Electric cutting tool with automatic emergency stop

Abstract

A safety cutting tool includes an electrically conductive cutting member, an electric motor for actuating the cutting member, a gripping handle electrically insulated from the cutting tool, a cutting trigger control that can be actuated by the hand of the user gripping the gripping handle, the cutting trigger control being electrically insulated from the cutting tool, an emergency stop device sensitive to a user coming into contact with the cutting member. The emergency stop device is controlled by a comparator comparing an impedance of a first electric circuit comprising the cutting tool and a second electric circuit used as a reference, the first and second circuits having manual contact electrodes provided on at least one of the gripping handle of the tool and the cutting trigger control. A method for controlling such a tool is also disclosed. The tool and method are is-applicable to electric secateurs and shears.

Claims

1. A safety cutting tool comprising: an electrically conductive cutting element; an electrically controlled actuating motor cooperative with said electrically conductive cutting element; a gripping handle electrically insulated from said electrically conductive cutting element, said gripping handle adapted to be gripped by a hand of an operator; a cutting trigger control electrically insulated from said electrically conductive cutting element, said cutting trigger adapted to be activated by the hand of the operator; and an emergency stop device adapted to be responsive 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 electrically insulated from each other and positioned on at least one of said gripping handle and said cutting trigger control; a first electrical circuit having a first manual contact and a first electrical impedance of said electrically conductive cutting element, said first electrical circuit adapted to close upon a simultaneous contact of the operator with said at least one first manual contact electrode and said electrically conductive cutting element; a second electrical circuit having said at least one first manual contact electrode and the at least one second manual contact electrode and the first electrical impedance and a second electrical impedance, said second electrical circuit adapted to close upon a simultaneous contact of the operator with said at least one first manual contact electrode and said at least one second manual contact electrode; at least one measuring device that measures an impedance characteristic of said first electrical circuit and an impedance characteristic of said second electrical circuit; and a comparator circuit of the impedance characteristic of the first electrical circuit and of at least one threshold characteristic dependent on the impedance characteristic of said second electrical circuit, said comparator circuit being connected to said emergency stop device so as to cause said electrically conductive cutting element to stop when the at least one threshold characteristic is crossed.

2. The safety cutting tool of claim 1, wherein the second electrical circuit includes a switch, the switch being driven by said cutting trigger control in order to open the second circuit during a cutting trigger and close the second circuit in the absence of a cutting trigger, wherein said at least one measuring device is configured to measure an impedance characteristic of said first electrical circuit when said second electrical circuit is open and measure the impedance characteristic of said second electrical circuit when the second electrical circuit is closed.

3. The safety cutting tool of claim 1, wherein said at least one measuring device has a source of electrical current in series with said first electrical circuit and said second electrical circuit and at least one of a voltmeter connected in parallel with terminals of the first electrical impedance and an ammeter or an ohmeter connected in series with the first electrical impedance.

4. The safety cutting tool of claim 1, wherein the source of electrical current is an alternating power source.

5. The safety cutting tool of claim 1, wherein the first electrical impedance has an Ohmic value equal to an Ohmic value of the second electrical impedance and is greater than 20 kΩ and in which the threshold characteristic of said first electrical circuit is equal to the impedance characteristic of said second electrical circuit.

6. The safety cutting tool of claim 1, further comprising: a memory cooperative with said emergency stop device and adapted to store the threshold characteristics.

7. The safety cutting tool of claim 1, further comprising: a control electrode electrically connected to said electrically conductive cutting element.

8. The safety cutting tool of claim 1, further comprising: a monitoring circuit that monitors an electrical potential of said electrically conductive cutting element, said emergency stop device of said electric motor being driven by the monitoring circuit in order to cause a stop of said electric motor when the electrical potential of the cutting element is outside a set point range.

9. The safety cutting tool of claim 1, wherein said emergency stop device has at least one of a power cutoff switch of the electrically controlled actuating motor and an electronic control card for the electrically controlled actuating motor and a cutoff switch of the cutting trigger control and an emergency brake for the electrically conductive cutting element.

10. The safety cutting tool of claim 1, wherein the at least one first manual contact electrode and the at least one second manual contact electrode are positioned on opposite sides of said gripping handle.

11. The safety cutting tool of claim 1, wherein said second electrical impedance has an Ohmic value grater than 20 kΩ.

12. The safety cutting tool of claim 1, wherein the first electrical impedance has an Ohmic value equal to an Ohmic value of the second electrical impedance.

13. The safety cutting tool of claim 1, wherein said electrically conductive cutting element is a pruning shears.

14. The safety cutting tool of claim 1, wherein said electrically conductive cutting element is a scissors.

15. A process of controlling a cutting tool in which the cutting tool has an electrically conductive cutting element and an electrically controlled drive motor that actuates the electrically conductive cutting element and a gripping handle, the gripping handle being electrically insulated from the electrically conductive cutting element, the gripping handle having at least one first manual contact electrode and at least one second manual contact electrode, the process comprising comparing an impedance characteristic of the first electrical circuit and a threshold characteristic when an operator grasps the gripping handle and touches the first manual contact electrode and the second manual contact electrode, the first electrical circuit having the first manual contact electrode and a first electrical impedance and the electrically conductive cutting element, the threshold characteristic being dependent on an impedance value of a second electrical circuit, the second electrical circuit having the first manual contact electrode and the second manual contact electrode and the first electrical impedance; and triggering a stop of the electrically conductive cutting element when the impedance characteristic of the first electrical circuit crosses the threshold characteristic.

16. The process of claim 15, further comprising: causing an emergency stop of an electrically controlled actuating motor when a non-zero voltage potential is detected on the electrically conductive cutting element.

17. The process of claim 16, further comprising: cutting off a power supply to the electrically controlled actuating motor when the step of triggering occurs.

18. The process of claim 16, further comprising: braking the electrically controlled actuating motor or the electrically conductive cutting element when the step of triggering occurs.

19. The process of claim 16, further comprising: initiating an emergency movement of the electrically controlled actuating motor when the step of triggering occurs.

20. The process of claim 16, wherein the electrically controlled actuating motor is stopped or deactivated when an accidental opening of the second electrical circuit is detected.

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 from one figure to the other.

DETAILED DESCRIPTION OF IMPLEMENTATIONS OF THE INVENTION

(4) FIG. 1 schematically shows a safe tool 10 provided with a cutting element 12. FIG. 2 shows the particular example of an electric pruning shears 10, the cutting element of which is formed by a mobile blade 14 capable of closing onto a hook 16 and any reference to this particular construction refers to FIG. 2.

(5) The blade and the hook are metallic and electrically conductive parts. They may possibly be covered with an electrically insulating polymer coat 13.

(6) The cutting element 12 is electrically connected to the ground 18 of the pruning shears constituting a reference potential.

(7) The pruning shears also include an electric motor 20 that is mechanically connected to the cutting element 12, through a transmission mechanism 22. In the example of FIG. 2, it makes the mobile blade 14 pivot between an open position and a closed position on the hook 16.

(8) The motor is associated to an electric power supply 24 and to an electronic control card 26 of the motor. The electronic control card 26 can receive signals of a cut release command 28 activated by a hand of the operator grasping a gripping handle 30 of the electric pruning shears 10. The cut release control, for example a trigger control as shown in FIG. 2, is positioned in proximity of the gripping handle 30.

(9) The gripping handle 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 the operator's hand comes necessarily into contact with when the safe tool is seized by its handle. They are electrically insulated from each other and are electrically insulated from the cutting element 12. The gripping handle 30 may be made of insulating plastic material for this purpose.

(10) An emergency stop device 40 of the electric motor 20 of the pruning shears is governed by two electric circuits 42, 44. The electric circuits 42, 44 include components of the pruning shears but may also include parts of a human body using the pruning shears 10.

(11) The first electric circuit 42 includes, in series, a component forming a first electrical impedance 52, a first manual contact electrode 32 and the cutting element 12.

(12) The first electrical impedance 52, the value which is noted Zi may be a simple electronic component such as an electric resistance. Its value is preferably defined to be equal or greater than 100 kΩ.

(13) The first electric circuit 42 is normally an open circuit having consequently a quasi-infinite global impedance.

(14) 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.

(15) Whereas, when the operator also touches the cutting element 12, for example with a finger of his free hand, he closes the first electric circuit 42. In this case, the first electrical impedance 52 finds itself successively in series 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.

(16) One may note here that the circuit would also be closed if the operator indirectly touched the cutting element through the intermediary of another conductive item such as a branch, a wine stalk or even an iron trellis wire which would be in the process of being cut by the cutting element. This situation only adds a supplemental impedance in the circuit and does not put the invention in question. Only the case of a direct contact of the finger with the cutting element will be dealt with below.

(17) 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 and Z.sub.D respectively.

(18) Thus, when the first circuit is closed, a total impedance Z.sub.42 is such that:
Z.sub.42=Z.sub.1+Z.sub.M1+Z.sub.C+Z.sub.D

(19) The impedance of wiring and the cutting element are ignored here.

(20) A measuring device of an impedance characteristic of the first electric circuit is provided. In the example of implementation of FIG. 1, it includes a source of alternating current 56 in series with the first circuit 42, and a voltmeter 58 connected in parallel with the first impedance 52. The value of voltage measured by the voltmeter is, in this case, the impedance characteristic of the first circuit, in the sense of the invention.

(21) A second electric circuit 44 includes the first electrical impedance 52, the first manual contact electrode 32, the second manual contact electrode 34 and a second electrical impedance 54. Just like the first electrical impedance, the second electrical impedance may be formed by an electronic component such as a simple resistance of a defined value, preferably above 100 kΩ. Its impedance value whether real or complex, is marked Z.sub.2.

(22) The second electric circuit is also an open circuit when the operator does not seize the gripping handle 30 of the electric pruning shears 10. The manual contact electrodes 32, 34 are in effect electrically insulated from each other.

(23) However, when the operator seizes the gripping handle 30, his hand does electrically connect the first and the second manual contact electrode 32, 34. A contact impedance 60 of the operator's hand with the first manual contact electrode and a contact impedance 61, of a value noted as Z.sub.M2 of the operator's hand with the second manual contact electrode thus are added in series in the second circuit.

(24) In this case, the impedance Z.sub.44 of the second electrical circuit 44 is such:
Z.sub.44=Z.sub.1+Z.sub.2+Z.sub.M1+Z.sub.M2

(25) Now, the values Z.sub.M1 and Z.sub.M2 are coming from contacts of the same kind and their variations are therefore similar. The same is the case for the value Z.sub.D in the first circuit and each circuit thus includes two values of contact impedance of the same kind. A comparison of Z.sub.42 and Z.sub.44 makes it possible to eliminate the variations of the contact impedances with the manual contact electrodes, and to measure the occurrence of a contact of the operator's finger with the cutting element by evaluating Z.sub.C and Z.sub.D opposite Z.sub.2 and Z.sub.M2.

(26) A measuring device is also provided to establish an impedance characteristic of the second electric circuit. In the example of implementation described here, this is the source of the alternating current 56 and the voltmeter 58 previously mentioned and also used in connection with the first electric circuit 42. The voltage measured at the terminals of the first electrical impedance 52 is a value of the characteristic of the second electric circuit 44.

(27) Since a single measuring device is provided to measure the impedance characteristics of the first and second electric circuit 42, 44, an alternate measure is provided.

(28) For this purpose, the second electrical circuit 44 includes a switch 70 to open or close the second electric circuit. The switch 70 may be of the electromechanical type or, preferably, an electronic transistor switch.

(29) The switch 70 is servo-driven by the cutting trigger control 28 so as to be closed in the absence of a cutting command and so as to open in case of a cutting command.

(30) In the absence of a cutting command, the risk of the operator being cut by the cutting element does not exist. The first circuit is open, supposing that the operator does not touch the cutting element. Switch 70 of the second circuit is then closed and enables a measurement of the characteristic of the second electric circuit 44. However, an electrical characteristic of singular value can be measured in the case where the operator does not simultaneously touch the two manual contact electrodes, and therefore when the circuit 44 is open in spite of the switch 70 being closed.

(31) When the operator activates the operation of the cutting element, for example, by reinforcement of a trigger connected to the cutting release control 28, the switch 70 of the second electric circuit 44 is open so as to enable a measurement of the impedance characteristic of the first electric circuit 42.

(32) Each time the impedance characteristic of the second electrical circuit 44 is measured, the value of this impedance characteristic, or a value proportional to the impedance characteristic, is updated and stored in a memory 72 until a next measurement. The measurement of the impedance characteristic of the second electric circuit 44 and the update of the memory 72 can be made at each closing of the switch 70 of the second electric circuit or at predetermined intervals when said switch remains closed.

(33) The stored value constitutes a threshold characteristic.

(34) The memory 72 is connected to an input of a comparator 74 so as to supply the threshold characteristic as reference value.

(35) A second input of the comparator 74 receives the impedance characteristic of the first electric circuit at the time of a cutting release, which is to say when the switch 70 of the second electric circuit 44 is open.

(36) The impedance characteristic of the first electrical circuit is thus compared to the threshold value.

(37) An output of the comparator 74 is connected to the electronic control card 26 of the motor 20. If the threshold value is deviated from, by a higher or lower value, according to the impedance characteristic selected, the comparator emits a signal in the direction of the electronic control card of the motor. The comparator can also emit such a signal in case of the presence of a single value in the memory 72, signifying a fault of simultaneously maintaining contact with the two manual contact electrodes and thereby obscuring the possibility of detecting a contact between the finger and the cutting element. The signal is used by the electronic card to trigger an emergency stop of the motor and the cutting operation. The emergency stop may include, for example, the opening of a circuit of electric power supply, or a servo-drive of the motor to block the movement of the cutting element.

(38) The electronic control card 26 of the motor is thus a part of the emergency stop device 40. The emergency stop device may also include a switch 27 for opening a circuit of an electric power supply of the motor.

(39) In a particularly simple implementation of the cutting tool, the first electric impedance 52 and the second electric impedance 54 may be set at a same value Z.sub.1=Z.sub.2 in the order of 100 kΩ or 200 kΩ. These values are clearly much higher than an estimated impedance of the human body and to an estimated impedance of contact of the finger with the cutting element.

(40) An impedance of the human body is estimated to be less than 10 kΩ. The same is the case for the impedance of the finger with the cutting element, which is estimated to be below 10 kΩ,

(41) and rather by an order of magnitude of 1 kΩ, especially after an emerging injury.

(42) In this example of implementation, the retained threshold characteristic Z.sub.threshold is directly the measured impedance of the second circuit, namely:
Z.sub.threshold=Z.sub.44=Z.sub.1+Z.sub.2+Z.sub.M1+Z.sub.M2

(43) When the first electronic circuit is open, its impedance Z.sub.42 is quasi infinite and Z.sub.42>Z.sub.threshold

(44) On the other hand, when the operator holding the handle simultaneously touches the cutting element, the impedance of the first electric circuit becomes Z.sub.44=Z.sub.1+Z.sub.M1+Z.sub.C+Z.sub.D

(45) The comparator 74 thus compares: Z.sub.1+Z.sub.M1+Z.sub.C+Z.sub.D and Z.sub.1+Z.sub.M1+Z.sub.M2+Z.sub.2.

(46) By eliminating Z.sub.1 and Z.sub.M1 in the two total impedances, this amounts to comparing the sum of the impedance of the operator's body and the contact impedance with the cutting element to the electrical impedance Z.sub.2 increased by a value Z.sub.M2.

(47) Thus, as Z.sub.2 is chosen to be greater than the estimated values of Z.sub.C and Z.sub.D, the sum of Z.sub.M2+Z.sub.2 is greater than the sum of Z.sub.C and Z.sub.D and one gets: Z.sub.42<Z.sub.threshold

(48) The threshold has been exceeded and the comparator delivers an emergency stop signal.

(49) 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.

(50) A monitoring circuit 78 of the potential of the cutting element is also provided. It is built around a voltmeter and is also linked to the electronic control card 26 of the electric motor 20 to cause an emergency stop when an electric potential of the cutting element becomes different from a set value. In the example of implementation shown, one verifies that the electric potential of the cutting element is at the reference potential of the tool.

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

(52) FIG. 2 illustrates the contact of the hand with the manual contact electrodes 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 and more precisely the middle finger, the ring finger and the pinky come into contact with the first manual contact electrode 32 of the first electrical circuit 42. 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.

(53) A part of the palm comes into contact with the second manual contact electrode 34 positioned here on an upper part of the gripping handle 30.

(54) The index finger of the hand is free to actuate a controller 28 which is here constituted by a control trigger for triggering the cutting element.

(55) The thumb is located in front of the section and bent around the handle of the tool for a good grip on the tool is not shown.

(56) The operator's free hand is shown in a position where one finger comes into contact with the mobile blade 14 of the cutting element 12. 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.

(57) The electronic control card 26 of the electric motor is connected to the electric motor 20 by three wires. This is a three-phase control. It can be used to control the rotation of the motor in a direction causing the closure 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.

(58) 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.