Rotary striking tool

1. A rotary striking tool, comprising:

a motor;

an impact unit having a driving part and an output part, the driving part of the impact unit being driven by the motor;

an output shaft that is coupled to the output part of the impact unit such that a tip tool can be attached to the output shaft;

an impact detection unit that detects an impact generated at the impact, unit; and

a control unit configured to control the motor such that a force of a striking by the impact unit is smaller than a force of a previous striking by the impact unit at a predetermined timing.

2. The rotary striking tool of claim 1 ,

wherein the control unit is configured to control the motor such that the force of the striking by the impact unit is smaller than the force of the previous striking by the impact unit when the impact detected by the impact detection unit reaches a prescribed value.

3. A rotary striking tool comprising:

a brushless motor including a stator including windings and a rotor including a permanent magnet and configured to rotate within the stator;

an impact unit having a driving part and an output part, the driving part of the impact unit being driven by the brushless motor;

a plurality of rotation position detection elements configured to detect a rotation position, of the rotor disposed, along a circumferential direction with predetermined intervals therebetween; and

a control unit configured to control the brushless motor based on a signal from the rotation position detection element,

wherein the control unit detects a striking by the impact unit based on the signal from the rotation position detection element.

4. The rotary striking tool of claim 3 , further comprising an output shaft that is coupled to the output part of the impact unit such that a, tip tool can be attached to the output shaft,

wherein the plurality of rotation position detection elements output pulses at a predetermined, cycle, when a rotation of the output shall substantially synchronizes with a. rotation of the rotor, and

wherein the control unit detects the striking by the impact unit when the pulses are not output at the predetermined cycle.

5. The rotary striking tool of claim 4 ,

wherein a predetermined number of pulses are output between a previous striking and a next striking, and

wherein the control unit determines that the previous striking is abnormal when a number of pulses output between the previous striking and the next striking is larger than a predetermined number.

6. A method of operating a rotary striking tool including an impact unit having a driving part and an output part, the driving part of the impact unit being driven by a motor; an output shaft that is coupled to the output part of the impact unit such that a tip tool can be attached to the output shaft; and an impact detection unit that detects an impact generated at the impact unit, the method comprising steps of:

rotating the motor such that a force of a confirmation striking is smaller than a force of a previous striking performed prior to the confirmation striking;

controlling the impact unit to perform a confirmation striking when the impact detected by the impact detection unit reaches a prescribed value;

detecting a rotation angle of the output shaft at the confirmation striking;

determining whether a fastening operation is completed when the detected rotation angle is equal to or smaller than a predetermined angle; and

continuing the fastening operation when the detected rotation angle is larger than the predetermined angle.

7. The method of claim 6 , further comprising the step of:

calculating the rotation angle based on outputs of the rotation position detection elements, wherein:

the motor is a brushless DC motor, and

rotation position detection elements are provided at the brushless DC motor.

8. The method of claim 7 , further comprising the step of:

calculating the rotation angle based on variation in the outputs of the rotation position detection elements during a period from a previous striking to a next striking.

9. The method of claim 8 , wherein:

the brushless DC motor includes a rotor having plural permanent magnets of pairs of N and S poles, and

the position detection elements are hall elements or hall ICs which are provided at a predetermined interval so as to face the permanent magnets.

10. The method of claim 8 , further comprising the step of:

performing the confirmation striking in a state where a duty ratio of a signal supplied to an inverter circuit for supplying a driving current to the brushless DC motor is reduced.