Field-oriented sensorless brushless motor control in a power tool

1 . A power tool comprising:

a housing;

a brushless motor disposed within the housing, the motor comprising a stator having a plurality of windings and a rotor;

a power switch circuit that supplies power from a power source to the brushless motor; and

a controller configured to apply a drive signal to the power switch circuit to control the supply of power to the brushless motor,

wherein the controller is configured to determine an angular position of the rotor using a high-frequency injection (HFI) scheme at a motor speed below a speed threshold and transition from the HFI scheme to a sensorless commutation scheme different from the HFI scheme during a transition period when the motor speed exceeds the speed threshold,

wherein the HFI scheme includes injecting a plurality of voltage pulses to the motor and detecting corresponding high-frequency current components to determine the angular position of the rotor, and

wherein, during the transition period, the controller is configured to:

calculate an error value between a detected rotor angle from the sensorless commutation scheme and the determined angular position of the HFI scheme;

gradually adjust a commutation sequence associated with the sensorless commutation scheme by ramping the commutation sequence up or down to reduce the error value;

determine that the error value has fallen below a predefined convergence threshold such that the detected rotor angle from the sensorless commutation scheme is substantially close to the determined angular position of the rotor from the HFI scheme; and

in response to determining that the error value has fallen below the predefined convergence threshold, gradually ramp down injection of the plurality of voltage pulses associated with the HFI scheme.

2 . The power tool of claim 1 , wherein the controller is configured to apply a vector-space pulse-width modulation (VSPWM) control to the drive signal for field-orientated control of the brushless motor.

3 . The power tool of claim 2 , wherein the controller is configured to compute a position difference between the detected angular position of the rotor and a target position associated with a target speed reference, generate an error-correction signal as a function of the computed position difference, and apply the VSPWM control accordingly.

4 . The power tool of claim 1 , wherein the power switch circuit comprises a plurality of low-side switches, a plurality of high-side switches, and a shunt component to which at least one signal associated with a phase current of the motor is coupled.

5 . The power tool of claim 4 , wherein the shunt component is provided on an output node of the power switch circuit.

6 . The power tool of claim 4 , wherein the shunt component is provided in series with one of the low-side switches.

7 . The power tool of claim 1 , wherein the power switch circuit comprises a plurality of low-side switches and a plurality of high-side switches, wherein at least one signal associated with a phase current of the motor is coupled to a plurality of low-side switches and the controller measures the motor phase current using an internal resistance of the plurality of low-side switches.

8 . The power tool of claim 7 , wherein the controller is configured to determine an ON-state of the plurality of low-side switches and measure a shunt voltage of one of the low-side switches that in the ON-state to calculate the motor phase current.

9 . The power tool of claim 1 , wherein the controller is configured to operate the motor using the HFI scheme and the sensorless commutation scheme in a variable speed range of zero to at least 15,000 rotations-per-minute (RPM) and within a power output range of zero to at least 1500 watts.

10 . The power tool of claim 1 , wherein the sensorless commutation scheme includes a sliding-mode observer (SMO) step of estimating a back electromotive force (back-EMF) voltage of the motor based on a phase current of the motor and determining the angular position of the rotor based on the estimated back-EMF voltage.

11 . The power tool of claim 1 , wherein the controller is configured to operate the motor using the HFI scheme and the sensorless commutation scheme in a variable torque range of zero to at least 15 Newton-meters (N.m.) and within a power output range of zero to at least 1500 watts.

12 . A method of operating a brushless motor including a stator having a plurality of windings and a rotor via a controller, comprising:

applying a drive signal to a power switch circuit to control a supply of power to the brushless motor;

determining an angular position of the rotor using a high-frequency injection (HFI) scheme at a motor speed below a speed threshold, wherein the HFI scheme includes injecting a plurality of voltage pulses to the motor and detecting corresponding high-frequency current components to determine the angular position of the rotor; and

transitioning from the HFI scheme to a sensorless commutation scheme different from the HFI scheme during a transition period when the motor speed exceeds the speed threshold, wherein the transition includes:

calculating an error value between an angular position of the rotor from the sensorless commutation scheme and the determined angular position of the rotor from the HFI scheme;

gradually adjusting a commutation sequence associated with the sensorless commutation scheme by ramping the commutation sequence up or down to reduce the error value;

determining that the error value has fallen below a predefined convergence threshold such that the angular position of the rotor from the sensorless commutation scheme is substantially close to the determined angular position of the rotor from the HFI scheme; and

in response to determining that the error value has fallen below the predefined convergence threshold, gradually ramping down injection of the plurality of voltage pulses associated with the HFI scheme.