Open-loop Control System

Report
BMFS 3373
CNC TECHNOLOGY
Lecture 2
Lecture Objectives
 At the end of the lecture, you will be able to:
 Describe the two types of control systems used to output tool
movement
 Explain the two types of loop systems used with CNC
controller
 Explain how points are located using Cartesian coordinate
system and the CNC axis of motion
 Explain the significant difference between incremental and
absolute positioning
CNC Components
 CNC system required two interfaces for it to function
properly.
 The Operator interface that consists of the control
panel and the various connection for the input
media.
 The Machine tool that consists of a controller
interface , the axis control and a power supply.
CNC Terminology
BLU: basic length unit 
smallest programmable move of each axis.
Controller: (Machine Control Unit, MCU) 
Electronic and computerized interface between operator and m/c
Controller components:
1. Data Processing Unit (DPU)
2. Control-Loops Unit (CLU)
Part Program: 
A part program is a series of coded instructions required to produce a part. It controls the movement of
the machine tool and the on/off control of auxiliary functions such as spindle rotation and coolant. The
coded instructions are composed of letters, numbers and symbols and are arranged in a format of
functional blocks as in the following example
N10 G01 X5.0 Y2.5 F15.0
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Feed rate (15 in/min)
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Y-coordinate (2.5")
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X-coordinate (5.0")
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Linear interpolation mode
Sequence number
CNC Components
CNC System Elements
A typical CNC system consists of six elements
1. Part program
2. Program input device
3. Machine control unit
4. Drive system
5. Machine tool
6. Feedback system
NC System Elements
Controller Components
Data Processing Unit:
Input device [RS-232 port/ Tape Reader/ Punched Tape Reader]
Data Reading Circuits and Parity Checking Circuits
Decoders to distribute data to the axes controllers.
Control Loops Unit:
Interpolator to supply machine-motion commands between data points
Position control loop hardware for each axis of motion
Machine Control Unit
The machine control unit (MCU) is the heart of a CNC system.
It is used to perform the following functions:
 Read coded instructions
 Decode coded instructions
 Implement interpolations (linear, circular, and helical) to
generate axis motion commands
 Feed axis motion commands to the amplifier circuits for
driving the axis mechanisms
 Receive the feedback signals of position and speed for each
drive axis
 Implement auxiliary control functions such as coolant or
spindle on/off, and tool change
Drive System
 A drive system consists of amplifier circuits, stepping
motors or servomotors and ball lead-screws. The MCU
feeds control signals (position and speed) of each axis to
the amplifier circuits. The control signals are augmented
to actuate stepping motors which in turn rotate the ball
lead-screws to position the machine table.
Types of CNC machines
Based on Motion Type:
Point-to-Point
or
Continuous path
or
Closed loop
or
Hydraulic
Based on Control Loops:
Open loop
Based on Power Supply:
Electric
Based on Positioning System
Incremental
or
Absolute
or
Pneumatic
Open-loop Control System
 In open-loop control system step motors are used.
 Step motors are driven by electric pulses.
 Every pulse rotates the motor spindle through a
certain amount.
 By counting the pulses, the amount of motion can
be controlled.
 No feedback signal for error correction.
 Lower positioning accuracy .
Operational Features of CNC Machines
Closed-loop Control Systems
 In closed-loop control systems DC or AC motors are
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used.
Position transducers are used to generate position
feedback signals for error correction.
Better accuracy can be achieved.
More expensive.
Suitable for large size machine tools.
Operational Features of CNC Machines
Operational Features of CNC Machines

NUMERICAL DATA MAY BE FED INTO THE SYSTEM BY STORED DATA.
A TRANSLATNG UNIT READ THE DATA AND CHANGES IT INTO AN ELECTRICAL
FORM THAT THE MACHINE TOOL CAN UNDERSTAND.
 A MEMORY SYSTEM STORES THE DATA UNTIL IT IS NEEDED.
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ROTARY RESOLVERS OR SERVO UNITS (TRANSDUCERS) ON THE MACHINE TOOL
CONVERT THE DATA INTO THE REQUIRED MACHINE MOVEMENTS.
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A GAGING DEVICE MEASURES THE MACHINE MOVEMENTS TO DETERMINE IF
THE SERVO UNITS HAVE GIVEN THE CORRECT COMMANDS.

A FEEDBACK UNIT FEEDS INFORMATION BACK FROM THE GAGING DEVICE FOR
COMPARISON TO SEE THAT THE MACHINE HAS MOVED THE REQUIRED
AMOUNT.
Open Loop VS Closed Loop Controls
output
input
output
Open Loop VS Closed Loop Controls
Cartesian Coordinate System
 The machine tool movements used in producing a
product consists of two types.
1. Point-to-point ( straight line movements)
2. Continuous Path (Contouring Movements)
 Any specific point can be described in mathematical
terms from any other point along three
perpendicular axis.
 This concepts fits machine tools perfectly since their
construction is generally based on three axis of
motion (X,Y,Z) plus an axis of rotation.
Cartesian Coordinate
CNC Vertical Spindle Machine
Positioning System
Absolute Positioning
 In the absolute system (G90), all dimensions or positions are given
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from a fixed zero or reference point.
The zero or origin point may be a position on the machine table, such
as the corner of the worktable or at any specific point on the workpiece.
In absolute dimensioning and programming, each point or location on
the workpiece is given as a certain distance from the zero or reference
point, measured along an axis.
By referring to the next slide, all positions on the part are taken from
the X0/Y0 point at the lower corner of the part.
The first hole would have coordinates of X1.000, Y1.000; the second
hole coordinates are X2.000, Y1.000; the third hole coordinates are
X3.000, Y1.000.
Absolute Positioning
Positioning System
Incremental Positioning
 In the incremental system (G91), dimensions or positions are given
from a previous known point.
 Incremental program locations are always given as the distance and
direction from the immediately preceding or latter point.
 By referring to the next slide, the first hole coordinates are X1.000,
Y1.000; the second hole coordinates are X1.000,Y1.000.
 After each machine move, the current location is reset to X0/Y0 for the
next move. The machine controller does not reference any common
zero point.
Incremental Positioning
Setting the Machine Origin
 Most CNC machinery have a default coordinate
system.
 The origin of this system is called the machine origin
or home zero location.
 Home zero is usually located at the tool change
position of a machining center.
 There are three ways a zero point can be set on CNC
machines.
1. Operator
2. Program absolute zero shift
3. Work Coordinates
Setting the Machine Origin
Manual setting
 The setup person positions the spindle over the desired
part zero and zeros out the coordinate system on the MCU
console. The actual keystroke sequence for accomplishing
this varies from controller to controller.
Absolute Zero Shift
 An absolute zero shift is a transfer of the coordinate system
inside the NC program.
 The programmer commands the spindle to the home zero
location. Next, a command is given that tells the MCU how
far from the home zero location the coordinate system
origin is to be located.
Setting the Machine Origin
Work Coordinates
 A work coordinate is a modification of the absolute zero
shift.
 Work coordinates are registers in which the distance from
home zero to the part zero can be stored.
 The part coordinate system does not take effect until the
work coordinate is commanded in the NC program.
Dimensioning
Datum Dimensioning
 All dimensions on a drawing are placed in reference
to one fixed zero point. Datum dimensioning is
ideally suited to absolute positioning equipment.
Dimensioning
Delta Dimensioning
 Each location is dimensioned from the previous one.
 Delta drawings are suited for programming
incremental positioning machines.
CNC Safety
 The use of CNC machine tools requires at least the same
precautions as with conventional machine tools, including eye
protection and standard setup procedures and operation.
 Do not let others distract you from your work. Your
concentration is critical when setting-up and operating
machine tools. Likewise, do not distract others they need to
concentrate.
 You should never work inside a CNC machine tool when
someone else is touching the controls. They might
accidentally start the machine and cause serious injury to you.
In fact, it is a good idea to put the machine in a locked mode
such as “edit” before changing the tools or workpiece.
CNC Safety
 You should always calculate speeds and feeds for CNC
machining because you will not be able to feel when the
cutting conditions are correct.
 Special care should be used when using rapid transverse
on a CNC machine tools, including allowing an adequate
distance above the workpiece and not using rapid traverse
below the surface of the part.
 Part programs should be tested by first using a computer
simulation on the PC, simulating on the control , and then
dry-running above the workpiece at a reduced rate and in
single block mode.
End
Chapter 2

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