FUNDAMENTALS OF DIRECT NUMERICAL CONTROL

By EIA definition, direct numerical control (DNC) is a

system connecting a group of numerically controlled machines

to a common computer memory for part-program storage, with

provision for on-demand distribution of machining data.

Typically, additional provisions are available for collection,

display, or editing of part programs, operator instructions, or

data related to the NC process.

The original DNC concept was forwarded (circa 1965) as a

means of reducing NC control costs through use of one

powerful controller for a group of machines, rather than a

separate controller for each machine tool. The cost of electronic

control equipment was much higher in the early 1960’s than it is

today; so high that the DNC concept was driven by a need to

reduce control cost.

Proponents of the DNC concept believed that punched paper

tape and tape readers at the machine tool could be completely

eliminated by driving the NC machines directly from the

memory of a central computer. Because NC machines of the day

typically were not equipped with memory, the central computer

would drive the machines in real time—that is, the computer

would send NC data to the machine tools in sequence or pulse

form during the actual machining operation. By operating in

this manner, it was expected that maintenance costs of tape

readers at the machine tool could be eliminated and that input

errors caused by improper operation of tape readers at the

machine tool could be avoided.

In addition, the original DNC concept promised simpler

management of NC programs and an elimination of a need for

costly libraries of punched paper tape. It was believed that by

storing NC programs and monitoring NC machines using a

central computer, the optimization of NC programs and

simulation of numerical control functions (verification of tool

path using a plotter, for example) would be more easily

performed. Also, it was thought that collection and reporting of

system operating data such as downtime, production, and

maintenance information would be more easily accomplished.

The original concepts of DNC worked reasonably well in a

few isolated applications; however, some of the promises of

DNC never were realized on a broad scale. For example, the

thought that DNC could eliminate the need for tape readers or

other input devices at the machine tool proved unrealistic. Tape

readers or other alternate input devices at the machine tool were

found to be useful in the early DNC systems as backups to the

computerized system. Such manual backup was required

because occasional downtime of the central computer caused

the entire DNC system to go down, sometimes idlinga dozen or

more expensive NC machine tools.

With the advent of computer numerical control, came the

availability of relatively inexpensive computer memory at the

machine tool. The nature of the DNC concept was altered as a

consequence. With computer memory at the machine tool, it is

no longer required that the CNC machines of a DNC network

be driven in real time. Instead, NC programs can be downloaded

in total from the memory of the central computer to the

memory of the computer at the machine tool; the connection

between the central computer and the individual machine tools

in the system need only be maintained for a short period of

time—the time necessary to transmit the NC program. In this

way, the uptime of individual machine tools is less dependent

upon the uptime of the central computer and, because the

machines are not driven in real time by the central computer,

program editing at the machine is made much easier. This

concept, known as distributed numerical control, is growing in

usage. In fact, the acronym DNC, originally defined as direct

numerical control, is now used by many experts in the controls

industries to describe distributed numerical control.

THE FUTURE OF DIRECT NUMERICAL

CONTROL

Direct numerical control systems of the future are expected

to perform more and more sophisticated tasks which will aid in

production management and help increase machine tool utilization

and productivity. In the coming years, the use of DNC

will not be limited merely to tasks of monitoring but will play an

ever-increasing role in controlling the manufacturing operation

and optimizing production efficiency

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