Tech Briefs Magazine - August 2021 - MD-10

es the spool back to the normal position,
supplying air to the retract side of the
press cylinder.
SUPPLY
A
SOL06
P R
A B
VLV05
SUPPLY
SUPPLY
VLV07
A
R
P
VLV08
A
R
P
R P S
I
CYL03
While only one button must be re -
leased to retract the press cylinder, if
either pneumatic pushbutton is tied
down (held pressed), only one button
could be used to operate (extend) the
press. This would not be safe. In most
systems with high press force, additional
safety systems would need to be added to
check that both buttons have been
released after each cycle and that both
buttons are pressed at the same time,
within about a quarter of a second,
before supplying pilot air to the direction
valve VLV03. Control reliable features
could also be added to ensure a
single failure doesn't allow the system to
operate unsafely.
Figure 4. Continuous cycle cylinder. Start cycle by energizing 3-way solenoid valve. The cylinder will cycle
until valve is turned off. Cycle always ends with cylinder retracted.
The key pneumatic logic components
of this circuit are the 4-way air-piloted
valve (VLV05), which is the directional
control valve, and the two 3-way rolleractuated
valves (VLV07 and VLV08),
which do the same job as electrical solenoids
to control the spool position of
VLV05. Instead of electricity, pilot air
alone controls VLV05. Valves VLV07 and
VLV08 are configured like limit switches
with a mechanical arm. Cams or flags on
the cylinder are used to actuate the
valves. When not activated, the valves
spring return to their normal position.
Two-Hand Control Circuit
The pneumatic circuit in Figure 5
details a pneumatic two-hand safety control
system for a press application. The
SUPPLY
P
RR P S
A B
A
P
R
A
VLV01 VLV02
VLV03
circuit combines pneumatic buttons
VLV01 and VLV02 configured as 3-way
valves. These valves feed pilot air to a 4way
valve (VLV03). This circuit also highlights
how small valves - the pneumatic
pushbuttons - can use pilot air to operate
a large valve controlling a large press
cylinder with high airflow requirements.
Although not shown in this circuit, antitiedown
checks could be added to further
improve the safety of this design.
Both pneumatic buttons must be
pressed simultaneously to cascade pilot
air to the directional control valve
VLV03. The supply of pilot air switches
the valve spool, causing the double-acting
press cylinder CYL01 to extend.
When either pushbutton is released, the
spring return function of VLV03 switchCYL01
As
discussed above, one-way flow control
valves throttle the air exiting the
cylinder to control the speed. While only
the press extend speed is controlled in
this circuit, a second speed control could
be added to control the retract speed.
An important note when controlling
air exiting the cylinder is that air must
be present to control the speed. If all the
air is exhausted due to an emergency
stop or idle air leakage, the cylinder may
move very fast the first cycle. To eliminate
this problem, sometimes the flow of
air into instead of out of the cylinder is
used to control its motion.
Additional upgrades to this circuit
that are not shown include adding a
pressure regulator to control the extend
pressure (force) of CYL01. A pressure
switch could also be added for errorproofing.
The switch would sense that
minimum press pressure was met and
provide a press force okay signal to a
PLC, for example.
The variety of pneumatic circuits is
endless but these four basic pneumatic
circuit examples show how common
pneumatic components can be combined
to perform useful automation
functions. With a little imagination,
FRLs, valves, flow controls, cylinders,
buttons, actuators, and other pneumatic
components can be combined in a variety
of ways to meet the needs of almost
any pneumatic system.
This article was written by Pat Phillips,
Figure 5. Two-hand control circuit diagram. This pneumatic circuit uses two-hand control methods, via manual
pneumatic pushbuttons, to improve the safe operation of small presses.
10
Cov
ToC
Product Manager, Fluid Power & Mechanical
Products, AutomationDirect (Cumming, GA).
For more information, visit http://info.
hotims.com/79416-283.
Motion Design, August 2021
http://info.hotims.com/79416-283 http://www.abpi.net/ntbpdfclicks/l.php?202108MDNAV

Tech Briefs Magazine - August 2021

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