Automatic Assembly Control Method And Device Therefor

Abstract

An automatic assembly control device employs a holding mechanism for holding a piston coupled to a positioning mechanism through elastic coupling means, so that the positioning mechanism may be controlled in response to signals representative of the displacement or deflection of the elastic coupling means. The piston is first placed upon the external part out of a range of variation in position of the center of a cylinder into which is fitted the piston, and then moved toward the center while the piston is pressed against the external part with a predetermined pressure. The completion of the above coarse positioning or adjustment is detected by detecting the partial insertion of the piston into the cylinder, and then the piston is gradually inserted into the cylinder while its position is corrected by the positioning mechanism.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and device for automatically and sequentially assemblying together an external part having a bore or hole (hereinafter referred to as a cylinder) and an internal part having a projection (hereinafter referred to as a piston) to be fitted in the bore or the hole and, more particularly to an automatic insertion control system for inserting the piston into the cylinder with a precision on the order of terms of microns.

2. Description of the Prior Art

In automatic assembly control systems there have been not devised and successfully demonstrated automatic insertion control devices for automatically inserting a piston into a cylinder, except those for providing pressure fits and shrinkage fits. The various fits are almost provided manually because the automation for inserting the piston into the cylinder, with a precision on the order of tens of microns has been almost impossible.

Moreover, there has been a strong demand for automatic assemblying systems for automatically assembling a piston and a cylinder in view of saving man hours, but such systems as described above cannot be provided only by improving the accuracy in positioning the piston, with respect to the cylinder. The correction of the position of the piston with respect to the cylinder and the control of the force with which the piston is inserted into the cylinder must be adjusted with an extremely higher degree of accuracy and sensitivity equal to that of a human being depending upon the condition of engagement of the piston with the cylinder.

SUMMARY OF THE INVENTION

One of the objects of the present invention is, therefore, to provide an automatic assembly control device for automatically assembling together a piston and a cylinder, to provide precision fits.

One embodiment of the present invention for accomplishing this object is characterized in that a holding mechanism for holding the piston or cylinder is coupled to a positioning mechanism through elastic or resilient coupling means, such as springs. In the instant invention, the term of a holding mechanism is used to refer to various mechanisms for releaseably holding a part for assembly such as artificial arms and fingers, electromagnets, screws, bolts and nuts, and so on. Furthermore, it will be understood that the term resilient or elastic coupling means is used to refer not only to means for elastically or resiliently coupling the holding mechanism with the positioning mechanism but to an arrangement in which play or clearance is provided between the two mechanisms. The holding mechanism is coupled through the resilient or elastic coupling means to the positioning mechanism in such a manner that when the resilient or elastic coupling means is not displaced or deflected, the axis of the piston held by the holding mechanism coincides with that of the positioning mechanism.

Briefy stated, according to the automatic assembly method of the present invention, first the piston is placed into contact with an external part of the cylinder in such a manner that the center axis of the piston is not coincident with that of the cylinder, and then the piston is moved toward the center of the cylinder while the piston is pressed against the cylinder under a predetermined pressure.

Another feature of the present invention resides in the fact that a control circuit is provided for controlling the positioning mechanism in response to the signals derived from means for detecting the displacement or deflection of the resilient or elastic coupling means in such a manner that the displacement or deflection may be eliminated, thereby correcting the displacement or deviation between the center axes of holding mechanism and the positioning mechanism.

According to the automatic assembly method of the present invention, the piston is placed upon the outer surface of the cylinder at a position out of the range of variation of the center thereof, In this first step, the holding mechanism may hold either the piston or cylinder. In a second step, the positioning mechanism is moved toward the center of the cylinder, while the piston is pressed against the outer surface of the cylinder under a predetermined pressure until the piston is partially inserted into the cylinder under the pressure of the elastic or resilient coupling means. In a third step, the movement toward the center of the cylinder of the positioning mechanism is terminated in response to the detection of the partial insertion of the piston into the cylinder, and the displacement or deflection of the elastic or resilient coupling means is detected, and thus the positioning mechanism may be corrected in position to eliminate the deflection or displacement.

The above and other objects, features and advantages of the present invention will become more apparent from the following description of some preferred embodiments thereof taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIGS. 1A - 1F are views used for the explanation of the underlying principle of the present invention;

FIGS. 2A - 2C are views used particularly for the explanation of a second step of the automatic assembly method in accordance with the present invention;

FIG. 3 is a schematic perspective view of a first embodiment of the present invention;

FIG. 4 is a circuit diagram of a control circuit therof;

FIGS. 5A - 5C are views used for the explanation of the characteristics of elastic or resilient coupling or supporting means therof;

FIG. 6 is a perspective view illustrating a variation thereof:

FIG. 7 is a diagram of a switching circuit included in the control circuit shown in FIG. 4;

FIGS. 8A, 8B, 8C, 9, 10 and 11 are views illustrating some variations of elastic or resilient coupling or supporting means of the embodiment shown in FIG. 3;

FIG. 12 is a graph used for the explanation of the variation shown in FIG. 11;

FIG. 13 is a circuit diagram of a variation of the control circuit shown in FIG. 4;

FIG. 14 is a graph used for the explanation thereof;

FIG. 15 is a diagram illustrating a differential amplifier included in the control circuit shown in FIG. 13; and

FIGS. 16 and 17 are views illustrating a second embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Underlying Principle

Referring to FIGS. 1A - 1F, the automatic assembly control method in accordance with the present invention will be described. In the instant embodiment, external and internal parts 1 and 2 to be assembled together are a cylinder having a through bore and a piston in the form of a rod respectively, but it will be understood that the present invention may be applied in order to fit an internal part having a projection having any cross sectional configuration into a mating hole or bore of an external part.

A part holding assembly generally indicated by 3 is coupled through elastic means 5, such as springs, to a part positioning assembly generally indicated by 4 which includes a drive shaft 4a which, in turn, is driven in the X-, Y- and Z-directions by step motors to be described hereinafter. In the instant embodiment the X-, Y- and Z-directions correspond to the X-, Y- and Z-coordinate axes, respectively, of a Cartesian coordinate system in which the XY plane is parallel with the upper surface 1c of the cylinder 1 and the Z coordinate axis is perpendicular to the XY plane.

According to the present invention, first the coarse adjustment of the position of the piston 2 relative to the cylinder 1 is effected, and then the piston 2 is fitted into the bore 1a of the cylinder 1 while the position of the piston 2 is controlled in the three steps to be described hereinafter.

1. FIRST STEP:

The cylinder 1 is assumed to be delivered to a predetermined position by a conveyor (not shown). Then the holding assembly 3 holds the cylinder 2 and places it at a predetermined position P.sub.A out of the range of variation in position of the center of the bore 1a as shown in FIGS. 1A and 1B. Assume that the center of the bore 1a is varied in position within a circle 1b (See FIG. 2A) when the cylinder 1 is delivered from the conveyor to a predetermined assembly position. Then the piston 2 is placed at the position P.sub.A on the left side of the circle 1b. The positioning of the piston 2 at a predetermined position P.sub.A according to the present invention has very significant features. First of all, the direction in which the piston 2 is to be moved so as to be fitted into the bore 1a is uniquely determined. That is, the piston 2 may be moved toward the center P.sub.B of the circle 1b or the range of variation and, hence, toward the center of the bore 1a.

The second feature resides in the fact that the step for detecting whether the piston 2 is inserted into the bore or not may be eliminated because when the piston 2 is placed at a predetermined position P.sub.A, it is clear that the piston 2 is not inserted into the bore 1a. Therefore, the overall sequential control system may be simplified.

2. SECOND STEP

In the second step, the piston 2 is moved toward the center P.sub.B of the bore 1a while some pressure is exerted to the piston 2 in the Z-direction. Since the part holding assembly 3 is coupled through elastic supporting means 5 to the positioning assembly 4, when the driving shaft 4a is moved in the X-direction, the cylinder 2 is inclined as shown in FIG. 1C. When the center of the piston 2 coincides with the center P.sub.B of the bore 1a, it is forced into the bore 1a under the force of elastic supporting means 5. Thus, the coarse positioning of the piston 2 relative to the bore 1a of the cylinder 1 is accomplished.

Since the leading edge of the inclined piston 2 is inserted into the bore 1a under the force of elastic supporting means 5, the coarse adjustment may be accomplished without fail even when the initial position of the piston 2 is slightly displaced from a predetermined position P.sub.A. The greater the force of elastic or resilient supporting means 5, the greater the tolerable deviation of the initial position of the piston 2 from a predetermined position P.sub.A becomes. This will be described in more detail hereinafter with reference to FIG. 2C. A hatched area 1c denotes a range of tolerable initial position P.sub.A of the piston 2 having a width in the Y-direction, on the order 10 - 15 mm. Therefore, the piston 2 may be moved along a linear path as shown in FIG. 2A so as to be fitted into the mating bore 1a. However, when the elasticity of resilience of supporting means 5 is not sufficient, it becomes difficult to insert the piston 2 into the bore 1a. To overcome this problem, it is preferable to reciprocate the piston 2 in the Y-direction with a relatively small constant amplitude as shown in FIG. 2B. The movement of the piston 2 accompanied with the reciprocation in the direction perpendicular to the direction of the movement toward the bore 1a is included in the method for moving the piston 2 toward the bore 1a according to the present invention.

3. THIRD STEP:

In the third step, the position of the drive shaft 4a is adjusted in the X- and Y-directions so as to coincide with the axis of the piston 2. For this purpose, the deviation of resilient supporting means from the normal position is detected and fed back to a control circuit included in the positioning assembly 4 so that the deviation becomes zero. The pressure is applied to the piston 2 in the Z-direction while the fine adjustment is being made in the manner described above so that the piston 2 may be gradually forced into the bore 1a. However, when the piston 2 is positioned as shown in FIG. 1F, it is locked so that even when the greater fitting force is applied to the piston 2, the latter will not be inserted into the bore 1a any longer. In the worst case the piston 2 and/or the cylinder 1 are broken. To overcome this problem, the reaction force of supporting means 5 is detected so that when the reaction force becomes in excess of a predetermined level, the fitting force is released and then the fine adjustment for making the axis of the piston 2 coincident with that of the bore 1a is effected again. The piston 2 may be completely fitted into the bore 1a when the above steps are repeated.

FIRST EMBODIMENT

Next, referring to FIG. 3, the first embodiment of a fit control device in accordance with the present invention will be described. The part positioning assembly 4 includes the step motors 4x, 4y and 4z for driving the drive shaft 4a in the X-, Y- and Z-directions. The step motors 4x, 4y and 4z are controlled in response to the control signals from drive circuits 6x, 6y and 6z, respectively. In the instant embodiment, the Cartesian coordinate system is employed, but it will be understood that any suitable coordinate system such as a cylindrical coordinate system may be used. The part holding assembly 3 includes arms 3b for holding a piston 2 and a support 3a for supporting the arms 3b, and is coupled to the positioning assembly 4 through crossed-leaf springs 5 upon which are bonded strain gauges 7. Four rods 9 extending from the four corners at the undersurface of a base plate 10 fixed to the drive shaft 4a are fitted into four through bores 3c in the support 3a, and coiled springs 8 are fitted over the rods 9 between the base plate 10 and the support plate 3a. Microswitches 11 mounted over the undersurface of the support plate 3a are so arranged as to be actuated by the free ends of the rods 9 extending through the through bores 3c.

The part holding assembly 3 is suspended by the four leaf springs 5 from the drive shaft 4a of the positioning assembly 4 when the piston 2 is out of contact with cylinder 1. When the piston 2 is gradually lowered as the Z-direction step motor 4zis driven, the leaf springs 5 as well as the coiled springs 8 are gradually contracted. In this case, the position of the part holding assembly 3 relative to the positioning assembly 4 as well as the deflections of the leaf springs 5 may be detected from the signals from the strain gauges 7. When the drive shaft 4a is further lowered by the Z-direction step motor 4z, the free ends of the rods 9 make contact with the limit switches 11, thereby actuating them. In response to the signals from the mircroswitches 11, the Z-direction step motor 4z is controlled so as to prevent excess pressure from being exerted to the cylinder 1 from the piston 2.

Next, referring to FIG. 4, a control circuit for controlling the part holding assembly 3 and the positioning assembly 4 will be described. The positive and negative pulse trains generated by a pulse generator 22 are applied to one input terminal of signal switching circuits 29x, 29y and 29z through a switching circuit 31 comprising switches SW-1 - SW-10. To the other input terminals 27x, 27y and 27z of the signal switching circuits 29x, 29y and 29z are applied the external control signals for controlling the movement of the piston to a predetermined supply position or to a predetermined initial position from which the control for fitting of the piston into the bore is started. The outputs of the signal switching circuits 29x, 29y and 29z are selectively derived in response to the signal g applied to a terminal 28. For example, when the "on" signal g is applied, the signal switching circuits 29x, 29y and 29z transmit signals from the switching circuit 31, whereas when the "off" signal g is applied, the signals applied to the other input terminals 27x, 27y and 27z are transmitted. The outputs of the signal switching circuits 29x, 29y and 29z are applied to driving circuits 6x, 6y and 6z of the step motors 4x, 4y and 4z, respectively. In response to the positive pulse train, the drive circuits 6x, 6y and 6z control the step motors 4x, 4y and 4z so as to effect rotation in the clockwise direction, whereas in response to the negative pulse train, they are rotated in the counterclockwise direction. The signals for controlling the part holding assembly 3 are applied to a terminal 27.sub.H and then to a drive circuit 6H for controlling a step motor 4H which, in turn, controls the actuation of the arms 3b of the part holding assembly 3. (The step motor 4H is not shown in FIG. 3).

The deflection in the X-direction is detected by the strain gauges 7a and 7c whereas the deflection in the Y-direction, by the strain gauges 7b and 7d. The signals derived from the strain gauges 7a and 7c are applied a comparator 20. When the relative position of the part holding assembly 3 with respect to the positioning assembly 4 in the X-direction is in excess of a predetermined value X.sub.1, the signal a (See FIG. 5A) is obtained, but when the deviation of the part holding assembly 3 is in excess of a predetermined value -X.sub.1 the signal b is obtained. The signals derived from the strain gauges 7b and 7d are applied to a comparator 21 so that when the deviation in the Y-direction is in excess of a predetermined value Y.sub.1, the signal c (See FIG. 5B) is obtained but when the deviation is in excess of a predetermined value -Y.sub.1, the signal d is obtained. The switches SW1 - SW4 in the switching circuit 31 are actuated in response to these signals a, b, c and d.

The deviation of the drive shaft 4a in the Z-direction of the positioning assembly 4 is detected by the microswitches 11 which outputs the signal e (See FIG. 5C) when the deviation is in excess of a predetermined value Z.sub.2, but delivers the signal f when the deviation is less than a predetermined value Z.sub.1. Flip-flops 23, 24 and 25 are actuated in response to the signals e and f, and in response to the outputs of the flip-flops 23, 24 and 25 the switches SW6 - SW10 are actuated.

Next, the mode of operation of the device with the above construction will be described. In response to the control signals applied to the external terminals 27x, 27y and 27z the step motors 4x, 4y, 4z and 4H are actuated so that the part holding assembly 3 holds a piston 2 and brings it to the position shown in FIG. 1A. Next, the "on" signal g is applied to the terminal 28 so that the signal switching circuits 29x, 29y and 29z are switched into the mode for transmitting therethrough the output of the switching circuit 31, and the flip-flop 24 is set so that its Q output becomes "1" to close the switch SW9. As a result, the positive pulse train is applied from the pulse generator 22 through the switch SW9 and the signal switching circuit 29z to the drive circuit 6z so that the step motor 4z is rotated so as to cause the drive shaft 4a to move down in the direction -Z-direction. When the drive shaft 4a is driven downwardly further even after the piston 2 has made into contact with the cylinder 1, the free ends of the rods 9 actuate the microswitches 11, so that the signal e is applied to the flip-flop 24. As a result, the flip-flop 24 is reset whereas the flip-flop 23 is set so that the switch SW9 is opened, and the switch SW5 closes its contact b in response to the Q output of the flip-flop 23. The input to the step motor 4z is therefore interrrupted so that the piston 2 is forced into contact with the cylinder 1 with a predetermined constant pressure.

When the flip-flop 24 is reset, its Q output becomes "1" so that the flip-flop 25 is set. In response to the Q output of the flip-flop 25, the switch SW6 is closed so that the positive pulse train from the pulse generator 22 is applied to the drive circuit 6x of the X-direction step motor 4x. The drive shaft 4a of the positioning assembly 4 is caused to move in the X-direction from the position shown in FIG. 1B to the position shown in FIG. 1C, and the lower end of the piston 2 is partly inserted into the bore 1a as shown in FIG. 1D, so that the leaf springs 5 and the coiled springs 8 are returned to their normal position. As a result, the signal f is derived from the microswitch 11 and applied to the flip-flop 25 so that the latter is reset. Both Q outputs of the flip-flops 24 and 25 become "1" so that, in response to the output of the AND gate 26, the switches SW7, SW8 and SW10 are closed. The Q output of the flip-flop 23 which is reset in response to the signal f becoming "0" so that the switch SW5 has its contact a.sub.1 closed. The actuation of the switches SW1 - SW4 is controlled in response to the output signals of the comparators 20 and 21 which vary in response to the deviation of the part holding assembly 3 from the positioning assembly 4. For example when the deviations in both X- and Y-directions of the part holding assembly 3 with respect to the positioning assembly 4 are in excess of predetermined values X1 and Y1, the comparators 20 and 21 deliver the signals a and c in response to which the switches SW2 and SW4 are closed. As a result, the negative pulse train is applied from the pulse generator 22 to the drive circuits 6x and 6y of the step motors 4x and 4y so that the positioning assembly 4 is driven in both -X and -Y directions until the deviations in the X- and Y-directions are within the ranges between X1 and -X1 and between Y1 and -Y1, respectively. The strain gauges are generally not sensitive to the deflections within these ranges. The pulse train is applied through the switches SW5 and SW10 to the step motor 4z until the microswitches 11 deliver the signal e, so that the piston 2 is normally pressed against the cylinder 1 under a constant force.

Since the constant pressure is applied to the piston 2 when the position of the piston 2 is adjusted by the positioning assembly 4, it is gradually inserted into the bore 12. When the piston 2 is inserted to a predetermined depth within the bore 1a, the "off" signal g is applied to the terminal so that the assembly is completed.

In the instant embodiment, the drive shaft 4a has been described as being driven in the X-, Y- and Z-directions by the step motors 4x, 4y and 4z, but the present invention is not limited to the arrangement. In a variation shown in FIG. 6, cylinder 1 is mounted on a pallet 12 which may be moved in the X- and Y-directions by the step motors 4x and 4y, respectively, and the movement in the Z-direction of a piston 2 is accomplished by the step motor 4z mounted in the positioning assembly 4. The X- and Y-direction step motors 4x and 4y are controlled in response to the signals from the strain gauges (not shown) bonded to the elastic or resilient supporting members 5 between the part holding assembly 3 and the positioning assembly 4.

The insertion or fitting of a piston 2 into a bore 1a may be faciliated by a switching circuit 31 of the type shown in FIG. 7. The switch SW7 is interlocked with the switch SW11 whereas the switch SW9 is interlocked with the switch SW12, and one of the terminals or contacts of the switches SW11 and SW12 are connected to one of the contacts of the switches SW7 and SW9, whereas the other terminals or contacts are connected to terminals 32 and 33, respectively. To these terminals 32 and 33 are applied the signals with different frequency which swing between the positive and negative polarities so that the step motors 4x and 4y are reversed in rotation through a very small angle at a predetermined frequency. Consequently, even when the signals from the strain gauges 7 are with the non-sensitive ranges as shown in FIGS. 5A and 5B the drive shaft 4a of the positioning assembly 4 may be swung with an amplitude equal to the width of the non-sensitive range so that the controlability of the drive shaft 4a may be always ensured. Furthermore, a gap may be provided between the strain gauges 7 and the part holding assembly 3 or the positioning assembly 4 so that they may be swung with an amplituide equal to the width of the non-sensitive range of the signal derived from the strain gauges 7. Then, a very small deviation of the piston from the bore due to the pressure exerted to the piston in the Z-direction may be automatically corrected, so that the fitting of the piston into the bore may be much facilitated.

Next, the coupling between the part holding assembly 3 and the positioning assembly 4 will be described in more detail. During the precision fitting, the pressure applied to the piston 2 must be sufficient to overcome the frictional forces encountered when the piston is fitted into the bore, but when the pressure is applied to the piston when the latter is not coincident with the bore, it sticks to the bore wall. As a result, it beomes impossible to further insert the piston into the bore. Therefore, it is necessary to detect the magnitude and direction of the force exerted to the piston held by the part holding assembly.

Referring to FIG. 8A illustrating one embodiment of resilient supporting means, a base 15, which is securely fixed to the drive shaft 4a, has a cylindrical rocking member disposed therein with balls 102 and securely connected to the support 3a of the part holding assembly 3. The leaf spring 5 secured to the support 3a with a detent 103 is brought into contact with a spacer 101 which, in turn, is fixed to the base 15.

In response to the external force exerted on the piston 2, the rocking member 17 may be resiliently displaced relative to the base 15. That is, the rocking member 17 is caused to move in the Z-direction and th X- and Y-directions perpendicular to the Z-direction. The leaf spring 5 is provided in order to return the rocking member 17 to its normal position when the external force is released and to provide a resilient displacement thereof. The size of the balls 102 is suitably proportioned so that a predetermined play may be provided in the X- and Y-directions.

In order to detect the magnitude and direction, detecting means may be interposed between the base 15 and the rocking member 17 or support 3a. In the instant embodiment in which the leaf spring is used, means for detecting the deflection thereof may be used.

A variation of the resilient supporting means 5 is shown in FIG. 8B. The drive shaft 4a of the positioning assembly 4 is securely fixed to the casing 15 having an opening 15a, and the arms 3b of the part holding assembly 3 are pivoted to a member 19 which, in turn, is fixed to a supporting plate 17 suspended from the upper wall of the casing 15 through springs 16. A winding 18a of a differential transformer 18 is mounted upon the supporting plate 17, and a core 18b of the differential transformer 18 is extended from the upper wall of the casing 15 and fitted into the winding 18a.

In response to the magnitude of the force acting in the Z-direction upon the piston 2, the winding 18b is displaced in the Z-direction, so that the signal representing this displacement may be derived from the differential transformer 18. In response to the output signal derived from the differential transformer 18, the Z-direction step motor is driven to continuously control the relative displacement of the piston 2 in the Z-direction. Of course, the force acting on the piston 12 in the Z-direction may be maintained constant. In addition to the arrangement for detecting the displacement in the Z-direction, means (not shown) are provided for detecting the displacement in the X- and Y-directions.

In another variation shown in FIG. 8C four means such as differential transformers are provided in order to detect the displacement in the X-, Y- and Z-directions. The displacement in the Z-direction may be detected from the overall analysis of the outputs of the four detecting means.

The resilient supporting means 5 of the type shown in FIGS. 8A, 8B and 8C are provided in order to continuously detect the displacement in the X-, Y- and Z-directions, thereby causing the axis of the piston to coincide with the axis of the bore and to exert to the piston 2 such force which will not cause sticking of the piston 2 to the bore wall. However, it is not necessary to continuously detect the displacement in the X-, Y- and Z-directions, as will be described hereinafter.

In a variation shown in FIG. 9 the displacement in the Z-direction is detected in two steps. That is, response to the displacement of the supporting member 17 in the Z-direction a first limit switch 42 is actuated by a projection 41 extended upwardly from the upper surface of the supporting member 17, and when the displacement is further increased a second limit switch 42b is also actuated by the projection 41. Since the supporting member 17 is coupled through the springs 16 to the casing 15, the displacement in the Z-direction is in proportion to the magnitude of the reaction force acting upon the piston 2. Therefore, when both limit switches 42a and 42b are off the piston 2 is inserted into the bore, but when both are on the piston 2 is moved away from the bore 1a. When the first switch 42a is on while the second switch 42b is off, the position of the piston 2 in the X- and Y-directions is adjusted. Thus the piston 2 may be inserted into the bore 1a without causing sticking. The system shown in FIG. 9, in which the displacement or the reaction in the Z-direction is detected by the on-off action of the limit switches, is advantageous in that the system is stable in operation and the level of the output signals is high as compared with the arrangements shown in FIGS. 8A and 8B. The arrangements shown in FIGS. 8A and 8B may be modified so that the "on" and "off" signals may be derived by establishing a suitable threshold level.

In the arrangement shown in FIG. 9, the reliable operation of the limit switches 42a and 42b cannot be ensured when the projection 41 is inclined due to the displacement of the supporting member 17 in the X- and Y-directions. To overcome this problem, there is provided an arrangement shown in FIG. 10 in which the reaction only in the Z-direction may be detected by the first and second limit switches 42a and 42b. When the arms 3b of the part holding assembly 3 are displaced in the X- and Y-directions, the supporting member 17a is displaced about a ball-and-socket bearing 43 so that the casings 15a and 15b are not inclined, but the casing 15a is displaced in the Z-direction when the reaction in the Z-direction is acting upon the piston 2, so that the limit switches 42a and 42b are sequentially closed. The arrangement shown in FIG. 10 is advantageous in that the displacement only in the Z-direction may be detected with a higher degree of accuracy.

Next, referring to FIG. 11, illustrating still another variation of the resilient supporting or coupling means 5, the supporting plate 17 of the part holding assembly 3 is coupled to the driving plate 10 of the positioning assembly 4 through coiled springs 45a and 45b fitted over bolts 44 extending from the driving plate 10 through thorough bores of the supporting plate 17, so that the spacing between the supporting plate 17 and the driving plate 10 is variable. Differential transformer type detecting means 46a and 46b are mounted in order to detect an angle of inclination of the piston 2. Between a pair of spring seats 47 and 48, there is loaded a coiled spring whose spring constant is smaller than that of the coiled springs 45a and 45b and whose free length is shorter than the spacing between the driving plate 10 and the supporting plate 17. The upper end of the coiled spring 50 is normally spaced apart from the driving plate 10. When the outputs of the detecting means 46a and 46b are not balanced as the pressure is exerted to the piston 2 in order to insert it into the bore 1a, the position of the drive shaft 4a is corrected or adjusted. When the supporting plate 17 is further raised, so that the coiled spring 50 is compressed, a pin 49 fixed to the upper spring seat 47 makes contact with a microswitch 51, thereby closing it. In other words, the sticking of the piston 2 to the bore wall is detected. Therefore the drive shaft 4a is raised so as to release the sticking, and then lowered again to exert the force to the piston 2.

FIG. 12 shows a graph illustrating the relation between the displacement of the supporting plate 17 with respect to the driving plate 10 and spring tension. When the relative displacement reaches a predetermined value D, then the compression of the coiled spring 50 is started, and the relative displacement must be further increase by an amount d before the microswitch 51 is closed. Because of this non-linear spring tension or constant, the deviation of the piston 2 may be detected with a relatively higher degree of accuracy in the positioning step, because the piston 2 is supported by the relatively weak springs 45a and 45b. In the step in which the piston 2 is forced into the bore 1a, a relatively greater spring tension is applied to the piston 2. When the piston 2 sticks to the bore wall, the sticking is detected so that the force in excess of a predetermined magnitude will not be exerted to the piston 2 in a manner substantially similar to that described hereinbefore.

According to the method of the present invention, the positioning of the piston 2 is accomplished in the second step, and then the fine adjustment is made in the third step so that the detection of the completion of the coarse positioning or the second step is required. The partial insertion of the piston 2 into the bore 1a may be detected in response to the reaction in the Z-direction encountered by the piston 2 in the manner described hereinbefore, but this detection is very difficult in practice because only a small portion of the piston 2 is inserted into the bore. Furthermore, there is a problem that when the fine positioning is started before the completion of the coarse positioning the piston 2 cannot be inserted into the bore 1a.

In order to overcome this problem in the control circuit shown in FIG. 4, when the reaction in the Z-direction is detected, that is when the microswitch 11 provides the output signal f, the flip-flops 23 and 25 are reset for a relatively short time so that pulses may be supplied to the X- and Y-direction drive circuits for a very small time interval, thereby accomplishing the positioning of the positioning assembly 4. When the piston 2 is partially inserted into the bore 1a, it is further inserted because of the above operation so that the signal f is derived again. In response to the signal f the flip-flops 23 and 24 are set and then reset for a predetermined short time, respectively, so that the fine positioning is continued. When the signal f is obtained in response to the noise even through the piston 2 is not inserted into the bore 1a, the fine positioning is made only one time and then the coarse adjustment or positioning is continued.

However, there is the problem that when the signal f is continuously derived for some reason, even when the piston 2 is partially inserted into the bore 1a, the coarse positioning is carried out to such an extent that the piston 2 is completely moved away from the bore 1a. To overcome this problem, when the difference in the output signals between the strain gauges 7a and 7c or between 7b and 7d is in excess of a predetermined value, that is, when an abnormal inclination of the piston 2 is detected, the flip-flops 23 and 25 are reset for a predetermined short time interval, so that the correction of the position of the positioning assembly 4 is effected in the manner described hereinbefore until the outputs of the strain gauges 7a - 7d become zero. Therefore, the correction is made before the piston 2 is completely moved away from the bore 1a, so that the piston 2 may be positively inserted into the bore 1a.

Next, referring to FIG. 13, a variation of a control circuit will be described. The outputs a', b', c' and d' of the strain gauges 7a - 7d, bonded on the springs 5, are applied to decision circuits 50 and 51, each comprising a differential amplifier and a threshold circuit, and the pulse trains are transmitted through gate circuits 60x1, 60x2, 60y1, 60y2, 60z1 and 60z2 to the drive circuits 6x, 6y and 6z of the step motors 4x, 4y and 4z from a pulse generator 53. A decision element 52 is adapted to compare the sum of the outputs a', b', c' and d' with a predetermined threshold value and, in response the output signal g from the decision element 52, a gate control circuit 54 opens or closes the gate circuits 60z1 and 60z2. A gate control circuit 55 is adapted to open or close a gate circuit 56 in response to the output of the decision element 54. A control circuit for driving the step motors 4x, 4y and 4z in accordance with a predetermined sequence in addition to the driving for position control or correction is not shown. The outputs a', b', c' and d' are not obtained until the piston 2 is brought into contact with the cylinder 1 by the drive of the Z-direction step motor 4z, and their magnitudes are increased as the force with which the piston 2 is exerted on the cylinder 1 is increased.

When the sum of these output signals reaches a predetermined value which corresponds to a predetermined force to be applied to the piston 2, the increase in the pushing force in the Z-direction is terminated, and the X- and/or Y-direction step motors 4x and 4y are driven, so that the axis of the piston 2 may be moved to be made coincident with that of the cylinder bore 1a. When they are generally coincident with each other, the leading end of the piston 2 is inserted into the bore 1a as the pressure is exerted to the piston 2. From the decision element 52, there is obtained an output signal respresenting that the piston 2 is pressed against the cylinder 1, but this output signal g is varied because of the instantaneous change in the sum of the outputs a', b', c' and d'. As described hereinabefore when the decision circuits 50 and 51 are actuated at this stage, the fine positioning of the piston 2 can be affected, while it is engaged with the bore 1a. Therefore, the positive insertion of the piston 2 into the bore 1a in cooperation with the circuit 54 may be ensured.

However, the variation in magnitude of the output signal g occurs in response to noise, in practice. In the instant embodiment the gate control circuit 54 comprises a comparator having a hysteresis curve as shown in FIG. 14. The magnitude of the output signal g of the decision circuit 52 is gradually increased as the Z-direction step motor is driven to increase the pressure exerted to the piston 2. When the magnituide of the output signal g is increased in excess of a predetermined value P, the output Q is derived from the control circuit 54 to stop increasing the pressure exerted on the piston. When the coarse adjustment or positioning is carried out by the X- and Y-direction step motors 4x and 4y while the piston is pressed against the cylinder 1, the output signal g varies due to the surface roughness of the cylinder. However, the output signal g of the decision circuit 54 remains constant until the variation in the magnitude of the output signal g will not exceed the point R or S. When the piston is dropped into the bore, the variation in magnitude of the output signal g exceeds the point S so that the output of the decision circuit 54 drops from the point S to the point T. Therefore, the insertion of the end of the piston into the bore may be detected. In response to this output drop, the control circuit 55 and the gate circuit 56 are actuated, so that the correction of the position of the piston in the X- and Y-directions may be carried out and the pressure applied to the piston is increased again. If the pressure applied to the piston exceeds a predetermined value because of sticking or the like so that the output signal g reaches a point R (See FIG. 14), the Z-direction step motor 4z is reversed in rotation, thereby releasing the pressure exerted on the piston. Thus, the piston may be positively inserted into the bore. In this embodiment, when the point R is reached, the limit switch 11 (See FIG. 3) is closed. It will be easily understood by those skilled in the art that the comparator 54 having such a hysteresis may be provided by positive-feeding back the output of the flip-flop 542 which is actuated in response to the output of a differential amplifier 541 to the input thereof at a suitable level, as shown in FIG. 15.

SECOND EMBODIMENT, FIGS. 16 AND 17

Next, the second embodiment of the present invention will be described with reference to FIGS. 16 and 17. Parts 151 transported by a conveyor means 150 are to be assembled into a jig comprising a shaft 152b fixed to a base 152a and rods 152c. Each part 151 is provided with holes 151a and 151b into which are fitted the shaft 152b and the rods 152c, respectively. A part inserting member 155, which is coupled through a resilient means 154 to one end 153 of a positioning assembly or mechanism (not shown), is provided with means such as a magnet 156 for holding the part 151, which is substantially equal in outer diameter to the shaft 152b and is provided with a tapered lower end in order to faciliate the insertion of the inserting member 155 into the hole 151a of the part 151. The positioning assembly connected to the end 153 may be displaced in the X-, Y- and Z-directions and the end member 153 may be rotated in either direction, as shown in FIG. 16. Means (not shown) for detecting the relative displacement between the two members 153 and 155 are mounted on the resilient coupling means 154 in a manner substantially similar to that described hereinbefore.

As shown in FIG. 16 the positioning mechanism is moved in the Z-direction so that the inserting member 155 is inserted into the hole 151a and the holding means 156 holds the part 151 to move it toward the assemblying position. Thereafter, according to the fitting control method of the present invention, the assembly is carried out. In the instant embodiment, the movement in the .theta. direction of the member 153 of the positioning mechanism must be controlled. This is accomplished by the cooperation of the inserting member 155 with the shaft 152b of the jig. That is, an engaging hole 155a formed in the lower end portion of the inserting member 155 is fitted over a cylindrical projection 152d extended from the upper end of the shaft 152b. In other words, the two parts are not assembled by one-to-one correspondence between them, but assembled by one-to-one correspondence between one of the parts to be assembled and means for holding the other part for assembly. This method is advantageous especially when it is difficult to establish one-to-one correspondence between two parts to be assembled.

As described hereinbefore, the present invention includes the method in which one of the two parts to be fitted into a hole or bore of the other part, or, as shown in FIG. 17, a part having a hole to be fitted over a mating part, is held by the resilient coupling or supporting means.

Depending upon the types of the mechanisms or assemblies, coupling or supporting means which is not provided with a spring or the like and is capable of the displacement with respect to the positioning assembly or mechanism may be employed. In addition to the detecting means used in the embodiments and variations described hereinbefore, any suitable mechanical, magnetic, electrostatic or optic detecting means may be employed, and its output may be continuous or intermittent. The continuous output may be converted into the intermittent signals such as on-off signals as in the case of the embodiment shown in FIG. 4, in order to eliminate the adverse effect of the output drift when the displacement is almost zero. It will be understood that variations and modifications of the positioning assembly or mechanism and its driving system and the actuators may be effected within the spirit of the present invention. Furthermore, it will be understood that the assembly control may be achieved in a similar manner to those of the embodiments described hereinbefore even when a part having a hole or bore into which is fitted another part or projection thereof is displaced in the X-, Y- and Z-directions while the ends of the springs for supporting or suspending a mechanism for holding the another part.

Claims

What is claimed is:

1. An automatic assembly control device for assembling together first and second members, at least a portion of one of which is to be disposed internally of the other member in its assembled condition, comprising:

first means for holding one of said first and second members;

second means for driving a drive shaft along the respective axes of a selected coordinate system;

third means for elastically coupling said first means to said drive shaft;

fourth means, responsive to the displacement of said third means, relative to said first means, for generating a signal representative thereof; and

fifth means, responsive to said signal generated by said fourth means, for controlling said second means in accordance therewith.

2. An automatic assembly control device as defined in claim 1, wherein said third means comprises

four leaf springs interconnected in the form of a cross, and said fourth means comprises

respective strain gauge means bonded to the surface of each of said four leaf springs for generating a signal representative of the deflection of said leaf springs.

3. An automatic assembly control device as defined in claim 1, wherein said fourth means comprises

first detecting means for detecting the relative displacement between said drive shaft of said second means and one of said first and second members along first and second orthogonal coordinate axes; and

second detecting means for detecting the relative displacement between said drive shaft and one of said first and second members in the direction of an axis along which said first and second members are assembled together.

4. An automatic assembly control device as defined in claim 3, wherein said first detecting means has a prescribed response characteristic so that it provides an output signal only when the absolute value of said relative displacement detected thereby is in excess of a preselected value.

5. An automatic assembly control device as defined in claim 3, wherein said second detecting means includes means for generating a first output signal when the relative displacement detected thereby exceeds a first predetermined value, and for generating a second output signal when the relative displacement detected thereby is less than a second predetermined value.

6. An automatic assembly control device as defined in claim 3, wherein said fifth means comprises

a pulse generator for generating respective trains of positive and negative pulses;

first and second switching circuits for selectively transmitting therethrough one of said positive and negative pulse trains in response to the output of said first detecting means,

a third switching circuit for selectively transmitting therethrough one of said positive and negative pulse trains in response to the output of said second detecting means, and

respective stepping motors adapted to be rotated in a direction corresponding to the polarity of said positive and negative pulse trains transmitted through said first, second and third switching circuits, thereby effectively moving said drive shaft of said second means in the respective directions of said axes.

7. An automatic assembly control device as defined in claim 1, wherein said fourth means comprises a differential transformer with one of the core and winding of which is fixed to said drive shaft while the other is fixed to a portion of said first means, whereby a continuous signal representative of the displacement of one of said members in the direction of said axis may be derived.

8. An automatic assembly control device as defined in claim 1, wherein said fourth means includes a pressure sensitive element interposed between said first and second means, whereby a continuous signal representative of the displacement of one of said members in the direction of said axis may be derived.

9. An automatic assembly control device as defined in claim 4, further including means for causing said second means to oscillate with an amplitude within a range of values less than said preselected value.

10. An automatic assembly control device as defined in claim 1, wherein said third means has a non-linear characteristic corresponding to the relationship between the force exerted thereon and the displacement thereof.

11. An automatic assembly control device as defined in claim 1, wherein the output of said fourth means bears a non-linear relationship relative to said relative displacement.

12. An automatic assembly control device as defined in claim 4, wherein said second detecting means includes means for generating a first output signal when the relative displacement detected thereby exceeds a first predetermined value, and for generating a second output signal when the relative displacement detected thereby is less than a second predetermined value.

13. An automatic assembly control device as defined in claim 12, wherein said fifth means comprises

a pulse generator for generating respective trains of positive and negative pulses;

first and second switching circuits for selectively transmitting therethrough one of said positive and negative pulse trains in response to the output of said detecting means,

a third switching circuit for selectively transmitting therethrough one of said positive and negative pulse trains in response to the output of said second detecting means, and

respective stepping motors adapted to be rotated in a direction corresponding to the polarity of said positive and negative pulse trains transmitted through said first, second and third switching circuits, thereby effectively moving said drive shaft of said second means in the respective directions of said axes.

14. An automatic assembly control device as defined in claim 13, wherein said fifth means further includes a logic control circuit, responsive to the first and second output signals of said second detecting means, for controlling the delivery of said pulse trains through said switching circuits.

15. An automatic assembly control device comprising:

first means for holding a first member to be inserted into a mating bore of a second member;

second means for effecting the displacement of a drive shaft in the direction in which said first member is inserted into said mating bore of said second member, which direction corresponds to the Z axis of a three dimensional coordinate system;

third means for elastically coupling said first means to said second means;

fourth means for effecting the displacement of a stand, upon which said second member is disposed, in first and second orthogonal directions corresponding to the X and Y axes of said coordinate system;

fifth means, responsive to the displacement of said third means along said X, Y and Z axes, for generating a signal representative thereof; and

sixth means, responsive to the output of said fifth means, for controlling said second and fourth means in accordance therewith.

16. An automatic assembly control device for assembling together first and second members, at least a portion of one of which is to be disposed internally of the other member in its assembled condition, comprising:

first means for holding one of said first and second members;

second means for effecting the displacement of a drive shaft along the respective axes of a selected coordinate system;

third means for elastically coupling said first means to said second means;

fourth means for generating a signal representative of the displacement of said third means; and

fifth means for controlling said second means in response to signals from said fourth means.

17. An automatic assembly control device for assembling together first and second members, at least a portion of one of which is to be disposed internally of the other member in its assembled condition, comprising:

first means for holding one of said first and second members;

second means for elastically coupling an end member to said first means;

third means for generating a signal representative of the displacement of said second means;

fourth means for effecting the displacement of one of said first and second members along the respective axes of a selected coordinate system; and

fifth means for controlling said fourth means in response to the output of said third means.

18. An apparatus for effecting the insertion of a projection extending from a first member into a mating bore of a second member comprising:

first means for holding one of said first and second members;

a positioning mechanism moveable relative to said first means;

second means for holding the other of said first and second members;

third means for elastically coupling said second means to said positioning mechanism;

fourth means for causing said positioning mechanism to move in parallel with said first means, while said first and second members are brought into contact with each other under a predetermined pressure, thereby moving said first member along the surface of said second member in which said bore is formed;

fifth means for detecting the movement of said third means in response to the parallel movement of said positioning mechanism;

sixth means for controlling said fourth means, in response to the output of said fifth means, so as to effect the restoration of the displacement of said third means;

seventh means for effecting the movement of said positioning mechanism in a direction perpendicular to said first means;

eighth means for detecting the displacement of said third means in response to the perpendicular movement of said positioning mechanism; and

ninth means for effecting said perpendicular movement only when the output of said eighth means is less than a predetermined value and to interrupt said perpendicular movement when the output of said eighth means exceeds said predetermined value.

19. A method of controlling an automatic assembly control device for assembling together first and second members, at least a portion of one of which is to be disposed internally of the other member in its assembled condition, said device comprising:

first means for holding one of said first and second members;

second means for driving a drive shaft along the respective axes of a selected coordinate system;

third means for elastically coupling said first means to said drive shaft;

fourth means, responsive to the displacement of said third means, relative to said first means for generating a signal representative thereof; and

fifth means, responsive to said signal generated by said fourth means, for controlling said second means in accordance therewith;

said method comprising the steps of:

a. bringing said first member into contact with said second member at a position which is out of the range of variation in position of the center of that internal portion of the one of said members into which the other member is to be assembled;

b. effecting relative movement between said first means and said second means, so that said drive shaft may be caused to move toward said center while said member which is held by said first means is pressed against the other member under a predetermined pressure, thereby effecting a coarse alignment of the axis of each of said members; and

c. moving said drive shaft along prescribed directions so that the displacement of said third means interposed between said first and second means may be restored, thereby correcting the position of the member held by said first means and assembling said first and second members together.

20. A method according to claim 19, wherein said first and second means are provided with mechanical clearance therebetween with said steps (a) - (c) being carried out during oscillation motion caused as a result of said mechanical clearance.

21. A method of controlling an automatic assembly control device for assembling together first and second members, at least a portion of one of which is to be disposed internally of the other member in its assembled condition, said device comprising:

first means for holding one of said first and second members;

second means for driving a drive shaft along the respective axes of a selected coordinate system;

third means for elastically coupling said first means to said drive shaft;

fourth means, responsive to the displacement of said third means, relative to said first means for generating a signal representative thereof; and

fifth means, responsive to said signal generated by said fourth means, for controlling said second means in accordance therewith, said further means including

first detecting means for detecting the relative displacement between said drive shaft of said second means and one of said first and second members along first and second orthogonal coordinate axes; and

second detecting means for detecting the relative displacement between said drive shaft and one of said first and second members in the direction of an axis along which said first and second members are assembled together,

wherein said second detecting means includes means for generating a first output signal when the relative displacement detected thereby exceeds a first predetermined value, and for generating a second output signal when the relative displacement detected thereby is less than a second predetermined value,

said method comprising the steps of:

a. bringing said first member into contact with said second member at a position which is out of the range of variation in position of the center of that internal portion of the one of said members into which the other member is to be assembled;

b. effecting relative movement between said first means and said second means, so that said drive shaft may be caused to move toward said center while said member which is held by said first means is pressed against the other member under a predetermined pressure, thereby effecting a coarse alignment of the axis of each of said members; and

c. moving said drive shaft along prescribed directions so that the displacement of said third means interposed between said first and second means may be restored, thereby correcting the position of the member held by said first means and assembling said first and second members together,

wherein said step (b) is carried out in place of step (a) in response to the first output signal of said second detecting means, and step (c) is carried out in place of step (b) in response to the second output signal from said second detecting means.