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ABSTRACT
This paper describes in detail an indigenous state-of-the-art DNC system designed, developed,
fabricated and installed at DRDL NC Centre. It describes the hardware and software interfaces
designed and developed in-house. At present, it supports a total of 16 CNC machines, 8 in serial
port and the balance in parallel port. This system has the capability of extension up to 64 machines.
During last one year of its installation and working it has been found to be extremely reliable.
INTRODUCTION
Distributed numerical control (DNC) can be defined
as a manufacturing system in which a number of
machines are connected by a computer through direct
connection. The tape reader is eliminated in DNC, thus
relieving the system of its least reliable component.
Instead of using the tape reader, the part program is
transmitted to the machine tool directly from the
computer memory .One of the most important demands
of our time is making correct information available at
the right time and right place. This demand is satisfied
by use of a DNC system.
Installation of a DNC system is the first step towards
computer integrated manufacture. A DNC system
receives CL data from a CAD/CAM workstation and
transfers it to individual machines after post-processing,
thus, saving precious workstation time and carrying out
error-free data transmission.
One of the main tasks to be carried out by DNC is
the provision of unequivocal information paths ensuring
efficient supply of information to machines and retrieval
of such information as and when required. A condition
for moving over from NC technology towards integrated
manufacturing is the manner in which information is
linked. The current situation characterised by increasing
investments in complex manufacturing cells, the linking
of machining processes, the reduction in batch sizes and
need for shorter deliveries, demands an information
supply and retrieval system in which the machines and
men are fully integrated.
Components of a DNC System
The term distributed numerical control is evolved
from the direct numerical control. A direct numerical
control system consists of four basic components : (i)
central computer, (ii) bulk memory , which stores the
NC part programmes, (iii) communication lines, and
(iv) machine tool.
The computer calls the part program instructions
from bulk storage and sends it to the individual machines
as and when required. It also receives data back from
the machines. This two-way information flow occurs in
real-time. This means that each machine requests for
instructions which must be satisfied almost
instantaneously.
Similarly, the computer must always be ready to
receive information from the machines and to respond
accordingly. The remarkable feature of the DNC system
is that the computer is servicing a large number of
separate machine tools, all in real-til..e.
Depending upon the number of machines and the
computational requirements imposed on the computer ,
it is sometimes necessary to make use of satellite
computers. These satellites are minicomputers, and they
serve to take some burden off the central computer .
Each satellite controls several machines. Groups of part
program instructions are received from the central
computer and stored in buffers. They are then dispensed
to the individual machines as required. Feedback data
from the machines are also stored in the satellite's buffer
before it is collected by the central computer .
NEC~SITY OF DNC
There are several functions which a DNC system is
designed to perform. These functions are unique to
DNC and cannot be accomplished with either
conventional NC or CNC system. The functions and
advantages of DNC are explained in the following
sections
NC without Punched Tape
The major objectives of DNC system is to eliminate
the use of punched tape. The problems of tape reader
are: (i) unreliability of the tape reader, (ii) fragile nature
of paper tape, and (iii) difficulties in making corrections
and changes in the program contained on the punched
tape.
In addition, certain expenses are associated with the
equipment that produces the punched tape. AU of these
costs anc! inconveniences can be eliminated with the
DNC approach. The other advantages are data transfer
at high baud rate and data redundancy.
NC Part Program Storage
An important function of the DNC system is storing
the part programs. The program storage subsystem must
be structured to satisfy several purposes :
(a) The progIams should be made available for down
loading to the NC machine tools,
(b) The subsystem should allow for editing of
programs,
I The DNC soItware should accomplish the postprocessing
function,
' " The part program in a DNC system is typically
stored as the CLFILE (cutter location data files).
The CLFILE is converted into instructions for a
particular machine tool (i.e. post-processing),
The storage system must be structured to perform certain data processing and management functions
such as file security, display of programs,
manipulation of data etc, and
The computational flexibility (edit and graph).
2.3 Data Collection, Processing and Reporting
The data collection, processing and reporting
functions of DNC concern the direct link from the
central computer to the machine tool in the factory.
Another important function of DNC involves the
opposite function of transferring data from the machine
tools back to the central computer. Thus the DNC
involves a two-way data transfer .
The basic purpose behind the data collection,
processing and reporting functions of DNC is to monitor
online production in a factory .