CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a national stage filing under 35 U.S.C. 371 of International Patent Application Serial No. PCT/EP2018/065972, filed Jun. 15, 2018, entitled “SEMI-SYMMETRIC SWITCHING”, the content of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to electric motor control, and more particularly to a power tool, a controller for controlling a driver circuit used to drive an electric motor, a method of controlling a driver circuit, a computer program and a non-transitory storage medium.

BACKGROUND ART

Many powered devices, that is, devices being supplied with electric power to operate, use electric motors to function. Powered devices are advantageous because they rely on power supplied by an electric source, either alternating current (AC) or direct current (DC), which is easily generated and controllable.

In powered devices, the operation of an electric motor is controlled by regulating the amount of electric power supplied to the motor. More specifically, the current flowing through the motor phase windings is controlled to generate a rotating electromagnetic field, which causes the rotor of the motor to rotate.

For electronically commutated electric motors, a driver circuit having switching elements is used to control the supply of electric current to each phase winding of the motor. Brushless motors are a known type of electric motors which may be electronically commutated using a power source (e.g. a DC or AC power source) and which do not require electric power to be supplied to the rotor. Types of brushless motors includes Brushless DC motors (BLDC) having a permanent magnet rotor, Switched Reluctance motors (SRM) etc.

Switching elements are electronic components that can selectively allow electric current to flow. In the present disclosure, a switching element will generally be defined to be ON when it allows electric current flow, and OFF when it does not.

A common configuration for driver circuits used with brushless motors is called a bridge configuration (also called Half bridge). Pairs of switching elements which are connected in series are connected to the terminals of the power source, the pairs being connected in parallel to each other, and the common node of the switching elements in each switching elements pairs is connected to a respective one of the phase windings of the motor.

FIG. 1shows a conventional bridge configuration driver circuit60connected to a DC power source40on one side, and a motor80on the other side.

In driver circuits having a bridge configuration, electric current is provided to a phase winding of the motor by switching ON two switching elements in different pairs, each switching element being connected to opposite terminals of the DC power source (either the ‘+’ or ‘−’ terminal). This creates a series connection between the first switching element, the phase winding of the motor and the second switching element, allowing electric current to flow between the DC power source terminals through the phase winding of the motor. The desired current level in a phase winding is achieved by controlling the amount of time current is allowed to flow through the phase winding, typically by driving one or both of the switching element with a pulse-width modulation, PWM, signal, that is, a pulsing signal whose pulse width may be controlled.

A known control method used with half-bridge driver circuits is asymmetric switching, in which a switching element connected to one of the power source terminals (e.g. the ‘+’ terminal) is kept ON during a commutation period, and another switching element from another pair connected to the other power source terminal is driven by a PWM signal during the commutation period, to regulate the current supplied to the phase windings. The switching element of each pair connected to the same power source terminal is kept ON in succession, each during a respective commutation period of the motor.

However, asymmetric switching causes the switching element driven by the PWM signal to be excessively affected, because it suffers from larger switching losses, and consequently from a higher temperature increase than the other switching element in the pair. This may lead to the switching elements to deteriorate at different rates, or differ in their performance. Additionally, conduction losses in the switching elements and its diode or body diode are also unequally affected.

Another known control method is symmetric switching, in which the involved switching elements are switched simultaneously. That is, both switching elements are ON simultaneously and likewise OFF.

However, symmetric switching requires both switching elements to be repeatedly switched ON and OFF, leading to a higher total power loss and therefore a significantly reduced overall efficiency which is undesirable.

There is therefore a need for an electric motor control allowing for the power losses to be more evenly distributed on switching elements in an half bridge driver circuit whilst achieving a desirable overall efficiency level.

At the same time, powered devices emit electromagnetic energy when operating, which can negatively affect surrounding equipment, by causing electromagnetic interference, for example. Extensive testing must therefore be carried out to ensure that the level of energy emitted by a powered device does not exceed an acceptable threshold (i.e. that the powered device has electromagnetic compatibility), such as thresholds defined in standards developed by various international bodies. For example, European standard EN 55014 defines the electromagnetic compatibility requirements for household appliances, electric tools and similar apparatus.

A factor affecting the electromagnetic energy emitted by powered devices is the frequency of electric signals in the powered devices including the harmonic components caused by switching ON or OFF the elements in the device.

It is therefore desirable to regulate the emitted electromagnetic energy to be lower than a predefined threshold.

SUMMARY

Embodiments of the present invention have been devised which address at least some of the above-identified problems.

In a first aspect, there is provided a power tool with a direct current (DC) power source (for example a constant DC power source or pulsating DC power source), a driver circuit and a brushless motor. The power tool comprises a controller arranged to alternately switch a first and a second switching elements that are part of the driver circuit and that are coupled to respective terminals of the DC power source and coupled to respective phase windings of a brushless motor.

The power tool has a DC power source which is coupled to the driver circuit. The power source may be an independent DC power source, such as an internal DC battery, or may be a dependent power source drawing power from an external AC or DC power source, for example if the power tool is connected to an AC power plug (in which case the AC power is converted into DC power to form the DC power source).

The controller may receive information about a desired operation of the motor including a desired rotation speed, torque and/or other characteristics known to a skilled person, for example if a user actions a switch provided on the power tool causing a signal to be transmitted to the controller, and the controller may generate one or more control signals for controlling the state of the switching elements accordingly.

More specifically, the controller is coupled to each switching elements of the driver circuit (for example, by way of an electric connection to the gate or the base of a transistor), so as to controllably form electric paths through the switching elements.

When an electric path is formed between the terminals of the DC power source through switching elements which are ON, a voltage is obtained between separate points on phase windings of the brushless motor. The obtained voltage leads to a current flowing through the switching elements which are ON and the phase windings between these points, which, in turn, leads to a magnetic field being generated in the motor. The generated magnetic field interacts with the rotor (for example with a magnetic field generated by permanent magnets on the rotor), causing the rotor to operate as desired.

With the above power tool comprising the controller, the first and second switching elements are not switched at the same time, therefore the switching losses occurring in each switching element are limited, and the efficiency of the system is maintained when compared, for example, to an asymmetric switching scheme, or even improved when compared, for example, to a symmetric switching scheme.

Additionally, because the switching elements are switched alternately, or in turn, the switching losses are distributed between the switching elements.

Accordingly, the above controller allows for the power losses to be distributed on switching elements in a half bridge driver circuit and for a desirable overall efficiency to be achieved.

In an embodiment, the controller is arranged to alternately switch a first switching element which is in a first switching element pair and coupled to a first phase winding of the brushless motor, and a second switching element which is in a second switching element pair and coupled to a second phase winding of the brushless motor, during a same switching period.

At least one of the switching elements is driven by a pulsing signal to regulate the current in phase windings. The frequency of the pulsing signal may be defined as a switching frequency (f_sw), and the corresponding period (1/f_sw) as a switching period (p_sw). In pulse-width modulation, the pulse signal is high for a part of the switching period, generally defined as the duty cycle and expressed in terms of percentage. For example, a duty cycle of 50% means the switching element is controlled to be ON for half of the switching period and OFF for the other half.

In this embodiment, because the first and second switching elements are alternately switched during a same switching period, the effective switching frequency of each switching element is reduced.

More specifically, this allows for the first and second switching elements to be switched only once per switching period (either OFF to ON, or OFF to ON), while obtaining the desired current level in the phase windings.

Consequently, the effective switching frequency of each switching element is substantially halved, and the effect of the switching elements on the emitted energy occurs at a much lower frequency, thereby reducing the level of energy emitted by the system at higher frequencies.

In another embodiment, the controller is arranged to keep a first switching element ON during a first commutation step of the brushless motor and control a second switching element with a pulsing signal (for example a pulse-width modulation, PWM, signal), and to keep the second switching element ON during a second commutation step of the brushless motor and control the first switching element with a pulsing signal. In this embodiment, the first switching element is in a first switching element pair and coupled to a first phase winding of the brushless motor, and the second switching element is in a second switching element pair and coupled to a second phase winding of the brushless motor.

When considering both the first and second commutation periods, the switching losses are distributed between the two switching elements, such that neither switching element is disproportionately affected.

The first switching element is switched ON and OFF at each switching period during the first commutation period, and the second switching element is switched ON and OFF at each switching period during the second commutation period. Consequently, switching the switching elements cause more energy to be emitted at higher frequencies than in some of the previously described embodiments.

As the total energy emitted depends on other components of the system as well, this may be preferable in some situations.

In an embodiment, the controller is arranged to control the switching elements in at least one of the switching element pairs synchronously.

In other words, the switching elements within the same pair (e.g66) which are synchronously driven are driven by opposite PWM signals (with the required dead time being inserted), such that one is switched ON as the other one is switched OFF and that at any given time, exactly one of the switching elements in the pair is ON (except during a dead time, in which case both switching elements are OFF). The dead time would be understood by a skilled person to be a period between the moment one the switching elements in a pair is turned off and the moment the other switching element in the pair is turned on, to take into account to account the switching elements' fall time and avoid causing a short-circuit between the terminals of the DC power source.

For a MOSFET the conduction losses due to the on-resistance are typically lower than the losses in the body diode, hence using synchronous switching the overall system efficiency is improved since the power losses in the body diode are replaced by the smaller power losses due to the on-resistance of the MOSFET.

Then, for each switching element pairs controlled synchronously the controller only needs to determine one control signal to control one of the switching elements, because the second switching element in that pair is to be controlled by the opposite signal.

In an embodiment, the controller is arranged to control the first switching element and second switching element based on a same control signal, and to adjust the phase of the control signal to control the second switching element.

In this embodiment as well, the first switching element is in a first switching element pair and coupled to a first phase winding of the brushless motor, and the second switching element is in a second switching element pair and coupled to a second phase winding of the brushless motor. Additionally, the first switching element and the second switching elements are coupled to a respective terminal of the DC power source.

The controller therefore only needs to generate one control signal for controlling both the first and the second switching elements (which are in separate switching element pairs). By adjusting the phase of the control signal driving the second switching element, relative to the phase of the control signal driving the first switching element, the controller can control the width of the pulse of the voltage across the motor windings.

The phase of the control signal may be adjusted, for example by means of a delay.

Such as controller may allow for a reduced complexity, which may be particularly useful for applications having limited processing and/or spatial resources.

In an embodiment, the controller is arranged to alternately switch the first switching element N times and the second switching element M times over a predefined period, N and M being integer numbers, and the controller is arranged to adjust at least one of N and M.

Accordingly, the controller may dynamically adjust the ratio of N and M, therefore determining how the switching losses are distributed on the first and second switching elements.

In an embodiment, the controller is arranged to adjust at least one of N and M based on information indicating an operating condition of at least one of the first switching element and the second switching element.

Accordingly the controller may determine, based on the information indicating its operating condition, that the effect of switching losses in one of the switching element should be reduced, and can therefore decrease the number of times that switching element is switched. Optionally, the information may indicate a temperature of the switching element as an operating condition and/or an internal resistance of the switching element as an operating condition.

Therefore, the controller may, based on the information indicating the temperature of the switching element, determine that the switching element should be switched less frequently, and either decrease the number of times it switches (M or N), and/or increase the number of times the other switching element switches (N or M). As a consequence, the controller may adjust the ratio of N and M, thereby determining to what extend each of the switching elements is to be affected by the switching.

In a second aspect, there is provided a controller for controlling a driver circuit driving a brushless motor in a power tool, the driver circuit being coupled to a direct current (DC) power source (for example a constant DC power source or pulsating DC power source) and including a first switching element pair coupled to a first phase winding of the brushless motor and a second switching element pair coupled to a second phase winding of the brushless motor; and the controller being arranged to alternately switch a first switching element of the first switching element pair and a second switching element of the second switching element pair, wherein the first switching element and the second switching elements are coupled to a respective terminal of the DC power source.

The controller is arranged to generate one or more control signals for controlling the state of the switching elements, so as to drive the motor in accordance with a desired operation.

With the above controller, the first and second switching elements are not switched at the same time, therefore the switching losses occurring in each switching element are limited, and the efficiency of the system is maintained when compared, for example, to an asymmetric switching scheme, or even improved when compared, for example, to a symmetric switching scheme.

Additionally, because the switching elements are switched alternately, or in turn, the switching losses are distributed between the switching elements.

Accordingly, the above controller allows for the power losses to be distributed on switching elements in a half bridge driver circuit and for a desirable overall efficiency to be achieved.

In a third aspect, there is provided a method for controlling a driver circuit driving a brushless motor in a power tool, wherein the driver circuit is coupled to a direct current, DC, power source and includes a first switching element pair coupled to a first phase winding of the brushless motor and a second switching element pair coupled to a second phase winding of the brushless motor. The method comprises alternately switching a first switching element of the first switching element pair and a second switching element of the second switching element pair, wherein the first switching element and the second switching elements are coupled to a respective terminal of the DC power source.

With this method, the switching elements are not switched at the same time, therefore the switching losses occurring in each switching element are reduced, and the method can maintain the efficiency of the system.

Additionally, because the switching elements are switched alternately, or in turn, the method allows for the switching losses to be distributed between the switching elements.

Accordingly, the above method allows for the power losses to be distributed on switching elements in a half bridge driver circuit and for a desirable overall efficiency to be achieved.

In a fourth aspect, there is provided a computer program comprising instructions, which, when executed by a processor, cause the processor to carry out the methods described herein.

A processor as described herein may be any known processor such as a central processing unit (CPU), a microprocessor, capable of executing a computer program stored in memory.

In a fifth aspect, there is provided a non-transitory storage medium storing a computer program comprising instructions which, when executed by a processor, causes the processor to carry out the methods described herein.

The non-transitory storage medium may comprise a non-volatile memory, for example in the form of a read-only-memory (ROM), a programmable ROM (PROM), an erasable or electrically erasable programmable read-only memory (EPROM or EEPROM), flash memory, a magnetic computer storage device (for example a hard disk) or an optical disk, may be pre-loaded with the computer program, so that the computer program may be executed by a processor when the processor is coupled with the non-transitory storage medium.

Alternatively, a computer program product, such as a non-transitory computer-readable storage medium250(for example an optical disk such as a CD-ROM, DVD-ROM, etc.) or a computer-readable signal may carry information used to input the computer program to writeable volatile or non-volatile memory, such as random access memory (RAM), from which a processor may execute the computer program.

Hence, the above-described power tools, controllers, methods, computer programs and non-transitory storage mediums, it is possible to provide an electric motor control allowing for the power losses to be distributed on switching elements in a driver circuit and having a desirable overall efficiency whilst operating the motor according to a desired operation.

In embodiments described herein, the controller may be implemented in software, hardware or any combination thereof.

For example, the controller may comprise a processor and a memory comprising computer program instructions. The processor may be coupled to a power source (such as the DC power source) and be arranged to execute the computer program instructions so as to generate control signals at varying voltage levels.

In embodiments and implementations described herein, the controller may have a control signal generator for generating control signals to control the first and second switching elements, so as to alternately switch the first and the second switching elements.

In embodiments and implementations described herein, the switching elements may be transistors, such as Field-effect transistors (FETs) (e.g. Si MOSFETS, GaN MOSFETs, SiC MOSFETs, etc.), Bipolar junction transistors (BJTs), insulated-gate bipolar transistors (IGBTs), thyristors, or other known types of switches.

In embodiments and implementations described herein, the rotor topology of the brushless motor is not limited, and the motor may be a BLDC motor, a switched reluctance motor, or any other brushless motor.

