1. Technical Field
The present invention relates to processes for determining torque output and controlling power impact tools. The invention also relates to a mechanical impact wrench having electronic control.
2. Related Art
In the related art, control of power impact tools has been accomplished by directly monitoring the torque of impacts of the tool. For instance, in U.S. Pat. Nos. 5,366,026 and 5,715,894 to Maruyama et al., incorporated herein by reference, controlled impact tightening apparatuses are disclosed in which complex processes involving direct torque measurement are used. Direct torque measurement involves the measurement of the force component of torsional stress, as exhibited by a magnetic field about a tool output shaft, at the point in time of impact. From this force component, related art devices directly determine the torque applied during the impact, i.e., torque T=force F times length of torque arm r. As exemplified by FIG. 10 of U.S. Pat. No. 5,366,026, however, torque measurements fluctuate, even after a large number of impacts are applied. This phenomena is caused by the inconsistent nature of the force component of the impact. In particular, some devices measure torque at a given point in time, such that the torque measured is based on whatever force is being applied at that point in time. In other cases, the force is monitored as it rises, and is measured for peak at a point in time at which a force decrease is detected. In either case outlined above, the force may not be the peak force and, hence, the peak torque derived may not be accurate.
To rectify this problem, related art devices use weighting factors, or peak and/or low pass filtering of torque peak measurement, and/or assume, even though it is not the case, a constant driving force from the motor. For instance, in U.S. Pat. No. 5,366,026, torque measurements are used to calculate a clamping force based on the peak value of a pulsatory torque and an increasing coefficient that represents an increasing rate of a clamping force applied. Unfortunately, torque measurement accuracy remains diminished. Accordingly, there exists a need for better processes of operating power impact tools and, in particular mechanical impact tools (i.e., those with mechanical impact transmission mechanisms), with greater accuracy of torque measurement. There also exists a need for more accurate torque measurement.
Another shortcoming of the related art is the lack of an electronic control in a mechanical impact wrench.