Method and system for prevention of motor reversal
A power tool includes a brushless DC motor housed inside a tool housing, an input unit, and a control unit. The motor includes a rotor, a stator, and at least one sensor positioned to sense a state of the rotational position of the rotor inside the stator. A control unit controls commutation of the motor through a series of power switches coupled to the power supply. The control unit receives a current sensor state from the sensor and a user-selected speed from the input unit and determines whether the motor has reversed direction using the user-selected speed and the current sensor state. The control unit disables commutation of the motor if the motor has reversed direction until a proper current sensor state is received from the sensor.
1. A power tool comprising:
a housing;
a brushless DC motor housed inside the housing, the motor comprising a rotor, a stator, and at least one sensor positioned to sense a state of the rotational position of the rotor inside the stator;
a power supply;
an input unit actuated by a user comprising a variable-speed actuator; and
a control unit configured to control commutation of the motor through a plurality of power switches coupled to the power supply, the control unit receiving a current sensor state from the sensor and a user-selected speed from the variable-speed actuator, the control unit further configured to determine whether the motor has reversed direction using the user-selected speed and the current sensor state and to disable commutation of the motor if the motor has reversed direction until a proper current sensor state is received from the sensor.
2. The power tool of claim 1 , wherein the control unit is further configured to utilize a sensor transition table to determine an expected sensor state based on a previous sensor state and to determine that the motor has reversed direction if the current sensor state is different from the expected sensor state.
3. The power tool of claim 1 , wherein the control unit is further configured to commutate with angle-advancing if the user-selected speed is above a predetermined speed threshold and without angle-advancing if the user-selected speed is below the predetermined speed threshold.
4. The power tool of claim 1 , wherein the sensor is a Hall Effect sensor.
5. The power tool of claim 1 , wherein the input unit further comprises a forward-reverse switch, and the control unit is configured to determine whether has inadvertently reversed using a position of the forward-reverse switch in addition to the user-selected speed and the current sensor state.
6. The power tool of claim 1 , wherein the control unit comprises a programmable micro-controller and the power supply comprises a battery.
7. A method of commutating a brushless DC motor in a power tool, the brushless motor comprising a rotor, a stator, and at least one sensor positioned to sense a state of the rotational position of the rotor inside the stator, the method comprising:
receiving a user-selected speed from a variable-speed actuator;
receiving a current sensor state from the sensor;
determining whether the motor has reversed direction using the user-selected speed and the current sensor state; and
disabling commutation of the motor if the motor has reversed direction until a proper current sensor state is received from the sensor.
8. The method of claim 7 , further comprising:
determining an expected sensor state based on a previous sensor state utilizing a sensor transition table; and
determining that the motor has reversed direction if the current sensor state is different from the expected sensor state.
9. The method of claim 7 , further comprising:
commutating with angle-advancing if the user-selected speed is above a predetermined speed threshold and without angle-advancing if the user-selected speed is below the predetermined speed threshold.
10. The method of claim 7 , further comprising using a position of the forward-reverse switch in addition to the user-selected speed and the current sensor state to determine whether the motor has reversed direction.