PLC and CNC with a PC style

When designers at Mannesmann Rexroth Indramat were faced with the design of their next generation machine controller, they eschewed the ideas of their earlier proprietary architecture in favour of a PC/AT- based architecture. While retaining the software compatibility of the earlier design, the new architecture offered a lower cost solution that offered customers the opportunity to customise the design with a plethora of off-the-shelf solutions, should they choose to.

The older design, dubbed the MT-CNC, comprised a proprietary rack that housed the CNC controller, PLC, I/O and axis processors. The subsystem communicated to an industrial PC based controller over an RS-232/485 link. In the new MT-200 design (Figures 1 and 2), all the cards have been built to an AT form factor.

A centralised PC card communicates with a PLC card (the MTS-P) and a CNC card (the MTS-C) that resides on the same AT bus through a dual ported memory design. Each CNC controller card has the possibility to be expanded through the use of additional PC 104 based daughter cards.

Each daughter card can control up to 8 axes, and three daughter cards can be added to the CNC AT-based controller card. While one might think that this would allow 24 axes to be controlled, in fact the system is limited to 20 axes maximum due to the limitations of the software. The PLC itself can also be expanded by the addition of one PC 104 based card that allows for interconnection to the users choice of Interbus-S or Profibus interconnection schemes.

HOW DOES IT WORK?

In operation, a DOS based front end runs on the PC/AT CPU card in the rack, while the CNC and PLC cards each have their own dedicated real-time kernels running on them. Programs can be developed for the PLC card using a number of IEC 1131-3 programming languages, such as sequential function chart, ladder logic, function block and statement list, while software for the CNC card is developed using the familiar G-code programming language. The PLC supports up to 8K I/O points and computing time is typically 2.5ms per 1000 instructions.

System integrators can build their own graphical user interface for the system using off-the shelf drawing packages that provide BMP, PCX extensions, or a proprietary package supplied by Indramat. The graphical user interface can represent the operation of the machine in a dynamic format, showing the user the operating statistics and variables as they occur.

This is handled as follows: after the developer has developed his code for the PLC, it is compiled and then executed on the PLC. At the same time, a map file containing a live dynamic record of the registers in the PLC (that might be required by the graphical user interface) is retained on the system hard disk in a map file. The status of the map file is continuously updated by the PLC when AT bus usage time is low. The DOS based CPU front end in the system uses the information contained in the map file to update the values of the icons displayed on the screen.

OLD VERSUS THE NEW

In earlier closed loop feedback CNC designs, positional feedback from a motor was sent to the CNC controller while velocity feedback information was sent to the drive. With newer digital drives using the IEC 1491 Sercos interface, the picture has changed: both position and velocity is processed by the drive, only position command data is sent to the drive, offloading the positioning task from the CNC controller. Now, the CNC is solely responsible for the processing of G-code blocks while the drive handles the positioning requirement. The Sercos position loop update time, in the case of the MT-200, is 0.25ms.

In addition to handling up to 20 axes, the CNC controller can also handle up to seven independent tasks as standard. Essentially then, the systems integrator could be able to control up to seven machines (depending on the number of axes of each) just using the one CNC controller. NC cycle time for the MTC-200 system is 2ms, independent of the number of axes being controlled and the block preparation time of the G-code is between 2ms to 4ms.

Because it is modular, the MTS-200 can be upgraded. The system integrator can choose from the level of processing power that is available in the chassis: be it 486, or Pentium based. A range of external options are also available, including a configurable operator panel, and a passive backplane I/O system.

Mannesmann Rexroth

Tel: South Cerney (01285) 863004