EP2886262A2

EP2886262A2 - Electric power tool and a restart prevention system therefor

Info

Publication number: EP2886262A2
Application number: EP14183269.1A
Authority: EP
Inventors: Jeroen Schroer
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2013-10-29
Filing date: 2014-09-02
Publication date: 2015-06-24
Also published as: RU2014139650A; EP2886262A3; NL2011696C2

Abstract

The present invention relates to an electric power tool, in particular a grinder. The invention comprises a mains connector to be selectively connected to a power supply by a user, a hardware switch, which is operable by a user and an electric drive, in particular an electric motor, wherein the switch is arranged in a connection between the connector and the drive to selectively allow supply of power to the drive with the connector connected to the supply, a restart prevention system, comprising at least one opto-coupler, CHARACTERISED IN THAT the restart prevention system is configured to prevent restart of the drive after temporary disconnection from or interruption of power supply with the switch in a state to supply power to the drive, wherein the opto-coupler is connected to conductors of the connection and there arranged in series with a normally conducting switch part of the switch, which is opened to a non-conductive state in conjunction with user operation of the switch to activate the drive, and the opto-coupler is arranged in a by-pass excluding the switch part of the switch, when the switch part of the switch is in said non-conductive state, to maintain current flow through the opto-coupler while connected via the connection to the power supply. Furthermore the invention relates to a system configured to prevent restart of an electric power tool.

Description

The present invention relates to an electric power tool having a system to prevent restart of an electric power tool, in particular a grinder, in particular after inadvertent temporary disconnection or interruption of a connector, like a plug, from a power source, like a mains power supply socket.
The invention further relates to a system to prevent restart of power tools in general also on its own.
A prior art situation, to which the present invention relates, is schematically represented in figure 1. Therein, a power tool 1 is depicted, comprising a housing 3, from which a power cord 7 extends to a plug 2, which can be inserted into a mains power supply socket 6. The tool 1 further comprises a switch 5 extending from housing 3. The switch 5 is user operable to activate the drive in the form of a motor 4. When the plug 2 is inserted (arrow A) into the socket 6, preferably, switch 5 is in a position to keep the motor 4 deactivated. However, this is often not secure. For instance in a situation where plug 2 is taken out of socket 6 (arrow C) while motor 4 has been activated by operation of switch 5 (arrow B), for instance by accident, many less experienced users will tend to simply insert plug 2 into the socket 6 (arrow D). If this is attempted, then motor 4 will be activated immediately upon inserting plug 2 into socket 6. This results in a dangerous situation for the user. Likewise, a similar situation may occur after a fuse is blown, or an (extension) cord is cut, and/or the like. Consequently, where herein after reference is made to interruption or disconnection of power supply to a tool, any one or a combination of such or other causes is intended.
To the best knowledge of the inventors of the present development, which is disclosed herein below and protection for which is defined in the appended claims, no adequate solution to this problem is available. Namely, a solution to such a problem must be sturdy and be able to withstand disrupting or even wear and tear promoting dust.
Notwithstanding such a consideration, the person skilled in the art in the technical field of power tools is most likely to envisage a solution in the direction of mechanical safety switches or the like, using mechanical transmissions or the like, or incorporate moving components in general. Such solutions will always be susceptible to the disrupting and where promoting influence of dust.
Moreover such potential approach to solving this problem requires considerable space, which is not available, in particular in smaller power tools, for instance for domestic users.
The above approach to the present problem and/or similar solutions, that may be expected from the skilled person from within the range of his skill, are all anticipated to be complex and costly, whereas especially for smaller electric power tools for the domestic market, required space and simplicity should be as low as possible. If such requirements for solving the above identified problem can be met within the context of smaller hand tools, for instance for the domestic market, then such a solution should also be expected to be suitable for larger and more professional power tools, where price considerations and required space or volume are less decisive with respect to potential embodiments of solution to address the above-mentioned problem.
Although the above reference is made to a problem with respect to potentially dangerous activation of a drive when replugging a power tool into a mains power supply (or after a short or longer power supply interruption), and to potential solutions with respect to such problems, none of the potential solutions referred to constitutes known as prior art, with which the inventors of the below disclosed solution are familiar.
However, reference is made here to US-2009/160371 , which is acknowledged to constitute the closest prior art, relative to which at least the features in the characterising portions of the appended independent claims are novel. It is to be noted in this context that US-2009/160371 relates to a system for blocking operation of the drive or motor, during setting of speed changes, which is basically different from embodiments of the present disclosure.
In order to provide a solution to the above-mentioned problems, embodiments of electric power tools, in particular grinders, are provided as comprising all of the features of the appended power tool oriented independent claim 1.
The use of an opto-coupler, which is completely insensitive to influences like dust, vibrations, and the like, results in a power tool like a grinder and a system to prevent inadvertent activation of a drive, in particular a motor, after re-plugging a plug of the tool into the mains power supply with a switch thereof in a motor activating state. The provided solution allows for minimum volume and/or space requirements and an optimal simplicity, and is even applicable in the context of professional, larger and more powerful power tools in view of the robustness of the presented solution.
Further, the opto-coupler (20) is connected to conductors of the connection (7) and there arranged in series with a normally conducting switch part (19) of the switch (15), which is opened to a non-conductive state in conjunction with user operation of the switch (15) to activate the drive. The combination of the opto-coupler with the normally conducting switch part of the switch allows for a very simple realisation of a very effective solution to the posed problem.
Also, the opto-coupler (20) is arranged in a by-pass excluding the switch part (19) of the switch (15), when the switch part (19) of the switch (15) is in said non-conductive state, to maintain current flow through the opto-coupler (20) while connected via the connection (7) to the power supply. Once the opto-coupler has been activated with the switch part in a conductive state thereof, it will take over to ensure a continued conductive state of the circuit, regardless of the actual state of the normally conductive switch part. When power supply is then interrupted, supplying power to the drive for motor will continue to be disabled, as long as the switch part is not returned to the normally conductive state thereof, i.e. as long as the switch for power supply to the drive or motor is not returned first to the state thereof in which direct power supply from the plug to the drive for motor is interrupted.
In preferred embodiments, the electric power tool exhibit the feature that the restart prevention system comprises a rectifier. This is in particular useful and relevant in case an opto-coupler requires DC power supply, which is common, although embodiments of opto-couplers may function also on mains AC power. In any case, when a rectifier is provided, for DC power supply to the opto-coupler, this appears to run against the desire for a compact design and minimum space or volume requirements. However, embodiments of this feature may be implemented quite advantageously in a very simple circuit design, for instance on a compact printed circuit board, for instance together with the opto-coupler.
In specific embodiments, electric power tools may additionally or alternatively relative to the preceding features exhibit the feature that the restart prevention system is arranged between the mains connector and the switch. Such a feature runs against a first inclination of a skilled person to implement any solution in conjunction with the drive for motor, and emphasises that are very compact and elegant design of a circuit or system can be arranged practically anywhere within the housing of the power tool or electric hand tool.
In specific embodiments, electric power tools may additionally or alternatively relative to the preceding features exhibit the feature that the opto-coupler is associated with a relay in at least one conductor from the connector to either or both of the switch and the drive. Only when the opto-coupler is in a conductive state, potentially bypassing the switch part, if provided, can the relay be driven to enable power supply from the plug to the drive for motor. In such an embodiment it can be preferable to provide the feature that the relay is a solid state relay. Thereby, the use of moving parts can be avoided. Further also in embodiments exhibiting a relay, the relay itself can comprise at least one further opto-coupler. Thereby, the use of moving parts, and a required space therefore, can be avoided.
In specific embodiments, electric power tools may additionally or alternatively relative to the preceding features exhibit the feature that the switch comprises a switch part in connection with the opto-coupler and a further switch part in connection with at least one conductor from the connector to the drive. Consequently, when a user activates the switch, the switch part associated with the opto-coupler can be changed from the normally conductive state thereof to the non-conductive state, with as a result a high degree of reliability and efficiency. In such an embodiment, the electric power tool may further exhibit the feature that another of normally at least two conductors from the connector to the drive forms a direct connection between the connector and the drive, without any switch part of the switch therein. Thereby, the number of switch parts can be reduced relative to the situation where a two switch parts are provided, one for each conductor between the plug and the drive or motor, with additionally a further switch part for association with the opto-coupler; only a single switch part needs to be provided for one of normally two conductors between the plug and the drive or motor, with additionally also the further switch part for association with the opto-coupler. Thus, a further simplification can be achieved.
In specific embodiments, electric power tools may additionally or alternatively relative to the preceding features exhibit the feature that the opto-coupler in the bypass comprises a light emitting element and an associated light receiving element in a series configuration. This enables a very simple and elegant configuration, with which the desired functionality of main embodiments of the present disclosure are capable of allowing current to keep on flowing through the opto-coupler, even after the switch part of the switch is made to be non-conductive, and for as long as power is available aand current flows through the opto coupler.
Above, embodiments of solutions for the posed problem have been outlined in relatively generic terms, corresponding with the features of the below appended claims. Below, a description of more detailed embodiments will follow, on the basis of the appended drawing. It is, however, emphasised here that such embodiments are merely exemplary and can by no means impose any limitation on the interpretation of the scope of protection, since the scope of protection for the embodiments is defined in the appended claims, in particular the appended independent claim. In the drawing:

figure 1 shows a schematic view, described above, to set out the challenge of solving the problem identified above;
figure 2 shows a schematic representation of a first embodiment;
figure 3 shows a more detailed embodiment, which essentially corresponds with figure 2 and exhibits more detail thereof;
figure 4 shows an implementation on a simple circuit board of an embodiment with an adaptation to the tools switch; and
figure 5 shows a more detailed embodiment as in figure 3, having the adapted switch of figure 4.

Figures 2 and 3 exhibit an embodiment of circuitry, to be arranged within a housing 13 of a power tool. The circuitry comprises a rectifier circuit 17, arranged between conductors of connection or power cord 7. The rectifier circuit 17 is shown in more detail in figure 3, where for the sake of clarity the plug 2 and drive or motor 14 are omitted.
A rectified power is supplied by the rectifier circuit 17 to a detection circuit 23. The detection circuit 23 comprises opto-coupler 20. Further, the detection circuit 23 is connected to a switch part 19, which forms part of or is associated with the user operable hardware switch 15. The switch part 19 is normally conductive, i.e. when switch 15 has not been operated by the user, and the drive or motor 14 is in rest in the absence of a power supply. Consequently, when an AC voltage is applied over the rectifier 17 after plug 2 has been inserted into a mains power supply source, like a socket 6 in figure 1, the rectifier 17 provides a rectified voltage of for instance 5 V (or any other voltage appropriate for the opto-coupler 20) to the detection circuit 23 comprising the opto-coupler 20. Opto-couplers are known to be able to function also at AC voltages, in which case the rectifier 17 can be omitted and/or replaced by for instance a transformer, to only adapt the voltage level to an AC voltage value required for the opto-coupler to work. As soon as the appropriate voltage from the rectifier 17 is arranged over the opto-coupler 20, switch part 19 can be bypassed, but current will flow through the switch part 19 as well as the bypass. Normally, the user will only thereafter activate switch 15 to set the drive or motor 14 into motion. Simultaneously with closing by the user of the switch parts of the switch 15 to the drive for activation of the motor 14, switch part 19 will go into a non-conductive state thereof. However, the bypass along the switch 19 will ensure a flow path for current to end from the rectifier 17. This flow of current is only interrupted, after plug 2 is taken from socket 6. If this occurs, then power supply to the rectifier 17 is stopped, and the opto-coupler 20 becomes nonconductive, the same as the state of switch part 19. Consequently, when the plug is reinserted into the socket, the detector 23 will remain powerless, as the opto-coupler 20 requires a predetermined current to flow therethrough to become conductive and the switch part 19 is also still non-conductive in correspondence with the position of switch 15, after switch 15 has been operated to allow power transmission to the drive.
The detector 23 also comprises a further opto-coupler 21 forming part of a relay circuit 18. With the current flowing through the opto-coupler 20, the opto-coupler 21 is also conductive to allow power to be transferred along the conductors to the drive for motor 14, if switch 15 has been manually operated by the user accordingly. When the plug 2 has been taken from the socket 6, without returning the switch 15 and the switch part 19 to the normal position thereof, reinserting the plug 2 cannot lead to uncontrolled reactivation of the drive or motor 14. More in particular, the detector circuit 23 comprising the opto-coupler 20 will in those circumstances prevent power from being transported from plug 2 to the drive or motor 14, when the relay circuit 18 is in a non-conductive state. As long as the detection circuit 23 remains powerless, the relay circuit 18 will prevent power from being provided to the drive for motor 14. The detection circuit 23 comprising the opto-coupler 20 can only be returned to the original state thereof by reverting the switch 15 and thereby the normally conductive switch part 19 to establish flow of current through the detection circuit 23 and allow the opto-coupler with the bypass to take over the current flow, regardless of the position or state thereafter of the switch part 19. Consequently, when the user subsequently again operates the switch 15 to set the drive for motor 14 into motion, this can be achieved and maintained as long as plug 2 is in the socket 6 and the switch 15 is set to power the drive or motor 14.
Figure 4 exhibits an embodiment of a printed circuit board 24, which allows a considerable freedom of design, for instance with respect to the shape of the printed circuit board 24 to allow the printed circuit board 24 to be arranged within a housing of a power tool.
Likewise, the design of the circuits in figures 2 and 3 exhibits considerable freedom with respect to design parameters. For instance, a solid state relay could be employed instead of the relay circuit 18, provided the costs thereof allow for a commercially viable implementation. However, the relay circuit 18 in figures 2 and 3 comprises for instance a TRIAC 22, which allows the design to be adapted to a power requirement of the motor. Normally, solid state relays do not allow such flexibility in the design of the circuitry, which is why a tailor made solution on a printed circuit board 24 may be preferred over a standardised solid state relay, even though a standard solid state relays may also form part of implementations of present embodiments.
Further, the embodiment of figure 4 exhibits the two-part switch design of the switch 25 comprising a normally non-conductive switch part 26 and the normally conductive switch part 19. Further, switch part 26 of switch 25 acts on a single conductor leading to the drive or motor 14, as exemplified in figure 5. The other conductor comprises the relay circuit 18. Consequently, an essentially traditional two- part 19, 26 switch can be employed to embody the switch 25, provided one of the poles is switch part 19, which is redesigned to be normally conductive. Consequently, a considerable simplification can be achieved using the embodiment of figures 4 and 5, which correspond with respect to essentially only the configuration of the switch 25.
A capacitor 27 is arranged in figure 5 between the two conductors leading to drive or motor 14. The capacitor is designed and dimensioned for RFI protection to prevent disruption of exterior devices by reducing radio frequency interference that could be caused by the drive or motor. Further, the capacitor 27 may serve to suppress supply voltage ripples. The capacitor 27 further may serve for ESD (electro-static discharge) protection of the circuitry of at least one of detector 20, relay 18 and rectifier 17, in conjunction and/or cooperation with other electrical components, in particular other capacitors in rectifier 17, relay 18 and/or detector 20 (such other capacitors are not individually indicated in figures 2, 3 or 5 with individual reference numbers).
One connection of the capacitor is arranged between switch part 26 and the drive. Consequently, a discharge of the capacitor 27 over the pins of plug 6 is prevented.
For remaining parts and components, the printed circuit board 24 could comprise a circuit according to figure 5, or according to a combination of figures 2 and 3, but also other practical circuits and designs can be implemented.
From this preceding consideration and also the disclosure above and in the appended drawings as a whole it is directly and immediately evident, that many alternative and additional embodiments are possible within the framework of a solution to the problem commonly underlying all described and specifically shown embodiments, from which it is considered self evident that no specific features or functions or components or circuits of the above specific embodiment description, referring to the appended drawings, can be taken as a limitation on the scope of protection for embodiments as defined in the appended claims, in particular the independent claims thereof. For example, other power tools then grinders can benefit from a restart prevention system. The relay circuit 18 can be embodied in any manner, without any limitation to the specific embodiment of figures 2, 3 and 5, for instance using solid-state relays, which have already been mentioned above as a potential alternative. It is in principle possible to arrange the relay 18 and switch part 26 of switch 25 in one of the conductors between the connector or plug 6 and the drive or motor 14, even though in such an embodiment inductive effects on the circuitry need to be addressed using additional components and/or features, which are in themselves well within the realm of the skilled person's capabilities. From these considerations it is evident that only the terms and definition of embodiments according to the appended claims can limit the scope of protection therefore.

Claims

An electric power tool, in particular a grinder, comprising:
- a mains connector (2) to be selectively connected to a power supply (6) by a user;

- a hardware switch (15), which is operable by a user; and

- an electric drive, in particular an electric motor (14),

- wherein the switch (15) is arranged in a connection (7) between the connector (2) and the drive (14) to selectively allow supply of power to the drive (14) with the connector (2) connected to the supply (6); and

- a restart prevention system, comprising at least one opto-coupler (20),
CHARACTERISED IN THAT

- the restart prevention system is configured to prevent restart of the drive (14) after temporary disconnection from or interruption of power supply with the switch (15) in a state to supply power to the drive (14),
wherein the opto-coupler (20) is connected to conductors of the connection (7) and there arranged in series with a normally conducting switch part (19) of the switch (15), which is opened to a non-conductive state in conjunction with user operation of the switch (15) to activate the drive, and the opto-coupler (20) is arranged in a by-pass excluding the switch part (19) of the switch (15), when the switch part (19) of the switch (15) is in said non-conductive state, to maintain current flow through the opto-coupler (20) while connected via the connection (7) to the power supply.
The electric power tool of any of the preceding claims, wherein the opto-coupler (20) is associated with a relay (18) in at least one conductor from the connector (2) to either or both of the switch (15) and the drive (14).
The electric power tool of claim 1 or 2, wherein the restart prevention system comprises a rectifier (17), via which the opto-coupler (20) is connected to the conductors of the connection (7).
The electric power tool of claim 1, 2 or 3, wherein the restart prevention system is arranged between the mains connector (2) and the switch (15).
The electric power tool of claim 2, 3 or 4, wherein the relay (18) is a solid state relay.
The electric power tool of claim 2 or 5, wherein the relay (18) comprises at least one further opto-coupler (21, 22).
The electric power tool of any of the preceding claims, wherein the switch (15) comprises a switch part (19) in connection with the opto-coupler (20) and a further switch part in connection with at least one conductor (7) from the connector (2) to the drive (14).
The electric power tool of claim 7, wherein another of at least two conductors (7) from the connector (2) to the drive (14) forms a direct connection between the connector (2) and the drive (14), without any switch part of the switch (15) therein.
The electric power tool of claims 2 and 7, wherein another of at least two conductors (7) from the connector (2) to the drive (14) comprises the relay (18).
The electric power tool of any of the preceding claims, wherein the opto-coupler (20) in the bypass comprises a light emitting element and an associated light receiving element in a series configuration.
A system configured to prevent restart of an electric power tool, in particular a grinder, more in particular of a drive (14) of the tool after temporary disconnection of a connector (2) to the power supply (6) with a user operable switch (15) of the tool in a state to supply power to the drive (14), the system comprising the restart prevention system features of at least one of the preceding claims.