Under-speed detection 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 speed signal from a variable-speed input mechanism mounted on the power tool;

determining a target speed of the motor corresponding to the speed signal;

controlling a supply of power to the motor based on the target speed;

determining an underspeed threshold corresponding to at least one of the target speed or the speed signal;

determining a rotational speed of the motor;

interrupting a supply of power to the motor if the rotational speed of the motor meets a criterion in relation to the underspeed threshold;

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

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

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

6 . 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.

7 . 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 receive a speed signal from the variable-speed input mechanism, determine a target speed of the motor corresponding to the speed signal, control a supply of power to the motor based on the target speed, determine an underspeed threshold corresponding to at least one of the target speed or the speed signal, determine a rotational speed of the motor, and interrupt the supply of power to the motor if the rotational speed of the motor meets a criterion in relation to the underspeed threshold,

wherein the controller is further configure to control the rotational speed of the motor in a closed-loop speed control if the target speed is greater than or equal to a threshold target, wherein the rotational speed of the motor is maintained at the target speed even with increased torque; and control the rotational speed of the motor in an open-loop speed control if the target speed is not greater than or equal to the threshold target, wherein the rotational speed of the motor varies with increased torque.

8 . The power tool of claim 7 , 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.

9 . The power tool of claim 8 , 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.

10 . The power tool of claim 9 , wherein the timer is decremented every time the rotational speed of the motor is greater than or equal to the underspeed threshold.

11 . The power tool of claim 7 , wherein in closed-loop speed control, a pulse-width modulation (PWM) duty cycle for driving the motor is set according to the target speed and at least one of a measured current of the motor or a detected change in a rotational speed of the motor.

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

13 . The power tool of claim 7 , wherein the controller is configured to determine the target speed of the motor based on the speed signal using a look-up table, and calculate the underspeed threshold as a function of the target speed.

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

receiving a speed signal from a variable-speed input mechanism mounted on the power tool;

determining a target speed of the motor corresponding to the speed signal;

controlling a supply of power to the motor based on the target speed;

determining an underspeed threshold corresponding to at least one of the target speed or the speed signal;

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 for a predetermined period of time by utilizing a timer, wherein the timer is incremented every time the rotational speed of the motor is less than the underspeed threshold, and the timer is reset every time a change in the position of the variable-speed input mechanism is detected.