Application: Unusual CNC Design
Milling center has highly dynamic 3-axis processing with exceptional stability, high energy, efficiency, and compact dimensions, using industrial PC (IPC) and industrial Ethernet automation technologies.
Milling center design can extend beyond the traditional Cartesian coordinate system. Vienna University of Technology developed the X-Cut, a highly dynamic 3-axis processing machine with exceptional stability, high energy efficiency, and compact dimensions, according to the project team. From the CPU to the motors, the sophisticated control concept was implemented using industrial PC and industrial Ethernet technologies, proving their suitability for unusual tasks.
Scientists often look at the usual answers and ask new questions, such as, "Why should machine tools always be constructed the same way, just because it's been done that way for decades?" That was the approach of a team led by professor Friedrich Bleicher at the manufacturing laboratory at the Institute for Manufacturing Technology and High Power Laser Technology at the Vienna University of Technology. "The laboratory takes a scientific approach to solving concrete problems faced in industry," explained Falko Puschitz, the laboratory's project manager for mechatronics. "This ranges from manufacturing technology and production automation to calculation, construction, and even implementation of machine tools." Scientific-approach opportunities are found wherever conventional development methods fall short.
New machine manufacturing
The X-Cut machine design is one such new approach, where the machine’s main spindle has freedom of movement on two axes to position itself in the available space. A third and fourth axis could be added by moving the tool carrier laterally on the spindle or by moving the workpiece carrier. The X part of the name comes from the high level of parallel kinematics, with a parallelization degree of 2 for positioning the spindle. The spindle is located at the apex of a triangle formed by two arms, whose other ends are moved parallel in opposite directions on a common track, which moves the spindle in the x and y directions.
When this triangle is stretched to an extreme angle (very flat or very sharp) where the two arms become nearly parallel (what scientists call a singularity), rigidity is lost in one direction. For this reason a second pair of arms makes the triangle into an X (see image). At any point, the weakness of one pair of arms is compensated by the strength of the other pair. The design has four advantages over a conventional approach.
1) The vertical tracks in which the arms move give the machine a very small footprint, so it takes up less floor space.
2) It has exceptional stability in the z direction, much more than conventional solutions.
3) Stability in the x and y directions can be directly controlled by independently moving the two triangles, "wedging" them to achieve extreme rigidity.
4) Compared to machines with a Cartesian axis structure, the kinematic translations of the X-Cut allow accelerations of nearly 2g, thanks to the small amount of mass to be moved, which also increases energy efficiency.
Puschitz said, "The Institute for Manufacturing Technology and High Power Laser Technology has been working for years on new machine tool concepts, including testing and development of special kinematics such as parallel kinematics.” Quickstep and the Quickstep Neon, two tripod structures, have been presented at the EMO trade fair with Krause & Mauser Machine Tools. The highly parallel X-Cut structure offers “a clear step forward in relation to conventional structures," Puschitz said.
Math-based machine control
The control of the X-Cut's parallel kinematics goes beyond sequential programming of individual movement steps.
"Rather than programming linear axis movements, it's better to use the mathematical model of the kinematic transformation and also use it for all necessary path corrections," explained Puschitz. Using automation with a development environment allowed the developer to program the transformations. With many manufacturers of controller hardware, such functions must be programmed into the firmware. Algorithms generally are programmed into the hardware by the manufacturer. "This would have been unacceptable," said Puschitz, delaying each step of the project and passing valuable expertise to the controller manufacturer.
Since production of the complex individual components of the machine is expensive and public financing limited, the Vienna University of Technology team used simulations prior to constructing the prototype. With machinery construction, distribution of forces needed to be tested under all possible load scenarios using finite element analysis. It also applies to the open-loop and closed-loop control logic, which is tested with simulation software before it is allowed on the machine as a program. The automation software generates program code from the simulation model and transfers it to the controller.
"This saves us valuable time and increases safety," said Puschitz. After changes, “we can have the finished program on the machine only minutes after performing a successful simulation. And we do so without potentially introducing new errors to the simulation result during programming." Using one automation provider allowed easier integration, said Puschitz.
An industrial PC (IPC) is connected to smart I/O modules via industrial Ethernet. Additional scales connect to the controllers via a second control card. The modules perform the machine's switching commands, such as turn on motor, switch tools, and switch motor spindle. The machine does not yet have any protective equipment in place, but when the time comes, this can be integrated using the safe I/O components and programmed in the same development environment. The internal Ethernet network “has proven itself in several ways," said Puschitz, reducing wiring and switching cabinet space, while handling “high data throughput that results from the high dynamics and precision of the parallel kinematics, and from the fact that feedback and diagnostics data also runs on the same network."
Also connected to the CPU via Ethernet are the drive components. Compact and powerful servo drives provide maximum power with minimum volume. Embedded intelligence provides diagnostics and safety functions without external connection. Five three-phase synchronous motors have 31.6 Nm rated torque and 36.4 Nm stall torque. These are electronically commutated synchronous motors with excellent dynamic characteristics, positioning precision, compact size, and reduced weight.
The machine uses a CNC panel human-machine interface (HMI) with a hand operating device, developed primarily for milling centers. Equipped with operation functions designed specifically for CNC processing, the control elements are customized for user interface and are suitable for a visualization application programmed by the customer. This ergonomic combination of operating units is equipped with function switches and buttons designed for operation in a harsh environment, even by a gloved operator wearing gloves.
Vienna University of Technology
Vienna University of Technology: T.U. Vienna is described as one of Europe’s most successful technical universities. With over 20,000 students and nearly 2,000 researchers, it is Austria‘s largest applied science–technical research and educational institution.
The Institute for Manufacturing Technology at the Vienna University of Technology covers a broad range of production technology and machine tools. In regard to process development and the associated machine engineering, the institute is considered important for production technology research.
Automation technologies used in this application
- B&R Automation Studio development environment allowed programming of the transformations
- Matlab/Simulink was used to test open-loop and closed-loop control logic
- B&R Automation Studio Client for Simulink provides the ability to generate the program code from the simulation model and transfers it to the controller
- B&R CPU, motors, APC 620 industrial PC
- B&R X20 I/O interface modules
- Powerlink Ethernet protocol
- Acoposmulti drive components
- Five B&R three-phase synchronous motors from the 8LS series
- B&R CNC Panel with a hand operating device
- Safety could be integrated with B&R’s X20 Safe I/O components and programmed in the same development environment using SafeDesigner.
- Edited by Mark T. Hoske, CFE Media, Control Engineering; Posted by Kelsey Kirkley, Control Engineering, www.controleng.com