Rexroth gives car manufacturers a lift

14 November 2008

As the automotive industry becomes more competitive, car manufacturers are increasingly looking for ways to boost efficiency, reduce costs and enhance productivity.

Rodney M. Rush and Richard Vaughn, of Bosch Rexroth, site an example where a Rexroth Programmable Lift Platform (PLP) has boosted flexibility at an automotive plant.

The car manufacturer in question used a PLP to lift and locate a bumper into an assembly station, where a tightening power head is then moved into position to attach it to the body of a car.

For lift and locate applications, using a six-axis robot is unnecessary, since no more than three-axis of motion are required. Furthermore it is inappropriate, as it does not provide the required payload and accuracy. The PLP can perform these applications both efficiently and effectively, and is simpler to program and implement into an assembly line. Almost like a linear robot, the PLP is a lift and position platform. Instead of just lifting the car, the X- and Y-axes are used to position different end-effectors such as a weld gun or fastening tool.

Because of the volumetric accuracy and the required repeatability in a three-axis plane, Rexroth’s TKK ball rail tables were used in a Cartesian configuration. Post configuration of the Z-axis was necessary to accommodate the maximum payload of 113kg. The specified speed was a maximum of 300mm per second, with 300mm of travel on the Z-axis and just over 200mm on both the X- and Y-axis. Positional repeatability of less than 1mm was achieved, and the volumetric accuracy accounted for a programmed stop position with settling time. Bosch Rexroth also considered the weld slag within the environment where the lift would be used and that the lift would be operating continuously, six days a week.

Based on the sizing of the actuators to handle the heavy payloads, Rexroth’s MKD servomotors and IndraDrive servo drives were used. The flexibility offered by IndraDrive enabled the Rexroth engineers to configure for more co-ordinated motion, up to five-axis capability as requested during the initial specification. It also enabled the final PLP system to connect with all the standard communications protocols, including SERCOS III, Profibus and Ethernet. Rexroth’s IndraMotion was used in the drive platform for handling operations, which, in essence, allowed it to perform local logic algorithms.

To achieve the maximum 24 axes, the final design needed to control up to eight Cartesians at a time, In addition, all the Z-axes on the system were synchronised. For the operator interface, a teach pendant was used alongside Rexroth’s BT0V HMI.

Mechanical features of the system included four runner blocks per carriage. The runner blocks were 8 per cent pre-load, to achieve the required high stiffness. In the original design class seven ball screws were used but the specification was modified to class five in order to increase the volumetric accuracy. Double-float bearings were also used to achieve the necessary high speed and thrust capacities.

Rodney M. Rusk is automotive industry manager at Bosch Rexroth, Electric Drives and Controls.
Richard Vaughn, is robotics product engineer at Bosch Rexroth, Linear Motion and Assembly Technologies.

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