GB2121561A - Multi-axis industrial robot - Google Patents

Abstract

A multi-axis industrial robot (1) cancels the positioning error between the multi-axis robot and the parts (9,11) to be assembled and synchronizes its movement with that of the assembly line conveyor belt (8). The positioning error can be cancelled by freeing predetermined movable parts (3,4,5,) of the robot (1) for a particular period of time during a critical assembly operation of the robot to allow the freed movable parts to conform to the shape and movement of the part to be assembled while the remaining movable part(s) is driven under a predetermined active control procedure which performs the actual assembly operation. <IMAGE>

Description

been made free to move for a predetermined interval of time during the position-intensive assembly operation of the robot and the overall predetermined assembly operation is carried out by controlling the remaining movable part(s) in cooperation with the freed positioning part(s) and the movement of conveyor belt so as to synchronize the two.

In more detail, the position and/or movement of the robot and the workpiece are known well enough to recognize that at a certain point in the operation, the working points of the two will be within a known range of position error. At that point in the operation, the positioning parts of the robot affected by the position error are rendered free to move while other positioning parts remain under the active control of the robot. Guiding means is provided so that adjustment of one of the remaining activelycontrolled positioning parts causes the freelymovable positioning parts to be moved to eliminate the position error and/or to track further movement of the workpiece during the operation. The guiding means may take the form of a tapered surface centered over the working point of the workpiece.

This passive control of the freely-movable positioning parts is especially advantageous for moving workpieces, such as those on an assembly line, since once the working points of the robot and the workpiece are aligned by the guiding means, the force driving the workpiece can also be used to drive the robot under passive control.

Brief description of the drawings A more complete understanding of the present invention may be obtained from the following detailed description taken in conjunction with the drawings in which like reference numerals designate corresponding elements and in which: Figure 1 shows an arrangement of a three-axis robot using a cylindrical coordinate system and a conveyor belt in an assembly line applicable to the present invention, specifically illustrated in a state where a tool fixture of the robot is in its operation start position; Figure 2 is an enlarged cross-sectional view of a workpiece consisting of first and second parts shown in Figure 1; Figure 3 shows the projection of the range of movement of the working end of the three-axis robot in relation to the conveyor belt shown in Figure 1; Figure 4 is a cut-away view of a nut runner attached to the tool fixture shown in Figure 1;; Figures 5(A) and 5(B) form a circuit diagram of a first preferred embodiment of a control system for the robot according to the present invention; Figures 6(A), 6(B), and 6fC) form an operational flowchart of the control system in the first preferred embodiment shown in Figures 5(A) and 5(B); Figure 7 is a timing chart of the control unit shown in Figures 5(A) and 5(B); Figure 8 is a cut-away view of the nut runner and workpiece when the nut runner has achieved proper insertion into the workpiece;; Figures 9 and 10 show the extremes of movement of the robot in the operational position wherein rotary and extensile positioning parts of the robot SPECIFICATION Multi-axis industrial robot Background of the invention Field of the invention The present invention relates to a multi-axis industrial robot incorporated into an automatic parts assembly system.

Description of the prior art In most industrial robots which have recently been developed for in-line assembly applications, it is important that bolt fastening and similar partsassembly operations be repeated quickly and accurately without failure, since these operations comprise the major portion of some part assembly processes. In addition to this, it is important how well and easily such industrial robots can be adapted to such product assembly lines, especially in the case where an industrial robot is installed beside a conveyor belt which carries sub-assembly parts of a product, e.g., an automotive vehicle.

In other words, to quickly and accurately perform part-assembly operations such as fastening of bolts, it is important to improve the repeatability of such an industrial robot. An important factor in such a quick and accurate assembly operation is how simply positioning errors of the robot relative to the position oftheworkpiece can be eliminated. In addition, when the robot is installed on a conveyor assembly line, it is particularly important to synchronize the movement of the robot with that of the conveyor assembly line. It is very complex and difficult, however, to link the control system of the robot itself to the synchronization control system of the conveyor belt since a complicated dynamic analysis of both robot and conveyor assembly line would be required.

Summary of the invention With the above-mentioned problem in mind, it is an object of the present invention to provide a multi-axis industrial robot which can simply correct for positioning errors such as described hereinabove by means of appropriate control of the robot itself and means for easily synchronizing the movements of the robot and the conveyor assembly line with each other.

This can be achieved by providing a multi-axis industrial multi-axis robot having a plurality of positioning parts, at least two of which can freely be moved by external forces when drive power to corresponding electrical or hydraulic actuators (motors or hydraulic cylinders), which actuate re spectively the positioning parts, is interrupted, the robot comprising means for interrupting the drive power to each of the actuators so as to make the corresponding positioning part freely movable and means for operating the interrupting means for a predetermined interval of time during a specified stage of operation of the multi-axis robot.

Specifically, the positioning error is cancelled by an adjustment operation performed on the involved positioning part of the multi-axis robot, which has are moved by means of mechanical force transmitted through the nut runner; Figure 11 shows a second preferred embodiment using hydraulic cylinders for the positioning actuators; and Figure 12 is an explananatory cut-away view of the nut runner and third and fourth parts, the fourth part being fixed to a stationary bench.

Detailed description of the preferred embodiments Reference will be made to the drawings in order to facilitate understanding of the present invention.

Figure 1 shows a cooperative system of a threeaxis robot using a cylindrical coordinate system and a conveyor belt applied to the present invention.

In Figure 1, numeral 1 denotes the three-axis robot using the cylindrical coordinate system (hereinafter, simply referred to as a "robot").

The robot 1 shown in Figure 1 comprises a rotary positioning part 3 which rotates in the arrow-marked directions A with respect to a base 2, a vertical positioning part4which moves upward and downward in the arrow-marked directions B with respect to the rotary positioning part 3, an extensile positioning part 5 which extends and retracts in the arrowmarked directions C with respect to the vertical positioning part 4, and a tool fixture 7, located at the free end of the extensile positioning part 5, to which a nut runner 6 to be described hereinafter is attached.

The rotary positioning part3 is driven in the arrow-marked directions A about its axis via a reduction gear having a very low gear ratio by means of a first motor M1, acting as an electrical actuator, installed within the base 2. The vertical positioning part4 is moved upward and downward in the arrow-marked directions B via another reduction gear and rotation-translation transducing mechanism by means of a second motor M2 located within the rotary positioning part 3.

Finally, the extensile positioning part 5 is driven in the arrow-marked directions C via another reduction gear having a very low gear ratio by means of a third motor M3 installed within the rotary positioning part 3 via another rotation-translation transducing mechanism (for example, a screw bearing).

The rotary and extensile positioning parts 3 and 5 in the robot 1 are designed so that they can freely be moved in response to external forces when the drive signals to the first and second motors M1 and M2 are interrupted to deenergize the corresponding motor.

It should be noted that robots having this freelymovable feature are well known. It should also be noted thatthefirst, second, and third motors M1, M2, and M3 which drive the corresponding positioning parts of the robot 1 are provided additionally with first, second, and third pulse generators PG1, PG2 and PG3 and first, second, and third tachogenerators TG1, TG2, and TG3 each for detecting the position and speed of the corresponding positioning part of the robot 1.

On the other hand, a chain-driven conveyor belt 8 carries at a constant speed in the arrow-marked direction D a first part 9 clamped in a predetermined position along the conveyor belt 8 and a second part 11 with a countersink 1 1a within which a stud bolt 10 screwed partway into the first part 9 stands vertically as shown in Figure 2.

A pair of photoelectric part passage detectors 12 and 13 are installed on opposite sides and just above the conveyor belt 8 so as to bridge same. When the edge of the first part 9 passes through a line shown by a broken line photoelectrically connecting a light-emitting section 12a and a light-receiving section 13a and blocks the light emitted by the lightemitting section 12a, the pair of photoelectrical part passage detectors 12 and 13 detects the passage of the first part 9.

The range of movement of the nut runner 6 provided by means of the rotary and extensile positioning parts 3 and 5 of the robot 1 is shown in hatching in Figure 3 with the position of the belt 8 illustrated in dot-and-dash lines.

The nut runner 6 will hereinafter be described with reference to Figure 4.

As shown in Figure 4, a head 6c is fixed to the lower end of a bit 6b which is rotationally driven by means of a motor installed within a cap portion 6a.

The head Sc is spline-coupled to a sleeve so as to slide along the inner casing 6e of the sleeve while being prevented from rotating within the inner casing, which is slidable and rotatable with respect to an outer casing 6d.

The inner casing 6e is biased toward the free, open end of the outer casing 6d by means of a spring 6f inserted between the inner casing 6e and head 6c. A nut 14 is held within a nut-retaining portion 69 at the free end of the inner casing 6e.

In this drawing, numeral 6h denotes a stopper which holds the bit Sc and the seat of spring 6f within a predetermined distance, and numeral 6i denotes a bore into which stud bolt 10 shown in Figure 2 projects while the nut 14 is being threaded onto the stud bolt 10.

In addition, the nut 14 is held within the nut retaining portion 69 by means of a spring or magnet provided within the nut retaining portion 6g. The robot 1 before each operation cycle picks up one of the nuts 14 provided within a nut box (not shown in the drawings) located adjacent to the robot 1.

It should also be noted that the nut runner 6 is provided with a well-known torque limit detector for detecting the completion of the operation of screwing the nut 14 onto the stud bolt 10.

Figures 5(A) and (5(B) are electrical circuit block diagrams showing one preferred embodiment according to the present invention.

In these drawings, a main control unit 15 comprising, by way of example, a microcomputer reads sequentially positioning instruction data for the robot 1 from a memory unit 16, computes move instructions for the rotary positioning part 3, vertical positioning part 4, and extensile positioning part 5 in the robot 1, respectively, on the basis of the corresponding positioning instruction data, and sends the move instructions to respective first, second, and third position control units 17, 18, and 19.

Each of the first, second, and third position control units 17,18, and 19 receive move instructions from the main control unit 15 and position data fed back from the first, second, and third pulse generators PG1 through PG3 and sends actuation commands to first, second, and third speed-control amplifiers 20, 21, and 22 according to the remaining positional error indicated by the position data from the first, second, and third pulse generators PG1, PG2, and PG3.

The first, second, and third speed-control amplifiers 20, 21, and 22, connected to the first, second, and third position control units 17, 18, and 19, output drive signals to the first and third motors M1 and M3 via normally-closed contacts NC11 and NC21 and to the second motor M2 directly according to the amount of deviation between the values of the corresponding actuation commands received from the first, second, and third position control units 17, 18, and 19 and speed data generated by the first, second, and third tachogenerators TG1, TG2, and TG3 via other normally-closed contacts NC12, NC22 or fed back directly. These normally-closed contacts NC11, NC21, NC12, and NC22 are open when their associated magnetic relay coils MC1 and MC2, to be described later, are energized.The first drive signal is sent to the first motor M1 via the first normally closed contact NC11. The second drive signal is sent to the second motor M2 directly. The third drive signal is sent to the third motor M3 via the second normally closed contact NC21.

The first motor M1 rotates to actuate the rotary positioning part 3 of the robot 1 to rotate in either of the directions A shown in Figure 1 in response to the first drive signal, the second motor M2 rotates to actuate the vertical positioning part 4 to move upward or downward as denoted by B of Figure 1 in response to the second drive signal, and the third motor M3 rotates to actuate the extensile positioning part 5 to extend or retract in either of the directions C in response to the third drive signal. The nut runner 6 in the tool fixture 7 located at the tip of the extensile positioning part 5 moves consequently on a predetermined trajectory determined by the preprogrammed position instruction data in the memory unit.

Furthermore, the main control unit 15 reads another move instruction from the memory unit 16 which the vertical positioning part 4 carries out at a timing to be described later in order to fasten the nut 14 to the stud bolt joined to the first and second parts 9 and 11 shown in Figure 2. Thereafter, the control unit 15 sends the move instruction described above to the second position control unit 18 to move the vertical positioning part 4 downward by a predetermined distance. It will be shown that this causes the tip of nut runner 6 to shift and enter a countersink 11 a of the second part 11 shown in Figure 2.

It should be noted that the normally-closed contacts NC11 and NC21 open by means of magnet relay coils MC1, MC2 which are energized respectively when two transistor drive signals a and b outputted simultaneously from the main control unit 15 turn corresponding transistors Tr1 and Tr2 on.

When the normally-closed contacts NC11, NC21 are open, the first and third motors M1 and M3 are deenergized so that the rotary and extensile positioning parts 3 and 5 can move freely in response to external forces. The reason for the normally-closed contacts NC11, NC22, which open at the same time the other normally-closed contacts NC11, NC21 open, is to prevent the speed detection data produced by the first and third tachogenerators TG1, TG3 from reaching the first, and third speed control amplifiers 20 and 22.Similarly, other normally-closed contacts (not shown) are provided in the lines connecting each of the first and third pulse generators PG1, PG2 to the first and third position control units 17 and 19 to prevent the position detection data produced by the first and third pulse generators PG1, PG3 from reaching the first and third positional control units 17 and 19. It should be noted that the control unit 15 always receives the position data produced by the first, second, and third pulse generators PG1, PG2, and PG3. The control unit 15, therefore, always monitor the respective present positions of the rotary positioning part 3, vertical positioning part 4, and extensile positioning part 5 on the basis of these position data.

In addition, the main control unit 15 also controls the normally-closed contacts NC11, NC12, No21, and NC22 so that these contacts are open for a particular period of time associated with the bolt-and-nut fastening operation of the robot 1. This will be described in more detail later. The control unit 15 receives a passage signal c which turns to a high "H" level only when the first part 9 passes through a beam of light passing from the light-emitting part 12a to the light-receiving part 13a of the pair of photoelectric part passage detectors 12, 13.

In more detail, the pair of photoelectric part passage detectors 12, 13 comprises a photo coupler based on the light-emitting part 12a and the lightreceiving part 13a. The light-emitting part 12a may comprise a photo diode. The light-receiving part 13a may comprise a photo transistor. As long as the light from the photo diode is received by the photo transistor, the photo transistor 13a remains on so that a transistor Tr3 connected to the photo transistor 13a remains on. Therefore, the input and output voltage levels of a buffer 23 grounded by way of transistor Tr3 will be low "L" level. When the light from the photo diode described hereinabove is interrupted by the first part 9, the photo transistor turns off and the transistor Tr3, in turn, turns off. At this time, the output voltage level of the buffer 23, i.e., the passage signal c, changes to a high "H" level.

The control unit 15 sends a signal d to a flip-flop circuit 24 (hereinafter referred to simply as F/F) at the critical timing to be described later to set the F/F 24.

When the F/F 24 is set, the motor Mn is energized to rotate in order to rotate and depress the bit 6c, thereby threading nut 14 onto stud bolt 10. The motor Mn stops when the F/F 24 is reset in response to a completion signal e from the torque limit detector TQ indicative of the completion of the bolt-and-nut fastening operation. The completion signal e is also inputted to the control unit 15 for use in other control processes later.

The detailed operation of the main control unit 15 will be described hereinafter with reference to the flowchart of Figures 6(A), 6(B), and 6(C) and the timing chart of Figure 7.

In a first step SP1, the main control unit 15 checks to see if the passage signal c is turned to a high "H" level indicating that the first part 11 is passing through the pair of photoelectric part passage detectors 12 and 13. When the first part 11 passes therethrough at time tq shown in (a) of Figure 7, the control unit 15 receives the active ("H" level) passage signal c. The control unit 15 starts to read a sequence of positioning instruction data from the memory unit 16 shown in Figures 5(A) required to move the free end of the nut runner 6 from its present position to an operation start position OS shown in Figure 1 at the time t1 when the passage signal c of the high level "H" is received, in a step SP3. The control unit 15 searches for a move instruction for each positioning part of the robot 1.

The result is, in turn, sent sequentially to the first, second, and third position control units 17, 18, and 19.

Consequently, the positioning parts of the robot 1 are moved so that the tip of the nut runner 6 moves from its origin to the operation start position OS (referto (b) of Figure 7). It should be noted that the operation start position OS is defined by a predetermined position above a vertical line which passes through the center of the countersink 11 a of the second part 11 at a height slightly above the countersink 11 a.

The control unit 15, almost simultaneously with time t, actuates an internal timer at a step SP2 as shown in (c) of Figure 7 to measure the interval of time Tx required for the countersink 1 lea of the second part 11 to arrive at the operation start position OS described above which is constant due to the constant speed of the conveyor belt 8. In a subsequent step SP4, the control unit 15 checks to see if the free end of nut runner 6 has arrived at the operation start position OS within the measured time Tx.If the free end of nut runner 6 has not yet arrived, the control unit 15 stops the entire robot 1 due to malfunction in a step SPs. It should be noted that the arrival of the free end of nut runner 6 at the operation start position OS is recognized on the basis of each position data from the first, second, and third pulse generators PG1, PG2, and PG3.

The main control unit 15, in the subsequent step SP6, reads a move instruction from the memory 16 to thread the nut 14 onto the stud bolt 10 immediately upon completion of the measurement of the interval of time Tx at a second time t2 in (c) of Figure 7 and sends it to the second position control unit 18.

Consequently, the associated vertical positioning part 4 is moved downward by a predetermined distance from the operation start position OS so that the free end of the nut runner 6 is inserted all the way through the countersink 1 1a of the second part 11 (Refer to (d) of Figure 7). Simultaneously in a step SP6 the control unit 15 sends two transistor drive signals a and b as shown in (e) of Figure 7 at the second time t2 to the transistors Tr and Tr2 to energize the magnet coils MC1 and MC2. Consequently, the normally closed contacts NC11, NCr2, No21, and NC22 are opened.

Immediately before the free end of the nut runner 6 is completely inserted into the countersink 1 1a of the second part 11, the rotary positioning part 3 and extensiie positioning part 5 are rendered free to move in response to external forces. Therefore, if the position of the nut runner 6 with respect to the countersink 11 a deviates from the predetermined operation start position OS as shown by a dot-anddashed lines of Figure 2, the nut runner 6 can still be inserted into the countersink 11 a by following the taper of the countersink 11 a and thereafter can follow the movement of the second part 11 on the conveyor belt 8 without any active electronic control.

The interval oftime Tx described hereinabove includes the interval of time during which the nut runner 6 is lowered from the operation start position OS into and through the countersink 1 1a.

After recognizing in a step SP8 that the tip (free end) of the nut runner 6 has reached the position shown in Figure 8 on the basis of the position data received from the second pulse generator PF2, the control unit 15 sends a motor drive signal d to the F/F 24 shown in Figure 5(A) in a step SPg so that the nut runner 6 is actuated to screw the nut 14 onto the stud bolt 10 shown in Figure 2 (Referto (g) of Figure 7).

Next, the routine goes to a step SP,0. in this step, the control unit 15 checks to see if the above fastening operation has been completed. If a completion signal e is received from the torque limit detector at time t4 in (h) of Figure 7, the F/F 24 shown in Figure 5(A) is reset at time 4 in (h) of Figure 7 so that the nut runner 6 is stopped.

Thereafter, the main control unit 15 in a step SP12 reads a move instruction from the memory 16 to remove the nut runner 6 from the countersink 11 a following the threading operation, and sends itto the second position control unit 18.

The second position control unit 18, in turn, operates to lift the vertical positioning part 4 by the predetermined distance as described in the step SP12. Consequently, the free end of the nut runner 6 is separated from the countersink 11 a of the second part 11 (refer to (d) of Figure 7).

Figures 9 and 10 depict the movement of the robot 1 and first and second parts 9 and 11 along the conveyor belt 8 through which the rotary and extensile positioning parts 3 and 5 passively follow the movement of the second part 11 while the free end of the nut runner 6 is inserted into the countersink 1 1a of the second part 11.

In the subsequent step SP13, the control unit 15 checks to see if the free end of the nut runner 6 has been removed from the countersink 11 a.

After the step SP13, the control unit 15 turns off the transistor drive signals a and b in a step SP14 and reads sequentially the positioning instruction data from the memory unit 16 required to return the free end of the nut runner 6 to its origin in a step SP15 and determines a move instruction of each positioning part of the robot 1. The result is outputted sequentially to the first, second, and third position control units 17, 18, and 19. It should be noted that the normally-closed contacts NC11, NC21, NC12, and NC22 are closed again when the transistor drive signals a and b are turned off at the step SP14.

In the final step SPa5, the control unit 15 returns the free end of nut runner 6 to its origin. Each positioning part of the robot 1 is accordingly actuated and the free end of the nut runner 6 is returned to its origin and stops as described in a step 15 to wait until the start of the subsequent bolt-and-nut fastening operation (refer to (b) of Figure 7).

In this way, the second part 11 has been fixed by means of the stud bolt 10 and nut 14 to the first part 9 carried by the conveyor belt 8, thus completing one fastening operation.

The above-described embodiment concerns a motor-driven multi-axis robot. In the case of a hydraulic cylinder-driven robot using hydraulic actuators, the structure and function can be substantially the same.

In this case, solenoid operated valves are used to communicate the right and left chambers of each hydraulic cylinder at the same timing as the normally closed contacts are opened as described inbefore.

Figure 11 shows an example of a hydraulic circuit which actuates hydraulically the three-axis robot shown in Figure 1.

In this drawing, hydraulic cylinders 26 and 27 serve as actuators which drive respectively the rotary positioning part 3 and the extensile positioning part 5. The solenoid operated valves 28, 29 serve to free the associated cylinder rod by connecting the right and left chambers of the hydraulic cylinders 26, 27, respectively.

It should be noted that the linear movement of the cylinder rod in the hydraulic cylinder 26 is converted to rotational movement by means of a translationrotation converting mechanism (e.g. screw bearing).

The rotary positioning part 3 is thereby driven.

In this drawing, a hydraulic cylinder 25 is used to actuate the vertical positioning part 4 of the robot 1.

Numeral 30 denotes a check valve. Numerals 31,32, and 33 denote servo valves, numeral 34 denotes another solenoid operated valve for switching between positioning-intensive operation and repetitive operation of the robot 1, numeral 35 denotes a throttling valve for the positioning-intensive operation, numeral 36 denotes a check valve, numeral 37 denotes a relief valve for controlling hydraulic pressure, numeral 38 denotes a cooler, numeral 40 denotes an oil pump, numeral 41 denotes an accumulator, numeral 42 denotes a second relief valve for venting accumulated pressure within the accumulator 41, numeral 43 denotes a valve leading to a pressure gage 44, and numeral 45 denotes a pressure switch. The robot 1 is activated when the pressure switch 45 is turned on.

In the above-described example, such a robot is applied to the bolt-and-nut fastening operation to join the two parts carried by the conveyor belt 8. The robot can be applied equally well to a bolt-and-nut fastening operation for two parts, one part being fixed on a stationary bench.

In more detail, as shown in Figure 12, if the nut runner 6 deviates from its normal position with respect to a countersink 49a of a fourth part 49 laid on top of a third part 48 fixed to the bench 47, the nut runner 6 is moved directly downward so that the free end of the nut runner 6 is inserted into the countersink 49a along the taper thereof with the rotary and extensile positioning parts 3 and 5 being made freely movable immediately before the nut runner 6 is inserted into the countersink 49a.

In these cases, there is no need to equip the robot with a special attachment to compensate for insuffi cient accuracy in a previously programmed (play back type) rotot. Thus, the structure of the robot is simple.

It should be noted that the free movement of the rotary and extensile positioning parts 3 and 5 is returned to the regular movement under the normal active control when the free end of the nut runner 6 is removed from the countersink 49a upon completion of the bolt-and-nut fastening operation.

Although the above-described embodiments describe the bolt-and-nut fastening operation in the case where the nut runner 6 is attached to the robot 1, such a bolt-and-nut fastening operation may also be performed by means of an automatic screwdriver provided in place of the nut runner 6.

Furthermore, the above-described embodiments describe the three-axis robot using the cylindrical coodinate system to which the present invention is applied. However, the present invention can be applied equally well to two-axis or other multi-axis robots, a multi-articulated robot having a mechanical hand corresponding to the vertical positioning part described above, a robot using an orthogonal coordinate system, and so on.

As described hereinbefore, the robot acccording to the present invention cannot only cancel simply the positioning error of the robot between the position set in the robot's programming during development and the position under working conditions but also can synchronize the movement of the robot with that of the conveyor belt with a simple passive control method. Consequently, such a robot can be installed practically in such a parts assembly line.

Claims (14)

1. A multi-axis industrial robot, comprising: (a) a plurality of positioning parts each capable of moving in predetermined directions in response to corresponding actuation commands and at least two of which are capable of moving freely in predetermined directions in response to external forces in the absence of the corresponding actuation command; (b) means for selectively interrupting said actuation commands to make said positioning parts free to move; and (c) means for operating said interrupting means for a predetermined period of time within a predetermined stage of operation of the robot in which external forces are known to be available to move said parts in a desired manner.

2. A multi-axis industrial robot as set forth in claim 1, wherein said positioning parts include a vertically positioning part which moves upward and downward and at least two other positioning parts capable of cooperating to position a working point of the robot to an arbitrary position in a plane orthogonal to said vertical direction.

3. A multi-axis industrial robot as set forth in claim 2, wherein said interrupting means selectively interrupts the actuation commands to said at least two other positioning parts while allowing transmission of a corresponding actuation command to said vertically positioning part.

4. A multi-axis industrial robot as set forth in claim 3, wherein the working point of the robot is provided with a fastening tool and said operating means operates said interrupting means for the period of time from immediately before said fastening tool is inserted into a tapered working point of a workpiece carried by an conveyor belt assembly line and at least until said tool is removed from the working point of the workpiece upon completion of the fastening operation.

5. A multi-axis industrial robot as set forth in claim 3, wherein the working point of the robot is provided with a fastening tool and said operating means operates said interrupting means for the period of time from immediately before said fastening tool is inserted into a tapered working point of a workpiece fixed to a stationary base and at least until the fastening operation of the workpiece by means of said tool has been completed.

6. A multi-axis industrial robot for a part assembly operation, comprising: (a) a plurality of positioning parts and one tool fixture having a tool at a free end thereof, the positioning parts including at least two positioning parts capable of cooperatively moving said tool through a predetermined area and being capable of being moved freely within the predetermined area by means of external forces; (b) a plurality of actuators, each associated with one of the positioning parts for actuating the associ ated positioning part to move in a specified direction in response to a drive signal inputted thereto; (c) means for operatively interrupting the drive signals inputted to said actuators associated with said freely-movable positioning parts; and (d) means for operating said interrupting means for a predetermined interval of time during a predetermined portion of the part assembly operation of the robot, whereby said at least two positioning parts which are made free conform to the shape and movement of the part to be assembled.

7. A multi-axis robot as set forth in claim 6, wherein said plurality of positioning parts include a vertical positioning part which moves perpendicular to said predetermined area and said vertical positioning part moves so that said tool provided at the tip of said tool fixture performs the part assembly operation in response to a drive signal inputted thereto during said predetermined period of time.

8. A multi-axis robot as set forth in claim 7, wherein the part to be assembled has a tapered countersink through the axis of which a stud bolt projects vertically, said tool is a nut runner, at the tip of which a nut is held and wherein said predeter mined period of time during which said operating means operates said interrupting means is from immediately before said tool is inserted into said countersink of the part until immediately after said tool is removed from said countersink upon completion of the bolt-and-nut fastening operation.

9. A multi-axis robot as set forth in claim 8, wherein the parts to be assembled are conveyed through the vertical projection of said predetermined area by a conveyor belt, the conveying direction being parallel to the plane of said predetermined area.

10. A multi-axis robot as set forth in claim 8, wherein the parts to be assembled are stationarily located on a horizontal surface and said predetermined area is parallel to the horizontal plane.

11. A multi-axis robot as set forth in claim 6, wherein said actuators are motors and said interrupting means are normally closed contacts which simultaneously open so as to interrupt the drive signals to be inputted to said motors associated with said at least two freely-movable positioning parts.

12. A multi-axis robot as set forth in claim 11, wherein said operating means are electromagnetic relay coils associated with said normally closed contacts.

13. A multi-axis robot as set forth in claimS, wherein said actuators are hydraulic cylinders and said interrupting means are solenoid-operated valves which can connect right and left chambers of said hydraulic cylinders associated with said at least two freely-movable positioning parts.

14. An automatic assembly system comprising a multi-axis robot and a workpiece, the robot having at least one positioning member capable of both active position control by way of the force of the robot and passive position control by way of forces external to the robot, means for switching between the active and passive position control modes, and an assembly tool capable of acting on the workpiece to assemble same and the position of which is determined by said positioning member and at least one other positioning member, the workpiece having a working point at which said assembly tool is effective to assemble the workpiece and means for guiding said assembly tool from any position within a predetermined area to said working point, said switching means switching said positioning member to passive position control when said assembly tool is within said predetermined area, whereby any positioning error between said assembly tool and said working point small enough to leave said tool within said predetermined area will be corrected by said guiding means.