WO2011154041A1

WO2011154041A1 - Electric power tools

Info

Publication number: WO2011154041A1
Application number: PCT/EP2010/058164
Authority: WO
Inventors: Johan Svennung; Mikael Racov; Magnus Lundgren
Original assignee: Husqvarna Ab
Priority date: 2010-06-10
Filing date: 2010-06-10
Publication date: 2011-12-15

Abstract

An electric power tool, such as a hedgetrimmer, comprising: a plurality of blades (2) arranged for reciprocal motion relative to one another; an electric motor (3) having an output shaft (6); and a coupler (7) coupled to the blades (2) and arranged to convert rotational movement at an input of the coupler (7) into reciprocating relative linear movement of the blades (2), in which the output shaft (6) is directly coupled to the input of the coupler (7). A flywheel (9) may be provided on the output shaft (6), and the motor (3) may be a brushless permanent magnet motor, having an angular position sensor (14) such as a plurality of Hall Effect sensors (15) for the angular position of the motor rotor (10) or the output shaft (4).

Description

ELECTRIC POWER TOOLS

This invention relates to electric power tools, such as, non-exclusively, battery operated hedgetrimmers.

Battery operated hedgetrimmers and other battery-powered electric tools are known from such documents as the United States patent application publication number US2007/0245575. Such tools typically comprise a pair of elongate blades. A motor is provided, powered by a battery, which is connected by means of a gearbox or other such transmission to a coupler which converts rotary motion generated by the motor into reciprocating linear movement of the blades along their length. It is desirable to make such tools lighter, quieter and cheaper to manufacture. According to a first aspect of the invention, there is provided an electric power tool comprising a plurality of blades arranged for reciprocal motion relative to one another, an electric motor having an output shaft and a coupler coupled to the blades and arranged to convert rotational movement at an input of the coupler into reciprocating relative linear movement of the blades, in which the output shaft is directly coupled to the input of the coupler.

Thus, by directly coupling the output shaft of the motor to the coupler, the gearbox or other transmission means of the prior art is excluded; there will be no change of gear ratio between the motor and the coupler. Frictional losses in the gearbox may be eliminated. The tool may be cheaper to produce, smaller and lighter, because of the omission of a complex part. The tool may be quieter, as the noise from the gearbox is eliminated. This will therefore mean that the motor will be operating at a reduced speed compared with prior art tools. Such tools rely upon the angular momentum of the rotating parts of the motor in order to overcome the peaks in the moment required of the motor in such tools as hedgetrimmers. Given that the angular momentum of the motor is proportional to the angular speed at which it is running, the angular momentum of the motor of the present tool will be reduced.

As such, the tool may be provided with a flywheel coupled to, or directly mounted upon, the output shaft. The flywheel may provide the moment of inertia required to overcome the peaks in moment required by the motor. The flywheel may be sized so as to provide sufficient moment of inertia to overcome the largest expected peak moment. Typically, the moment of inertia of the rotating parts of the motor, the output shaft and the flywheel may be such that, at operating speeds (typically 1500 to 2000 rpm), the rotational energy stored in those components is at least 20 Joules, maybe as much as 50 Joules or even higher. These values have been found to provide sufficient moment of inertia to allow a hedgetrimmer to operate satisfactorily, and correspond to moments of inertia of those parts of at least 3.51 x 10^{" 1 1} kg m², 5.62 x 10^{" 1 1} kg m², 8.27 x 10^{" 11} kg m² and 1.06 x

10 -^" 10 kg m 2 respectively. Controlling electric motors down to speeds of the region of 2000 rpm can be difficult. It is therefore proposed to use a brushless permanent magnet motor. In order to run at relatively low speeds, the motor may comprise a plurality of phases, a control circuit arranged to selectively apply electric current to each of the phases and an angular position sensor arranged to sense the angular position of the output shaft, the angular position sensor being coupled to the control unit such that the control unit is able to use the angular position of the output shaft in order to determine the electric current that is applied to each phase. The angular position sensor may comprise at least one magnetic field sensor such as a Hall effect sensor, arranged to determine the position of magnets on the rotor of the motor. Typically, the tool is a hedgetrimmer. The tool will also typically comprise a battery arranged to power the motor, and a controller (which may be simple on-off, or variable whether in steps or continuously) for controlling the operation of the motor. In the preferred embodiment, there are two relatively reciprocating blades.

There now follows, by way of example only, an embodiment of the present invention described with reference to the accompanying drawings, in which:

Figure 1 shows a perspective view of a hedgetrimmer according to an embodiment of the invention;

Figure 2 shows a plan view of the handle end of the hedgetrimmer of Figure 1 ;

Figure 3 shows a longitudinal cross section through the view of Figure 2;

Figure 4 shows a cross section through the motor of the hedgetrimmer of Figure 1 ;

Figure 5 shows a similar view to that of Figure 3, with the coupler viewed in more detail; and

Figure 6 shows a perspective view of the motor and coupler of the hedgetrimmer of Figure 1.

The accompanying drawings show a hedgetrimmer according to an embodiment of the invention. The hedgetrimmer comprises a handle portion 1 , from which extends a pair of toothed blades 2. These blades 2 are mounted slidingly on the handle portion 1. Also supported by the handle portion 1 is an electric motor 4. This is powered by a battery pack 3 which is also mounted on the handle portion 1. A trigger 5 , operating an electrical switch (not shown) is provided by means of which electric current can be applied to the motor.

Whilst the accompanying Figures show the battery pack 3 and motor exposed, the handle portion 1 will be provided with a housing (not shown) surrounding these integers, so as to contain them.

The motor 4 has an output shaft 6, which is a continuation of the rotor of the motor. This is connected to the input of a coupler 7, which is connected to the blades 2 so as to convert the rotational movement of the output shaft 6 of the motor into reciprocating linear motion of the blades. The coupler is shown in more detail in Figures 5 and 6 of the accompanying drawings.

Typically, this coupler would be of the form of a disc 8 mounted on the output shaft 6. The disc 8 has a stud 20 eccentrically formed in each of its two faces. Each stud is mounted in a yoke 21 by means of rotating bearings 22. Each yoke 21 is pivotally mounted to one of the blades 2 by another pivoting coupling 23. As the blades 2 are constrained to move only along their lengths, this arrangement converts the rotational movement of the output shaft into relative linear movement of the blades 2, with the disc acting as the input to the coupler.

As such, this arrangement allows for the conversion of the rotary motion of the output shaft 6 of the motor 4 into to reciprocating relative linear motion of the blades 2, without the need for a gearbox or other such transmission between the motor 4 and the coupler 7. This means that the hedgetrimmer can be made lighter, smaller and quieter than if a gearbox were present, as the gearbox is large, heavy and a source of noise. Similarly, the absence of a gearbox means that the frictional losses in the gearbox are removed, and the tool can potentially be used continuously for longer time as there is no gearbox to overheat as well as the tool consumes less electric energy as the overall efficiency is higher. Finally, the hedge trimmer can be made cheaper to produce, as the gearbox is a costly item.

However, the inventors have appreciated several additional features that are advantageous in order to make best use of this "direct drive" design. Firstly, they have appreciated that the moment of inertia in the motor will be lower than a motor and gearbox combined, which means the ability to deal with peaks in the moment or torque required from the motor (due, for example, by cutting thick branches) may be reduced. In order to ameliorate this, the present hedgetrimmer includes a steel flywheel 9 directly connected to the output shaft 6. This will increase the moment of inertia of the system. With the motor 4 operating at its working speed of 1500 to 2000 revolutions per minute (rpm), the rotational kinetic energy of the motor 4, output shaft 6 and flywheel 9 will be at least 20 Joules; this has been found to provide adequate performance, although storing up to 50 Joules or even more at 2000 rpm is proposed to provide increased resistance to peaks in the load.

Secondly, the motor 4 used is a low-speed permanent magnet motor. As shown in Figure 4 of the accompanying drawings, this comprises a rotor 10, which is free to rotate within stator 1 1. Stator 1 1 has a plurality (here, six) of phase windings 13, which are independently switchable by a control circuit 12; the phase windings shown in dotted lines in the accompanying figures are shown as such in order to indicate merely that they are hidden by other features. The rotor 10 comprises a plurality (here, three) of pairs of alternating permanent magnet poles 16, and is coupled to the output shaft 6. A position sensor 14 comprising three Hall Effect sensors 15 spaced around the rotor 10 is provided, fixed relative to the stator. Each Hall Effect sensor 15 senses the magnetic field in its vicinity, and so can determine whether a pole is nearby. By comparing the outputs of the three Hall Effect sensors 15 in the control circuit 12, a rough measurement can be made of the position of the rotor 10 and from that a decision can be made as to the appropriate windings to pass current through. Thus, the motor can be used at low speeds, where it is important to correct switch or commutate the current to the appropriate windings 13. Whilst such motors are known from such publications as United States Patent number 5 610 457, we are not aware of such motors being used in hedgetrimmers.

Claims

1. An electric power tool comprising: a plurality of blades (2) arranged for reciprocal motion relative to one another; an electric motor (4) having an output shaft (6); and a coupler (7) coupled to the blades (2) and arranged to convert rotational movement at an input of the coupler into reciprocating relative linear movement of the blades (2), in which the output shaft (6) is directly coupled to the input of the coupler (7).

2. The tool of claim 1 , in which there is no change of gear ratio between the motor (3) and the coupler (7).

3. The tool of claim 1 or claim 2, comprising a flywheel (9) coupled to the output shaft.

4. The tool of any preceding claim, in which the motor (4) is a brushless permanent magnet motor.

5. The tool of any preceding claim, in which the motor (4) comprises a plurality of phases ( 13), a control circuit (12) arranged to selectively apply electric current to each of the phases (13) and an angular position sensor (14) arranged to sense the angular position of a rotor (10) of the motor or the output shaft (6), the angular position sensor (14) being coupled to the control unit ( 12) such that the control unit (12) is able to use the angular position of the output shaft (6) or the motor rotor ( 10) in order to determine the electric current that is applied to each phase (13).