Harmony at high speed

Photo | Bosch Rexroth AG
Marginal Column

Photo | Bosch Rexroth AG

Three articulated arm robots insert the blanks in the part carriers. Spring action holds them exactly in place there.

Photo | Bosch Rexroth AG

A 3D scanner controls the laser beam as it passes across the component.

July 2012

 

ERLAS builds a fully automatic manufacturing system for the automotive industry, incorporating laser cells that combine various joining technologies. This multi-technology automation solution made by Rexroth ensures that everything meshes perfectly, even when cycles are short.

 
 

The orange-colored robots turn on their own axes, quick as greased lightning, to pick up and position tiny components. Conveyor belts resupply as necessary. Flashes of light flicker in a machining cell. There, optical scanners guide the laser beam as it welds parts together. Crimping, clipping in place, laser welding. Seven individual parts are joined at a tier 1 automotive supplier to create a safety-critical assembly for steering systems. That is all done fully automatically, in less than seven seconds. 54 real axes and five virtual master axes make this complex sequencing possible.

The IndraMotion MLC control concept made by Rexroth monitors all the processes. “In addition to the extremely short cycle of 6.7 seconds for the components – to be welded, tested and placed in small product carriers – the customer had specified that no more than two operators be required,” is how Dr. Eng. Bernd Pögel of ERLAS Erlanger Lasertechnik GmbH describes the demanding assignment. As the engineering manager for this project, he designed the control concept. The result is a finely tuned combination of joining technologies, executed by seven articulated robots and two laser welding cells.

Everything under control

The system carries out a large number of process steps in a very brief period. “It quickly became clear to us that we could not achieve the required cycle times with controls that worked sequentially,” Pögel recalls. The solution: Controls regulate various motion sequences in parallel and, when a parameter changes, automatically adjust the other axes involved. “The motion sequences for the robots have to be integrated into the materials flow pattern so that no interruptions occur when there are changes in the speed of the master axes,” is how Pögel describes the requirements for the controls.

The IndraMotion MLC system made by Rexroth satisfies these needs. It is based on a PLC complying with IEC 61131-3 and the PLCopen libraries of functions. Acting via the sercos automation bus, the hardware coordinates 59 axes in real time. The FlexProfile software function modifies all the motions involved whenever there are changes in the parameters and takes account of timed external parameters.

Quick and sure joining

The manufacturing process for the component groups begins with automatic positioning of the individual parts in the system. A machine with two synchronized hydraulic axes, set up to develop 50 kilonewtons of process power, crimps the punched parts together while vibratory conveyors feed other components to the system. Three articulated arm robots with six axes lay those blanks in a carrier where they are precisely positioned by way of spring force. An intelligent system, comprising conveyor systems and shuttles, moves the carriers to the lift unit just ahead of the two laser machining cells, which are served by overhead feeders.

Then the 6 kilowatt disk laser and its 3D scanner go to work. It welds 24 seams at each part, with a total length of 224 millimeters. And it needs only 4.9 seconds to do so, including the movements needed to orient the part carrier. While processing takes place in the first cell, a finished assembly is removed from the second cell and a part carrier holding a component to be processed is positioned. A diverter transfers the laser beam between the two cells, increasing the laser’s duty cycle to almost 100 percent.

Zero tolerance: The component fits perfectly

In the next step, five tiny plastic parts are clipped in place on the welded component. Vibratory conveyors separate the parts, the pick-and-place units put them on the grab fingers which chuck the parts in place, and with a swinging movement press them into the welded component. “According to the drawings, the tolerance here is zero,” Pögel explains. Ten servo axes, which follow a virtual master axis, ensure that everything seats precisely. Then the assembled part is checked for dimensions and measurements relevant to proper functioning. Ten additional servo axes support fully automated quality inspection and marking. “Using Rexroth automation components made it possible to drastically reduce the amount of effort invested in engineering and assembly,” Pögel stresses.

Integral safety

Rexroth supplies not only the controls, but the IndraDrive servo drives, linear conveyor components and pneumatics, and all the hydraulics, as well. As regards the control systems, ERLAS also banks on electronic components made by Rexroth, right down to the control panels. “In this way were able to slash the number of suppliers and now have a single contact person, responsible for most of the components and interfaces,” the head engineer notes. The multi-technology solution saves time for the laser specialist and his customers time – in two ways. When compared with sequential function chart programming, ERLAS reduces the cycle time by a factor of three and, in addition, shortens the throughput time between order receipt and transferring the system to the customer.

Seeking support during commissioning, ELRAS contracted parts of the work to Fritz Automation GmbH, the Rexroth systems integrator. That gives ERLAS the assurance that everything will run fault-free, around the clock. The IndraDrive units are equipped with “Safety on Board”. The advantage: ERLAS can separate out the safety functions for individual axes and for defined zones. The sequences of motions at various virtual master axes can thus be modified and adapted simultaneously. Dynamization of the IndraDrive power units is effected online, in the background, and avoids any downtimes during fault analysis. Thus the system will join components reliably and quickly – 24 hours a day.