GB2485276A

GB2485276A - Switching off an electrical machine tool electronically when a current limit is reached

Info

Publication number: GB2485276A
Application number: GB1119030.3A
Authority: GB
Inventors: Bernd Wirnitzer; Andreas Schlegel; Juergen Dietel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2010-11-04
Filing date: 2011-11-03
Publication date: 2012-05-09
Also published as: GB201119030D0; CN102528772B; DE102010043361A1; CN102528772A

Abstract

The electrical machine tool (1, Figure 1), such as a hammer drill, includes a tool shaft 3 which can be driven by an electric motor 2. The electrical tool also includes an electronic control unit 4 which brakes or switches off the electric motor if the tool shaft is blocked, depending on an actual current value (IIST). The tool may be used in a method of switching off an electric motor 2 of an electrical machine tool, in particular a hammer drill, with a tool shaft 3 which can be driven by the electric motor. The electric motor being switched off or braked if an actual current value which is required to drive the electric motor exceeds a current limit (IGRENZ) over a defined period (TDEF). The control unit may include a measuring unit to capture the current value and a counter to capture the defined period.

Description

Switching off an electrical machine tool electronically when a current limit is reached

Prior art

This invention concerns an electrical machine tool, in particular a hammer drill, with a tool shaft which can be driven by an electric motor, and a method of switching off an electric motor of the electrical machine tool, which in particular is according to the invention.

For drilling machines, providing a safety coupling in such a way that if the drill tilts, e.g. in a wall or other workpiece, or if the tool holder is blocked, the drilling machine does not continue to rotate out of control, is known. Such a drilling machine is disclosed in DE 100 02 748 Al, for example. Such a safety coupling is in the form of a locking coupling, and is opened from a specified torque, so that a tilted drill or blocking drill bit is no longer rotated even if the electric motor continues to be driven. In this way, the user is protected.

Limiting the motor torque which typically occurs in the case of blocking is also known. This is disclosed, for example, in the published specification DE 103 41 975 Al, which teaches limiting the motor current to a maximum permitted value, depending on the rotational speed.

Disclosure of the invention

The object of this invention is to create a very compact electrical machine tool which can be produced inexpensively, and in which the safety coupling which makes blocking the drill and/or the bit safe is device-specifically configurable.

The object is achieved with an electrical machine tool, in particular a hammer drill, with a tool shaft which can be driven by an electric motor, the electrical machine tool including an electronic control unit, which brakes or switches off the electric motor if the tool shaft is blocked, depending on an actual current value of a motor current of the electric motor.

In the sense of the invention, braking the electric motor means short-circuiting it, and switching off the electric motor means idling it. If the tool shaft is driven rigidly, therefore, both braking the electric motor and switching it off result in stopping the tool shaft, and thus stopping the tool.

Such an electrical machine tool therefore requires no mechanical safety coupling, which is replaced by an electronic control unit. Therefore, the mechanical safety coupling is omitted from the electrical machine tool according to the invention, and the space which is required on the tool shaft in the case of traditional electrical machine tools, the assembly cost for fitting the mechanical safety coupling, and the component itself are saved.

To capture the actual current value, the control unit preferredly includes a measuring unit. The electric motor is preferably braked or switched off if the actual current value exceeds a current limit over a defined period. To capture the defined period, the control unit preferredly includes a counter.

The current limit and/or the defined period are preferredly configurable, so that the control unit, which replaces the safety coupling, is device-specifically configurable.

In a preferred embodiment, the current limit and/or the defined period are provided so that after the current limit is exceeded, the tool shaft continues to rotate for less than one revolution, preferredly less than a half revolution, so that in the case of blocking, the electrical machine tool is as far as possible not rotated while uncontrolled.

Specially preferredly, the defined period is provided so that it corresponds to not more than a half revolution of the tool shaft, quite specially preferredly not more than a quarter revolution of the tool shaft. This takes into account that the electric motor, just because of the rotatory energy of the electrical machine tool, but also because of further influences such as the user's posture and/or material properties of the workpiece being worked on, continues to rotate after the electric motor is switched off.

It is also preferred that the defined period is not more than 80 ms, specially preferredly not more than 40 ms, and quite specially preferredly not more than 20 ms. Depending on the properties of the electrical machine tool, in particular depending on its mass moment of inertia, this period corresponds approximately to a half to a quarter revolution of the tool shaft. However, depending on the electrical machine tool, other periods are also preferred.

In a preferred embodiment, the control unit includes a memory unit, from which the current limit and/or the defined period can be taken. The current limit and/or the defined period can be specified within development, so that in this embodiment, preconfiguring the electrical machine tool is preferred. Providing exactly one current limit and/or exactly one specified period for control is also preferred. However, it is also preferred that the current limit and/or the defined period depend on a rotational speed list value and/or a torque course, i.e. the change of torque over time, of the electric motor. To capture such a torque course and/or such a rotational speed list value, the control unit preferredly includes a further measuring unit. A current limit and/or a defined period, corresponding to a torque course and/or a rotational speed list value, can be taken from the memory unit, e.g. via a

-lookup table.

However, additionally or alternatively to the memory unit, it is also preferred that the control unit includes an arithmetic-logic unit to control the control unit, and/or to calculate the current limit and/or the defined period, depending on the rotational speed list value and/or the torque course. This embodiment has the advantage that no device-specific preconfiguration of the electrical machine tool is required, since the calculation of the current limit and/or the defined period can be held by formula in the control unit, at a higher level than the device.

The electric motor of the electrical machine tool according to the invention is preferredly in the form of a direct current motor.

The object is also achieved by a method of switching off an electric motor of an electrical machine tool, in particular a hammer drill, which in particular is according to the invention, with a tool shaft which can be driven by the electric motor, the electric motor being switched off or braked if an actual current value which is required to drive the electric motor exceeds a current limit over a defined period.

The invention makes an electronic safety coupling for an electrical machine tool, which in particular is according to the invention, possible. Advantageously, the current limit and/or the defined period for carrying out the method are configurable, so that the control unit, which replaces the safety coupling, is device-specifically configurable.

It is preferred that the defined period is provided so that it corresponds to not more than a half revolution of the tool shaft, specially preferredly not more than a quarter revolution of the tool shaft, so that after the motor current is switched off, the electric motor as far as possible does not continue to rotate while uncontrolled.

In a preferred embodiment, the current limit and/or the defined period of the electrical machine tool are calculated depending on the rotational speed list value and/or the torque course of the electric motor, so that the preconfiguration of the electrical machine tool is also omitted.

S

Below, the invention is described on the basis of figures.

The figures are used only as examples, and do not restrict the general idea of the invention.

Fig. 1 shows an electrical machine tool according to the invention, here a hammer drill, Fig. 2 shows the electrical machine tool with the control unit schematically, and Fig. 3 shows, in Figs. 3(a) and 3(b), two possible courses for the actual current value.

Fig. 1 shows an electrical machine tool 1 according to the invention, here a hammer drill driven by a battery 9.

Below, the terms electrical machine tool 1 and hammer drill are used synonymously.

The hammer drill 1 has a tool shaft 3, which can be driven by an electric motor 2 (see Fig. 2) . The tool shaft 3 is drawn schematically here. A transmission which may be arranged between a motor shaft (not shown) of the electric motor 2 and the tool shaft 3 is not shown here and below.

On the tool shaft 3, a tool 31, here for example a drill 31 (see Fig. 2), can be arranged for co-rotation by means of a tool holder 13, the drill 31 rotating together with the tool shaft 3. in one direction of rotation 32 when the latter is driven. Below, the terms tool 31 and drill are used synonymously.

The electric motor 2, which here is in the form of a direct current motor, can be switched on by a user via an operating switch 11. The hammer drill 1 also has a changeover switch 12, by which the direction of rotation 32 in which the driven tool shaft 3 rotates can be reversed.

If the drill 31 tilts or the tool holder 13 blocks, the electric motor 2 is still driven, although the tool shaft 3 no longer rotates. Consequently, the hammer drill 1 rotates. To protect the user, this hammer drill 1 has a controller 4 (see Fig. 2) Fig. 1 also shows schematically an angle of rotation a of the tool shaft 3, here a quarter revolution.

Fig. 2 shows the electrical machine tool 1 with the control unit 4 schematically.

The control unit 4 includes a measuring unit 51, by which an actual current value 1ST of a motor current I is measured. If the actual current value 1ST exceeds a current limit GRENZ, a counter 6, which counts along a defined period TDEF, begins to run. If the actual current value 1ST exceeds the current limit GRENZ throughout this defined period TDEF, an arithmetic-logic unit 8, which is provided to control the control unit 4, opens a switch 10 in the circuit 91 of the motor current I, so that the latter is interrupted and the electric motor 2 is no longer driven.

Fig. 2 therefore shows the electric motor 2 being switched off by interruption of the circuit I, and thus the electric motor 2 idling. Since when the electric motor 2 is switched off in this way it is not actively braked, it runs until the stored kinetic energy is consumed. Alternatively, the electric motor 2 is braked electrically by the control unit 4, by being short-circuited or its polarity being reversed, e.g. by use of a NOSFET (metal oxide semiconductor field-effect transistor) . For this case, it is preferredly possible to feed the stored residual energy back into the battery 9, e.g. a lithium-ion battery which drives the S electric motor 2.

An embodiment in which the defined period TDEF continues to be counted even if the actual current value 1ST, after exceeding the current limit GRENZ, undershoots it again, if appropriate by a defined amount (not shown), but then exceeds it again within a shorter defined period (not shown) and/or by a different defined amount (not shown), is also preferred.

The current limit GRENZ and/or the defined period TDEF are preferredly provided so that the rotation of the electric motor 2, measured after the current limit GRENZ is exceeded, corresponds to less than an angle of rotation a (see Fig. 1) of one revolution of the tool shaft 3, preferredly less than a half revolution of the tool shaft 3. This angle of rotation a consists, from the instant at which the actual current value 1ST exceeds the current limit GRENZ, of a first part, corresponding to the defined period TDEF, and a second part, after the switch 10 is opened, resulting from the rotatory energy which is still stored in the hammer drill 1. This angle of rotation a is also influenced by the user, in particular his or her posture or how firmly he or she holds the hammer drill 1, and the workpiece being worked on (not shown), in particular its material properties. The angle of rotation a also depends on the properties of the hammer drill 1, in particular its mass moment of inertia, so that in the case of a heavier hammer drill 1 and identical defined period TDEF, a different angle of rotation a results.

The defined period TDEF is preferredly provided so that it corresponds to not more than an angle of rotation a of a half revolution of the tool shaft 3, specially preferredly not more than a quarter revolution of the tool shaft 3.

Depending on what hammer drill is used, an angle of rotation a of a half to a quarter revolution corresponds approximately to a defined period TDEF of 20 to 80 ms.

Device-specifically, however, a different defined period TDEF may be preferred.

If the actual current value 1ST of the motor current I rises above the current limit GRENZ, the control unit 4 switches off the motor current drive train, in particular the tool shaft 3, to limit the moment acting on the user.

Depending on the chosen defined period TDEF, the hammer drill 1 can rotate around the tool axis only by the fraction of a revolution corresponding to the defined period TDEF, while maintaining this moment. Additionally, it continues to rotate with less moment, by the amount resulting from the rotatory energy and the further influences.

Since the angle of rotation a, by which the hammer drill 1 runs on, depends on the stored rotatory energy and/or the properties of the hammer drill 1, it is preferred either to define the current limit and/or the defined period TDEF device-specifically, and to hold them in a memory unit 7 of the control unit 4 so that they can be called up. In a further embodiment, holding different current limits TGRENZ and/or defined periods TOEF, depending on properties of the operated hammer drill, in particular depending on a torque course M(t) and/or a rotational speed nI5T of the electric motor 2, is preferred. Alternatively or additionally, calculating, in the arithmetic-logic unit 8, the current limit GRENZ and/or the defined period TDEF depending on these properties, in particular the torque course M(t) and/or the rotational speed of the electric motor 2, is preferred.

For this purpose, the control unit 4, in a preferred embodiment, also includes a measuring unit 52, 53 to measure the properties, in particular to measure the torque course M(t) and/or the rotational speed np of the electric motor 2.

It is preferred that at least the arithmetic-logic unit 8 and the counter 6, specially preferredly also the memory unit 7, are arranged in a single integrated component. In this case it is preferred that the arithmetic-logic unit 8 is in the form of a microprocessor (CPU, central processing unit), and the integrated component is a microcontroller.

However, the arithmetic-logic unit 8 can also be implemented as a different, programmable electronic component, e.g. a PLD (programmable logic device) or FPGA (field programmable gate array). The counter 6 and/or the memory unit 7 can also be implemented as discrete components.

Fig. 3 shows, in Figs. 3(a) and 3(b), two possible courses for the actual current value I$T 33 As soon as the actual current value 1ST exceeds a current limit GRENZ, the counter 6 (see Fig. 2) begins to count along the defined period TDEF. If the actual current value 1ST exceeds the current limit GRENS over the whole defined period TDEF, a switch 10 in the circuit 91 of the motor current I is opened by means of the control unit 4, so that the motor current I is switched off and the electric motor 2 is no longer operated.

S

Fig. 3(a) shows, as well as the course of the actual current value lIST and the current limit GRENZ, the maximum permitted motor current I<, which is controlled electronically, and a maximum permitted motor current N if the electronics fail, according to Standard EN 60745.

The motor current I is thus limited to the maximum permitted motor current I. In principle, it is possible that the motor current I, in the case of infinitely steep rise from the current limit GRENZ, is at the current limit for the whole defined period TDEF. This is shown in Fig. 3 (b) In the case of blocking, the required torque and therefore the actual current value 1ST of the motor current I rise.

However, in the case of the hammer drill 1 according to the invention, this rise is permitted electronically only up to the maximum permitted current limit I.

Claims

Claims 1. An electrical machine tool, in particular a hammer drill, with a tool shaft which can be driven by an electric motor, characterized in that it includes an electronic control unit, which brakes or switches off the electric motor if the tool shaft is blocked, depending on an actual current value.
2. The electrical machine tool according to Claim 1, characterized in that to capture the actual current value, the control unit includes a measuring unit.
3. The electrical machine tool according to any one of the preceding claims, characterized in that the electric motor is switched off if the actual current value exceeds a current limit over a defined period.
4. The electrical machine tool according to any one of the preceding claims, characterized in that to capture the defined period, the control unit includes a counter.
5. The electrical machine tool according to any one of the preceding claims, characterized in that the current limit and/or the defined period are preconfigurable.
6. The electrical machine tool according to any one of the preceding claims, characterized in that the current limit and/or the defined period are provided so that after the current limit is exceeded, the further rotation of the electric motor corresponds to less than one revolution, preferredly less than a half revolution, of the tool shaft.
7. The electrical machine tool according to any one of the preceding claims, characterized in that the defined period is provided so that it corresponds to not more than a half revolution of the tool shaft, preferredly not more than a quarter revolution of the tool shaft.
8. The electrical machine tool according to any one of the preceding claims, characterized in that the defined period is not more than 80 ms, preferredly not more than 40 ms.
9. The electrical machine tool according to any one of the preceding claims, characterized in that the control unit includes a memory unit, from which the current limit and/or the defined period can be taken.
10. The electrical machine tool according to any one of the preceding claims, characterized in that the control unit includes an arithmetic-logic unit to control the control unit, and/or to calculate the current limit and/or the defined period.
11. The electrical machine tool according to any one of the preceding claims, characterized in that the current limit and/or the defined period depend on a rotational speed list value and/or a torque course of the electric motor.
12. The electrical machine tool according to any one of the preceding claims, characterized in that the electric motor is a direct current motor.
13. A method of switching off an electric motor of an electrical machine tool, in particular according to any one of the preceding claims, in particular a hammer drill, with a tool shaft which can be driven by the electric motor, characterized in that the electric motor is switched off or braked if an actual current value which is required to drive the electric motor exceeds a current limit over a defined period.
14. A method according to Claim 13, characterized in that the defined period is provided so that it corresponds to not more than a half revolution of the tool shaft, preferredly not more than a quarter revolution of the tool shaft.
15. A method according to any one of Claims 13 to 14, characterized in that the current limit and/or the defined period are calculated depending on the rotational speed list value and/or a torque course of the electric motor.
16. An electrical machine tool substantially as herein described, with reference to the accompanying drawings.
17. A method of switching off an electric motor of an electrical machine tool substantially as herein described, with reference to the accompanying drawings.