Electric power tool configured to detect twisted motion

1. An electric power tool comprising:

a housing;

a motor that is housed in the housing;

an output shaft that is housed in the housing and includes a first end for attachment to a tool bit, the output shaft being configured to be rotatively driven by the motor and configured to rotate the tool bit about a Z-axis; and

a twisted-motion detector that is configured to detect twisting of the housing,

wherein the twisted-motion detector includes:

a three-axes acceleration sensor configured to measure accelerations of the housing including a first axis acceleration along a first axis, a second axis acceleration along a second axis, and a third axis acceleration along a third axis; and

an acceleration detection circuit configured to:

determine an X-axis acceleration along an X-axis orthogonal to the Z-axis without a gravity acceleration component based on an input signal from the three-axes acceleration sensor;

calculate an angular acceleration of the housing about the Z-axis from the X-axis acceleration;

calculate a change in an angular velocity based on integrating the angular acceleration for a most recent period;

determine a Z-axis angular velocity about the Z-axis as equal to the change in the angular velocity, without adding a previous change in the angular velocity from before the most recent period; and

detect a twisted-motion of the housing based on the Z-axis angular velocity.

2. The electric power tool according to claim 1 , wherein:

calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during a first period while including a first weighting factor; and

integrating the angular acceleration during a second period while including a second weighting factor,

the second period occurs before the first period,

the most recent period equals the second period plus the first period, and

the second weighting factor is smaller than the first weighting factor.

3. The electric power tool according to claim 2 , wherein:

the first weighting factor is constant, and

the second weighting factor monotonically increases from zero to the first weighting factor during the second period.

4. The electric power tool according to claim 3 , wherein the second weighting factor linearly increases from zero to the first weighting factor during the second period.

5. The electric power tool according to claim 1 , wherein:

calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as integrating the angular acceleration during the most recent period while including a weighting factor in the most recent period, and

the weighting factor increases in a step-wise manner during the most recent period.

6. The electric power tool according to claim 1 , wherein:

calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during a first period while including a constant first weighting factor;

integrating the angular acceleration during a second period while including a constant second weighting factor;

integrating the angular acceleration during a third period while including a constant third weighting factor; and

integrating the angular acceleration during a fourth period while including a constant fourth weighting factor,

the fourth period occurs before the third period,

the third period occurs before the second period,

the second period occurs before the first period,

the most recent period equals the fourth period plus the third period, the second period, and the first period,

the fourth weighting factor is smaller than the third weighting factor,

the third weighting factor is smaller than the second weighting factor, and

the second weighting factor is smaller than the first weighting factor.

7. The electric power tool according to claim 1 , wherein calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during the most recent period while including a weighting factor monotonically increasing during the most recent period.

8. The electric power tool according to claim 1 , wherein calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during the most recent period while including a weighting factor linearly increasing during the most recent period.

9. The electric power tool according to claim 1 , wherein calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during the most recent period while including a weighting factor non-linearly increasing during the most recent period.

10. The electric power tool according to claim 1 , wherein the acceleration detection circuit is configured to filter the input signal from the acceleration sensor by a digital filter, and determine the X-axis acceleration without the gravity acceleration component based on the input signal filtered.

11. The electric power tool according to claim 10 , wherein the digital filter includes a high-pass filter.

12. The electric power tool according to claim 1 , wherein detecting the twisted-motion of the housing based on the Z-axis angular velocity is defined as:

calculating a change in angle based on integrating the Z-axis angular velocity for the most recent period,

determining a present rotation angle about the Z-axis as equal to the change in angle, without adding a previous change in angle from before the most recent period, and

detecting the twisted-motion of the housing based on the present rotation angle.

13. The electric power tool according to claim 12 , wherein detecting the twisted-motion of the housing based on the present rotation angle is defined as:

calculating an estimated rotation angle based on the present rotation angle and a present Z-axis angular velocity, the estimated rotation angle being a rotation angle during a time until when the tool bit stops, and

detecting the twisted-motion of the housing based on comparing the estimated rotation angle and a threshold rotation angle.

14. An electric power tool comprising:

a housing;

a motor that is housed in the housing;

an output shaft that is housed in the housing and includes a first end for attachment to a tool bit, the output shaft being configured to be rotatively driven by the motor and configured to rotate the tool bit about a Z-axis; and

a twisted-motion detector that is configured to detect twisting of the housing, wherein the twisted motion detector includes:

a three-axes acceleration sensor is configured to measure accelerations of the housing including a first axis acceleration along a first axis, a second acceleration along a second axis, and a third acceleration along a third axis; and

an acceleration detection circuit configured to:

calculate an angular acceleration of the housing about the Z-axis based on an input signal from the three-axes acceleration sensor;

calculate a change in an angular velocity based on integrating the angular acceleration for a most recent period;

determine a Z-axis angular velocity about the Z-axis as equal to the change in the angular velocity, without adding a previous change in the angular velocity from before the most recent period; and

detect a twisted-motion of the housing based on the Z-axis angular velocity, and wherein:

calculating the change in an angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during a first period while including a first weighting factor; and

integrating the angular acceleration during a second period while including a second weighting factor,

the second period occurs before the first period,

the most recent period equals the second period plus the first period, and

the second weighting factor is smaller than the first weighting factor.

15. The electric power tool according to claim 14 , wherein:

the first weighting factor is constant, and

the second weighting factor monotonically increases from zero to the first weighting factor during the second period.

16. The electric power tool according to claim 15 , wherein the second weighting factor linearly increases from zero to the first weighting factor during the second period.

17. The electric power tool according to claim 14 , wherein the second weighting factor increases in a step-wise manner from zero to the first weighting factor during the second period.

18. The electric power tool according to claim 14 , wherein calculating the change in the angular velocity based on integrating the angular acceleration for the most recent period is defined as:

integrating the angular acceleration during the most recent period while the second weighting factor monotonically increases during the second period.

19. The electric power tool according to claim 14 , wherein detecting the twisted-motion of the housing based on the Z-axis angular velocity is defined as:

calculating a change in angle based on integrating the Z-axis angular velocity for the most recent period;

determining a present rotation angle about the Z-axis as equal to the change in angle, without adding a previous change in angle from before the most recent period; and

detecting the twisted-motion of the housing based on the present rotation angle.

20. The electric power tool according to claim 19 , wherein detecting the twisted-motion of the housing based on the present rotation angle is defined as:

calculating an estimated rotation angle based on the present rotation angle and a present Z-axis angular velocity, the estimated rotation angle being a rotation angle during a time until the tool bit stops; and

detecting the twisted-motion of the housing based on comparing the estimated rotation angle and a threshold rotation angle.