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Slide 3
Speed control The only way to change the extension speed of the
piston of a single acting cylinder is to restrict the flow of air
at the inlet and use the spring to determine the speed of
retraction, Therefore, a one way flow control valve is placed in
the circuit to control the speed. It is always necessary to reduce
the speed of cylinder from maximum speed based on selected size of
final control valve to the nominal speed depending on the
application. Speed control of pneumatic cylinders can be
conveniently achieved by regulating the flow rate supply or exthaus
air. The volume flow rate of air can be controlled by using flow
control valves which can be either two way flow control valve or
one way flow control valve.
Slide 4
Flow control valve One way flow control valve is often used to
achieve independent speed control of cylinder in the forward and
return motion. This has a variable restrictor and a non return
valve in paralel. Two flow control valve is essentially a valve
with variable restrictor which offers resistance to passage of air
in both direction. OPENCLOSE
Slide 5
One way flow control valve Generally used for speed control of
cylinder and is installed in the working pressure line, between the
final control valve and the cylinder ports. When the flow takes
place in the reverse direction, pressure exerted by the compressed
air from the bottom of the diaphragm, opens it up against the seat
and directly by passes the air without undergoing throttling. Then
compressed air has to pass through the gap between needle and
orificle of the valve which results in throttling. When compressed
air is admitted in the direction of throttling (left to right) it
exerts force above the diaphragm and holds it against the seat.
This prevents by passing of air through the gap between diaphragm
and seat.
Slide 6
Quick exhaust valve In many applications especially with single
acting cylinders, it is a common practice to increase the piston
speed during retraction of the cylinder to save the cycle time.
This is carried out by incorporating a quick exhaust valve. The
quick exhaust valve has essentially three ports. Supply port 1, is
connected to the out put of the final control element (directional
control valve). The output port, 2 of this valve is directly fitted
on to the working port of cylinder. The exthaus port, 3 is left
open to the atmosphere. Symbol for Quick Exhaust Valve
Slide 7
Quick Exhaust Valve forward motion During forward movement of
piston, compressed air is directly admitted behind the piston
through ports 1 and 2, port 3 is closed due to the supply pressure
acting on the diaphragm. Port 3 is usually provided with a silencer
to minimise the noise due to exhaust.
Slide 8
Quick Exhaust Valve return motion During return movement of
piston, exhaust air from cylinder is directly exhausted to
atmosphere through opening 3 (usually larger and fitted with
silencer). Port 2 is sealed by the diaphragm. Thus exhaust air is
not required to pass through long and narrow passages in the
working line and final control valve.
Slide 9
Control of single actuated cylinder with flow control valve To
control the speed of the piston rod, flow restrictors are placed in
the pipes close to each of the valves. Adjustment of the
restrictors will slow down the flow rate thereby giving independent
outstroke and instroke speed control 10121012 OUTIN 2 1 1 2 2/2
directional valve
Slide 10
Control of single actuated cylinder with flow control valve By
repeated operation of either button during movement the piston rod
can be moved in small steps for approximate positioning. This will
only be successful under slow speeds. 10121012 OUTIN 2 1 1 2 2/2
directional valve
Slide 11
Control of single actuated cylinder with flow control valve
With any compressed air system that intentionally traps air, the
potential hazard of this must be recognised. Unintended release or
application of pressure can give rise to unexpected movement of the
piston rod. A pressure indicator or gauge must be fitted to warn of
the presence of pressure. 2 10 1 12 1 2 1012 OUTIN 2/2 directional
valve
Slide 12
Control of single actuated cylinder with flow control valve To
generally slow the cylinder speed an adjustable. bi- directional
flow regulator or fixed restrictor can be used. The flow regulator
setting will be a compromise as the ideal outstroke speed may not
produce the desired results for the instroke speed. 3/2 directional
valve 1 2 3 12 10
Slide 13
Control of single actuated cylinder with flow control valve To
control the outstroke speed of a single acting cylinder without
controlling the instroke speed, a uni-directional flow regulator is
used. The flow into the cylinder closes the non return valve and
can only pass through the adjustable restrictor. By adjusting the
restrictor the outstroke speed of the cylinder can be set. 3/2
directional valve 1 2 3 12 10
Slide 14
Control of single actuated cylinder with flow control valve For
independent speed control in each direction two flow regulators are
required. Installed in opposite directions to each other. Upper
regulator controls the outstroke speed. Lower regulator controls
the instroking speed. 3/2 directional valve 1 2 3 12 10
Slide 15
Control of single actuated cylinder with flow control valve A 3
port valve provides the inlet and exhaust path and is the normal
choice for the control of a single acting cylinder. In the normal
position produced by the spring, the valve is closed. In the
operated position produced by the push button the valve is open.
The push button must be held down for as long as the cylinder is
outstroked. 3/2 directional valve 1 2 3 12 10
Slide 16
Control of double actuated cylinder with flow control valve The
advance and return movement speeds of a double acting cylinder are
to be regulated. Separately adjustable exhaust throttling for
advance and return movement. Initial joil until forces are
equalised, but then better possibility for regulating (independent
of load). With 4/2 way valves, one-way flow control valves must be
used. With 5/2 way valves, two flow control valves are
adequate.
Slide 17
Control of double actuated cylinder with flow control valve
Separately adjustable supply air throttling for advance and return
movement. Steady initial motion, but poor possibility of
regulation. Cannot be applied for pulling loads.
Slide 18
Raising the speed of single acting and double acting cylinder
The return speed of a single acting cylinder is to be increased.
The speed of advance movement of a double acting cylinder should be
increased.
Slide 19
Example I. Liquid metal is drawn from a smelting crucible by a
casting ladle and cast in moulds. The raising and lowering of the
ladle is controlled by separate manual push buttons. The raising
and lowering speed is separately adjustable. Design a pneumatic
control circuit for this application.
Slide 20
Solution
Slide 21
Example II. Slow forward movement rapid return A double acting
cylinder is to travel out slowly and on reaching the end position
return rapidly to its initial position.
Slide 22
Solution When valve 1.2 is operated, the piston of the cylinder
travels out. The forward speed can be set on the one-way flow
control valve 1.02. Switchover of valve 1.3 causes the piston to
return to its initial position and the speed is increased by quick
exhaust valve 1.03. 1.21.3 1.1 1.02 1.03
Slide 23
Logic functions for poppet and spool valves To meet the
requirement of various conditions in pneumatic applications, signal
processing devices are often used. The following gates or valves
are used, depending on the required conditions. OR gate shuttle
valve used to select one of the two input signals. AND gate two
pressure valve to combine two input signals i.e to satisfy two
conditions at the same time. NOT gate 3/2 way, normally open, pilot
operated directional control valve used to negate the
function.
Slide 24
Shuttle valve logic OR An input signal (1) can be applied on
either side of the valve to obtain an out put signal at port 2. A
small aluminium or plastic ball or spool is used as the shuttle
which blocks the port opposite to the input signal. 11 22
Slide 25
Shuttle valve logic OR Source X and Y can be remote from each
other and remote from the destination of Z When X or Y is operated
the shuttle valve seal moves across to prevent the signal Z from
being lost through the exhaust of the other valve X Y Z 1 2 3 12 10
1 2 3 12 10
Slide 26
Control with shuttle valve The advance movement of a single
acting cylinder is to be effected from two different points. When
valve 1.2 is operated, the compressed air flows from P to A, and
with valve 1.6 from X to A, to the cylinder. The same occurs when
valve 1.4 is operated. The compressed air flow from P to A, on
valve 1.6 from Y to A, to the cylinder. If shuttle valve 1.6 were
not there, the air would escape through the exhaust of the other
non-operated valve on operation of 1.2 or 1.4. 1.2 1.4 PP AA RR XY
A 1.6
Slide 27
Control with shuttle valve The piston rod of a double acting
cylinder is to advance if one of the two pushbutton is actuated.
The piston rod is to return when the pushbutton is released. 1.21.3
1.4 1.5
Slide 28
5/2 OR function The valve at position a is reversed connected
and supplied from the valve conventionally connected at position b
The cylinder can be controlled from either position a OR position b
1 24 53 14 12 1 24 53 14 12 a b
Slide 29
Two pressure valve logic AND AND circuit is the circuit that
generates output only if all inputs exist in the circuit that has
more than two input circuits. It is called a serial circuit or
simultaneous operating circuit. The AND circuit usually is used for
safety control and inspection functions. This circuit is used to
work in a case that several workers press each of their buttons to
run the presser for the workers safety. If a second signal is
applied on the opposite side at the same time, it will be
communicated to the out put port.
Slide 30
AND gate combination I. The piston rod of a single acting
cylinder may move out only if two 3/2-way valves are operated.
Operation valves 1.2 and 1.4 produces a signal X and Y on the two
pressure valve (1.6), and thus the compressed air is applied to the
cylinder. X Y A 1.21.4 1.6
Slide 31
AND gate combination II. The piston rod of a single acting
cylinder may move out only if two 3/2-way valves are operated.
Valves 1.2 and 1.4 must be operated before the single acting
cylinder can move out. (Series connection) 1.2 1.4
Slide 32
AND gate combination III. The piston rod of a single acting
cylinder may move out only if two 3/2-way valves are operated.
Valves 1.2 and 1.4 mus be operated. Valve 1.6 assumes the AND
function. Valve 1.6 is switched over by valve 1.2 at control port
Z, compressed air from valve 1.4 flows through the port P. 1.21.4
1.6 Z P PP RR R A A A
Slide 33
OR, AND, NOT A single 3/2 pilot operated spring return valve
can be use for any of these logic functions x OR y gives output z x
AND y gives output z x gives NOT z 1 2 3 12 10 1 2 3 12 10 1 2 3 12
10 AND OR NOT x y x y z x z
Slide 34
Single pulse maker Converts a prolonged signal x into a single
pulse z Signal z must be removed to allow the valve to reset then x
can be applied again The duration of the pulse can be adjusted with
the flow regulator 1 2 3 12 10 x z
Slide 35
Single pulse control Each time the foot operated valve is
pressed the cylinder will single stroke + and - alternately First
foot operation the cylinder moves out Second foot operation the
cylinder moves in Third.. out and so on
Slide 36
Air conservation Power stroke in the instroke direction only
Differential area of the piston gives an outstroke force when the
pressure is balanced Air used to outstroke is equivalent to a
cylinder with only the same bore as the rod diameter Assumes the
cylinder is not loaded on the plus stroke and low friction 24 15 14
12 24 15 14 12
Slide 37
Double flow Where a larger 3/2 valve is not available Two flow
paths in a 5/2 valve each with a separate supply can be arranged to
give double flow or supply separate devices Ensure the tube size to
the cylinder is large enough to take the double flow 42 13 1214 51
42 13 1214 5
