Page 706 - Mechanical Engineers' Handbook (Volume 2)

3 Numerical Controllers 697

desired path. In 1952, the U.S. Air Force provided funding for a project at the Massachusetts
Institute of Technology (MIT) that developed the Whirlwind computer. In a subsequent
project, the Servomechanisms Laboratory at MIT developed the concept of the first workable
numerical control (NC) system. The NC architecture was designed to exploit the Whirlwind
computer with emphasis on five-axis NC for machining complex aircraft parts.
The MIT NC architecture identified three levels of interaction with the numerical con-
troller. 13–15 At the highest level is a machine-independent language, called APT (Automati-
cally Programmed Tools). APT provides a symbolic description of the part geometry, tools,
and cutting parameters. The next level, called the cutter location (CL) level, changes the
symbolic specification of cutter path and tool control data to numeric data. The CL level is
also machine independent. The lowest level, called the G-code level, contains machine-
specific commands for the tool and the NC axis motions.
The conversion from APT to CL data involves the computation of cutter offsets and
resolution of symbolic constraints. The conversion from CL data to G-code is called post-
processing. 13,15 Postprocessing transforms the tool center line data to machine motion com-
mands, taking into account the various constraints of the machine tool such as machine
kinematics and limits on acceleration and speed. The APT-to-CL data conversion and the
compilation of CL data to G-code are computationally intensive; these computationally in-
tensive functions were envisioned to be performed by the Whirlwind computer. The numer-
ical controller works with simple G-codes to keep computational requirements low. The
G-codes, punched on perforated paper tape, would be the input medium to the numerical
controller.
Since their inception in the 1950s, numerical controllers have followed a similar pattern
in the evolution of controller technology as the computer industry in the past 30 years. The
first numerical controllers were designed with vacuum tube technology. The controllers were
bulky and the logic inside the control was hard wired. The hard-wired nature of the controller
made it very difficult to change or modify its functionality. Vacuum tubes were replaced by
semiconductors in the early 1960s. In the early 1970s, numerical controllers started using
microprocessors for control. The first generation of numerical controllers with microproc-
essor technology were mostly hybrid, with some hard-wired logic and some control functions
in software. Today, most NC functionality is in the software. Microprocessor-based NC is
also called computer numerical control (CNC).
The concept of distributed numerical control (DNC) was introduced in the 1960s to
provide a single point of programming and interface to a large number of numerical con-
trollers. Most NC users agree that the paper tape reader on a numerical controller suffers
the most in reliability. DNC can transfer a program to NC through a direct computer link,
bypassing the paper tape reader. DNC has two primary functions: (1) computer-assisted
programming and storage of NC programs in computer memory and (2) transfer, storage,
and display of status and control information from the numerical controllers. DNC can store
and transfer programs to as many as 100 numerical controllers. Distributed numerical con-
trollers commonly connect to numerical controllers through a link called Behind the Tape
Reader (BTR). The name BTR comes from the fact that the connection between DNC and
CNC is made between the paper tape reader and the control unit.
The use of paper tape is no longer the main form of storing or updating in CNC. With
the advent of computer technology that is now incorporated into CNC, the CNC now has
memory storage capability such that the programs are stored in the CNC as files (similar to
a computer). Thus, loading new programs or storing old programs is now done via electronic
connection through RS232, Ethernet, or memory storage devices such as flash memory cards.
DNC is still used, especially in die-cutting machines where programs are too long and the