Two types of configurations of computer controlled machine tools are commonly found (i) where a dedicated small microcomputer is used for control of one machine tool (referred to as computer numerical control or (CNC), and (ii) where a large computer controls a number of separate NC machine tools, (referred to as direct numerical control or DNC).
Type # 1. CNC System:
In a CNC machine the part programs are initially entered either through a punched tape reader or could be entered through a video control unit. The main difference between CNC and conventional NC control is thus that while in conventional system the punched tape is cycled through the reader for every workpiece in the batch but in CNC, the program is entered once and then stored in computer memory. CNC of course, has additional flexibility and computational capability. Reprogramming capability, both for part programs and system control, makes the system quite flexible and versatile.
While computer is meant mainly for machine tool control, it also performs other functions like in-process compensation, improved programming and operating features, and diagnostics. In addition, computer can also be used to optimise the machining process (adaptive control strategy) i.e. it measures one or more process variables (like cutting force, temperature, horsepower, etc.) and manipulates speed and/or feed to compensate for undesirable changes in the process variables.
Machine tool control involves conversion of the part program instruction into machine tool motions through the computer interface and servo system. Sometime, it is found advantageous to use hardwired logic circuits for some of the functions, and thus hybrid system consisting of soft-wired system (for motion interpolation and servo-system) and hardwired logic and interface is also used.
With computer control, in-process compensation facilities (like adjustments for error sensed by in-process inspection probes and gauges; offset adjustments for tool radius and length, computation of predicted tool life and appropriate action for change of tool, adaptive control) are easily provided.
Further several improved and convenient programming and operating features are incorporated. Some of these are: editing of part programs at the machine, permitting correction and optimisation of program; graphic display of the tool path to verify the tape; several types of interpolation like circular, parabolic, cubic, etc.; storage of a large number of part programs; manual data input; use of specially written subroutines which simplify the task of programming; etc.
In view of high initial cost of CNC machines it is imperative that the downtime should be as least as possible. In other words, mean time between failures (MTBF) should be high and mean time to repair (MTTR) be less.
This is achieved by incorporating suitable diagnostics features which assist the operator in maintaining and repairing the system by giving timely information of exact nature of fault. For very critical components, redundancy is incorporated so that the standby part takes over whenever main part fails and machine operation continues uninterruptedly. At the same time, alarm is given for the failed part which can be replaced on-line.
Advantages of CNC System
The advantages of CNC over conventional NC system are:
(i) Tape reader, a weak component of conventional NC system is used only once initially for program entry.
(ii) NC tape can be updated at machine site during the tape tryout.
(iii) Risk of obsolescence is less since new control options (like interpolation schemes, other updates, etc. can be introduced with relative ease.
(iv) Mean time to repair is low due to incorporation of diagnostics features.
(v) It is possible to optimise working conditions by using techniques like adaptive control and in-process compensation.
(vi) CNC system is more compatible in a total factory automation.
Application of Computer Numerical Control (CNC):
More sophisticated software is required to increase productivity in various areas of metal working. Some of the applications of CNC in this area are described in brief.
It is usually a time consuming process to locate the work pieces (particularly heavy ones) on the right position to be machined. In ordinary NC machines, the deviation of
actual work position from programmed position has to be found and new NC command has to be reproduced which takes lot of time.
In CNC application some points are measured with inspecting probe and measured data is stored in the memory. Then CNC program directs to machine the workpiece by transforming the part program co-ordinate into the actual work coordinate by calculating the deviation using the measured data.
The measuring scales themselves have some errors. These can be calibrated using laser measuring device and the errors stored in memory. Thus the errors can be compensated automatically according to the position of machine using stored data. It is possible to incorporate double check system (redundancy) so that expensive work will not be spoiled even on malfunction of NC and machine motion is stopped at once.
It is possible to introduce ‘Users macro’ in the CNC software so that operator can carry out the capabilities desired by him in the system.
In fact a vast number of applications are possible using CNC equipment by developing proper software.
Type # 2. DNC Control:
Another configuration of computerised control system could be use of a centralised computer to directly control in real time more than one number of machine tools. (Refer Fig. 41.13). The part programs of all machine tools are stored in main memory of control computer and transmitted on direct telecommunication lines.
The tape reader is thus totally eliminated. Such a computerised control system is known as Direct Numerical Control (DNC) system. The computer provides necessary instructions to each machine tool on demand. Data is also collected from individual machine tools and processed and displayed on control computer. Two-way information flow occurs in real time. Various machine tools can communicate with computer via telecommunication line.
Thus a central computer is able to service a large number of separate machine tools and all in real time. However, if the number of machines to be attended are high and the computation requirements are also high, then use of satellite computers can be made as shown in Fig. 41.14.
These satellite computers (usually minicomputers) take some burden of the central computer. The information collected from the machine tools is stored in memory buffer and passed on to central computer as and when required.
Unique features of DNC are:
(i) Total Elimination of Punched Tape:
In DNC system, the NC part programs are keyed in directly thus eliminating all the disadvantages of punched tape, viz. fragile nature of tape, unreliable tape reader, fixed nature of program i.e. no flexibility in modification, need of separate tape puncher, etc.
(ii) Storing the Part Programs:
It is possible to store various part programs for different components. The desired program is down loaded to NC machine for producing a desired component. New programs can be entered, old ones deleted or modified/updated. DNC software can also accomplish post processing functions to covert the program into a form compatible for given machine tool. It is also desirable to perform certain data processing and management functions, such as file security, display of programs, manipulation of data, etc.
(iii) Data Collection, Processing and Reporting:
With the facility of data collection (regarding number of workpieces produced, tool usage, machine utilisation, stores inventory etc.), it is possible to measure the performance of the shop. After processing raw data, system provides necessary information for the management to take appropriate decisions.
A communication network is required between central computer and machine tools, NC part programmer terminals, and bulk memory. For computer integrated factory communication network is also required between DNC system and CAD system, shop floor control system, remote maintenance diagnostics system, corporate data processing computer, etc.
Advantages of DNC are:
DNC system offers following advantages:
(i) Elimination of punched tapes and tape readers.
(ii) Greater computation and data processing function.
(iii) Flexibility in chaining programs.
(iv) Convenient editing and diagnostic features.
(v) Storage of NC part programs.
(vi) Storing of programs in more general format.
(vii) Reporting of shop performance.
(viii) Enabling total computerisation of factory.
Direct Numerical Control (DNC) Vs Computer Numerical Control (CNC):
Earlier developments in NC machines were with hardware system. Such systems however were not flexible and different machines were required for different jobs. With fast developments in computers and reduction in their price, hardwired concept was replaced with soft-wired system.
Thus same machine could be used for many jobs simply by changing the program, making system highly flexible. Earlier trend in adoption of computers was the use of a central computer for the whole machine shop (Direct numerical control).
The computer used to be located in a central place and commands from it were sent to machines located away from it and information from all these machines was fed to the central computer to take necessary corrective action. DNC system is quite expensive requiring highly skilled software knowledge and its installation is justified in large workshops. The software of DNC also serves, as a part of the management information system.
Satellite minicomputers with memory buffers are arranged for controlling a group of machine tools and these minicomputers via telecommunication lines are connected with central computer. With the development of microprocessors (micro-computers), the trend changed. Microcomputers are used (having dedicated software) with each machine tool.
Each microcomputer is thus located close to each machine tool. The software augments the capabilities of a particular machine tool. Such system is called CNC. Of course, it is possible to have a communication network so that information about all machine tools is available with a central system.
Hierarchical Approach in Combining DNC/CNC System:
If one traces history of computers, it would be realised that mainframe computers were in vogue initially. These were big central computer systems. With advance in microelectronics, the size of computer went on decreasing and it was possible to use one microcomputer dedicated to each machine tool.
Thus the trend today is to employ hierarchical approach in which CNC computers have direct control over the machine tools and report to satellite minicomputers, which in turn report to a central DNC computer. These systems are quite flexible. With this approach the system can be gradually expanded and the information system can be tailored to the specific requirements of the user.
The piece-by-piece installation of the computer integrated manufacturing system is a more versatile and economic approach. The changes and corrections can be carried out easily as the system is being integrated in stages.
The cost of the system can be spread over a longer period of time and benefit of each subsystem reaped soon after its installation. The central computer provides useful reports on production operations to management in real time.
The central computer downloads the program directly to the CNC computer memory in one instance (instead of block-by- block procedure) since CNC controller has sufficient capacity to accept the entire part program.
The amount of communication between central computer and CNC system is thus reduced. A single failure does not lead to catastrophes since necessary backup devices can be incorporated. Further the overall communication system is considerably improved since computers can communicate easily with other computers than the hard-wired devices.