Computer Numerical Control (CNC) SYSTEM
Numerical control (NC) is a method employed for controlling the motions of a machine tool slide and its auxiliary functions with an input in the form of numerical data. A computer numerical control (CNC) is a microprocessor based system to store and process the data for the control of slide motions and auxiliary functions of the machine tools.
A modern NC machine tool is a microprocessor controlled and programmed using its own programming language. Various terms are used to describe the type of control such as NC, CNC, DNC.
Basic Configuration of CNC system
- Central Processing Unit (CPU)
- Servo control
- Operator control panel
- Machine control panel
- Other peripheral devices.
- Programmable Logic Controller (PLC)
Central Processing Unit:- Central processing unit is the heart and brain of a CNC system. This translates the part program which stored in memory to position control signals, it also check the movement of control axis or spindle whenever does not match with the program signal, a corrective action is taken. The CPU also checks whether all safety conditions on the machines have been full filled and whenever this does not happen. It takes necessary corrective action, if the situation becomes beyond the control of CPU is takes the final action of shutting down the machine.
All the compensations required for machine accuracy (like lead screw pitch error, tool wear out, backlash etc) are calculated by the CPU depending upon the corresponding inputs made available to the system. The same will be taken of during the generation of control signals for the axis movement.
Servo Control Unit:- Servo control unit is which receives the feed back signal of the actual movement of the machine tool slides from the feed back devices. For example: Digital encoder. And which passes this information to CPU for processing. The actual sense, the servo control units perform the data communication between the machine tool and CPU. And also achieved actual movements of slides on machine space tool through servo drives.
All movements controlled by the CNC machines and depending upon the type of feedback system used in the system.
Types of Feedback System
Closed Loop system
Open loop system
Operator Control Panel:- The below figure shows the typical Fagor control panel. Operator control panel provides the interface to facilitate two way Communication between the user and the CNC system, this consists of two parts.
- Video Display Unit
- Video display unit: Visual displays are the interface between controller and user, good informative displays are very important for error free machine operation. Early NC displayed only the current values of individual axes but user was at least able to see, from almost any position, whether the axes were still moving or already reached their target positions. This was major improvement over the mechanical scales normal on machine tools at that time.
- Keyboard:- The keyboard is the most common input device used for computers. It is, however, awkward for entering geometric data for computer aided design. It can be used as a string device, pick device, a valuator, a button or a locator and it is most flexible of all the input devices. The keyboard is usually used to enter commands. Commands may be to read the file name for a previously stored part drawing, to input the X-Y co-ordinates for a point or to describe some other geometry which is to picked or input into the computes. With the ASCII keyboard, the user enters the program block word by word, and the CNC system stores the program in its program memory. Errors are corrected in the same way with the aid of editor. CNC system especially designed for shop floor programming have between five and ten so called soft keys in addition to the normal keys. These soft keys are programmable to offer regularly used special functions and their functions being displayed on the VDU screen.
Machine Control Panel (MCP):-The features contained on the control panel of the MCU are the direct interface between the machine operator and the CNC system. The more common switches available on the control panel are listed below.
- Auto mode
- Manual data input mode (MDI)
- Reference mode
- Discrete jogging switches
- Block by Block mode
- Input / Output mode
- Other peripherals
- Jogging switches
Programmable Logic Controller [PLC]:- A PLC matches the NC to the Machine PLC’s were basically introduced as replacement for hard wired relay control panels. They were developed to be reprogrammed without hardware changes when requirements were altered and thus are reusable.
It is a general purpose device and it can be programmed to control a different types of machines. The programs can be very easily modified for a new jobs. The concept of hardwired in shift to the software based and the consequent replacement of Electromechanical devices by PLC improves reliability of Industrial controls and reduces maintenance effect greatly.
Configuration of PLCs:-
- Input / Output
- Programming device or user
- Controller:- It is a microprocessor based and it is constantly monitoring input and output signals from controlled devices and memory is provided for firmware that is manufacturers software and for a Erics Programs.
- Input and Output:- The circuit converts incoming signals from the controlled devices into the ones which are compatible with microprocessor. The output circuitry perform the opposite function of adjusting processor’s signals to those of the controlled devices.
- User:– It is generally a keyboard with a display device, many times it is possible to use a personal computer for programming the PLC.
- Monitoring:– This is an order to identifying faults in the NC machine tool, the machine and interface control at an early stages to largely prevent damages occurring to the work piece, tool or machine. If any faults occurs, first the machining sequence is interrupted and the drives are stopped, similarly the cause of the fault is stored and displayed as an alarm and at the same time the PLC is informed that an NC machine tool alarm exists in Hinumeric CNC system
The following can be monitored
- Position encoders and drives
- Spindle speed
- System program memory
- User program memory
- Serial interfaces
- Transfer between NC and PLC
- Diagnostics:- PLCs can perform self checking similar to computers and CNC systems with the help of in built firmware. Then can also be programmed to diagnose and identify faults in problem areas of Electromechanical devices like limit switches, transducers, relays, motor starters and variety of peripheral equipment, there capabilities result in lower maintenance effort and costs.
- Machine Data:- A CNC system is designed as general purpose the CNC interfaced to the machine by means of machine data. The machine data means specific information that are fed into the NC controller at the time of commissioning it on a particular CNC machine tool.