Cnc, dnc & adaptive control

CNC, DNC &
Adaptive Control
Arvind Deshpande

Problems with Conventional NC

1. Partprogramming mistakes
2. Nonoptimal speeds and feeds
3. Punched tape
4. Tape reader
5. Controller
6. Management information

4/10/2012 Arvind Deshpande(VJTI) 2

Computer Numerical Control
 Conventional hard- wired NC controller
unit replaced by computer.
 NC system that utilizes stored programs in
a dedicated computer to perform some or
all NC functions
 Soft-wired
 Flexibility


CNC


Functions of CNC
1. Machine tool control
 Hybrid CNC –Hard-wired logic circuits for
functions like feed rate generation , circular
interpolation etc. in addition to computer
Mass production of circuits and less expensive
computer
 Straight CNC – Computer to perform all NC
functions


Hybrid CNC


Straight CNC


Functions of CNC
2. In-process compensation – Dynamic correction of
machine tool motion for changes or errors that occur
during processing
 Adjustment of errors sensed by in-process inspection
probes and gauges
 Recomputation of axis positions when an inspection
probe is used to locate a datum reference on the work
part
 Offset adjustments for tool radius and length
 Adaptive control adjustments to sped and feed
 Computation of predicted tool life and selection of
alternate tooling when indicated.


Functions of CNC
3. Improved programming and operating features
 Use of tape and tape reader only once
 On-line editing of part programs at the machine
 Special canned cycles.
 Graphic display of tool path to verify the tape
 Various types of interpolation: circular, parabolic, cubic
 Support of various units. Conversion from one unit to another
unit.
 Use of specially written subroutines or macros
 Manual data input (MDI)
 Several part programs in bulk can be stored.


Functions of CNC
4. Diagnostics – Equipped with diagnostic
capability to assist in maintaining and
repairing the system
 Identification of reason for downtime
 Indication of imminent failure of certain
component
 Redundancy of components


Direct Numerical Control
 A manufacturing system in which no. of
machines are controlled by a computer
through direct connection and in real time.


DNC with satellite computer


DNC – Drip Feeding

Very complex part shapes  very large NC program

NC controller memory may not handle HUGE part program

computer feeds few blocks of
NC program to controller

When almost all blocks executed,
controller requests more blocks


Behind the Tape Reader (BTR)
 Computer is linked
directly to regular NC
controller unit
 The connection is
made behind the tape
reader
 Two temporary
storage buffers
 Less cost


Special Machine Control Unit
 Regular NC controller is
replaced by special MCU
 More accuracy in circular
interpolation and fast
material removal rates
than BTR systems
 Most CNC machines are
sold with computer


NC, CNC and DNC


Functions of DNC
1. NC without punched tape
2. NC part program storage
 Programs must be made available for downloading to CNC machine tools
 Part program can be uploaded after editing from CNC machine
 Entry of new programs. Editing of programs , deletion of programs
 Tool management
 Tool offsets can be downloaded in to MCU
 Postprocessor
 Data processing and management functions
 Primary storage and secondary storage
 Syntax checking and graphic proving of programs on CNC computer
 CNC can be operated directly from DNC computer
 Flexibility in shop floor scheduling
 Part program preparation
 Machinability database for calculating speed/feed


Functions of DNC
3. Data collection, Processing and reporting
 Monitor production in the factory
 Data processing and report generation by DNC computer
 Getting the data about health of the machine in the form of sensor
signals or diagnostic messages which can be used for
preventive/predictive maintenance
 Metrological data in the form of dimensional acceptance
4. Communications
 Central computer and machine tools
 Central computer and NC part programmer terminals
 Central computer and bulk memory, which stores the NC
programs
 CAD system
 Shop floor control system
 Corporate data processing
 Remote maintenance diagnostics system

Justification of DNC
 Interconnected CNC machines are large in
number
 Very large program size. Can not be
accommodated in the part program
memory of MCU
 Large variety of part programs and small
batch sizes
 Frequent changes in program designs

Advantages of DNC
1. Elimination of punched tape and tape reader
2. Greater computational capability and flexibility
3. Convenient storage of NC part programs in computer
files
4. Programs stored as CLFILE
5. Reporting of shop performance
6. Establishes the framework for evolution of future
computer automated factory (CIM)
7. 2-5 % increase in operational efficiency of CNC
machine tools. Cost of DNC installation can be
recovered quickly.


Combined CNC/DNC systems
 Development of hierarchical computer systems in
manufacturing
 Flexibility
 Ability to gradually build the system
 More versatile and economic approach
 Distributed Numerical System
 Part program downloaded only once
 Redundancy
 Improved communication between central computer and
shop floor


Adaptive Control
 A control system that measures certain output process
variables like spindle deflection, force, torgue, cutting
temperature, vibration amplitude, horse power and uses
them to control speed or feed
 NC reduces non productive time in a machining
operation
 AC determines proper speeds and feeds during
machining as a function of variation in work piece
hardness, width or depth of cut, air gaps in part
geometry etc.
 Increased metal removal rate and reduced cost per
volume of metal removed


Where to use adaptive control?
1. In-process time consumes significant
portion of the machining cycle time.
(>40%)
2. Significant sources of variability in the job
3. Higher cost of operation of machine tool
4. Work material – steel, titanium, high
strengh alloys


Sources of variability in machining

1. Variable depth/width of cut
2. Variable workpiece hardness and
variable machinability
3. Variable workpiece rigidity
4. Toolwear
5. Air gaps during cutting


Adaptive Control Optimization
(ACO)
 Index of performance is a measure of overall
process performance such as production rate or
cost per volume of metal removed.
 Objective is to optimize the index of performance
by manipulating speed or feed in the operation
 IP = MRR/TWR
MRR – Material removal rate
TWR – Tool wear rate
 Sensors for measuring IP not available


Adaptive control Constraint (ACC)

 Nearly all AC systems is of this type
 Less sophisticated and less expensive
than research ACO systems
 Objective is to manipulate speed or feed
so that measured process variables are
maintained at or below their constraint limit
values.


Operation of ACC system
 Profile or contour milling on NC machine tool
 Feed is controlled variable
 Cutter force and horsepower are used as measured
variables
 Hardware components
1. Sensors mounted on the spindle to measure cutter
force
2. Sensors to measure spindle motor current
3. Control unit and display panel to operate the system
4. Interface hardware to connect the AC system to
existing NC/CNC system


Relationship of AC software to APT
program


Operation of ACC system during
machining process


Benefits of AC
1. Increased production rate
2. Increased tool life
3. Greater part protection
4. Increases machine life
5. Less operator intervention
6. Easier part programming


Cnc, dnc & adaptive control

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