Abstract:
A method and an apparatus for positioning a workpiece to a pallet on a working line. The line transports plural kinds of workpieces. Plural positioning pins are provided at various locations on each pallet to enable all the plural kinds of the workpieces to be carried by the pallets. Each positioning pin has a set position and a reset position. All positioning pins are first reset to the reset position. Thereafter, a selected positioning pin is set to the set position to accommodate a particular kind of workpiece. The positioning of the workpiece is accomplished by fitting a positioning hole defined by the selected workpiece onto the selected positioning pin.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for positioning a work piece to a pallet in a working line for machining or assembly of the workpiece. 
     BACKGROUND OF THE INVENTION 
     For automatic assembly of a workpiece which is carried in a working line, there has been known a method for positioning the workpiece at a predetermined position on a workpiece carrying pallet by means of providing a positioning pin on the pallet and then fitting a positioning hole of the workpiece to the positioning pin. Such a prior method is disclosed in, for example, the Japanese Utility Model Publication No. 59-120685. In case of different kinds of workpieces which have different configurations carried on the same working line, the position of the positioning pin on the pallet has to be changed or relocated in accordance with each workpiece. Therefore, it is necessary to prepare a plurality of different kinds of pallets which have the position pin arranging at a variety of suitable locations each designed to support a different workpiece and to select one of these pallets in accordance with the particular workpiece passing along the working or production line. As an alternative, one could provide a plural of detachable pins on the pallet and to repeat the steps of setting and removing the pins according to each workpiece passing along the working or production or mass assembly line. In either case, changing the pallets or the pins according to each workpiece passing along the working line is troublesome and to be avoided if possible. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method and apparatus for positioning a workpiece to a pallet in a working line which avoids the foregoing trouble and problem encountered with prior art practices. 
     Another object of the present invention is to provide a method and apparatus for positioning a workpiece to a pallet in a working line in such a way that it is possible to position different kinds of workpieces which have different configurations to the same pallet. 
     A further object of the present invention is to provide a method and apparatus for positioning workpieces to pallets in a working line whereby positioning the workpieces to the pallets is effected easily and automatically regardless of the fact that the workpieces may be of different configurations. 
     In order to accomplish the above described objects of the present invention, a method for positioning a workpiece to a pallet in working line is herein taught which comprises the steps of providing a plurality of positioning pins, each of which is able to take either a set position such that the positioning pin protrudes from a workpiece set surface of the pallet or a reset position such that the positioning pin sinks or is retracted under or beneath the workpiece set surface of the pallet. The positioning pins are distributed at suitable locations in the pallet sufficient to support all the different kinds of workpieces intended to pass along the working line. Initially the positioning pins are all reset to the reset position, then, a particular or preselected positioning pin selected from the plurality of position pins is set to the set position in accordance with the workpiece intended to be put on the pallet, and then, the intended workpiece is positioned on the pallet by fitting a positioning hole defined by that workpiece to the particular or preselected set positioning pin. 
     The apparatus of the present invention achieves the objects stated above by a structure that comprises a pallet having a workpiece set surface on which a workpiece is to be set, a positioning means for positioning the workpiece on the pallet which has a plurality of positioning pins provided at suitable locations in the pallet sufficient to support all the different kinds of workpieces. Each of the positioning pins is structurally arranged to be able to take either a set position in which the positioning pin protrudes from the workpiece set surface of the pallet or a reset position in which the positioning pin sinks or is retracted under or beneath the workpiece set surface of the pallet. A resetting means is provided for resetting all the positioning pins to the reset position, and a setting means is provided for moving a preselected or particular positioning pin selected from the plurality of positioning pins to the set position from the reset position in accordance with a designated workpiece intended to be put on the pallet so that the particular positioning pin registers, is received in, or fits into a positioning hole defined by the designated workpiece. The present invention, obviates the problems and troubles of prior art practices by providing a single pallet passing along a working line which can serve a plurality of different kinds or configurations of workpieces. 
     The above, and other objects, features and advantages of the present invention will become apparent from the following detailed description which is to be read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is plan view of a novel pallet according to the present invention; 
     FIG. 2 is a fragmentary plan view of a working line carrying the novel pallet shown in FIG. 1; 
     FIG. 3 is a sectional view taken along a line III--III of FIG. 2; 
     FIG. 4 is a front view of a workpiece detecting structure; 
     FIG. 5 is a sectional view taken along line V--V of FIG. 1 showing a set position of a positioning pin; 
     FIG. 6 is a sectional view showing a reset position of the positioning pin; 
     FIG. 7 is a front view of a positioning pin reset structure; 
     FIG. 8 is a sectional view showing a positioning pin set structure; 
     FIG. 9 is a fragmentary plan view showing the positioning set structure; 
     FIG. 10 is a sectional view taken along line X--X of FIG. 3; and 
     FIG. 11 is a sectional view showing three different setting positions of the memory unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention will now be described by way of example with reference to a preferred embodiment and the accompanying drawings. 
     In FIG. 1, a pallet 1 has an oblong base plate 1a, with cutouts 1b 1  and 1b 2  provided at each corner of long sides 1a 1  and 1a 2  of the base plate 1a. A sub plate 1c is fixed on one short side 1a 3  of the base plate 1a where the cutouts 1b 1  and 1b 2  are located, and extends vertically upwardly. Each side of the sub plate 1c has cutouts 1c 1  and 1c 2  at each lower corner so that the width of the lower side of the sub plate 1c is the same as the width of the short side 1a 3  of the base plate 1a. The width of the upper side of the sub plate 1c is the same as the width of the other short side 1a 4  of the base plate 1a (as shown in FIG. 1 and FIG. 3). A bracket 2a is fixed on an upper surface 1a 5  of the base plate 1a in parallel to the long side 1a 1  where the cutout 1b 1  is located. Four memory pins 2b 1-4  are slidably mounted in the bracket 2a. Each memory pin 2b 1-4  is arranged along the long side 1a 1  and slides in parallel to the upper surface 1a 6  and the short side 1a 3  of the base plate 1a. The memory pins 2b 1-4  and the bracket 2a are used as a memory unit 2 which indicates the type or kind of a workpiece to be put on the pallet, as will be described hereinafter. 
     Three positioning pins 5, 6 and 7 are fixed on the upper surface 1a 5  of the base plate 1a, and extend vertically upwardly, respectively. As shown in FIG. 1, FIG. 7 and FIG. 9, each of the fixed positioning pins 5, 6 and 7 has a pillar shape provided with a bevel or taper on top. The locations of the fixed positioning pins 5, 6 and 7 on the pallet are determined according to the locations of positioning holes W 1a , W 2a  and W 3a  which are provided at under surfaces of workpieces W 1 , W 2  and W 3 , such as a transmission case, respectively, so that when one fixed positioning pin (for example, W 5 ) is inserted into one positioning hole (for example W 1a ), the other fixed positioning pins (for example 6 and 7) do not interfere with the workpiece mounted on the pallet. 
     Three movable positioning pins 8, 9 and 10 are provided slidably mounted in the base plate 1a. The movable positioning pins 8, 9 and 10 are able to be moved upwardly and downwardly to be projected and sunk or retracted with respect to the upper surface 1a 5  of the pallet which serves as a workpiece set surface. The locations of the movable positioning pins 8, 9 and 10 are determined according to locations of positioning holes W 1b , W 2b  and W 3b  which are provided at the under surfaces of workpieces W 1 , W 2  and W 3 , respectively. 
     As shown in FIG. 2, the pallet 1 is transported by a working line 11. The working line 11 comprises first line 11T, second 11Y which is at right angles to the first line 11T, third line 11X which is at right angles to the second line 11Y and is in parallel to the first line 11T, and fourth line 11Z which is at right angles to the third line 11X and is in parallel to the second line 11Y. These lines 11T, 11Y, 11X and 11Z form a closed circulating line. 
     An unloaded pallet is transported to a starting point SP of the first line 11T from the fourth line 11Z. The pallet 1 is transported to a memory station 12 which is provided in the middle of the first line 11T. In the first line 11T, the pallet 1 is transported in the condition that the short side 1a 4  of the base plate 1a is a leading side to a transportation direction A of the first line 11T. On each pallet and workpiece, one positioning hole W 1a , W 2a , or W 3a  and one of the fixed positioning pin 5, 6 or 7 which is inserted into the positioning hole W 1a , W 2a  or W 3a  are located at opposite sides of the pallet and workpiece from positioning hole W 1b , W 2b  or W 3b  and one of the movable positioning pin 8, 9 or 10 which is inserted into the positioning hole W 1b , W 2b  or W 3b . Accordingly, as shown in FIG. 1 the workpiece W 1  is firmly positioned by use of the fixed positioning pin 6 and the movable positioning pin 9. The workpiece W 2  is also firmly positioned by use of the fixed positioning pin 5 and the movable positioning pin 10 (as shown in FIG. 9). And, the workpiece W 3  is also firmly positioned by use of the fixed positioning pin 7 and the movable positioning pin 8 (as shown in FIG. 9). 
     As shown in FIG. 3, the long sides 1a 1  and 1a 2  of the pallet 1 are guided by stationary guide members 11a 1  and 11a 2  of the working line 11. Lower surface 1a 6  of the base plate 1a is supported by upper rollers 11b 1  and 11b 1  of a free flow type chain conveyor mechanism 11b. Lower rollers 11b 2  and 11b 2  of the chain conveyor mechanism 11b are freely rotatably supported on supporting rails 11b 3  and 11b 3 . And, each of the upper rollers 11b 1  and 11b 1  is connected with the lower rollers 11b 2  and 11b 2  respectively by chain links 11b 4  and 11b 4  which are driven in the transportation direction A. 
     A stopper device 11c is provided at a leading end of the memory station 12. The stopper device 11c is composed of a cylinder 11c 1 , a piston rod 11c 2 , pressure passages 11c 5  and 11c 6  and a switching valve 11c 7 . 
     The cylinder 11c 1  is fixed to a base or frame member 11d of the working line 11. The piston rod 11c 2  is slidably mounted with the cylinder 11c 1  in order to move upwardly and downwardly. The lower end of the piston rod 11c 2  is received in the cylinder 11c 1  slidably and divides cylinder 11c, into two chamber 11c 3  and 11c 4 . The upper end of the piston rod 11c 2  is positioned below the lower surface 1a 6  of the base plate 1a when the piston rod 11c 2  is moved downwardly, and is positioned above the lower surface 1a 6  of the base plate 1a when the piston rod 11c 2  is moved upwardly. Pressure lines, conduits or passages 11c 5  and 11c 6  are connected to the chambers 11c 3  and 11c 4 , respectively, and supply a positive pressure by means of a hydraulic or air pump (not shown) to the chambers 11c 3  and 11c 4  respectively, or relieve the pressure from the chambers 11c 3  and 11c 4  to a drain tank (not shown), respectively. Switching valve 11c 7  is provided in the pressure passages 11c 5  and 11c 6 , and controls the supply and relief of pressure to the chambers 11c 3  and 11c 4  according to switching signals s 1  and s 2  of a controller 13. 
     A bracket 11e is fixed on the guide member 11a 2  which guides the long side 1a 2  of the base plate 1a. A roller 11f is rotatably supported by bracket 11e, and is able to contact with the upper surface 1a 5  in the vicinity of the long side 1a 2  of the base plate 1a for preventing upward movement of the pallet 1. The cutout 1c 2  of the sub plate 1c prevents that the sub plate 1c interferes with the bracket 11e and the roller 11f. At the memory station 12, a bracket 20 is fixed on an upper surface of the guide member 11a 1 . A memory set device 21 is fixed to the bracket 20. As shown in FIG. 10, the memory set device 21 is composed of four cylinder 21a 1 , 21a 2 , 21a 3  and 21a 4 , four piston rods 21c 1 , 21c 2 , 21c 3  and 21 c 4  which are mounted slidably to the cylinder 21a 1 , 21a 2 , 21a 3  and 21a 4 , respectively, and pressure lines or passages 21d 1 , 21d 2 , 21e 1 , 21e 2 , 21f 1 , 21f 2 , 21g 1  and 21g 2 , and a switching valve 21h. The cylinders 21a 1-4  lie in a horizontal plane and are fixed to an upper surface of the bracket 20. The cylinders 21a 1-4  are arranged to register or align with the bracket 2a of the memory unit 2 relative to the memory pins 2b 1-4 . The piston rods 21c 2 , 21c 2 , 21c 3  and 21c 4  are mounted slidably in the cylinders 21a 1 , 21a 2 , 21a 3  and 21a 4 , respectively. 
     The received ends of the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  within the cylinders 21a 1 , 21a 2 , 21a 3  and 21a 4 , respectively, divide each cylinder 21a 1-4  into two chambers 21i 1  and 21i 2 , 21j 1  and 21j 2 , 21k 1  and 21k 2 , and 21l 1  and 21l 2 , respectively. 
     The longitudinal axes of the cylinders 21a 1 , 21a 2 , 21a 3  and 21a 4  and the piston 21c 1 , 21c 2 , 21c 3  and 21c 4  are aligned with longitudinal axes of the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4 , respectively. The projecting or inner ends of the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  face toward the outer ends of the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4 , respectively. The inner ends of the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  keep a predetermined spacing from the outer ends of the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4 , respectively , when the piston rods 21c 1-4  are drawn into the cylinders 21a 1-4  (as shown in FIG. 10). When the piston rods 21c 1-4  are moved inwardly, that is, toward the pins; the inner ends of the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  are able to contact with the outer ends of the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4 , respectively, and to push the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4  inwardly to a set position. 
     The pressure passages 21d 1 , 21d 2 , 21e 1 , 21e 2 , 21f 1 , 21f 2 , 21g 1  and 21g 2  are connected to the chambers 21i 1 , 21i 2 , 21j 1 , 21j 2 , 21k 1 , and 21 l  and 21l 2 , respectively and supply a positive pressure which is produced by an air or hydraulic pump (not shown) to the chambers 21i 1 , 21i, 21j 1 , 21j 2 , 21k 1 , 21k 2 , 21l 1 , 21l 2  respectively, or relieve the pressure from the chambers 21i 1 , 21i 2 , 21j 1 , 21j 2 , 21k 1 , 21k 2 , 21l 1  and 21l 2  to outside or a drain tank (not shown), respectively. 
     The switching valve 21h is provided in the pressure passages 21d 1 , 21d 2 , 21e 1 , 21e 2 , 21f 1 , 21f 2 , 21g 1  and 21g 2 , and controls the supply and relief of the pressure to the chambers 21i 1 , 21i 2 , 21j 1 , 21j 2 , 21k 1 , 21k 2 , 21l 1  and 21l 2  according to switching signals S 3  and S 4  of the controller 13. 
     A lift device 22 is fixed to the bracket 20. The lift device 22 is composed of a cylinder 22a, a piston rod 22b, a guide rod 22c, pressure passages 22d 1  and 22d and a switching valve 22e. The piston rod 22b is attached to the cylinder 22a which is fixed to the upper surface of the bracket 20 between the cylinders 21a 2  and 21a 3  of the memory set device 21 in order to be moved upwardly and downwardly. The lower end of the piston rod 22b is slidably received in the cylinder 22a and divides cylinder 22a into two chambers 22a 1  and 22a 2 . The upper end of the piston rod 22b is fixed to a lower surface of a cylinder 23a of a memory reset device 23. The guide rod 22c is arranged in parallel to the piston rod 22b and is slidably received in a guide hole 22a 3  which is provided in a side wall of the cylinder 221a. The upper end of the guide rod 22c is fixed to the lower surface of the cylinder 23a of the memory reset device 23 so as to be moved upwardly and downwardly according to the movement of the piston rod 22b. Therefore, the guide rod 22c prevents the piston rod 22b from leaning or tilting due to the weight of the memory reset device 23. 
     Pressure passages 22d 1  and 22d 2  are connected to chambers 22a 1  and 22a 2 , respectively, and supply a positive pressure from an air or hydraulic source (not shown) to the chambers 22a 1  and 22a 2 , respectively, or relieve the pressure from the chambers 22a 1  and 22a 2 , respectively. Switching valve 22e is provided in the pressure passages 22d 1  and 22d 2 , and controls the supply and relief of the pressure to the chamber 22a 1  and 22a 2  according to switching signals S 5  and S 6  of a controlled 13. 
     Piston rod 23b of the memory reset device 23 is received in the cylinder 23a in order to be moved inwardly and outwardly. Received or outer end of the piston rod 23b is provided in the cylinder 23a slidably and divides cylinder 23a into two chambers 23a 1  and 23a 2 . An upper side of an oblong shaped reset plate 23c is fixed a the projecting end of the piston rod 23b. The reset plate 23c extends vertically downwardly from the projecting end of the piston rod 23b and extends parallel to the bracket 2a of the memory unit 2. 
     Pressure passages 23d 1  and 23d 2  of the memory reset device 23 are connected to the chambers 23a 1  and 23a 2 , respectively, and supply a positive pressure from an air or hydraulic source (not shown) to the chamber 23a 1  and 23a 2 , respectively, or relieve the pressure from the chamber 23a 1  and 23a 2 , respectively. Switching valve 23e of the memory reset device 23 is provided in the pressure passages 23d 1  and 23d 2 , and controls the supply and relief of the pressure to the chamber 23a 1  and 23a 2  according to switching signals S 7  and S 8  of the controller 13. 
     As shown in FIG. 3, the reset plate 23c is able to be positioned in five positions P 1 , P 2 , P 3 , P 4  and P 5 . When the piston rod 22b of the lift device 22 is drawn into the cylinder 22a of the lift device, and also, the piston rod 23b of the memory reset device 23 is drawn into the cylinder 23a of the memory reset device 23, the reset plate 23c is located in the position P 1 . In the position P 1 , the reset plate 23c and an inner end of the cylinder 23a of the memory reset device 23 are located in an area corresponding to the cutout 1c 1  of the sub plate 1c so that no interference exists between the memory reset device 23 and the sub plate 1c of the pallet 1, even if the sub plate 1c passes beside the memory reset device 23. Also, the predetermined spacing between the piston rods 21c 1-4  of the memory set device 21 and the memory pins 2b 1-4  is set, so no interference exists between lower side of the set plate 23c and the inner ends of the piston rods 21c 1-4  whenever the piston rods 21c 1-4  are drawn into the cylinders 21a 1-4  of the memory set device 21. In case the piston rod 22b of the lift device 22 is moved upwardly from the above-mentioned condition, the position P 2  is carried out or effected. And, the position PP 3  is carried out when the piston rod 23b of the memory reset device 23 is moved inwardly from the above-mentioned condition. Between the position P 2  an the position P 3 , a lower edge of the reset plate 23c is positioned above the bracket 2a of the memory unit 2 for preventing any interference, between the reset plate 23c and the bracket 2a. The position P 4  is carried out by the condition which is shown in FIG. 3. And, when the piston rod 23b of the memory reset device 23 is drawn a predetermined stoke into the cylinder 23a of the memory reset device 23 from the condition which is shown in FIG. 3, the position P 5  is carried out. 
     In the position P 4 , the lower side of the reset plate 2c is able to be faced to and keep a predetermined space from the inner ends of the memory pins 2b 1-4  even if the memory pins 21b 1-4  are positioned in any set position. 
     Before the pallet 1 is transported into the memory station 12, the piston rod 11c 2  of the stopper device 11 is memory set device 21 are also drawn into the cylinders 21a 1-4  respectively. The piston rod 22b of the lift device 22 and the piston rod 23b of the memory reset device 23 are drawn into the cylinder 22a and the cylinder 23a, respectively, so that the reset plate 23c is positioned in the position P 1 . When the pallet 1 is transported into the memory station 12 by the chain conveyor mechanism 11b, the controller 13 sends the switching signal S 1  to the switching valve 11c 7  in order to supply pressure to the chamber 11c 4  by the pressure passage 11c 6  and relieve th4e pressure from the chamber 11c 3  by the pressure passage 11c 5  so that the piston rod 11c 2  of the stopper device 11c is moved upwardly. Therefore, the short side 1a 4  of the pallet 1 contacts with the upper end of the piston rod 11c 2 , and the pallet 1 is stopped in the memory station 12. 
     When the pallet 1 is stopped in the memory station 12 completely, the controller sends the switching signal S 5  to the switching valve 22e of the lift device 22 in order to supply pressure to the chamber 22a 2  by the pressure passage 22d 2  and relieve the pressure from the chamber 22a 1  by the pressure passage 22d 1  so that the piston rod 22b is moved upwardly. Therefore, the reset plate 23c is moved from the position P 1  to the position P 2 . Then, the controller 13 sends the switching signal S 7  to the switching valve 23e of the memory reset device 23 in order to supply pressure to the chamber 23a 2  by the pressure passage 23d 2  and relieve the pressure from the chamber 23a 1  by the pressure passage 23d 1  so that the piston rod 23b is moved inwardly. Therefore, the reset plate 23c is moved from the position P 2  to the position P 3 . 
     Then, the controller 13 sends the switching signal S 6  to the switching valve 22e of the lift device 22 in order to supply pressure to the chamber 22a 1  by the pressure passage 22d 1  and relieve the pressure from the chamber 22a 2  by the pressure passage 22d 2  so that the piston rod 22b is moved downwardly. Therefore, the reset plate 23c is moved from the position P 3  to the position P 4 . And, the controller 13 sends the switching signal S 8  to the switching valve 23e of the memory reset device 23 for a predetermined time in order to supply pressure to the chamber 23a 1  by the pressure passage 23d 1  and relieve the pressure from the chamber 23a 2  by the pressure passage 23d 2  so that the piston rod 23b is moved outwardly a predetermined stroke. Therefore, the reset plate 23c is moved from the position P 4  to the position P 5 . When the reset plate 23c is moved from the position P 4  to the position P 5 , the lower side of the reset plate 23c contacts with the memory pins 2b 1-4  and pushes the memory pins 2b 1-4  outwardly so that all memory pins 2b 1-4  are positioned in a preset position which is shown in FIG. 10. After the above-mentioned memory reset operation, the controller sends the switching signals in order of S 7 , S 5  and S 2  so that the reset plate is returned to the position P 2 . 
     In this embodiment, three kinds of workpieces W 1 , W 2  and W 3  will be loaded on the pallet 1 according to a predetermined working program or order. The memory unit 2 will be able to memorize the kind of workpiece by positions of the memory pins 21b 1-4 . The workpiece W 1 , W 2  and W 3  are represented by conditions which are shown in FIG. 11(a), (b) and (c) respectively. 
     The predetermined working program or order is memorized in the controller 13 in order to send memory set signals or the switching signals S 3  and S 4  to the memory set device 21. 
     After the reset plate 23c is returned to the position P 2 , the controller 13 decides the kind of workpiece which will be loaded on the pallet 1. When the controller 13 decides that the workpiece W 1  will be loaded on the pallet 1, the controller 13 sends n switching signal to the switching valve 21h and a memory set operation is finished. Therefore, the memory pins 21b 1-4  are positioned in the set condition which is shown in FIG. 11(a). When the controller 13 decides that the workpiece W 2  will be loaded on the pallet 1, the controller 13 sends the switching signal S 3  to the switching valve 21h of the memory set device 21 in order to supply the pressure to the chambers 21j 1  and 21k 1  by the pressure passages 21e 1  and 21f 1 , respectively, and relieve the pressure from the chambers 21j 2  and 21k 2  by the pressure passages 21e 2  and 21f 2 , respectively, so that the piston rods 21c 2  and 21c 3  are moved inwardly and push the memory pins 2b 2  and 2b 3  inwardly respectively. Then, the switching valve 21h is switched in order to supply pressure to the chambers 21j 2  and 21k 2  by the pressure passages 21e 2  and 21f 2 , respectively, and relieve the pressure from the chambers 21j 1  and 21k 1  by the pressure passages 21e 1  and 21f 1 , respectively, so that the piston rods 21c 2  and 21c 3  are drawing to the cylinders 21a 2  and 21a 3 , respectively. Therefore, the memory pins 21b 1-4  are positioned in the set condition which is shown in FIG. 11(b). 
     When the controller 13 decides that the workpiece W 3  will be loaded on the pallet 1, the controller 13 sends the switching signal S 4  to the switching valve 21h of the memory set device 21 in order to supply pressure to the chambers 21i 1 , 21j 1 , 21k 1  and 21l 1  by the pressure passages 21d 1 , 21e 1 , 21f 1  and 21g 1 , respectively, and relieve the pressure from the chamber 21i 2 , 21j 2 , 21k 2  and 21l 2  by the pressure passages 21d 2 , 21e 2 , 21f 2  and 21g 2 , respectively, so that the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  are moved inwardly and push the memory pins 2b 1 , 2b 2 , 2b 3  and 2b 4 , respectively. Then, the switching valve 21h is switched in order to supply pressure to the chambers 21i 1 , 21j 2 , 21k 2  and 21l 2  by the pressure passages 21d 2 , 21e 2 , 21f 2  and 21g 2 , respectively, and relieve the pressure from the chambers 21i 1 , 21j 1 , 21k 1  and 21l 1  by the pressure passages 21d 1 , 21e 1 , 21f 1  and 21g 1 , respectively, so that the piston rods 21c 1 , 21c 2 , 21c 3  and 21c 4  are drawn into the cylinders 21a 1 , 21a 2 , 21a 3  and 21a 4  respectively. 
     Therefore, the memory pins 2b 1-4  are positioned in the set condition which is shown in FIG. 11(c). After the above-mentioned memory reset operation, the controller 13 sends the switching signal S 6  to the switching valve so that the reset plato 23C is moved from the position P 2  to the position P 1 . 
     Then, the controller 13 sends the switching signal S 2  to the switching valve 11C 7  in order to supply pressure to the chamber 11C 3  by the pressure passage 11C 5  and relieve the pressure from the chamber 11 4  by the pressure passage 11C 6  so the pallet 1 is relieved from the stopper device 11C and, is transported by the chain conveyer mechanism 11b to a read station 13 which is provided at a leading edge of the first line 11T. 
     Each of the other stations which are described hereinafter has the same type of the stopper device 11C at the leading end of the station for stopping the pallet 1. 
     As shown in FIG. 2 and FIG. 4 the kind of the workpiece which is memorized in the memory unit 2 is read by a read device 30 when the pallet 1 is stopped in the read station 13. The read device 30 is fixed on the guide member 11a 1  of the working line 11 by a bracket 31 in a manner that four sensors 30a 1 , 30a 2 , 30a 3  and 30a 4  (30a 1-3  are not shown in FIG. 4), such as, a magnetic non-contact sensor of the read device 30 are faced to the outer ends of the memory pins 2b 1 , 2b 2 , 2b 3  b and 2b 4 , respectively. The read device is located in an area corresponding to the cut out 1C 1  of the sub plate 1C so that no interference exists between the read device 30 and the sub plate 1C even if the sub plate 1C passes beside the read device 30. The sensors 30a 1-4  detect the positions of the memory pins 2b 1-4 . The set positions show the kind of workpiece. The read device sends a &#34;kind&#34; signal S 9 , S 10  or S 11  to a shift register 32 corresponding to the workpiece W 1 , W 2  or W 3 , respectively carried on the pallet 1. 
     Thereafter, the pallet 1 is transported to a movable positioning pin reset station 14 which is provided at the second line 11Y. In the second line 11Y, the pallet 1 is transported in the condition that the long side 1a 2  of the base plate 1a is a leading side to the transportation direction A of the second line 11Y. As shown in FIG. 5 and FIG. 6, the pallet 1 has three through holes 1f which penetrate the base plate 1a from the upper surface 1a 5  to the lower surface 1a 6 . A holder 40 is fixed into each through hole 1f and has a through hole 40a which is provided at a right angle to the upper surface 1a 5  of the base plate 1a. One of the movable positioning pins 8, 9 and 10, which are formed in pillar shape, is slidably fitted in the through hole 40a. 
     The holder 40 has a threaded hole 40b which is provided at a right angle to the through hole 40a. In the threaded hole 40b, there is a ball or detent 41 which is urged by a spring 42 for maintaining the position of the movable positioning pin 8, 9, or 10. A screw 43 is threaded into the threaded hole 40b for adjusting the urging force of the spring 42. The peripheral surface of each movable positioning pin 8, 9, or 10 has two annular grooves 44 and 45 which are spaced along the longitudinal axis of the movable positioning pin 8, 9, 10. As shown in FIG. 5, when the ball or detent 41 is engaged with the annular groove 45, the movable positioning pin 8, 9 or 10 is maintained at a set position such that the movable positioning pin 8, 9 or 10 protrudes from the upper surface 1a 5  of the base plate 1a. Also, as shown in FIG. 6, when ball 41 is engaged with the annular groove 44, the movable positioning pin 8, 9, or 10 is maintained at a reset position such that the movable positioning pin 8, 9 or 10 sinks under or is retracted even with or beneath the upper surface 1a 5  of the base plate 1a. The holder 40 has an annular groove 40c at lower edge of the through hole 40a. In the annular groove 40c there is a snap ring 46 so that the movable positioning pin 8, 9 or 10 is prevented from falling out. 
     As shown in FIG. 7 a positioning pin reset device 50 is provided at the positioning pin reset station 14. The positioning pin reset device 50 comprises a support bracket 51, a cylinder 52, a push plate 53 and two slide bars 54 and 55. The support bracket 51 is disposed at the positioning pin reset station 14. The cylinder 52 is fixed vertically to a horizontal member 51a of the support bracket 51 above the second line 11Y. The longitudinal vertical axis of the cylinder 52 is perpendicular to the upper surface 1a 5  of the pallet 1 which is stopped at the positioning pin reset station 14 by a stopper device as described. The cylinder 52 has a piston rod 52a which reciprocates vertically by hydraulic or air power. A free or projecting end of the piston rod 52a is connected with the push plate 53. As shown in FIG. 1, the push plate 53 has an area sufficient to cover all of the movable positioning pins 8, 9 and 10 at once and not to interfere with the fixed positioning pins 5, 6 and 7. The slide bars 54 and 55 which are parallel to the cylinder 52 are slidably supported to the horizontal member 51a of the support bracket 51, and the lower ends of the side bars 54 and 55 are connected to the push plate 53. Therefore when the push plate 53 is stroked downwardly by the piston rod 52a, the positioning pins 8, 9 and 10 are pressed downwardly by the push plate 53 so that all of the positioning pins 8, 9 and 10 are moved at once to the reset position as shown in FIG. 6. After the above-mentioned positioning pins reset operation, the pallet 1 is transported to a positioning pin set station 15 which is provided at a starting point of the third line 11X by the chain conveyor mechanism 11b. At the positioning pin set station 15, the pallet 1 is in the condition that the sub plate 1c of the base plate 1 is leading to the transportation direction A of the third line 11X. 
     As shown in FIG. 8 and FIG. 9, a positioning pin set device 60 is provided at the positioning pin set station 15 below the plate 1a which is stopped, in the positioning pin set station 15, by a stopper device as described. The positioning pin set device 60 is composed of three cylinders 61a, 61b and 61c fixed vertically to fixed support bracket 62 which is located in parallel to the base plate 1a. Three piston rods 63a, 63b and 63c are slidably mounted in the cylinders 61a, 61b and 61c, respectively; pressure passages 64a 1 , 64a 2 , 64b 1 , 64b 2 , 64c 1  and 64c 2  and a switching valve 65 provide pressure and relief. The piston rods 63a, 63b and 63c extend vertically upwardly from the cylinders 61a, 61b and 61c. Upper ends of the piston rods 63a, 63b and 63c are registered with, faced, oriented, aligned, or indexed to lower ends of the movable pins 8, 9 and 10, respectively, so that the piston rods 63a, 63b and 63c are able to push the movable pins 8, 9 and 10 upwardly from the reset position to the set position, respectively, when the piston rods are moved upwardly. Lower ends of the piston rods 63a, 63b and 63c are provided in the cylinders 61a, 61b and 61c, respectively, and divide the cylinders 61a, 61b and 61c into two chambers 61a 1  and 61a 2 , 61b 1  and 61b 2  and 61c 1  and 61c 2 , respectively. The pressure passages 64a 1 , 64a 2 , 64b 1 , 64b 2 , and 64c 1  and 64c 2 , are connected, respectively, in order to supply a positive pressure which is produced from an air or hydraulic pump 66 or relieve the pressure to outside or a drain tank 67. The switch valve 65 is provided in the pressure passages 64a 1 , 64a 2 , 64b 1 , 64b 2 , 64c 1  and 64c 2  in order to switch the supply and relief of the pressure according to switching signals S 9 , S 10  and S 11  of the shift register 32. 
     When the pallet 1 is transported to the positioning pin set station 15, the shift register 32 sends one of the switching signals S 9 , S 10  and S 11  which is correlated with the kind of workpiece to be mounted on pallet 1 to the switching valve 65 in the order of memorized signal on first-memorized-first sent basis. When the switching signal S 9  which shows that the workpiece W 1  will be loaded on the pallet 1 is sent to the switching valve 65, the switching valve 65 is switched so that the pressure is supplied to the chamber 61b 2  by the pressure passage 64b 2  and the pressure in the chamber 61b 1  is relieved by the pressure passage 64b 1 . Therefore, the piston rod 63b is moved upwardly so as to push the movable position pin 9 upwardly from the reset position to the set position. The pressure in the chamber 61a 1  and the chamber 61c 1  are kept so that the piston rod 63a and the piston rod 63c  are kept in a down position so that the upper ends of the piston rods 63a and 63c keep a predetermined spacing from the lower surface 1a 6  of the base plate 1a. 
     When the switching signal S 10  is sent to the switching valve 65 which shows that the workpiece W 2  will be loaded on the pallet 1, the switching valve 65 is switched so that the pressure is supplied to the chamber 61c 2  by the pressure passage 64c 2  and the pressure in the chamber 61c 1  is relieved by the pressure passage 64c 1 . Therefore, the piston rod 63c is moved upwardly so as to push the movable positioning pin 10 upwardly from the reset position to the set position. The pressure in the chamber 61a 1  and the chamber 61b 1  is kept so that the piston rod 63a and the piston rod 63b are kept in the down position. 
     When the switching signal S 11  is sent to the switching valve 65, which shows that the workpiece W 3  will be loaded on the pallet 1, the switching valve 65 is switch so that the pressure is supplied to the chamber 61a 2  by the pressure passage 64a 2  and pressure in the chamber 61a 1  is relieved by the pressure passage 64a 1 . Therefore, the piston rod 63a is moved upwardly so as to push the movable position pin 8 upwardly from the reset position of the set position. The hydraulic pressure in the chamber 61b 1  and the chamber 61c 1  is kept so that the piston rod 63b and the piston rod 63c are kept in the down position. 
     In the condition that one of the piston rods 63a, 63b and 63c has pushed one of the movable positioning pins 8, 9 and 10 into the set position by means of the above-mentioned positioning pin set operation, one of the workpieces W 1 , W 2  and W 3 , which is corresponding to the switching signal S 9 , S 10 , S 11 , is loaded to the base plate 1a and is positioned by one of the fixed positioning pins 5, 6 and 7 and one of the movable positioning pins 8, 9 and 10. After the workpiece positioning operation the switching valve 65 is switched in order to move downwardly the piston rod 63a, 63b or 63c which has pushed one of the movable pins 8, 9 and 10 upwardly. Then the pallet 1 is transported to working stations 16, 17 and 18 in successive order. At the working stations 16, 17 and 18, other parts are fitted to the workpiece, or the workpiece is machined by an automatic machining device or any other operation is carried out. 
     It is possible to guide the sub plate 1c by the chain conveyor mechanism 11b in order to change the position at a right angle comparing with the position in the first line 11T and the second line 11Y at the working stations 16, 17 and 18. In the above-mentioned condition, even if the upper side surface of the sub plate 1c is guided by the same type of roller device as the roller 11f and the bracket 11e, interference between the roller device and the base plate 1a is prevented by the cutouts 1b 1  and 1b 2  of the base plate 1a. 
     After the above-mentioned assembly or machining operation, the pallet 1 is transported to a workpiece removing station 19 which is provided at the edge of the third line 11X in order to remove the workpiece from the pallet 1. Then, the pallet 1 which is now unloaded is transported to the fourth line 11Z in the condition that the long side 1a 1  of the base plate 1a is a leading side to the transportation direction A of the fourth line 11Z. 
     In the embodiment described above, the fixed positioning pins 5, 6 and 7 are fixed to the base plate 1a. These fixed positioning pins 5, 6 and 7, however, may be changeable to positioning pins such as the movable positioning pins 8, 9 and 10 so that all positioning pins are projectable and retractable 
     Although the invention has been described with reference to a preferred embodiment, changes are possible which will be evident to those skilled in the art. Such changes are deemed to fall within the purview of the invention as claimed.