Pneumatics and electronics

Systems of low complexity and those in use in hazardous areas, not compatible with electronics, will probably be designed as pure pneumatic systems.

A purely pneumatic system can be viewed as three main sections:

1 Generation and preparation of the compressed air source.

2 Power actuation of pneumatic cylinders through directional control valves.

3 Pneumatic signal processing or logic control.

Electronics can influence all of these sections, for example:

(a) By electronic management control of compresors and controlled pressure regulation.

(b) In section 2 there are solenoid valves that provide proportional flow and pressure, together with air cylinders having electronic proportional feedback.

(c) In section 3, for many systems pneumatic logic has been replaced completely by electronic sequence or logic control.

Programmable sequence controllers (sequencers) and programmable logic controllers (PLCs) are commonly used devices and offer a wide range of features such as timing, counting, looping and logic functions. If a proposed scheme involves a sequence of events more complicated than that shown in Fig. 27.38, then electronic possibilities should be explored. In addition to sequence operations there may be the additional complications from long counting operations, or a number of time delays, requiring a high degree of repeatable accuracy. Here the electronic controller will usually be the better choice. Inputs to the controller indicate the completion of the cylinder movement.

These are most conveniently achieved by using a magnetic cylinder fitted with reed switches. The reed switch consists of two spring like metal reeds within a sealed enclosure. When a magnet around the piston is within range, the reeds are magnetized, each having a N and S pole. As the free ends will be of the opposite polarity they snap together. For environments where strong magnetic fields exist mechanical limit switches may be used.

The scope of such a system will be appreciated from Fig. 27.39. Programming methods vary with the type of controller and for someone with no experience it is generally easier than they think. Sequencers are designed to be easy to program and are a good choice for machines where the actions are performed in a one-after-the-other interlock. Sequencers are able to jump from one part of the sequence to another, run sections of a sequence in a repeating loop, time, count and perform logic functions such as AND, OR, NOT, etc. It may also be possible to hold several sequences in a memory and select the desired one for a particular task. Sequencers will have a built in range of control buttons to provide facilities such as, run/end cycle, emergency stop, single cycle, auto cycle and manual over-ride.

It takes a little longer to program a PLC. This is produced by keying in a list of logic statements first determined by drawing a ladder diagram. A ladder diagram for a PLC is a logic circuit of the program as it relates to a machines function and sequence. The ladder diagram illustrated in Fig. 27.40 is derived from, a0 a1

Table 27.3

Out t

Electronic controller

a1 a0 b1 b0

c0 d1 d0

+24V DC

27.39

X400

S601

Hstl

S602

Hstl

S603 -|STL

S604 -|STL

S605 -ISTL

S606 -|STL

X 401 condition 1

X 402 condition 2

X 403

X 404

X 405

X 406

X 407

Fig.

Fig. 27.40

and similar to the ladder electrical circuits used to design electro mechanical relay systems.

Pneumatic and electronic systems play an important part in production engineering and typical applications are the control of the main axes of variable pick and place arms and robotics.

The authors wish to express their thanks for the assistance given and permission to include examples of applications of pneumatic controls manufactured by Norgren Martonair Limited, Campden Road, Shipston-on-Stour, Warwickshire.

BS 1533 specifies graphical symbols for use in general engineering. Within the European Community, many additional symbols are in common use and a selection of these are included here for reference purposes.

-[ S601 ]— -( Y431 )— -[SS602]-_[SS604]— -( Y432 -[SS603]-

Equipment Labelling positioning or flow direction steps on off stand-by

(stand-by position)

day (normal mode), sun, brightness cooling, frost, cold (below 0°C) heating flame wind, wind influence (international) manual actuation, manual control safety insulation, electrical protection Class II

extra-low voltage (up to 50 volts), protection Class III

dangerous electrical voltage (voltage indication)

flow arrow, indicating passage, entry and exit of important substances indicator arrow temperature increase, increase temperature temperature reduction, reduce temperature

Table 27.3 (continued)

Systems symbole

Systems symbols

heating energy, energy demand heat exchanger, general with substance flows crossing heat exchanger, general without substance flows crossing tank, general (pressureless)

tank with convex bottom, general (for high pressure)

-[X|- isolating valve (general) two way valve "C^b three way valve

-00- four way valve

flow symbols:

- variable

- constant 3 mixing

1 diverting shower, nozzle

steam trap

-m- filter

manual actuator self operated actuator (or actuator in general) (j^H electromotoric actuator

| ^ electrothermic actuator I H= hydraulic or pneumatic actuator Q-- diaphragm actuator o cam control electromagnetic actuator example:

1 LlUi 3 magnetic valve showing flow liquid pump, circulating pump, general fan (general) compressor (general)

compressor, 4 step air damper air filter (general) heating coil cooling coil

device or function unit, general modulating controller (general)

keys, keyboard sensor with on-off function (e.g. thermostat, hygrostat pressure switch etc.)

sensor with on-off function (e.g. thermostat, hygrostat pressure switch etc.) with immersion, duct or capillary pocket immersion thermostat for temperature

Other references examples: x absolute humidity h enthalpy p pressure aq air quality (SCS)

Ap differential pressure V flow, volume flow rate v velocity

occupancy etc.

Table 27.3 (continued)

Symbols for electrical schematics DC-current, also DC-voltage (general)

alternative (use this symbol only where there is a risk of confusion on diagrams)

AC-current, also frequency in general AC-voltage (frequency indicated where necessary - on the right of the symbol, e.g. ~50 Hz)

suitability for use on either DC or AC supply positive polarity

Symbols for electrical schematics combined wires, general, any sequence on each side (wires should be coded)

combined wires, general, as above but single line representation general symbol denoting a cable example: 2 core cable example: 2 core cable 'screened' (general) coaxial line, screened

L2t L3 f

negative polarity

~ definitions of electrical conductors

- PE Earth

AC-current with m phases, frequency f and voltage U

Example:

three-phase AC-current with neutral wire, 50 Hz, 380 V (220 V between phase wire and neutral wire)

one wire or a group of wires flexible wires line showing the number of wires e.g. 3 wires numbers of wires = n example: 8 wires

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