Patent Publication Number: US-7900541-B2

Title: Detection system for power equipment

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/487,717, filed Jul. 17, 2006, issuing as U.S. Pat. No. 7,421,315, on Sep. 2, 2008, which is a continuation of U.S. patent application Ser. No. 10/292,607, filed Nov. 12, 2002, issued as U.S. Pat. No. 7,077,039 on Jul. 18, 2006, which claims the benefit of and priority from U.S. Provisional Patent Application Ser. No. 60/335,970, filed Nov. 13, 2001. The above applications are hereby incorporated by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to detecting contact between a body part and a sensor and distinguishing such contact from contact between the sensor and other objects. 
     BACKGROUND 
     There are many circumstances where it is beneficial to be able to distinguish contact with a human body from contact with other objects or materials. One area where such a capability is especially important is in the guarding of dangerous power equipment. For instance, a system adapted to detect accidental contact between the user of a saw and the saw blade is described in U.S. Provisional Patent Application Ser. No. 60/225,200, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,426, filed Aug. 13, 2001, which are incorporated herein by reference and are assigned to the assignee of the present application. The system of that application relies on the inherent capacitance of the human body to change the voltage on a saw blade carrying a high frequency signal. The system monitors the voltage on the blade, and when it drops suddenly due to contact with a body, the system signals a high speed brake to stop the blade. 
     While the above-incorporated applications describe various configurations and features which allow the system to distinguish voltage drops caused by contact between the blade and a person from voltage drops caused by other events, additional configurations are possible as describe below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of a machine with a fast-acting safety system according to the present invention. 
         FIG. 2  is a schematic diagram of an exemplary safety system in the context of a machine having a circular blade. 
         FIG. 3  is an equivalent circuit model that generally characterizes the electrical system formed by an exemplary detection subsystem and blade when the blade is in contact with a human body. 
         FIG. 4  is a graph schematically illustrating exemplary drive and sensed signals according to the present invention. 
         FIG. 5  is similar to  FIG. 4  but shows the sensed signal reduced in amplitude due to increase apparent capacitance of the blade. 
         FIG. 6  is a graph schematically illustrating an alternative drive signal having a period substantially equivalent to the RC-time constant of a human body, and showing the corresponding sensed signal when the human body is coupled to the blade. 
         FIG. 7  is a graph schematically illustrating an alternative drive signal having a period equal to approximately five RC-time constants of a human body, and showing the corresponding sensed signal when the human body is coupled to the blade. 
         FIG. 8  is a graph schematically illustrating an alternative drive signal having a period equal to approximately 1/10 th  of an RC-time constant of a human body, and showing the corresponding sensed signal when the human body is coupled to the blade. 
         FIG. 9  is a flowchart illustrating an exemplary method for configuring a detection subsystem to distinguish contact between a cutting tool and a person from contact between a cutting tool and other materials. 
         FIG. 10  is a graph showing an exemplary frequency response graph corresponding to the sensed signal from an exemplary cutting tool in contact with a person. 
         FIG. 11  is a graph showing an exemplary frequency response corresponding to the sensed signal from an exemplary cutting tool in contact with a material other than a person. 
         FIG. 12  is an equivalent circuit model that generally characterizes the electrical system formed by a blade and an alternative exemplary detection subsystem having a test electrode. 
     
    
    
     DETAILED DESCRIPTION 
     A machine according to the present invention 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. 
     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 . 
     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 . 
     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. 
     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. 
     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 Ser. No. 60/182,866, filed Feb. 16, 2000 and U.S. patent application Ser. No. 09/676,190, filed Sep. 29, 2000, the disclosures of which are 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. 
     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, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,226, filed Aug. 13, 2001, the disclosures of which are 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, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,242, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference. 
     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. 
     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, brake 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, filed Aug. 14, 2000 and U.S. patent application Ser. No. 09/929,425, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference, describe other systems for stopping the movement of the cutting tool. U.S. Provisional Patent Application Ser. No. 60/225,057, filed Aug. 14, 2000 U.S. patent application Ser. No. 09/929,238, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,058, filed Aug. 14, 2000, and U.S. patent application Ser. No. 09/929,235, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference, describe safety system  18  in the context of particular types of machines  10 . 
     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 U.S. Provisional Patent Application Ser. No. 60/225,200, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,426, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,211, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,221, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/270,011, filed Feb. 20, 2001, U.S. Provisional Patent Application Ser. No. 60/298,207, filed Jun. 13, 2001, and U.S. Provisional Patent Application Ser. No. 60/302,937, filed Jul. 2, 2001, the disclosures of which are herein incorporated by reference. 
     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, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,237, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,094, filed Aug. 14, 2000, and U.S. patent application Ser. No. 09/929,234, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference. 
     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  might 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. 
     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. 
     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, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,240, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,169, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,241, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,170, filed Aug. 14, 2000, and U.S. patent application Ser. No. 09/929,227, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference. 
     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, filed Aug. 14, 2000, U.S. patent application Ser. No. 09/929,236, filed Aug. 13, 2001, U.S. Provisional Patent Application Ser. No. 60/225,212, filed Aug. 14, 2000, and U.S. patent application Ser. No. 09/929,244, filed Aug. 13, 2001, the disclosures of which are herein incorporated by reference. 
     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. 60/157,340, filed Oct. 1, 1999, U.S. Provisional Patent Application Ser. No. 60/182,866, filed Feb. 16, 2000, and U.S. patent application Ser. No. 09/676,190, filed Sep. 29, 2000, the disclosures of which are herein incorporated by reference. 
     Considering detection subsystem  22  in more detail, the references incorporated above describe a variety of different exemplary detection subsystems adapted to detect contact between a person and blade  40 . For example, several detection subsystems described in U.S. Provisional Patent Application Ser. No. 60/225,200, filed Aug. 14, 2000, and U.S. patent application Ser. No. 09/929,426, filed Aug. 13, 2001 are configured to detect any change in the apparent electrical capacitance of the blade.  FIG. 3  schematically illustrates the basic electrical circuit equivalent of a typical detection subsystem  22  and saw blade. The detection subsystem includes a drive portion  100  coupled to the blade (represented by capacitor  102 ). Drive portion  100  is configured to couple a drive signal onto the blade. A sense portion  104  of the detection system is also coupled to the blade to monitor the signal on the blade. As shown in  FIG. 4 , the drive signal (represented by solid line  106 ) typically has a voltage amplitude that varies with time such as a sine wave, square wave, delta function, pulse, etc. The sensed signal coupled to the blade (represented by dash line  108 ) essentially mirrors the drive signal except that the amplitude of the sensed signal V S  is less than the amplitude of the drive signal V D . In addition, any resistance in the cabling between the drive/sense portions and the blade may cause a small charge/discharge delay on the blade. However, it will be appreciated by those of skill in the art that the resistance, if any, will typically be in the milli-ohm range so that the RC-time constant of the cabling/blade assembly will be on the order of a few picoseconds or less. (It should be noted that in  FIGS. 4-8 , sensed signal  108  is shown with a greatly exaggerated charge/discharge delay due to the RC-time constant of the cabling/blade assembly.) 
     When a person contacts the blade, the impedance of the person&#39;s body, indicated at  110  in  FIG. 3 , is coupled to the blade. The human body impedance can be modeled by a resistor  112  in series with a capacitor  114 . Thus, a portion of the charge on blade  102  is transferred through resistor  112  to capacitor  114 , thereby decreasing the amplitude of the sensed voltage V S  to a lower level as indicated at V SC  in  FIG. 5 . The exemplary detection subsystem described in the above-identified references detects this decrease in the sensed voltage amplitude and sends a signal to the control subsystem which triggers the reaction subsystem. It will be appreciated that the RC-time constant of resistor  112  and capacitor  114  will determine how fast a portion of the charge on capacitor  102  is transferred to capacitor  114 . Typical values for the human body capacitance are 50-300 pF, while typical values for the human body resistance are approximately 1 k-ohm. As a result typical values for the RC-time constant of resistor  112  and capacitor  114  will be 50-300 nanoseconds. As described in the references incorporated above, the drive signal typically has a frequency of approximately 100-500 kHz, giving a signal period of 2-10 μsec. Thus, the RC-time constant of a person&#39;s body is substantially invisible to the exemplary detection subsystems described in the above-incorporated references. 
     As also described in the above-identified references, other materials may cause a change in the sensed signal when placed in contact with the blade. For example, when very green wood is being cut by the blade, the relatively high dielectric constant of the wood may also cause the apparent capacitance of the blade to increase as the air dielectric around the blade is replace by green wood dielectric. In other words, the capacitance of capacitor  102  in  FIG. 3  is increased, thereby reducing the amplitude of sensed voltage V S  for a given drive voltage V D . In some instances, the sensed voltage V S  may be decreased to the level V SC  as shown in  FIG. 5 , thereby causing control subsystem  26  to trigger reaction subsystem  24 . The change in apparent blade-capacitance experienced when cutting green wood accumulates (i.e., increases to a maximum amount) over tens, hundreds or even thousands of milliseconds as more and more of the green wood is moved into contact with the blade. However, the RC-time constant of the cabling/blade assembly will remain very low. 
     In view of the effect of high-dielectric materials such as green wood on the sensed voltage V S , some of the exemplary embodiments of detection subsystem  22  described in the above-mentioned references are configured to distinguish contact between the blade and a person from contact between the blade and green wood to prevent erroneously triggering the reaction subsystem when cutting green wood. These detection subsystems typically identify contact between a person and the blade based on a predetermined decrease in the sensed voltage which occurs over several microseconds (V D  has a frequency of a few hundred kHz). As discussed above, this time frame is much larger than the RC-time constant associated with the human body impedance. Therefore, the person&#39;s body is fully charged and discharged during a small fraction of each cycle of the signal. In contrast, the change in apparent blade-capacitance due to contact with green wood changes only slightly over a time frame of several microseconds. Therefore, while cutting green wood can ultimately cause a comparable decrease in the amplitude of sensed voltage V S , the decrease occurs over many cycles of the signal. In other words, these detection subsystems distinguish a human body from green wood based on the rate at which the apparent capacitance of the blade changes. If the apparent capacitance of the blade decreases to a predetermined threshold within several microseconds or tens of microseconds, then detection subsystem  22  recognizes the decrease as a human body contact. However, if the apparent capacitance of the blade decreases to the predetermined threshold within several hundred or thousand microseconds, then detection subsystem  22  does not recognize the decrease as a human body contact. 
     Alternatively, detection subsystem  22  may be configured in any of a variety of other ways to distinguish contact between the blade and a person from contact with high-dielectric materials such as green wood. For example, the detection subsystem may be configured to detect the charging and/or discharging of the person&#39;s body (i.e., capacitor  102 ) that occurs separately from charging and discharging of the blade. Since green wood merely increases the capacitance of the blade rather than adding an additional capacitor to the detection circuit, no separate charging or discharging occurs when green wood contacts the blade. In other words, detection subsystem  22  may be configured to a change in the apparent capacitance of the blade, but also a change in the apparent frequency response of the blade. 
     Thus, in one exemplary embodiment of detection subsystem  22 , drive portion  100  is configured to drive a signal onto the blade having a frequency and/or shape adapted to output a signal to sense portion  104  that indicates the charging and/or discharging of the person&#39;s body. For example, drive signal V D  may be a signal having a period comparable to the RC-time constant of the human body impedance. It will be appreciated that the drive signal may be any type of alternating signal such as a sine wave, square wave, delta signal, etc., or may be repeating pulses (either periodic or non-periodic) having rise and/or fall times much shorter than the RC-time constant of the human body impedance. Additionally, the frequency of the drive signal may be varied, stepped or swept over a range of frequencies to emphasize the charge/discharge delay that is coupled to the blade when a human body is placed into contact with the blade. The detection subsystem may be configured to analyze the sensed signal at each frequency to distinguish human body contact from contact with a high-dielectric material. Alternatively, the drive signal may have several frequency components (e.g., a 50 MHz component, a 10 MHz component, a 1 MHz component, and a 500 kHz component). In such case, the detection subsystem may be configured to filter or otherwise separate out each component from the sensed signal to determine the frequency response of the blade circuit. Optionally, multiple sense portions  104  may be employed to analyze and/or sense each frequency component. 
       FIG. 6  illustrates an exemplary drive signal  106  including a pulse width (T) on the order of approximately one RC-time constant of the human body impedance. When the blade is in contact with a person&#39;s body, sensed signal  108  initially rises to a level V S  corresponding to the capacitance of the blade alone. However, as a portion of the charge on the blade discharges into the person&#39;s body, the sensed signal decreases toward V SC  corresponding to the apparent capacitance of the blade when coupled to the person&#39;s body. In contrast, the voltage of sensed signal  108  does not vary during the period T when the blade contacts a high-dielectric material because: 1) the RC-time constant of the cabling/blade assembly is much less than T; and 2) there is no secondary RC circuit to charge. 
     Similarly,  FIG. 7  illustrates an exemplary drive signal  106  including a pulse width (T) on the order of approximately five RC-time constants of the human body impedance. When the blade is in contact with a person&#39;s body, sensed signal  108  initially rises to V S , and then decreases to V SC  well within period T. In  FIG. 8 , drive signal  106  has a pulse width on the order of 1/10 th  of the RC-time constant of the human body impedance. Consequently, sensed signal  108  does not discharge toward V SC  during time period T even when a person&#39;s body is in contact with the blade. 
     It will be appreciated by those of skill in the art that detection subsystem  22  may be configured in any of a variety of ways to distinguish contact between a blade and a person from contact with other materials based on changes in the apparent frequency response of the blade. For example, the drive signal may be configured with multiple frequencies, one or more having periods approximately equal to or greater than the RC-time constant of a person&#39;s body, and one or more frequencies having periods approximately equal to or less than the RC-time constant of a person&#39;s body. In such case, a drop in the sensed signal voltage level at low frequencies but not at high frequencies may indicate contact with a human body rather than other materials. Alternatively or additionally, the detection subsystem may be configured to utilize a drive signal with a single frequency (or rise/fall time) comparable to the maximum frequency response of the blade when contacted by a human body. In which case, a decrease in the voltage level of the sensed signal from an initially high level during the period of the pulse (e.g., as shown in  FIGS. 6 and 7 ) may indicate contact with a human body rather than other materials. It will be understood that while the examples used herein employ positive voltage pulses, the detection subsystem may additionally or alternatively be configured to employ negative voltage pulses to detect contact. 
     The exemplary detection subsystem described above may also be used to distinguish contact between the blade and a person from contact with conductive materials such as aluminum which may electrically ground the blade to other portions of machine  10 . Thus a sensed signal having a zero voltage would not indicate contact between the blade and a person. Since the detection subsystem may be unable to detect contact between the blade and a person when the blade is grounded, it may be desirable to configure control subsystem  26  to turn off power to machine  10  if the sense signal is grounded (unless a bypass control is provided as described in the above-incorporated references). 
     It will be appreciated that the frequency characteristics of the drive signal employed by detection subsystem  22  to distinguish contact between the blade and a person from contact between the blade and other materials may vary depending on the type of other materials likely to come into contact with the blade. For example, wood products including green, wet and/or pressure treated materials are typically the workpiece materials most likely to come into contact with the cutting tools of woodworking machines. Alternatively, many similar machines are used for cutting other building or manufacturing materials (e.g., plastics, foams, ceramics, etc.), food products (e.g., meats, etc.), textiles, paper, etc. Therefore, depending on the particular application, it may be desirable to analyze the frequency response of the workpiece materials (or other materials) likely to contact the blade relative to the frequency response of the human body to determine the optimal frequency parameters for use by the detection subsystem. 
     An exemplary method for conducting such an analysis is illustrated in  FIG. 9  and indicated generally at  116 . The method includes measuring the frequency response of the cutting tool when in contact with a person, as indicated at  118 , and measuring the frequency response of the cutting tool in contact with one or more other materials, as indicated at  120 . For example,  FIG. 10  illustrates a sample frequency response graph corresponding to an exemplary cutting tool in contact with a person, while  FIG. 11  illustrates a sample frequency response graph corresponding to the same cutting tool in contact with another material. Method  116  continues with comparing the measured frequency responses to determine one or more optimal frequencies at which to detect contact, as indicated at  122 . Detection subsystem  22  may then be configured to sense for contact between the blade and a person at the determined frequencies, indicated at  124 . For example, in the exemplary embodiments illustrated in  FIGS. 10 and 11 , detection system  22  might be configured to sense for contact at one or more frequencies from f 1  to f 2 , as well at f 0  or less. In such case, attenuation of the sensed signal at f 0  but not at f 2  would indicate contact between the blade and a person rather contact between the blade and a material having the frequency response shown in  FIG. 11 . 
     In an alternative embodiment, detection subsystem  22  may be configured to automatically “tune” to the frequency response of a particular person. For example,  FIG. 12  illustrates an exemplary detection subsystem which includes a test electrode  126  connected to both drive portion  100  and sense portion  104 . When a person&#39;s body is placed into contact with test electrode  126 , the drive signal charges and discharges the person&#39;s body. The detection subsystem analyzes the sensed signal to determine the RC-time constant of the person&#39;s body. The detection subsystem then adjusts the frequency characteristics of the drive signal correspondingly so that only materials having a substantially similar frequency response will be detected as a dangerous condition (i.e., contact between the blade and a person). The test electrode may be a separate, dedicated structure or may be built into any suitable portion of machine  10  which a user typically touches such as control buttons, knobs, handles, cranks, fences, etc. Control subsystem  26  may be configured to require a user to contact test electrode  126  prior to enabling operation of machine  10 . 
     As described herein, safety system  18  includes a detection subsystem adapted to detect contact between a person and the cutting tools of various types of woodworking machines. The detection subsystem is adapted to detect when contact occurs between the cutting tool and a human body, while distinguishing contact between the cutting tool and other materials which may change the electrical characteristics of the cutting tool. While several exemplary embodiments of safety system  18  and detection subsystem  22  are described above, the particular embodiments that have been described serve to illustrate that many different modifications and alterations are possible within the scope of the invention. The particular electrical implementation of detection subsystem  22  may utilize any of a variety of different electronic components and configurations which are known to those of skill in the art. 
     It will be appreciated that safety system  18  and detection subsystem  22  may be adapted for use on a variety of different woodworking machines. Several examples of such woodworking machines, as well as further detailed descriptions of alternative safety systems may be found in the references incorporated above, as well as in the following references, the disclosures of which are herein incorporated by reference: PCT Patent Application Serial No. PCT/US00/26812, filed Sep. 29, 2000; U.S. patent application Ser. No. 09/955,418, filed Sep. 17, 2001; U.S. patent application Ser. No. 09/929,221, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,226, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,227, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,234, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,235, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,236, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,237, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,238, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,240, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,241, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,242, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,244, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,425, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/929,426, filed Aug. 13, 2001; U.S. patent application Ser. No. 09/676,190, filed Sep. 29, 2000; U.S. Provisional Patent Application Ser. No. 60/312,141, filed Aug. 13, 2001; U.S. Provisional Patent Application Ser. No. 60/324,729, filed Sep. 24, 2001; U.S. Provisional Patent Application Ser. No. 60/323,975, filed Sep. 21, 2001; U.S. Provisional Patent Application Ser. No. 60/308,492, filed Jul. 27, 2001; U.S. Provisional Patent Application Ser. No. 60/307,756, filed Jul. 25, 2001; U.S. Provisional Patent Application Ser. No. 60/306,202, filed Jul. 18, 2001; U.S. Provisional Patent Application Ser. No. 60/302,916, filed Jul. 3, 2001; U.S. Provisional Patent Application Ser. No. 60/292,100, filed May 17, 2001; U.S. Provisional Patent Application Ser. No. 60/292,081, filed May 17, 2001; U.S. Provisional Patent Application Ser. No. 60/279,313, filed Mar. 27, 2001; U.S. Provisional Patent Application Ser. No. 60/275,595, filed Mar. 13, 2001; U.S. Provisional Patent Application Ser. No. 60/275,594, filed Mar. 13, 2001; U.S. Provisional Patent Application Ser. No. 60/273,902, filed Mar. 6, 2001; U.S. Provisional Patent Application Ser. No. 60/273,178, filed Mar. 2, 2001; U.S. Provisional Patent Application Ser. No. 60/273,177, filed Mar. 2, 2001; U.S. Provisional Patent Application Ser. No. 60/270,942, filed Feb. 22, 2001; U.S. Provisional Patent Application Ser. No. 60/270,941, filed Feb. 22, 2001; U.S. Provisional Patent Application Ser. No. 60/233,459, filed Sep. 18, 2000; U.S. Provisional Patent Application Ser. No. 60/225,210, filed Aug. 14, 2000; U.S. Provisional Patent Application Ser. No. 60/225,058, filed Aug. 14, 2000; U.S. Provisional Patent Application Ser. No. 60/225,057, filed Aug. 14, 2000; U.S. Provisional Patent application Ser. No. 60/182,866, filed Feb. 16, 2000; U.S. Provisional Patent Application Ser. No. 60/157,340, filed Oct. 1, 1999; and U.S. Pat. No. 4,267,914, issued May 19, 1981 to Saar. 
     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 sub-combinations 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. 
     It is believed that the following claims particularly point out certain combinations and sub-combinations that are directed to one of the disclosed inventions and are novel and non-obvious. Inventions embodied in other combinations and sub-combinations 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.