Power tool anti-kickback system with rotational rate sensor

Abstract

A control system is provided for use in a power tool. The control system includes: a rotational rate sensor having a resonating mass and a controller electrically connected to the rotational rate sensor. The rotational rate sensor detects lateral displacement of the resonating mass and generates a signal indicative of the detected lateral displacement, such that the lateral displacement is directly proportional to a rotational speed at which the power tool rotates about an axis of the rotary shaft. Based on the generated signal, the controller initiates a protective operation to avoid further undesirable rotation of the power tool. The controller may opt to reduce the torque applied to shaft to a non-zero value that enables the operator to regain control of the tool.

What is claimed is: 
     
       1. A method for initiating a protective response in a power tool having a rotary shaft, comprising:
 monitoring rotational motion of the power tool about a longitudinal axis of the rotary shaft using a rotational motion sensor disposed in the power tool;   computing angular displacement of the power tool about the axis of the rotary shaft using a controller disposed in the power tool and based on input from the rotational motion sensor;   initiating a protective operation by the controller when an operating condition of the power tool exceeds a threshold and the angular displacement of the power tool falls within a range of angular displacements; and   initiating a protective operation by the controller when the operating condition of the power tool is less than the threshold but the angular displacement of the power tool exceeds the range of angular displacements.   
     
     
       2. The method of  claim 1  further comprises initiating a protective operation when angular velocity of the power tool about the axis exceeds a velocity threshold and the angular displacement of the power tool falls within the range of angular displacements. 
     
     
       3. The method of  claim 1  further comprises initiating a protective operation when angular displacement of the power tool falls within a range of angular displacements and angular acceleration of the power tool about the axis exceeds an acceleration threshold. 
     
     
       4. The method of  claim 1  further comprises arranging the rotational motion sensor at a location in the power tool spatially separated from the rotary shaft. 
     
     
       5. The method of  claim 1  further comprises employing a rotational motion sensor that measures rotational velocity based on Coriolis acceleration. 
     
     
       6. The method of  claim 1  wherein the protective operation when angular displacement of the power tool falls within a range of angular displacements is different than the protective operation when angular displacement of the power tool exceeds the range of angular displacements. 
     
     
       7. The method of  claim 1  wherein the protective operation is selected from the group consisting of pulsing a motor of the power tool, braking the rotary shaft, braking the motor, disengaging the motor from the rotary shaft, discontinuing power delivered to the motor and reducing slip torque of a clutch disposed between the motor and the rotary shaft. 
     
     
       8. A method for initiating a protective response in a power tool having a motor drivably coupled to a rotary shaft to impart rotary motion thereto, comprising:
 monitoring rotational motion of the power tool about a longitudinal axis of the rotary shaft using a rotational motion sensor disposed in the power tool;   determining angular displacement of the power tool about the axis of the rotary shaft from a baseline using a controller disposed in the power tool and based on input from the rotational motion sensor;   initiating a protective operation in the power tool by the controller when a first operating condition of the power tool exceeds a first operating threshold and angular displacement of the power tool falls within a first range of angular displacements; and   initiating a protective operation in the power tool by the controller when a second operating condition of the power tool exceeds a second operating threshold and angular displacement of the power tool falls within a second range of angular displacements, where the second operating condition is different than the first operating condition and the second range of angular displacements is mutually exclusive of the first range of angular displacements.   
     
     
       9. The method of  claim 8  further comprises initiating a protective operation when angular velocity of the power tool about the axis exceeds a velocity threshold and angular displacement of the power tool falls within the first range of angular displacements. 
     
     
       10. The method of  claim 9  further comprises initiating a protective operation when angular velocity of the power tool is less than the velocity threshold and angular displacement of the power tool falls within the second range of angular displacements. 
     
     
       11. The method of  claim 8  further comprises arranging the rotational motion sensor at a location in the power tool spatially separated from the rotary shaft. 
     
     
       12. The method of  claim 8  further comprises employing a rotational motion sensor that measures rotational velocity based on Coriolis acceleration. 
     
     
       13. The method of  claim 8  further comprises periodically resetting the baseline when angular velocity of the power tool about the axis is less than a velocity threshold. 
     
     
       14. The method of  claim 8  wherein the protective operation is selected from the group consisting of pulsing a motor of the power tool, braking the rotary shaft, braking the motor, disengaging the motor from the rotary shaft, discontinuing power delivered to the motor and reducing slip torque of a clutch disposed between the motor and the rotary shaft. 
     
     
       15. A method for initiating a protective response in a power tool having a rotary shaft, comprising:
 monitoring rotational motion of the power tool about a longitudinal axis of the rotary shaft using a rotational motion sensor disposed in the power tool;   computing angular displacement of the power tool about the axis of the rotary shaft from a baseline using a controller disposed in the power tool and based on input from the rotational motion sensor;   periodically resetting the baseline when angular velocity of the power tool about the axis is less than a velocity threshold;   initiating a protective operation by the controller when an operating condition of the power tool exceeds a threshold and the angular displacement of the power tool falls within a range of angular displacements; and   initiating a protective operation by the controller when the operating condition of the power tool is less than the threshold but the angular displacement of the power tool exceeds the range of angular displacements.   
     
     
       16. A method for calibrating a power tool having a rotational rate sensor, comprising:
 mounting the power tool to a test fixture, the power tool having a rotary shaft and a rotational rate sensor configured to detect rotational motion of the power tool about a longitudinal axis of the rotary shaft;   rotating the power tool at a known angular velocity about the longitudinal axis using the test fixture;   measuring, by the rotational rate sensor, an angular velocity of the power tool rotating about the longitudinal axis; and   computing a first difference between the measured angular velocity and the known angular velocity.    
     
     
       17. The method of claim 16 further comprises measuring output of the rotational rate sensor when the power tool is stationary on the test fixture to obtain an offset calibration value.  
     
     
       18. The method of claim 16 further comprises
 adjusting the angular velocity measured by the rotational rate sensor using the difference; and   comparing the adjusted angular velocity to the known angular velocity to verify calibration of the tool.    
     
     
       19. The method of claim 16 further comprises
 rotating the power tool at the known angular velocity in an opposite direction about the longitudinal axis using the test fixture;   measuring, by the rotational rate sensor, an angular velocity of the power tool rotating in the opposite direction about the longitudinal axis; and   computing a second difference between the measured angular velocity and the known angular velocity.    
     
     
       20. The method of claim 16 further comprises measuring angular velocity based on Coriolis acceleration.  
     
     
       21. The method of claim 16 further comprises
 storing the differences in a memory of the power tool;   removing the power tool from the test fixture; and   adjusting, during operation of the power tool, output reported by the rotational rate sensor using the difference values.    
     
     
       22. A method for calibrating a power tool having a rotary shaft, comprising:
 removing a test module from the power tool, the test module detachably couples to the power tool and houses a rotational rate sensor configured to detect rotational motion of the power tool about a longitudinal axis of the rotary shaft;   mounting the test module to a test fixture;   rotating the test module at a known angular velocity about the longitudinal axis using the test fixture;   measuring, the rotational rate sensor, an angular velocity of the test module rotating about the longitudinal axis; and   computing a first difference between the measured angular velocity and the known angular velocity.    
     
     
       23. The method of claim 22 further comprises measuring output of the rotational rate sensor when the test module is stationary on the test fixture to obtain an offset calibration value.  
     
     
       24. The method of claim 22 further comprises
 adjusting the angular velocity measured by the rotational rate sensor using the difference; and   comparing the adjusted angular velocity to the known angular velocity to verify calibration of the tool.    
     
     
       25. The method of claim 22 further comprises
 rotating the test module at a known angular velocity in an opposite direction about the axis using the test fixture;   measuring, by the rotational rate sensor, an angular velocity of the test module rotating in the opposite direction about the longitudinal axis; and   computing a second difference between the measured angular velocity and the known angular velocity.    
     
     
       26. The method of claim 22 further comprises measuring angular velocity based on Coriolis acceleration.  
     
     
       27. The method of claim 26 further comprises
 storing the first and second differences in a memory of the test module;   removing the test module from the test fixture;   re-installing the test module in the power tool; and   adjusting, during operation of the power tool, output reported by the rotational rate sensor using the first and second differences.    
     
     
       28. A method for operating a power tool having a rotational rate sensor, comprising:
 mounting the power tool to a test fixture, the power tool having a rotary shaft and a rotational rate sensor configured to detect rotational motion of the power tool about a longitudinal axis of the rotary shaft;   rotating the power tool at a known angular velocity about the longitudinal axis using the test fixture;   measuring, by the rotational rate sensor, an angular velocity of the power tool rotating about the longitudinal axis;   computing a difference between the measured angular velocity and the known angular velocity;   storing the difference in a memory of the power tool   monitoring rotational motion of the power tool about a longitudinal axis of the rotary shaft using a rotational motion sensor disposed in the power tool;   computing angular displacement of the power tool about the axis of the rotary shaft using a controller disposed in the power tool, the computation being based on input from the rotational motion sensor and the difference in the memory;   initiating a protective operation by the controller when an operating condition of the power tool exceeds a threshold and the angular displacement of the power tool falls within a range of angular displacements; and   initiating a protective operation by the controller when the operating condition of the power tool is less than the threshold but the angular displacement of the power tool exceeds the range of angular displacements.