UNIT-V

DESIGN & SELECTION

Design of Pneumatic Logic Circuits• Pneumatic control is ideal for a vast variety of low-cost industrial automation

applications, both simple and complex. Welding and fabricating machines, presstools, automatic machine tools, assembling, material handling, packaging, foodand beveraging, printing and mining are some of the fields where pneumaticpower can be applied to get its full advantage.

• The main advantage of a pneumatic control system is the use of the same powermedium throughout the system for performing both work function and controlfunction thus avoiding the need for additional interface elements.

1. Classic method2. Cascade method3. Step-counter method4. Logic Design method (with Karnaugb-Veitch Maps)5. Combinational circuit design

Classic Method• The classic method is based on the knowledge of pneumatic logic elements and

application of the logics. As already mentioned, each pneumatic element is a logicelement performing various logic functions like AND, NAND, OR, NOR etc.

• The principle of Boolean algebra and De Morgan theorem are employed to solveproblems in the analysis of control logics.

• In boolean algebra, input signals and output signals are expressed by means ofletters. The logical Illocutions are denoted by symbols. For example, OR is usedas {+) or U, AND is used as (.) or fl.

If input signal A or signal B gives an output Y, the logic equation is written as A + B Y.Some common logic nomenclature along with logic equations are shown in figure 14.1 inat a bular form.Step 1: Analyses the pre-conditions of the systemStep 2: As per the pre-conditions, write the control equationStep 3: If needed, De Morgan’s theorem is applied to simple the control equationStep 4: Based on the control equation, the logic block diagram can be drawn Step 5: Thelogic equations are selected and the pneumatic circuit diagram can be drawn

Cascade MethodThe cascade method is simple to apply and results in reliable and easily understoodcircuits.Steps involvedStep 1: Each cylinder is given a code letter and theft sequence is determined. Forexample K, B A-, B- etc. ‘+‘ and represent extension and refraction of the cylinderrespectively.Step 2: The sequence is split into minimum number of groups. Care should be taken tosee that no letter is repeated within any group. Next the circuit is draswiup using thefollowing steps.Step 3: Each group is assigned a pressure manifold line which must be pressurised onlywhile that particular group is active. So the number of pressure lines equals the number ofgroups.Step 4: Selecting the valves.a. The limit valves are denoted as a a b b etc. where the suffix 0 corresponds to valveswhich are actuated at the end of return stroke and the suffix I corresponds to valves whichare actuated at the end of forward stroke. Each cylinder requires two limits valves and itequals twice the number of cylinders. Each manifold line supplies air pressure to thoselimit valves within its particular group.b. In order to pressurise the various manifold lines in the proper order, one or more groupchanging valves or cascade valves are employed. The number of group valves alwaysequals the number of groups minus one.c. For each cylinder, a pilot operated direction control valve is selected. The number ofcylinder acting valves equals the number of cylinders.Step 5: The valves are connected as follows. The output of each limit valve is connectedto the pilot input corresponding to the next sequence step with one exception. The limitvalve corresponding to the last step of the given group is ‘not’ connected to the actuatingvalve of the next cylinder, but rather to the pilot line of a group changing valve so as topressurise the manifold of the next group. This manifold line is then connected to thepilot line corresponding to the first step of the next group.

Step-Counter Method• A step-counter is a digital modular pounter, constructed from stepping units

which serve as sequence steppers for asynchronous sequential fluid powercontrols. A basic stepping unit as shown in Fig 14.5 is built from a signal outputMEMORY valve and a pre-switched AND valve with two inputs. Achieving anoutput signal from the stepping unit requires

a. a signal input at line 1, which serves as a preparation signal arising from the precedingstepping unit andb. line 2, a confirmation signal which confirms the completion of the cylinder movementby the preceding stepping unitc. line 4 resets the memory valve of the preceding stepping unitd. line 5 serves as a preparation signal to prepare the next stepping unite. line 6 is the reset signal from the next stepping unitf line 3 switches the D.C.V

Steps InvolvedStep 1: Draw the position-step diagram. Position-step diagram is used to representmovement of drive elements. In position-step diagram the X-axis represents steps i.e., thesequence of operation is divided into a number of steps which are expressed as 1, 2, 3, 4etc. The Y-axis represents position and is indicated as 0, 1. The functional lines are drawnin thick lines and they determine the position of the driving unit. Any change of positionof a member has to start or stop at a corner of the squares.Step 2: From the position-step diagram, the number of stepping units are decided.Step 3: Draw the cylinders and their memory valves (D.C.Vs). Connect the memories tothe cylinders.Step 4: Draw the step-counters.Step 5: Draw the position sequence valves.Step 6: Connect the stepping units and valves.

Electrical Control of Fluid PowerComponents Of Electrical ControlsThe basic devices commonly used in the control of fluid power systems are1. Switchesa) Push button switchesb) Pressure switchesc) Limit switchesd) Temperature switches

2. Solenoids3. Relays4. TimersSwitchesThese are control members used to make or break the electrical circuit.a. Push button switches These switches make or break contact only as long as they areheld pressed and hence they are called momentary switches. They are used mainly forstarting and stopping the operation of a machine.The three common types of push button switches are

b. Pressure switches Pressure switches open or close their contacts based on systempressure. They generally have high pressure and low pressure settings

c. Limit Switches Limit switches, open and close circuits when they are actuated either atthe end of the retraction or extension stroke of hydraulic or pneumatic cylinders. Thelimit switches are mechanically actuated by various elements like plunger, cams, rollerand levers. The speed at which the limit switch is operated is very important from thecontrol point of view.The symbols for limit switches are

d. Temperature switches A temperature switch is an instrument that automatically sensesa change in the temperature and opens or closes an electrical switch when a pre-determined temperature is reached. Temperature switches can be used to protect a fluidpower system from serious damage when components such as a pump or strainer orcooler begin to malfunction. The resulting excessive build-up in oil temperature is sensedby the temperature switch which then shuts off the entire system.The symbols used for temperature switches are

Solenoids• A solenoid is an electromechanical device which can convert electrical power into

mechanical force and motion. Solenoids provide a push or pull force to remotelyoperate fluid power valves.

• There are two types of solenoids: AC and DC. It consists mainly of a plunger,wire or coil and a body. The working principle is such that when an electriccurrent is passed through a coiled wire, a magnetic field is set up around the wire.

• The force due to the magnetic field pulls or pushes the plunger and this force isfed to the spool.

Relays• Relays are switches whose contacts (one or more) open or close when their

corresponding coils are energised. Relays are used for energising and de-energising solenoids which operate at a high current level.

• A low voltage circuit can be used to energise relay coils which control the highvoltage contacts used to open and close the circuits containing solenoids.

• As shown in the figure, when i- SW is closed, the coil is energised. This pulls onthe spring loaded relay arm to open the upper set of normally closed contacts andclose the lower set of normally open contacts.

Electro-Hydraulic/ Pneumatic Circuits• When drawing electro-hydraulic or electro pneumatic circuits, separate circuits

are drawn for the fluid system and the electrical system. Each component islabelled to show exactly how the systems interface.

• Electrical circuits use Ladder Diagrams. The ladder diagram is a representation ofhardware connections between switches, relays, solenoids etc., which constitutethe basic components of an electrical control system.

• The left leg of the ladder is connected to the power and the right to the ground.The operation of the total system can be understood by an examination of the

fluid power circuits and the ladder diagram as they show the interaction of all thecomponents.

Reciprocation of a Cylinder using Pressure Switches• The figure shows a system which uses two pressure switches to control the

operation of a double acting hydraulic cylinder. Each pressure switch has a set ofnormally open contacts.

• When switch 1-SW is closed the cylinder reciprocates continuously until 1-SW isopened.

The sequence of operation is as follows assuming solenoid A was last energised.i) When solenoid A is energised, the oil from the pump flows through the valve into theblank end of the cylinder.i When the cylinder has been filly extended the pressure builds up to actuate pressureswitch 1-PS. This energises SOL B.iii) When solenoid B energises, the oil then flows to the rod end of the cylinder.iv) Upon fl.ill retraction, the pressure builds up to actuate 2-PS. During retraction of thecylinder, 1-PS has been de-actuatc’d to de-energise SOL B.v) The closing of the contact 2-PS energises SOL A to once again begin the extendingstroke of the cylinder

Control of a Cylinder using a Single Limit Switch• The figure shows a system which uses a single solenoid and a single limit switch

to control a double acting hydraulic cylindet In the ladder diagram, one relay witha coil designated 1-CR and two noimally open sets of contacts I-CR (NO) areused.

• The limit switch is labelled l-LS (NC) and the two push button switches, onenormally closed (labelled stop) and one normally opened (labelled start), are alsoincluded.

The sequence of operations is as followsi) When the START button is momentarily pressed, the relay coil 1-CR is energisedwhich closes both the contacts of I-CR.i Upper 1-CR contacts serves to keep the coil I-CR energised even though the STARTbutton is released.119 Lower I-CR contacts closes to energise solenoid A to extend the cylinder.iv) At the end of the forward stroke, the piston rod cam actuates I -LS (NC) and opens tode-energise coil I-CR.v) The de-energisation coil 1-CR causes both the I-CR contacts to open.vi) Due to the opening of the lower I-CR contact, the solenoid A de-energises and thevalve returns to its spring mode. The cylinder then retracts.vii) The retraction ofcylinder again closes the 1 -LS but coil I-CR is ‘not’ energisedbecause the START button and upper I-CR contacts are in the open position. Thecylinder stops at the end of the retraction stroke.viii) This cycle is repeated each time the START button is momentarily pressed. TheSTOP button is a panic button and when pressed the extension stroke is immediatelystopped and it hilly retracts the cylinder.

Dual Cylinder Sequencing CircuitThe figure illustrates a circuit that gives a cycle sequence of two pneumatic cylinders.The cycle sequence initiated by the momentary pressing of the START button is asfollows.I. Press start button2. Cylinder I extends3. Actuate limit switch l-LS4. Cylinder 2 extends while cylinder I retracts5. Actuate limit switch 2-LS6. Cylinder 2 retracts7. Cycle is completed

The operation is as follows:i When the START button is depressed momentarily, SOL A is energised to allow theflow through valve Ito extend cylinder I and actuate l-LS.ii. As limit switch 1-LS is a double pole single throw type actuation of limit switch 1 -LSopens the holding circuit for relay I-CR and simultaneously closes the holding circuit forrelay 2-CR.iii This de-energises SOL A and simultaneously energises SOL B. This returns valve I toits spring offset mode and switches valve 2 into its solenoid actuated mode. As a result,cylinder I retracts while cylinder 2 extends.iv. When 2-LS is actuated, the holding circuit for relay 2-CR is broken, which deenergises solenoid B to shift valve 2 back to its spring offset mode thus causing theretraction of the cylinder 2.v. The STOP button is used to break the source of electrical power in the circuit and itinstantly retracts both cylinders.

Regenerative Circuit• A regenerative circuit is used to speed up the extending speed of a double acting

hydraulic cylinder.• The figure illustrates a regenerative circuit controlled with two solenoid valves, a

pressure switch and two check valves.The circuit operation is as followsi) When switch 1-SW is manually turned to the extend position, solenoid A is energisedwhich causes the cylinder to extend.ii) Oil from the rod end flows through check valve 4 tojoin the incoming oil from thepump to provide a rapid cylinder extension in the regenerative condition.iii) When oil pressure builds up due to loadings on the cylinder, it actuates a normallyopen pressure switch I-PS. As a result, coil 1-CR and solenoid C become energised. Rodend oil is vented directly back to the oil tank throughi valve 2. As a result, the cylinderextends slowly as it drives a high resistive load. Relay contact 1-CR-A provides a holdingcircuit for relay coil 1-CR and

Microelectronic Control of Fluid Power

PLC Construction• A PLC can be defined as a digital electronic device that uses a programmable

memory to store instructions such as logic, sequencing, timing, counting andarithmetic to control machines or proce,sses.

• It is a software based instrument and hence it can be programmed using an easy-to-learn programming language.

The three basic elements of PLC areI. Central processing unit (CPU) with an associated memory2. Input modules3. Output modules

Central Processing Unit• The CPU receives input signals from the various input modules and based on the

programs stored in the memory, decides on the appropriate signals, which ittransmits to the respective output modules.

MemoryIn choosing a PLC, the available memory capacity plays an important role. The differentmemory types used in PLC of both volatile and non-volatile type are given below.1. RAM (Random access memory) — Volatile to — prograimdevelopment2. Read/Write memory—Non volatile3. ROM (Read only memory) —Non-volatile to store execution program4. PROM (Programmable read only memory)5. EPROM (Erasable PROM)6. I3EPROM (Electrically erasable PROM)Programming The PLC

• PLC is programmed by means of a programming device. The programmingdevice is usually detachable from the PLC and it can be shared between thedifferent controllers.

• Different devices ranging from simple teach pendent type devices to specialprogrammed keyboards and CRT displays are adopted.

• Most of the programming methods used today for PLCs are based on the ladderlogic diagram. There are various approaches for entering the program into thePLC.

1. Ladder diagram based2. Low-level language based on Boolean expressions3. Functional blocks4. High-level languagesLadder Diagram Based

• The ladder logic diagram is converted into a PLC ladder diagram by using theconventions of PLC ladder diagram construction.

This method requires the use of a keyboard and a CRT with limited graphic capability todisplay the symbols, representing components and their inter-relationships in the ladderlogic diagram.Low-Level Language Based on Boolean Functions

• The second method makes use of tow-level language using the Boolean AND,OR, NOT functions.

• Using the language instructions, the programmer constructs the ladder diagram byspecify the various components and their relationships for each rung. Somecommon instructions are

Functional Blocks• Functional blocks provide another means to input high-level instructions.

However the format in which the instructions are entered is the same as the ladderlogic diagram.

• The instructions are composed of operational blocks. Each block has one or moreinput and output. Within the block, certain operations take place on the inputs totransform the signals into the desired outputs.

• The functional blocks include operations such as timers and counters, controlcomputations using equations, data manipulations and data transfer to othercomputer based systems.

High-Level Languages• The principal advantage offered by the high-level language for programming the

PLC is their capability to perform data processing and calculations on valuesother than binary.

• This permits the use of a more complex control algorithm for communication withcomputer based systems and to display the data on a CRT control.

PLC Operation• In the PLC, the program steps defined by the ladder diagrams are executed

simultaneously and continuously. First the inputs to the PLC are sampled by theprocessor and the contents are stored in memory. Next the control program isexecuted.

• The input values stored in memory are used in the control logic calculations todetermine the value of the output. Finally the outputs are updated to agree withthe calculated values.

• This cycle consisting of reading inputs, executing the control program andrevising the outputs is referred to as a scan.

Microprocessor Construction• The word microprocessor is popularly used when referring to a microcomputer.

But a microprocessor forms only a part of a microcomputer and must be usedalong with other devices.

• The basic elements of a microcomputer are (i) Microprocessor (ii) Memory (iii)Input/output device (iv) Power supply. This system is on a single board and it isinterfaced with the fluid power system and provided with a power supply and aprogram. A typical arrangement of a microcomputer is shown in Fig 16.4.

Microprocessor• The microprocessor is the brain of a microcomputer. Microprocessors are

described as large scale integrated circuits.• This is due to the many thousands of tiny electronic circuits from which they are

made. However, to use them they are represented by groups of registers each withits own purpose. Typically a register consists of 8 bits.

Memory• The memory of a microcomputer holds the program or programs currently being

used and provides storage for information such as input values, results ofcalculations etc.

DC Power Supplies• A complete fluid power system with a microcomputer will require various power

supplies at different voltage levels. For example, atypical microcomputer needs astable supp of 5V while a solenoid will have a moderately stable 24V DC supplyor even an AC voltage supply.

• Power provided by the mains need to be converted to produce DC voltage. Thereare two main types of power supply.

1. Linear (low frequency)2. Switch mode (high frequency)Bus

• An interconnecting structure of the microcomputer is called the bus. An 8-bitmicroprocessor will have 8 lines for transferring words of data, perhaps 16 linesfor carrying a memory address and a few lines for control and power supply.

• Data are transferred through the bus system to the various devices. Information onthe locations of the addresses are transferred via the address bus.

Interfacing• Microcomputers usually work with different levels ofvoltage and currents than

those used by sensors and actuators. The means of effecting the connectionbetween items which are not directly compatible is called interfacing.

• For example, the microcomputer is incapable of driving the solenoid directly andan interface is needed.

Programming the MicroprocessorA microprocessor can be programmed using either a low-level language or a high-levellanguage or a combination of the two.

Low-Level Language Programming• Both machine code and assembly language are commonly referred to as low-level

language.• Machine code is the one that the microprocessor executes. Hexadecimal (HEX)

number system is used for this.• As the hexadecimal number is to a base of 16, the digits are from Ito 9 and A, B,

C, D, E, F are used to represent the rest of the numbers upto 16. Assemblylanguage is a symbolic or mnemonic form of machine code.

High-Level Language Programming• There are many high-level languages available. A program which is written in

high- level language is the easiest and therefore cheaper to produce.• In applications concerning control and monitoring of machinery, it is invariably

found that certain portions of the program require precise time or extra speed.• In these circumstances, it may be necessary to write these portions as subroutines

in machine code and then insert them into the program in high-level language. Asprocessing speed of processors increases so the proportion of programmingneeded to be done in machine code will decrease.

• A high-level language must also be translated into machine code. This is done bya program called a ‘compiler’ or alternatively executed at the time of use by aprogram called an ‘interpreter’.

Microprocessor Operation• Microprocessor does its work by moving data or information between itself, the

memory and the 1/0 device. It performs simple logical operations on data.• The sequence in which it performs the operations is stored in the memory. Data

are coded in the form of binary digits called bits.• Information is handled in units of a fixed number of bits called bytes. The rate at

which a microprocessor executes instructions is governed by a timing devicereferred to as a clock.

Difference Between PlCs And Microcomputers• The question usually is whether to use a microcomputer or a PLC. So it might be

useflul to know the difference between PLCs and microcomputers.