A Special Pneumatic Actuator

Copyright © IFAC Pneumatic Wanaw. Poland 1980

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Hydraulic Components

A SPECIAL PNEUMATIC ACTUATOR L. Helm and A. SZllCS Computer and Automation, Institute Hungarian Academy of Sciences, Hungary

Abstract.

It has been developed by our Institute a special robustic

electropneumatic stepping motor in order to make possible an additional remote control over valves - operated by hand before. The development seems to be sucsessfull and has been approved by different experiments. Keywords.

Actuators; analog-digital conversion; centralised plant control;

computer applications; industrial control; manipulation; nonlinear systems; pneumatic control equipment; robots; stepping motors.

It is known, that valves play a particuler

excluded before.

role in a control loop. They are organic parts of the technology itself at the same

Theoretically the universal actuator block

time when sensors and central units are

can be featured by continuous rotating and by

fairly independent. The steady state of

any kind of auxiliary power supply. However

valves is a restriction at some flow - but

practically electropneumatic stepping motor,

when they are in a transient, it means a

where a pneumatic mechanism in connected to

control operation. The valves can be ac-

an electrical control logic - seems to be one

tuated either by human operator (hand

of the most economic solutions. (Of course

valves) or automatically. It is a tradition

logic control may be full pneumatic as well

using a hand valve in a manually controlled

if it is needed in certain cases).

loop only - but on the contrary - an automatic value has generally a hand-wheel too.

As it is shown on figure I. the output shaft

In many cases it is necessary to insist on

of the stepping motor is mounted on a gear.

the tradition but perhaps in much more cases

The gear is attached by simple pneumatic

there is quite an other way also for appro-

actuators having retaining spring. One of

aching the problem, namely to automatize the

these actuators can rotate the gear in po-

hand-wheel itself. Automation of the work of

sitive, the other can rotate it in negative

a human operator rotating a wheel can be

direction. Having lost air-pressure, springs

realized by a stepping motor characterized

retain the actuator stems in null-position

by human power furthermore by insensibility

and in this case gear can be rotated freely.

to overload and heavy circumstances. Some

If one of the actuators is operated by an

basic types of this kind of stepping motors

air-pressure impulse, gear is rotated in a

can overcontrol many types of hand-operated

following angular position by the actuator

valves. Automation can be spread in such

stem. Consequently the output shaft driven

fields by this method in which it has been

by the air-pressure impulses rotates step by 229

L. Helm and A. Szucs

230

step from one angular position into an other

In such a way velocity is a function of load.

in a positive or a negative direction. The

In some cases however time-based logic cont-

output shaft manipulates the wheel of the

rol may also be possible. The universal ac-

hand valve with suitable pins or mechanical

tuator block can be used in remote control

fingers fastened to the shaft itself. In a

and normal control loops as well, but natu-

nUll-position the output shaft is fixed by

rally every essential points of the concrete

the friction of the gasket of the valve. I t

situation must be taken in account.

is also possible to realize the output shaft as a tube. The advantage of doing so is easy

The most successful application of the uni-

application in the cases of long wormed

versal actuator block may be using it for

spindle with fixed positiona1 wheel. The

additionaZ automation. Many times control

spindle moves inside the tube. The air-

system of some technology goes out of date

pressure impulses are provided by solenoid

earlier than the machinery type equipments,

valves connected to control logic and to air-

included valves, do. The demand for making

supply.

control more complete, is limited, when doing so, expensive valves of high quality are to

The main function of the universal actuator

be changed. Much more economical solution of

block is to actuate the hand-wheel of a valve.

the problem is to mount the stepping motor

Its mechanical fitting on the valve or on

to that equipments which are essentially good.

something else fixed closely to the valve in

In such a way vas t remote control systems can

suitable

be built long time after the original in-

form - may have many variations.

It is a simple mechanical adaptation of an

stallation, when automation becomes reasonable

outside type. It may be realized without

even the exact data and control - requirements

bothering the valve just working as a re-

become more clear. Even complex automation can

strictor of the technology. After having

be realized so, because this solution is very

applied the stepping motor to a valve, the

suitable to computerized systems generally.

hand-wheel can be operated automatically but

Furthermore this kind of actuator may have

at the same time manually as well. At maun-

great importance in certain ad hoc cases in

ting it is necessary to conduct air-supply

which certain valves must be remote-controlled

conduit and a compound-cable of low vo1tages

but for a limited time only. There are such

to the spot. The normally available actua-

technologies manually controlled because of

tors are tailor-made parts of same seria of

their low noise - their steady-state opera-

va1v.es designed in order to operate with them

tion almost never changes - but their putting

together. The universality of that new type

into work is a very hard task requiring mani-

of actuator is the high capability for driving

pulation of many valves. Taking a special set

almost any equipment mounted with hand-wheel.

of universal actuator blocks and of course

It depends on the transfer and on the construc-

their logic control boxes - this kind of work

tion, that driven wheel rotates several angles

lasting several days or weeks may become much

only or perhaps 270

0

or more by one impulse.

There were examined ISO, 300 and 270 0 angular

easier. At last this method can be very profitable in experimental examinations.

rotatings in the experiments. The algorithm of appearing and disappearing of impulses

FieZds of appZication are almost unlimited.

depends on the system as a whole, the mode of

Naturally there are requirements of different

application and on the requirements. The nu11-

technologies which allow full electric reali-

position and the contrary limit-position of

zation but there are such ones which demand

the stems actuating the gear are observed by

explosion-proof solutions for example.

limit switches. Therefore point-to-point logic control can be chosen as most simple method.

231

A Special Pneumatic Actuator Experiments were carried out in an industrial

control influences reliability and cost at

refrigirator system in the oil-mining indus-

the least. It is possible to build a unic

try, furthermore in the water-preparation and

logic control - the actuator itself and its

boiler equipments of a power station. There

logic control box is entirely stand-alone in

were used valves as follows: bore 50 mm,

this case. In a batch control the volues be-

pressure 25 bar; bore 80 mm, pressure 40 bar;

longing to one group are connected each to

bore 125 mm, pressure 40 bar; bore 250 mm,

other by a given algorithm (for example auto-

pressure 6 bar. The universal actuator blocks

clav control or start-stop systems). At last

were mounted on the valves during the normal

in komplex sys terns this logic control is ef-

working state. Experiments proved that the

fected by the whole of technolog y

developed devices can be operated for long

certain parameters of production.

even

periods among industrial circumstances and their velocity and torque is acceptable.

The universal actuator block has always a basic logic control which is necessary for

Beside experiments the field of remote

the activit y

itself. Higher levels form cer-

heating and the so called peripherical

tain degree of comfort of remote control and

equipments (little installations, not well

at a very high level they operate as inter-

known technologi e s) are pointed out as good

faces connected to a computer .

fields of application. As transport, distribution and control of electricity is reali-

The most advantageous feature of that additi-

zed in the electrical power industry just

onal method written above is high versatility

with the help of switching techniques only -

of application. Regarding to this character

to have a similar possibility for distri-

it may be observed some kind of relationship

buting of heating energy it would be very

between the so called industrial robots well

advan tageous.

known in the machinery and this kind of actuator. Realizing a development conception con-

Generally the problems of automation of

nected to any plant when universal actuator

peripherical little installations are

blocks get into application there is a possi-

dropped in the desicnirc period already. I t

bility to use valves in such a felxible manner

is recognized later, during the everyday

as i t is a custom to day in the field of da te

operation the need of automation. Often

processing.

designers have not routine enough to get an, easy choice of automatic valves demanded by

The developed device remains in its last posi-

the technolog y being not frequently on the

tion when auxiliary energ y is l ost. In many

designer's table. - So they rather like to

cases this is an advantage. It is possible

prescribe a manual control . However using

that technology must not be taken into an

the universal actuator block designer's work

extreme state if for example electricity drops

becomes eas ier - they get free hand doing

for a short time.

so. On the one hand the traditional manual system may remain but on the other hand

Logic control needs low dc voltages (24V=) -

remote control or high-levelled control may

it can be operated by storage battery (accu-

be specified.

mulator). Mechanism is operated by airpressure: 2,5 bar. Therefore - if main elect-

The universal actuator block can be used in

rical supply drops - ie. at a situation of

the most simple remote control and in the

emergency - the system can operate without

komplex automation as well. The level of app lication deterrnins a level of logic cont-

trouble.

rol too. To day a high-s ofisticated logic PHC IAC .

a

232

L. Helm and A. Szucs

Using stepping motor in control - techniques

a seria deeply structured in system engine-

it is given a good chance for realizing spe-

ering because of the different needs of the

cial methods too. At processes featured by

different fields of application.

long dead time and non-linear characteristics this methods are particularly useful. Digitized stepping form of restrictin3 in a slow manner - it is a significant factor of sucsessful control in such cases. As it is shown on the figure 2. - the controlled parameter is limited by a higher and a lower limitvalue. When the controlled parameter overflows the one or the other limit-value, then the controller gives a command for the stepping motor to step in the suitable positive or negative direction with a determined frequency and stepping or angular digit. Naturally the system can be realized in a full automatic way as well, so, that every essential parameter included the widthof the zone limited by the limit-values can be changed automatically. Generally it is an important feature of the developed device that it can be stepped into any prescribed position with very fine digits and it can be stopped anywhere. It was one of the main points during the development work to create a simple and consequently inexpensive construction . A pushforward of high fidelity is not needed in the volve-techniques of this kind. The pregnant advantages of pneumatics must be dominant in this actuator: robustic, big static force, capability of overloading and using a storage type sort of supply energy. Reproducing the mechanism in a mass production - it becomes very cheap and it dos not demand a high technology even in this case. During the experiments carried out in the industry - the reason and usefulness of the fundamental idea has been proved. Finally it must be mentioned, that it is necessary to work out a more distinct seria of the universal actuator block mainly in regarding to its logic control in order to have

A Special Pneumatic Actuator

233

pnelJ'TlC1tic actuators shaft wormed spindle

logical cCX'ltrol

gear command ·

s1e~ing

motor wheel air supply

Fig.l. Schema of the universal actuator block

p~ocess

Xo

- X, -Xo -Xo switching signal

I

~

s,lgnal

"

~

rime -

. . , command signal time

Cl

ono

-

stepping impulses

uuu

-

time

steps of the valve

Fig.2. The main control characteristics