Claims
- 1. A power tool having an electric motor, having associated therewith a response time, for driving an output spindle having a tool holder operatively coupled thereto, an operator actuable switch for controlling the amount of power applied to the motor, and a control circuit for modulating the power supplied to the motor in accordance with the position of said switch by varying the duty cycle of a constant frequency, pulse width modulated (PWM) direct current (d.c.) control signal generated by the control circuit to thereby control the speed of the motor; the improvement wherein the period of the PWM d.c. control signal generated by said control circuit is greater than the response time of the motor.
- 2. A power tool having an electric motor for driving an output spindle having a tool holder operatively coupled thereto, an operator actuable switch for controlling the amount of power applied to the motor, and a control circuit for modulating the power supplied to the motor in accordance with the position of said switch by varying the duty cycle of a constant frequency, pulse width modulated (PWM) direct current (d.c.) control signal generated by the control circuit to thereby control the speed of the motor; the improvement wherein said control circuit generates said PWM d.c. control signal at a frequency that is sufficiently low to cause said motor to provide, over a substantial portion of the duty cycle range of said control signal, bursts of torque to said output spindle that cause substantial variation in the speed of rotation of said output spindle between successive bursts of torque.
- 3. The power tool of claim 2 wherein said frequency is within a range between 10 Hz and 50 Hz.
- 4. The power tool of claim 2 further including a second operator actuable device for selectively setting the frequency of said PWM control signal.
- 5. The power tool of claim 4 wherein said second operator actuable device comprises a second switch for selectively setting the frequency of said PWM control signal to either a first high frequency greater than 1 KHz or a second low frequency less than 50 Hz.
- 6. The power tool of claim 4 wherein said second operator actuable device is adapted to selectively vary the frequency of said PWM control signal within a range that includes 10 Hz-50 Hz.
Parent Case Info
This application is a continuation of application Ser. No. 08/834,774, filed Apr. 3, 1997 now U.S. Pat. No. 6,424,799, which is a continuation of application Ser. No. 08/369,497, filed Jan. 6, 1995 (now abandoned), which is a continuation-in-part of application Ser. No. 08/087,932, filed Jul. 6, 1993, now U.S. Pat. No. 5,440,215.
US Referenced Citations (22)
Foreign Referenced Citations (5)
Number |
Date |
Country |
3230642 |
Feb 1984 |
DE |
0291215 |
Nov 1988 |
EP |
0340999 |
Nov 1989 |
EP |
0680808 |
Apr 1995 |
EP |
2269025 |
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Continuations (2)
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Number |
Date |
Country |
Parent |
08/834774 |
Apr 1997 |
US |
Child |
10/166805 |
|
US |
Parent |
08/369497 |
Jan 1995 |
US |
Child |
08/834774 |
|
US |
Continuation in Parts (1)
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Number |
Date |
Country |
Parent |
08/087932 |
Jul 1993 |
US |
Child |
08/369497 |
|
US |