Scientists are people who look past the usual answers and ask themselves questions, like: “Why should machine tools always be constructed the same way, just because it’s been done that way for decades?” Answers to such questions lead to new approaches and innovations in the industry. Customers are found in all branches of the industry – wherever conventional development methods fall short, because new problems demand an innovative approach.
Innovations in CNC technology has taken a new scientific approach to solve concrete problems faced in the industry, and one such is the X-Cut, a CNC solution with a difference, developed by Vienna University of Technology supported by B&R Industrial Automation. The CNC functions can be implemented on a general-purpose hardware platform together with the standard control and safety technology systems. This results in a significant increase in the performance; decrease in costs and at the same time protecting know-how of the developers.
New approaches to machine manufacturing
The search for new approaches to machine manufacturing will lead you directly to the X-Cut, a machine whose main spindle has complete freedom of movement on two axes to position itself in the available space. A third and potentially fourth axis can be added by moving the tool carrier laterally on the spindle or by moving the work piece carrier. The X in the name comes from the high level of parallel kinematics, with a parallelisation 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, moving 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 is used to make the triangle into an X. At any one point, the weakness of one pair of arms is compensated by the strength of the other pair.
The four differentiating factors
This construction has four advantages over a conventional approach:
Vertical tracks in which the arms move and give the machine a very small footprint, so it takes up less of the valuable floor space in a production hall
Construction of the machine gives it exceptional stability in the z direction, much more than conventional solutions
Stability in the x and y directions can be directly controlled by independently moving the two triangles and more or less “wedging” them to achieve extreme rigidity
Finally, compared to machines with a Cartesian axis structure, the kinematic translations of the X-Cut allow high accelerations of nearly 2g, thanks to the small amount of mass to be moved, which also increases energy efficiency
CNC path calculation
The core of this CNC system is the path calculation, which is performed on the CNC channel and requires close attention. Path calculation is performed stepwise on the CNC channel and can be broken down into separate functional units.
The CNC channel consists of five functional units. These include the interpreter, which interprets the CNC program; the path planner, which plans the path described in the CNC program; the cutter diameter compensation, which corrects the generated path; the dynamics calculation, which determines the path velocity; and the path generator, which converts the path information into cyclic set points for the individual axes.
Interpreter: The interpreter reads CNC programs and compiles the G-code they contain into a symbolic representation for further processing in the system.
Path planner: The path planner generates a geometric representation of the programmed path movements.
Cutter diameter compensation: The cutter diameter compensation function interprets the geometric representation as the outer contour of a work piece and uses the tool dimension data provided by the user to create a new geometric representation of the programmed path movements.
Dynamics calculation: The dynamics calculation creates a velocity profile to match the geometric description of the path movement, taking into consideration the limit values defined by the user (velocity, acceleration, jolt) for the axes and the path.
Path generator: The path generator represents the “cyclic processing” (real-time processing) functional unit of ARNC0 path generation. Its task is to generate output and set points for the axes involved. To do this, it uses the data it receives from the “background processing” functional units of ARNC0 path generation.
Machine control as a mathematical task
The control of the X-Cut’s parallel kinematics is therefore a task that goes way beyond sequential programming of individual movement steps. Rather than programming linear axis movements, the mathematical model of the kinematic transformation is used, and it can be extended for all necessary path corrections. The development environment allows the developer to program these transformations together with the application program.
The model-based development is based on the principle of simulation. A complex process is first depicted in an algorithmic model. Here, it is important that the processes that are necessary for and relevant to the development are contained in the model, because further development is based on the simulation model. During the test phase, continuous improvements due to the test results can be made on the model.
Since production of the complex individual components of the machine is expensive, simulating the system prior to constructing the prototype reduces engineering time and cost. This applies to the machinery construction, where the distribution of forces needs 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 using MATLAB / Simulink before it is allowed on the machine as a program. The target for Simulink provides the ability to generate the program code directly from the simulation model and transfer it to the controller. This saves time and increases safety. When there are changes, the finished program is downloaded on the machine only minutes after performing a successful simulation. In addition, it is done without potentially introducing new errors to the simulation result during programming.
General-purpose automation – the heart and brain of the machine
Gone are the days when a dedicated controller was needed for CNC control. General purpose automation controller has the necessary processing power to compute G-codes, interpret the commands and define a profile suited for any application. Using the real-time communication, such as, Powerlink has proven itself to deliver a much-needed latency in high-speed CNC operation. Not only does it require very little wiring and a small amount of switching cabinet space, it can also handle the high data throughput that results from the high dynamics and precision of the parallel kinematics, and from the fact that feedback and diagnostics data runs on the same network.
Vision out-of-the box
The milling centre doesn’t necessarily have to consist of axes boxed up in the Cartesian coordinate system. A little outside-the-box thinking at the Vienna University of Technology resulted in the development of the X-Cut, a highly dynamic three-axis processing machine that boasts of exceptional stability, high energy efficiency and compact dimensions. From the CPU to the motors, the sophisticated control concept was implemented using general-purpose automation technology from B&R and goes to validate a claim that for truly complex applications, one should look beyond customary solutions.