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| Case Study : Robot Assembly of Pneumatic Cylinders | ||
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| Published: Jun.28.2006 @ 3:28 pm | ||
| This is a manufacturing study that I was asked to carry out for a
Swedish world leading manufacturer of compressors, generators,
construction and mining equipment, industrial tools and assembly
systems. The company wanted to create a database of observed operation
times for robot assembly tasks. INTRODUCTION The robot assembly of the pneumatic cylinder has been analyzed using the video taken recently. Activity times were related to the digital clock display on the video. The object of analyzing the assembly process was to create a data base. Information could be extracted from this database to evaluate the robot assembly of other products manufactured by the client. PNEUMATIC CYLINDER ASSEMBLY There are fifteen parts used in the assembly of the pneumatic cylinder and some of these are actually sub-assemblies. All the parts are presented to the robot on a pallet, with the exception of the screws. The cover screws and piston rod screws are automatically handled by vibratory linear feeders. The cycle time for the complete assembly is 166 seconds. This is substantially longer than predicted by academic estimation methods. The speed of the robot is set at 60 percent of the maximum. An electric current in excess of that tolerated by the drive motor circuitry, at 100 percent, makes this action necessary. This high power consumption, at start-up, is caused by the mass of the turret being approximately five times that of a conventional gripper. The robot manufacturer is replacing the relevant circuitry to allow full speed of the robot. Additionally, they have modified the feedback circuit to compensate for the larger mass. The robot programmer estimates that an increase in speed, from 60 percent to 100 percent, would provide a reduction in the cycle time of no more than 20 percent. The activity times obtained from the current analysis are used for the purpose of design for robotic assembly, and the evaluation of the client’s other products. SYNTHESIS OF ROBOTIC ASSEMBLY TIMES The time taken to assemble a part by the robot has four periods : 1) The movement of the gripper from the previous assembly position to above the current part to be assembled. 2) The picking up of the part. 3) The movement of the part to above the place of insertion. 4) The insertion, and subsequent release, of the part. The above time periods, when added together, make up the basic operation time for a single activity. RESULTS OF THE STUDY The total assembly time for the pneumatic cylinder is broken down into 54 steps to quantify the 4 constituent periods in each activity. The time study sheet is shown later in this article. From the time study sheet, a basic operation time of eight seconds is derived. It can be seen from the table that this basic operation time is equally divided between the four constituent periods. Notable deviations from the basic operation time are : End-piece - The end-piece is used to form a sub-assembly with the half -piston. There is no insertion time for this part because it is integrated with the half-piston. Cover Screw - There is a 50 percent increase in the basic operation time for this part. It is caused by the extra time involved with screw fastening and the transportation distance between the linear feeder and the work fixture. Piston Rod - A significant increase in the basic operation time for this part is due to the additional operation of 'knocking down' the piston rod after insertion. This is necessary because of the technique used to lift this part from the pallet. Piston Rod Screw - A 50 percent increase in the basic operation time is caused by the screw fastening operation and the transportation time from the linear feeder to the work fixture. Completed Cylinder - The gripper is in an adverse position from the previous operation and this increases the operation time. PREDICTION OF CYCLE TIME FOR THE ROBOT ASSEMBLY OF THE PNEUMATIC CYLINDER The cycle time for the robot assembly of the pneumatic cylinder is predicted by using a basic operation time, multiplied by a factor. assembly time = basic operation time * assembly process factor where, basic operation time = 8 seconds assembly process factor = 1.0 for straightforward insertion 1.5 for screw fastening operations 1.5 for long parts requiring two insertions Using the above approximations, a cycle time of 164 seconds is predicted. This is within one percent of the actual time of 166 seconds. It is not suggested that such a simple method could always achieve this accuracy. However, in the present case, the predictions for 9 out of 10 parts are within plus/minus 1 sec. PART ACTUAL PREDICTED DEVIATION OUTSIDE DESCRIPTION TIME TIME 1 SECOND CYLINDER BARREL 7 8 NONE END-PIECE 5 *4 * NONE HALF-PISTON 7 8 NONE COVER SCREW 48 48 NONE PISTON ROD 13 12 NONE HALF-PISTON 8 8 NONE PIN ROD SCREW 12 12 NONE END-PIECE 8 8 NONE COVER SCREW 12 12 NONE COMPLETED CYLINDER 10 8 2 SECONDS 166 164 **NOTE** The predicted time of 4 seconds for the end-piece allows for the fact that it is not inserted into the part-built assembly. This part forms a sub-assembly with the half-piston. There is a negligible amount of time lost due to gripper changing. The turret is indexed during movement from one operation to the next. A typical programming chart for a component is given at the end of this article and it shows that the basic assembly operation takes 8 program steps. Additional steps are required for screw fastening. TIME STUDY FOR ROBOT ASSEMBLY OF PNEUMATIC CYLINDER 0:01 Gripper above cylinder barrel 0:02 Pick up cylinder barrel 0:05 Cylinder barrel above fixture 0:07 Release cylinder barrel 0:10 Gripper above end-piece 0:14 Gripper above half-piston 0:15 Insertion of end-pieces and half-piston completed 0:17 Half-piston and end-piece above barrel 0:19 Release half-piston 0:21 Arm 2 above cover screw 0:24 Pick up cover screw 0:26 Cover screw above barrel 0:34 Arm 2 above cover screw 0:36 Pick up cover screw 0:38 Cover screw above barrel 0:46 Arm 2 above cover screw 0:48 Pick up cover screw 0:50 Cover screw above barrel 0:58 Arm 2 above cover screw 1:00 Pick up cover screw 1:02 Cover screw above barrel 1:09 Gripper above piston rod 1:11 Pick up piston rod 1:14 Piston rod above fixture 1:20 Completion of piston rod assembly to cylinder 1:22 Gripper above half-piston 1:24 Pick up half-piston 1:26 Half-piston above fixture 1:28 Completion of half-piston assembly to barrel 1:30 Gripper above piston rod screw 1:32 Pick up piston rod screw 1:35 Piston rod screw above fixture 1:40 Completion of piston rod screw assembly to piston rod 1:42 Gripper above end-piece 1:44 Pick up end-piece 1:46 End-piece above fixture 1:48 Completion of end-piece assembly to barrel 1:50 Gripper above cover screw 1:52 Pick up cover screw 1:55 Cover screw above barrel 2:02 Gripper above cover screw 2:04 Pick up cover screw 2:07 Cover screw above barrel 2:15 Gripper above cover screw 2:17 Pick up cover screw 2:19 Cover screw above barrel 2:27 Gripper above cover screw 2:29 Pick up cover screw 2:32 Cover screw above barrel 2:36 All cover screws inserted 2:40 Gripper above barrel 2:42 Pick up completed cylinder 2:44 Completed cylinder above pallet 2:46 Completed pneumatic cylinder in pallet BASIC OPERATION TIME OF THE ROBOT FROM PREVIOUS PICK MOVE TO INSERTION OPERATION OPERATION UP PLACE OF AND TIME TO ABOVE PART INSERTION RELEASE (SECONDS) PART 10) CYLINDER BARREL 1 1 3 2 007 09) END-PIECE 3 2 0 0 005 08) HALF-PISTON 2 1 2 2 007 07) COVER SCREW 3 2 2 5 012 07) COVER SCREW 3 2 2 5 012 07) COVER SCREW 3 2 2 5 012 07) COVER SCREW 3 2 2 5 012 06) PISTON ROD 2 2 3 6 013 05) HALF-PISTON 2 2 2 2 008 04) PISTON ROD SCR 2 2 3 5 012 03) END-PIECE 2 2 2 2 008 02) COVER SCREW 3 2 2 5 012 02) COVER SCREW 3 2 2 5 012 02) COVER SCREW 3 2 2 5 012 02) COVER SCREW 3 2 2 5 012 01) COMPLETED CYLINDER 4 2 2 2 010 TOTAL 166  |
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