Numerical control has caused a virtual revolution in the discrete metal parts manufacturing industry. The success of NC has led to number of extensions of Numeric Control concepts and technology. Four of the important developments are the following:
There are number of problems inherent in conventional machiningwhich are motivated machine tool builders to seek imprivements in the basic NC system. Among the difficulties encountered in using conventional numerical control machine are the following Part programming mistakes,Punched tape, Tape reader, Controller, Management information.
DNC defined
It can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC thus releiving the system of its least reliable component. Instead of using tape reader the part program is tranmitted to the machine tool directly from the computer memory. In principle,one computer can be used to control more than 100 seperate machines. Figure below illustrates the general DNC configuration
The system consists of four components.
1. Central computer
2. Bulk memory which stores the NC part programs.
3. Telecommunication lines.
4. Machine Tools.
The computer calls the part program instructions from
bulk storage and sends data back from the machines. This two way information
flow occurs in real time,which means that machine's requests for instructions
must be satisfied almost instantaneously. Similarly, the computer must
always be ready to receive information from the machines and respond accordingly.
The remarkable feature of the DNC system is that the computer is ervicing
a large number of seperate machine tools,all in real time.
Depending upon the number of machines and computational
requirements that are imposed on the computer it is sometimes necessary
to make use of satellite computersas shown in figure.
Computer Numerical Control (CNC)
CNC is a NC system that utilizes a dedicated, stored program
computer to perform some or all of the basic numerical control functions.
As of this writing the typical CNC system uses a minicomputer as the controller
unit. It is expected that in future generations of computer numerical control,microcomputers
will become predominant.
Because a digital computer is used in both CNC and DNC
there is often confusion surrounding the two systems.
Figure below shows the general configuration of CNC
1. DNC computers distribute instructional data to and
collect from a large number of machines. CNC computers control only one
machine or a small number of machines.
2. DNC computers occupy a location that is typically
remote from the machines under their control. CNC computers are located
very near their machine tools.
3. DNC softwrae is developed not only to control individual
pieces of production technology,but also to serve as part of management
information system in the manufacturing sector of the firm. CNC software
is developed to augment the capabilities of a particular machine tool.
Except for the fact that a digital computer is used,
CNC machines are similar and the part programs are entered in a similar
manner. Punched tapes are still the common device for entering the part
program into the system
Adaptive Control Machining
One of the priciples reasons for using numerical control(including
DNC and CNC) is that NC reduces the nonproductive time in manufacturing.
This is accomplished through a reduction in the following elements,which
constitue a significant portion of totla production time.
1. Workpiece handling.
2. Setup of the job.
3. Lead times between receipt of a order and production.
4. Tool changes.
5. Operater delays.
For a machining operation the term adaptive control means a control system that measures output process variables and uses these to control system that measures certain uotput process variables nad uses these to control speed and or feed. Some of the process variables and uses these to control speed and feed. Some of the process variables that have been used in adaptive control machining systems include spindle deflection or force,torque,cutting temperature viration amplitude and horsepower .In other words all the metal-cutting variables have been tried in experimental control systems.
A number of potential befitis accure to the user of an
adaptive control machine tool . The advantage gained will depend upon the
particular job under consideration. There are obviously many machining
situations for which adaptive control system cannot be
justified.
1. Increased production rates
2. Increased tool life.
3.Greater part protection.
4.Less operater intervention.
5.Easier part programming.
Industrial Robots
An industrial robot is a general purpose programmable
machine possesing certain anthropomorphic charesteristics. It is most typically
used for parts handling tasks but can also be used in conjuction with a
variety of manufacturing processes. The robots can be programmed to carry
out a sequence of mechanical movements. It will perform that sequence over
and over again until reprogrammed to carry out a sequence of mechanical
movements .
General Physical Configuration
There are two principal robot configuration :polar
and cylindrical. The two types are illustrated in figure In the polar configuration
the body of the robot pivots either horizontally vertically or both. Attached
to body pivots is the are that moves by body motion. The are of the robot
in the horizontal orientation and can be made to moive up and down and
in or out with respect to the body.
Arm and body motions
1. Vertical traverse Up and down motion of the arm
2.Radial traverse Extension and retraction of the arm
3.Rotational traverse Rotation about the vertical axis
Wrist Motions
4.Wrist swivel Rotation of the wrist
5.Wrist bend Up or down movement of the wrist which
also involves a rotational movements.
6.Wrist yaw Right or left swivel of the wrist.