This document summarizes a lecture on position control systems for numerical control (NC). It discusses two types of positioning systems - open loop and closed loop control. Open loop systems operate without feedback to verify the tool reaches the desired position, while closed loop systems use feedback like optical encoders to confirm accurate positioning. The lecture also covers engineering analysis of NC positioning systems and precision.

1. ME-418 CAD/CAM
Lecture #: 17
Position Control System of NC
Ref: Groover (Chap – 06)
Lecture By: Muftooh Ur Rehman Siddiqi
Research Associate
Office # G-17
Email: muftooh@giki.edu.pk
Faculty of Mechanical Engineering
Ghulam Ishaq Khan Institute of Engineering Sciences & Technology, Pakistan.

2. NC Positioning Systems
•
The NC positioning system converts the coordinates axis values
in the NC part program into relative positions of the tool and
workpart during processing.
•
The worktable moves linearly by means of a rotating leadscrew
which is driven by a stepping motor or servomotor.
•
The leadscrew has a certain pitch p (in/thread, mm/thread).
The table moves a distance of p at every rotation of the lead
screw.
Figure shows a typical
motor and lead screw
arrangement in NC
positing system
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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3. NC Positioning Systems
There are two types of positioning systems used in NC
systems
(a)
Open loop system
(b)
Closed loop control
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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4. Open Loop system
Operates without verifying that the actual position
achieved in the move, is the same as the desired position.
Open loop systems cost less than the closed loop systems
and are appropriate when the force resisting the actual
motion is minimal.
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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5. Closed Loop Control
Uses a feedback measurements to confirm that the final
position of the worktable is the location specified in the
program
Normally specified for machines that perform continuous
path operations such as milling or turning, in which there
are significant forces resisting the forward motion of the
cutting tool.
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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6. Closed Loop Control – Optical Encoder
Frequency of the pulse is
computed to determine
the position and velocity
of worktable.
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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7. Engineering Analysis of NC positioning Systems
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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8. Engineering Analysis of NC positioning Systems
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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9. Precision in NC Positioning
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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10. Assignment
Solve the following problems from Groover Chapter 6
Submission date 12nd Dec 2011.
Open loop
24, 25, 26, 27
Closed Loop
29, 31, 32, 34, 36
Resolution and Accuracy
38, 39, 40, 41
Batch Production, 8th Dec 2011.
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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11. Thank you…
© Muftooh Ur Rehman Siddiqi, FME GIK Institute, Pakistan.
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