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Fast gantry robot stacks bumpers.

Fast gantry robot stacks bumpers A Cartesian gantry robot capable of linear motion as fast as 10 fps (3 mps) has automated the formerly labor-intensive task of stacking automobile bumpers after manufacture. The robot was placed in service in early 1988 as the final station in a flexible, automated, bumper-forming and welding line at Deco Automotive, Rexdale, Ontario. Deco Automotive, a division of Magna International (Canada) Inc, is a leading producer of bumpers for Canadian and US automakers.

Believed to be the fastest robot of its type and size yet developed, the five-axis machine picks up a completed bumper and places it into a shipping bin in an eight-second cycle. At present, the robot accommodates four different stacking patterns for four different bumper configurations. However, no practical limit exists. Switching between bumper types requires only a five-minute software and gripper change over.

Flexibility dictated design

The robot was developed by Trans Automated Equipment (TAE), Concord, Ontario, another Magna International company. "Our customer selected a robot rather than a dedicated stacking device to accommodate a variety of bumper styles and stacking patterns," comments Michael VanderVeen, TAE project engineer. "Had the line been designed to produce just a single bumper style, it would have been much less expensive to build some sort of pushing equipment rather than a gantry."

The robot operates by picking up bumpers--one at a time--using two mechanical grippers. (Bumpers arrive from the manufacturing line's welding station individually on chain conveyor fixtures). Once lifted by the robot, each bumper is quickly brought to a pallet bin, lowered, and released. Individual bumpers typically are staggered slightly to nest together better. Alternating layers usually are rotated 180 degrees to improve container stability.

Stacking patterns are customized using the robot's teach mode. Placement of individual bumpers or layers of bumpers may be shifted from the one preceding by any angle between 0 and 360 degrees and between any linear (X, Y) coordinates. To permit continuous stacking, the machine alternates between two bins; while one bin is being filled, the other is exchanged for an empty. The Cartesian robot is generic in design and could be modified to stack other products as well.

Servos move minimum mass

"We are always fighting weight when designing high-speed gantry robots," Vanderveen states. "To minimize inertia, every moving component must be as light as possible. The Deco machine, for instance, makes extensive use of aluminum."

At 680 kg (1499 lb), the overhead bridge (tandem X-axis travel) is the heaviest moving component. The carriage that traverses the bridge (Y axis) weighs only 225 kg (496 lb) yet contains drives for raising and lowering (Z axis) and rotating (O axis) the grippers that pick up the bumpers. Vertically moving elements total just 110 kg (243 lb). VanderVeen says the dynamic parts are extremely light for a gantry robot having a working cube of 3.2 m x 2 m x 1.7 m (10.5 ft x 6.6 ft x 5.8 ft) corresponding to the X, Y, and Z axes, respectively.

For powering the robot elements, TAE compared the capabilities of both AC and DC servomotors having resolvers and working through 6:1 cycloidal reducers on four of the axes and a worm gear on the fifth (Z axis). An absolute resolver system was desired to preclude rezeroing when electric power is lost.

Engineers selected five Modicon Cyberline brushless AC servo drives and amplifiers, a Modicon 3220 eight-axis flexible automation controller, and a Modicon TR-130 control station, all supplied by Modicon (Canada) Ltd, Mississauga, Ontario, and Modicon Motion, Pittsburgh, PA. This equipment proved ideal because of its high speed, tandem drive synchronization, and available Cyber robot operating system programming (C/ROS).

The eight-axis closed-loop controller combines the power of an industrial computer with the flexibility of a programmable logic controller. All servo compensation and program parameters are entered digitally with a few keystrokes at the control station. A menu-driven operator interface minimizes training and errors.

Robot programming speeded

C/ROS is a Modicon-developed programming environment for applying the Model 3220 in robot systems. "When writing a teach-mode robot program from scratch, it is imperative that some sort of programming aid be used as a shortcut," VanderVeen relates. "Otherwise, programming time is prohibitively long. As a small OEM, we have neither the time nor the manpower to develop every detail. Further, had we written a dedicated program, Deco probably would need to call us every time a program change was desired."

C/ROS permits users of the eight-axis controller to quickly create, maintain, and execute robot function sequences. Its English phrase, menu-driven executive continuously monitors machine operations in real time. Should problems occur, a trouble shooting guide displays fault conditions to speed corrections. Program changeover by tape takes only five minutes.

Speed without vibration

Critical to the design of the robot is the tandem drive system for X-axis motion. Two synchronous drives, one at either end of the bridge, generate parallel motion through racks and pinions. They reportedly are superior to a single, large motor with shafts and gear boxes. Avoided are the shaft resonance and vibration seen with a high-speed single-motor drive. Also, the design saves weight.

"The two motors must be synchronized very closely with an absolute, resolver-based, true tandem control such as the 3220 controller provides," the project engineer reports. Bridge design speed is 10 fps (3 mps), about 70 percent of its maximum capability. Speed is limited by acceleration and deceleration forces on the gantry frame rather than by the power of the drives. Everything possible is done to smooth velocity changes through the tuning parameters of the feedback loop. Acceleration and deceleration are nonlinear to maintain point-to-point coordination of a straight-line path on diagonal moves.

Other features

To minimize the weight of vertical (Z-axis) components, the only moving elements are a square, vertical support beam with its toothed rack plus the bumper gripper bar. Both vertical and rotary (O-axis) motions of the gripper bar are packaged together. The servodrive for vertical travel mounts on a plate that rotates with the vertical beam. The drive rotating the plate mounts on the bridge. The packaged design obviates the need to lift the rotary drive.

"To assure positive pickup, bumpers are positioned very accurately when they arrive from the welding station," VanderVeen adds. "Further, zero position for the robot is the bumper-pickup position. Because the robot stacks bumpers, laydown positioning is not critical."

Random X, Y laydown point accuracy is plus or minus 5 mm; repeatability, plus or minus 2 mm. The Cartesian gantry robot is said to be capable of significantly greater accuracy through application of low-backlash gear reducers or precision ball screws.

TAE is investigating the same robot technology for palletizing boxes of mufflers in variable patterns according to muffler type and size. This application may require a second, independently operating bridge to boost output. Another possibility is waterjet cutting, a job that would make use of most of the eight axes of control available in the Modicon 3220 servo controller because of the need for wrist action at the end effectors.

PHOTO : View from above of bumper stacking cartesian gantry robot designed and built by Trans

PHOTO : Automated Equipment, Concord, Ontario, and installed at Deco Automotive, Rexdale, Ontario.

PHOTO : Four of the five Modicon Cyberline AC servomotor drives that power the robot are visible.

PHOTO : In the foreground, at either end of the bridge, are the tandem drives for the bridge (X

PHOTO : axis). Nested between the bridge supports are the gripper bar's rotary drive (O axis) on

PHOTO : the left and the carriage drive (Y axis) on the right (out of sight).

PHOTO : The upright drive on the carriage produces the gripper arm's vertical motion (Z axis).

PHOTO : Note that a bumper is held just above a chain conveyor that brings bumpers to the robot.

PHOTO : on the mezzanine above the bumper line's welding station are Brian DeMoe, Modicon (Canada)

PHOTO : Ltd, (left) and Bo Lindgren, electrical planning manager. Trans Automated Equipment. In

PHOTO : the cabinet behind them is the Modicon 3220 eight-axis flexible automation controller and

PHOTO : Modicon Cyberline servo amplifiers for the servodrives.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Publication:Tooling & Production
Article Type:product announcement
Date:Jul 1, 1989
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