If you’re a global automation supplier with a product line as broad as Bosch Rexroth’s, it makes sense to shoulder some of the engineering burden for customers by melding those diverse technologies into creative designs.
That’s just what Bosch Rexroth’s automotive team did in developing a prototype Programmable Lift Platform (PLP), which it is now demonstrating to the auto industry.
In a simple, 3-axis gantry design, the PLP offers lifting capabilities now performed with expensive pallets and robots. Designed for versatile duties, along automotive assembly lines, these programmable platforms integrate a wide range of Bosch Rexroth components, including: ball screws, linear elements, PC control and electric servo motors and drives.
“The PLP concept came as a request from one of the automakers,” says Rodney Rusk, automotive industry manager for Bosch Rexroth Electric Drives and Controls. “They were looking to incorporate flexible tooling so they could build multiple vehicles on the same assembly line.”
Manufacturing to Meet Customer Demands
With so many models and custom features now being offered, this move toward a “flexible body shop” has become a major trend in the auto industry, adds Rusk. Automakers now need to employ nimble strategies to allow faster changeover in vehicle production. As a result, they need new technologies that will reduce their dependence on hard tooling.
Some older lines, for instance, still use expensive pallets to hold specific body parts in place for different cars, or the parts may be held in place by complex programmable robots. In such setups, four fixturing robots might be programmed to lift a vehicle body for spot welding. If the next vehicle were a different model, the robots would need to be programmed to accommodate the new model.
In contrast, the new PLP is specifically designed for lift and locate applications and does not need more than three programmable axes of X, Y and Z. So it is far simpler to program and implement into an assembly line. The number of PLPs needed on the job is based on the weight and size of the part or body being handled. A full car body may require a minimum of four PLPs in one work cell, while a fender application might require just one or two PLPs.
Rusk notes the flexibility of this novel lift platform allows multiple vehicles or parts to be processed on the same production line without any downtime to reset parameters. Along the “body in white” line, the PLP concept could be applied as an integrated cell, with the ability to move locking pin points on each PLP to adjust for multiple body or component sizes without resetting parameters. For example, the PLP could be programmed to lift a vehicle so that a welding robot could perform its tasks on car underbodies, fenders or frames.
The PLP could also be used to lift and locate components such as fenders into an assembly station, where a tightening power head could be moved into position to attach it to the body of car. The lift could be programmed to automatically change locking pin parameters to the proper position according to the next vehicle coming down the line. The change for different body types could be calculated automatically by bar code scan, or the operator may make the body type selection from a handheld pendant.
For a flexible body shop, the range of body styles one line can typically accommodate is no more than four. Six PLP units could handle the needs of that line, notes Rusk, all of them operating under the same Rexroth controller. This represents a significant reduction in the number of control components, along with related costs and space requirements.
Bringing It All Together
What was the takeoff point for developing the PLP? “We started with the Rexroth ‘LOSTPED’ concept,” says Richard Vaughn, robotics product engineer for Bosch Rexroth Linear Motion and Assembly Technologies. “This involves analyzing the customer’s application for Load, Orientation, Speed, Travel, Positional Accuracy, Environment and Duty cycle.”
Application engineers in the Bosch Rexroth linear motion and assembly technologies group then began working with these design criteria. Concurrently, engineers in the electric drives and controls group tackled motors and motion control challenges.
Because of the volumetric accuracy and the required repeatability in a three-axis plane, engineers choose the company’s most accurate TKK ball rail tables in a Cartesian configuration.
The load that the PLP needed to move was a maximum 113 kg, to be carried along the Z axis. The engineers needed to design a post configuration for the Z. Other requirements included a maximum speed of 300 mm per second and 300 mm of travel on the Z axis, along with just over 200 mm of travel on both the X and Y axes. Positional repeatability was just under 1 mm.
“There was also a volumetric accuracy spec we needed to meet where we had to account for a programmed stop position with settling time,” says Vaughn. “We also had to consider the weld slag within the environment where the lift would be used, and the fact that the lift would be operating 24 hours a day, six days a week.”
Based on the sizing of the actuators to handle these payloads, engineers selected the Rexroth MKD servo motors and Rexroth IndraDrive servo drives. The flexibility with IndraDrive allows operators to configure for more coordinated motion (up to five-axis capability as requested during specification). It also allows connection to variety of communications networks, such as SERCOS III, Profibus and Ethernet. The drive platform also incorporates Bosch Rexroth’s IndraMotion for Handling, which is basically onboard PLC.
In addition, the PLP application called for controlling eight of these cartesians at a time, which meant a maximum of 24 axes. And all Z axes in the system needed to be synchronized together. For the operator interface, engineers selected the Bosch Rexroth BT0V HMI and worked with the electric drives and controls division to create screens and do the programming.
Mechanical features of the system included four runner blocks per carriage. Engineers originally used Class 7 ball screws, but changed that spec to class 5 to increase volumetric accuracy. They also opted for double-float bearings for high speed and thrust capacities. Because of the weld slag in this application, the design features weld-resistant bellows.
To reduce the PLP’s footprint, the motors were mounted to standard side drives. The Z-axis motor also includes a brake. For extra design touches, engineers added a base on which to mount the system, as well as lifting rings to help install the lift in a facility.
As for design tools, the engineering team relied on Pro/Engineer3D solid modeling software for overall system design. They also used Bosch Rexroth IndraWorks, which offers commonality in an engineering software package.
“IndraWorks allowed us to keep to one software suite, instead of using separate packages to handle programming, commissioning and other needs,” says Vaughn.
The Urge to Integrate
While the PLP is still in the concept stage with prototypes under review by automakers, Bosch Rexroth believes that such integrated automation designs will be a bigger part of the company’s future.
“Customers – especially automotive customers – are becoming so lean now that they are outsourcing a lot of their engineering work,” says Rusk. “More and more, they are seeking a supplier that can engineer a product solution to help save them time and money.”
Rusk adds that mechatronics design also is a key part of the company’s strategy. “We call it the drive and control philosophy, where we offer multiple technology options under one roof,” he says. “Integrators and end-users are demanding that everything come from one source. If we can integrating technologies, we can give the customer one part number for an entire system.”
Bosch Rexroth distributors are already offering customers several versions of these integrated packages. Among them: Cartesian Motion System with IndraMotion for Handling; Easy-2-Combine pneumatic systems; linear motion and electric control subassemblies for semiconductor production; and the camoLINE handling system that combines linear motion, pneumatics and electric.