Patent Publication Number: US-10758989-B2

Title: System and method for sensing cable fault detection in a saw

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Application No. 62/131,977, which is entitled “SYSTEM AND METHOD FOR CONTROL OF A DROP ARM IN A TABLE SAW,” and was filed on Mar. 12, 2015, the entire contents of which are hereby incorporated by reference herein. This application also claims priority to U.S. Provisional Application No. 62/132,004, which is entitled “TABLE SAW WITH DROPPING BLADE,” and was filed on Mar. 12, 2015, the entire contents of which are hereby incorporated by reference herein. 
    
    
     CROSS REFERENCE 
     This application cross-references U.S. application Ser. No. 15/060,649 (issued as U.S. Pat. No. 9,914,239), which was filed on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference herein. This application further cross-references copending U.S. application Ser. No. 15/060,656, which was filed on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference herein. This application further cross-references copending U.S. application Ser. No. 15/060,664, which was filed on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference herein. This application further cross-references copending U.S. application Ser. No. 15/060,670, which was filed on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference herein. This application further cross-references copending U.S. application Ser. No. 15/060,742, which was filed on Mar. 4, 2016, the entire contents of which are hereby incorporated by reference herein. 
     FIELD 
     This disclosure relates generally to power tools, and, more specifically, to systems and methods for detecting contact between a blade and objects in a saw. 
     BACKGROUND 
     Detection or sensing systems have been developed for use with various kinds of manufacturing equipment and power tools. Such detection systems are operable to trigger a reaction device by detecting or sensing the proximity or contact of some appendage of an operator with some part of the equipment. For example, existing capacitive contact sensing systems in table saws detect contact between the operator and the blade. 
       FIG. 1  depicts a prior art capacitive sensing based detection system  90  that is incorporated with a table saw  1 . The detection system  90  drives an excitation voltage that is electrically coupled to a movable blade  22  of the saw  1 , and detects the current drawn from the blade  22 . The amplitude or phase of the detected current and/or excitation voltage changes when the blade  22  comes into contact with an electrically conductive object (such as an operator&#39;s hand, finger or other body part, as well as work pieces). The characteristics of the changes are used to trigger the operation of a reaction system  92 . The reaction system  92  disables operation of the blade  22  by, for example, applying a brake to cease motion of the blade  22  and/or by moving the blade  22  below the cutting area. One example of a reaction system  92  uses an explosive charge to drive a brake (not shown) into the blade  22  to arrest the motion of the blade  22 . In addition, or instead, an embodiment of the reaction system  92  collapses a blade support member (not show) to urge the blade  22  below the surface of the table  14 . 
     The embodiment of the detection system  90  shown in  FIG. 1  includes an oscillator  10  that generates a time-varying signal on line  12 . The time-varying signal is any suitable signal type including, for example, a sine wave, a sum of multiple sine waves, a chirp waveform, a noise signal, etc. The frequency of the signal is chosen to enable a detection system to distinguish between contact with the first object, such as a finger or hand, and a second object, such as wood or other material, to be cut by the power tool. In the embodiment of  FIG. 1 , the frequency is 1.22 MHz, but other frequencies can also be used, as well as non-sinusoidal wave shapes. The oscillator  10  is referenced to the saw table  14  or other metallic structure as a local ground. As shown in  FIG. 1 , the blade  22  is disposed vertically in an opening defined by the saw table  14  (or work surface or cutting surface or platform). 
     The oscillator  10  is connected to two voltage amplifiers or buffers  16 ,  18  through the line  12 . The first voltage amplifier  16  has an output connected to line  20 , which operatively connects the output of the oscillator to the saw blade  22 . A current sensor  24  operatively connects a signal from line  20  onto line  26  that is fed to an amplifier  28 , which is connected to a processor  30  by line  32 . The current sensor  24  is, for example, a current sense transformer, a current sense resistor, a Hall Effect current sense device, or other suitable type of current sensor. An output line  34  from the processor  30  is operatively connected to the reaction system  92  so that the processor  30  triggers the reaction system  92  if predetermined conditions are detected indicating, for example, contact between the blade  22  and the first object. 
     The signal on line  26  is indicative of the instantaneous current drawn by the blade  22 . Because the saw blade  22  is in motion during operation of the table saw, the connection is made through an excitation plate  36 , which is mounted generally parallel to the blade  22 . The plate  36  is driven by the first voltage amplifier  16 , and is configured with a capacitance of approximately 100 picoFarad (pF) relative to the blade  22  in the embodiment of  FIG. 1 . The plate  36  is held in a stable position relative to the side of the blade  22 . The excitation plate  36  is configured to follow the blade  22  as the height and bevel angle of the blade  22  are adjusted during operation of the saw  1 . 
     The capacitance between the first object and the saw table  14  (or power line ground if one is present) is in the range of approximately 30-50 pF in the embodiment of  FIG. 1 . When the capacitance between the excitation plate  36  and the saw blade  22  exceeds the capacitance between the first object and the saw table  14 , the detection thresholds are not unduly affected by changes in the plate-to-blade capacitance. In the configuration of  FIG. 1 , the plate  36  is arranged in parallel with the blade  22  on the side where the blade  22  rests against the arbor  37 , so that changes in blade thickness do not affect the clearance between the blade  22  and the plate  36 . Other methods of excitation, including contact through the arbor bearings or brush contact with the shaft or the blade, could be used to the same effect. 
     In the detection system  90 , the second-amplifier  18  is connected to a shield  38 , and the amplifier  18  drives the shield  38  to the same potential as the excitation plate  36 . Also, sensors in the detection system  90  optionally monitor the level of electrical current drawn by the shield  38 . The shield  38  extends around the blade  22  underneath the table  14 , and is spaced some distance away from the blade  22  on the top of the table  14  in the configuration of  FIG. 1 . The configuration of the shield  38  reduces the static capacitance between the blade  22  and the table  14 , which acts as a ground plane if the table is not electrically connected to an earth ground. In various embodiments, the shield  38  is a continuous pocket of mesh, or some other type of guard that is electrically equivalent to a Faraday cage at the excitation frequencies generated by the oscillator  10 . The shield  38  optionally includes a component that moves with the blade adjustments, or is large enough to accommodate the blade&#39;s adjustment as well as the various blades that fitted on the table saw. In the configuration of  FIG. 1 , the shield  38  moves with the blade adjustments, and includes a throat plate area of the table top  14 . 
     The processor  30  performs various pre-processing steps and implements a trigger that enables detection of conditions indicative of contact between the first object and the blade  22 . The processor  30  optionally includes one or more associated analog-to-digital (A/D) converters. The blade current signal from the current sensor  24  is directed to one or more of the A/D converters, which generate a corresponding digital signal. A blade voltage signal representing the voltage difference between the blade  22  and the excitation plate  36  is directed an A/D converter to generate a digital blade voltage signal in some embodiments. The processor  30  receives the digitized signal and performs various digital signal processing operations and/or computes derivative parameters based on the received signal. The processor  30  analyzes or otherwise performs operations on the conditioned blade signal to detect conditions indicative of contact between the first object and the blade  22 . 
     The prior art saw requires that the blade  22  be formed from an electrically conductive material that is also electrically connected to the arbor  37 . Non-conductive blades and blades that include non-conductive coatings prevent proper operation of the contact detection system in the prior art saws. Additionally, the blade  22  and arbor  37  must be electrically connected to a ground plane for the contact detection system to operate effectively. The requirement for a ground connection to the blade also requires the saw  1  to be electrically connected to a proper ground, such as a ground spike, metal pipe, or other suitable ground, which requires that the table saw  1  remain in a fixed location. Other types of table saws include portable table saws that are transported between job sites where providing a ground connection may be inconvenient or impractical. Additionally, the requirement for a ground connection increases the complexity of setup and operation of non-portable table saws. Consequently, improvements to contact detection systems that do not require an electrical ground connection for the blade in portable and non-portable table saws would be beneficial. 
     SUMMARY 
     In one embodiment, a method for detection of a fault in a sensing cable within a saw has been developed. The method includes generating, with a signal generator, a predetermined excitation signal transmitted through a first conductor and a second conductor in the sensing cable, the first conductor being electrically connected to a plate in the saw and the second conductor being electrically connected to an implement in the saw positioned at a predetermined distance from the plate, detecting, with a controller, a return signal corresponding to the excitation signal through the first conductor and the second conductor in the sensing cable, identifying, with the controller, a signal-to-noise ratio (SNR) of the return signal, and generating, with the controller and a user interface device in the saw, an output indicating a fault in the sensing cable in response to the SNR of the return signal being below a predetermined value. 
     In a further embodiment, the method includes deactivating, with the controller, a motor in the saw prior to generating the predetermined excitation signal to enable identification of the fault in the cable while the motor remains deactivated. 
     In a further embodiment, the method includes generating, with a clock source in the saw, a sinusoidal signal at a predetermined frequency. 
     In a further embodiment, the method includes generating, with an amplifier, an amplified sinusoidal signal based on the sinusoidal signal from the clock source, and transmitting the amplified sinusoidal signal through the sensing cable. 
     In a further embodiment, the method includes generating, with a clock source in the saw, a series of delta pulses at a predetermined frequency. 
     In a further embodiment, the method includes disabling, with the controller, operation of a motor in the saw in response to the SNR being below the predetermined threshold. 
     In a further embodiment, the method includes transmitting, with a first winding in a transformer in the saw, the predetermined excitation signal from the signal generator to the first conductor and the second conductor in the sensing cable. 
     In a further embodiment, the method includes receiving, with the controller, the return signal through a third conductor in the sensing cable, the third conductor being electrically connected to the plate and to an analog to digital convertor associated with the controller. 
     In another embodiment, a system for detecting faults in a sensing cable in a saw has been developed. The system includes a sensing cable including a first conductor and a second conductor, a plate that is electrically connected to the first conductor in the sensing cable, an implement, the implement being positioned at a predetermined distance from the plate and the implement being electrically connected to the second conductor in the sensing cable, a signal generator configured to generate a predetermined excitation signal transmitted through the first conductor and the second conductor of the sensing cable, a user interface device, and a controller connected to the signal generator, the user interface device, and the first conductor and the second conductor of the sensing cable. The controller is configured to operate the signal generate to generate the predetermined excitation signal, detect a return signal corresponding to the excitation signal through the first conductor and the second conductor in the sensing cable, identify a signal-to-noise ratio (SNR) of the return signal, and generate an output with the user interface device indicating a fault in the sensing cable in response to the SNR of the return signal being below a predetermined value. 
     In further embodiment, the system includes a motor configured to move the implement during operation and the controller is operatively connected to the motor. The controller is configured to deactivate the motor prior to operation of the signal generator to generate the predetermined excitation signal to enable identification of the fault in the cable while the motor remains deactivated. 
     In a further embodiment, the signal generator includes a clock source configured to generate a sinusoidal signal at a predetermined frequency as the excitation signal. 
     In a further embodiment, the signal generator includes an amplifier configured to generate an amplified sinusoidal signal based on the sinusoidal signal from the clock source. 
     In a further embodiment, the signal generator includes a clock source configured to generate a series of delta pulses at a predetermined frequency. 
     In a further embodiment, the system includes a motor configured to move the implement during operation, and the controller is operatively connected to the motor. The controller is further configured to disable operation of the motor in response to the SNR being below the predetermined threshold. 
     In a further embodiment, the system includes a transformer with a first winding connected to the first conductor in the sensing cable and the second conductor in the sensing cable, and the signal generator is configured to transmit the predetermined excitation signal to the first conductor and the second conductor through the first winding. 
     In a further embodiment, the system includes a third conductor in the sensing cable, the third conductor is electrically connected to the plate and to an analog to digital convertor associated with the controller. The controller is configured to receive the return signal through the third conductor in the sensing cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a prior art table saw including a prior art detection system for detecting contact between a human and a saw blade. 
         FIG. 2  is a schematic diagram of a table saw including an object detection system configured to identify if a saw blade in the saw contacts an object during rotation of the saw blade. 
         FIG. 3  is an external view of one embodiment of the table saw of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of selected components including the blade, arbor, and sensor plate in the saw of  FIG. 2 . 
         FIG. 5A  is an external view of a user interface device in the saw of  FIG. 2 . 
         FIG. 5B  is a view of the user interface device of  FIG. 5A  with an external housing removed. 
         FIG. 5C  is a profile view of the user interface device of  FIG. 5B . 
         FIG. 5D  is an exploded view of components in the user interface of  FIG. 5A - FIG. 5C . 
         FIG. 6A  is an exploded view of a charge coupled plate and arbor assembly in one embodiment of the saw of  FIG. 2 . 
         FIG. 6B  is a profile view of the components depicted in  FIG. 6A . 
         FIG. 7  is a schematic diagram depicting additional details of the object detection system and other components in one embodiment of the saw of  FIG. 2 . 
         FIG. 8A  is a diagram depicting a sensing cable installed in one embodiment of the saw of  FIG. 2 . 
         FIG. 8B  is a cut away diagram of components in a coaxial sensing cable. 
         FIG. 8C  is a diagram depicting a connection of a first conductor in the sensing cable to a plate in the saw of  FIG. 8A . 
         FIG. 8D  is a diagram depicting a mount at one location for connection of a second conductor in the sensing cable to an implement enclosure in the saw of  FIG. 8A . 
         FIG. 8E  is a diagram depicting a mount at another location for connection of a second conductor in the sensing cable to an implement enclosure in the saw of  FIG. 8A . 
         FIG. 9A  is a schematic diagram of capacitive sensors arranged in a throat plate around a blade in one embodiment of the saw of  FIG. 2 . 
         FIG. 9B  is a block diagram of a process for operation of a table saw using the capacitive sensors of  FIG. 9A . 
         FIG. 10  is a block diagram of a process for monitoring activity of the implement reaction mechanism in one embodiment of the saw of  FIG. 2  and disabling the saw for maintenance after the number of activations of the implement reaction mechanism exceeds a predetermined number. 
         FIG. 11  is a block diagram of a process for measuring profiles of different types of materials used in work pieces for the object detection system in the saw of  FIG. 2 . 
         FIG. 12  is a block diagram of a process for measuring the capacitance in the body of an operator of the saw to adjust operation of the object detection system in the saw of  FIG. 2 . 
         FIG. 13A  is a schematic view of components in the motor of one embodiment of the saw of  FIG. 2 . 
         FIG. 13B  is a block diagram of a process for measuring wear on a brush in the motor depicted in  FIG. 13A  based on electrical resistance in the brush. 
         FIG. 13C  is a block diagram of a process for measuring wear on a brush in the motor depicted in  FIG. 13A  based on a pressure measurement for a spring that biases the brush to a commutator in the motor. 
         FIG. 14  is a block diagram of a process for diagnosing faults in the sensing cable of one embodiment of the saw of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the embodiments described herein, reference is now made to the drawings and descriptions in the following written specification. No limitation to the scope of the subject matter is intended by these references. This patent also encompasses any alterations and modifications to the illustrated embodiments as well as further applications of the principles of the described embodiments as would normally occur to one skilled in the art to which this document pertains. 
     As used herein, the term “power tool” refers to any tool with one or more moving parts that are moved by an actuator, such as an electric motor, an internal combustion engine, a hydraulic or pneumatic cylinder, and the like. For example, power tools include, but are not limited to, bevel saws, miter saws, table saws, circular saws, reciprocating saws, jig saws, band saws, cold saws, cutters, impact drives, angler grinders, drills, jointers, nail drivers, sanders, trimmers, and routers. As used herein, the term “implement” refers to a moving part of the power tool that is at least partially exposed during operation of the power tool. Examples of implements in power tools include, but are not limited to, rotating and reciprocating saw blades, drill bits, routing bits, grinding disks, grinding wheels, and the like. As described below, a sensing circuit integrated with a power tool is used to halt the movement of the implement to avoid contact between a human operator and the implement while the implement is moving. 
     As used herein, the term “implement reaction mechanism” refers to a device in a saw that retracts an implement, such as a blade or any other suitable moving implement, from a location with potential contact with a work piece or a portion of the body of a human operator, that halts the motion of the implement in a rapid manner, or that both retracts and halts the implement. As described below in a table saw embodiment, one form of implement reaction mechanism includes a movable drop arm that is mechanically connected to an implement, such as a blade, and an arbor. The implement reaction mechanism includes a pyrotechnic charge that is operated by an object detection system in response to detection of contact between a portion of the body of an operator and the blade during operation of the saw. The pyrotechnic charges force the drop arm and blade below the surface of the table to retract the blade from contact with the operator in a rapid manner. In other embodiments of the implement reaction mechanism, a mechanical or electromechanical blade brake halts the movement of the blade in a rapid manner. 
       FIG. 2  depicts a schematic view of components in a saw  100 , while  FIG. 3  depicts an external view of one embodiment of the saw  100 . The table saw  100  includes a table  104  through which a saw blade  108  extends for cutting work pieces, such as pieces of wood. The table saw  100  also includes an electric motor  112  that rotates an arbor  109  to drive the saw blade  108 , an implement enclosure  118 , and an implement reaction mechanism  132 . While  FIG. 2  depicts a cutting blade  108  for illustrative purposes, those of skill in the art will recognize that the blade  108  may be any implement that can be used in the saw  100  and that the references to the blade  108  are for illustrative purposes. In the saw  100 , the implement enclosure  118  includes a height adjustment carriage and a bevel carriage that surround the blade  108 , and the implement enclosure  118  is alternatively referred to as a blade enclosure or “shield” that surrounds the blade  108  or other suitable implement in the saw  100 . As depicted in  FIG. 3 , a portion of the blade  108  extends upward through an opening in the throat plate  119  above the surface of the table  104 . A riving knife  330  and blade guard  332  are positioned over the blade  108 . 
     Within the saw  100 , the implement enclosure  118  is electrically isolated from the blade  108 , arbor  109 , the top surface of the table  104 , and a plate  120 . In one embodiment, the implement enclosure  118  includes a throat plate  119  that is formed from an electrical insulator, such as thermoplastic. The throat plate  119  includes an opening to enable the blade  108  to extend above the surface of the table  104 . The throat plate  119  is level with the surface of the table  104  and provides further electrical isolation of the blade  108 , height adjustment carriage, and bevel carriage in the implement enclosure  118  from the surface of the table  104 . The general configuration of the table  104 , blade  108 , and motor  112  are well known to the art for use in cutting work pieces and are not described in greater detail herein. Some components that are commonly used in table saws, such as guides for work pieces, blade height adjustment mechanisms, and blade guards are omitted from  FIG. 2  for clarity. 
     The saw  100  further includes an object detection system  102  that is includes a digital controller  140 , memory  142 , clock source  144 , amplifier  146 , transformer  150  and demodulators  143 A and  143 B. The object detection system  102  is electrically connected to the plate  120  and to the blade  108  via the implement enclosure  118  and arbor. The controller  140  in the object detection system  102  is operatively connected to the user interface device  110 , motor  112 , and implement reaction mechanism  132 . During operation of the saw  100 , the blade detection system  102  detects electrical signals that result from changes in the capacitance levels between the blade  108  and the plate  120  when an object contacts the rotating blade  108 . An object can include a work piece, such as a piece of wood or other material that the saw  100  cuts during ordinary operation. The object detection system  102  also detects contact between the blade  102  and other objects, including potentially a hand or other portion of the body of the operator of the saw, and activates the implement reaction mechanism  132  in response to detection of contact between the blade  108  and objects other than work pieces. Additional structural and operational details of the object detection system  102  are described in more detail below. 
     In the saw  100 , the table  104  is electrically isolated from the saw blade  108 , arbor  109 , and other components in the saw enclosure  118  as depicted in  FIG. 2  and  FIG. 3 . In one embodiment, the surface of the table  104  is formed from an electrically conductive metal, such as steel or aluminum. At the surface of the table  104 , the electrically non-conductive throat plate  119  isolates the blade  108  from the surface of the table  104 . Under the table  104 , one or more electrically insulated mounts that secure the table  104  to the frame of the saw  100  but electrically isolate the table  104  from other components within the saw. As depicted in  FIG. 2 , in some embodiments the table  104  is electrically connected to ground  182  with an electrical cable. The ground connection reduces or eliminates the buildup of static electricity on the table  104 , which prevents errant static discharges that can reduce the accuracy of object detection during operation of the saw  100 . 
     In addition to the ground connection for the table  104 , the blade  108  and implement enclosure  118  are connected to the ground  182  through high resistance cables that incorporate large resistors  180  (e.g. 1 MΩ resistors). The implement enclosure  118  is connected to ground  182  through a first cable and a resistor  180  that provides a high-resistance connection to ground. The blade  108  is also connected to the ground  182  via the arbor  109  through a second cable and resistor  180 . The high-resistance connections to ground for the blade  108  and implement enclosure  118  also reduce the buildup of static charge on these components. While prior art detection devices require a low-resistance ground connection (e.g. a direct connection using an electrical cable with a resistance of less than 1Ω) in order to detect contact between a blade and an object using a low-impedance connection directly to earth ground, the high-resistance ground cables in the saw  100  are not required for operation of the object detection system  102 . Instead the high-resistance cables merely reduce the effects of static electricity in the saw  100  to reduce potential false-positive detection events, but the object detection system  102  is still fully functional to detect contact between the blade  108  and an object without any ground connection. Alternative embodiments use different materials for either or both of the plate  120  and blade  108  to reduce the buildup of static electricity in the saw  100  and do not require any connection between the blade  108  or implement enclosure  118  and ground. 
     The table saw  100  includes a rip fence  304  that is mounted on rails  310  and  312 . The rip fence  304  is configured to move to a predetermined position over the table  304  with an orientation that is parallel to the blade  108  to guide work pieces through the saw  100  during operation. In the saw  100 , the rip fence  304  is electrically isolated from the table  104 . For example, in  FIG. 3  an electrically insulated thermoplastic rail mount  306  couples the rip fence  304  to the rail  310 . A plastic guard (not shown) on the bottom of the rip fence  304  and another guard  320  on the top of the rip fence  304  electrically isolate the rip fence  304  from the table  104  in the saw  100 . In some embodiments, the rip fence  304  includes another electrical insulator positioned on the side of the rip fence  304  that faces the blade  108  to ensure electrical isolation between the rip fence  304  and the blade  108  when a work piece engages both the rip fence  304  and blade  108  simultaneously. 
     Referring again to  FIG. 2 , the saw  100  also includes the detection system  102  that detects contact between objects and the blade  108  during operation of the saw  100 . In one configuration, some or all of the components in the detection system  102  are mounted to one or more printed circuit boards (PCBs). In the embodiment of  FIG. 2 , a separate PCB  172  supports a power supply  106  and a control TRIAC  174 . The power supply  106  receives an alternating current (AC) electrical power signal from an external power source, such as a generator or electrical utility provider, and supplies electrical power to the motor  112  through the TRIAC  174  and to supply electrical power to the components in the sensing system  102 . The separate PCBs for the sensing system  102  and power supply  172  isolate the digital controller  140  from the power supply  106  and TRIAC  174  to improve cooling of the digital electronics in the controller  140  and to isolate the controller  140  from electrical noise. In the embodiment of  FIG. 2 , the power supply  106  is a switched power supply that converts the AC power signal from an external power source to a direct current (DC) electrical power signal at one or more voltage levels to supply power to the controller  140 , clock source  144 , and amplifier  146 . The detection system  102  and the components mounted on the detection system  102  are electrically isolated from an earth ground. The power supply  106  serves as a local ground for the components mounted to the detection system  102 . 
     In the saw  100 , the plate  120  and the blade  108  form a capacitor  124  where a small air gap between the plate  120  and the blade  108  acts as a dielectric. The plate  120  is an electrically conductive plate such as a steel or aluminum plate that is positioned at a predetermined distance from the blade  108  with a parallel orientation between the plate  120  and the blade  108  to form two sides of the capacitor  124  with an air gap dielectric. The transformer  150  includes a first winding  152  and a second winding  154 . In the saw  100 , the plate  120  is a metallic planar member that is electrically connected to the winding  152  in the transformer  150 . The plate  120  is otherwise electrically isolated from the implement enclosure  118  and is electrically isolated from the blade  108  by a predetermined air gap to form the capacitor  124 . The plate  120  is also referred to as a charge coupled plate (CCP) because the plate  120  forms one side of the capacitor  124  in conjunction with the blade  108 . In one embodiment, a plastic support member holds the plate  120  in a predetermined position with respect to the blade  108 . The blade  108  and blade arbor  109  are electrically isolated from the enclosure  118 , plate  120 , the drop arm in the implement reaction mechanism  132 , and other components in the saw  100 . For example, in the saw  100 , one or more electrically insulated plastic bushings isolate the arbor  109  and blade  108  from the implement enclosure  118 , the drop arm in the implement reaction mechanism  132 , and other components in the saw  100 . Additionally, the saw blade  108  and arbor  109  are electrically isolated from ground. Thus, the blade object detection system in the saw  100  operates in an “open loop” configuration where the capacitor  124  is formed from the plate  120  and the blade  108  while the blade  108  and arbor  109  remain electrically isolated from the other components in the saw  100 . The open loop configuration increases the capacitance between the plate  120  and the saw blade  108  in comparison to the prior art sensing systems where the saw blade is electrically grounded. The larger capacitance in the saw  100  improves the signal to noise ratio for detection of a signal that indicates contact between a human operation and the saw blade  108 . 
     As depicted in  FIG. 2 , the plate  120  is electrically connected to one side of the first winding  152  in the transformer  150  while the implement enclosure  118  is electrically connected to the other side of the first winding  152 . In one embodiment, the saw  100  includes a single coaxial cable that includes two electrical conductors to establish the two electrical connections. In one configuration, the center conductor element of the coaxial cable is connected to the plate  120  and the first terminal of the first winding  152  in the transformer  150 . The outer sheath of the coaxial cable is electrically connected to the blade  108  through the enclosure  118  and the arbor  109  and to the second terminal of the first winding in the transformer  150 . The structure of the coaxial cable provides shielding to transmit the electrical signals from the plate  120  and implement enclosure  118  while attenuating electrical noise that is present in the saw  100 . 
       FIG. 4  depicts a cross-sectional view of the blade  108 , arbor  109 , and plate  120  in more detail. In  FIG. 4 , electrically nonconductive bushings  404  and  408  engage the arbor  109 . The electrically nonconductive bushings  404  and  408  include, for example, layers of electrically insulated plastic, ceramic, or other insulators that electrically isolate the arbor  109  from other components in the saw  100 . In the illustrative example of  FIG. 4 , the bushings  404  and  408  include bearings that enable the arbor  109  to rotate during operation. The blade  108  only physically engages the arbor  109 , and remains electrically isolated from other components in the saw  100 . In  FIG. 4 , a plastic support member  412  holds the plate  120  in position at a predetermined distance from the blade  108  while electrically isolating the plate  120  from other components in the saw  100 . 
       FIG. 6A  and  FIG. 6B  depict an exploded and front view, respectively, of the components depicted in  FIG. 4 .  FIG. 6A  depicts the plate  120  and the support member  412 , which are affixed to the support frame that holds the arbor  109  using a set of screws. To maintain electrical isolation between the plate  120  and the arbor  109  and other components in the enclosure  118 , the screws are either electrically non-conductive or the threaded holes in the support frame include electrically non-conductive threadings to maintain the electrical isolation. The support member  412  includes a lip  612  that surrounds the outer perimeter of the plate  120  and extends outward past the surface of the plate  120 . The lip  612  provides additional protection and electrical isolation to the plate  120  during operation of the saw  100 . In particular, the lip  612  prevents contact between the blade  108  and the plate  120  due to potential transient wobbles in the rotation of the blade  108  as the blade  108  cuts work pieces during operation of the saw  100 .  FIG. 6B  further depicts the lip  612  of the support member  412  that extends around the plate  120 . 
       FIG. 7  depicts additional details of one embodiment of the object detection system  102  and power supply and control PCB  172  of  FIG. 2  in more detail. In the configuration of  FIG. 7 , some of the cables connecting different components in the saw  100  include ferrite chokes, such as ferrite chokes  708 ,  738 , and  740  that are coupled to cables  724 ,  736 , and  742 , respectively. The cable  742  connects the TRIAC  174  to the motor  112  and the ferrite choke  740  reduces noise in the electrical current that passes through the cable  742  to supply power to the motor  112  upon activation of the TRIAC  174 . As discussed in more detail below, the ferrite chokes  708  and  738  reduce noise in the data and power cables  724  and  736 , respectively, which connect the object detection system  102  to the power supply and control PCB  172 . In the configuration of  FIG. 7 , the sensing cable  720  that includes the first conductor connected to the plate  120  and the second conductor electrically connected to the saw blade  108  does not pass through a ferrite choke. Similarly, a motor tachometer cable (not shown) connecting the motor  112  to the controller  140  does not pass through a ferrite choke. As is known in the art, the ferrite chokes filter high-frequency noise from the cables that are connected to the controller  140  and other components in the object detection system. 
       FIG. 7  also depicts thyristors  743 A and  743 B. The thyristor  743 A connects the third terminal of the transformer  150  to the demodulator  143 A for demodulation of the in-phase component of the sensing signal. The thyristor  743 B connects the fourth terminal of the transformer  150  to the second demodulator  143 B for the quadrature phase component of the sensing signal. The thyristors  743 A and  734 B are “two lead” thyristors, which are also referred to as Shockley diodes, that switch on in response to an input signal that exceeds a predetermined breakdown voltage but do not require a separate gate control signal to be placed in the switched on state. The thyristors  743 A and  743 B are configured with a breakdown voltage that is somewhat higher than the normal voltage amplitude of sensing signal to reduce the effects of random noise in the inputs of the demodulators  143 A and  143 B. However, if an object such a human hand contacts the blade  108 , then the input voltages exceed the breakdown threshold level of the thyristors  743 A and  743 B and both the thyristors  743 A and  743 B switch on to enable the spike and the sensing signal to pass to the demodulators  143 A and  143 B, respectively. The thyristors  743 A and  743 B are optional components in the embodiment of  FIG. 7  and alternative configurations of the object detection system  102  omit these thyristors. 
     In  FIG. 7 , the data cable  724  that connects the controller  140  to the power supply  106  and TRIAC  174  on the power supply PCB  172  passes through the ferrite choke  708 . Additionally, a pull-down resistor  732  connects the data cable  724  between the controller  140  and the power supply PCB  172  to a local ground (e.g. a copper ground plane on the PCB of the object detection system  102 ) to provide additional noise reduction in signals that are transmitted over the cable  724 . The pull-down resistor and ferrite choke enable the data cable  724  to carry control signals using a predetermined command protocol, such as I 2 C, over a long distance between the first PCB of the object detection system  102  and the second PCB  172  of the power supply  106  and TRIAC  174 . For example, in one configuration of the saw  100 , the data cable  724  has a length of approximately 0.75 meters and transmits the I 2 C signals from the controller  140  to the power supply  108  and command logic associated with the TRIAC  174 . The power cable  736  that provides electrical power from the power supply  106  to the controller  140  and other components in the object detection system  102  passes through the ferrite choke  738 . While  FIG. 7  depicts a separate data cable  724  and power cable  736 , in another embodiment a single cable provides both power and data connectivity between power supply PCB  172  and the components in the object detection system  102 . The single cable embodiment also uses a ferrite choke to reduce the effects of noise in a similar manner to the configuration of  FIG. 7 . 
       FIG. 8A - FIG. 8E  depict the coaxial cable that connects the plate  120  and blade  108  to the detection system  102  in more detail.  FIG. 8A  depicts an enclosure  802  that contains the PCB and other components in the SCU that implements the object detection system  102  and other control elements of the saw  100 . The sensing cable  720  is electrically connected to both the sensing plate  120  and the blade  108 . As depicted in  FIG. 8A  and  FIG. 8B  the sensing cable  720  is a coaxial cable with a first internal conductor  852 , an electrical insulator  856  that surrounds the inner conductor  852  and separates the inner conductors from a second metallic conductor  862 , and an exterior insulator  864  surrounding the second conductor  862 . In the configuration of  FIG. 8A , the first conductor  852  is connected to the plate  120  and to the first terminal of the transformer  150  in the object detection system  102  as depicted in  FIG. 2 . The second conductor  862  is electrically connected to the blade  108  and to the second terminal of the transformer  150  in the object detection system  102  as depicted in  FIG. 2 . 
     While  FIG. 8B  depicts a coaxial cable, an alternative embodiment employs a twisted pair cable that includes two different conductors that are twisted around one another in a helical pattern. One or both of the conductors in the twisted pair cable are surrounded by an electrical insulator to isolate the conductors from each other. Additionally, a shielded twisted pair cable includes an external shield, such as a metallic foil, that is wrapped around the twisted pair cable and reduces the effects of external electrical noise on the conductors in the twisted pair cable. 
       FIG. 8A  depicts the connection of the single sensing cable  720  to the plate  120  at location  832  and to the bevel carriage and height adjustment carriage of the implement enclosure  118  at locations  836  and  838 .  FIG. 8C  depicts the connection of the first conductor in the sensing cable  720  to the plate  120  at location  832  in more detail. A metal retention clip  866  is affixed to the plate  120  and to the first conductor  852  in the sensing cable  720  to establish the electrical connection. In the configuration of  FIG. 8C , the retention clip  866  is inserted between the plate  120  and the support member  412  to ensure a stable connection between the sensing cable  720  and the plate  120 . In some embodiments, the retention clip  866  is soldered to the plate  120 . 
     The second conductor  862  is electrically connected to the blade  108 , but since the blade  108  rotates during operation of the saw and since the blade  108  is typically a removable component, the second conductor  862  is not physically connected to the blade  108  directly. Instead, the second conductor is connected to the implement enclosure  118 . In some saw embodiments, the enclosure  118  actually includes multiple components, such as the height adjustment carriage and bevel carriage in the saw  100 . To ensure a consistent electrical connection, the second conductor in the single sensing cable  720  is connected to each of the height adjustment carriage and the bevel carriage to maintain a reliable electrical connection with the blade  108 . For example, in  FIG. 8  the second conductor in the sensing cable  720  is connected to the height adjustment carriage at location  836  and to the bevel carriage at location  838 . 
       FIG. 8D  and  FIG. 8E  depict two different mount locations that connect the second conductor in the sensing cable  720  to the implement enclosure  118  at two different locations including both the height adjustment carriage and bevel carriage. As depicted in  FIG. 8D , the second conductor is electrically and physically connected to the implement enclosure  118  at location  836  using a connection mount  872 . The outermost insulator  864  is removed from the sensing cable  720  within the connection mount  872  to establish an electrical connection with the implement enclosure  118 . In some embodiments, the connection mount  872  is formed from a metal sleeve that surrounds and engages a portion of the second conductor  862  in the sensing cable  720 . As described above, the implement enclosure  118  is electrically connected to the arbor  109  and the blade  108 , and the cable mount  872  provides a reliable electrical connection between the second conductor  862  in the sensing cable  720  and the blade  108  through the height adjustment carriage.  FIG. 8E  depicts another configuration of a connection mount  876  that secures the sensing cable  720  at location  838  to the bevel carriage and provides an electrical connection between the second conductor  862  in the sensing cable  720  and the implement enclosure  118 . In one embodiment, the connection mount  876  is also formed from a metal sleeve that surrounds a portion of the second conductor in the sensing cable  720  to establish the electrical connection with the blade  108  through the implement enclosure  118 . 
     As depicted in  FIG. 2  and  FIG. 7 , the controller  140  is operatively connected to the power supply  106  and TRIAC  174  on the separate PCB  172  through a data line. In the embodiment of the saw  100 , the data line is a multi-conductor cable such as an HDMI cable and the controller  140  transmits command messages to the PCB  172  using the I 2 C protocol. The controller  140  optionally receives status data or data from sensors, such as onboard temperature sensors, from the PCB  172  using the I 2 C protocol. The ferrite choke  708  reduces electrical noise in the data cable  724  and the ferrite choke  738  reduces electrical noise in the power cable  736 . The tamp resistor  732  also reduces noise through the data cable  724 . In one embodiment, the data cable  724  includes a physical configuration that conforms to the High-Definition Multimedia Interface (HDMI) standard, which includes multiple sets of shielded twisted-pair conductors, although the data cable  724  does not transmit video and audio data during operation of the saw  100 . In the embodiment of  FIG. 2 , the data cable has a length of approximately 0.75 meters to connect the separate PCBs  102  and  172 . 
     During operation, the controller  140  signals the TRIAC  174  to supply electrical current to the motor  112  through a gate in the TRIAC. Once triggered, the TRIAC  174  remains activated for as long as at least a predetermined level of electrical current from the power supply  106  passes through the TRIAC  174  to power the motor  112 . The power supply  106  varies the amplitude of the current that is delivered to the motor  112  to adjust the rotational speed of the motor  112  and saw blade  108 . To deactivate the motor  112 , the power supply reduces the level of power supplied to the TRIAC  174  below a predetermined holding current threshold and the TRIAC  174  switches off. In the embodiment of  FIG. 2 , the TRIAC  174  enables operation of the motor  112  at varying speed levels and activation/deactivation without requiring relays that are typically needed in prior art power saws. In the illustrative example of  FIG. 2 , the TRIAC  174  passes an AC electrical signal to the motor  112 , although alternative embodiments include DC motors that receive DC electrical power instead. 
     The controller  140  and associated components in the detection system  102  are sometimes referred to as a saw control unit (SCU). The SCU is electrically isolated from other components in the saw  100  with the exception of the power, control, and sensor data connections between the detection system  102  and other components in the saw  100 . In the saw  100 , the controller  140  also handles control of other operations in the saw  100  that are not directly related to the detection of object contact with the blade  108 , such as activating and deactivating the motor  112 . In the embodiment of  FIG. 2 , the SCU is located outside of the implement enclosure  118 , the detection system  102  is mounted to a non-conductive plastic support member, and the detection system  102  is oriented to avoid placing the ground plane of the detection system  102  in parallel with any metallic members within the saw  100  to reduce the transfer of electrical noise to the electrically conductive traces in the detection system  102 . 
     In the saw  100 , the clock source  144  and driving amplifier  146  in the sensing circuit generate a time varying electrical signal that is directed through a first winding  152  in the transformer  150 , the capacitive coupling plate  120 , the blade  108 , and the implement enclosure  118 . The time varying electrical signal is referred to a “sensing current” because the controller  140  senses contact between the blade  108  and a portion of a human body with reference to changes in the amplitude of the sensing current. The time varying electrical signal is a complex valued signal that includes both an in-phase component and quadrature component. The sensing current passes through the first winding  152  in the transformer  150  to the plate  120 . The changes in the first winding caused by discharges between the plate  120  and the blade  108  produce an excitation signal in the second winding  154  of the transformer  150 . The excitation signal is another complex valued signal that corresponds to the sensing current passing through the first winding  152 . 
     The controller  140  in the sensing circuit is operatively connected to the motor  112 , the second winding  154  in the transformer  150 , a mechanical implement reaction mechanism  132 . The controller  140  includes one or more digital logic devices including general purpose central processing units (CPUs), microcontrollers, digital signal processors (DSPs), analog to digital converters (ADCs), field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs) and any other digital or analog devices that are suitable for operation of the saw  100 . The controller  140  includes a memory  142  that stores programmed instructions for the operation of the controller  140 , and data corresponding to a threshold of max-min variations, a variance threshold, or a frequency response threshold that are used to identify if samples obtained from a sensing current flowing through the blade  108  indicate that the saw blade  108  is rotating or is halted. 
     During operation of the sensing circuit, the clock source  144  generates a time varying signal, such as sinusoidal waveform, at a predetermined frequency. In the embodiment of  FIG. 2 , the clock source  144  is configured to generate a signal at a frequency of 1.22 MHz, which is known to propagate through the human body. The amplifier  146  generates the sensing current as an amplified version of the signal from the clock source  144  with sufficient amplitude to drive the transformer  150  and capacitor  124  for detection by the controller  140 . In the embodiment of  FIG. 2 , the saw  100  generates the sensing signal using amplitude modulation (AM), but in alternative embodiments the sensing signal is generated with a frequency modulation, phase modulation, or other suitable modulation technique. 
     During operation of the sensing circuit, the controller  140  receives the in-phase component I of the excitation signal in the second winding  154  through a first demodulator  143 A and the quadrature component Q of the excitation signal through a second demodulator  143 B. The transformer  150  isolates the sensing current flowing through the first winding  152 , plate  120 , saw blade  108 , and implement enclosure  118  from demodulators  143 A and  143 B that supply the in-phase and quadrature phase components of the signal, respectively, to the controller  140 . Since the demodulators  143 A and  143 B generate electrical noise, the transformer  150  reduces or eliminates the effects of the noise on the first winding  152  and sensing current. In one configuration, the transformer  150  is a 1:1 transformer where the first winding  152  and second winding  154  have an equal number of turns. In alternative configurations, the ratio of windings in the first winding  152  and second winding  154  are selected to either step-up or step-down the signal for demodulation and monitoring by the controller  140 . The controller  140  includes one or more ADCs, filters, and other signal processing devices required to generate digital representations of the amplitude of the in-phase signal I and quadrature signal Q. The controller  140  identifies an amplitude of the sensing current A at a given time as a Pythagorean sum of the in-phase and quadrature components in each sample, as illustrated in the following equation: A=√{square root over (I 2 +Q 2 )}. The controller  140  measures the demodulated signal at a predetermined frequency, such as a 100 KHz sampling rate with a 10 μsec period between each sample, to identify changes in the amplitude A of the complex valued signal. 
     As the motor  112  rotates the blade  108 , the rotating blade  108  comes into contact with different objects, including blocks of wood and other work pieces. A small portion of the charge that accumulates on the blade  108  flows into the work piece. The electrical conductivity of the wood work piece is, however, quite low, and the controller  140  in the sensing circuit continues to enable the motor  112  to rotate the saw blade  108 . For example, when the blade  108  engages a block of wood, the controller  140  typically measures a small change in the sensing current A, but the change in the sensing current is identified as corresponding to wood or another material with low electrical conductivity. 
     While work pieces, such as wood, have low electrical conductivity, another object, such as a part of the human body, has a much higher electrical conductivity and absorbs a much greater portion of the charge on the blade  108  as the part approaches the blade  108 . In  FIG. 2  a portion of a human body  164 , such as a hand, finger, or arm, is represented by a charge cloud indicating the flow of charge from the blade  108  to the human body. The contact between the human body and the blade  108  effectively changes the capacitance level, since the human body and saw blade  108  both receive charge from the sensing current. The controller  140  identifies contact between the human body  164  and the blade  108  as a rapid increase in the amplitude A of the sensing current at the time when the human body  164  contacts the blade  108 . In response to the rapid increase in the amplitude of the sensing signal, the controller  140  deactivates the motor  112 , engages the implement reaction mechanism  132  to halt the motion of the blade  108 , and optionally retracts the blade  108  before the blade contacts the human body  164 . 
     In the configuration of  FIG. 2 , the human body has sufficient conductivity and capacity to draw charge from the blade  108  even when the detection system  102  is isolated from earth ground and when the human body  164  is isolated from earth ground, such as when a human operator wears shoes with rubber soles. Thus, while the detection system  102  and the human  164  do not share a common electrical ground, the controller  140  continues to identify contact between the human  164  and the blade  108  through identification of a rapid increase in the identified sensing current amplitude A. While the absolute value of the amplitude A may vary during operation of the saw  100 , the controller  140  can still identify contact with the human  164  in response to the amplitude and time of the increase in the relative value of the amplitude A. During operation of the saw  100 , the controller  140  is configured to identify contact with the human  164  and to deactivate the motor  112  and engage the implement reaction mechanism  132  to halt the saw blade  108  in a time period of approximately 1 millisecond. 
     In the saw  100 , the controller  140  deactivates the electrical motor  112  in response to identification of contact between the blade  108  and a portion of a human. In the saw  100 , the saw blade  108  generally continues rotating for a period of several seconds due to the momentum that the saw blade  108  accumulates during operation. The implement reaction mechanism  132  is configured to either halt the saw blade  108  in a much shorter period of time, to drop the saw blade  108  below the table  104 , which retracts the saw blade  108  from contact with the human, or to both halt and retract the blade  108 . In the saw  100 , the implement reaction mechanism  132  includes a drop arm that is mechanically connected to the saw blade  108 . The implement reaction mechanism  132  also includes a pyrotechnic charge that is configured to push the drop arm down into the housing of the saw and away from the surface of the table  104 . The controller  140  operates the pyrotechnic charge to move the drop arm and blade  108  downward in response to detection of contact between a portion of the body of the operator and the blade  108 . The implement reaction mechanism retracts the blade  108  below the surface of the table  104 . 
     In some configurations of the saw  100 , the controller  140  is configured to lock out operation of the saw  100  after the pyrotechnic device is fired a predetermined number of times. For example, in the configuration of the saw  100  the implement reaction mechanism  132  includes a dual-pyrotechnic charge with a total of two “shots”. Each operation of the implement reaction mechanism consumes one pyrotechnic charge a in a “monoshot” operation. The operator removes and re-inserts the pyrotechnic device to place the second pyrotechnic charge in position to move the drop arm in a subsequent operation of the implement reaction mechanism  132 . The controller  140  stores a record of the number of activations of the implement reaction mechanism  132  and prevents the saw  100  from being activated in a lockout process after the number of activations exceeds a predetermined number, such as one, two, or a larger number of activations. The controller  140  optionally sends a network notification to a service or warranty provider in embodiments of the saw  100  that are connected to a data network, such as the Internet, in the lockout operation. The lockout process enables service providers to diagnose potential issues with the operation of the saw  100  or procedures for use of the saw  100  in response to operation of the implement reaction mechanism  132  on a frequent basis. 
     In addition to sensing contact between an object and the saw blade  108  when the saw blade  108  is moving, the sensing circuit in the saw  100  is configured to identify if the saw blade  108  is moving when the motor  112  is deactivated. For example, the controller  140  identifies a period of time when the saw blade  108  continues to rotate after an operator operates the user interface  110  to activate the saw  100  to cut one or more work pieces, and subsequently opens the operates the user interface  110  to deactivate the motor  112 . The user interface  110  includes, for example, an activation/deactivation switch to operate the saw  100 , a speed control input device, and status indicator lights that provide information about the operational status of the saw  100 , such as if the saw is ready for operation or has developed a fault. The user interface device  110  is also referred to as a human machine interface (HMI). 
     The saw  100  is configured to be operated with the blade  108  and blade arbor  109  being isolated from electrical ground. The control electronics on the boards  102  and  172 , the plate  120  and the implement enclosure  118  may not be connected to a true earth ground in some configurations, but these components share a common ground plane formed by, for example, a metal chassis of the saw or a ground plane formed on the circuit boards of  102  and  172 . As described above, during the contact detection process the controller  140  identifies a spike in the current level for the sensing signal. However, electrical noise that is generated within the saw  100  could produce false positive or false negative detection events since noise interferes with detection of the sensing signal. In the saw  100 , the PCBs  102  and  172  include ferrite core chokes that act as low-pass filters to reduce the effects of noise. Additionally, the power cabling and data cabling pass through ferrite cores to reduce the noise. The power supply  106  includes a ferrite choke and a thyristor to reject low-speed transient noise in the electrical power signal that is received from the electrical power grid, a generator, or other electrical power source. 
       FIG. 5A - FIG. 5D  depict a portion of one embodiment of the user interface device  110  in more detail.  FIG. 5A  depicts an exterior view of a device status display including an external housing  502 , indicator lights  528 A- 528 D, and a covering for a short-range antenna  508 . During operation, the controller  140  activates one or more of the lights  528 A- 528 D to indicate different status information about the saw  100 . For example, the light  528 A indicates that the saw  100  is ready for operation. The light  528 B indicates that the implement reaction mechanism  132  has operated and that the pyrotechnic charge in the implement reaction mechanism  132  should be reset. The light  528 C indicates that the user should look up a fault code. The light  528 D indicates that the saw  100  requires maintenance to replace a component in the saw, such as a motor brush, or that the saw  100  requires maintenance after the implement reaction mechanism has operated for more than a predetermined number of times. As depicted in  FIG. 5A , the indicator lights  528 A- 528 D provide a simplified interface. Alternative embodiments include a different arrangement of indicator lights or include additional input and output devices including, for example, video display screens, touch input devices, and the like. 
     While the display indicator lights  528 A- 528 D provide simplified direct output feedback to the operator for regular use with the saw  100 , in some circumstances the saw  100  transmits more complex diagnostic and configuration data to external devices. The controller  140  and user interface device  110  optionally transmit more complex diagnostic data and other information about the saw  100  to an external computing device via the short-range wireless antenna under the cover  512 . Examples of diagnostic data that the controller  140  collects and optionally transmits with the wireless transceiver and antenna  516  include, presence of voltage in the sensing circuit, the level of the sensor signal, status information to indicate if the pyrotechnic device (pyro) in the implement reaction mechanism  132  is armed or disarmed, generate a test signal for the pyro firing line without sending a signal with sufficient amplitude to trigger monoshot operation of the pyro, detect presence or absence of the pyro, check a resistance range for corrosion or wire damage in the sensor cable connected to the plate  120  and implement enclosure  118  or other cables in the saw  100 , generate a “tackle pulse” to identify a wire break in the line that provides power to the motor  112 , and identify faults in the motor  112  during a power on self-test. 
     As depicted in  FIG. 5B , the short-range wireless antenna  516  is formed from a predetermined arrangement of conductive traces on a PCB that supports the indicator lights  528 A- 528 D.  FIG. 5B  and  FIG. 5C  depict optically translucent caps  504 A- 504 D that form the exterior visible surface of each of the lights  528 A- 528 B, respectively. The external housing  502  protects the antenna  516  from external elements while enabling the antenna to be located on the exterior of the saw  100  to communicate with external electronic devices. The antenna  516  is operatively connected to a wireless transceiver, such as an NFC, Bluetooth, IEEE 802.11 protocol family compatible (“Wi-Fi”), or other suitable short-range wireless transceiver. An external electronic device, such as a smartphone, tablet, portable notebook computer, or other mobile electronic device receives data from the saw via a wireless communication channel and optionally transmits information to the saw  100  using the wireless communication channel. For example, a smartphone receives diagnostic data from the saw  100  and a software application that is run on the smartphone displays detailed diagnostic information to an operator or maintenance technician to assist in maintenance of the saw  100 . The software application optionally enables the operator to input configuration information for operational parameters of the saw  100  that are not directly accessible through the simplified input device  110 . For example, in one configuration the software application enables the operator to input a maximum RPM rate for the motor  112  and blade  108 . In another configuration, the software application enables the operator to transmit an identifier for a type of material that the saw  100  will cut during operation, such as different types of wood, ceramics, plastics, and the like. 
     In another configuration, the saw  100  includes a lockout mechanism to prevent operation of the saw  100  unless a mobile electronic device with an appropriate cryptographic key is within a predetermined distance of the saw  100 . The mobile electronic device transmits an encrypted authorization code to the saw  100  in response to a query from the saw  100  to unlock the saw  100  for operation. When the mobile electronic device is removed from proximity from the saw  100 , a subsequent query fails and the saw  100  remains inactive. 
       FIG. 5C  depicts a profile view of the indicator lights  528 A- 528 D. Each light includes an optically translucent cap, such as the cap  504 A on the light  528 A, and an optically opaque body member  524 A directs light from a light source, such as an LED, to the translucent cap. In the indicator light  528 A, an LED  552  that is mounted on the PCB projects light through an opening in the opaque body member  524 A and the translucent cap  504 A. The opaque body member  524 A has a tapered shape with a narrow end surrounding a first opening for the LED  552 A and a wider end with a second opening that engages the translucent cap  504 A. The optically opaque member  524 A prevents the light from the LED  552 A from bleeding and producing false illumination in any of the other indicator lights  528 B- 528 D. The configuration of  FIG. 5C  enables the indicator lights in the user interface device  110  to operate in direct daylight conditions and prevents false illumination of incorrect indicator lights during operation. 
       FIG. 5D  depicts an exploded view of select components from  FIG. 5A - FIG. 5C .  FIG. 5D  depicts the indicator cap assembly  540 , which is formed from a molded plastic member that includes the translucent indicator light caps  504 A- 504 D for the lights  528 A- 528 D. The indicator cap assembly  540  also includes an attachment member, such as the hook  506  that is formed from the molded plastic member of the indicator cap assembly  540 , to secure the caps to other components in the user interface device  110 . The body member assembly  544  is another molded plastic member that includes the optically opaque body members  524 A- 524 D corresponding to the caps  504 A- 504 D. Each of the optically opaque body members  524 A- 524 D includes a first opening that aligns with one of the LEDs  552 A- 552 D and a second opening that engages one of the caps  504 A- 504 D. The body member assembly  544  also includes attachment members, such as the hooks  526 , which connect the opaque body members to other components in the user interface device  110 . The PCB  550  includes physical mounting locations and electrical connections for the operation of the user interface device  110 . In particular,  FIG. 5D  depicts light emitting diodes (LEDs)  552 A- 552 D that are aligned with first openings in the corresponding opaque members  524 A- 524 D and that provide light for the caps  504 A- 504 D of the indicator lights  528 A- 528 D. The PCB  550  also includes the antenna  516 , which is formed from a predetermined pattern of conductive traces on the PCB, to enable wireless communication with the user interface device  110 . In some embodiments, the PCB  550  also supports a wireless transceiver directly, while in other embodiments the wireless transceiver is integrated with the controller  140 . The indicator cap assembly  540 , body member assembly  544  and PCB  550  are mounted to a base member  560 , which is a molded plastic member in the embodiment of  FIG. 5D . The base member  560  secures the components of the user interface device  110  to the exterior housing of the saw  100 . 
       FIG. 3  depicts the user interface device  110  mounted on the exterior of the housing of the saw  100 . The base member  560  attaches the components in the user interface device  110  to the exterior of the housing in the saw  100  where the indicator lights  528 A- 528 D are easily visible to the end user. Furthermore, the antenna  516  on the PCB  550  is positioned outside of the electrical shielding of the saw  100 , which provides both a clear view to enable communication with short-range external wireless devices and isolates the antenna  516  and any wireless transceivers on the PCB  550  from sources of electrical noise within the saw  100 . A data cable (not shown) connects the controller  140  mounted on the PCB within the housing of the saw  100  to the user interface device  110  on the exterior of the saw. 
     While the user interface device  110  depicted above includes lights and a wireless data interface, in some configurations the saw  100  includes additional data interface devices. For example, in one embodiment a universal serial bus (USB) or other suitable wired data connector is operatively connected to the controller  140 . The saw  100  includes a USB port near the rear of the bevel carriage. The USB port is hidden from ordinary operators, but maintenance personnel access the USB port by moving the bevel carriage to either the left or right extreme tilt position and locating the USB port through an opening at the back of the housing of the saw  100 . The USB port is connected to an external computing device to perform diagnostic and maintenance operations. The USB connection also enables maintenance personnel to update stored software programs in the memory  142  that the controller  140  executes during operation of the saw  100 . 
     Referring again to the saw configuration of  FIG. 2 , in one operating mode the controller  140  in the saw  100  employs an adaptive thresholding process to identify the current spikes that correspond to the contact between an operator and the blade  108  to control operation of the implement reaction mechanism  132 . During the adaptive threshold process, the controller  140  identifies an average signal level for the sensing signal over a predetermined period of time (e.g. 32 sampling periods that last 320 μsec at a sampling rate of 100 KHz). The controller  140  applies a predetermined bias value to the detected average level and uses the sum of the average and the bias level as an adaptive threshold. The controller  140  updates the average threshold based on comparatively small changes in the average level of the sensing signal that occur due to electrical noise, which prevents detection of a false positive contact event when the level of the sensing signal only changes due to electrical noise in the sensing signal. If contact between the operator and the blade  108  occurs, the rapid spike in the sensing current exceeds the predetermined bias level and the controller  140  detects the contact and activates the implement reaction mechanism  132 . 
     In an optional embodiment of the adaptive threshold detection process, the controller  140  also identifies the signal to noise ratio (SNR) in the sensing signal in response to detecting a spike in the sensing signal current to further reduce the likelihood of false positive detection. The controller  140  identifies the SNR with reference to a mean value of the signal over a predetermined time window divided by the variance of the signal level over the same time window. In one configuration, the controller  140  performs a block computation process to reduce the computational complexity of identifying the SNR, which enable the controller  140  to identify the SNR within the operational timing constraints for operation of the implement reaction mechanism  132 . In the block computation process, the controller  140  identifies the mean values of the signal over comparatively short blocks (e.g. 32 sampling periods that last 320 μsec at a sampling rate of 100 KHz) and stores the computed block mean values in a memory. The controller  140  then identifies the SNR over a series of the blocks, such as eight consecutive blocks of time over a period of 2560 μsec in one embodiment. 
     The controller  140  identifies a single variance value for all of the blocks based on the difference between the eight “local” mean values that occur in each of the eight blocks and a single “global” average mean value for all eight blocks. The controller  140  identifies the SNR based on only the eight mean values an d the identified variance value of instead of identifying the mean and variance over a total of 256 separate samples. The block computation process greatly reduces the computational power that is required to identify the SNR. The controller  140  continues identification of additional samples over time and updates the SNR sample after the oldest block is removed from the set of eight blocks to accommodate newer samples during operation. After identification of the SNR, the controller  140  identifies if the SNR level is below a predetermined minimum threshold at the time of detecting a sensing current spike that exceeds the detection threshold for contact between the operator and the blade  108 . If the SNR level is too low, which indicates a weak signal level in comparison to the detected noise level, then the controller  140  does not operate the implement reaction mechanism  132  to prevent false positive operation when the operator is not actually in contact with the blade  108 . 
     Another optional configuration of the adaptive thresholding process includes an operation to detect static discharge from the blade  108  and prevent a static discharge event from being incorrectly identified as contact between the operator and the blade  108 . During operation of the saw  100 , the rotating blade may accumulate static and discharge the static to components within the saw  100  or to an external object such as a work piece. The static discharge often produces a momentary positive or negative voltage spike in the sensing signal that is similar to the spike that occurs in response to contact between the operator and the blade  108 . However, the amplitude of the spike due to static discharge is often several times larger than any spike that is generated due to contact with the operator. Consequently, in some embodiments the controller  140  identifies human contact not only in response to the amplitude of the sensing signal exceeding the adaptive threshold, but also in response to the amplitude of the spike being below an upper bound threshold that is higher than the initial detection threshold to avoid a false-positive operation of the implement reaction mechanism  132  in response to a static discharge event. 
     The adaptive thresholding process is useful in multiple operating modes of the saw  100  including operating modes in which the saw  100  performs “DADO” cuts. As is known to the art, during a DADO cut operation the blade  108  cuts a trench through all or a portion of a work piece, but does not completely cut the work piece into two separate parts. Many DADO cuts produce trenches that are thicker than a single saw blade, and the saw  100  operates with multiple saw blades placed together on the arbor  109  to form the thicker trenches. The multiple saw blades act as an antenna and receive electrical noise from various sources inside and outside of the saw  100 , which reduces the signal to noise ratio during DADO cuts. 
     In some embodiments, the controller  140  also detects contact between the operator and the blade  108  over a longer period of time during DADO cutting operations to account for the increased noise levels that are present in the detection signal. For example, in one configuration the controller  140  identifies a spike in the current level that exceeds the adaptive threshold for contact detection in a first sample period. In a high-noise environment, a noise spike may also produce the large spike that exceeds the adaptive threshold level. However, a true contact event produces a relatively consistent spike in the current that remains above the threshold for several sampling periods (e.g. up to 10 periods at a sampling rate of 100 KHz). The controller  140  identifies the change in the spike level over multiple sampling periods. If the amplitude of the spike remains high and does not change level by a large amount over several sampling periods, then the controller  140  identifies that the blade  108  is in contact with the operator and activates the implement reaction mechanism  132 . If, however, the controller  140  identifies large variations in the level of the sensing current spike, then the controller  140  identifies that the changes in the sensing current are due to noise and does not operate the implement reaction mechanism  132 . Even with the longer detection period, the total detection and operation time of the object detection system  102  occurs within a period of only a few milliseconds to maintain the effectiveness of the implement reaction mechanism  132 . 
     The adaptive thresholding process improves the accuracy of contact detection during the DADO cut. However, the adaptive thresholding process is not strictly required for use during DADO cut procedures, and the adaptive thresholding process is also effective for use in other modes of operation of the saw  100 . 
     During operation of the saw  100 , the controller  140  optionally performs a fault detection process to identify faults in the cable that connects the sensor plate  120  or implement enclosure  118  to the detection system  102 . The controller  140  identifies hard faults, such as full breaks in the cable, via a continuity test. So-called “soft faults” occur when the cable is at least intermittently connected, but the quality of the connection does not enable the sensing signal to reach the sensor plate  120  and for the controller  140  to detect the sensing current through the capacitor  124 . In one configuration, the controller  140  identifies soft faults prior to activation of the motor  112 . The controller  140  generates the sensing current through the sensing cable while the motor  112  remains deactivated and the level of electrical noise in the saw  100  is relatively low. If the amplitude or noise levels of the sensing signal deviate from expected values by more than a predetermined operational tolerance threshold, then the controller  140  identifies a soft fault in the sensing cable. The controller  140  produces an error signal through the user interface device  110  and prevents activation of the motor  112  in response to detection of hard or soft faults in the sensing cable until the sensing cable is repaired or replaced. 
     In some embodiments, the saw  100  characterizes the capacitance levels of different operators through contact with a capacitive sensor at a predetermined contact location in the saw. For example, in one embodiment the saw  100  includes a metal handle that registers the capacitance, conductance, and other electrical properties of the hand of an operator when the operator grips the handle. In other embodiments, a capacitive sensor is located in a rail or other surface of the saw  100  that an operator contacts during typical operation of the saw  100 . The controller  140  receives sensor data corresponding to the electrical properties of each operator and adjusts the blade contact detection thresholds and other operating parameters to improve the accuracy of blade contact detection for each operator. 
     In some embodiments, the saw  100  performs pattern detection with the sensing signal to identify the state of the blade  108  during operation. For example, in one embodiment the controller  140  identifies elements of the sensing signal that correspond to tooth strikes between the blade  108  and a work piece. The controller  140  optionally uses a tachometer or other RPM sensor to identify the rotational rate of the blade  108 , and the controller  140  receives data corresponding to the size and number of teeth on the blade  108  to identify an expected frequency of tooth strikes as the blade  108  engages the work piece. The controller  140  uses the expected tooth strike frequency to assist in identification of sensing signals that may correspond to contact between the operator and the blade  108  or that merely corresponding to electrical noise that is produced when a tooth strikes the work piece. 
     In some embodiments of the saw  100 , the controller  140  stores identified profiles of the sensing signal while the saw  100  cuts different types of material. For example, the saw  100  cuts through different varieties of wood or pieces of wood with varying moisture levels to identify the amplitude and noise levels for the sensing signal that are detected while cutting a plurality of different types of wood or other material. The profile generation process optionally occurs at a factory prior to shipment of the saw  100 . During subsequent operation, the operator provides input to characterize the type of material that the saw  100  will cut, and the controller  140  retrieves a stored profile of the expected sensing signal parameters from a memory to assist in identification of the expected sensing signal when cutting the work piece. 
       FIG. 9A  depicts another embodiment of an object detection sensors that is suitable for use in conjunction with the object detection system  102  in the saw  100  or in another saw embodiment. In  FIG. 9A , the throat plate  119  incorporates capacitive sensors  904 ,  908 , and  912 . Each of the sensors  904 ,  908 , and  912  are capacitive sensors that can detect the presence of a human hand or other body part either in contact or close proximity to the surface of the corresponding capacitive sensor due to a change in capacitance around the sensor. By contrast, a work piece such as wood produces a much different change in capacitance to enable a controller, such as the controller  140  depicted in  FIG. 2 , to distinguish the work piece from a human body part. The capacitive sensors  904 - 912  are arranged along the cut direction  920 , which corresponds to the direction that a work piece travels as the blade  108  cuts the work piece. The capacitive sensor  904  is arranged in an area across the front of the saw blade  108 . The capacitive sensors  908  and  912  are arranged conformal to the saw blade  108  on the left and right hand sides, respectively, as viewed from the front of the saw blade  108 . 
     As depicted in  FIG. 9A , each of the capacitive sensors  904 - 912  occupies a predetermined region of the throat plate  119 , such as the rectangular regions depicted in  FIG. 9A  or another geometric shape. In some embodiments, the capacitive sensors  904 - 912  detect not only the presence of a human body part proximate to the corresponding sensor, but also a location of the human body part over the surface of the sensor and a velocity and direction of movement of the human body part over time. The thermoplastic throat plate  119  isolates the capacitive sensors  904 - 912  from the blade  108 , the surface of the table  104 , and from other components within the saw. 
       FIG. 9B  depicts a process  950  for operation of the capacitive sensors  904 - 912  in the saw  100 . In the description below, a reference to the process  950  performing a function or action refers to an operation of a controller to execute stored program instructions in association with other components in the saw to perform the function or action. The process  950  is described in conjunction with the embodiment of  FIG. 9A  and the saw  100  for illustrative purposes. 
     Process  950  begins as the saw  100  is activated and the motor  112  moves the blade  108  to cut work pieces (block  954 ). During operation, the capacitive sensors  904 - 912  generate the capacitive sensing signals to detect the presence of objects that are proximate to the surfaces of the capacitive sensors  904 - 912  in the throat plate  119  around the blade  108  (block  958 ). 
     If the controller  140  identifies a change in the capacitance level of one or more of the capacitive sensors  904 - 912  based on a change in an RC time constant of the capacitive sensing signal, then the controller  140  detect the presence of an object, such as a work piece or human body part, in the region around the saw blade  108  prior to contact between the object and the saw blade (block  962 ). For example, in some embodiments, the capacitive sensors  904 - 912  include capacitive sensing elements that form one plate in a capacitor and an electrically non-conductive dielectric that covers the capacitive sensing element and covers the surface of the capacitive sensors  904 - 912 . An oscillator in the capacitive sensors generates a time-varying capacitive sensing signal using an RC circuit formed from the capacitive element in each sensor and a predetermined resistor. As is known in the art, the RC time constant changes in response to a change in the size of the capacitance C in the RC circuit, and the capacitive sensor or an external control device identifies contact with objects based on changes in the time-varying signal. An object positioned over the surface one of the sensors  904 - 912  acts as the second plate in a capacitor and produces a change in the capacitance level of the sensor. 
     If the controller  140  identifies that there is no object proximate to the capacitive sensors (block  962 ) or that a detected object produces a minimal change in capacitance that corresponds to a work piece but not a human body part (block  966 ) then the saw  100  continues operation to cut a work piece (block  970 ). Electrically conductive objects, such as a finger or other body part of a human operator, produce comparatively large changes in capacitance while electrically nonconductive objects, such a wood work pieces, produce small changes in the capacitance level. As described above, the characteristics of a work piece such as wood generate a change in capacitance in the sensors  904 - 912  that is sufficiently distinct from a human body part to enable the controller  140  to distinguish between the work piece and a human body part that is in close proximity to the capacitive sensors  904 - 912 . 
     During process  950 , if the capacitive sensors generate a signal corresponding to a sufficiently large change in capacitance that corresponds to the presence of a hand or other body part in close proximity to the capacitive sensors  904 - 912 , then the controller  140  generates a warning output, deactivates the motor  112 , or activates the implement reaction mechanism  132  prior to the object contacting the blade  108  (block  974 ). In a configuration where the detected object has not actually touched the blade but has moved within a predetermined distance of the blade, the controller  140  deactivates the motor  112  to enable the saw blade  108  to come to a halt but does not engage the implement reaction mechanism  132  unless the object actually contacts the blade as detected using the object detection system  102  described above. In other embodiments, if the capacitive sensors  904 - 912  detect an object corresponding to a human body part, the controller  140  generates a warning signal, such as a light that is visible to the operator on the table  104 , for the operator prior to deactivating the motor  112  or operating the implement reaction mechanism  132 . In some embodiments the object detection system  102  operates the implement reaction mechanism  132  if the object contacts the blade  108  prior to the blade  108  coming to a complete halt or prior to the object coming into contact with the blade  108 . 
     In some embodiments of the process  950 , the capacitive touch sensors  904 - 912  each include a two-dimensional grid of sensing elements that enable the touch sensors to generate multiple capacitive detection signals corresponding to the position within the two-dimensional region covered by each of the capacitive sensors. In some configurations, the controller  140  generates a warning signal if a human body part object is detected at a first position that is over one of the sensors  904 - 912  but beyond a first predetermined distance from the blade  108  and then deactivates the motor  112  if the object moves with the predetermined distance of the blade  108 . Furthermore, the controller  140  or other control device identifies a path of movement and velocity of the object based on a series of object locations that the individual sensing elements in the capacitive sensors  904 - 912  generate over time. If the path of movement indicates that an object, such as a human hand, has a high likelihood of contacting the blade  108  at some point along the path, then the controller  140  deactivates the motor  112  or generates the warning output as described above. Additionally, in some configurations the controller  140  activates the implement reaction mechanism  132  to retract the blade  108  or other implement prior to actual contact between the hand or other body part of the operator. For example, if the detected location of a hand of the operator is within a predetermined distance of the blade  108  or the path of movement of the hand over the capacitive sensors has a trajectory that could with the blade  108 , then the controller  140  optionally activates the implement reaction mechanism  132  before the contact with the blade  108  actually occurs. Of course, the capacitive sensors  904 - 912  and the process  950  can be implemented in tandem with the operation of the object detection system  102  that is described above to detect the presence of a body part of the operator in proximity to the blade  108  and to detect actual contact between the body part and the blade  108 . 
     In addition to the operation of the object detection system  102  that is described above, the saw  100  is further configured to perform different configuration and diagnostic processes to maintain reliability and enable operation of the saw with a wide range of different materials. For example, the saw  100  is configured to maintain a record of the number of times that the implement reaction mechanism has been activated to ensure that the saw  100  receives proper maintenance. 
       FIG. 10  is a block diagram of a process  1000  for monitoring the operation of the implement reaction mechanism in the saw. In the discussion below, a reference to the process  1000  performing a function or action refers to the operation of a controller to execute stored program instructions to perform the function or action in association with one or more components in the saw. The process  1000  is described in conjunction with the saw  100  for illustrative purposes. 
     Process  1000  begins upon activation of the implement reaction mechanism (block  1004 ). In the saw  100 , the controller  100  activates the implement reaction mechanism  132  in response to detection of contact between an object, such as the hand of the operator, other than a work piece. In one embodiment of the saw  100 , a pyrotechnic charge in the implement reaction mechanism  132  fires to retract the blade  108  below the level of the table  104 . The controller  140  increments a counter held in a non-volatile portion of the memory  142  to maintain a record of the number of times that the implement reaction mechanism has been activated during operation of the saw  100  (block  1008 ). As is known to the art, the non-volatile memory, such as a solid-state or magnetic data storage device, retains stored data over a long period of time even when the saw  100  is deactivated and disconnected from electrical power. 
     The process  1000  and the operation of the saw  100  continues while the total number of activations of the implement reaction mechanism  132  remains below a predetermined threshold (e.g. five activations of the implement reaction mechanism  132 ) (block  1012 ). If the number of activations of the implement reaction mechanism exceeds the predetermined threshold (block  1012 ) then the controller  140  disables operation of the saw  100  until the saw  100  undergoes a maintenance procedure (block  1016 ). For example, in one configuration the controller  140  ignores any input signals from the user interface  110  to activate the saw  100 , and the motor  112  remains deactivated while the saw  100  is disabled. The controller  140  optionally generates an output indicated signal via the user interface  110  to alert the operator that the saw  100  is disabled and requires maintenance. 
     The process  1000  continues during a maintenance operation. In addition to any required maintenance to repair or replace mechanical or electrical components in the saw  100 , the maintenance operation further includes resetting the counter value in the memory of the saw  100  to return the saw to operation (block  1020 ). In one embodiment, the maintenance process includes connection of an external programming device, such as a PC or other computerized programming device, to a maintenance port within the saw  100 , such as a universal serial bus (USB) port, to both retrieve diagnostic data from the memory  142  and reprogram the memory  142  to reset the counter that stores the number of times that the implement reaction mechanism has been activated. The use of the external programming device enables the saw  100  to be re-enabled for use after the maintenance process while remaining disabled until the saw undergoes proper maintenance. 
     The process  1000  ensures that the saw  100  remains disabled until receiving maintenance in the event of an unusually large number of activations of the implement reaction mechanism  132 . The maintenance operation ensures that all components within the saw  100  are operating properly and that the object detection system  102  accurately detects contact between an object other than a work piece and the saw blade  108 . 
     As described above, the object detection system  102  receives input signals in response to contact between the blade  108  and any object, including both work pieces which the saw cuts during normal operation, and other objects including potentially a body part of the saw operator that results in activation of the implement reaction mechanism. During operation of the saw  100 , the object detection system  102  receives input signals corresponding to changes in the capacitance levels in the capacitor  124  formed by the plate  120  and blade  108  corresponding to both contact between work pieces and potential contact with objects other than work pieces. For example, in some situations wood with high moisture content has the potential to be confused with a portion of the body of a human operator during operation of the saw.  FIG. 11  depicts a process  1100  that generates profiles of the signals generated by different types of material in various work pieces to improve the accuracy of object detection. In the discussion below, a reference to the process  1100  performing a function or action refers to the operation of a controller to execute stored program instructions to perform the function or action in association with one or more components in the saw. The process  1100  is described in conjunction with the saw  100  for illustrative purposes. 
     Process  1100  begins as the saw operates with the object detection system  102  enabled but the implement reaction mechanism  132  disabled (block  1104 ). The operation of the saw  100  without the implement reaction mechanism being enabled occurs under controlled conditions such as at a facility of the manufacturer or an approved maintenance facility. During the process  1100 , the saw cuts various materials in work pieces that are suitable for use with the saw  100  but have the potential to produce sensing signals that could be misinterpreted as corresponding to a human body part or other object that should trigger the implement reaction mechanism  132  upon contact with the spinning blade  108 . 
     Process  1100  continues as the saw  100  records sensing signals in the object detection system  102  that are produced at predetermined times when the work piece initially contacts the blade  108 , during the cut as the work piece moves past the blade  108 , and at the time of completion of the cut when the work piece disengages from the blade  108  (block  1108 ). The recorded sensing signal information typically includes spikes in the sensing signal that correlate to changes in the capacitance level in the capacitor  124 . For example, the initial spike that occurs when the work piece initially contacts the rotating blade  108  may be similar to the initial spike that is generated when an object other than the work piece initially contacts the rotating blade  108 . 
     In another embodiment of the process  1100 , the saw  100  includes additional sensors other than the capacitive sensor formed by the capacitor  124  that can detect characteristics of the work piece material that can be distinguished from the body of a human operator. For example, one embodiment further includes one or more infrared sensors that are mounted on the riving knife  330  that is depicted in  FIG. 3 . The infrared sensors generate a profile of the frequency response of infrared light that is reflected from the work piece. The controller  140  is operatively connected to the infrared sensors to record the frequency response of the material in the work piece. 
     Process  1100  continues as the controller  140  or a processor in an external computing device identifies differences between the recorded sensing signal and the predetermined sensing signal profile for an object that would trigger the implement reaction mechanism in the saw  100  (block  1112 ). For example, as described above the controller  140  uses an adaptive thresholding process to identify spikes in the sensing current that correspond to a hand or other portion of the human body when in contact with the blade  108 . The spike corresponding to the contact with the human hand includes both an amplitude profile and time profile. The controller  140  identifies differences in the amplitude and duration between the predetermined profile for a human body part and the initial spike that occurs when the work piece first contacts the rotating blade  108  and any subsequent spikes that occur as the blade  108  cuts the work piece and disengages from the work piece. 
     The controller  140  or external processor then generates a detection profile that is specific to the test material based on the differences between the recorded sense signals and the predetermined object detection profile for the human body (block  1116 ). In one embodiment, the controller  140  generates a profile with a range of amplitude values around the amplitude of the recorded spike in the sensing signal when the blade  108  engages the predetermined material in the work piece. The range of values for the amplitude of the spike does not include the threshold amplitude for the spike amplitude for the predetermine profile of the human operator to ensure that the controller  140  does not incorrectly identify a sensing signal corresponding to a human operator as a work piece. Thus, the size of the range of amplitude values that correspond to different work pieces varies based on the differences between the recorded spikes produced through contact between the blade  108  and the work piece material and the predetermined profile corresponding to a human body. The controller  140  similarly generates a time range corresponding to the duration of the spike in the sensing signal from the work piece based on differences between the time range of the spike from the work piece and an expected duration of the spike in the profile for contact with a human body. The updated profile enables the controller  140  to distinguish between sense signals from the capacitor  124  that correspond to contact between the blade  108  and a work piece of a predetermined type of material compared to potential contact with a portion of the human body. 
     As noted above, in an alternative embodiment the controller  140  generates a profile based on the data from the infrared sensors to identify the frequency response range of the material in the work piece and distinguish the frequency response range from a predetermined frequency response range that is associated with a human operator. The controller  140  uses a predetermined response range for the operator that is stored in the memory  142  to ensure that the frequency response range in the profile of the material does not overlap the predetermined profile for the human operator. For example, in one configuration the memory  142  stores a frequency response profile for near infrared responses that have a peak response at wavelengths of approximately 1080 nm and a minimum response at wavelengths of approximately 1580 nm for a wide range of human skin tones. Other types of materials for various work pieces have peak and minimum infrared frequency responses at different wavelengths, and the controller  140  generates a profile with a range of frequency responses for both peak and minimum response values at wavelengths that correspond to work pieces but do not overlap with the wavelengths corresponding to the responses of human skin. 
     During process  1100 , the updated profile for the test material is stored in the memory  142  (block  1120 ). During subsequent operations with both the object detection system  102  and implement reaction mechanism  132  being enabled, the controller  140  uses the stored profile information for the test material to reduce potential occurrences of false positive detection events when changes in the sensing signal that occur due to contact between the work piece and the saw blade  108  are misinterpreted as corresponding to contact between the operator and the saw blade. For example, if the saw  100  is cutting a particular type of material that is stored in a profile in the memory  142 , then the controller  140  continues to operate the saw  100  as long as any spikes in the sensing signal in the object detection system  102  remain within the amplitude and time duration ranges for the stored profile corresponding to the material type. In some configurations, the memory  142  stores profiles for multiple types of material that the saw  100  cuts during operation. The operator optionally provides an input to the saw  100  that specifies the type of material to be cut to enable the controller  140  to use a stored profile for the appropriate type of material in the work piece. 
     As described above, the object detection system measures changes in the sensing signal through the capacitor  124  in response to contact between objects and the rotating saw blade  108 . The memory  142  stores predetermined threshold information that the controller  140  uses with the adaptive threshold process described above to detect contact between the body of the human operator and the blade  108 . However, the bodies of individual human operators may exhibit different capacitance levels both between individuals and the capacitance level of one individual may vary over time for a variety of reasons. Examples of factors that affect the capacitance levels of operators include, but are not limited to, the temperature and ambient humidity in the environment around the saw, the physiological makeup of each operator, the perspiration level of the operator, and the like.  FIG. 12  depicts a process  1200  for measuring the level of capacitance of an individual operator during operation of the saw  100  to enable the saw  100  to adjust the object detection thresholds for different individuals. In the discussion below, a reference to the process  1200  performing a function or action refers to the operation of a controller to execute stored program instructions to perform the function or action in association with one or more components in the saw. The process  1200  is described in conjunction with the saw  100  for illustrative purposes. 
     Process  1200  begins as the saw  100  measures a capacitance level of the operator through a capacitive sensor that is formed in a handle or other predetermined contact location on a surface of the saw  100  that the operator touches during operation of the saw (block  1204 ). Using the illustration of  FIG. 3  as an example, capacitive sensors in one or more of the rip fence  304 , the front rail  310 , the bevel adjustment handle  352 , the height adjustment handle  354 , or other surfaces of the saw that the operator touches during operation generate measurements of the capacitance level in the hands of the operator. The operator does not need to remain in continuous contact with the capacitive sensor during operation of the saw  100 , but the controller  140  optionally updates the measured capacitance level when the operator touches one or more of the capacitive sensors. 
     Process  1200  continues as the controller  140  modifies the threshold level for detection of contact with an object, such as the body of the operator, other than the work piece (block  1208 ). The controller  140  decreases the default spike amplitude detection threshold for the sensing signal in response to a measured capacitance level that is less than a predetermined default level, which can occur when the operator has unusually dry skin or other environmental factors reduce the effective capacitance in the body of the operator. The controller  140  modifies the threshold based on the difference between a default capacitance level that is appropriate for a wide range of human operators and the measured capacitance level that may be higher or lower than the default level. Reducing the threshold level effectively increases the sensitivity for detection between the operator and the saw blade  108  in the saw  100 . The controller  140  optionally increases the threshold in response to an identification of a large capacitance value in the operator. In some embodiments the controller  140  limits the maximum threshold level for object detection to ensure that the object detection system  102  retains the capability to detect contact between the operator and the blade  108  since increasing the detection threshold level effectively reduces the sensitivity of the object detection system  102 . 
     Process  1200  continues as the saw  100  operates to cut work pieces and the object detection system  102  uses the modified detection threshold to detect potential operator contact with the blade  108  (block  1212 ). As described above, if the hand or other body part of the operator contacts the rotating blade  108 , the controller  140  compares the amplitude of the measured spike in the sensing signal through the capacitor  124  to the modified threshold using the adaptive thresholding process described above. Since the controller  140  modifies the detection threshold based on the measured capacitance of the operator, the process  1200  enables the saw  100  to detect contact between the operator and the saw blade  108  with improved accuracy. 
     In the saw  100 , the motor  112  includes one or more brushes that engage a commutator. The use of brushes in electric motors is well-known to the art. Over time, brushes experience wear, which reduces the efficiency of the motor and worn brushes often generate sparks. The sparks can be detrimental to operation of the motor  112  and in some circumstances the sparks generate electrical noise that is detected by the object detection system  102 .  FIG. 13A  depicts an example of the shaft  1350 , commutator  1354 , and brushes  1358 A and  1358 B in the motor  112 . Springs  1362 A and  1362 B bias the brushes  1358 A and  1358 B, respectively, into contact with the commutator  1354 . In many embodiments, the brushes  1358 A and  1358 B are formed from graphite. In the motor  112 , the mounts  1366 A and  1366 B are formed in a housing of the motor  112  and engage the springs  1358 A and  1358 B, respectively. In one embodiment, the mounts  1366 A and  1366 B include pressure sensors that measure the compressive force applied through the springs  1362 A and  1362 B. In another embodiment, the mounts  1366 A and  1366 B generate sensing currents through the springs  1362 A and  1362 B and the corresponding brushes  1358 A and  1358 B to identify the electrical resistance levels through the brushes. 
     Since worn brushes not only reduce the efficiency of operation of the motor  112 , but may introduce additional electrical noise into the sensing signal for the object detection system  102 , the saw  112  optionally detects brush wear in the motor  112  and generates an output to indicate that worn brushes should be replaced via the user interface  110 .  FIG. 13B  depicts a first embodiment of a process  1300  for measuring brush wear in the motor  112 . In the description below, a reference to the process  1300  performing an action or function refers to the operation of a controller, such as the controller  140  in the saw  100 , to execute stored program instructions to perform the function or action in conjunction with other components in the saw  100 . 
     During process  1300 , an electrical source positioned in each of the mounts  1366 A and  1366 B generates an electrical current through the corresponding brushes  1358 A and  1358 B (block  1304 ). In one embodiment, the current passes through the cables that are connected to the brushes  1358 A and  1358 B for normal operation of the brushes  1358 A and  1358 B in the saw  100 . In another configuration, the current passes through the springs  1362 A and  1362 B and the corresponding brushes  1358 A and  1358 B. The current is generated during a diagnostic mode when the saw motor  112  is otherwise deactivated and the level of the electrical current used in the process  1300  is well below a drive current that produces rotation in the motor shaft  1350  during operation of the motor  112 . During process  1300 , the controller  140  or a controller that is integrated with the motor  112  measures an electrical resistance level through the brushes and compares the measured electrical resistance level to a predetermined resistance threshold (block  1308 ). The measurement of the electrical resistance level includes, for example, a measurement of a voltage level or current level for the electrical current that flows through each of the brushes  1358 A and  1358 B in the diagnostic mode and an application of Ohm&#39;s law to find the resistance (e.g. R=E/I for a measured voltage E and predetermined current I or predetermined voltage E and measured current I). Once the resistance drops below a predetermined threshold, the controller  140  generates an output signal via the user interface  110  to indicate that the brushes should be replaced (block  1312 ). The resistance drops as the brushes wear and grow thinner, which reduces the total resistance through the springs  1362 A and  1362 B and the corresponding brushes  1358 A and  1358 B. In some configurations, the controller  140  also disables operation of the saw  100  until any worn brushes have been replaced and the controller  140  performs the process  1300  again to confirm that the new brushes are no longer worn. 
       FIG. 13C  depicts a second embodiment of a process  1320  for measuring brush wear in the motor. In the description below, a reference to the process  1320  performing an action or function refers to the operation of a controller, such as the controller  140  in the saw  100 , to execute stored program instructions to perform the function or action in conjunction with other components in the saw  100 . 
     In the process  1320 , the spring mounts  1366 A and  1366 B each include a pressure sensor that measures the compressive force of the corresponding springs  1362 A and  1362 B during a diagnostic mode when the motor  112  is deactivated (block  1324 ). As the brushes  1358 A and  1358 B experience wear, the corresponding springs  1362 A and  1362 B expand to bias the brushes onto the commutator  1354 . The compressive force in the springs  1362 A and  1362 B decreases as the springs expand. The controller  140  or a controller in the motor  112  is operatively connected to the pressure sensors and compares the measured pressure levels from the pressure sensors to a predetermined pressure threshold (block  1328 ). Once the pressure sensors in the mounts  1366 A and  1366 B measure that the compressive force of the springs  1362 A and  1362 B has dropped below a predetermined threshold, the controller  140  generates an output signal via the user interface  110  to indicate that the brushes should be replaced (block  1332 ). In some configurations, the controller  140  also disables operation of the saw  100  until any worn brushes have been replaced and the controller  140  performs the process  1320  again to confirm that the new brushes are no longer worn. 
     As described above, during operation the object detection system  102  receives sensing signals through a single sensing cable, such as the coaxial cable  720  depicted in  FIG. 8B , that includes two different conductors. Within a high vibration environment such as the saw  100 , the sensing cable  720  may experience wear and faults over time that eventually require cable replacement during saw maintenance. If the sensing cable  720  breaks or is disconnected from any of the PCB of the object detection system  102 , the plate  120 , or implement enclosure  118 , then the PCB does not detect any sensing signal and can disable the saw  100  until the single sensing cable  720  is repaired. However, in some circumstances the sensing cable  720  experiences a “soft fault” in which the cable is not completely disconnected, but the cable continues to operate with greatly degraded performance in the saw. The PCB  102  still receives the sensing signal, but the fault within the sensing cable  720  introduces noise or attenuates the sensing signal, which reduces the accuracy of the object detection system  102 .  FIG. 14  depicts a process  1400  for diagnosing soft faults in the sensing cable  720 . In the description below, a reference to the process  1400  performing an action or function refers to the operation of a controller, such as the controller  140  in the saw  100 , to execute stored program instructions to perform the function or action in conjunction with other components in the saw  100 . 
     Process  1400  begins as the object detection system  102  generates a predetermined excitation signal during a diagnostic mode (block  1404 ). In one embodiment, the controller  140  activates the clock source  144  to generate the same sinusoidal sensing signal that is used during operation of the saw  100  using amplitude modulation. In another embodiment, the clock source  144  produces an impulse train including a series of delta pulses at a predetermined frequency to enable the controller  140  to receive an output corresponding to the unit impulse response of through the sensing cable  720  and capacitor  124 . In further embodiments, the clock source  144  generates any suitable predetermined signal that enables diagnosis of potential faults within the sensing cable  720 . During the diagnostic mode, the motor  112  in the saw  100  is deactivated and there is minimal electrical noise present within the saw. 
     Process  1400  continues as the controller  140  identifies a signal to noise ratio (SNR) of the detected excitation signal (block  1408 ). In the saw  100 , the controller  140  detects a return signal in response to the excitation signal from the clock source  144  and the amplifier  146  that passes through the sensing cable  720  and the plate  120  and saw blade  108  of the capacitor  124 . Since the clock source  144  and driving amplifier  146  generate the excitation signal with a predetermined amplitude and modulation, the controller  140  identifies the SNR using a predetermined measurement technique that is otherwise known to the art. Of course, even in a an otherwise deactivated saw the excitation signal experiences some degree of attenuation through the sensing cable  720  and capacitor  124 , and some degree of noise, such as Johnson-Nyquist noise, is always present within the sensing circuit. As used in the context of the process  1400 , a measurement of SNR also includes a measurement of signal strength attenuation that does not include a direct measurement of noise. For example, the predetermined excitation signal is generated with a predetermined amplitude and the controller  140  measures the amplitude of the return signal. Some level of attenuation in the return signal is expected and a predetermined amplitude level for the signal strength of the return signal for a properly functioning sensing cable is identified empirically and stored in the memory  142 . However, if the amplitude of the return signal drops below a predetermined level, then the controller  140  identifies a potential fault in sensing cable  720 . 
     In an alternative configuration, the sensing cable  720  includes a third conductor that is electrically isolated from the first conductor and second conductor in the sensing cable. In one embodiment, the third conductor is formed as part of a second twisted pair in the sensing cable  720 , while in another embodiment the sensing cable includes two coaxial elements that form three separate conductors. One end of the third conductor is connected to the plate  120  in a similar manner to the first conductor as depict in  FIG. 8C . The other end of the third conductor is connected to an analog to digital converter (ADC) that is mounted to the PCB of the object detection system to provide a digitized version of the sensing signal to the controller  140 . During the process  1400 , the controller  140  measures a return signal based on the excitation signal through the third conductor instead of through the first conductor and the second conductor. 
     The controller  140  identifies if the measured SNR for the excitation signal drops below a predetermined minimum SNR ratio that is suitable for operation of the object detection system  102  (block  1412 ). A fault in the sensing cable  720  attenuates the level of the received signal, introduces additional noise into the sensing cable  720 , or produces both an attenuation of signal strength and increase in noise that degrades the SNR. If the SNR remains above the predetermined threshold, then the sensing cable  720  is considered functional and the saw  100  continues with operation (block  1416 ). However, if the measured SNR falls below the predetermined threshold, then the controller  140  generates an output indicating a potential fault in the sensing cable (block  1420 ). In the saw  100 , the controller  140  generates the output via the user interface  110  to alert an operator to a potential cable fault. In some configurations, the controller  140  disables operation of the saw  100  until the sensing cable  720  is repaired or replaced. 
     It will be appreciated that variants of the above-described and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the following claims.