Problem Description:
The industry uses many control circuits. In many industrial control
systems, for instance, there can be many more than a pair of pneumatically actuated operators that must perform their functions in a
specific sequence. Such a process is called sequential control. Here, our goal
is to show how to control two double-acting pneumatic actuators in a
predetermined sequence.
Hence, control valves,
limit valves, and pneumatic logic will be employed
in the control of two pneumatic
cylinders that are in parallel and undergo two-action movements. In this way,
we understand the importance of
sequence control to understand pneumatic logic.
Diagram:
Condition 1: A+ (Cylinder A Extends)
When the start push button
is pressed, compressed air enters at the pilot port of 5/2 valve in cylinder A.
The spool will shift, allowing air to enter the left chamber of Cylinder A.
The air inside
is exhausted through
port R/S.
Consequently, Cylinder A will move forward (A+).
Condition 2: B+ (Cylinder
B Extends)
The limit valve A1 is actuated when Cylinder A reaches its fully forward
position.
This sends a pneumatic signal
to the pilot port of Cylinder B's 5/2 valve.
The spool moves and air enters the front side of Cylinder
B.
Thus, Cylinder B extends (B+).
Condition 3: A– (Cylinder Retracts)
Thus, when cylinder
B attains its forward position,
limit valve B1 is actuated.
This signal
is fed back to pilot port of Cylinder A’s valve.
When this vale detects
motion, it switches
position.
Air now enters
through the rod side of Cylinder A and exhausts
from the other side.
Therefore, Cylinder
A retracts (A−).
Condition 4: B− (Cylinder B Retracts)
When Cylinder A fully retracts,
the limit valve A0 gets actuated.
This signal
causes Cylinder B's valve to go to its return
position.
Air enters on the rod side of Cylinder B and exhausts
at the forward side.
Finally, Cylinder
B retracts (B−).
Solution:
We could understand from the figure the use of pneumatic logic to
find the sequential action of two double-acting cylinders. This in turn
controls cylinder 'A' and cylinder 'B' in a fixed sequence by the use of limit
valves and directional control valves.
Under the first
condition, upon pressing the start
push button, compressed air is fed to the directional control valve of cylinder A. For this reason,
cylinder A moves in a forward direction, which is represented as A+.
When cylinder A extends to its full position, it actuates a limit
valve. The limit valve sends a pneumatic signal to the control valve of
cylinder B. This allows the cylinder B to extend, which is represented as B+.
When cylinder B arrives at the forward position, another limit
valve is triggered. The signal shifts the
position of the directional control valve of cylinder A, and cylinder A begins
to retract-at position A−.
When it is fully retracted, cylinder A turns on another limit
valve. The signal goes to the control valve of cylinder B. Cylinder B retracts
back to the initial position denoted as B−.
In this manner, the entire sequence A+ B+ A− B− is realized employing exclusively pneumatic devices.
List of Inputs
1 -Start
Push Button
2- Limit Switch of Cylinder A (Forward)
3- Limit Switch of Cylinder B
(Forward) 4– Limit Switch
of Cylinder A (Backward)
Program Description:
·
In the first step, when the start push button
is pressed, the output is activated, and cylinder A extends.
·
When cylinder A reaches the forward position, the limit switch
labeled is activated, so the output labeled activates cylinder B.
·
When cylinder B is at forward position, limit switch will turn ON, and
cylinder A will retract.
·
Also, when cylinder
A is in backward position, limit switch is ON and cylinder B retracts.
·
Thus,
the whole process of A+ B+ A- B- occurs smoothly.