Trajectory optimization on the automobile press line:

The CNC PC control as a pacemaker

The PC is generally considered to be a universal machine, but does this also apply in industrial automation, where separate PLC and CNC control-lers are firmly established? SCHLEICHER is stepping up to the mark to prove this with an integrated controller concept based on the ProNumeric IPC system. The technological solution is completely new in the press control and will be implemented when the Bernsbach press plant (supplier for the automobile industry) based in the Ore Mountains is modernized. The solution is based on a combination of quick linear feeders, trajectory optimization with the CNC control integrated in the IPC and also IPC-based visualization. The PC is also the "communication center" that acti-vates the field bus (CANopen), Ethernet and Sercos. The concept ensures quicker handling, safe, balanced operation of the presses and an increase in flexibility to ensure that the press plant can meet the increasingly de-manding requirements of the motor industry (just in time, express orders) even more reliably.

Flexible Specialization

It should be possible to operate each press individually or in a sequence while being able to process the entire spectrum of parts. The key is "Flexible Spe-cialization": After all, the decisive advantage small specialists have over the large mass producers is their flexibility. They are more customer-oriented and react more quickly to changes in the market. However, until now it has only been possible to gain this advantage by lowering productivity. The automation solution presented here allows the production of both small series and medium-sized to large volumes and virtually cancels out any disadvantages there have been until now.

Fig.: 1

Press line Bernsbach

Linear feeder and trajectory optimization 

The main feature of the hardware used in the new handling solution at the press plant in Bernsbach are STROTHMANN linear feeders, which each have three degrees of freedom. One of these feeders is installed on both the feed and the discharge side. The linear axes of the feeders driven by servo motors reach accelerations of up to 2.5 m/s at speeds of 150 m/min. The trajectory control optimizes the movement of the feeders, which already reach considerable speeds. Motion Control is managed by an IPC-based CNC, which interpolates nine NC axes per feeder (eight servo axes and the leading axis) – a task to which a CNC controller is much more suited than a PLC. Outside the press, the CNC funs free. The press only needs to stop when there are particularly com-plicated insertion operations. Master encoder, feeders and I/O modules are networked with the controller via CANopen.

Fig.: 2

Position of the linear feeders on the press

The CNC data and simulation of the motion sequences are calculated in ad-vance. Afterwards, the supporting points for the sheets are reduced and an optimized NC program is generated using the SCHLEICHER NERTHUS proc-ess (data reduction with freeform contours). The graphic simulation of all com-ponents involved (press, tools, work piece and feeder) helps achieve a reliable press cycle with quick and accurate movement sequences. The executable programs are transmitted via a network to the ProNumeric controllers, which then interpolate the motion sequence.

The press cycle is also optimized:

The CNC controller minimizes the residence time of the feeder in the press, which means that the withdrawal feeder is already retracting while the press is still opening. As soon as it has removed the blank and exits from the press, the loading feeder follows it in from the other side as close behind as possible. While still exiting, this feeder initiates a new sequence through movements co-ordinated with one another, resulting in a lift time close to the gross rated power of the machine.

NERTHUS – Data minimization with freeform contours

The Schleicher NERTHUS process reduces the number of supporting points and therefore the NC sets for any freeform contours, subject to an arbitrary contour tolerance. The motion sequence can be simulated on a standard PC. The CNC control in turn generates the freeform from these few NC sets using the Online Curve Interpolator (OCI).

Fig.: 3

Example of data minimization with NERTHUS

A constant path with no jolting is therefore achieved at an optimized axis speed. The three axes of the entry feeder and the three axes of the discharge feeder as well as the two of the loading feeder are linked to the "upright shaft" on the press using the program. For this purpose, the key value encoder is read in the interpolation cycle via the CANopen field bus and the axes are synchronized with one another, depending on the position and speed of the press. The five individual presses can also be run in combination as a press line. Here it is possible to link the entire network with a higher level virtual master encoder to attain the best overall production speed.

The IPC in the center of the controller configuration

An IPC offers the advantage of constructing solutions based on hardware and software stan-dards, which are then easy to integrate into communication structures. However, the condi-tions for PC use are not exactly simple, especially in a press plant. In the case at issue, the control center comprises a ro-bust industrial PC with a high performance 400 MHz Celeron processor without a fan and an integrated USV. Software secu-rity with simultaneous openness is achieved by combining the VxWorks and Windows NT4.x operating systems. A connec-tion with the CANopen field bus is made using a plug-in card. There is also a Sercos interface on this card, to which the In-dramat digital servo frequency converters are coupled, as well as the MMU for generating inter-rupts for VxWorks. The I/O pe-ripherals are connected via the RIO field bus coupler and  CANopen, the command level via Ethernet and TCP/IP.

Fig.: 4

Controller layout

ProNumeric = IPC + PLC + CNC 

The ProNumeric system works with a soft PLC to avoid the additional expense of adding a co-processor and in any case, processors today usually have suffi-cient power reserves. The CNC is responsible for motion control, whereas the PLC running in parallel controls and monitors all the peripheral devices. The PLC is programmed according to IEC 61131-3 with AWL, FBS, KOP, ST and Sequential Function Chart (SFC). PLC and CNC functions are directly coupled with one another in the interpolation cycle. The graphic programming system Prodoc Plus is used for programming. The CNC part of the ProNumeric reaches an interpolation cycle of 1 ms with 8 NC axes and offers up to 64 axes. Pro-gramming covers the extended scope of functions according to DIN 66025.

Fig.: 5

Switchgear with drive engineering

Task distribution between two operating system levels

ProNumeric is based on the two operating systems Windows NT4.x and VxWorks and combines the advantages from both areas. Each operating sys-tem has its own memory area. An MMU uses a non-maskable interrupt (NMI) to ensure that VxWorks is regularly activated to work through time-critical tasks.

This process guarantees a strictly deterministic time response and the hard realtime capability of the PLC runtime and the CNC functions. A communica-tions buffer is used to synchronize CNC and PLC.

Windows is used for the non time-critical tasks, such as the visualization and processing of operator dialogs for online manuals and standard programs. The visualization package VisiWinStudio by INOSOFT seamlessly fits in with the universal press automation structure. Internal communication is achieved using OPC-Client/Server mechanisms; the modular visualization is compiled from ActiveX components, which means that integration in higher priority (ERP) sys-tems is possible without any problems – the IPC with standard Ethernet con-nection plays to its strengths here. In addition, use of the Windows-based con-figuration tool Prodoc Plus makes the adaptation to new tools much simpler and results in reduced set-up times that increase productivity.

Fig.: 6

Internal design of the ProNumeric controller

Conclusion

When the Press Center Bernsbach was modernized, the concept of a continu-ous IPC-based controller was implemented in combination with an efficient handling solution. The speed improvements attained in production change-overs, the wide spectrum of processed parts and the actual increase in produc-tivity underline the strengths of the IPC as a standardized base for flexible and open control and communication solutions - also in respect to other possible applications.