Safety mechanisms for power tools, including magnetorhelogical brake for blade

Abstract

Various safety systems for power tools, and in particular table saws include detection systems for detecting a dangerous condition relative to a blade of the power tool, reaction systems for taking mitigation action in response to detection of a dangerous condition. The safety system may detect, prevent, and/or mitigate a dangerous condition associated with the power tool.

What is claimed is: 
     
         1 . A table saw comprising:
 a cutting surface;   a motor-driven rotatable shaft positioned below the cutting surface;   a blade for cutting a workpiece on the cutting surface, wherein the blade spins when the shaft rotates, and wherein a portion of the blade is extendable above the cutting surface;   a detection system for detecting a dangerous condition relative to the blade;   a reaction system in communication with the detection system for taking a reaction in response to detection of the dangerous condition, wherein the reaction system comprises a magnetorheological rotary brake connected to the shaft that brakes the shaft to thereby brake the spinning of the blade in response to detection of the dangerous condition by the detection system.   
     
     
         2 . The table saw of  claim 1 , wherein the blade is mounted on the rotatable shaft 
     
     
         3 . The table saw of  claim 1 , wherein the magnetorheological rotary brake comprises:
 a rotor fixed to the shaft;   a housing, wherein the housing and the rotor define a spacing;   magnetorheological fluid in the spacing; and   a magnetic field-producing coil that is energized in response to detection of the dangerous condition by the detection system to produce a magnetic field that causes the magnetorheological fluid to increase its viscosity to brake the rotor, thereby braking the shaft, thereby braking the blade.   
     
     
         4 . The table saw of  claim 3 , wherein the blade is mounted on the rotatable shaft. 
     
     
         5 . The table saw of  claim 1 , further comprising a kickback detection system for detecting kickback of the workpiece during cutting of the workpiece. 
     
     
         6 . The table saw of  claim 5 , wherein the reaction means further comprises a downstream kickback mitigation means for mitigating kickback of the workpiece in response to detection of kickback of the workpiece. 
     
     
         7 . The table saw of  claim 1 , further comprising a clutch coupled to the rotatable shaft, such that when the clutch is engaged and the motor is running, the blade shaft is rotated, and wherein the reaction means comprises a motor shut-down circuit connected to the clutch and the brake system, wherein the motor shut-down circuit disengages the clutch and actuates the brake system to brake the blade when the motor is turned off. 
     
     
         8 . The table saw of  claim 1 , further comprising a blade-spin detection means for detecting whether the blade is rotating based on energy from the blade, wherein the blade-spin detection system is in communication with the reaction means and provides an output to arm the reaction means when the blade-spin detection system detects that the blade is spinning. 
     
     
         9 . The table saw of  claim 1 , further comprising an electrical generator connected by one or more gears to the rotatable shaft, wherein the electrical generator is further connected to the reaction means and generates electricity when the shaft rotates to power at least one of the reaction means and the detection means. 
     
     
         10 . The table saw of  claim 1 , wherein the blade comprises:
 an outer peripheral portion that comprises a first material; and   an inner portion that comprises a second material that is different from the first material, wherein the first material is denser than the second material.   
     
     
         11 . The table saw of  claim 1 , further comprising a passive downstream safety member for mitigating kickback of the workpiece, wherein the passive downstream safety member is not actuated by the detection means. 
     
     
         12 . The table saw of  claim 11 , wherein the passive downstream safety member comprises angled snells that impede kickback of the workpiece. 
     
     
         13 . The table saw of  claim 1 , wherein the detection means comprises a blade contact sensor means for sensing contact with by a foreign object with the blade, wherein the blade contact sensor is in communication with the reaction means, and wherein the reaction means is for taking the reaction in response to detection of contact by a foreign object with the blade upon detection by the blade contact sensor means. 
     
     
         14 . The table saw of  claim 13 , wherein:
 the blade comprises:
 a first electrically conductive blade portion; 
 a second electrically conductive blade portion that comprises teeth for cutting the workpiece; and 
 a dielectric between the first and second electrically conductive blade portions, and 
   the blade contact sensor means is connected to the first electrically conductive blade portion and drives the first electrically conductive blade portion with a drive signal, and wherein the blade contact sensor means comprises a processor for detecting contact with the blade by the object based on an electrical signal from the first electrically conductive blade portion.   
     
     
         15 . The table saw of  claim 1 , further comprising a blade proximity sensor means for sensing proximity of a foreign object to the blade, wherein the blade proximity sensor means is in communication with the reaction means, and wherein the reaction means is for taking the reaction in response to detection of proximity by a foreign object to the blade upon detection by the blade proximity sensor means. 
     
     
         16 . The table saw of  claim 1 , further comprising sensor means connected to the cutting surface for sensing a characteristic of the workpiece during cutting of the workpiece.