Method of detecting failure to tighten screws against works and device therefor

Abstract: A method of detecting failure to tighten screws against works (3) includes steps of detecting every screw tightening operation against the work (3) after start of a screw tightening step, counting the number of times of the screw tightening operations detected in the previous step, detecting the completion of the screw tightening step, and comparing a counted value at the time of the completion of the screw tightening step with a predetermined value, thereby informing of a result of comparison. A device for carrying out the method includes a current relay (17) responsive to a large load current during the tightening operation of an electric screwdriver (11) in a screw tightening step, a presettable counter (12) for counting the number of operations of the current relay (17), photoelectric switches (4, 5) detecting completion of the screw tightening step for the work (3), and an alarming device (23).

Description: This invention relates to a method of detecting failure to tighten screws against works and a device therefor, suitable for preventing occurrence of failure to tighten the screws in working step for tightening the screws against the works. DE-A-2930430 discloses a method and apparatus for monitoring the screwing home of a threaded element into a work by a power tool. In this apparatus each screw tightening revolution is detected after the start of a screw tightening step and the number of screw tightening revolutions are counted. The counted value at completion of screw tightening is compared with a predetermined value. In an assembly line for electrical equipments, for example, various parts are mounted on an electrical equipment body as work by means of screws in a plurality of working steps. Generally, a worker in charge for each step tightens the previously allotted number of screws against each electrical equipment body sequentially conveyed along the assembly line with a tool such as an electric screwdriver or pneumatic screwdriver. Whether or not the screws are correctly tightened against each work depends upon the degree of skillfulness and carefulness of the worker. Accordingly, it is inevitable that the failure to tighten the screws occurs at a certain rate. Particularly, various types of one electrical equipment have recently been produced in a small number in order that a diversity of consumers' needs can be met. In such circumstances, the number of screws to be used differs from one type of the electrical equipment to another and positions where the screws are tightened are changed with design changes even in one type. Accordingly, contents of work in the screw tightening step are also changed at relatively short intervals. Consequently, the workers cannot sometimes cope with the changes in the work contents, which has caused frequent occurrence of failure to tighten the screws. Conventionally, the products are checked in a final inspection at the assembly line. However, the products are visually inspected, which inevitably causes oversight. Therefore, an object of the present invention is to provide a device for detecting a failure in tightening screws against a work. According to one aspect of the present invention, there is provided a device for detecting a failure in tightening screws against a work having a plurality of portions into which the screws are tightened, the device being characterized by: a) means for generating first and second signals, respectively, at the times of entry of the work to and departure of the work from, respectively, a screw tightening station; b) detection means for generating an operation detection signal every time a load current supplied to an electrically driven screw tightening tool with which a screw is tightened into each portion of the work exceeds a predetermined value; c) means for setting a predetermined value representative of the number of the portions of the work into which the screws are to be tightened; d) counter means for automatically counting the operation detection signals sequentially generated by the detection means for a period between the generation of the first signal and the generation of the second signal; e) comparison means for electronically comparing a counted value obtained by the counter means with the predetermined value and producing a comparison result signal; and f) means supplied with the comparison result signal for informing the operator as to whether or not the counted value is smaller than the predetermined value. According to another aspect of the present invention, there is provided a device for detecting a failure in tightening screws against a work having a plurality of portions into which the screws are tightened, the device being characterized by: a) means for generating first and second signals, respectively, at the times of entry of the work to and departure of the work from, respectively, a screw tightening station; b) detection mean comprising a switch for generating an operation detection signal in response to operation of a manually operated member of a pneumatically driven screw tightening tool with which a screw is tightened into each portion of the work, the manually operated member allowing and disallowing compressed air to be supplied thereto; c) means for setting a predetermined value representative of the number of the portions of the work into which the screws are to be tightened; d) counter means for automatically counting the operation detection signals sequentially generated by the detection means for a period between the generation of the first signal and the generation of the second signal; e) comparison means for electronically comparing a counted value obtained by the counter means with the predetermined value and producing a comparison result signal; and f) means supplied with the comparison result signal for informing the operator as to whether or not the counted value is smaller than the predetermined value. Desirably, the first and second signal generating means comprise first and second position detecting devices disposed on a line along with the works against which the screws are tightened are conveyed, with a predetermined distance therebetween in the direction in which the works are conveyed along the line and the first and second position detecting devices detect the work passing the respective first and second positions on the line, thereby generating the respective first and second signals. Alternatively, the first and second signal generating means comprise a support on which the work is placed and a switch element provided on the support for generating the first signal in response to the placement of the work on the support and the second signal in response to the removal of the work from the support. In another alternative, the first and second signal generating means comprise a holding member for holding the screw tightening tool at a non-operating position and a switch provided on the holding member for generating the second signal when the screw tightening tool is held by the holding member and the first signal when the screw tightening tool is removed from the holding member. The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is an electrical circuit diagram employed in a first embodiment of the invention; FIG. 2 is a perspective view of a detecting device in accordance with the first embodiment; FIG. 3(a) to 3(l) are time charts for explaining the operation of the detecting device; FIG. 4 is a view similar to FIG. 1 showing a second embodiment of the invention; FIG. 5 is a longitudinal section of the major part of the detecting device in accordance with the second embodiment; FIG. 6 is a perspective view of the major part of a third embodiment of the invention; FIG. 7 is a perspective view of the major part of a fourth embodiment of the invention; and FIG. 8 is a perspective view of the detecting device in accordance with a sixth embodiment of the invention. A first embodiment of the present invention will now be described with reference to FIGS. 1 to 3(a)-3(l) of the accompanying drawings. Referring to FIG. 2, an assembly line 1 for electrical equipments such as microwave ovens is provided with a belt conveyor 2 driven at a predetermined speed. Works such as electrical equipment bodies 3 are sequentially conveyed by belt conveyor 2. For the purpose of confirming a period of a screw tightening work by detecting the completion of a screw tightening step, a displacement detecting device is provided at one of sides of belt conveyor 2 for detecting the displacement of electrical equipment body 3. The displacement detecting device comprises first and second reflection type photoelectric switches 4 and 5 disposed with a predetermined distance therebetween in the direction in which electrical equipment bodies 3 are sequentially conveyed on belt conveyor 2. The distance between photoelectric switches 4 and 5 is determined to be shorter than the dimension of electrical equipment body 3 in the direction in which it is conveyed on belt conveyor 2. Each of photoelectric switches 4 and 5 is of the built-in contact type. Photoelectric switch 4 disposed at the upper side has a normally closed sensor contact 4a (see FIG. 1) which is opened when electrical equipment body 3 being conveyed on the belt conveyor 2 is detected. Photoelectric switch 5 has a normally open sensor contact 5a (see FIG. 1) which is closed when electrical equipment body 3 is detected. Cables 4b and 5b for drawing outputs from contacts 4a and 5a are connected to photoelectric switches 4 and 5 at one ends and to plugs 4c and 5c at the other ends, respectively. A detecting device body 6 is provided in the vicinity of belt conveyor 2 of assembly line 1. Detecting device body 6 is provided, at one side thereof, with jacks 4d and 5d to which plugs 4c and 5c are coupled. A door 6a is provided for closing the front opening of detecting device body 6. A twin plug socket 7, reset button 8 and counter unit 9 as counting means are mounted on the outer side of door 6a and an alarming buzzer 10 is mounted on the inner side thereof. A power supply plug 11a of an electric screwdriver 11 as a tool for tightening screws is inserted into one of the plug sockets of twin plug socket 7. Electric screwdriver 11 comprises a body portion 11b which is held by a worker so that the worker performs the screw tightening job with screwdriver 11. A switch knob 11c of a power supply switch (not shown) is mounted on the body portion 11b. Knob 11c is operated so that power supply switch is closed, thereby driving an electric motor (not shown) provided within body portion 11b. Since twin plug socket 7 is employed, two electric screw drivers having different tightening torques may be used together. Two reset switches 8a and 8b (see FIG. 1) are mounted on the inner side of door 6a of detecting device body 6. Reset switches 8a and 8b are combined with reset button 8 such that both switches are opened when reset button 8 is depressed. Counter unit 9 comprises a presettable counter 12 and a power supply section (both shown in FIG. 1). Presettable counter 12 comprises setting knobs 12a and 12b for setting a two digit preset value Ns and a display 12c each digit of which is formed from seven segments each consisting of one light-emitting diode. Display 12c is externally operated to selectively display the digital preset value Ns or a digitized value Nr counted by counter 12. Detecting device body 6 is supplied with electrical power from a AC power source through a power supply plug 14. Various control equipments such as a relay, current relay and timer are provided within alarming device body 6. The arrangement of a control circuit including these control equipments, counter unit 9 and so on will be described with reference to FIG. 1. A DC power (100 V, for example) is supplied through a pair of power supply lines 15 and 16 and plug 14. Plug socket 7 is connected to a current relay 17 as a current detector between power supply lines 15 and 16. Consequently, when plug 11a of electric screwdriver 11 is connected to plug socket 7, electric screwdriver 11 may be energized. Upon energization of electric screwdriver 11, a load current is caused to flow through current relay 17. Current relay 17 is provided for detecting a starting current as the load current flowing into the built-in motor of screwdriver 11. In the case where the motor of screwdriver 11 draws approximately 0.75 amps. at start-up, for example, current relay 17 is operated to close normally open relay switch 17a when a current of 0.5 amps. or more is drawn. Counter unit 9 is connected between power supply lines 15 and 16. Counter 12 of the counter unit is adapted to count up one step every time a voltage signal is stepped down after the voltage signal is supplied to a clock terminal CK. Counter 12 is adapted to initialize the counted value when the voltage signal is supplied to a reset terminal R. Counter 12 has a normally open counter switch 12d and a normally closed counter switch 12e. When the counted value Nr reaches the preset value Ns, switch 12d is closed and switch 12e opened. A first relay 18 is connected in series to a relay switch 17a of current relay 17 and a normally closed timer contact 19a of a timer 19 between power supply lines 15 and 16. First relay 18 has three normally open relay switches 18a, 18b and 18c. Relay switch 18a is connected in parallel with relay switch 17a of current relay 17. Relay switch 18b is connected between power supply line 15 and the clock terminal CK of counter 12. Timer 19 is connected in parallel with first relay 18. Timer 19 is adapted to open timer contact 19a thereof when energization thereof is continued for a predetermined period T (1.5 seconds, for example). Period T is set so as to be a little longer than a period necessary for tightening a screw with the screwdriver 11. A second relay 20 is connected in series to sensor contacts 4a and 5a between power supply lines 15 and 16. Second relay 20 has three normally open relay switches 20a, 20b and 20c. Relay switch 12a is connected between power supply line 15 and the reset terminal R of counter 12. A third relay 21 is connected in series to relay switch 18c of first relay 18, a normally closed relay switch 22a of a fourth relay 22 described later, and reset switch 8a between power supply lines 15 and 16. Relay 21 has three normally open relay switches 21a, 21b and 21c. Relay switch 21a is connected in parallel with relay switch 18c of first relay 18. Fourth relay 22 has a normally closed relay switch 22b as well as relay switch 22a. Fourth relay switch 22 is connected in series to counter switch 12d of counter 12, relay switches 20b, 21b and 22b between power supply lines 15 and 16. An alarm buzzer 10 is connected in series to counter switch 12e of counter 12, relay switches 20c and 21c and reset switch 8b between power supply lines 15 and 16, which series circuit constitutes an alarm circuit 23. Operation of the above-described arrangement will now be described with reference to FIG. 4 as well as FIGS. 1 and 2. In the case where four screws are to be tightened against each work with electric screwdriver 11 in the screw tightening step, the preset value Ns of counter 12 is set at "4." Display 12c may be switched so as to display the preset value Ns for confirmation thereof and need be switched again so as to display the counted value Nr after confirmation. When first reflection-type photoelectric switch 4 detects electrical equipment body 3 conveyed on the belt conveyor 2, sensor contact 4a of the photoelectric switch is opened, thereby detecting start of the screw tightening step. Subsequently, when electric screwdriver 11 is driven so that a first screw is tightened against a predetermined position of electrical equipment 3 (at time t1 in FIG. 3), a large starting current of approximately 0.7 amps. flows into built-in motor of screwdriver 11, thereby determining that the screw tightening operation has been performed. Upon detection of the starting current, current relay 17 causes relay switch 17a to be closed. Consequently, first relay 18 is energized with the result that relay switches 18a, 18b and 18c are closed (at time t2). In response to closure of relay switch 18a, first relay 18 is maintained in the self-holding state and timer 19 is continuously energized in response to the self-holding state of relay 18. Timer contact 19a is opened after timer 19 is energized for the predetermined period T, thereby releasing relay 18 from the self-holding state (at time t3). Consequently, since relay contact 18b is opened and then closed, that is, since the voltage applied to clock terminal CK of counter 12 is stepped down, counter 12 counts up one step at time t3 when relay switch 18b is opened. As described above, third relay 21 is energized to thereby close relay switches 21a, 21b and 21c at time t2 when relay switch 18c is closed. Third relay 21 is maintained in the self-holding state owing to closure of relay switch 21a. When the screw is tightened against electrical equipment body 3 by electric screwdriver 11, the load current flowing into the motor of screwdriver 11 varies as shown in FIG. 3(c). More specifically, upon start of drive of screwdriver 11, the relatively large starting current of approximately 0.7 amps. temporally flows into the motor of screwdriver 11. Thereafter, when the screw tightening is completed, a lock current of approximately 0.5 amps. is drawn. Accordingly, relay switch 17a of current relay 17 the operative current of which is set at 0.5 amps. could be closed when the lock current is drawn. As a result, counter 12 counts up at the occurrence of the lock current and there is the possibility that the counted value Nr does not correspond to the number of screws tightened against the work. However, first relay 18 is maintained in the self-holding state such that counter 12 counts up by one step, until the period T set in timer 19 elapses, the period T being set so as to be a little longer than the period necessary for tightening one screw with the screwdriver. Consequently, the counted value Nr corresponds to the number of screws tightened with screwdriver 11. Relay 18 is thus operated by timer 19 every time the screw tightening operation is executed. Counter 12 counts up by one step every time relay 18 is operated. When the counted value Nr of counter 12 reaches the preset value Ns or "4" or when the necessary number of screws are tightened, counter switch 12d is closed and counter switch 12e is opened (at time t4). On the other hand, electrical equipment body 3 is further conveyed on belt conveyor 2 and detected by second photoelectric switch 5. Sensor contact 5a of photoelectric switch 5 is closed to thereby detect the completion of the screw tightening step. Since the distance between photoelectric switches 4 and 5 is set so as to be shorter than the dimension of electrical equipment body 3 in the direction in which it is conveyed on belt conveyor 2, sensor contact 4a of first photoelectric switch 4 is still opened. When electrical equipment 3 is further conveyed on belt conveyor 2, sensor contact 4a of first photoelectric switch 4 is closed at time t5 corresponding to the time of completion of the screw tightening step. Then, both of sensor contacts 4a and 5a are closed and second relay 20 is energized, thereby closing relay switches 20a and 20b. Counter 12 is initialized when relay switch 20a is closed. Relay switch 21b of third relay 21 which is still in the self-holding state is in the on-state and counter switch 12d is closed. Consequently, fourth relay 22 is energized when relay switch 20b is closed. Then, since relay switches 22a and 22b are opened, third relay 21 is released from the self-holding state and second relay 22 is deenergized, thereby restoring the initial state. Since counter switch 12e is opened, alarm buzzer 10 is not energized even when relay switch 20c is closed, that is, alarm buzzer 10 is not driven when necessary four screws are tightened against electrical equipment 3, with the result that electrical equipment 3 is successively conveyed on belt conveyor 2. On the contrary, when all the screws are not tightened against electrical equipment 3 by the worker's mistake, counter switch 12d remains open and counter switch 12e remains closed. Accordingly, even when relay switch 20b is closed in response to energization of second relay 20 at the time of completion of the screw tightening step or when sensor contact 4a is re-closed, fourth relay 22 is not energized with the result that third relay 21 is maintained in the self-holding state. Consequently, when relay switch 20c of second relay 20 is closed, alarm buzzer 10 is energized through counter switch 12e, relay switches 20c and 21c and reset switch 8b, thereby informing of the occurrence of failure in the screw tightening. Upon alarming operation of buzzer 10, the worker can tighten one or more screws which have not tightened. Upon the alarming operation, reset button 8 is depressed to open reset switches 8a and 8b, whereby third relay 21 is released from the self-holding state and alarm buzzer 10 is deenergized, thereby restoring the initial state. According to the above-described embodiment, when all the screws are not tightened in the work step of tightening necessary number of screws against the electrical equipment 3, alarm buzzer 10 is immediately driven in the work step to thereby inform the worker of the occurrence of a failure. Consequently, such occurrence of the failure to tighten the screws against the electrical equipment 3 may safely be coped with. Furthermore, since the automatic execution of the alarming operation necessitates only the setting of the necessary number of screws as the preset value Ns in counter 12, the screw tightening efficiency is not reduced. FIGS. 4 and 5 illustrate a second embodiment of the invention. Although the screws are tightened against the electrical equipments 3 conveyed on belt conveyor 2 in the foregoing embodiment, the invention may be applied to a screw tightening step in which jigs are employed. Referring to FIG. 5, an electrical equipment panel 24 as a work is placed on a holding jig 25. A plurality of boss portions 24a are formed in panel 24. A necessary number of screws 27 is engaged with boss portions 24a so that, for example, a printed wiring board 26 is secured. A normally closed limit switch 28 serving as means for generating a screw tightening completion signal is mounted on jig 25 for the purpose of confirming the period necessary for the screw tightening step. Limit switch 28 is opened when panel 24 is placed on jig 25. Limit switch 28 is connected between power supply lines 4 and 5 through second relay 20 within alarming device body 6, as shown in FIG. 4. Provision of limit switch 28 eliminates first and second photoelectric switches 4 and 5 from alarming device body 6 in the foregoing embodiment. Connection of limit switch 28 is made by means of jack 4d of detecting device body 6 and jack 5d is short-circuited by, for example, a plug adapter (not shown). The predetermined number of screws 27 is set at counter 12 as the preset value Ns. In the condition that panel 24 is placed on jig 25, limit switch 28 is opened and accordingly, second relay 20 is deenergized. During deenergization of second relay 20, counter 12 counts up by one step every time one screw is tightened with electric screwdriver 11. When the necessary number of screws are tightened, counter switch 12d is closed and counter switch 12e is opened. Thereafter, when panel 24 is removed from jig 25 and limit switch 28 is closed, fourth relay 22 is energized through counter switch 12d and relay switches 20b, 21b and 22b. Consequently, alarm buzzer 10 is not driven. On the other hand, in the case that the necessary number of screws are not tightened against electrical equipment 3 by the worker's mistake or that counter contact 12d is opened and counter switch 12e is closed, fourth relay 22 is not energized even when panel 24 is removed from jig 25 and limit switch 28 is closed. Consequently, alarm buzzer 10 is energized to thereby inform the worker of the occurrence of failure to tighten screws. Although limit switch 28 is mounted on jig 25 for detecting completion of the screw tightening step, in the second embodiment, it may be mounted on a conventional reel disposed over the worker for holding electric screwdriver 11 at a standby position as shown in FIG. 6 as a third embodiment. Limit switch 28 is closed when a wire 29a suspending electric screwdriver 11 is taken up by reel 29 such that electric screwdriver 11 is lifted. Or, as illustrated in FIG. 7 as a fourth embodiment, limit switch 28 may be mounted on a stand 30 of electric screwdriver 11 disposed in the vicinity of the worker. Limit switch 28 is closed when electric screwdriver 11 is held on stand 30. The system shown in FIGS. 1 and 2 may be further modified as a fifth embodiment. In order to tighten the screws against one work with a plurality of electric screwdrivers in a single screw tightening step, n number of electric screwdrivers 11 are provided. Between power supply lines 4 and 5 are connected two or more screw-tightening detecting circuits each including plug socket 7 and current relay 17 connected in the same manner as shown in FIG. 1 and two or more circuits each including relay 18, timer 19 and relay switches 17a, 18a and 19a connected in the same manner as shown in FIG. 1. Two or more relay switches 18b of relays 18 of circuits are connected in parallel with one another between power supply line 15 and clock terminal CK of counter unit 19. In the fifth embodiment, failure to tighten the screws may be detected and informed when a plurality of workers are engaged in the screw tightening work against one work. Referring to FIG. 8 illustrating a sixth embodiment, a pneumatic screwdriver 32 is provided as a screw tightening tool so as to be driven by compressed air supplied through a hose 31 communicated to a compressed air source (not shown). Pneumatic screwdriver 32 has a lever 33 for closing and opening a valve which allows and disallows compressed air to flow to pneumatic screwdriver 32 and a first switch 34 connected in series to a lead wire 35. First switch 34 is interlocked with lever 33. When lever 33 is gripped by the worker for the screw tightening, the valve is opened, thereby driving pneumatic screwdriver and turning a first switch on by way of lever 33. A holder 36 is provided over assembly line 2 for holding pneumatic screwdriver 32 with completion of the screw tightening step. Holder 36 is provided with a second switch 37 responsive to the weight of pneumatic screwdriver 32. A circuit arrangement wherein first switch 34 is employed instead of switch 17a and second switch 37 instead of switches 4a and 5a in FIG. 1 performs the same operation of detecting failure to tighten screws as the device shown in FIG. 1. The foregoing disclosure and drawings are merely illustrative of the principles of the present invention and are not to be interpreted in a limiting sense. The only limitation is to be determined from the scope of the appended claims.

Claims: A device for detecting a failure in tightening screws against a work (3) having a plurality of portions into which the screws are tightened, the device being characterized by: a) means (4, 5) for generating first and second signals, respectively, at the times of entry of the work (3) to and departure of the work (3) from, respectively, a screw tightening station; b) detection means (17) for generating an operation detection signal every time a load current supplied to an electrically driven screw tightening tool (11) with which a screw is tightened into each portion of the work (3) exceeds a predetermined value; c) means (12) for setting a predetermined value representative of the number of the portions of the work (3) into which the screws are to be tightened; d) counter means (9) for automatically counting the operation detection signals sequentially generated by the detection means (17) for a period between the generation of the first signal and the generation of the second signal; e) comparison means (9, 12a) for electronically comparing a counted value obtained by the counter means (9) with the predetermined value and producing a comparison result signal; and f) means (6) supplied with the comparison result signal for informing the operator as to whether or not the counted value is smaller than the predetermined value. A device for detecting a failure in tightening screws against a work (3) having a plurality of portions into which the screws are tightened, the device being characterized by: a) means (4, 5) for generating first and second signals, respectively, at the times of entry of the work (3) to and departure of the work (3) from, respectively, a screw tightening station; b) detection means (17) comprising a switch (34) for generating an operation detection signal in response to operation of a manually operated member (33) of a pneumatically driven screw tightening tool (32) with which a screw is tightened into each portion of the work (3), the manually operated member (33) allowing and disallowing compressed air to be supplied thereto; c) means (12) for setting a predetermined value representative of the number of the portions of the work (3) into which the screws are to be tightened; d) counter means (9) for automatically counting the operation detection signals sequentially generated by the detection means (17) for a period between the generation of the first signal and the generation of the second signal; e) comparison means (9, 12a) for electronically comparing a counted value obtained by the counter means (9) with the predetermined value and producing a comparison result signal; and f) means (6) supplied with the comparison result signal for informing the operator as to whether or not the counted value is smaller than the predetermined value. A device according to claim 1 or 2, characterized in that the first and second signal generating means (4, 5) comprise first and second position detecting devices (4, 5) disposed on a line (1) along with the works (3) against which the screws are tightened are conveyed, with a predetermined distance therebetween in the direction in which the works (3) are conveyed along the line (1) and the first and second position detecting devices (4, 5) detect the work (3) passing the respective first and second positions on the line (1), thereby generating the respective first and second signals. A device according to claim 1 or 2, characterized in that the first and second signal generating means (4, 5) comprise a support (25) on which the work (3) is placed and a switch element (28) provided on the support (25) for generating the first signal in response to the placement of the work (3) on the support (25) and the second signal in response to the removal of the work (3) from the support (25). A device according to claim 1 or 2, characterized in that the first and second signal generating means (4, 5) comprise a holding member (36) for holding the screw tightening tool (32) at a non-operating position and a switch (37) provided on the holding member (36) for generating the second signal when the screw tightening tool (32) is held by the holding member (36) and the first signal when the screw tightening tool (32) is removed from the holding member (36).

Inventors: HIRABAYASHI JUNZOU NAGOYA WORK, HIRABAYASHI, JUNZOU, NAGOYA WORKS
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: B25B23
IPC codes: B25H1, B23P19, B25B23