EEIB413 Chapter 2

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Chapter 2

Discrete-State Process

Control

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Introduction

• Many industry processes to be controlled in

sequence. The term discrete state expresses that

each event in the sequence can be described by

specifying the condition of all operating units of

the process. For example:

– Valve A is open

– Valve B is closed

– Conveyer C is on

– Limit switch S

is closed

• A technique for designing and describing the

sequence of process events, call ladder diagram

which represents the electromechanical relays to

control the sequence in such process. The most

common control system for discrete control is

done by programmable logic controller (PLC).

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Definition of Discrete

State Process Control

• Figure above shows a manufacturing process and the controller for the process. Input variable (S₁, S₂, S₃) and output variable (C₁, C₂, C₃) can only be in two value. For example:

– Valves are open / closed – Motors are on / off

– Temperature is high / low

– Limit switches are closed / open

• If there are three input variables and three output variables, then the possible states are 64 because each variable can take on two values. • An event in the system is defined by a particular state of the system –

as long as the input variables remain in the same state and the output variables are left in the assigned sate.

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Characteristics of The System

• Industrial objective: to manufacture some

product from the input raw materials.

• The process involve many operations and

steps:

– Some steps occur in series.

– Some steps occur in parallel.

– Some events involve regulation of continuous

variable over the duration of event.

• The discrete-state process control system

is the master control system for the entire

plant operation.

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• The discrete-state input variables are

– Door open/closed

– Cooler temperature high/low

– Freezer temperature high/low

– Frost eliminator timer time-out/not time-out

– Power switch on/off

– Frost detector on/off

• The discrete-state output variables are

– Light on/off

– Compressor eliminator timer started/not

started

– Frost eliminator timer started/not started

– Frost eliminator heater and fan on/off

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• The event sequence are:

– a) If the door is opened, the light is turned on.

– b) If the cooler temperature is high and the frost

eliminator is off, the compressor is turned on and the

baffle is opened until the cooler temperature is low.

– c) If the freezer temperature is high and the frost

eliminator is off, the compressor is turned on until the

temperature is low.

– d) If the frost detector is on, the timer is started, the

compressor is turned off, and the frost eliminator

heater/fan are turned on until the timer times out.

• The events of (a) can occur in parallel with any of

the others.

• The event of (b) and (c) can occur in parallel.

• Event (d) can only be serial with (b) and (c).

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Continuous Control

• Consider for a moment the problem of liquid level in a tank.

Figure above shows a tank with a valve that controls flow of liquid into the tank and some unspecified low out of the tank.

• A transducer is available to measure the level of liquid in the tank. • The objective is to maintain the level of liquid in the tank at some

preset or setpoint value.

• If the outflow increases, the control system will increase the

opening of the input valve to compensate by increasing the input flow rate. The level is thus regulated.

• This is a continuous variable control system because both the level and the valve setting can vary over a range.

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Discrete-State Control

• Consider the revised problem shown in above; the variables, level and valve settings, are discrete because they can take on only two values. • The valves can only be open or closed, and the level is either above or

below the specified value.

• The objective is to fill the tank to a certain level with no outflow. The event of sequence:

i) Close the output valve.

ii) Open the input valve and let the tank fill to the desired level, as indicated by a simple switch.

iii) Close the input valve. iv) Open the output valve.

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Composite Discrete/Continuous

Control

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Process Specifications

• Specified into two parts:

– Objectives of the process.

– Hardware assembled to achieve the objectives.

• Process objectives:

– Statement of what the process is supposed to accomplish.

– Global objective is the end result of the plant. It broken in to many secondary objectives.

– Each sub objective may be independently in the whole operation. – A discrete-state control system then be applied to each independent

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• Process Hardware:

– Design the hardware such as conveyor

system, mixing tank, oven etc so that

these hardware can carry out the

designed process in-order to achieve

the objectives.

– Determination type of components such

as sensor, relay, motor etc be used in

the hardware design.

– The designer should be very familiar

with the components characteristic.

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• How these two states devices relate to the process?

• These devices give

information about the process to the controller.

• The state of the device can determine the step of the whole operation.

• The designer decided to use the state of device either NO or NC when design the

operation.

• Both NO or NC are valid, all depend on the designer.

• Input devices:

– Right box present. – Left box present.

– Feed conveyor right travel limit. – Feed conveyor left travel limit. – Hopper low.

– Feed conveyor center.

• Output devices:

– Hopper valve solenoid.

– Feed stock conveyor motor right.

– Feed stock conveyor motor left. – Right box conveyor motor.

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Event Sequence Description

• A process-control engineer may not have been

involved in the development of the system

hardware construction, but they must study the

hardware carefully and understand the

characteristic of each element. Then, describe how

this hardware will be manipulated to accomplish the

objective.

• A sequence of events must he described that will

direct the system through the operations to provide

the desired end result.

• Narrative Statements Specification of the sequence

of events starts with narrative descriptions of what

events must occur to achieve the objective.

• This specification describes in narrative form what

must happen during the process operation.

• In systems that run continuously, there are

typically a startup, or initialization phase and a

running phase.

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• The start-up phase is used to place the feed conveyor in a known

condition. This initialization might be accomplished by the following specification:

I. Initialization Phase

A. All motors off, feed valve solenoid off.

B. Test for right limit switch 1. If engaged, go to C. 2. If not, set feed motor

for right motion. 3. Start feed-conveyor

motor.

4. Test for right limit switch.

a. If engaged, go to C. b. If not, go to 4.

C. Set feed motor for left motion and start. D. Test for center switch

1. If engaged, go to E. 2. If not, go to D. E. Open hopper-feed valve. F. Test for left limit switch:

1. If engaged, go to G. 2. If not. go to F.

G. All motors off, hopper-feed valve closed.

17 • Completion of this phase means that the feed

conveyor is positioned at the left limit position and the right half of the conveyor has been filled from the feed hopper.

II. Running

A. Start right box conveyor. B. Test right box present switch:

1. If set, go to C. 2. If not, go to B.

C. Start feed-conveyor motor, right motion.

D. Test center switch:

1. If engaged, g to E. 2. If not, go to D. E. Open hopper-feed valve. F. Test right limit switch:

1. If engaged, go to G. 2. If not, go to F.

G. Close hopper-feed valve, stop feed conveyor.

H. Start left box conveyor. I. Test left box present switch:

1. If set, go to J. 2. If not, go to I.

J. Start feed conveyor, left motion. K. Test center switch:

1. If engaged, go to L. 2. If not, go to K. L. Open hopper-teed valve. M. Test left limit switch:

1. If engaged, go to A. 2. If not, go to M.

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Flowcharts of the Event

Sequence

• It is often easier to

visualize and

construct the

sequence of events if

a flowchart is used to

pictorially present the

flow of events.

• Part of the

initialization phase of

the conveyor system

is expressed in the

flowchart format.

Flowchart Symbol

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Start/End

Input/Output

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Binary State Variable

Descriptions

• It used to describe the sequence of events

in terms of the sequence of discrete states

of the system.

• Each of the state, including both input and

output variables be specified.

• The input variables cause the state of the

system to change because operations

within the system cause a change of one of

the state variables.

• The output variables are change in the

system state that are caused by the control

system itself.

• Construct a state variable description of the process shown in

Figure bellow. The timer output (TU) is initially low when its input (TM) is low. When TM is taken high the output stays low for five minutes and then goes high. It resets to low when TM is taken low. All level sensors become true when the level is reached. The

process sequence is:

1. Fill the tank to level A (LA) from valve A (VA) 2. Fill the tank to level B (LB) form valve B (VB) 3. Start the timer (TM), stir (S) and heater (H)

4. When five minute are up take stir (S) and heater (H) off 5. Open output valve (VC) until the tank is empty (LE) 6. Take the timer low (TM) and go to step 1.

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Input Output Description

LA LB LE TU VA VB VC TM S H

0 0 0 0 ~ 1 0 0 0 0 0 Starting state, open valve A

0 0 1 0 ~ 1 0 0 0 0 0 Reaches level LE, continue with A fill

1 0 1 0 ~ 0 1 0 0 0 0 Reaches Level A, close valve A, open valve B

1 1 1 0 ~ 0 0 0 1 1 1 Reaches Level B, close valve B, start timer, heater, stir

1 1 1 1 ~ 0 0 1 1 0 0 Time up, stop stir and heater, open valve C to empty

1 0 1 1 ~ 0 0 1 1 0 0 Reaches level B, continue with empty

0 0 1 1 ~ 0 0 1 1 0 0 Reaches level A, continue with empty

0 0 0 1 ~ 0 0 0 0 0 0 Tank empty, turn off timer, go to first state

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Boolean Equations

• It uses Boolean algebra techniques to

represent the process flow.

• It is necessary to write a Boolean equation

for each output variable in the system.

• This equation will then determine when

that variable is taken to its true state.

• The equation may depend not only on the

set of input variables, but on some of the

output variables.

• Figure bellow shows a pictorial view of an oven, along with the associated input and output signals. All of the inputs and outputs are two-state variables, and the relation of the states and the variable is indicated. Construct Boolean equation that implement the following events:

1. The heater will be on when the power-switch is activated, the door is closed and the temperature is below the limit.

2. The fans will be turned on when the heater is on or when the temperature is above the limit and the door is closed.

3. The light will be turned on if the light switch is on or whenever the door is opened.

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Relay Controllers and

Ladder Diagrams

• The hardware and the sequence of the event can be

combined and represent by a schematic

representation called as ladder diagram.

• It is an outgrowth of early controllers that operated

from ac lines and used relays as the primary switching

elements.

• The ladder diagram show how the hardware should be

driven so the proper sequence of events can be

accomplished.

• An industrial control system typically involves electric

motors, solenoids, heaters or coolers, and other

equipment that is operated from the ac power line.

• When a control system wants to turn on an ac motor,

this is done by a switch to energize a relay with

contact ratings that can handle the heavy load.

• Relay is the primary control element of discrete-state

control systems.

5-Pin Contact Relay

Symbol for Input Devices in Physical Ladder Diagram

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• The symbol for a motor is a

circle with a designation of M

followed by a number, as

shown in Figure a. The

control system treats this

circle as the actual motor,

although in fact, this may be

a motor start system.

• The solenoid symbol is shown

in Figure b. For example, it

may be a solenoid to open a

flow valve, or move material

off a conveyor, or a host of

other possibilities.

• The lights symbol is shown in

Figure c, is used to give

operators information about

the state of the system. The

color of the light is indicated

by a capital letter in the

circle; for example, R stands

for red, G for green, A for

amber, and B for blue.

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Internal Relays

• A relay coil is represented by a circle identified as CR (for control relay) and an associated identifying number. The contacts for that relay will be either normally open (NO) or normally closed (NC) and can be identified by the same number.

• A NO contact is one that is open when the relay coil is not energized and

becomes closed when the relay is

energized. Conversely, the NC contact is closed when the relay coil is not

energized and opens when the relay is energized.

• Time-delay relay as one for which the contacts do not activate until a

specified time delay has occurred. It designated of TR to indicate timer relay.

• An on-delay timer relay. When the coil is energized, the contacts are not

energized until the time delay has lapsed.

• There is also an off-delay timer relay. The contacts engage when the coil is energized. When the coil is

de-energized, however, there is a time delay before the contacts go to the de-energized state.

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• Figure above shows a relay used as a latch where a

green light is on when the relay is not latched and a

red light is on when the relay is latched.

• When the normally open (NO) push-button switch PB1

is depressed, control relay RL1 is energize and stays

closed.

• To de-energize or unlatch the relay, the normally

closed (NC) push button PB2 is depressed. PB2 opens

the circuit, and the relay is released.

34 PB₁ PB₂ R G 0 0 0 1 1 0 1 0 0 0 1 0 0 1 0 1

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• The elevator shown employs a platform to move objects up and down.

• The global objective is that when the UP button is pushed, the

platform carries something to the up position, and when the DOWN button is pushed, the platform carries something to the down position.

• Output Elements

M1 = Motor to drive the platform

up

M2 = Motor to drive the platform

down

• Input Elements

LSI = NC limit switch to

indicate UP position

LS2 = NC limit switch to indicate DOWN position

START = NO push button for START

STOP = NO push button for STOP

UP = NO push button for UP

command

DOWN = NO push button for

DOWN command

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Narrative Description

1.When the

START

button is pushed, the

platform is driven to the down position.

2.When the

STOP

button is pushed, the

platform is halted at whatever position it

occupies at that time.

3.When the

UP

button is pushed, the

platform, if it is not in downward motion,

is driven to the up position.

4.When the

DOWN

button is pushed, the

platform, if it is not in upward motion, is

driven to the down position.

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Programmable Logic

Controller Design

• The modern solution for the problem of how to provide

discrete-state control is to use a computer-based device

called a programmable controller (PC) or programmable

logic controller (PLC).

• The move from relay logic controllers to computer-based

controllers was an obvious one because:

i)

The input and output variables of discrete-state control

systems are binary in nature, just as with a computer,

ii)

Many of the ""control relays" of the ladder diagram can

be replaced by software, which means less hardware

failure.

iii)

It is easy to make changes in a programmed sequence

of events when it is only a change in software.

iv) Special functions, such as time-delay actions and

counters, are easy to produce in software.

v)

The semiconductor industry developed solid-state

devices that can control high-power ac/dc in response

to low-level commands from a computer, including

SCRs and TRlACs.

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Processor

• It is used to execute a program to perform the

operations specified in a ladder diagram or a set of

Boolean equations. The processor performs arithmetic

and logic operations on input variable data and

determines the proper state of the output variables.

• It can only perform one operation at a time. That is,

like most computers, it is a serial machine. Thus, it

must sequentially sample each of the inputs, evaluate

the ladder diagram program, provide each output,

and then repeat the whole process.

• The heart of a PLC is a microprocessor such as AMD

2901 and 2903, because much of the data in PLC

operation is processed bit by bit. With the great

increases in processor speed, it is now possible to

employ a desktop personal computer with data I/O

boards running PLC software to emulate PLC

Central Processing Unit (CPU)

• It containing one or more

microprocessors to handle ladder

diagram programming.

• It executes the operating system,

manages memory, monitors inputs,

evaluates the user logic, and turns

on the appropriate outputs.

• It can handle noise environment.

Memory

• Read Only Memory (ROM)

– Stalled the operating system which handle the conversion of ladder diagram to the computer language, control the data flows and the operation of the entire PLC.

– It is nonvolatile memory.

• User Memory

– It is the array for the I/O relays, internal relays, special relays, counter, timer.

– Each I/O, timer and counter has it own corresponding bit in the memory.

• Random Access Memory (RAM)

– Stalled the user’s program, timer/counter values, I/O status. – It is volatile memory.

– The memory can be sustained by the use of lithium battery. – CMOS-RAM which is the low power consumption devices.

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• Operation of the programmable controller can be

considered in two modes, the I/O scan mode and the

execution mode.

• I/O Scan Mode

– During the I/O scan mode, the processor updates all

outputs and inputs the state of all inputs one channel at a

time.

– The time required for this depends on the speed of the

processor.

• Execution Mode

– During this mode, the processor evaluates each rung of the

ladder diagram program that is being executed

sequentially.

– As a rung is evaluated, the last known state of each switch

and relay contact in the rung is considered, and if any TRUE

path to the output device is detected, then that output is

indicated to be energized - that is, set to ON.

– At the end of the ladder diagram, the I/O mode is entered

again, and all output devices are provided with the ON or

OFF state determined from execution of the ladder diagram

program. All inputs are sampled, and the execution mode

starts again.

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• Scan Time

– Is time required for one complete cycle of I/O

scan and execution. This depends on how

many input and output channels are involved

and on the length of the ladder diagram

program and also depends on the clock

frequency of the processor.

– The higher the clock frequency, the greater the

speed, and the faster the scan/execution time.

– The length of time for one scan consists of

three parts i) input time, ii) execution time,

and iii) output time. Most of the scan time

comes from the execution phase.

PLC Programming Devices

• Handheld Programmers

– It can be plugged into the PLC to monitor the status of inputs, outputs, variables, counters, timers.

– It can turn the inputs and outputs on or off.

• Computer

– It can be used for offline programming and storage of programs.

– Document the PLC program.

– Notes for technician, I/O devices.

– The document is for understanding and troubleshooting ladder diagrams.

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Programming

• Although the programmable controller can

be programmed directly in ladder diagram

symbols through the programming unit,

there are some special considerations.

• These considerations include the

availability of special functions and the

relation between external I/O devices and

their programmed representations.

• The programmable controller has no "real"

relays or relay contacts. The relay used in

the ladder diagram programming is just

the software symbols.

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Addressing

• The programmable controller uses a similar

method of identifying devices, but if is referred to

as the device address or channel. The addresses

are used to identify both the physical and

software devices according to the following

categories:

i)

Physical input devices - ON or OFF

ii) Physical output devices - energized (ON) or

de-energized (OFF)

iii) Programmed control relay coils and contacts

iv) Programmed time-delay relay coils and

contacts

v) Programmed counters and contacts

vi) Special functions

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Programmed Diagram

Interpretation

• There is an important difference between the interpretation

of a physical ladder diagram and a programmed ladder

diagram. This difference arises from the fact that the

programmed diagram bases the state of a rung on a logical

interpretation of the symbol rather than its physical state.

• In a

physical diagram

, the symbol for a NO contact

indicates a normally open contact through which current

cannot flow unless the contact has been closed. For the NC

contact, the idea is that current will flow until the contact

has been opened.

• In a

programmed diagram

, the symbol for a NO contact

indicates that the device should be interpreted as FALSE if

the contact is tested and found to be open, and TRUE if it is

found to be closed. Consider the programmed NC symbol.

This means if it is tested and found to be closed, then it is

FALSE, and if tested and found open, it is ON.

• A software

Control Relay

is denoted by a circle with an

identification number. It can have any number of NO or NC

contacts, which are identified by the same number as the

relay.

FPWin Pro/Trilogi/LogixPro

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Input Modules

• The input modules examine the state of physical switches and

other input devices and put their state into a form suitable for the processor. The PLC is able to accommodate a number of inputs, called channels.

• The input state systems are often designed to 230 Vac/24 Vdc to the input module. This type of connection assumes that switches, for example, are wired to the PLC, as shown in Figure above. If the switch is closed, the input will be 230 Vac/24 Vdc and it open; the input will be 0 Vdc. The input module converts this into the 1 or 0 state needed by the processor.

• The input modules have a certain number of channels per module. Each channel is often equipped with an indicator light to show if the particular input is ON or OFF.

Optical Isolation

• PLC must be protected from the outside world

but be able to receive input data from the input

module.

• Optical isolation means no electrical connection

exists between the input module and the CPU;

they are separated optically.

• The input module supplies a signal that turns on

a light in the input card, the light shines on a

receiver and the receiver turns on.

• It use for both input and output modules.

• LED is used to transmit the data.

• Input module also provide circuits that debounce

the input signal.

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Output Modules

• The output modules supply ac/dc power to external devices. If the required power is greater, an external relay may be used, as

shown in Figure above.

• Internally, the output module accepts a 1 or 0 input from the

processor and uses this to turn ON or OFF the control device such as a TRIAC. In this sense, the output module is a solid-state relay. • Programmable controllers also are designed with output modules

to provide other outputs, such as variable-rate pulse outputs (such as would be required by a stepping motor).

• An output module can have one or several channels per unit. Each channel is usually provided with an indicator light to show

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The Operation of the Internal Relay Contact

with Different External Input Devices

Type of External Input Devices Symbol of the External Input Devices Type of Internal Relay Contact Symbol of the Internal Relay Contact The Status of the Internal Relay Contact Before the External Input Devices be Pushed The Status of the Internal Relay Contact After the External Input Devices be Pushed NO NO NC NC NO NC

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The Operation of the Relay with Different Connection

OFF

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The Operation of the Relay with Different Connection

ON

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The Operation of the Relay with Different Connection

ON

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The Operation of the Relay with Different Connection

OFF

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• Figure above shows a relay used as a latch where a

green light is on when the relay is not latched and a

red light is on when the relay is latched.

• When the normally open (NO) push-button switch PB1

is depressed, control relay RL1 is energize and stays

closed.

• To de-energize or unlatch the relay, the normally

closed (NC) push button PB2 is depressed. PB2 opens

the circuit, and the relay is released.

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PLC

Programmed Ladder Diagram

Input Devices

PB

(NO) – X

PB

(NC) – X

Output Devices

L

(Green) – Y

L

(Red) – Y

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• The elevator shown employs a platform to move objects up and down.

• The global objective is that when the UP button is pushed, the

platform carries something to the up position, and when the DOWN button is pushed, the platform carries something to the down position.

• Output Elements

M1 = Motor to drive the platform

up

M2 = Motor to drive the platform

down

• Input Elements

LSI = NC limit switch to

indicate UP position

LS2 = NC limit switch to indicate DOWN position

START = NO push button for START

STOP = NO push button for STOP

UP = NO push button for UP

command

DOWN = NO push button for

DOWN command

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Programmed Ladder Diagram

Input Devices Output Devices Start Button (NO) X0 Motor 1 (Up Motion) Y0

Stop Button (NO) X1 Motor 2 (Down Motion) Y1

Up Button (NO) X2

Down Button (NO) X3

Up Limit Sensor (NC) X5

Down Limit Sensor (NC)

X6

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Up Motion

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Error

• During the initialization phase, up motion still can be

executed when X

(up motion button) is pushed. A NO R

contact can be located between R

and X

contact in rung 4

to overcome the error.

• Emergency stop can’t be executed during the initialization

phase. A NC R

contact can be located on the 1

_{rung to}

overcome the error.

• During the up motion, X

(down limit switch) will return to

NC condition after the stop away from it. This cause R

energized. NC R

contact will has connection. If the user

push X

; this will cause down motion immediately after end

of the up motion. If the user push X

; this will cause the up

motion stop immediately. A parallel NO R

contact and a

series NC R

contact can be located in rung 3 to overcome

this error.

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Example 8.7 (Pg 412)

I. Initialization (prefill of tank)

A. Conveyor stopped, output valve closed

B. Start the level-control system by operate a sufficient time to reach the setpoint C. Go to the running phase

II. Runing

A. Start the bottle conveyor

B. When a bottle is in the position (BP true)

1. Stop the conveyor (M1 off)

2. Open the output valve

C. When the bottle is full (BF true)

1. Close the output valve D. Go to step II.A and repeat

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Input Devices Output Devices Start Button (NO) X0 Motor Y0

Stop Button (NO) X₁ Valve Y₁ BP Sensor (NO) X2 Level Controller Y2

Example 8.9 (Pg 424)

• The objective of the system is as follow:

1. If PB1 alone is pushed, the red light turns on. 2. If PB2 alone is pushed, the green light turns on.

3. If both buttons are pushed at once, neither light turns on.

• Show the wiring connection to the PC and the ladder

program that will accomplish this task. Channels 01 and 02

are inputs, and 08 and 09 are outputs

.

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Timer

• The timer will have an address and the preset number of

ticks to count. The timer only counts while it has a true

input. If the input becomes False and then True again, the

timer will reset to zero and start to count again.

• Figure b shows an accumulating timer, which retain a tick

count when its input goes false. When the input goes true

again, the tick count will pick up where the previous one

left off. It is necessary to have a reset input to this timer so

that, when desired, the timer can be reset back to zero.

100

Counter

• A counter is a programmed function that counts (increments)

every time the input changes from False to True. This means

that, if in one scan the input is False and in the next scan the

input is True, the counter increments. No further counts will

occur until the input goes False again and then True.

• Counters are often drawn as a rectangle in programmed ladder

diagrams, like that shown in Figure above. There are two input

lines, one for the count input and another to reset the counter.

The counter has an address and a preset number of counts.

When the preset number of counts have been accumulated,

the counter becomes True and can activate some other part of

the ladder diagram.

104

Example 8.10 (Pg 426)

• A counter is to be used to count objects in the

conveyor delivery system.

• Show how a counter can be set up to count 200

objects and then turn off the conveyor motor.

• What is the maximum conveyor speed to assure

that no objects will be missed in the count if the

objects are 1 cm apart? Assume the scan time

is 30 ms for 1 cm.

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Example 8.11 (Pg 429)

• Show how a timer can be used to

turn a red light on for 2500 ms when

a NO start push button is pushed.

The PLC timer tick is 10 ms. A NC

stop button resets the system.

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Example 8.12 (Pg 429)

• A chemical vat in which a mixture must be cooked at a

temperature greater than 100 °C for 10 minutes. Due to

external influences, the temperature might fall below 100

°C periodically, and this should not be counted in the

cooking time. After the vat is filled, a Start push button

(NC) starts the cooking. It is terminated by a NC Stop push

button. A thermal switch goes high when the temperature

is above 100 °C. When the mixture has been cooked for 10

minutes at 100 °C, the heater is turned off and a drain

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Example 8.13 (Pg 431)

• Prepare the physical and programmed ladder diagram for the control problem shown in Figure 24. The global objective is to heat a liquid to a specified temperature and keep it there for 30 min.

• The hardware has the following characteristics: i) START push button is NO. STOP is NC.

ii) NO and NC are available for the limit switches. • The event sequence is

i) Fill the tank.

ii) Heat and stir the liquid for 30 min. iii) Empty the tank.

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• Develop the programmable ladder

diagram for a motor with the

following: NO start button, NC stop

button, thermal overload limit switch

opens on high temperature, green

light when running, red light for

thermal overload

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113

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• When turn ON, the tank system alternately fills to level L and then empties to level E. The level switch are activated on a rising level. Both NO and NC connections are available for the level switches and the ON/OFF push buttons. Prepare a physical ladder diagram for this system.

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Problem 8.8 (Pg 434) Solution

Problem 8.8 (Pg 434) Solution

• Prepare a ladder diagram for the system bellow. Assume a logic off-delay 5 minute timer (i.e., goes true when enabled and stays true for 5 min). Assume a NO start push button and an NC stop push button.

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Problem 8.10 (Pg 435)

The Process Sequence:

1.Fill the tank to level A (LA) from valve A (VA) 2.Fill the tank to level B (LB) form valve B (VB) 3.Start the timer (TM), stir (S) and heater (H)

4.When five minute are up take stir (S) and heater (H) off 5.Open output valve (VC) until the tank is empty (LE)

Problem 8.10 (Pg 435) Solution

• Prepare a PLC ladder program for the

system of Problem 8.8 with the

following added requirements:

a) the system should cycle 100 times and

then quit.

b) during each cycle, there should be a

1.5 minute delay after filling before the

empty phase starts. The PLC tick time

is 10 ms.

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Problem 8.14 (Pg 436) Solution

• The system has the following functions:

– Move a work piece into position, clamp it, start the drill, move it down to drill a hole in the work piece (as long as a thermal overload does not occur), back the drill out, turn the drill off, and then repeat for a new work piece. Assume master NO start and NC stop buttons and that limit switches have both NO and NC contracts. The system stops if a thermal overload occurs and turns on a red light (not shown). Develop the

programmable ladder diagram. 122

Problem 8.11 (Pg 436) Solution