Download - Pneumatic and Hydraulic Control 3

7/25/2019 Pneumatic and Hydraulic Control 3

1/18

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


2/18

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


3/18

3

Air gate

It must be supplied with a

relatively low pressure.

It produces a low pressure

signal that must be amplified.


4/18

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.


5/18

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.


6/18

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!


7/18

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!


8/18

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


9/18

,#le return %al%e


10/18


11/18

Alternating Controls

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

a - input signale - output signal


12/18

+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


13/18

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.


14/18

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.


15/18

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.


16/18

+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


17/18

'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.


18/18

#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.