Electrical power tool having a motor control circuit for providing control over the torque output of the power tool

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 selectively generates said PWM d.c. control signal at a first constant frequency that is sufficiently high to cause said motor to provide a substantially smooth application of torque to said output spindle over substantially the entire duty cycle range of said control signal, or at a second constant 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.

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 and thereby control the speed of the motor in accordance with the position of said switch;

• the improvement comprising operator controllable means for selectively causing said control circuit to supply power to said motor in either a first manner that results in a substantially smooth application of torque to said output spindle or, with said switch in a substantially fixed position, in a second manner that produces substantial cyclical variations in the torque applied to said output spindle independently of any changes in the load applied to said output spindle so as to produce substantial cyclical variations in the speed of said output spindle.

A method of controlling a power tool having an electric motor for driving an output spindle having a tool holder operatively coupled thereto and a control circuit that is responsive to a first operator actuable device for controlling the amount of power applied to the motor, the method comprising the steps of:

• modulating the power to the motor in accordance with the position of said first operator actuable device 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;
• and
• setting the frequency of said PWM d.c. control signal sufficiently low to cause the motor to provide, over a substantial portion of the duty cycle range of the 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.

A power tool having an electric motor and a control circuit for modulating the power supplied to the motor by varying the duty cycle of a constant frequency, pulse width modulated (PWM) direct current (d.c.) control signal generated at an output of the control circuit;

• the output of said control circuit being operative to switch from a first mode to a second mode when the magnitude of an input signal supplied thereto is less than a first threshold value and from said second mode to said first mode when the magnitude of said input signal exceeds a second threshold value greater than said first threshold value by a predetermined amount;
• an input circuit including a capacitor for producing said input signal in accordance with the charge on a first side of said capacitor and further including a charge circuit for charging the capacitor when the output of said control circuit is in said second mode and a discharge circuit for discharging said capacitor when the output of said control circuit is in said first mode;
• and a frequency control circuit including an operator adjustable device for controlling the frequency of said PWM d.c. control signal by varying the magnitude of a voltage signal supplied to the other side of said capacitor in accordance with the setting of said operator adjustable device.

A method of controlling a power tool having an electric motor for driving an output spindle having a tool holder operatively coupled thereto and a control circuit that is responsive to a first operator actuable device for controlling the amount of power applied to the motor, the method comprising the steps of:

• modulating the power to the motor in accordance with the position of said first operator actuable device by varying the duty cycle of a constant frequency, pulse width modulated (PWM) direct current (d.c.) control signal by the control circuit to thereby control the speed of the motor;
• and
• setting the frequency of said PWM d.c. control signal at a relatively low frequency so that at high duty cycle settings the motor operates in a first mode wherein a substantially smooth application of torque is provided to the output spindle and at low duty cycle settings the motor operates in a second mode in which bursts of torque are provided to said output spindle that cause substantial variation in the speed of rotation of said output spindle between successive bursts of torque.