Pneumatic and Hydraulic Control 3

7/25/2019 Pneumatic and Hydraulic Control 3

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1

Basic circuits with contactless signal

transmitters

It would be beyond the scope of this course to discuss all

contactless sensing systems in pneumatics. However, as an

example we will demonstrate 3 types:

reflex sensorair gateback-pressure nole


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2

Reflex sensor

It must be supplied with a relatively low pressure.It produces a low pressure signal that must be amplified.

A: Signal outputP: Connection for supply

line


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3

Air gate

It must be supplied with a

relatively low pressure.

It produces a low pressure

signal that must be amplified.


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4

Back-pressure nozzle

!he outlet opening must be closed completely.It can be supplied with normal pressure.

!he output signal is at normal pressure level.


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Amplifier

"ince the output signals from the air gateand the reflexsensorare low-pressure signals, it is necessary to

incorporate amplifiers.

#mplifiers must be supplied with low pressure in order to

attain the high level of amplification.

"ince a directional control valve is operated by the

amplified signal, the symbol of the amplifier may also be

drawn directly on the directional control valve.


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1! "he piston ro# of a #ou$le-acting cylin#er is to tra%el

out when an air $arrier is momentarily interrupte#& its

return mo%ement is to $e controlle# $y a proximity typeof limit switch 'reflex sensor( in the forwar# en#

position!


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1)! A #ou$le-acting cylin#er is to $e controlle# in $oth

#irections $y $ack-pressure nozzles! "he se*uence of

mo%ements is to $e initiate# an# terminate# $y a switch!"he initial position is always to $e the rear en# position!


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Signal +limination

"ometimes it is re$uired that a certain signal to beeliminated shortly after its existence %e. g. signals from limit

switches which are still operated when the opposing signal

occurs and then do not permit this opposing signal to be

effective&.

,#le return %al%e Pulse shortening


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,#le return %al%e


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Alternating Controls

!he characteristic of an alternating control is that the outputreverses with each arriving input signal.

a - input signale - output signal


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+xample: Alternating circuit with latching $y

means of limit switches

It has the disadvantage

that it will be reversed

immediately if the input

signal is applied for too

long. 'etter design


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etho#s for constructing a circuit #iagram

'asically there are two primary methods for constructingcircuit diagrams.

(.!he so-called )intuiti%e) methods, also fre$uently termed

con%entional or trial-an#-errormethods.

*.!he metho#icaldesign of a circuit diagram in accordancewith prescribed rules and instructions.

+ethodical design is independent of personal influences

from the designer such as ability, frame of mind, mood andso on. however, more devices will be re$uired in such a

control than in a circuit devised by the intuitive method.


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Constructing the circuit #iagram intuiti%ely

!he following procedure is then recommended in building upthe circuit diagram intuitively:

a(raw the working elements %actuators&.

$(raw the associated final control elements.

c(raw the re$uired signal elements without symbol. If

impulse valves are used as final control elements, * driving

signals and hence * signal elements are re$uired initially per

impulse valve.#& raw in the energy supply.

e(onnect control lines.

f(umber the elements.


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g(!ranspose the motion diagram into the circuit diagram

%programming of the circuit by positional allocation of the limit

switches to the respective working element&.

h(heck where signal cut-outs are re$uired. !his check can

be made in the motion and control diagram.

i( raw in the actuating controls of the valves.

k(/here applicable, incorporate the auxiliary conditions.


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+xercise 1 Package transfer:

0ackages arriving on a conveyor are lifted by a pneumaticcylinder and pushed onto another conveyor by a second

cylinder. ylinder ' may then perform a return stroke only

after cylinder # has reached the rear end position. !he start

signal should be provided by means of a manual button,

each signal initiating one cycle.

.isplacement-step #iagram


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'y adopting the procedure we get

/hen checking, it must be ensured that no signals influencing

the same cylinder are present at the same time, as this would

lead to a condition which is not uni$uely defined, i.e. one signal

blocks the other.


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#n interlock for (.* is incorporated by means of an additional

limit switch at the rear end position of cylinder '. !his ensures

that no new cycle will start unless the old cycle finishes.

Pneumatic and Hydraulic Control 3

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