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Programmable Logic Controllers Third Edition. Frank D. Petruzella McGraw-Hill. Chapter 7. Programming Timers.

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slide1

Programmable

Logic Controllers

Third Edition

Frank D. Petruzella

McGraw-Hill

slide2

Chapter 7

Programming

Timers

slide3

Time delay relays and solid-state timers are used to provide a time delay. They may have displays, pots or other means of operator interface for time settings and electromechanical or solid state outputs.

Solid-State

Timer

Time Delay

Relay

Timers

There are very few industrial control systems that do not need at least one or two timed functions. They are used to

activate or de-activate a device after a preset interval of

time.

slide4

On-Delay Timing Relay

Operating coil

Nontimed contacts are controlled directly by the timer coil, as in a general-purpose control relay.

Instantaneous

contacts

NO

NC

When the coil is energized, the timed contacts are are prevented from opening or closing until the time delay period has elapsed. However, when the coil is de-energized, the timed contacts return instantaneously to their normal state.

NO

Time control

contacts

NC

Time adjustment

slide5

Timed Contact Symbols

On-Delay Symbols

Normally closed, timed open contact (NCTO)

Normally open, timed closed contact (NOTC)

Contact is open when relay coil is de-energized

Contact is closed when relay coil is de-energized

When relay is energized, there is a time delay in opening

When relay is energized, there is a time delay in closing

slide6

Timed Contact Symbols

Off Delay Symbols

Normally closed, timed closed contacts (NCTC).

Normally open, timed open contacts (NOTO).

Contact is normally

open when relay coil

is de-energized.

Contact is normally

closed when relay coil

is de-energized.

When relay coil is energized, contact closes instantly.

When relay coil is energized, contact opens instantly.

When relay coil is de-energized, there is a time delay before the contact opens.

When relay coil is de-energized, there is a time delay before the contact closes.

slide7

S1 closes, TD energizes,

timing period starts,

TD1 still open, L1 is still off.

After 10 s, TD1 closes, L1 is switched on.

10 s

Input

ON

S1 is opened, TD de-energizes, TD1 opens instantly, L1 is switched off.

OFF

Output

Timing Diagram

On-Delay Relay Timer Circuit (NOTC Contact)

Sequence of operation

L2

L1

S1

S1 open, TD de-energized,

TD1 open, L1 is off.

L1

10 s

slide8

S1 closes, TD energizes,

timing period starts, TD1

is still closed, L1 is still on.

10 s

After 10 s, TD1 opens,

L1 is switched off.

Input

On

S1 is opened, TD de-energizes,

TD1 closes instantly, L1 is

switched on.

Output

Off

Timing Diagram

On-Delay Relay Timer Circuit (NCTO Contact)

L1

L2

L1

L2

Sequence of operation

S1

S1 open, TD de-energized,

TD1 closed, L1 is on.

L1

10 s

slide9

S1 closes, TD energizes,

TD1 closes instantly,

L1 is switched on.

S1 is opened, TD de-energizes,

timing period starts, TD1 is

still closed, L1 is still on.

10 s

Input

On

Off

After 10 s, TD1 opens, L1 is

switched off.

Output

Timing Diagram

Off-Delay Relay Timer Circuit (NOTO Contact)

L1

L2

Sequence of operation

S1

S1 open, TD de-energized,

TD1 open, L1 is off.

L1

10 s

slide10

S1 closes, TD energizes,

TD1 opens instantly,

L1 is switched off.

S1 is opened,

TD de-energizes, timing

period starts, TD1 is still

open, L1 is still off.

10 s

Input

On

Output

After 10 s, TD1 closes,

L1 is switched on.

Off

Timing Diagram

Off-Delay Relay Timer Circuit (NCTC Contact)

L1

L2

Sequence of operation

S1

S1 open, TD de-energized,

TD1 closed, L1 is on.

L1

10 s

slide11

1. Timers are used to activate or de-activate a

device after a preset interval of time.

(True/False)

2. With most timers the time delay period is fixed

and can not be varied. (True/False)

  • 3. ________ contacts are controlled directly by the
  • timer coil, as in a general-purpose control relay.
  • timed (c) instantaneous
  • NO (d) NC
slide12

5. Which of the following symbols represents a

normally open timed closed contact?

4. When a relay timer coil is de-energized, the timed contacts return instantaneously to their normal state. (True/False)

slide13

6. Which of the following symbols represents a

normally closed timed open contact?

slide14

7. The timed relay contact shown is designed to operate so that:

a. when the relay coil is energized, there is a

time delay in closing

b. when the relay coil is energized, there is a

time delay in opening

c. when the relay coil is de-energized, there is a

time delay in closing

d. when the relay coil is de-energized, there is a

time delay in opening

slide15

8. The timing diagram shown is that of an

a. on-delay timer circuit (NOTC contact)

b. on-delay timer circuit (NCTO contact)

c. off-delay timer circuit (NCTC contact)

d. off-delay timer circuit (NOTO contact)

10 s

Input

On

Off

Output

Timing Diagram

slide16

L1

L2

S1

L1

10 s

a. as long as S1 is closed

b. for 10 seconds after coil TD is energized

c. for 10 seconds after coil TD is de-energized

d. both a and c

9. In the circuit shown, the light will stay off

slide17

10. In the circuit shown, the light will stay on

  • as long as S1 is closed
  • as long as S1 is open
  • c. for 10 seconds after S1 is closed
  • d. both b and c
slide18

Some advantages of PLC timers:

  • their settings can be altered easily
  • the number of PLC timers used

can be increased or decreased by

programming changes without

wiring changes

  • timer accuracy and repeatability

are extremely high

Programmed Timer Instructions

PLC timers are output instructions that provide the same functions as timing relays and solid state timers.

slide19

RSLogic Timer Commands

TON

Timer/Counter

Counts time base

intervals when the

instruction is “true”

TON

Timer On-Delay

Command Name Description

slide20

RSLogic Timer Commands

TOF

Timer/Counter

Command Name Description

TOF

Timer Off-Delay

Counts time base

intervals when the

instruction is “false”

slide21

RSLogic Timer Commands

RES

RTO

Timer/Counter

Command Name Description

Counts time base intervals when the instruction is “true” and retains the accumulated value when the instruction goes "false" or when power cycle occurs

Retentive Timer

ON

RTO

When this instruction is "true" it resets the count of the RTO counter

RES

Reset

slide22

Quantities Associated with the Timer Instruction

Preset Time – Represents the time duration of the timing

circuit. For example, if a time delay of 10 s is required, the timer will have a preset of 10 s.

Accumulated Time – Represents the amount of time that

has elapsed from the moment the timing coil became energized.

Time Base– Timers can typically be programmed with

several different time bases: 1 s, 0.1 s, and 0.01 s are typical time bases. For example, if you enter 0.1 for the time base and 50 for the preset time the timer would have a 5 s delay (50 x 0.1 s = 5 s).

slide23

Contact determines

rung continuity

The timer assigned

an address

The type of timer

is specified

XXX

TON

Coil-Formatted Timer Instruction

Preset value PR:YYY

Time base 0.1 s

Accumulated value AC:000

When the timer rung has logic continuity, the timer's accumulated value increases. When accumulated value equals the preset value, the output is energized and and the timed output contact associated with the output is closed. The timed contact can be used as many times as you wish throughout the program as a NO or NC contact.

slide24

Control line controls the actual timing operation of the timer.

Whenever this line is true the timer will time.

Retentive timer block

Preset time

Time base

Accumulated time

Output line

The timer continuously compares its accumulated time with its preset time. Its output is logic 0 as long as the accumulated time is less than the preset time. When the two become equal the output changes to logic 1.

Reset line resets the the timer's accumulated value to zero.

Generic Block-Formatted Timer Instruction

Timers are most often represented by boxes in a ladder logic.

slide25

Input

Timer

True

Timer Sequence

False

Rung condition

Timed period

On delay

time duration

True

False

ON

Timed output bit

OFF

On-Delay Timer Instruction

The on-delay timer operates so that, when the rung containing the timer is true, the timer time-out period commences.

The timed output becomes true sometimes after the timer rung becomes true; hence the timer is said to have an on delay.

slide26

The control word uses three control bits: Enable (EN) bit,

Timer-Timing (TT) bit, and Done-Bit (DN).

TIMER TABLE

/EN

/TT

/DN

0

0

0

T4:0

Allen-Bradley On-Delay Timer Instruction

Allen-Bradley PLC-5 and SLC-500 controller timer elements each take three data table words: thecontrol word, preset word, and accumulated word.

slide27

TON

TIMER ON DELAY

EN

Timer T4:0

T4:0

Enable bit true

EN

Allen-Bradley On-Delay Timer Instruction

The Enable (EN) bit is true (has a status of 1) whenever the timer instruction is true. When the timer instruction is false, the enable bit is false (has a status of 0)

Enable bit false

slide28

TON

TIMER ON DELAY

EN

Timer T4:0

Preset 50

Accumulated 10

T4:0

Timer-Timing bit true

TT

Allen-Bradley On-Delay Timer Instruction

The Timer-Timing (TT) bit is true whenever the accumulated value of the timer is changing, which means the timer is timing.

slide29

50

Done-bit changes state

Allen-Bradley On-Delay Timer Instruction

The Done-Bit (DN) changes state whenever the accumulated value reaches the preset value. Its state depends on the type of timer being used.

TON

TIMER ON DELAY

EN

Timer T4:0

Preset 50

Accumulated 10

DN

T4:0

DN

slide30

TIMER TABLE

.ACC

/TT

/EN

/DN

.PRE

0 0 0 0 0

T4:0

Allen-Bradley On-Delay Timer Instruction

The preset value (PRE)word is the set point of the timer, that is, the value up to which the timer will time.

The accumulated value (ACC) word is the value that increments as the timer is timing. The accumulated value will stop incrementing when its value reaches the preset value.

slide31

TON

Time base 1.0

Timer T4:0

TIMER ON DELAY

Preset 15

EN

Accumulated 0

DN

Timer number which must come from the timer file.

Time base which is expressed in seconds.

Preset value which is the length of the time delay.

Accumulated value which is normally entered as 0.

Allen-Bradley On-Delay Timer Instruction

The information to be entered includes:

slide32

10

On-Delay Timer Program

Ladder Logic Program

L2

L1

TON

Input A

TIMER ON DELAY

Timer T4:0

Time base 1.0

Preset 10

Accumulated 0

Input A

Output B

G

EN

DN

Output C

R

Output B

T4:0

EN

Output D

Y

Output C

T4:0

TT

Output D

T4:0

DN

slide33

Timing Diagram

On

On

On

On

Timer-enable bit

Timer-done bit

Timer-timing bit

Input condition A

Off

Off

Off

Off

4 s

4 s

Timer accumulated

value

0

On-Delay Timer Program

slide34

Word

EN

Internal use

0

TT

DN

1

Preset value PRE

Accumulated value ACC

2

Word 0 is the control word

Word 1 stores the preset value

Word 2 stores the accumulated value

On-Delay Timer Program

Timers are 3-word elements

slide35

Relay Ladder Schematic Diagram

Start

Stop

L1

L2

1TD

1TD

1TD-1

(instantaneous

contact)

M

1TD-2

(5 s)

(timed contact)

On-Delay Timer With Instantaneous Output

slide36

Ladder logic program

Internal

relay

Inputs

Output

Stop

Start

L2

L1

Stop

M

Motor

Internal

relay

Motor

M

Start

Timer

PR: 5

TB: 1 s

Output

line

On-Delay Timer With Instantaneous Output

Programmed Circuit

slide37

Relay Ladder Schematic Diagram

L1

Start-up

L2

Reset

CR1

PB2

PB1

CR1-1

1TD

CR1-2

Horn

CR1-3

1TD-1

(10 s)

Start-Up Warning Signal Circuit

slide38

Programmed Circuit

Ladder logic program

Inputs

Output

PB1

PB2

TON

TIMER ON DELAY

Timer T4:0

Time base 1.0

Preset 10

Accumulated 0

Start-up

EN

PB1

T4:0

DN

Reset

Horn

PB2

10

EN

Horn

T4:0

T4:0

EN

DN

Start-Up Warning Signal Circuit

slide39

Timed Closed Solenoid Value Program

Ladder logic program

Output

Input

L2

L1

TON

Timer On Delay

Timer timer_1

Preset 12000

Accumulated 0

Switch

SW_1

EN

SW_1

Valve

DN

12000

Valve

timer_1.dn

slide40

Start

PB2

L1

L2

Stop

PB1

Lube oil

pump motor

OL

M1

Main drive

motor

M1-1

OL

PS1

M2

(lube oil

pressure switch)

1TD

OL

M3

1TD-1

(15 s)

Feed

motor

Automatic Sequential Control System

Relay Ladder

Schematic

Diagram

slide41

15

Automatic Sequential Control System

Programmed Circuit

Ladder logic program

Inputs

PB2

PB1

M1

Outputs

OL

PB1

M1

M1

PB2

OL

M2

PS1

M2

PS1

OL

M3

TON

TIMER ON DELAY

Timer T4:0

Time base 1.0

Preset 15

Accumulated 0

EN

DN

M3

T4:0

DN

slide42

Ladder logic program

Output

Input

L2

L1

TOF

TIMER OFF DELAY

TIMER T4:3

Time base 1.0

Preset 15

Accumulated 0

S1

I:1.0/0

I:1.0/0

O:2.0/1

PL

EN

DN

15

T4:3/DN

O:2.0/1

Off-Delay Programmed Timer

The off-delay timer (TOF) operation will keep the output energized for a period after the rung containing the timer has gone false.

slide43

Off-Delay Programmed Timer

Timing Diagram

True

Input condition

S1

False

Timed period

Off delay

timed duration

True (logic 1)

False (logic 0)

Timed output

O:2.0/1

Preset value = accumulated value

slide44

Off-Delay Timer Used To Switch Motors Off

Ladder logic program

Input

Output

L2

5000

10000

15000

L1

slide45

Pneumatic Off-Delay Timer

L1

L2

Relay Ladder

Schematic

Diagram

slide46

Programmed Pneumatic Off-Delay Timer

Equivalent Programmed Circuit

Ladder logic program

Input

Outputs

L1

L2

5

slide47

Operation

  • Before starting, PS1 must be closed.

Fluid Pumping Process

  • When the pump start button is pressed, the pump starts. The button can then be released and the pump continues to operate.
  • When the stop button is pushed, the pump stops.
  • PS2 and PS3 must be closed for 5 s after the pump starts. If either PS2 or PS3 opens, the pump will shut off and will not not be able to start again for another 14 s.
slide48

Ladder logic program

Inputs

Output

L1

L2

5

0

Fluid Pumping Process Program

slide49

Electromechnical Retentive Timer

Once power is applied, the motor starts turning the cam. The positioning of the lobes determines the time it takes to activate the contacts. If power is removed from the motor, the shaft stops but does not reset.

Cam operated

contact

Motor-driven

cam

Retentive Timer

A retentive timer accumulates time whenever the device receives power, and maintains the current time should power be removed from the device. Once the device accumulates time equal to its preset value, the contacts of the device change state. The retentive timer must be intentionally reset with a separate signal for the accumulated time to be reset.

slide50

Unlike the TON, the RTO will hold its accumulated value when the timer rung goes false and will continue timing where it left off when the timer rung goes true again. This timer must be accompanied by a timer reset (RES) instruction to reset the accumulated value of the timer to zero.

Same address

Retentive On-Delay Timer Program

The PLC-programmed RETENTIVE ON-DELAY timer (RTO) operates in the same way as the nonretentive on-delay timer (TON), with one major exception. This is a retentive timer reset (RTR) instruction.

slide51

0

9

Retentive On-Delay Timer Program

3

slide52

Enable bit is reset when input pushbutton PB1 is opened

Accum = Preset

Accum value retained

when rung goes false

When reset PB2 is closed, the T4:2/DN bit is reset to 0. Accumulated value is reset and held at zero until the reset pushbutton is opened.

Retentive On-Delay Timing Chart

slide53

60000

0

Retentive On-delay Alarm Program

Ladder logic program

L2

L1

The purpose of the RTO timer is to detect whenever a piping system has sustained a cumulative overpressure condition of 60 s. At that point, a horn is sounded automatically. You can silence the alarm by switching the key switch to the rest position.

slide54

Bearing Lubrication

Program

Sequence Of Operation

  • To start the machine, the operator turns SW on.
  • Before the motor shaft starts to turn, the bearings are supplied

with oil by the pump for 10 s.

  • The bearings also receive oil when the machine is running.
  • When the operator turns SW off to stop the machine, the oil

pump continues to supply oil for 15 s.

  • A retentive timer is used to track the total running time of the

pump. When the total running time is 3 h, the motor is shut down

and a pilot light is turned on to indicate that the filter and oil

need to be changed.

  • A reset button is provided to reset the process after the filter and

oil have been changed.

slide56

Cascading Timers

The programming of two or more timers together is called cascading. Timers may be interconnected, or cascaded to satisfy any required control logic.

Relay Schematic Diagram

Three motors started automatically in sequence with a 20-s time delay between each motor startup.

slide58

The oscillator circuit output is programmed in series with the alarm condition. If the alarm condition is true, the appropriate output indicating light will flash.

Annunciator Flasher Circuit

Two timers can be interconnected to form an oscillator circuit. The oscillator logic is basically a timing circuit programmed to generate periodic output pulses of any duration. They can be used as part of an annunciator system to indicate an alarm condition.

slide60

30000

12000

Cascading of Timers for Longer Time Delays

slide61

Control of Traffic in One Direction

Sequence of Operation

Green

25 s

Red

30 s

Amber

5 s

Control of Traffic Lights in One Direction

A typical application for PLC timers is the control of traffic lights.

slide63

11. The timed contact of a PLC timer can only

be used as a normally-open contact. (True/False)

12. The ______ bit operates the same as an instantaneous contact on a timer relay.

a. enable

b. done

c. timer-timing

d. timer number

slide64

13. The preset time of a PLC timer represents the

amount of time that has elapsed from the moment

the timing coil became energized. (True/False)

14. If the preset time of a timer is 150 and the time

base is 0.1 seconds, the time-delay period would

be 1500 seconds. (True/False)

slide65

15. In general, the three different types of PLC timers are:

a. TON, TOF, and PRE

b. TON, TOF, and RTO

c. TON, ACC, and RTO

d. TT, EN, and DN

slide66

16. The amount of time for which a timer is

programmed is called the

a. preset

b. set point

c. Done Time (DN)

d. accumulated time

slide67

17. The timer reset instruction must be addressed

to the same address as the ______ instruction.

a. TON

b. TOF

c. RTO

d. EN

slide68

18. Which of the following statements is not true for a retentive on-delay timer?

a. The timer accumulates time when it is energized.

b.The timer requires a reset instruction to reset the accumulated value of the timer to zero.

c. The timer does not reset to zero when it is de-energized.

d. The reset input to the timer will not override the control input.

slide69

19. The timer instruction is:

a. an input instruction c. either a or b

b. an output instruction d. both a or b

20. The interconnection of timers is commonly

called :

a. grouping

b. programming

c. sequencing

d. cascading

slide70

21. For the program shown, the pilot light will be:

  • on at all times
  • off at all times
  • c. switched on 15 s after the switch has been actuated
  • from the open to the closed position
  • d. switched off 15 s after the switch has been actuated
  • from the on to off position