Lecture 17 position systems of nc [compatibility mode]

Lecture By: Muftooh Ur Rehman Siddiqi

ME-418 CAD/CAM

Lecture #: 17

Position Control System of NC

Ref: Groover (Chap – 06)

Faculty of Mechanical Engineering

Ghulam Ishaq Khan Institute of Engineering Sciences & Technology, Pakistan.

Lecture By: Muftooh Ur Rehman SiddiqiResearch Associate

Office # G-17Email: muftooh@giki.edu.pk

NC Positioning SystemsNC Positioning Systems

• The NC positioning system converts the coordinates axis valuesin the NC part program into relative positions of the tool andworkpart during processing.

• The worktable moves linearly by means of a rotating leadscrewwhich 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

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The table moves a distance of p at every rotation of the leadscrew.

Figure shows a typical motor and lead screw arrangement in NC positing system

NC Positioning SystemsNC Positioning Systems

There are two types of positioning systems used in NCsystems

(a) Open loop system

(b) Closed loop control

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Open Loop systemOpen Loop systemOperates without verifying that the actual positionachieved in the move, is the same as the desired position.

Open loop systems cost less than the closed loop systemsand are appropriate when the force resisting the actualmotion is minimal.

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Closed Loop ControlClosed Loop Control

Uses a feedback measurements to confirm that the finalposition of the worktable is the location specified in theprogram

Normally specified for machines that perform continuouspath operations such as milling or turning, in which thereare significant forces resisting the forward motion of the

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are significant forces resisting the forward motion of thecutting tool.

Closed Loop Control Closed Loop Control –– Optical EncoderOptical EncoderFrequency of the pulse is computed to determine the position and velocity of worktable.

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Engineering Analysis of NC positioning SystemsEngineering Analysis of NC positioning Systems

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Engineering Analysis of NC positioning SystemsEngineering Analysis of NC positioning Systems

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Precision in NC PositioningPrecision in NC Positioning

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Assignment

Solve the following problems from Groover Chapter 6

Submission date 12nd Dec 2011.

Open loop

24, 25, 26, 27

Closed Loop

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Closed Loop

29, 31, 32, 34, 36

Resolution and Accuracy

38, 39, 40, 41

Batch Production, 8th Dec 2011.

Thank you…

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Thank you…