Control Systems
 Open-Loop Control
– Stepper motor system
– Current pulses sent from control unit to motor
– Each pulse results in a finite amount of revolution of the motor 001” is possible
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Open Loop System
Uses stepping motor to create movement. Motors rotate a fixed amount for each pulse received from the MCU. The motor sends a signal back indicating that the movement is completed. No feedback to check how close the actual machine movement comes to the exact movement programmed.
Ken Youssefi
Rensselaer Polytechnic Institute
Mechanical Engineering Department
Intro to CAM
Chiappone
3

Control Systems
 Open-Loop Limitations
– Control unit “assumes” desired position is achieved
– No positioning compensation
– Typically, a lower torque motor
 Open-Loop Advantages
– Less complex, Less costly, and lower maintenance costs
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Closed Loop System
AC, DC, and hydraulic servo-motors are used. The speed of these motors are variable and controlled by the amount of current or fluid.
The motors are connect to the spindle and the table. A position sensor continuously monitors the movement and sends back a single to
Comparator to make adjustments.
Ken Youssefi
Rensselaer Polytechnic Institute
Mechanical Engineering Department
Intro to CAM
Chiappone
5

Control Systems
 Closed-Loop Control
–
Variable DC motors - Servos
–
Positioning sensors -Resolvers
» Feedback to control unit
» Position information compared to target location
» Location errors corrected
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Control Systems
 Closed-Loop Advantages
– DC motors have the ability to reverse instantly to adjust for position error
– Error compensation allows for greater positional accuracy (.0001”)
– DC motors have higher torque ranges vs.. stepper motors
 Closed-loop limitations
– Cost
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Three Basic Categories of Motion Control
Systems
 Point to Point - No contouring capability
 Straight cut control - one axis motion at a time is controlled for machining
 Contouring - multiple axis’s controlled simultaneously
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Three Basic Categories of Motion Control
Systems
Rensselaer Polytechnic Institute
Intro to CAM
Chiappone

Ken Youssefi
Machine Control Unit (MCU) the brain of the NC machine.
The Data Processing Unit (DPU) reads the part program.
The Control Loop Unit (CLU) controls the machine tool operation.
Mechanical Engineering Department 10

Ken Youssefi
The NC controller is the brain of the NC system, it controls all functions of the machine.
• Motion control deals with the tool position, orientation and speed.
•
Auxiliary control deals with spindle rpm, tool change, fixture clamping and coolant.
Many different types of controllers are available in the market (GE, Fanuc, Allen-Bradley, Okuma, Bendix, …).
There are two basic types of control systems: point-to-point and continuous path .
Mechanical Engineering Department 11

Ken Youssefi
Point-to-point control systems cause the tool to move to a point on the part and execute an operation at that point only.
The tool is not in continuous contact with the part while it is moving.
Drilling, reaming, punching, boring and tapping are examples of point-to-point operations.
Mechanical Engineering Department 12

Continuous-path controllers cause the tool to maintain continuous contact with the part as the tool cuts a contour shape.
These operations include milling along any lines at any angle, milling arcs and lathe turning.
Ken Youssefi Mechanical Engineering Department 13

Ken Youssefi
• Develop or obtain the 3D geometric model of the part, using CAD.
• Decide which machining operations and cutter-path directions are required (computer assisted).
• Choose the tooling required (computer assisted).
• Run CAM software to generate the CNC part program.
• Verify and edit program.
• Download the part program to the appropriate machine.
• Verify the program on the actual machine and edit if necessary.
• Run the program and produce the part.
Mechanical Engineering Department 14

 Larger part program storage running to MB rather than kB or single blocks in the previous controllers.


Part program graphical proving and editing.
Part program generation using conversational part programming methods such as
FAPT TURN.
 Tool life management function, which includes larger number of tool offset registers as well as monitoring the life of the individual tools used.
 Background part programming methods.
 Drip feeding of part programs when they are very large in cases such as finish machining of 3D contours of dies and moulds.
16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 15





Better interfaces to outside world (serial as well as parallel communications)
Diagnostic facilities with the possibility of direct linking with the service centres using modems.
Enhanced DNC functions with links to factory networks
Use of standard operating systems such as Windows 95/98 with the associated use of the controller for other functions.
 Better shop floor control by the use of two-way linking through the PLC with the outside world.
 Enhanced machine control such as adaptive control, lead screw pitch error compensation, thermal compensation, etc.
 Enhanced machine control for high speed machining by having a look-ahead facility.
 Multiple axis machining with more axes simultaneously.
16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 16

 Enhanced part programming facilities such as
 Complex interpolations such as parabolic and helical
 Additional canned cycles (other than the drilling series G 80 to 89)
 Repetitive part programming using functions such as DO loops,
 Use of subroutines and macros
 Probes for inspection programs
 Use of parameters in part programming
 Help for operator instructions
 Special geometric calculation facilities
16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 17

16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 18

16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 19

16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 20

16 April 2020
(c) TMH, New Delhi, CAD/CAM Principles and
Applications by P N Rao, 3rd Ed 21