Under-speed and closed-loop speed control in a variable-speed power tool

1. A method of operating a power tool having a motor and a controller, the method comprising:

receiving a position of a variable-speed input mechanism mounted on the power tool;

determining a target speed of the motor corresponding to the position of the variable-speed input mechanism;

determining an underspeed threshold corresponding to the target speed;

determining a rotational speed of the motor; and

interrupting a supply of power to the motor if the rotational speed of the motor is less than the underspeed threshold.

2. The method of claim 1 , wherein the supply of power to the motor is interrupted if the rotational speed of the motor is less than the underspeed threshold for a predetermined period of time.

3. The method of claim 2 , further comprising utilizing a timer that is incremented every time the rotational speed of the motor is less than the underspeed threshold, wherein the timer is reset every time a change in the position of the variable-speed input mechanism is detected.

4. The method of claim 3 , wherein the timer is decremented every time the rotational speed of the motor is greater than or equal to the underspeed threshold.

5. The method of claim 1 , further comprising:

controlling the rotational speed of the motor in a closed-loop speed control if the position of the variable-speed input mechanism is greater than or equal to a threshold position, wherein the rotational speed of the motor is maintained at the target output speed even with increased torque; and

controlling the rotational speed of the motor in an open-loop speed control if the position of the variable-speed input mechanism is not greater than or equal to the threshold position, wherein the rotational speed of the motor varies with increased torque.

6. The method of claim 5 , wherein in closed-loop speed control, a pulse-width modulation (PWM) duty cycle for driving the motor is set according to the position and at least one of a measured current of the motor or a detected change in a rotational speed of the motor.

7. The method of claim 1 , wherein the variable-speed input mechanism includes at least one of a trigger switch, a speed dial, a capacitive switch, or a touch-switch.

8. A power tool comprising:

a housing;

a motor disposed within the housing;

a variable-speed input mechanism mounted on the housing; and

a controller configured to control a supply of power to the motor based on a position of the variable-speed input mechanism, wherein the controller is configured to receive a signal indicative of the position of the variable-speed input mechanism, determine a target speed of the motor corresponding to the position of the variable-speed input mechanism, determine an underspeed threshold corresponding to the target speed, determine a rotational speed of the motor, and interrupt the supply of power to the motor if the rotational speed of the motor is less than the underspeed threshold.

9. The power tool of claim 8 , wherein the supply of power to the motor is interrupted if the rotational speed of the motor is less than the underspeed threshold for a predetermined period of time.

10. The power tool of claim 9 , wherein the controller utilizes a timer that is incremented every time the rotational speed of the motor is less than the underspeed threshold, wherein the timer is reset every time a change in the position of the variable-speed input mechanism is detected.

11. The power tool of claim 10 , wherein the timer is decremented every time the rotational speed of the motor is greater than or equal to the underspeed threshold.

12. The power tool of claim 8 , wherein the controller is further configured to:

control the rotational speed of the motor in a closed-loop speed control if the position of the variable-speed input mechanism is greater than or equal to a threshold position, wherein the rotational speed of the motor is maintained at the target output speed even with increased torque; and

control the rotational speed of the motor in an open-loop speed control if the position of the variable-speed input mechanism is not greater than or equal to the threshold position, wherein the rotational speed of the motor varies with increased torque.

13. The power tool of claim 12 , wherein in closed-loop speed control, a pulse-width modulation (PWM) duty cycle for driving the motor is set according to the position and at least one of a measured current of the motor or a detected change in a rotational speed of the motor.

14. The power tool of claim 8 , wherein the variable-speed input mechanism includes at least one of a trigger switch, a speed dial, a capacitive switch, or a touch-switch.

15. The power tool of claim 8 , wherein the controller is configured to determine the target speed of the motor from at least one of a look-up table or a continuous equation, and calculate the underspeed threshold as a function of the target speed.