Power tools with multi-stage tightening torque control

Abstract

A power tool includes a DC motor for tightening a fastener through three consecutive stages. In the first stage, the motor is rotated at a high speed to tighten the fastener to a pre-seated condition which is acknowledged by the tool. Then, the motor is temporarily stopped and restarted at a low speed for tightening the fastener to its seated condition to complete the second stage. At the third stage, a control is made to gradually increase a field current in a feedback manner by constantly monitoring the field current flowing through the motor up to a predetermined current limit determined as directly related to an intended tightening torque at which the fastener is tightened past the seat condition. After the field current reaches the current limit, the motor is stopped to complete the tightening cycle. By monitoring the field current during the third stage to control the field current in the feedback manner, it is possible to accurately determine the final tightening torque which is related substantially directly to the field current, assuring to tighten the fastener accurately to a desired final torque. The tool is also capable of increasing the field current at differing rates up to the current limits of different values within a constant period of time so as to complete the tightening of the fastener within substantially the same perod of time irrespective of the differing requirements for the final tightening torque.