Electric cutting tool with automatic emergency stop

Abstract

A safety cutting tool has an electrically conductive cutting element, an electrically controlled actuating motor drivingly connected to the cutting element, a gripping handle electrically insulated from the cutting element and adapted to receive a hand of an operator therein, a cutting trigger control electrically insulated from the cutting element and adapted to be actuated by the hand of the operator, and an emergency stop device that is able to respond to a contact of the operator with the cutting element. The emergency stop device includes a first manual contact, electrode and a second manual contact electrode with an electrical monitoring circuit connected thereto.

Claims

1. A safety cutting tool comprising: an electrically conductive cutting element; an electrically controlled actuating motor drivingly connected to said electrically conductive cutting element; a gripping handle adapted to receive a hand of an operator, said gripping handle being electrically insulated from said electrically conductive cutting element; a cutting trigger control adapted to be activated by the hand of the operator, said cutting trigger control being electrically insulated from said electrically conductive cutting element; an emergency stop device adapted to respond to a contact of the operator with said electrically conductive cutting element, said emergency stop device comprising: at least one first manual contact electrode; at least one second manual contact electrode, said at least one first manual contact electrode and said at least one second manual contact electrode being positioned on either said gripping handle or said cutting trigger control, said at least one first manual contact electrode and said at least one second manual contact electrode adapted to be touched simultaneously by the hand of the operator when the hand of the operator grasps said gripping handle; an electrical monitoring circuit connected to said at least one first manual contact electrode and said electrically conductive cutting element, said electrical monitoring circuit adapted to close upon a simultaneous contact of the operator with said at least one first manual contact electrode and with said electrically conductive cutting element; an electrical generator connected to said electrical monitoring circuit and adapted to monitor a current in said electrical monitoring circuit; and a measuring device connected to at least one of said electrically conductive cutting element and said at least one second manual contact electrode and adapted to monitor voltage therebetween; and a comparator connected to said emergency stop device, said comparator adapted to compare at least one monitored electrical characteristic dependent on the monitor voltage and a threshold electrical characteristic dependent on an impedance value, said comparator causing said emergency stop device to cause an emergency stop of said electrically controlled actuating motor when the at least one monitored electrical characteristic crosses the threshold electrical characteristic.

2. The safety cutting tool of claim 1, wherein the at least one monitored electrical characteristic is a voltage dependent on the monitor voltage, the threshold electrical characteristic being a threshold characteristic dependent on the impedance value and a monitored current.

3. The safety cutting tool of claim 2, wherein the at least one monitored electrical characteristic is the monitor voltage, the threshold electrical characteristic being equal to a product of the impedance value and the monitored current.

4. The safety cutting tool of claim 1, wherein the at least one monitored electrical characteristic has an impedance value dependent on the monitor voltage and a monitored current, the threshold characteristic being the impedance value.

5. The safety cutting tool of claim 1, wherein the at least one monitored electrical characteristic is equal to a ratio of the monitor voltage and a monitored current, the threshold characteristic being equal to the impedance value.

6. The safety cutting tool of claim 1, wherein said electrical generator is an alternating current generator.

7. The safety cutting tool of claim 1, wherein said electrical generator has a current source.

8. The safety cutting tool of claim 1, wherein said electrical generator has a voltage source.

9. The safety cutting tool of claim 1, wherein said electrical monitoring circuit has an electrical adjustment impedance in, series therewith so as to present a predetermined impedance value and another measuring device that measures a voltage on terminals of the electrical adjustment impedance.

10. The safety cutting tool of claim 9, wherein a current source is connected in parallel with the electrical adjustment impedance.

11. The safety cutting tool of claim 1, further comprising: a monitoring device that monitors an electrical potential of said electrically conductive cutting element, said monitoring device being connected to said emergency stop device so as to cause an emergency stop when electrical potential of said electrically conductive cutting element is beyond a desired range.

12. The safety cutting tool of claim 1, wherein said electrically conductive cutting element is connected to a ground potential.

13. The safety cutting tool of claim 1, wherein said electrically controlled actuating motor is a heat engine.

14. The safety cutting tool of claim 1, wherein said electrically controlled actuating motor is an electric motor, said emergency stop device having a electric card for a servo-driven motor.

15. The safety cutting tool of claim 1, wherein a grounding impedance is electrically connecting said at least one second manual contact electrode to said electrically conductive cutting element, the ground impedance having a value greater than the impedance value, the grounding impedance adapted to cause an emergency stop or a deactivation when the monitor voltage of said measuring device is zero.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a simplified schematic representation of the main characteristics and functions of a cutting tool in accordance with the invention.

(2) FIG. 2 is a simplified schematic representation of a pruning shears in accordance with the invention.

(3) Identical or similar parts of FIGS. 1 and 2 have the same numerical references so as to facilitate the transfer of one figure to the other and so as to repeat its description.

DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION

(4) FIG. 1 shows schematic diagram of a cutting tool 10 provided with a cutting element 12.

(5) The cutting element 12 is electrically connected to the ground potential 18 of the tool.

(6) The cutting tool also includes an electric motor 20 mechanically connected to the cutting element 12, by a transmission mechanism 22. The transmission mechanism transmits to the cutting element the mechanical energy supplied by the motor. It drives the cutting element in a pivoting movement, a circular rotational movement or a translatory movement, depending on the type of the cutting element.

(7) The electric motor 20 is associated with an electric power supply 24 and an electronic control card 26 of the motor. The electronic control card 26 can receive signals from a cutting trigger element 28 that is activated by the hand of an operator grasping a gripping handle 30 of the cutting tool 10. The cutting trigger element 28, for example a trigger, is positioned in proximity of the gripping handle 30, so it can be actuated by the hand grasping the handle.

(8) The gripping handle 30 is made of an insulating plastic material and hence electrically insulated from the cutting element. It is also provided with a pair of manual contact electrodes 32 and 34 positioned respectively on two essentially opposite parts of the handle. The manual contact electrodes are metallic electrodes or electrodes in a conductive plastic material with which the operator's hand necessarily comes into contact when the pruning shears are grasped by their handle. They are electrically insulated from each other and are electrically insulated from the cutting element 12. Either the first or the second electrode may also be positioned on the trigger of the cutting trigger element, actuated by the hand grasping the handle.

(9) An emergency stop device 40 of the electric motor 20 of the pruning shears is governed by an electric monitoring circuit 42. The electric monitoring circuit 42 includes components of the cutting tool but may also include parts of the body of a human using the pruning shears 10. The electric circuit 42 particularly includes in series, an adjustment impedance 52, the first manual contact electrode 32, the cutting element 12 and an electric generator 56 for a monitoring current. The electric generator is for example a source of voltage, for example an electric battery, or a source of current. The adjustment impedance may be formed by one or several electrical components with a known electrical impedance value. It may be included in the electric generator 56. It is, for example, an electric resistance with a value, for example, of 100 kΩ. However, its value is not critical. It may range, for example, from 1Ω to 200 kΩ. A measurement of the voltage V.sub.1 at the terminals of the adjustment impedance can be used to determine the monitoring current Is flowing in the monitoring circuit.

(10) The adjustment impedance is not necessary if the electric generator 56 includes a current source which directly generates a current of defined value I.sub.S, when the monitoring circuit 42 is closed.

(11) In the absence of contact with an operator, the electric monitoring circuit 42 is normally an open circuit and therefore has quasi-infinite overall impedance and a current of zero.

(12) When an operator seizes the handle, his hand comes into contact with the manual contact electrodes and thus with the first manual contact electrode 32. The first electric circuit 42 remains open.

(13) However, when the operator also touches the cutting element 12, for example with a finger of his free hand, he closes the electric monitoring circuit 42. In this case, the adjustment impedance 52 finds itself in series successively with the first manual contact electrode, a contact impedance 60 of the operator's hand with the first manual contact electrode 32, an impedance 62 of the operator's body, a contact impedance 64 of the finger with the cutting element, and, finally the cutting element 12.

(14) The values of the contact impedance 60 of the hand, the impedance 62 of the body and the contact impedance 64 of the finger with the cutting element are noted Z.sub.M1, Z.sub.C et Z.sub.D respectively.

(15) Thus, when the first circuit is closed, a total impedance Z.sub.T is such that:
Z.sub.T=Z.sub.1+Z.sub.M1+Z.sub.C+Z.sub.D

(16) The impedance of wiring and the cutting element are ignored here. Also ignored is the impedance of the electric generator 56 which is considered here to be a source of voltage.

(17) When the electric monitoring circuit is closed, the generator 56 causes an Is monitoring current to flow in the circuit.

(18) The value of the current Is can be predefined by the current generator when it includes a current source. It can also be predetermined based on a voltage measurement made on the terminals of the adjustment impedance 52 when the generator includes a voltage source, for example. Measurement of voltage V.sub.1 at the terminals of the adjustment impedance 52, of value Z.sub.1, is taken by an integrated voltmeter 53.

(19) A voltage measuring device 44, for example another integrated voltmeter, is connected between the ground 18 of the cutting tool 10 and the second manual contact electrode. It measures a potential V.sub.E2, or more precisely a monitoring voltage V.sub.E2 between the ground 18 and the second manual contact electrode 34.

(20) The monitoring voltage V.sub.E2 as well as the voltage delivered by the integrated voltmeter 53 are supplied to a digital management unit 74. The digital management unit, for example a microcontroller, or a dedicated integrated circuit, allows various operations to be performed.

(21) A first operation consists of calculating the monitoring current Is by performing a ratio V.sub.1/Z.sub.1.

(22) A second operation may consist of calculating an impedance value Z based on the monitoring voltage and the monitoring current. By referring to the preceding description, it is recalled that:
Z=Z.sub.C+Z.sub.D=V.sub.E2/I.sub.S.

(23) Finally, and mainly, the digital management unit 74 constitutes a comparator.

(24) It can particularly be used to compare the impedance value Z to the threshold value Z.sub.threshold which increases the human body impedance value. It can also be used to compare the monitoring voltage V.sub.E2 to the threshold voltage V.sub.threshold such as V.sub.threshold=Z.sub.threshold×I.sub.S.

(25) When the voltage drops below the voltage threshold value or the impedance drops below the impedance threshold value, the threshold electrical characteristic is respectively crossed and the emergency stop is actuated.

(26) Comparisons can be made for other parameters depending on the aforementioned parameters and also be calculated by the digital management unit 74. For example, it is possible to compare conductances rather than impedances.

(27) Following the comparison, and when the threshold electrical characteristic has been crossed, by higher or lower values depending on the characteristic chosen, the emergency stop device 40 is triggered.

(28) The electrical threshold characteristics used by the comparator of the digital management unit, for example the values V.sub.threshold or Z.sub.threshold, can be stored in a memory 72 associated with a digital management unit 74.

(29) In the example of FIG. 1, the emergency stop device includes the electronic control card 26 of the electric motor. The electronic control card 26 receives the emergency stop signal as indicated by a dot-and-dash line. In this case, the motor electronic control card is configured to actuate a movement of the motor itself to counteract the movement of the cutting element and/or to cause an electromagnetic braking of the motor and the cutting element by using the inductive circuits of the electric motor.

(30) This results in an almost instantaneous stop of the movement of the cutting tool.

(31) After the cutting tool has stopped, the electric power supply can also be cut. It can be effectuated by a switch 27, and in particular a transistor switch, servo-driven by the digital management unit 74.

(32) It should be noted that the digital management unit 74, and in particular the comparator which it constitutes, as well as the electronic control card 26 of the actuating motor, can be produced in the form of a single integrated component.

(33) Reference 80 designates a ground impedance. In the example shown in FIG. 1, this is an electric resistance of more than 1 MΩ connected between the second manual contact electrode 34 and the ground 18 of the cutting tool. It prevents a floating voltage of the second manual contact electrode. It is also capable of setting the voltage of the second manual contact electrode at a value of zero when the second manual electrode is not in contact with the operator's hand.

(34) Measurement of a monitoring voltage of zero by the voltage measuring device 44 can thus be exploited by the digital management unit 74 to inhibit the operation of the tool or to cause an emergency stop. In the example shown in the figure, the electric power supply to the actuating motor 20 can simply be inhibited if the measurement of the monitoring voltage is zero.

(35) This makes it possible, for example, to prevent the operation of the tool when it is being held by an operator wearing insulating gloves that would prevent the detection of a contact with the cutting element.

(36) Almost no current flows through the impedance to ground or the part of the operator's hand in contact with the second manual contact electrode. Thus, the potential V.sub.E2 measured on the second manual contact electrode is quasi-identical to a potential V.sub.C which would be measured between an imaginary point C inside the body of the operator grasping the handle 30 and the cutting element, and therefore the ground of the tool.

(37) Reference 76 designates a control electrode electrically connected to the cutting element 12. It is provided to allow an emergency stop test without touching the cutting element. In fact, it suffices for the operator grasping handle 30 and simultaneously touching the control electrode 76 with his free hand to cause an emergency stop. The electronic control card 26 can possibly be configured to request such a periodic control operation, so as to ensure the proper functioning of the emergency stop device.

(38) A monitoring circuit 78 of the potential of the cutting element is also provided. It is built around a voltmeter and is also connected to the electronic control card 26 of the electric motor 20 to cause an emergency stop when an electric potential of the cutting element 12 becomes different from a set value. In the example of implementation shown, it is checked that the electrical potential of the cutting element is at the ground potential of the tool. The voltmeter can be an integrated component being part of the same electronic card as the digital management unit.

(39) FIG. 2 shows a pruning shears, with a partial cutaway showing the electric drive motor for the cutting element 12, as well as the transmission mechanism 22 between the motor 20 and a mobile blade 14 of the cutting element 12.

(40) The cutting element 12 includes, in the case of a pruning shears, a fixed blade or hook 16, and a mobile blade 14, moved by the electric motor 20 and closing on the fixed blade following a request by the cutting trigger control 28 which is here a trigger. A signal of the trigger is directed towards the symbolically represented motor control card 26.

(41) FIG. 2 illustrates the contact of the hand with the first and the second manual contact electrode 32 and 34 when the operator grasps the gripping handle 30 for a functional utilization of the pruning shears. It can be observed that the fingers, specifically the middle, ring and little fingers, come into contact with the first manual contact electrode 32 and therefore with the electrical monitoring circuit. The first manual contact electrode 32 is here positioned on a lower part of the gripping handle 30 in an operating position of the pruning shears.

(42) The palm P of the hand comes into contact with the second manual contact electrode 34 positioned here on an upper part of the gripping handle 30.

(43) The forefinger I of the hand is free to actuate the trigger 28.

(44) The free hand of the operator is shown in a position where one finger comes into contact with the cutting element. The hand holding the gripping handle of the pruning shears 10 and the hand coming into contact with the cutting element are connected by a drawn line symbolizing the human body.

(45) The electronic control card 26 of the electric motor is connected to the electric motor so as to control the electric power supply of the different phases of the motor. It can be used for controlling the rotation of the motor in a direction causing the closing of the mobile blade 14 on the hook 16. It can also be used, during an emergency stop, in an inverse direction of rotation, causing the opening of the cutting element, and counteracting its closing. Finally, it can be used as an electromagnetic brake, for example, by short-circuiting the motor phases.

(46) The electronic control card is also connected to an electric power supply 24, such as, for an example, an electric battery carried on the operator's back.

(47) FIG. 2 also shows the monitoring voltage V.sub.E2 measured between the ground of the pruning shears and the second manual contact electrode 34.

(48) Reference 13 indicates a thin anticorrosive and/or electrically insulating coating such as a PTFE coating (Polytetrafluoroethylene) which may be able to cover the blades 14, 16 of the pruning shears. The use of such insulating coating does not interfere with the operation of the tool of the invention and does not interfere, in particular, with the actuation of the emergency stop device when the electric generator 56 is an alternating current generator. In effect, an alternating monitoring current can flow in the case of contact of the operator's finger with blades 14, 16 of the pruning shears, in spite of the electrically insulating coating. In this case, the impedance Z.sub.D of the finger contact with the cutting tool presents a capacitive component.