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Robot logic: slower line speed, higher output.

Engineers at Fisher Guide, Div of General Motors Corp, Columbus, OH, automated a nine-press production line to manufacture door-frame reinforcements for midsize cars. In retrofitting a fully automated system for the nine presses, the firm installed 10 robots, eight positioning fixtures, and a variety of sensors. As a result, line speed had to be reduced from 290 to 275 pcs/hr. The slower rate, however, was more than offset by extended uptime through breaks, lunch hours, shift changes, and a third shift--thus producing a steadier and longer flow of good parts.

There are already many secondary benefits. "We have reduced scrap rates because there are fewer mishits," says Ralph Smith, superintendent of productivity engineering at the plant, "and our production supervisor says that the need for die maintenance is reduced considerably because of more precise die loading."

The Fisher Guide-Columbus press line is tended by GMF's cylindrical coordinate M-1A robots. One operator oversees the entire 140-ft press line, although the system doesn't need 100-percent surveillance. "At the end of the second shift," Smith explains, "we load the bank of blank pallets, and the line will run untended until it faults, or all of the pallets have been emptied."

Blanks are presented to the first robot by a nonsynchronous indexing conveyor equipped with 12 pallets, each holding nearly 100 pieces. 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 chosen for an application. Smith remarks, "Our parts are light. Electric robots are the only way to go as far as we're concerned. The electric robots are less costly, have greater accuracy, require less preventive maintenance, and have higher uptime than hydraulic robots."

The first robot, equipped with vacuum grippers, senses the top of the stack with a limit switch. A fiber-optic sensor also detects whether there are any remaining blanks on the pallet. If there are none, another pallet indexes into position.

As this robot turns to load the forming press, it passes the blank through the rollers of a sensing device mounted near the die opening. This sensor verifies that there is only one blank. If more than one blank is sensed, the robot discards the "batch" and goes back to pick up another. Upon confirmation of a single blank, the robot loads it into a draw die that creates the basic part form.

The second robot unloads the part and immediately turns to load a gap press for restrike. It then removes the part and places it on the first staging fixture. The third robot moves the workpiece from this fixture to the next press, then to the next fixture. Robots 4 through 9 repeat this action, transferring the part through trim and pierce dies.

The operation at the ninth die is the slowest. To minimize its effect on total line speed, the die was altered so the gripper can stay in the die area during the press cycle. The press doesn't have to wait for it to retract. Instead, the arm merely relaxes its grip on the workpiece during the press stroke. In some other stations, engineers had to give the dies slightly more clearance so the end-of-arm tooling would fit while inserting or removing the workpiece.

The last staging fixture collects four finished parts before a signal is sent to the 10th robot for palletizing. Standard GMF palletizing software allowed several pattern alternatives. With mechanical grippers, the 10th robot grasps the stack of four parts and palletizes them in shipping containers. Tooled with vacuum grippers, the robot separates layers of parts with cardboard sheets. When one basket is full, a flashing red light alerts the operator, and the robot immediately begins palletizing in the empty alternate container.

Orienting fixtures

Staging fixtures are integral elements in this automated press line. Equipped with 90-degree or 180-degree rotating axes, they orient workpieces for proper loading in the dies. Without this intermediate orientation, each robot would need additional axes.

The last fixture incorporates a fiber-optic sensor to check the quality of the first part placed on the stand. It senses if all five holes are pierced, eliminating the need for visual part inspection and minimizing the chances of running the line with a broken punch or button. The system automatically shuts the line down if it detects a missing hole.

A programmable logic controller (PLC), with elaborate interfacing, coordinates all communication and most work within the line. The controller displays location of a fault, whether it's a press fixture or robot that didn't operate. This fault-diagnostics system covers 128 possible errors. Its intelligent display scrolls alphanumeric 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 Gary Hay, senior manufacturing engineer at the Columbus plant. "Eight of the robots are mounted on a track. Before beginning a new run, we can 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 die changes." This feature is also helpful for robot repair and maintenance.

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 to the supervisory PLC instantaneously stops all equipment. Since there are so many areas where a person could enter the work envelopes without being seen, the light curtain consists of 15 zones. The operator must reset the curtain where it was violated and return to the main controller to restart the line. A loop chain keeps people from casually entering the light-curtain area.

Smith notes that GMF was responsible for all automation, but robot application involved joint cooperation between vendor and user. "When we first istalled the robots and fixtures, order volumes didn'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 producing parts. Instead, our system evolved with many changes after startup."

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 articulated arm robot made by Advanced Robotics Corp, Columbus, OH. The sixth axis eliminates the need for special torches or brackets which were previously required to achieve the proper welding angle in many applications. With the sixth axis, the proper torch attitude can be maintained even while welding complex joint profiles.

Also, the robot reduces or eliminates the need for an indexing table or multiple-axis positioner by providing more robot dexterity. This reduces the work cell cost while providing maximum joint accessibility.

The new unit features a weight carrying capacity of 22 lb at the torch-mounting face plate, harmonic drive and ballscrews for smooth motion, and demonstrated reliability (99 percent uptime with approximately 4000 hr mean-time-between-failure). Working envelope radius is 1400 mm, and the robot's six-axis offset wrist design provides a larger range of movement within that radius.

The work cell utilizes the recently introduced Cyro C-30 controller with program shift and translation features designed to reduce programming time. A CRT screen built into the C-30 control cabinet displays English diagnostic messages and useful production data. The C-30's expanded teach pendant includes all programming and control commands and controls the robot, the weld process equipment, and associated positioning equipment.

Other key features include weld path weave capability, torch-angle compensation, and the capability to manually fine-tune weld speed, current, and voltage during program execution. Its offset wrist design eliminates interference of the torch body with the arm, thereby providing improved range in the tilt axis. The work cell can be equipped 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 task can maximize efficiency of an application. The DeVilbiss Co, Toledo, OH, accomplished this for JI Case by designing an exclusive gripper assembly for the EPR-1000 arc-welding robot (see photo and sketch). The retooled assembly houses two welding torches and a pneumatic gripper.

When Case approached DeVilbiss, as well as several other robot builders, the concept of an integrated robot system that could both pick-and-place and weld hadn't yet been developed. John Treuschel, manager of process automation at DeVilbiss, explains that the small part size made it possible to combine both functions. "Our primary goal was to design a gripper that could be integrated with an arc-welding robot and perform both functions with accuracy and repeatability. The entire system would have to locate, clamp and transfer parts, then weld with 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 steel angles that will later be assembled into the Series 450, 850, 1150, and 1450 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 from 12" to 12 ft. The multiple combinations are a challenge for repeatability, but by carefully programming the gripper to adjust to each size, the robot successfully places each nut within 0.005".

The process begins as the nuts and cages are manually loaded into a gravity feed track that leads to the pickup station. Using a magnetic fixture, the operator aligns and secures the angles on the welding table. Once set, the operator presses the control button to rotate the tabletop 180 degrees, positoning the angle in front of the robot. Simultaneously, the robot's articulated arm picks up the proper nut, 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 two torches, which are positioned on either side of the gripper.

To maximize efficiency, the table is equipped with two sets of fixtures, allowing the operator to align and secure a second angle while the robot is welding nuts to the first. When the robot finishes welding, the operator again rotates the tabletop and replaces the assembly with a new angle, while the robot welds on the other side of the table.

Before the robot was installed, a single operator manually welded each encased nut to the angles. Bill DeVenny, welding engineer at Case, cites the advantage of automating, "Production rates are up 30 percent and reject rates are down 10 percent."

As a result, Case is looking at additional applications. "Arc welding is our primary interest," says DeVenny. "In relation to this need, we will be examining the potential of machine vision as a means to automate even further. Robots are definitely a part of our future plans."

For more information about welding with robots, circle E61.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Date:Mar 1, 1985
Previous Article:Software-based program development for programmable controllers.
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