Robot logic: slower line speed, higher output Essay

Engineers at Fisher Guide, Div of General Motors Corp, Columbus, OH,automated a nine-press production line to manufacture door-framereinforcements for midsize cars. In retrofitting a fully automatedsystem for the nine presses, the firm installed 10 robots, eightpositioning fixtures, and a variety of sensors. As a result, line speedhad to be reduced from 290 to 275 pcs/hr.

The slower rate, however, wasmore than offset by extended uptime through breaks, lunch hours, shiftchanges, and a third shift–thus producing a steadier and longer flow ofgood parts. There are already many secondary benefits. “We have reducedscrap rates because there are fewer mishits,” says Ralph Smith,superintendent of productivity engineering at the plant, “and ourproduction supervisor says that the need for die maintenance is reducedconsiderably because of more precise die loading.” The Fisher Guide-Columbus press line is tended by GMF’scylindrical coordinate M-1A robots. One operator oversees the entire140-ft press line, although the system doesn’t need 100-percentsurveillance.

“At the end of the second shift,” Smithexplains, “we load the bank of blank pallets, and the line will rununtended until it faults, or all of the pallets have been emptied.” Blanks are presented to the first robot by a nonsynchronousindexing conveyor equipped with 12 pallets, each holding nearly 100pieces. Each 9″ X 12″ blank weight 1-1/2 lb. Electric choice for light parts Size and weight of the part have a lot to do with the robot chosenfor an application. Smith remarks, “Our parts are light. Electricrobots are the only way to go as far as we’re concerned.

Theelectric robots are less costly, have greater accuracy, require lesspreventive maintenance, and have higher uptime than hydraulicrobots.” The first robot, equipped with vacuum grippers, senses the top ofthe stack with a limit switch. A fiber-optic sensor also detectswhether there are any remaining blanks on the pallet. If there arenone, another pallet indexes into position. As this robot turns to load the forming press, it passes the blankthrough the rollers of a sensing device mounted near the die opening.

This sensor verifies that there is only one blank. If more than oneblank is sensed, the robot discards the “batch” and goes backto pick up another. Upon confirmation of a single blank, the robotloads it into a draw die that creates the basic part form. The second robot unloads the part and immediately turns to load agap press for restrike.

It then removes the part and places it on thefirst staging fixture. The third robot moves the workpiece from thisfixture to the next press, then to the next fixture. Robots 4 through 9repeat this action, transferring the part through trim and pierce dies. The operation at the ninth die is the slowest. To minimize itseffect on total line speed, the die was altered so the gripper can stayin the die area during the press cycle. The press doesn’t have towait for it to retract. Instead, the arm merely relaxes its grip on theworkpiece during the press stroke. In some other stations, engineershad to give the dies slightly more clearance so the end-of-arm toolingwould fit while inserting or removing the workpiece.

The last staging fixture collects four finished parts before asignal is sent to the 10th robot for palletizing. Standard GMF palletizing software allowed several pattern alternatives. Withmechanical grippers, the 10th robot grasps the stack of four parts andpalletizes them in shipping containers.

Tooled with vacuum grippers,the robot separates layers of parts with cardboard sheets. When onebasket is full, a flashing red light alerts the operator, and the robotimmediately begins palletizing in the empty alternate container. Orienting fixtures Staging fixtures are integral elements in this automated pressline. Equipped with 90-degree or 180-degree rotating axes, they orientworkpieces for proper loading in the dies. Without this intermediateorientation, each robot would need additional axes. The last fixture incorporates a fiber-optic sensor to check thequality of the first part placed on the stand.

It senses if all fiveholes are pierced, eliminating the need for visual part inspection andminimizing the chances of running the line with a broken punch orbutton. The system automatically shuts the line down if it detects amissing hole. A programmable logic controller (PLC), with elaborate interfacing,coordinates all communication and most work within the line. Thecontroller displays location of a fault, whether it’s a pressfixture or robot that didn’t operate. This fault-diagnosticssystem covers 128 possible errors. Its intelligent display scrollsalphanumeric messages describing all 128 system errors to the operator.

Operators need troubleshoot only the origin of uncommon errors. Move over for die changes “Our production runs last about two weeks,” reports GaryHay, senior manufacturing engineer at the Columbus plant. “Eightof the robots are mounted on a track. Before beginning a new run, wecan move the robots aside by removing four bolts and two locating pins.One man can then easily push the robot aside to make room for diechanges.” This feature is also helpful for robot repair andmaintenance. The entire 140-ft line is surrounded by an infrared light curtain.When someone breaks a light beam by entering the work area, a signal tothe supervisory PLC instantaneously stops all equipment.

Since thereare so many areas where a person could enter the work envelopes withoutbeing seen, the light curtain consists of 15 zones. The operator mustreset the curtain where it was violated and return to the maincontroller to restart the line. A loop chain keeps people from casuallyentering the light-curtain area. Smith notes that GMF was responsible for all automation, but robotapplication involved joint cooperation between vendor and user.

“When we first istalled the robots and fixtures, order volumesdidn’t allow us to debug the line, so we made gripper, die,electrical, and part-positioner revisions between manufacturing runs.Ideally, as engineers, we wanted to debug the system before producingparts. Instead, our system evolved with many changes afterstartup.” For robot and automation information from GMF Robotcs Corp, Troy,MI, circle E2. Six-axis robot welding work cell Foundation of a new work cell is the six-axis Cyro 1000 articulatedarm robot made by Advanced Robotics Corp, Columbus, OH. The sixth axiseliminates the need for special torches or brackets which werepreviously required to achieve the proper welding angle in manyapplications. With the sixth axis, the proper torch attitude can bemaintained even while welding complex joint profiles. Also, the robot reduces or eliminates the need for an indexingtable or multiple-axis positioner by providing more robot dexterity.

This reduces the work cell cost while providing maximum jointaccessibility. The new unit features a weight carrying capacity of 22 lb at thetorch-mounting face plate, harmonic drive and ballscrews for smoothmotion, and demonstrated reliability (99 percent uptime withapproximately 4000 hr mean-time-between-failure). Working enveloperadius is 1400 mm, and the robot’s six-axis offset wrist designprovides a larger range of movement within that radius. The work cell utilizes the recently introduced Cyro C-30 controllerwith program shift and translation features designed to reduceprogramming time. A CRT screen built into the C-30 control cabinetdisplays English diagnostic messages and useful production data.

TheC-30’s expanded teach pendant includes all programming and controlcommands and controls the robot, the weld process equipment, andassociated positioning equipment. Other key features include weld path weave capability, torch-anglecompensation, and the capability to manually fine-tune weld speed,current, and voltage during program execution. Its offset wrist designeliminates interference of the torch body with the arm, therebyproviding improved range in the tilt axis. The work cell can beequipped with an X-axis rail system to greatly expand the work envelope.Standard configurations come in 2.

5-, 5.0-, and 7.0-meter lengths. For more information, circle E34. End effector combines welding and material handling Retooling a single function robot to perform a dual-function taskcan maximize efficiency of an application. The DeVilbiss Co, Toledo,OH, accomplished this for JI Case by designing an exclusive gripperassembly for the EPR-1000 arc-welding robot (see photo and sketch).

Theretooled assembly houses two welding torches and a pneumatic gripper. When Case approached DeVilbiss, as well as several other robotbuilders, the concept of an integrated robot system that could bothpick-and-place and weld hadn’t yet been developed. John Treuschel,manager of process automation at DeVilbiss, explains that the small partsize made it possible to combine both functions.

“Our primary goalwas to design a gripper that could be integrated with an arc-weldingrobot and perform both functions with accuracy and repeatability. Theentire system would have to locate, clamp and transfer parts, then weldwith two torches.” The EPR-1000 is installed at Case’s Bettendorf, IA, facility.Its assignment is to pick-and-place and weld encased nuts to steelangles that will later be assembled into the Series 450, 850, 1150, and1450 crawler bulldozers and loaders. Case has three sizes of encased nuts (3/8″, 1/2″,5/8″), and cages that are welded to more than 60 parts ranging from12″ to 12 ft. The multiple combinations are a challenge forrepeatability, but by carefully programming the gripper to adjust toeach size, the robot successfully places each nut within 0.005″. The process begins as the nuts and cages are manually loaded into agravity feed track that leads to the pickup station.

Using a magneticfixture, the operator aligns and secures the angles on the weldingtable. Once set, the operator presses the control button to rotate thetabletop 180 degrees, positoning the angle in front of the robot.Simultaneously, the robot’s articulated arm picks up the propernut, transfers it to the table, and places it over one of several holds.The EPR-1000 then welds the encased nut to the angle with its twotorches, which are positioned on either side of the gripper.

To maximize efficiency, the table is equipped with two sets offixtures, allowing the operator to align and secure a second angle whilethe robot is welding nuts to the first. When the robot finisheswelding, the operator again rotates the tabletop and replaces theassembly with a new angle, while the robot welds on the other side ofthe table. Before the robot was installed, a single operator manually weldedeach encased nut to the angles. Bill DeVenny, welding engineer at Case,cites the advantage of automating, “Production rates are up 30percent and reject rates are down 10 percent.” As a result, Case is looking at additional applications. “Arcwelding is our primary interest,” says DeVenny.

“In relationto this need, we will be examining the potential of machine vision as ameans to automate even further. Robots are definitely a part of ourfuture plans.” For more information about welding with robots, circle E61.

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