EP-3731406-B1 - LOW-SPEED SENSORLESS BRUSHLESS MOTOR CONTROL IN A POWER TOOL AND CONTROL METHOD THEREOF

EP3731406B1EP 3731406 B1EP3731406 B1EP 3731406B1EP-3731406-B1

Inventors

WAIKAR, SHAILESH P
QU, Zhi
Lin, Wing W.

Dates

Publication Date: 20260506
Application Date: 20200420

Claims (13)

A power tool (10) comprising: a brushless motor (16) having a stator defining a plurality of phases, a rotor rotatable relative to the stator, and a plurality of power terminals electrically connected to the plurality of phases; a power unit (206) having a plurality of power switches connected electrically between a power source and the plurality of motor terminals and operable to deliver power to the motor; and a control unit (208) interfaced with the power unit to output a drive signal to one or more of the plurality of motor switches to drive the plurality of phases of the motor over a plurality of sectors of the rotor rotation, wherein, the control unit is configured to detect an incorrect direction of rotation of the rotor by applying a first plurality of test voltage pulses for a present sector of the plurality of sectors and a second plurality of test voltage pulses for a previous sector of the plurality of sectors, wherein the previous sector is the sector before the present sector, measuring motor currents associated with the first and second pluralities of test voltage pulses, and comparing corresponding motor current measurements to detect a transition from the present sector to the previous sector, wherein the control unit (208) is further configured to compare slopes of the motor current measurements and detect the transition from the present sector to the previous sector when the current measurement associated with one of the first plurality of test voltage pulses has a larger slope than the current measurement associated with a corresponding one of the second plurality of test voltage pulses.
The power tool of claim 1, where the control unit (208) is further configured to measure the time it takes for each of the motor currents associated with the first and second pluralities of test voltage pulses to reach a preset threshold, and to compare time measurements to detect the transition from the present sector to the previous sector.
The power tool of any preceding claim, wherein the first plurality of voltage pulses and the second plurality of test voltage pulses have the same pulse width, the control unit (208) being further configured to compare amplitudes of motor current measurements and detect the transition from the present sector to the previous sector when the current measurement associated with one of the first plurality of test voltage pulses has a larger amplitude than the current measurement associated with a corresponding one of the second plurality of test voltage pulses.
The power tool of any preceding claim, wherein the control unit (208) is configured to transition commutation from the present sector to a next sector of the plurality of sectors by applying a third plurality of test voltage pulses for the next sector, wherein the next sector is the sector after the present sector, measuring motor currents associated with the first and third plurality of test voltage pulses, and comparing corresponding motor current measurements to detect a transition from the present sector to the next sector.
The power tool of claim 4, wherein, the second plurality of test voltage pulses are applied during approximately a first half of the present sector, and the third plurality of test voltage pulses are applied during approximately a second half of the present sector.
The power tool of claim 4 or 5, wherein the control unit (208) is configured to compare slopes of motor current measurements and transition commutation from the present sector to the next sector when the current measurement associated with one of the first plurality of test voltage pulses has a larger slope than the current measurement associated with a corresponding one of the third plurality of test voltage pulses.
The power tool of any of claims 4 to 6, wherein the control unit (208) is configured to calculate an output speed of the motor based on frequency of commutation transitions.
The power tool of claim 7, wherein, after the output speed of the motor exceeds a speed threshold, the control unit (208) is configured to measure a back-electromotive force (back-EMF) voltage of the motor on an open phase of the plurality of motor terminals and transition commutation from the present sector to the next sector in relation to the back-EMF voltage.
The power tool of claim 8, wherein the control unit (208) is configured to transition commutation from the present sector to the next sector when the back-EMF voltage is approximately half a maximum motor voltage.
The power tool of any preceding claim, wherein the control unit (208) is configured to drive the plurality of phases of the motor using a trapezoidal control scheme.
The power tool of any preceding claim, wherein the control unit (208) is configured to detect an initial position of the rotor at start-up using an Initial Position Detection (IPD) technique.
A method of controlling commutation of a brushless motor (16) of a power tool (10), the brushless motor (16) having a stator defining a plurality of phases and a rotor rotatable relative to the stator, the method comprising: outputting a drive signal to one or more of a plurality of motor switches to drive the plurality of phases of the motor over a plurality of sectors of the rotor rotation; and characterized by detecting an incorrect direction of rotation of the rotor by applying a first plurality of test voltage pulses for a present sector of the plurality of sectors and a second plurality of test voltage pulses for a previous sector of the plurality of sectors, wherein the previous sector is the sector before the present sector, measuring motor currents associated with the first and second pluralities of test voltage pulses, and comparing corresponding motor current measurements to detect a transition from the present sector to the previous sector; wherein the comparing step comprises comparing slopes of motor current measurements to detect the transition from the present sector to the previous sector when the current measurement associated with one of the first plurality of test voltage pulses has a larger slope than the current measurement associated with a corresponding one of the second plurality of test voltage pulses.
The method of claim 12, further comprising: transitioning commutation from the present sector to a next sector of the plurality of sectors by applying a third plurality of test voltage pulses for the next sector, wherein the next sector is the sector after the present sector, measuring motor currents associated with the first and third plurality of test voltage pulses, and comparing corresponding motor current measurements to detect a transition from the present sector to the next sector; optionally, applying the second plurality of test voltage pulses during approximately a first half of the present sector, and applying the third plurality of test voltage pulses during approximately a second half of the present sector; and optionally, comparing slopes of motor current measurements, and transitioning commutation from the present sector to the next sector when the current measurement associated with one of the first plurality of test voltage pulses has a larger slope than the current measurement associated with a corresponding one of the third plurality of test voltage pulses.

Description

RELATED APPLICATION FIELD This disclosure relates to sensorless motor controls, and in particular to sensorless control of brushless motors in power tools. BACKGROUND Power tools may be of different types depending on the type of output provided by the power tool. For example, a power tool may be a drill, hammer, grinder, impact wrench, circular saw, reciprocating saw, and so on. Some power tools may be powered by an alternating current (AC) power source while others may be portable and may be powered by a direct current (DC) power source such as a battery pack. Power tools may use AC or DC motors. Some power tools have a movable switch such as a trigger or a speed dial that can be used to vary the speed of the motor or the power output by the tool. The switch can be moved from a resting position where the power output of the tool is minimum (e.g., zero), and a fully activated (e.g., pulled) position where the power output of the tool is maximum. Thus, the tool can output the maximum power only when the trigger is fully activated. Also, after the trigger is fully activated, the tool's power output cannot be increased beyond its maximum power. The present disclosure addresses these and other issues related to power tools as described below in the detail. Use of Brushless Direct-Current (BLDC) motors in power tools has become common in recent years. A typical BLDC motor includes a stator including a series of windings that form three or more phases, and a rotor including a series of magnets that magnetically interact with the stator windings. As the phases of the windings are sequentially energized, they cause rotation of the rotor. BLDC motors generate more power and are more efficient that similarly-sized conventional brushes DC motors and universal motors. BLDC motors are electronically commutated, requiring a controller to commutate proper phases of the motor based on the angular position of the rotor. Conventionally, the motor is provided with a series of Hall sensors that detect a magnetic field of the rotor and provide signals to the controller indicative of the rotor position. Known techniques for sensorless control of BLDC motors are available in applications such as outdoor products where the motor operates at predictable speed ranges. One such technique involves monitoring the motor induced voltage generated by the back-electromotive force (back-EMF) of the motor in the motor windings to detect a rotational position of the motor. Specifically, as the rotor rotates it induces current through a non-active phase of the motor, which can be detected by the controller to estimate a rotary location of the rotor. Such techniques are suitable for motor applications designed to operate at high speed. For many power tool applications such as drills, impact drivers, etc. that operate over various speed and torque ranges, however, use of such techniques alone may not be suitable. This is particularly true for power tools operating at very low speed, where the user may turn the tool in a direction opposite the intended motor rotation, causing the rotor to rotate in an unexpected direction. What is thus needed is a sensorless control technique suitable for use with power tools that operate at low speed / high torque ranges. EP0856937A2 discloses determining commutation position by differentiating the current flowing within the stator coils and comparing the differentiated current waveforms to one another. US5191270A discloses a method for starting a polyphase by measuring the rise time of current on each of the stator coils and determining from the rise time measurement the position of the rotor. WO2019/056072A1 discloses operating a motor by activating a series of bottom-side power switches during an off-time interval of the pulse measurement for each of a series of measurement pulse combinations of said motor. WO 2018/088442 A1 discloses a method for detecting a magnetic field location in an electric motor, which comprises a rotor having permanent-magnet field and a stator having star-connected three-phase coils and which is started by 120° rectangular-wave conduction supplied from a constant-voltage DC source. The electric motor further comprises control means for storing six conduction patterns and field location information for assigning excitation switching sections of 120° conduction corresponding to each of the conduction patterns. After starting rotation of the rotor having the permanent-magnet field, the method comprises detecting the rotating direction by periodically sensing three conduction patterns corresponding to the present section and the adjacent sections in the forward rotational direction and the reverse rotational direction, and comparing the measurement data of them. Additionally, various techniques are known for increasing and optimizing power output of a BLDC motor. One such technique involves increasing a conduction band of the phase voltages applied to the motor. Typically, in trapezoid