CNC systems are used to control machine tools and machining centers. The CNC system is mainly responsible for generating a relative movement between a tool (e.g cutting tool) and a workpiece. Therefore, the CNC system implements functionality to provide setpoints to a machine tool’s drives that realize the generated movement physically.
CNC systems are in most cases executed in combination with Programmable Logic Controllers (PLC). Whereas the CNC is responsible for the tool path generation, the PLC implements auxiliary functionality (mostly logical operations like activating lubrication at a certain time) and controls the peripheral devices.
CNC systems as a whole consist of several hardware and software components as illustrated in Figure 1.
- Computerized Numerical Control Kernel (CNC kernel): The CNC kernel realizes the core functionality of a CNC system. It implements functionality to generate the axes movement for machining workpieces based on a geometrical and technological description given in a CNC part program. Therefore it provides software functions for decoding, interpreting and processing data of a CNC part program, for path planning and setpoint generation considering given constraints and depending on the system’s realization for positioning the machine’s axes.
- Programmable Logic Controller (PLC): The PLC handles all logical operations of a CNC system. It is closely coupled with the CNC kernel and enables to realize complex process sequences that involve different actors and sensors of the machine next to the machine’s drives. The PLC may also realize monitoring and safety functions and processes input signals from certain devices or the user interface. PLC programs are often implemented according to IEC 61131-3.
- CNC-PLC-Interface: The CNC-PLC-Interface links the CNC kernel and the PLC. Both modules have to be synchronized and therefore are closely coupled. The CNC-PLC-Interface is often realized as shared memory to allow high performing data exchange.
- Machine Interface: The Machine Interface allows applications to exchange data with a CNC system. Thus, the CNC system can be configured, operated and monitored. The Machine Interface is used primarily by the User Interface (UI) but additionally by other applications like engineering and production management systems. Many CNC systems offer more than one Machine Interface connection point so that different communication mechanisms or protocols can be used at a time.
- (Graphical) User Interface (UI): The UI, mostly realized as graphical user interface (GUI) is a software application that allows operators and commissioning engineers to interact with the CNC system and therefore to configure, operate and monitor the CNC system as well as the complete machine tool. The UI is connected to the Machine Interface of the CNC system. It may be executed on the same system platform than the CNC or on a separate device.
- Fieldbus Interface: The peripheral devices and the drives of a machine tool are connected to the CNC system by a fieldbus. For that reason both CNC and hardware device provide a Fieldbus Interface for connecting one with another. Most fieldbus systems allow to communicate data in a cyclical, time deterministic behaviour, as well as in an asynchronous mode.
- Drives and Motors: The drive unit, consisting of drive and motor, realizes physically the desired tool path, generated by the CNC kernel. The drive implements among others feedback controllers for current, velocity and mostly for position, as well. Generated setpoints are used for controlling the motor. The drive itself is connected to the CNC by a fieldbus to realize a time deterministic communication behaviour.
- I/O-Modules and Actors, Sensors, Input Devices: Apart from the drives, other peripheral devices (actors, sensors and input devices) are connected directly or through I/O-Modules to the CNC by a fieldbus.
Machine tools and machining centers can constitute complex systems regarding their kinematic structure. To cope with this complexity CNC systems provide the possibility to fragment the control task by using a software construct organizing kinematic chains and additional moving machine tool components like tool changers. So called “channels” are responsible for controlling smaller portions of the machine tool kinematic and its additional components. In general, all axes within a channel are interpolated in common.
A CNC system provides data through the Machine Interface for external application systems like UIs, SCADA and engineering systems among others. This data has its origin within the CNC system itself or is collected from the drives and peripheral devices. All available data can be divided into the following categories:
- Parameter: Configuration data of the CNC system, hardware devices and communication mechanisms. Parameters do not have any requirements regarding communication behaviour.
- State data: States of the hardware devices, CNC kernel and PLC. States may be built within the PLC system itself by summarizing data.
- Command data: Setpoint values and commands that are communicated from the Machine Interface to the CNC system as well as from the CNC system to the hardware devices. The first do not have requirements regarding the communication behaviour, the latter are communicated cyclically from the CNC system to the appropriate devices.
- Process data: Actual values of the hardware devices. Values are communicated cyclically from the appropriate devices to the CNC system.
- Alarms: Events of the CNC System, e.g. by the CNC kernel, the PLC, but as well by the UI, informing about errors or other notifications.
- Files: Files are transferred from the Machine Interface to the CNC kernel or the PLC, maybe as well to the drives and peripheral devices holding configuration data (e.g. tool parameters) or process information in form of part programs or similar.
CNC systems may be integrated in a network comprising various systems for controlling and supervising the production. Hence CNC systems may be connected next to UI and engineering software applications to different systems like
- ERP systems,
- PDA/MDA systems,
- diagnosis and condition monitoring applications,
- further external UI applications, e.g. smart device applications or similar.
A CNC system can be integrated into a production network using the appropriate interface, mostly the Machine Interface. Due to the diversity of systems and system manufacturers standardized interfaces could reduce efforts for linking these systems. This Information Model aims at achieving precisely this objective.
The focus of the “OPC UA Information Model for CNC systems” is on data that is situated within the CNC kernel but not within the PLC of a CNC system. This results out of the main objective of this Information Model to standardize an interface that provides and enables to access clearly defined raw data. Hence this Information Model addresses applications like UIs, PDA/MDA systems, diagnosis and monitoring applications, but not necessarily MES or ERP systems as the two latter ones mostly need summarized data.
For PLC data and in a wider context for MES and ERP data, please refer to suitable standards and OPC UA companion standards like OPC 30000 (PLCopen) or OPC 10030 (ISA 95).