Computer Numerical Control is a numerical control system in which a program is stored in the PC to perform the basic and advanced numerical control programs.
The Computer Numerical Control is referred as a soft wire Numerical control, in which the machining processes is used in the manufacturing division with the help of the computer, the machine tools or work piece are controlled. The input parameters are depth of cut, feed, speed, spindle rotation on or off and coolant on or off.
In the CNC system the control signal are in the binary word format. The words consist of fixed number of bits, which include 32 or 64 bits commonly used. In the controlled axis each bits produce data on basic length unit.Advertisement:
Components of CNC:
The basic components of the CNC system are
- Input devices
- Part program
- Machine control unit
- Machine tools
- Driving system
- Feedback devices
The input devices used in the CNC system are
- Floppy disk Drive
- USB flash drive
- Serial communication
- Ethernet communication
- Conversational programming
Floppy Disk Drive:
It is a small magnet; additionally it acts as a storage device for CNC inputs. In 1970 it provides most collective storage medium like transferring data, data handling, read and write ability, storage size etc. For proper programming of the data the floppy disk is easily edited to provide the better result. The floppy disks are sensitive because of the large magnetic field. On the shop floor scratches and dust particles are occurred.Advertisement:
USB Flash drive:
USB Flash drive is a hard drive with compact size and having high storage size when compare with the floppy disk.it is a removable and rewritable hard drive. It is used to transfer the data from one place to the other place. Data which is stored in the flash drive are resistant to the scratches and the dust particles. For read and write date in the computer the USB flash drives are used for support. We can observe the USB flash drives in the computer numerical control unit.
With the help of the serial communication port an individual can transfer the data from the computer system to, the computer numerical control machine tool. We can understand the international standards in the serial communications, and the information can be exchanged in order wise. The common interface used between the computer and the computer numerical control is ELA standards RS-232. But when coming to the natural ones like personal computer, plus the computer numerical control is RS 232 port and for the connection of the cable standard type of the cable, must be preferred for the connection of the computer and the computer numerical machine. The data transfer is considered to be done in an undependable manner. The part program must be downloaded and temporarily uploaded in the computer storage system.
With the help of the Ethernet connection we can transfer the part program from computer to the computer numerical control machine. In the storage and transmission of the part programming, the media depends more on the Ethernet communication, to make it capable for transmission of data. For the direct communication with the LAN the machine tools provides an Ethernet card option.
With the help of the keyboard the input part program must be controlled. In the system there is an option of in built software to permit the operator to enter the data step by step.
The part program is a detailed set of program, given to the computer system followed by the machine tool. Each and every command should have a specific position in the Cartesian coordinate system. Part program should be well familiar with the machine tools, process variables, machining process; limitation of computer numerical controls. By using the computer assisted language (APT) or manually the program should be developed.
Machine control Unit:
The machine control unit is a microcomputer that is used to store the programs. By using the machine tools the program needs be executed into action. The machine control units consists of two units they are:
- Data processing unit and
- Control loop unit
The Data processing unit consists of software system, translation software, algorithm calculation which converts the part program into the working format for the machine control unit. To obtain a smooth program in case of changes and errors, the interpolation algorithm is used. The DPU processes the data, which must be sent to the part program and then, later sent to the control logic unit. This operates the drives, and is attached to the machine lead screws, from which it receives the feedback signal. The feedback signal must be receive the velocity and actual position of the axes. To the lead screw DC motor and feedback signal is attached. The control loop machine consists of circuits for positioning and velocity control loops, spindle controls like ON or OFF, checking and counterattack take up.
The machine control unit performs the following functions:
- Coded instructions should be read and also decode.
- For the generation of axis commands interpolation should be implemented.
Various types of the machine tools are controlled by using the CNC Control. The machine tools are controlled in the X and Y axes along with the rotation of the spindle in the Z axis direction.
The driving system consists of
- DC servo motor
- Ac servo motor
- Stepping Motor
- Linear motor
DC servo Motor:
In the CNC machines DC servo Motor is the common type of the feed motor used. In the permanently energized magnetic field the rotation of the armature winding the principle operation must be based. The winding must be connected to the commutator, and the cylinder must be insulated with the copper segments are mounted on the shaft. The DC current must be passed through the commutator and then to the carbon bushes. This is in-turn connected to the machine terminals. By varying the armature voltage the motor speed must be changes, and by controlling the motor armature current motor torque must be achieved. To obtain a required velocity and the position feedback signal, the system must be operated in the closed loop.
Ac servo motor:
AC servo motor consists of a rotor which acts as a permanent magnet, and the stator must be equipped with the 3 phase windings. The rotor speed must be equal to the frequency of the stator magnetic field. The frequency is controlled with the help of the frequency converter.Ac motors are replaced by the DC servomotors, and the main reason for the replacement of the AC motor is that the maintenance is fundamentally not required. The AC motor gives faster response.
Advantages of Servo motor:
- Available in all speeds
- Work well in place of velocity controls
- Provides high torque and high speeds
- Quiet in operation
- At lower speeds smoother in rotation
Disadvantages of servo motor:
- Compare with the steeper motor servo is more expensive
- In vacuum and hazardous conditions the servo motors are not suitable
- Control loop parameters are required during the run
Stepping motor is used to convert the electrical pulses into the mechanical motion of the shaft. It can convert the digital data into the mechanical displacement. In this system there is no requirement of the feedback device and analog to digital convertor for the system. They are mainly suitable for the open loop systems.In the machine tools the stepping motors are not used because of the draw backs like Low torque, slow speed, low resolution, easy to slip in over loads.
Advantages of stepper motor
- Low cost and low maintenance
- Construction must be simple
- Work must be done in any environments
- Excellent start and stop
- Less likely to slip
- Reversing response
Disadvantages of Stepper Motor:
- Compare with the DC motor it shows low torque
- Synchronization will be broken because of the overloading
- At high speeds vibration and noise are occurs
- Limited speeds
A linear motor is AC motor laid out flat. The torque produced in the rotary motion is used to produce a force in the linear motors. The electrical energy must be converted into the mechanical energy to create a linear motion. The rotational motion must be replaced with the linear motion. The liner motion must have an advantage of high precision, high speed and fast response.
The feedback devices are
- Positional feedback devices
- Velocity feedback devices
Positional feedback devices:
The positional feedback devices are divided into two types they are
- Linear transducers
- Rotary encoders
Linear transducer is a direct potential measurement. It is a device used to mount on the machine table. It is used to measure the actual displacement of the slide. Similarly, the screw backlash, motor and other devices are not responsible for the error in the feedback data. The linear transducers are more expensive, but the result needs to be accurate.
Rotary encoder is an indirect or angular linear measurement. It is a device mounted on the end of the motor screw or shaft to measure the displacement. The device does not measure the displacement directly, so at the time of measurement errors are observed. In the machine calibration process the errors are to be compensated.
Velocity feedback devices:
The speed of the motor can be measured in the terms of generated voltage from the tachometer mounted on the motor shaft end. DC tachometer must acts as a generator to produce an output voltage, which is proportional to the speed. The generated voltage must be compared with the voltage corresponding to the required speed. The difference of the voltage can be used to activate the motor for the rejection of errors.
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