Power tools with multi-stage tightening torque control

A power tool capable of effecting multi-stage tightening torque control, comprising:

• a DC motor having a motor output shaft operatively connected to a drive bit for tightening fasteners such as a screw and a nut, said motor having a characteristic of increasing an output torque with an increase in a field current fed through said motor;
• a source voltage applying a voltage to drive said motor;
• speed/revolution detecting means detecting a speed and revolutions of said motor;
• drive means connected to drive said motor for varying the speed and torque thereof;
• speed control means controlling said drive means to rotate said motor selectively at high and low speeds for tightening said fastener;
• current sensing means monitoring said field current flowing through said motor to provide a feedback signal;
• current control means controlling to vary said field current based upon said feedback signal for correspondingly varying said output torque of said motor;
• pre-seat judge means determining a pre-seated condition where the fastener is just before seated and providing a pre-seat signal indicative of said pre-seated condition;
• seat detecting means detecting a seated condition where said fastener is actually seated and providing a seat signal indicative of said seated condition;
• input means capable of setting a final tightening torque at which said fastener is tightened past said seated condition and storing said final tightening torque as directly related to a corresponding current limit for said field current flowing through said motor;
• central control means selectively activating said speed control means and said current control means, said central control means operating to firstly activate said speed control means in such a manner as to drive said motor at said high speed, stop it in response to said pre-seat signal, and to restart said motor at said low speed, said central control means responding to said seat signal and activating said current control means instead of said speed control means in such a manner as to drive said motor by controlling to increase said field current in the feedback manner at a suitable rate up to said current limit in order to further tighten said fastener past said seated condition at said final tightening torque determined by said current limit, said central control means operating said current control means to stop feeding said field current after said field current reaches said current limit, thereby stopping said motor.