CNC controller

For a single machine tool, the CNC is a self-contained system which includes a special form of processor controlled by stored instructions for executing the essential numerical control functions. At least three programs compose the software of a CNC controller; the part program, the service program, and the control program. Regarding the part program, it is based on a description of the geometry of the workpiece being created and the cutting conditions. The service program aims at checking, editing, and correcting the part program. Finally, the control program receives the part program and generates signals in order to drive the rotational and translation axes (Pislaru, 2001).

The signal for moving the axis of a machine tool passes in sequence through the interpreter, the interpolator, the Acceleration/Deceleration controller. Finally is transmitted to the position controller for minimizing the position error. The three-tier CNC system architecture portrayed in the Error!

Reference source not found. represents the general architecture for the axis control module of a

CNC machine tool and consists of the following modules (Suh, Kang, Chung, & Stroud, 2008):

- adaptive control module - error compensation module - interpolation module - spindle control module - servo control module

47 The adaptive control module produces optimized cutting conditions such as spindle speed and feedrate. The error compensation module performs the compensation of error factors which cause deviation from the programmed path. Such errors can be the volumetric error of CNC machine tools, the heat from moving elements, tool wear and tool deflection. The interpolator receives the optimized feedrate from the adaptive control module and compensated position instructions for each axis from the error compensation module and sends axial commands to the servo control module. The spindle speed is optimized by the adaptive control module and is transmitted to the spindle control module. (Suh et al., 2008).

Although a fully functional CNC system includes the various control modules which have been described above, the majority of CNC systems consist of control systems containing only the interpolation module and the servo and spindle control modules (Suh et al., 2008). The fact that the CNC controller of the studied five-axis gantry CNC machine tool is a digital computer yields a lot of advantages such as the possibility to access the control and compensation of loops through time sharing. The required control response can be achieved by changing parameters in software. Supervisory applications, such as scheduling required applications, can be executed by the controller.

CNC CONTROLLER SINUMERIK 840D SL

Milling machines represent a large category of machine tools with many variations and configurations available to meet the needs of complexity, capacity and access. Milling machines have capability to machine various surfaces by feeding the work piece against the rotating cutter to enable the machining of closed angles and contoured surfaces. A GEISS 5-axis gantry CNC high-speed milling machine tool (described in methodology section - Research Methodology has been used as the case study in this research. The controller of this machine tool is the SIEMENS SINUMERIK 840D solution line. The machine tool controller processes the part program and realizes its conversion into signals which control a number of actions of the CNC machine tool. In addition, the CNC controller can generate the position demand signal from input instructions entered manually from the operator of the machine tool via the Machine Control Panel (MCP) which also contains data displays for the machine operator (Boothroyd & Knight, 2006).

The SINUMERIK 840D integrated into the SINAMICS S120 and SIMATIC S7-300 automation system forms a new complete digital system. The SINUMERIK 840D sl has a high number of applications for high-speed cutting that are managed by the Numerical Control Unit (NCU), which combines Numerical Control kernel (NCK), HMI, Programmable Logic Control (PLC). The corresponding HMI Human Machine Interface software is already integrated in the NCU software for operating, programming, and visualization. Regarding the connections, DRIVE-CLiQ cables from Siemens are used. The typical topology of the SINUMERIK 840D sl complete system is illustrated in the following Figure 3-2 (Siemens, 2012).

48 Figure 3-2 Typical topology of the SINUMERIK 840D sl complete system (Siemens, 2008)

Technical specifications for the SINUMERIK 840D sl with NCU 730.1 are presented in Siemens (2008). The drive-end computing performance for the SINAMICS drives within the SINUMERIK 840D sl can be increased with the numeric control extension NX10.

DRIVE-CLIQ REAL-TIME SERIAL DIGITAL INTERFACE

The power supply with required system components are connected via a series of interfaces of the control unit. The components of the SINAMICS S120 are connected with the control unit via DRIVE- CLiQ - depicted in the Figure 3-3. Drive click needs to be used to integrate Siemens terminal module. TM41 allows the encoder position actual value of another drive is output as incremental encoder emulation position measurement. TM41 can be implemented in the real time hardware in the loop system.

49 According to manufacturer’s manual, the rules for wiring DRIVE-CLiQ are the followings:

- Ring wiring is not allowed.

- Individual components must not be double-wired. - Up to eight nodes may be connected in one row.

- A maximum of one line module, six motor modules, and three direct measuring systems can be connected to one control unit.