Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

43.2.3 Direct/Distributed Numerical Control (DNC)

 

· Uses a few methods,

  1. - the oldest methods used modems, and a mainframe which emulated a tape reader, to control the NC machine (no storage)
  2. - a more recent advance used a local computer which acts as a storage buffer. Programs are downloaded from the main DNC computer, and then the local controller feeds instructions to the hardwired NC machine, as if they have been read from tape.
  3. - the newer methods use a central computer which communicates with local CNC computers (also called Direct Numerical Control)

 

· DNC controllers came before CNC machines, but as computer technology improved it became practical to place a computer beside the NC machine, and DNC changed in form.

 

· Characteristics of modern DNC systems are,

  1. - uses a server (with large storage capacity) to store a large number of part programs
  2. - the server will download part programs on demand to local machines
  3. - may have abilities to,
    1. - display and edit part programs
    2. - transmit operator instructions and other data needed at the machines
    3. - collect and process machine status information for management purposes

 

· Advantages are,

  1. - eliminates the need for NC tapes (the advantages are obvious)
  2. - design changes are immediate
  3. - NC programs may be edited quickly
  4. - can be used to support an FMS system
  5. - increase efficiency of individual machine tools
  6. - more shop up-time than with stand alone machines
  7. - simplifies implementation of group technology, computer aided process planning, and other CIM concepts
  8. - reduces peripheral costs with NC tapes

 

· A Brief History,

  1. · Mid 60's
    1. - concept proved by Cincinnati Milacron and G.E.
    2. - telephone links used to send instructions from large computers to hard wired NC machines. Basically replaced a tape reader.
  2. · 1970
    1. - several commercial DNC systems announced.
  3. · Mid 70's
    1. - Aerospace companies used DNC because of the large number of distributed machines in their facilities.

 

· Initial resistance to DNC technology was (previously) based on,

  1. - high cost of computer hardware
  2. - the number of machines which could be controlled by one computer was limited
  3. - computer software was limited for maintenance, scheduling, control, and data collection
  4. - a backup computer was usually required
  5. - was hard to justify on the basis of downloading parts programs

 

· when downloading programs there are two popular opinions,

  1. - a program should only be downloaded in part, this accommodates easy engineering changes in a real-time environment.
  2. - many programs should be downloaded to the local controller to provide protection against system failure, and eliminating the cost of real-time response in the DNC central computer.