2. Overview of presentation
1. CNC control system- An overview
2. Open loop and closed loop control system system
3. Feedback subsystem in CNC
4. Feedback for CNC
5. Block diagram for CNC
6. CNC interpolation
7. Hardware and software interpolation
8. Features of CNC
9. Advantages and disadvantages of CNC
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3. CNC control system
Fundamental aspects of control-
In general CNC machines are programmed and controlled to
accomplish the major three actions-
A. Positioning of tool in 2 or 3 or more axes.
B. Motion-relative velocity of tool wrt work piece
C. Switching function i.e. direction of rotation of spindle, coolant
ON/OFF etc. are to be controlled.
D. CNC systems require motor drives to control both the position and the
velocity of the machine axes. Each axis must be driven separately and
follow the command signal generated by the NC control. There are two
ways to activate the servo drives: the open-loop system and the closed-
loop system.
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4. Based upon the nature of the positioning and motion,CNC
control is classified into two groups,
1. Point to Point control-it is purely for positioning of the tool
at programmed points.e.g drilling,tapping,boring operations
2. Continuous path control- there is a functional relationship
between individual cordinate axes. i.e 3 axes,5 axes
machining.
5 axes machining- 3 translational and 2 rotational axes.
best suited for doubly curved surface maching.i.e in
spacecraft,aircraft and fluid handling devices.
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6. Open loop and closed loop cnc
system
Open loop-
Programmed instructions are fed into
the controller through an input device.
These instructions are then converted
to
electrical pulses (signals) by the
controller and sent to the servo
amplifier to energize the servo
motors.
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7. Drawbacks of open loop
The primary drawback of the open-loop system is that there is
no feedback system to check whether the program position
and velocity has been achieved. If the system performance is
affected by load, temperature, humidity, or lubrication then the
actual output could deviate from the desired output.
For these reasons, the open-loop system is generally used in
point-to-point systems where the accuracy requirements are
not critical. Very few, if any, continuous-path systems utilize
open-loop control.
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8. Closed loop system
The closed-loop system has a feedback subsystem to monitor the actual
output and correct any discrepancy from the programmed input.
Closed-loop systems are very powerful and accurate because they are capable
of monitoring operating conditions through feedback subsystems and
automatically compensating for any variations in real-time.
Most modern closed-loop CNC systems are able to provide very close
resolution of 0.0001 of an inch. Closed-looped systems would, naturally,
require more control devices and circuitry in order for them to implement both
position and velocity control. This, obviously, makes them more complex and
more expensive than the open-loop system
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9. Comparison b/w open and
closed loop system
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10. Feedback subsystems in closed loop system
Feedback subsystems are of two types-
Analogue feedback system: Variations in physical systems such as
position and velocity are being measured as voltage levels in analogue
feedback system. Tachometers are typically used to measure the
velocity, whereas the resolvers are used to measure position. There are
two feedback loops in CNC drive systems; position loop and velocity
loop.
NOTE-A resolver is a type of rotary electrical transformer used for
measuring degrees of rotation. It is considered an analogue device, and
has a digital counterpart, the rotary (or pulse) encoder.
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11. Digital feedback system: A digital feedback system
uses a digital position transducer to measure the
position. Encoders are popular digital position
transducers. Comparator and amplifier are replaced by
up-down counter and digital-to-analogue converter.
It monitor output variations by electrical pulses.
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12. Feedback for closed loop system
The feedback in a closed loop system is the information
delivered from the CNC machine to the controller.
Feedback data is data that either confirms or denies that
the motors have moved the machining table to the
correct position and at the correct speed.
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14. Feedback can be accomplished in one of two ways
Linear Scales
Encoders
Note- SERVOMOTOR-it is an automatic device that uses error sensing negative
feedback to correct the performance of a mechanism.
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15. Linear scale-
It measures the absolute/exact position of the slide directly.
It is a graduated scale made of either glass or stainless steel
along with a measuring surface and scanning unit.
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16. Encoder-
An encoder is a sensor of mechanical motion that
generates digital signals in response to motion. As an
electro-mechanical device, an encoder is able to provide
motion control system users with information concerning
position, velocity and direction.
They convert rotary movement into digital signals.
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17. Types of encoders
There are two different types of encoders: linear
and rotary.
A linear encoder responds to motion along a path, while
a rotary encoder responds to rotational motion.
An encoder is generally categorized by the means of its
output. An incremental encoder generates a train of
pulses which can be used to determine position and
speed. An absolute encoder generates unique bit
configurations to track positions directly.
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18. Difference between incremental and absolute encoder-
Incremental rotary encoder outputs the pulse corresponding
to the rotation angle only while rotating, and is the counting
measurement method that adds up the pulse from the
measurement beginning point.
Absolute rotary encoder outputs the signal of position
corresponding to the rotation angle by code. The Absolute
position from the starting point is measured by the output
code data. The position is memorized though the power
supply is cut off.
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22. Cnc interpolation
interpolation is a method of constructing new data points
within the range of a discrete set of known data points.
Estimation of an unknown quantity between two known
quantities .It is a mathematical process to determine new
points on curved surface within two end points.
It produces a series of intermediate data points between
given coordinate positions and computes the axial velocity
of an individual axis along the contour path.
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23. Types of interpolation-
To coordinate the axis movements in CNC control these types of
interpolation are used
Linear interpolation: This moves tool from start point to the target point
along a straight line. It can be implemented in a 2-D plane or 3-D space.
the programming command should indicate X, Y, Z coordinates of target
point, and feed rate.
Circular interpolation: It is programmed to cut circular arcs in three
principal planes; namely XY, YZ, ZX. Direction, target position, arc radius,
cutting plane, and feed rate must be specified in the program.
Helical interpolation: Helical interpolation combines the two-axis circular
interpolation with a linear interpolation in third axis. i.e. machining of
helical pockets and threads
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24. Parabolic interpolation: It uses three non-collinear points to
approximate curves that are of free forms. It reduces the
number of programmed points by as much as 50 times the
number required by the linear interpolation mode. It is
mainly used in mold and die making and curves of high
order and high degree.
Cubic interpolation: Cubic interpolation approximates the
surfaces defined by third-order geometry. It involves the
motion of three axes to machine complex shapes such as
automobile sheet metal dies.
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25. Modern approaches to interpolation-
1.hardware interpolation-
It consists of a pair of DDA integrators.
It is capable of doing linear and circular operations
according to instruction from the punched tapes.
The main feature of hardware interpolation is that it
controls simultaneously two axes which can be X and Y,X
and Z or Y and Z.
Hardware circuits generate the points that keep the tool
path within tolerance.
It is used for fine interpolation.
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26. 2.Software interpolation-
This is the computer based approach of hardware
interpolation.
Basically it is a computer program which simulates a single
cycle of hardware interpolator and the feed rate control.
The computer analysis program divides the tool path into
segments.
Used for coarse(rough) interpolation.
It is microprocessor based interpolation system,samples
about 100 times/second are to be taken in this system.
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27. Requirements of interpolation system-
1. The amount of data required for interpolation should be as
small as possible.
2. The velocity along the interpolated path should be constant.
3. The curve must pass accurately through the numerically
specified reference points to prevent any cumulative error.
4. The curve produced by interpolation must approximate as
closely as possible to the required work piece contour.
5. Data required for the servomotor should be made available
at a faster rate as demanded by the process.
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28. Features of CNC
For a CNC machine control unit (MCU) decides cutting speed, feed,
depth of cut, tool selection , coolant on off and tool paths.
The MCU issues commands in form of numeric data to motors that
position slides and tool accordingly.
The tool or material moves.
Tools can operate in 1-5 axes.[multi axis machining]
Larger machines have a machine control unit (MCU)
which manages operations
Movement is controlled by a motors .
Feedback is provided by sensors.
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29. besides previous features of CNC system we can go for
the following features also-
1. Construction of the control
2. Operation
3. Operating aids
4. Colour graphics
5. Graphic simulation
6. Programming aids
A. cutter radius compensation.
B. blueprint programming
C. polar coordinates
D. cylindrical interpolation
E. Subroutines
F. Turning/milling operations- programmed contour in Cartesian
coordinates while working in polar coordinates
G. Machining cycles- subroutines for often needed machining operations.
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30. 7. program archiving
8. Integrated interface control- use of PLCs
9. Computer link
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31. CNC machines-
Advantages
Reduces product cost
Improved product quality
Facilitate production planning and control
Increased productivity
Greater flexibility
Limitations
High initial investment
High maintenance
For low production it is costlier process
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