of change to trigger reaction subsystem  24 . 
         [0055]    Rather than rely on incidental eccentricities, safety system  18  may include an exemplary electrical sensor assembly adapted to detect a signal variation caused by a designed eccentricity or non-uniformity in the blade. Alternatively, the sensor assembly may be adapted to detect the signal from an eccentricity in some portion of cutting tool  14  that moves with the blade and is electrically coupled to the blade. One exemplary implementation of such a sensor assembly is indicated generally at  1020  in  FIG. 10 . Sensor assembly  1020  includes a detection electrode  1021  capacitively coupled to detect an electrical signal on arbor  42 . Electrode  1021  may be mounted in any suitable fashion to provide electrical insulation from arbor  42  as well as the remainder of cutting tool  14  and machine  10 . In the exemplary implementation, electrode  1021  is mounted in insulating tube  268  and arranged to extend to a point closely adjacent the arbor between charge plates  44  and  46 . Sensor assembly  1020  also includes one or more eccentricities  1022  disposed on the arbor and substantially aligned with electrode  1021  so as to pass by the electrode as the arbor rotates. 
         [0056]    It will be appreciated that eccentricities  1022  may be configured in any desired quantity, size, shape or form adapted to cause a variation in the capacitance between the arbor and the electrode as the arbor rotates. In the exemplary implementation, eccentricities  1022  take the form of beveled regions formed on the surface of arbor  42 . Thus, the space between the electrode and the arbor is greater (and therefore the capacitance is less) when an eccentricity is positioned beneath the electrode than when an eccentricity is not positioned beneath the electrode. Alternatively, eccentricities  1022  may take other forms adapted to vary the capacitance between the arbor and electrode, including raised regions, dielectric pads, etc. In any event, if an electrical signal is induced in the arbor (e.g., by charge plate  44  of contact detection subsystem  22 ), then electrode  1021  will detect variations in that signal if the arbor is rotating. Conversely, the electrode will detect no variations in the signal if the arbor is stationary. 
         [0057]    Turning attention now to  FIG. 11 , another exemplary implementation of electrical sensor assembly  1020  is shown in which electrode  1021  is disposed adjacent the teeth  1023  of blade  40 . Electrode  1021  may be mounted on arbor block  250  or any other suitable portion of machine  10 . Additionally, the electrode may be positioned at the side of the blade (as shown in  FIG. 11 ) or at the perimeter of the blade facing in toward the arbor. The size, shape and position of the electrode may vary depending on the position and size of teeth  1023 . In any event, as teeth  1023  pass by electrode  1021 , the capacitance between the blade and the electrode varies, thereby varying the amplitude of the signal detected by the electrode. Alternatively, a plurality of electrodes may be positioned at various points adjacent the teeth so that blade motion would be detected by modulations in the relative signal amplitudes at the electrodes. Such an alternative detection mechanism may also be used with other implementations of sensor assembly  1020 . 
         [0058]    While a few exemplary magnetic, EMF, optical and electrical sensor assemblies have been described for detecting blade motion, it will be appreciated that many modifications and variations to such sensor assemblies are included within the scope of the invention. Furthermore, safety system  18  may include other types of motion detection sensors such as mechanical sensors, sonic and ultra-sonic sensors, etc. In any event, the invention provides effective and reliable means for discriminating between conditions which are, and are not, likely to cause injury to a user of power machinery. 
         [0059]    It is believed that the disclosure set forth above encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the inventions includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. No single feature, function, element or property of the disclosed embodiments is essential to all of the disclosed inventions. Similarly, where the claims recite “a” or “a first” element or the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. 
         [0060]    It is believed that the following claims particularly point out certain combinations and subcombinations that are directed to one of the disclosed inventions and are novel and non-obvious. Inventions embodied in other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure.