Power tool with digital variable reluctance motor control

Abstract

A power tool includes a control for the motor of the power tool that senses an operating characteristic of the motor and controls the operation of the power tool based on the sensed characteristic. The sensed characteristic includes variations in reluctance of the motor. A controller may sense changes on load on the motor as a result of interaction between a working element such as a saw blade or drill bit and the work piece. The controlled operation may include variation in speed or torque or both, or may include stopping the motor. Emergency conditions may be sensed by changes in reluctance.

We claim: 
     
         1 . A method for operating a power tool, comprising:
 operating a motor of the power tool;   driving a working element of the power tool using the motor, the working element engaging a work piece during the driving of the working element;   sensing reluctance of the motor during the driving of the working element;   sensing a predetermined change in the reluctance of the motor during driving of the working element, the predetermined change in the reluctance corresponding to a change in a resistance to the driving of the working element wherein the change in the resistance to driving is a result of interaction between the working element and the work piece; and   automatically performing a predetermined action upon sensing the predetermined change, the predetermined action being an action selected from the group consisting of: stopping operation of the motor, reversing operation of the motor, increasing the speed of the motor, decreasing the speed of the motor, increasing the torque output by the motor, and decreasing the torque by the motor;   wherein the automatically performed predetermined action includes reading a programmed action from a memory.   
     
     
         2 . A method as claimed in  claim 1 , wherein the predetermined action includes operating the motor according to a predetermined operating profile. 
     
     
         3 . A method as claimed in  claim 2 , wherein the predetermined operating profile includes:
 increasing the motor speed according to the operating profile upon sensing a first predetermined change in the motor reluctance, and   decreasing the motor speed according to the operating profile upon sensing a second predetermined change in the motor reluctance.   
     
     
         4 . A method as claimed in  claim 3 , wherein the predetermined operating profile includes:
 reversing the motor operation upon sensing a third predetermined change in the motor reluctance.   
     
     
         5 . A method as claimed in  claim 3 , wherein the predetermined operating profile includes halting motor operation upon sensing a fourth predetermined change in the motor reluctance. 
     
     
         6 . A method as claimed in  claim 1 , further comprising:
 detecting an emergency condition by sensing a predetermined emergency change in the motor reluctance; and   automatically performing an emergency operation of the motor upon detection of the emergency condition.   
     
     
         7 . A method as claimed in  claim 6 , wherein the detecting of the emergency condition includes detecting release of the workpiece from an engaged condition. 
     
     
         8 . A method as claimed in  claim 6 , wherein the emergency condition sensed by a change in the motor reluctance includes at least one of sensing an unbalanced condition of the working element, sensing a fracture of the working element, or sensing a chip in the working element. 
     
     
         9 . A method as claimed in  claim 1 , wherein the predetermined action includes one of increasing the torque or decreasing the torque output by the motor while maintaining a speed of the motor. 
     
     
         10 . A method as claimed in  claim 1 , wherein the predetermined action includes inhibiting operation of the power tool upon sensing a load beyond a capacity of the power tool. 
     
     
         11 . A method as claimed in  claim 1 , wherein the sensing the predetermined change includes a first sensing of an increase in load on the working element upon an initial contact of the working element with the work piece and a second sensing of a decrease in load on the working element upon the working element reaching an end of the work piece. 
     
     
         12 . A method as claimed in  claim 1 , wherein the sensing the predetermined change includes sensing a decrease in load on the working element upon the working element reaching a void in the work piece. 
     
     
         13 . A method as claimed in  claim 1 , further comprising:
 emitting an indication upon performing the predetermined action, wherein the indication includes at least one of light, sound or vibration.   
     
     
         14 . A method as claimed in  claim 1 , wherein the sensing the change in the reluctance include sensing the change in reluctance as a result of at least one of: a change in thickness of the workpiece, a change in density of the workpiece, a knot in the work piece, or a void in the work piece. 
     
     
         15 . A power tool, comprising:
 a motor connected for selective operation;   a working element connected to an output the motor for driving the working element;   a control connected to the motor for sensing changes in reluctance by the motor as a result of changes in load on the working element, the control being operable to automatically perform an action upon sensing a predetermined change in the reluctance, the action including at least one of: stopping operation of the motor, reversing operation of the motor, increasing a speed of the motor, decreasing the speed of the motor, increasing an output torque by the motor, and decreasing the output torque by the motor; and   a memory and a processor in the power tool, the memory storing a motor operating profile and the processor being operable to execute the motor operating profile upon sensing predetermined changes in the reluctance.   
     
     
         16 . A power tool as claimed in  claim 15 , wherein the control is operable to operate the motor in alternating forward and reverse operation. 
     
     
         17 . A method for operating a drill, comprising:
 operating a motor of the drill;   driving a bit of the drill using the motor, the bit engaging a work piece during the driving of the bit;   sensing a reluctance of the motor during the driving of the bit;   sensing a predetermined change in the reluctance of the motor during the driving of the bit, the predetermined change in the reluctance corresponding to a change in a load on the bit by interaction between the bit and the work piece; and   automatically performing a predetermined action upon sensing the predetermined change, the predetermined action including increasing or decreasing a speed of the motor or reversing the motor upon sensing the predetermined condition;   wherein the automatically performed predetermined action includes reading a programmed action from a memory.   
     
     
         18 . A method as claimed in  claim 17 , further comprising:
 automatically increasing a speed of the motor upon sensing the bit reaching an exit surface of the work piece by a change in the reluctance.   
     
     
         19 . A method as claimed in  claim 17 , further comprising:
 automatically increasing a speed of the motor upon sending initial contact of the bit with a surface of the work piece by a change in the reluctance.