Abstract:
A safety system for a rotary saw or other dangerous machine tools features a flesh-sensing circuit ( 14, 19 ), located on a safety guard ( 16 ) surrounding a dangerous machine part, which generates an electronic signal to trigger emergency stopping of the machine before the operator can contact the dangerous machine part. The stopping means preferably employs DC injection, winding current direction reversal and/or electromagnetic motor braking, in the event of detection of operator contact with the safety guarding device. The system is adapted to be used both with newly-manufactured machine tools and by retrofitting onto previously-manufactured machine tools, by in-the-field modifications.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from my provisional application Ser. No. 61/525,613, filed 19 Aug. 2011. 
    
    
     FIELD OF THE INVENTION 
     The present invention is in the technical field of safety systems for machine tools. More particularly, the invention is in the field of sensing a dangerous condition, and protecting a machine tool operator from accidentally contacting a dangerous portion of a machine tool, by rapidly disabling or stopping the machine whenever a dangerous condition is detected. 
     BACKGROUND OF THE INVENTION 
     There exists an urgent need for improved safety features for machine tools, especially for machine tools such as table saws, which overall have an especially grim safety record, dating back more than a century. Each day, consumer-grade table saws cause life-changing trauma injuries in the United States and other nations, including an estimated ten amputations daily in the United States alone. Besides table saws, many machine tools, such as woodworking and metalworking machines including, but not limited to: band saws, drill presses, lathes, shapers, jointers, jig saws, disk sanders and spindle shapers, milling machines and grinders, require the operator to manually move a work piece and/or the operator&#39;s hands in close proximity to a cutting blade or to other potentially dangerous elements or features. It is not uncommon for machine operators to inadvertently contact the dangerous element, resulting in a very serious injury such as amputation of fingers or mutilation of hands, resulting in a life-altering disability and much pain and suffering as well, as great financial costs for medical treatment, rehabilitation and lost income. The estimated annual cost of such accidental table saw accidents, in the United States alone, has been estimated to be $2 billion, which places an economic burden on not only the injured persons, but also on employers, manufacturers, retailers, medical institutions, insurance companies and government. 
     In the US, consumer table saws alone presently account not only for ten accidental amputations daily, but for an aggregate of 35,000 visits annually to hospital emergency rooms, to treat injuries of varying levels of severity. 
     Over the past decade, there has been great interest in machine operator flesh-sensing, in order to quickly stop a table saw blade if the operator accidentally contacts the spinning blade. Until now, progress in the art has been largely due to the many inter-related patents of GASS, et al., such as U.S. Pat. No. 6,994,004, which have disclosed a method of destructively stopping a saw blade after direct contact between an operator and the saw blade. Although it is expected that the GASS et al. method, of capacitively sensing direct contact between an operator&#39;s flesh and a moving blade, will reduce the severity of injury to an operator, it will still likely result in some finite level of injury to the operator. U.S. Pat. No. 7,290,474, issued to KELLER, also discloses destructively stopping a saw blade, through the use of an explosive device. All of the presently available systems, for rapidly disabling a machine tool in case of emergency, have disadvantages of excessive cost, lack of convenience and/or lack of adaptability to previously-manufactured machine tools. Unfortunately, perhaps due to the aforementioned disadvantages, over the past decade, safety systems have not been widely implemented, and the accident statistics related to table saw injuries have shown no discernable improvement. 
     SUMMARY OF THE INVENTION 
     It would be desirable to have a system that uses flesh-sensing to improve operator safety on certain classes of machine tools, including table saws, by reacting prior to the start of an actual injury, and quickly enough to prevent the injury from happening or to mitigate injury to the operator. Furthermore, it would also be desirable to have a flesh-sensing safety system that may be used in conjunction with many previously-manufactured machine tools, requiring no modification and only a minor in-the-field electrical addition to the machine tool. Still further, it would be desirable to have an auxiliary flesh-sensing safety system that may be used to selectively control multiple machine tools. Therefore, currently there exists an urgent need in the industry for a system to bring flesh-sensing technology to various classes of machine tools including both existing machine tools as well as newly designed machine tools. 
     Accordingly, the present invention is a safety system for electric-motor powered machine tools and is intended to be used with existing machine tools, as well as for incorporation into new machine tool designs. The basic embodiment of this invention comprises an electrical operator control station, as well as a number of distinct and appropriate devices, adapted to interface with a respective type of machine tool, and incorporating a flesh-sensor which is electrically connected to the operator control station for precisely measuring operator contact or proximity to a dangerous portion of a machine tool. In addition, the present invention includes electric power control circuitry for rapidly braking a machine tool motor or initiating a machine-disabling electronic response as a result of flesh-detection. 
     The present invention addresses a novel amalgam of machine operator safety control and shielding devices including electronic flesh-detection, triggered in advance of an injury in order to electronically react in time to prevent or mitigate machine operator injury. While previous table saw machine tool flesh-sensing and electronic emergency response methods have been proposed, such as those of NIEBERLE-HAUER DE-19509771-A and DILS Published Application US-2004/0194594-A1, both have significant technical shortcomings. Both have been abandoned and neither has been shown to be commercially viable. The HAUER disclosure&#39;s proposed flesh-detection method is likely subject to electrostatic and electromagnetic noise in a machine tool environment and is complex and would be expensive to implement. The DILS abandoned patent application is unworkable due to various technical and electrical errors, including errors in the proposed method of rapidly stopping a machine motor. The NIEBERLE-HAUER patent disclosure proposed a proximity sensing method which is based upon the much earlier THEREMIN circuit disclosed in U.S. Pat. No. 1,661,058 of Feb. 28, 1928, entitled: Method of and Apparatus for the Generation of Sounds. Almost 90 years of no known commercial use, of the THEREMIN entertainment circuit method for human proximity detection, leads to the inescapable conclusion that the early device is impractical for such usage. The NIEBERLE-HAUER application, now abandoned, fails to teach or suggest how to successfully implement a THEREMIN circuit in the electrically noisy environment of an electric-motor-driven table saw. That disclosure is therefore non-enabling as a teaching of reliable capacitive flesh-sensing. A reliable capacitive detector must prevent electrical noise from causing false-positive triggering of emergency stopping measures. 
     The flesh-sensing method employed herein is a capacitive touch flesh-sensing method, which is based upon well-known and very reliable prior art, dating back at least fifty years, such as U.S. Pat. No. 2,704,339, WESCOTT &amp; GEIGER, granted 1955. There are many embodiments of capacitive flesh sensing circuit designs readily available, since the basic electronic principles are well understood and widely published. A large number of capacitive touch-sensing circuit variations are currently available and  FIG. 3  is merely one basic example of a capacitive touch-sensing circuit. More recent resistive, infra-red or surface-acoustic-wave (S-A-W) methods of flesh sensing are also within the scope of the present invention. 
     The present invention includes one or more of the following components: auxiliary machine operating controls, as well as electrical function and condition indicators, which are intended to enhance the safe operation of a machine tool by alerting both the machine operator and nearby observers of the present safety condition of the machine a locking mechanism to prevent use of the machine tool without employing the safety system features controls for arming or disarming the flesh-sensing circuit&#39;s electrical function and condition indicators appropriate to a particular class of machine tool operating power control circuits and emergency machine tool braking circuits. 
     There are many embodiments of capacitive flesh sensing circuit designs readily available, since the basic electronic principles are well understood and widely published. 
    
    
     
       BRIEF FIGURE DESCRIPTION 
       Preferred embodiments of the invention are illustrated in the drawings, of which: 
         FIG. 1  is a basic system block diagram of the invention; 
         FIG. 2  is a basic system block diagram of a multi-port embodiment of the invention; 
         FIG. 3  is a electrical schematic diagram of an exemplary capacitive touch sensing circuit; 
         FIG. 4  is a perspective view of the safety system of the invention, installed on a table saw; 
         FIG. 5  is a perspective view of the safety system of the invention, installed on a band saw; 
         FIG. 6  is a perspective view of an exemplary machine guard installed on a bench-top style table saw, wherein the flesh-sensing and control circuitry are built into a base portion of the saw; 
         FIG. 7  is a perspective view of exemplary machine tool guards and safety shields encompassed by this invention; 
         FIG. 8  is a perspective view of an exemplary machine guard for use with a table saw; 
         FIG. 9  is a perspective view of an exemplary machine guard for use with a band saw; 
         FIG. 10  is an exemplary electrical schematic diagram of a single-port embodiment of the invention for use with machine tools employing a single-phase induction motor; 
         FIG. 11  is an exemplary electrical schematic diagram of a multi-port embodiment of the invention for use with machine tools employing single-phase induction motors; and 
         FIG. 12  is an exemplary electrical schematic diagram of a single-port embodiment of the invention, for use with machine tools employing series-wound universal motors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The basic embodiment of the invention is block-diagramed in  FIG. 1  as a single machine tool safety system or in  FIG. 2  as a multi-port machine tool safety system. The system of the present invention is made up of the following two main components: an electrical control safety station  1  suitable for use with electric-motor powered machine tools and a flesh-sensing machine guard  16  suitable for sensing presence of the operator&#39;s hands or body within close proximity to a dangerous portion of a machine tool, the machine guard being electrically connected to the control station. These components work together to create an architecture for the system that offers control, machine status indicators and flesh-sensing emergency braking or an alternative reaction which improves machine tool safety. It should further be noted that this invention embodiment may be electrically modified, according to the requirements of various classes of machine tools, with respect to parameters such as electric motor size, type, power rating, motor braking requirements and design. The geometry, transparency and size of the safety guard(s) can also be modified in a manner appropriate to the particular machine tool, on which the guard(s) will be installed. 
       FIGS. 2 and 11  show schematically a multi-port (4-port) implementation of the invention, for selectively controlling more than one machine tool. 
       FIGS. 1 ,  2 ,  4 - 9 , illustrate one or more solid transparent machine tool guards, to prevent contact between an operator&#39;s hand or other body part and a dangerous machinery component such as a saw blade. In addition to the basic exemplary machine guard shown in  FIGS. 1 ,  2 ,  4 - 9 , an auxiliary transparent peripheral safety shield  10 , shown in  FIGS. 4-9 , may also be coupled to the machine guard, to further aid in preventing operator contact with a dangerous machine part. At least one of the machine guard  16  and the auxiliary safety shield  10  incorporates capacitive flesh-sensors in the present embodiment, for detecting operator contact or proximity. The capacitive flesh-sensors  14  are typically located along the vertical surfaces of the machine guard or the auxiliary safety shield  10 , as shown in  FIGS. 7-9 . 
     Both the machine guard and auxiliary shield&#39;s shape, size and geometry are determined by the combination of the intended machine&#39;s overall design, as well as the distance from the machine danger zone at which the operator&#39;s contact is intended to deactivate or emergency-stop the machine tool, by first sensing the operator&#39;s flesh and, in response thereto, generating an electronic signal to rapidly initiate appropriate action to prevent injury to the machine operator. In the present embodiment, the electrical circuitry depicted schematically in  FIG. 3  is employed to activate and sense the capacitive change to the flesh-sensors  14  when the operator is very close to contacting, or is touching, the flesh-sensors. According to a preferred embodiment of the  FIG. 3  circuit, namely model QTFS3X, commercially available from professional engineer (and patentee) David A. Johnson of Lubbock, Tex. USA, a 20 kHz oscillator provides a capacitance value sample rate of about every 50 milliseconds. A higher oscillator frequency could alternately be used, for faster touch response, if desired. Numerous variations are possible and will be apparent to those having ordinary skill in the capacitance sensing art. A suitable threshold value for triggering is a delta (capacitance change) of 10 picofarads. An output signal from the detection circuit  19  can then actuate a control circuit which generates appropriate control signals to perform the emergency electromagnetic braking of the motor driving the saw blade. In this manner, detection occurs the instant that the operator touches the sensor, before contact is made with the blade, and thus before the operator can be injured by the blade or other dangerous tool part. 
     The embodiment depicted in  FIG. 3  has been demonstrated to have sufficient electrical noise immunity to avoid false positive triggering (triggering when no body part is actually present), while being capable of detecting and responding to a capacitance change (caused by a touch of the saw operator) as low as 10 picofarads at the flesh-sensing conductor  14 . Setting the threshold value lower than 10 picofarads would increase the sensitivity of the detector, at the risk of causing false-positive events, while setting the threshold value too high would reduce the sensitivity of the device, at the risk of causing false-negative events, i.e. instances when the emergency stop should be triggered but is not triggered. Generally planar element  10 , shown in perspective view in  FIGS. 4-9 , is an exemplary machine safety shield  10 , being used on a sawing machine, together with a blade safety guard  16 . The sensitivity is adjusted using a variable resistor connected to the emitter and base terminals of a PNP switching transistor, preferably a Philips model 2907 as shown in  FIG. 3 . 
     An important feature, noted here, of the safety guard  16  intended for table saws, is that the elevation of the guard is specifically intended to be adjusted, to just above the work piece, and firmly secured in position, in order to prevent the dangerous table saw phenomenon known as kick-back which is believed to be the cause of most table saw related injuries. Kick-back refers to a common but dangerous phenomenon in which the wooden workpiece is lifted off the sawing table by the saw blade, and ejected, contrary to the direction of feed, back toward the operator at a high velocity. 
     The machine guard and safety shield may be made of any suitable solid transparent material, which will permit the machine operator to observe the workpiece. Sectors of the machine guard or safety shield surfaces are to be made electrically conductive, in order to serve as part of touch- or proximity-activated electronic sensors serving to detect operator body contact or proximity with the conductive portion of the safety shield. The conductive segments of the machine safety guard may or may not also be transparent to the operator. Suitable materials for conductive transparent electrodes are indium tin oxide (ITO) or other transparent film coatings (TFC) such as carbon nano-tubes (CNT). Such coatings can be applied to various suitable transparent substrates such as polycarbonate. 
     In the exemplary embodiment presented herein, all or a portion of the periphery of the machine guard and safety shield embodiments, shown as  14  in  FIGS. 4-9 , are electrically conductive and will enable an associated electronic circuit (such as the aforementioned Johnson QTFS3X) to detect operator contact with the edges of the generally-planar exemplary safety guard. As shown as  FIG. 7 , additional sectors  13  of the shield  10  or the guard  16  may also be conductive, so as to facilitate electrical connection between the periphery of the guard  16  and external electronic circuits. 
       FIGS. 5 and 9  depict an embodiment of the machine guard  16  and safety shield  10  to be employed on a band saw and in addition,  FIG. 7  depicts examples of other suitable variations in size and shapes of machine guards  16  and safety shields  10 . It should be understood that other machine guard and safety shield modifications, substitutions and alternatives will be apparent to a person having ordinary skill in the machine safety art. Such modifications, substitutions and alterations can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
       FIGS. 4-5  depict an exemplary auxiliary or external machine control station enclosure  5 , a machine guard  16 , a machine guard support  17  ( FIGS. 4 and 6 ) and a safety shield  10  with a flesh-sensing conductive surface  14 , all mounted on a table saw  9 . The machine control station enclosure  5  typically contains the circuitry diagramed in  FIGS. 1 ,  2 ,  10 - 12 , including lighted indicators  6 , a locking key  7 , and a start-stop switch  8  as shown in  FIGS. 4-5  and any other controls and circuit components which may be useful to the machine operator. 
       FIGS. 4-5  depict a similar exemplary machine control station enclosure  5 , machine guard  16  shown in  FIGS. 1 ,  2 ,  4 - 8 , and safety shield  10 , attached to a band saw  11  ( FIG. 5 ). The functionalities, of the control stations depicted in  FIGS. 4 and 5 , are essentially identical, in that they are intended to emergency-stop the machine&#39;s induction motor upon detection of operator proximity to, or direct contact with, the flesh-sensing surface  14  of either the machine guard  16  or the safety shield  10 . 
       FIG. 6  is a perspective view of exemplary machine tool guards and safety shields which are encompassed by this invention. Element  13 , shown in  FIG. 7 , is a typical conductive electrical sector of a safety shield, used to connect the conductive surfaces  14  to the external electrical sensing circuit  19  as shown in  FIGS. 10 ,  11 , and  12 . 
       FIG. 6  is an exemplary perspective view of safety system of this invention as factory-installed on a bench-top style table saw  15 . The machine guard  16 , safety shield  10  and machine guard support  17  are shown here, while the control station electronics are factory-installed within the table saw base and therefore not visible in the drawing. 
       FIG. 8  is a perspective close-up view of an exemplary blade guard  16  and safety shield  10  for a table saw. 
       FIG. 9  is a perspective close-up view of an exemplary blade guard  16  and safety shield  10  for a band saw which also includes a blade light and switch  20  and a brake arming push-button switch  21 . 
       FIG. 10  is an exemplary electrical schematic diagram of the invention, intended for use with a single machine tool which employs a single-phase induction motor where the exemplary emergency-braking method employed is DC injection, also referred to as “dynamic braking.” Both the mechanical relays and contactors shown in the drawings (of a prototype) could be replaced with solid-state devices, in order to Improve the circuit characteristics and to reduce volume production cost.  FIG. 10  shows two Contactors, C 1  and C 2 . For normal motor running, contactor C 1  is closed to apply AC line power to the motor windings. For emergency stopping, contactor C 2  is closed, to apply rectified 120 volts. As shown in  FIG. 10 , these contactors C 1  and C 2  are controlled by a START/STOP six-relay interlock chain, to control starting and stopping the saw motor. In addition, there is a capacitive touch switch module, Johnson QTFS3X, a 12 volt DC power supply, and control switches and LED indicators. The circuitry to the left of the lower cable connector is contained within the blade enclosure for an exemplary machine tool and all the remaining circuitry is contained within the control unit. C 1  is the contactor supplying 120 VAC to the machine motor. C 2  will supply a short burst of the rectified 120 VAC to an induction machine motor, to act as an emergency brake. This short burst of DC power is typically 200 milliseconds in duration, as specified by the ARTISAN® 4310A timer shown or by a similar circuit. 
     Emergency machine stopping is initiated whenever (A) the brake circuit is armed by the operator, and (B) the operator subsequently contacts the proximity plates shown in  FIG. 10  or sensors  14  shown in other figures. Touching the sensor will trigger the Johnson QTFS3X capacitive touch circuit, or a similar circuit, to activate a Relay  1 , shown at top right. The proximity switch module is connected to metal foil sensors  14  along the bottom edges of the saw blade enclosure. Should the operator touch, or come very close to, these sensors, the circuit will sense an increase in capacitance and activate R 1 , which will initiate braking. R 1  NC (Normally Closed) is in the 12-volt DC motor start series chain, along with the Start-Stop switches, S 1 , C 2  and relays R 3  and R 4 , as shown in  FIGS. 10-12 . A third relay R 3 , together with the R-C components and transistor, acts as a bi-stable circuit or flip-flop, controlled by the toggle Brake Arm-Disarm switch, as well as the C 1  NC auxiliary contacts closure signal when the motor power is removed. This circuit arrangement requires the operator to always arm the braking circuit, before each successive Start, and will always automatically disarm the brake when the saw is manually or automatically stopped and C 1  is de-energized. Contactors C 1  and C 2  are mechanically interlocked, to insure they cannot be activated simultaneously. Alternatively, braking could be accomplished by using a variable frequency drive, such as the model D700 available from Mitsubishi Electric Company. 
       FIG. 11  is an exemplary electrical schematic diagram of the invention as a multi-port control station, intended for use with up to four machine tools employing respective single-phase induction motors. This embodiment employs a port selector switch  18 , shown on the lower right of the diagram. Both embodiments show contactors C 1 , C 2  at lower left. 
       FIG. 12  is an exemplary electrical schematic diagram of the invention, intended for use with a single machine tool which employs a single-phase series-wound universal motor where the exemplary emergency motor braking method employed is counter-current produced by reversing current flow through either the stator winding or the rotor winding, while short-circuiting the disconnected power line. In  FIG. 12 , elements comparable to those used in  FIG. 10  and  FIG. 11  bear similar designations. This method of motor braking, also referred to as plugging, is applicable to many tools that are driven by universal motors, such as the table saw depicted in  FIG. 6 . For emergency braking, after power supply to the motor is removed by opening contactor C 1 , contactor C 2  is engaged for a brief period of time, whose duration is specified by a digital timer, preferably an ARTISAN® model 4310A timer as shown. The C 2  contacts reverse the direction of current flow in the stator winding of the motor, and short the motor power leads to complete the circuit, causing a high current flow, as the rotor and stator fields magnetically oppose one another, until the motor comes to a stop. After the rotor comes to a stop, the C 2  contacts open, so that the circuit and motor return to a configuration which permits normal motor operation. 
     If electro-mechanical braking is desired, instead of electromagnetic braking, the flesh-sensing circuit could be configured to trigger operation of a conventional brake motor, such as those commercially available from Baldor Electric Company of Fort Smith, Ark., USA (member of the ABB Group) or KEBCO Power Transmission of St. Paul, Minn., USA. 
     Various changes and modifications are possible, within the scope of the inventive concept, as will be apparent to persons having ordinary skill in the machine tool safety art.