Transcript cnc_machine_tool_diagram_examples

CNC Machine Tool
(G&M Code Program)
Computer Numerical Control
Master Control Unit
The Axis amplifier
increases the
electrical current
from the CNC
control unit
The CNC program contains all the G&M code of that is interpreted by the motion control unit. When the machine
is instructed to move to a position in X,Y (lathe), or X,Y and Z (for a 3 axis machine) then a designated number of
pulses are sent to the axis drive amplifier. This in turn pulses the drive motor which moves the machine table or
axis. There is axis feedback from the drive motor which is fed back into the axis drive amplifier to ensure that the
motor has moved the correct amount of divisions. As the machine table or tool moves there is a linear encoder
which reads the actual position of the table/tool and feeds this back to the motion control unit. This will be
compared to the original value and the control unit will adjust the amount of pulses needed to move the tool/table to
the correct position
A Typical CNC Lathe/Milling Machine
PLC = Programmable Logic Controller, This is a
small computer with a built-in operating system
.
(OS) This means that at the heart of the CNC
machine is a computer. It may have a specialist
operating system.
NC = Numerical Control
(This is the part program which is read
/ used by the CNC machine. It contains
lines of instructions (G01;G00;T01;
)
M04, etc
This is usually transferred to the
machine via disk.
Machine Control Unit =
This part of the CNC machine is what
interprets (breaks down) the part
program and then instructs the various
parts of the machine to switch on/off,
move, etc
Axis Servo Drive
This will amplify the small electrical
signals sent from the control unit into
larger electrical signals which will then be
transferred to the servo motor on the axis
(X,Y,and Z)
Servo Motor
The servo motor will turn a number of
degrees each time a pulse is sent to it from
the axis servo drive. Each pulse may only
move the machine 0.01mm. In order to move
the machine 1mm the servo motor may have
to be pulsed 100 times.
Axis Positional
Encoder
This will send a positional
value back to the machine
control unit.
Spindle Speed Encoder
This will send a speed value
back to the machine control
unit.
CNC Block Diagram
A Typical CNC Lathe/Milling Machine Program Being Interpreted by the Machine Control Unit
CNC Program
G and M codes
G00
X20 z30
Send signals to
servo interface for
X and Z axis
Servo interface
to amplify signals
Servo motors
move axis
Encoder sends
Positional data
To control unit
Are axis
at position?
yes
T01
Request machine
to have T01 fitted
Is T01 Fitted?
Electrical pulse to
Tool change
yes
M04
S200
Send signals to
Spindle servo
interface
Servo interface
to amplify signals
Is T01 Fitted?
yes
no
Servo motors to
rotate spindle
Encoder sends
Positional data
To control unit
Is spindle
at speed?
yes
no
G01
X20 Z0
Send signals to
servo interface for
X and Z axis
Servo interface
to amplify signals
Servo motors
move axis
Encoder sends
Positional data
To control unit
Are axis
at position?
yes
Program Continues
Machine Control Unit =
This part of the CNC machine is what
interprets (breaks down) the part program
and then instructs the various parts of the
machine to switch on/off, move, etc. It is a
series of microcontrollers with an operating
system. (typically a windows interface)
Axis Servo Drive
This will amplify the small electrical
signals sent from the control unit into
larger electrical signals which will then
be transferred to the seperate servo
motors on each of the axis slides
(X,Y,and Z)
Axis / Spindle Servo Motors
The servo motors are mounted onto each
slideway (X,Y,Z) and move the tool or the table
of the machine.
The servo motor will turn a number of degrees
each time a pulse is sent to it from the axis
servo drive. Each pulse may only move the
machine 0.01mm. In order to move the machine
1mm the servo motor may have to be pulsed
100 times.
Spindle servo motors may operate at much
higher speeds than axis servo motors.
Axis Positional Encoder
The CNC machine tools operate with a closed loop control system. For this purpose it is necessary to provide
appropriate feedback in order to achieve accurate control of the movement of the axes. The feedback that are
in general utilised are the displacement and velocities of the individual axes in the machine tool. The typical
positional sensors used in the CNC machine tools are following:
1.
Linear scales
To know the exact position of the axis (slide position) it is important to have the absolute position read in a direct way
(physically on the slide). The linear scale is a good method to do this. Linear means ‘in a straight line’.
The linear scale consists of a finely graduated grating made of either glass or stainless steel that is the measuring
surface attached to one part of the slide. A scanning unit is fixed to other part. The scanning unit consists of a light
source (such as infra-red LED’s), which shine through a glass grid along graduated windows & some photo diodes as
receptors to detect how many pulses of light are sent through the small holes in the ‘moire fringes’
The basic principle employed in such measurements is that when two gratings overlap each other, a Moirè fringe
pattern is formed corresponding to the displacement. The actual distance moved can be calculated by measuring
the shift in the fringe pattern
CAD CAM – Principles and Applications - EBook
http://books.google.co.uk/books?id=QaY_sQC45YC&pg=PT274&lpg=PT274&dq=cnc+moire+fringes&source=bl&ots=G7mQtEvmHe&sig=9e4FOYHzpwc1zNBc3QjrOm2R3o&hl=en&sa=X&ei=r5ouUcHiA6qX1AWXtYGABw&ved=0CF8Q6AEwBw
2. Encoders
The encoder is a transducer that is linked directly to the rotor or the lead screw and therefore does the simplest
arrangement need no additional gearing. An optical rotary encoder changed the rotary motion of the motor into a
sequence of digital pulses. The pulses counted to change to the position measurement. The optical encoder contains a
disc having a number of accurately etched equidistant lines or slots along the periphery. The encoder disc is associated
to the shaft of the machine whose rotary position has to be measured. The disc is placed between a light source
(generally infra red LED) and a light measuring device (photo diode) as shown in Figure 6. When the disc rotates the
lines are interrupted and the light measuring device counts the number of times the light is interrupted. By a careful
counting and proper calculations it is possible to know the position traversed by the shaft.
The rotary encoder is normally mounted on the servo motor shaft or at the end of the lead screw as illustrated.
This allows the control to calculate the actual distance moved from the rotary motion by using the lead of the
lead screw. If the lead screw contains any backlash, then that will be reflected in the position indicated by the
encoder. Therefore it becomes mandatory to remove the backlash in the lead screw to correctly get the position
of the axis. Also it requires that the pitch of the lead screw be more correct over its entire length for exact sensing
of the position.
http://www.expertsmind.com/topic/cnc-machine-tools/feedback-elements-99896.aspx
Advanced Positional Encoder (Increased Accuracy)
This will send a positional value back to the machine control unit.
How Do They Work?
Uniquely coded inserts are placed between the precision balls in the scale. The inserts contain a small magnet that can
be detected by a series of hall sensors contained in the reader head. The density of the inserts and the detectors within
the reader head allows the encoder to determine fully absolute position.
Once the encoder has determined the true absolute position it is then a matter for the Digital Signal Processor (DSP) to
handle communication of the positional data to the control unit. Being a DSP based absolute system capable of a high
level of processing; the encoders are error mapped during manufacturing against a laser interferometer. This error map
is stored in FLASH memory allowing it to be applied in real-time thus resulting in a highly accurate encoder.
http://www.newall.co.uk/products/?name=Absolute+Encoders+for+Feedback+for+CNC%2FPLC&catid=69
Spindle Speed Rotary Encoder
This will send a series of pulses back to the machine control unit
each time the spindle of the machine rotates once (as a measured
number) to be converted by the control unit CPU and then
compared to the requested speed (RPM or revolutions in m/min).
If the spindle is running too fast or slow then the machine control
unit will increase or decrease the amount of pulses it then sends
to the spindle servo driver and then the spindle servo motors.
It is called an incremental rotary encoder. The location of
this encoder is to be mounted on the end of the spindle.
When the spindle rotates the encoder shaft rotates at
the same speed.
An incremental
encoder produces a
known number ‘n’
of digital pulses for
each 360-degree
rotation of the
encoder's shaft. (Each
rotation of the shaft
may produce 50 – 500
pulses)
Manufacturers of Rotary Encoders and Positional Systems
http://www.dynapar.com/Technology/Encoder_Basics/Incremental_Encoder/
http://www.pepperl-fuchs.co.uk/great_britain/en/classid_195.htm
Overview of CNC Machine
The machine operator has to be safely protected from the hazards of the machine. If at any time the door is opened
then a brpken signal is sent to the control unit which prevents the machine from being started, or it will stop if in-cycle
and the door is opened. If during the program coolant is required then the operator can switch on the coolant pump
manually using the keyboard or it may be started/stopped automatically in the program (M08=ON, M09=OFF)
G&M Program
Keyboard /
Front Panel /
Operators interface
Machine Control Unit
VDU Screen
(Monitor)
Safety switches
(Door interlocks)
Coolant Pump
Magnetic (reed)
switches
Door interlocks
Axis servo
/ motors
Spindle servo
/ motors
The VDU screen typically displays the program and any feedback from the machine control unit.