• Taking the guesswork


  •   
  • FileName: ClippardWhitePaper.pdf [read-online]
    • Abstract: Taking the guessworkout of pneumatic controlHere’s a step-by-step approach toefficientlydesigning systemsthat work rightthe first time.Designers have several options whenconstructing pneumatic-control systems.

Download the ebook

Taking the guesswork
out of pneumatic control
Here’s a step-by-
step approach to
efficiently
designing systems
that work right
the first time.
Designers have several options when
constructing pneumatic-control systems.
But modular air-logic systems are often a
good bet when a compact, economical unit
is a must. They typically consist of a series
of valves mounted onto standard manifold
subplates. Such systems speed assembly
and piping, and all interconnections be- Pneumatic sequence controllers provide step-by-step
tween valves and controls in the circuit are machined into system operation. Sequence valves and other
the manifold subplate. This cuts design and installation components mount to the manifold subplates.
time, reduces the number of fittings, and eliminates piping
and the routing errors that often go with it.
For instance, compared with discrete air-valve control
systems, a modular system features: Hall-effect switches, back-pressure cylinder sensors, and
• Lower component costs. manual pushbuttons. Feedback signals provide positive,
• Simple plumbing and troubleshooting. safe operation. If no signals are sent (due to a component
• Lower air consumption. failure, missing or jammed parts, and so on) the sequence
• No air locks. stops and an indicator pinpoints where to troubleshoot. In-
• A smaller total package. ternal interlocks prevent out-of-sequence feedback signals.
Compared with electrical-relay control, a modular A pneumatic output signal at each step actuates air-pi-
system offers: loted devices, including power valves, hydraulic valves,
• An explosionproof system with no danger of burnouts. pressure switches, and other components that may control
• Lower power consumption.
• Lower costs by eliminating solenoids and relays.
• A single air supply. Start options
• No heat buildup. Manual start A selector
Only a few manufacturers offer modular, manifold- valve gives
mounted pneumatic control systems. For instance, Clip- the option of
pard’s Pneumatic Programmable Controller is a sequen- Start one-cycle
tial controller that provides step-by-step system opera- Manual Auto 8 7 3 4 operation or
tion. It consists of a clear acrylic manifold for mounting 6 continuous
sequence valves and other such components in a com- S 1 cycling.
pact, efficient package. Auto
start 5 2
The system is designed so that when an operation fin-
ishes, feedback signals automatically initiate the next step.
Many types of sensors can generate feedback signals, in-
cluding limit valves, proximity sensors, pressure sensors,
1
Two-cylinder sequence
air cylinders. The last sequence valve resets the Actuating
control Step number Function Limit sensor
system to repeat the cycle of operations.
Start valve 1. Extend cylinder A A+ LVA+ (limit valve)
Design basics LVA+ 2. Retract cylinder A A– LVA– (limit valve)
Modular systems can contain just a few LVA– 3. Extend cylinder B B+ LVB+ (Hall-effect
valves or dozens, with many built-in func- switch)
tions, permitting a systematic approach to cir- LVB+ 4. Retract cylinder B B– LVB– (Hall-effect
switch)
cuit design. As with any control system, it is LVB– 5. Reset sequence Reset
essential to outline system requirements to
save time and reduce the chances of missing a S S
critical step.
The designer must also have a clear under- + – + –
standing of the sequence of operations. This
should include a brief description of the ma-
chine’s actions along with a sketch or drawing. Power Power
LVB– Hall-effect
Note pressure, temperature, filtration, and other valve A A valve B B
operating conditions as well as control require- switches
LVB+
ments, including manual, automatic, start, stop,
LVA–
and so on. Then list available input signals
from limit valves, sensors, and other controls. LVA+
Determine all mechanical and safety interlocks
that are required. Finally, numerically list a Programming guiDelines
step-by-step sequence of operations that indicates function After defining the overall functions and requirements
and sensing method. of a system, engineers need to configure the system.
As a final check of circuit operation, consider proper ac- Here are some basic guidelines:
tuation during all conceivable events. This includes Step one is to label all components in the pneumatic cir-
startup, shut down, loss of air, panic stops in midcycle, re- cuit. A practice Clippard recommends is to label each cyl-
starts in midcycle, and control during any other event inder with a letter of the alphabet, starting with “A.” The
likely to occur. same holds for air motors and other controlled devices.
Manual jog control
Jog
Jog CONTROLLING Manual Auto BINARY
Jog
On Off AIR TO LIMIT TRIGGER
VALVES S CIRCUIT
Binary
trigger
Auto
start To 2 8
LV R
LVA+ LVA– LVB– LVB+ supply
Manual start LVA+ LVA– LVB– LVB+
Manual Auto
7 3 4
8 S In In In In In Sequence
6 SM1 SM2 SM3 SM4 SM5
modules
Auto start 1 R Out Out Out Out Out
5 2
R
Jog control lets users manually step through machine operations. This option can be added by
controlling flow to limit valves or adding a binary trigger.
2
sequence controller System shutoff
primer System
Off On
Indicator Emergency
stop
Sequence controllers provide a straightforward, step-
by-step approach to pneumatic system design and oper-
ation. For example, the pneumatic circuit shown here
includes five sequence modules, two modular power Supply
valves, and two cylinders, both with dual limit valves.
The circuit extends and returns Cylinder A, then ex-
tends and returns Cylinder B. Pressure
To SM1
In the inactive mode, LVA– and LVB– are held pass- pressure In Reset Out
switch
ing, supplying pressure to Port 8 of sequence modules S valves
SM3 and SM5. The valves will not actuate because System shutoff prevents accidental machine
supply pressure to Port 6 connects to a larger pilot. operation.
Pushing the start button applies pilot pressure to SM1,
shifting the valve and passing a pneumatic signal to
power module PVA at Port 4. This shifts PVA and ex- Next label the valve controlling the cylinder with the same
tends the cylinder. When SM1 shifts, it also removes letter. Label the pilot of the valve that extends the cylinder
pressure from SM2 at Port 6, preparing it for Step 2. (or activates a device) with a “+” symbol, and the pilot of
Extending Cylinder A actuates LVA+. This shifts the valve that retracts the cylinder (or turns off a device)
SM2 and provides pressure to Port 6 of PVA, which with a “–” symbol. Label the limit sensor the cylinder rod
shifts and powers the retraction of Cylinder A. Shifting strikes with the letters LV (limit valve), the letter of the
SM2 also removes supply pressure from Port 4 of valve cylinder, and position of the sensor for extending or re-
PVA. This lets the cylinder retract and removes supply tracting. LVA+, for example, would mean the limit valve
pressure from Port 6 of SM3, preparing it for Step 3. of an extended cylinder A.
As a result, when LVA– is in the passing position, no The second step is to list in detail every sequence of opera-
pressure is on the larger pilot of SM3. SM3 shifts and tion. This includes the action or control that initiates a step,
supplies Port 4 of PVB. The power module shifts and what function takes place during that step, and the limit sensor
the resultant flow extends Cylinder B. The sequence that ends the operation. An example of a simple two-cylinder
continues unless the start button remains actuated. Re- sequence is shown in the accompanying table.
traction of Cylinder B actuates LVB– causing a chain In step three, select components for the control system.
reaction that resets the sequence valves for the next cy- Modular systems offer a lot of flexibility because users can
cle. Replacing the start button with a toggle or selector choose from many options when selecting sequence valves.
valve permits continuous cycling. For instance, in the “Two-cylinder sequence” example,
This is an example of a typical sequence circuit. five steps — extending and retracting the two cylinders
Most applications require additional functions, such as plus the reset — require five sequence valves. A basic
delay-in and AND, that increase the number of se- valve could be used for each step, or designers could
quence valves, but procedures for connections and add- choose valves with special features. For example, the first
ing steps remain the same. two steps could use a valve that provides a sequence reset
Input signals can come from many devices, includ- lock if the start button is held down or if the limit valve
ing limit valves LVA+ is
and feedback de- locked
vices such as Pneumatic sequence controller down.
proximity, pres- Start Reset
LVA+ LVA– LVB+ LVB–
sure, Hall-effect, lock
and liquid-level S S S S S means
sensors. The se- 8 7 4 8 7 4 8 7 4 8 7 4 8 7 4 the se-
quence circuit 6 6 6 6 6 quence
SM1 SM2 SM3 SM4 SM5
can also be used will not
5 2 5 2 5 2 5 2 5 2
with larger air- reset and
power valves or bypass
air-piloted hy- the valve
4 S 6 4 S 6
draulic valves. + – + – being ac-
8 2 8 2 tuated.
PVA PVB After
A B choosing
LVA– LVB–
compo-
LVA+ LVB+ nents, it
3
Multiple functions
SM1 SM2 SM3 SM4
Out Out Out Out
A+
S
A– B+
S
B–
nary-trigger circuit that controls the input to the sequence
circuit. This approach requires actuating the jog signal for
each step. A delay-out valve resets the binary trigger when
there are an odd number of sequence steps.
A B Power valves
reset. This control, when actuated, returns the se-
quence to the start position. Reset can also place power
A shuttle valve to the pilot of a power valve lets a valves in a home position. A reset circuit should only be
function actuate twice during a sequence. used when the control is in manual mode.
emergency stop. E-stop controls can halt system oper-
ations in several ways:
1. Stop the sequence only.
Back-pressure sensing 2. Stop the sequence and relieve pressure from the
power valves.
S 3. Stop the sequence and activate reset controls.
To next A latching mushroom-head button is commonly used
step for the emergency-stop control because it gives users a
positive response when activated.
system shutoff. These controls turn off the main air
Back-pressure supply. This prevents harm to personnel or product from
sensing indicates accidentally operating the machine. If the system has
cylinder position large power valves, a piloted three-way main supply
without limit valves. valve can control a specific machine section. When elec-
trical circuits are part of the system, an air-piloted pres-
sure switch ensures the system has electric power only if
is just a matter of assembling fittings and modular valves the air is on.
into the subplate. Connect the air supply and lines from the multiple outputs. When two (or more) functions
limit valves and electrical connections from the Hall-effect start with the same step, connect the output from that
sensors to the inlet connections. And connect air lines from sequence step to both power valves.
the outlet to the air pilots of the power valves. multiple functions. Functions that actuate twice dur-
ing the sequence call for the system to connect a shuttle
changing oPerations valve (OR function) to the pilot of the power valve.
Modular systems can be quickly tailored to meet spe- multiple inputs. When two functions actuate at the same
cific requirements. Sequence conditions can be altered or time, a piloted three-way valve (AND function) ensures both
adapted to the application by using different control mod- functions are complete before the next step begins.
ules. Here’s a look at some common options. Delays. Applications that require a delay before a
start options. If the application demands an input for step can use a delay-in module between the limit valve
each cycle of operations, use a pushbutton input signal and the input to the step being delayed.
and a sequence valve that provides a reset lock if the but- back-pressure sensing. Many air-cylinder applications
ton is held actuated. If continuous cycling is required, use cannot use mechanical limit valves for sensing because of
a selector or toggle valve at the input of the first step physical interference, temperature extremes, or other con-
along with a valve that permits continuous cycle se- ditions. A method called back-pressure sensing indicates
quences. For applications that require the choice of select- cylinder position without limit valves. For example, as a
ing one cycle or continuous cycling, add a selector valve cylinder retracts, it creates a back pressure behind the pis-
that determines the type of operation. ton. Restricting exhaust air at the control valve further in-
manual jog. Another consideration is manual jog. This creases pressure and slows return of the cylinder rod. This
control option lets users jog (step-by-step) through the back pressure holds the pilot down on a three-way valve.
machine sequences. One way to do this is to control air- When the cylinder fully returns, back pressure diminishes
flow to the limit valves, with the jog control a pushbutton at the pilot port of the three-way valve, letting a spring
or spring-return selector. If held actuated, the sequence shift the valve and send a pneumatic signal to the next step.
continues until released. If the system requires a delay, substitute a delay-out mod-
Another method for manual-jog control is to use a bi- ule for the three-way valve.
4


Use: 0.3109