ME3242 Industrial Automation Reference Text: Lecturer: Please: Industrial Automation, by David W. Pessen John Wiley & Sons, 1990. Prof Poo Aun Neow [email protected] Room: EA-07-32 When emailing me, please indicate ME3242 or TM3242 in Subject. IA - Introduction
1 Sequencing with purely pneumatic logic Objectives: To learn to design purely pneumatic logic circuits for the sequencing of pneumatic cylinders so as to achieve automatic operation. We will learn to use two approaches: the Cascade Method and the Lucas Method. We will also learn about some miscellaneous topics in pneumatics such as speed control of cylinders, operational and emergency stop operations. IA - Introduction 2 Why Pneumatics?
Air is clean, convenient and readily available in the factory floor. No problem with exhaust. Preferred for small or medium-size systems where actuators are pneumatic cylinders. Often preferable to use same power medium throughout system, avoiding need for interface. Several methods available for systematic design of purely pneumatic sequencing systems. IA - Introduction 3 Review:- Pneumatic Cylinder with Flow-control valve For the 4/2 valve, only one box can be operative at any time.
The right(left) box will be in operation when the last pressure signal is -(+). If there is no pressure signal on either side, the valve remains at rest in its current position. There should not be air pressure signals to BOTH sides at the same time. 4 port, 2 position flow control valve, or 4/2 valve When the right (left) box is in operation, the cylinder retracts(extends). IA - Introduction 4
Review: Notation for specifying sequence Sample requirement: An automatic drilling machine is designed to have three pneumatic cylinders for automatic operation. A typical cycle of operation is initiated by actuating a START pushbutton. A clamp cylinder A then retracts thereby actuating a clamp mechanism to hold the workpiece. The drill cylinder B then extends to perform the drilling operation at the end of which it immediately retracts. The clamp cylinder A extends after which a third cylinder C extends to push the workpiece away and then retracts. The cycle is then complete. Description of sequence: START A+ B+ B- A- C+ CIA - Introduction 5
Notation: Important note By convention, A- is always used for the at-rest position of Cylinder A, and A+ the other position. Thus in any sequence, the first action of any cylinder X is always X+, i.e. moving from rest to the other position. Note that when at rest (initial state), a cylinder could be either retracted or extended depending upon the application. A + A - + A-
A+ IA - Introduction A- - A+ 6 End of Introduction IA - Introduction 7
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