Programable Logic controller questions 1-3

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MODULE TITLE: PROGRAMMABLE LOGIC CONTROLLERS

TOPIC TITLE: PROGRAMMABLE FACILITIES

LESSON 2: TIMERS AND COUNTERS

PLC - 6 - 2

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION ________________________________________________________________________________________

The main object of this lesson is to provide the general basis for attempting

slightly more ambitious programming exercises. This is achieved by

incorporating into the ladder diagrams devices such as timers, counters and

combinations of the two. The use of timers allows what would otherwise be a

spontaneous on/off set of characteristics, to become a controlled sequence

occurring in a predictable fashion.

The challenge in designing, reading and understanding ladder diagrams for

specific applications does not, therefore, merely rest at the Boolean logic level.

To be "good" at applications, programmers need to possess intimate knowledge

of the behaviour of their machine's functions. They also need to remember the

historical state of the devices included within the ladder diagram.

________________________________________________________________________________________

YOUR AIMS ________________________________________________________________________________________

On completing this lesson you should be able to:

• relate a written algorithm to the ladder diagram which will satisfy its

requirements

• follow the operation of timer/timer combinations

• understand typical operation of counters used in PLCs

• analyse more complex ladder diagrams to determine correct

operation

• devise suitable ladder diagrams to fulfil operational specifications.

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________________________________________________________________________________________

CIRCUIT APPLICATIONS OF TIMERS ________________________________________________________________________________________

In the previous lesson of this topic, ladder diagram programming and the

operation of PLC timers were introduced. Brief mention was also made of the

requirements of part of a pelican crossing control circuit. At this stage the

circuit for the crossing can be expanded and improved by the addition of some

timers.

The following is a description of the required operation – note that a flashing

amber light is not included at this time.

When a pedestrian pushes the button the PLC must remember the event as well

as lighting up the WAIT sign. A check of the road sensors begins in order to

determine whether a vehicle is passing. The road sensors are to be checked

over a five second period. If no vehicle is sensed within this time then the road

lights can be taken through the light change sequence. If, on the other hand, a

vehicle is sensed then a further five seconds must elapse whilst the check is

made again.

The "normal" lighting requirement is that the GREEN road traffic light and the

RED man light should be on. The lights must return to this set of conditions

after the sequence change has been run through once.

The input designations will be:

X001 for the pedestrian push switch

X004 for the road traffic sensor.

The output designations will be:

Y000 for the red man light

Y001 for the green man light

Y004 for the wait sign

Y005 for the amber road light

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Y006 for the red road light

Y007 for the green road light.

The required light change sequence can be represented on the chart below.

The points to note from the chart are:

(a) The chart is broken up into six time change periods. The conditions in

time period 1 are the same as in time period 6 and are to be regarded as

the "normal" conditions.

(b) The red man light and the green man light are always in opposite states.

during the sequence.

(d) The amber road light has two changes of state during the sequence. This

means that two sets of conditions must bring on this output. One set of

conditions OR a second set of conditions i.e. the amber light output rung

in the ladder diagram is likely to possess an OR statement.

TIME CHANGE PERIODS

Green road light

Amber road light

Red road light

Red man

Green man

Wait sign

off

* see note (d)

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Other constraints are:

The change from period 2 to period 3 can only be made after at least one five

second check of the road sensors.

The conditions in period 3 should last for only four seconds before changing to

those of period 4.

The conditions in period 4 should last for ten seconds.

The conditions in period 5 should last for three seconds.

Specifying four distinct time durations tends to indicate the need for four

timers.

A ladder diagram for this sequence can now be examined. It must be

remembered that, as was shown in the previous lesson, many solutions to this

problem are likely to work. The one provided in this lesson is not necessarily

the best solution. It has been constructed and tested in a way which allows it

to be broken down into simple sections which can be recognised from previous

work. The explanation of the operation of the diagram is, unfortunately, long

and drawn out but you should follow it by examining the diagram as the

sequence is analysed.

The ladder diagram of the pelican crossing is shown in FIGURE 1.

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FIG. 1

TIM00 (5 secs) X004

Y005 Y007

Y006

R002

Y005

Y006 Y001Y000

TIM03

R003

X001 R000

R000

R004

R000

Y004

Y006 Y004

R000

TIM00

R001

R003 R001

Y005

Y007 Y001 Y000

R001 Y006 Y005

TIM01

Y006

TIM01 (4 secs)

R003 Y006

Y006 R003 TIM02 (10 secs)

TIM02

R002

R003 R002

R002 TIM03 (3 secs)

R003Y000

Y007

Y001 R004Y006

WAIT SIGN

GREEN LIGHT

RED MAN

AMBER LIGHT

RED LIGHT

GREEN MAN

Y005

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You may be able to follow the diagram of FIGURE 1 without the need for an

explanation. However, for those who can't, and for the sake of completeness,

we shall analyse this diagram by working down from top to bottom checking

through each rung as we go.

Before doing this in great detail just take a few moments to scan the diagram to

note that none of the rungs is, in any way, more complicated than those

examined in other lessons. So understanding the circuit operation should

present no real problems.

In this analysis we shall assume that every "normal" state of circuit elements

applies unless we have information to the contrary. Hence, if we come to, say,

a N/C set of contacts we assume that they are closed unless we definitely know

otherwise.

Rung one consists of a simple latch (which is used frequently throughout the

circuit). When the pedestrian presses the push switch X001 closes and because

R004 is assumed closed the R000 relay will switch on. Once on, all R000

contacts will change state. The first of these sets of contacts is a N/O set in

rung one. The closing of these holds the R000 relay in the on state. If the

pedestrian now releases the push the R000 relay will not switch off.

In the second rung the closing of the R000 contacts switches on the Y004

physical output (the N/C Y006 contacts assumed closed) to which is connected

the WAIT sign.

The lighting up of the wait sign signals recognition to the pedestrian of the

switch having been pressed.

In the third rung the closing of the R000 contacts enables the first timer to

begin the five second check for the passage of a road vehicle. If a vehicle is

detected then the N/C contacts of X004 input will open and immediately

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disable the timer. The timer value (representing five seconds) will be reloaded

into the timer so that the timing can begin again after the vehicle has passed. If

no vehicle is sensed within the five seconds the timer output will switch on.

In rung four, the closing of the TIM00 contacts switches on the R001 memory

relay (the N/C R003 contacts assumed closed). The R001 relay will now be

latched on by the R001 contacts connected, in parallel, across the TIM00

contacts. This arrangement ensures that the five second check is completed

and is remembered. If a vehicle subsequently passes the road sensor the

TIM00 timer will be disabled but this will have no effect on the R001 relay.

The fifth rung represents an unbroken path to switch on the Y007 output. This

output controls the green road light which is required to be "normally" on.

Y007 would be off if either of Y005 or Y006 outputs were on but at this point

in time we have not proved this.

In rung six two possible paths exist for the switching on of the Y000 output.

This output supplies the red man indicator which should "normally" be on and

therefore in rung six the Y007 contacts will be closed to bring on the Y000

(red man) output.

After reading the first six of the fifteen rungs we can pause to assess the

situation.

Rungs five and six proved that the green road light and the red man would

normally be on. Rungs one and two prove that the pressing of the push can be

recognised and remembered. Rungs three and four show that a five second

check can be made and that the system will remember having made the check.

We can now continue with the analysis.

The seventh rung presents two possible paths for the switching on of the output

which controls the amber road light (Y005). Both of these paths are

"normally" open circuits and so the amber light is normally off. In one of the

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paths, however, the R001 contacts could be complete, as we have already

shown by the proving of rung four. Therefore, if the pedestrian presses the

push, the wait sign illuminates and if no vehicle is detected for a five second

period then the amber road light (Y005) will come on.

This also means that the N/C Y005 contacts in rung five will open, thus

extinguishing the green road light (Y007). The loss of Y007 would switch off

the red man output too early and so the parallel connected Y005 contacts

across the Y007 contacts in rung six take over this task.

We have now completed the move through the sequence from period 1 to

period 2 and on to period 3.

The red man is still on but the green road light has gone off so that the amber

can come on.

When the amber light comes on the Y005 contacts in rung eight close to

enable the second timer TIM01. This timer is set with a value of four seconds

after which time all TIM01 contacts will change state.

In rung nine, we have the only set of TIM01 contacts in the diagram. When

these close the Y006 output (red road light) will come on and stay on by its

own latching contacts Y006 across the TIM01 contacts.

There are many sets of Y006 contacts used in the diagram.

In rung two the N/C Y006 contacts open to unlatch the Y004 (wait sign)

output.

In rung seven the loss of the Y006 contacts will cause the loss of the amber

road light (Y005). With the loss of Y005 in rung five the green road light

would come back on; however, the series connected N/C Y006 contacts keep

the green light off.

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With the switching off of both Y005 and Y007 rung six can no longer retain

the Y000 (red man) in the on state.

So, at rung ten, Y006 is closed and Y000 is not on, therefore the Y001 (green

man) output will come on to replace the loss of the red man.

The last Y006 contacts to mention at this stage are in rung eleven. These

enable the timer TIM02. While this timer is timing out no other changes are

made.

We are now in the conditions of period 4.

After the ten seconds of the TIM02 timer have elapsed the TIM02 contacts in

rung 12 will close. This allows R002 to switch and latch itself on.

In rung seven, the closing of the N/O parallel connected R002 contacts brings

the Y005 (amber road light) on for the second time. (Remember the mention

on page 3 about the likely ORed conditions.) The green man, red road light

and the amber road light are all on at this time. We are, therefore, in time

period 5.

The closing of the R002 contacts in rung thirteen enables the last timer TIM03.

After three seconds TIM03 output comes on to cause the R003 relay to switch

on. This relay is being used as a memory latch so that the system knows that

the red and amber road light combination has been on for three seconds.

The four sets of N/C R003 contacts within the diagram all open causing:

• the resetting of TIM02 (rung eleven)

• the unlatching of relays R001 and R002 (rungs four and twelve) thus

turning off the amber road light Y005 (rung seven)

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• the switching off of Y006 (the red road light) in rung nine, which in

turn causes the green man to go off (rung ten).

The loss of Y005 and Y006 restores Y007, the green road light (rung five).

Rung fifteen is a reset for the R000 relay. R004, the resetting element, will

switch on only when Y001, Y005 and Y006 are all on. This condition arises in

period 5 and ensures that the sequence will only run once (because R000 is

disabled) before the next five second check is carried out. This lockout effect

ensures that someone continuing to press the push does not cause the sequence

to freeze and that a new push press can only be accepted after the sequence is

virtually complete.

You should notice that, certainly at the beginning of the ladder diagram, there

was a relationship between working from the top of the diagram and the

requirements of the start of the sequence. The sequence was executed as we

worked our way down through the diagram. In this case the diagram does

not need to be presented in this way for it to work correctly, i.e. rung

seven could have been placed first instead of rung one, followed (say) by

rung ten, etc., etc. As long as all rungs were in the diagram somewhere it

would have worked. This is not, unfortunately, always the case and problems

can arise due to incorrect rung positioning.

Any unnecessary jumbling up of the rungs makes the diagram that much more

difficult for someone to read. That someone may be you at some time in the

future!

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TIMER/TIMER COMBINATIONS

Perhaps the diagram of FIGURE 1 could be improved even further by the

addition of a flashing amber output. One method used to obtain such action

requires the use of a further two timers. These two timers would need to be

arranged so that they worked together to produce a regular on/off switching

operation to drive a physical output. Suppose a selected output, say Y003, was

required to be switched on for five seconds then switched off for ten seconds

before being switched on again to recommence the five seconds on/ten seconds

off freerunning operation. The on/off/on sequence will be initiated and

maintained by the closing of input X006. The two timers will be identified as

TIM04 and TIM05. In Lesson 1 we looked at the operation of both a delay-in-

on timer and a delay-in-off timer. This problem effectively requires one of

each of these; one to delay the output from switching off (i.e. keep it switched

on for five seconds) and the other to delay the output from switching on (i.e.

keep it switched off for ten seconds).

The ladder diagram of FIGURE 2 can now be examined.

FIG. 2

TIM04

X006 TIM05 TIM04 5 seconds

TIM05 10 seconds

Y003X006 TIM04

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The X006 input contacts in rungs one and two are the enable/disable operation

control elements. If rung one is examined first then when input X006 contacts

close the Y003 output will switch on (the N/C TIM04 contacts are assumed to

be closed).

In rung two the X006 contacts are closed to enable timer TIM04 to begin

timing.

Rung three is presently under open circuit conditions.

When the Y003 output has been on for five seconds the TIM04 output will

switch on. All TIM04 contacts will change state.

In rung one the N/C TIM04 contacts will open switching off the Y003 output.

In rung three the N/O TIM04 contacts will close to enable the second timer

TIM05.

When the Y003 output has been off for ten seconds the TIM05 output will

switch on. All TIM05 contacts will change state.

In rung two the N/C TIM05 contacts will open to disable the TIM04 timer. All

TIM04 contacts will then change back to their original "normal" state. Output

Y003 will switch on again and the TIM04 contacts in rung three will now

disable the TIM05 timer.

TIM05 output is therefore on for a very short duration. It switches off

microseconds after it was switched on.

FIGURE 3 shows the timing diagram for the arrangement.

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FIG. 3

We will now leave the application of timers for the time being but you will

need to remember the work we have covered for future programming tasks.

Closed X006

Y003 On

TIM04 output

TIM05 output

Time axis

Off

1 second time periods

Small duration pulse

X006 contacts

open

Open

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________________________________________________________________________________________

COUNTERS ________________________________________________________________________________________

Counters are undoubtedly a valuable addition to any control system. In

everyday life we tend to take counters for granted. In a car, for instance, a

counter (the mileometer) is used to keep a record of the number of miles the

vehicle has travelled i.e. it is a mile counter. In the filling station the fuel

pumps count how many litres of fuel are put into a tank. In the home an audio

or video cassette recorder uses a counter to locate a specific position on the

tape. These, and many others, are everyday examples of counters in use.

Quite often, someone may say something such as "my vehicle has clocked up

80 000 miles" when really they should say that the vehicle mile counter has

counted 80 000. This association between clocks and counters seems to stem

from the construction of the original counters, the rows of cogs in the

mechanical manufacture being reminiscent of clockwork mechanisms.

Of course mechanical counters are quite acceptable in certain applications

especially when the display dials are required to accompany the mechanism.

If, however, a remote indication is necessary then the linkage medium between

the event and the display may present major problems. Additionally the

maximum speed of mechanical counters may not match the requirements of

the application. Electronic counters, on the other hand, can cope with the

remote requirements quite easily and can have very high maximum speeds.

The possible disadvantages are that they may, in certain circumstances, be

more expensive and they do require a supply of electricity.

Counters incorporated into PLCs may utilize counter integrated circuits. In

fact the same I.C. may also house the timers. To be able to use the counters

does not, however, mean that the user needs to know anything about this

hardware because the programming software makes all the necessary

arrangements.

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Reasonably high counting speeds can be achieved, limited mainly by the scan

time of the software. Where a PLC has separate counter input terminals count

speeds of 2000 Hz (2000 count pulse events per second) are quite realistic –

mechanical counters may be hard pushed to match such count rates.

PLC COUNTERS

Counters within PLCs are used to count specific events and compare the

number of events with a preset value specified by the program listing. The

preset value is often copied from the program listing and held in a specified

RAM location which is reserved with that counter's identity. When the count

event takes place the preset value, held in the counter, is decremented (i.e.

decreased) by subtracting one. Immediately after the decrementing operation

the value is checked to see if it has decreased to zero. If it has then the count is

complete and the counter output switches on. At which point all contacts

within the program, which carry that counter's identity, will switch i.e. N/O

counter contacts will close and N/C counter contacts will open. The counting

sequence can be interrupted and the preset value reinstated back into the

counter at any time by the application of a RESET condition.

A PLC may possess several counters so each must be identified to ensure that

the software handles the correct one.

Generally speaking there are four items which are required to be specified for

each counter being used:

(a) the event which is to be counted

(b) the identity of the counter being used

(d) the RESET (or DISABLE) conditions.

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The method used to represent counters on a ladder diagram and the

programming syntax used vary from manufacturer to manufacturer, as does the

counter operation. Here again the warning must be given to would-be users to

check the manual before assuming the type of operation.

FIGURE 4 shows a section of a ladder diagram which we can use as an

example to describe typical operation.

FIG. 4

In all cases of using counters the manufacturer's manual must be used to ensure

that the elements are specified in the correct order.

R059

X006

X001 R023

CNT2

Count input

CNT2

10 counts

Reset input

Y002

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The program listing for the above extract would be:

STEP OP CODE OPERAND

020 LD X001 021 AND R023

022 LD X006 023 OR R059

024 CNT2 010 counter identiity and preset value

025 LD CNT2 026 OUT Y002

We can now attempt to clarify the operation of the diagram of FIGURE 4.

A quick look at the diagram shows that each rung is in the open circuit

condition if the assumed "normal" conditions apply.

We'll start the explanation at rung two. This rung is the reset input for the

counter. A circuit is made if either X006 OR R059 is closed. Either of these

will cause the counter to reset back to its preset value (10 in this case). If

either input contacts X006 or memory relay contacts R059 close the counter

value is reset. Both of these sets of contacts must be opened before the counter

is enabled to count. Assuming then that the counter is reset and enabled we

can turn our attention to rung one.

Rung one contains the count input conditions. In the diagram these conditions

are X001 AND R023. This means that if both of these contacts are closed at

the same time the input will recognise this as a count event. Recognition of the

event is "edge sensitive" to ensure that only one count will be made. The term

edge sensitive can be best illustrated by the use of a timing diagram

(FIGURE 5).

a physical output driven by the counter outpuut{

specification for the reset conditions{

specification for the input count event{

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FIG. 5

Therefore, each time R023 ANDed with X001 are newly closed the

recognition of a count event is made and the counter value is decremented.

After ten count events (in this case), assuming that the reset line has not been

enabled, the counter (CNT2) output will switch on. In the diagram of FIGURE

4 this means that the N/O CNT2 contacts in rung 3 will close and physical

output Y002 will switch on. Any further count events will not be recognised

while the counter output is switched on.

If now, or at any time, either X006 or R059 contacts are closed then the count

value reverts back to ten and, because it is no longer zero, the counter output

will switch off. In our circuit this means that output Y002 will also switch off.

As with relays and timers any number of counter contacts can be included

within a ladder diagram.

R059 Open

X006

X001

R023

Closed

Open

Counter value

Counter enabled

Leading edge recognised

Time axis

10 10 9 9 8 8 7

Reset

X001 AND R023

Open

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A further point to remember is that the counter reset input normally has

precedence over the count up input i.e. if the reset line is enabled then no count

events will be recognised.

COUNTER VARIATIONS

What has been described so far about counters holds true for a number of

PLCs. However, the reader should understand that, as mentioned earlier, the

manufacturer's manual should be checked because variations do exist between

manufacturers.

This section of the lesson gives some indication of the extent of that variation.

In some cases the counter operation is not that of a down counter i.e.

decrementing to zero, but is instead that of an up counter. This means that

after reset and when enabled, the counter begins at zero and increments with

every input count event until the counter value equals the preset value, at

which time the counter output switches on.

In other cases the counter operation may be that of a ring counter. This counts

up from zero (after having been reset) to the preset value before turning the

output on, but then continues to recognise further count events which causes

the counter value to spill over back to zero before starting the count again. The

output remains on only while the counter value is the same as the preset value.

Yet a further variation can be an up/down counter. Such a counter has an up

count input and a separate down count input. When the counter is enabled a

suitable signal on either input makes the counter either increment or decrement

the count value. As with the previous counter the output switches on when the

count value equals the preset value. A very graphic description of the use of

such a counter would be in the control of the number of vehicles allowed to

enter a multistorey car park. If, for example, the car park had space for

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five hundred vehicles then an up/down counter could be used to count them in

and illuminate a CAR PARK FULL sign when five hundred have entered.

Should a vehicle leave by the exit then the count would be decremented and

the sign turned off.

In other types of counters, typically high speed types, multiple outputs may be

possible. The multioutputs do not normally drive physical outputs but will

control internal relays which can switch on or off the physical outputs.

Outputs from such counters are fully programmable by having two count

values specified. When the counter reaches the first value an output will

switch on and the counter will continue to recognise input count signals until a

second value is reached. At this time the counter output will switch off.

Couple this with the fact that the counter may be able to simultaneously

control numerous different outputs, all with different values, and you may

begin to appreciate how powerful this counter is.

Another variation relates to the operation of the reset line. Sometimes the reset

line must have its contacts held closed before the counter will be enabled. If

the reset line is opened at any time then the counter resets. This operation of

the reset line is the opposite of that explained earlier.

Hopefully, by now, you will appreciate the advice directing users to consult the

manufacturer's manual before launching into the use of counters.

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________________________________________________________________________________________

TIMER/COUNTER CASCADES ________________________________________________________________________________________

PLC manufacturers are faced with the problem of deciding values for both the

timing increments and the maximum value which a timer (or counter) will

accept.

The maximum value may well be dictated by the word size of the processor

used. For example, a value of between 0 and 255 (00 to FF Hex) is

understandably common for eight bit processors. Another common range is 0

to 99 where binary coded decimal (BCD) codes are employed. These two

number ranges fit quite readily into the number handling techniques used by

software engineers.

The problem of the value of the timing increments may seem a little more

arbitrary. When previously mentioning timers the incremental values of 0.1

second and 1 second were stated as common values but when these time values

are used with the maximum timer range the total time period seems very

limited. For example, a timer value of, say, 255 with a time increment of

0.1 second only spans a total time of 25.5 seconds. Even if a 1 second time

interval used the time span only increases to 255 seconds (4 minutes

15 seconds).

For many applications this fine control of small timed periods will be

excellent. A flexible manufacturing system, for example, where a conveyor

carries items from operation to operation may require an operation to be

carried out within, say, 5.5 seconds if the expected production rate is to be

maintained.

For other applications, however, such small values of time could be totally

inadequate. For example, a furnace may be allowed 45 minutes to heat soak an

ingot of metal before a rolling operation commences. Clearly 255 seconds

would not suffice.

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PLC manufacturers want to be able to claim that their PLC will be suitable for

any control application and need, therefore, to ensure that as many different

possibilities in programming exist without additional accessories being

required.

For this reason the timers are made with small time increments to be able to

handle the fine timing problems and yet they can be expanded by the use of a

timer/counter combination to obtain much larger timed periods.

Consider what most people would do if they were given a stop watch which

only timed up to one minute and were asked to time a five minute period. The

majority would break up this time period into five one minute intervals. After

each one minute they would add one to a count until their count value was five

and the time was up. The very same idea can be applied to timing operations

controlled by the PLC. For example, a timer set for 60 seconds could have its

output recognised and counted before being reset to time again. When the

counter reaches five then the counter output, and not the timer output, would

signal the end of the five minute period. The circuit of FIGURE 6 illustrates

this arrangement.

FIG. 6

X002

TIM02

X004 TIM02

CNT1

5 counts

Y002

TIM02

60 seconds

Count

Reset

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CIRCUIT OPERATION

In the diagram of FIGURE 6 all rungs are in the open circuit condition if the

"normal" state of each element is assumed. To reset the counter the X002

contacts will need to be closed and then opened to enable the count inputs to

be recognised. If we assume that this is the case then a starting point for

analysis purposes is established.

The only other set of input contacts is X004. If these contacts are closed and

are left closed then the TIM02 timer will be enabled to begin timing out. After

sixty seconds the timer output will switch on causing all TIM02 contacts to

change state. In the second rung the N/O TIM02 contacts will close to apply a

signal to the count input. The counter will, therefore, recognise the first 60

second time interval. In the first rung the N/C TIM02 contacts will open to

disable the timer and cause its output to switch off. Once off, the timer

contacts will revert to the normal state. The timer will be enabled to begin the

next 60 second time interval and the counter input will be open circuit once

again.

The operation of the timer is therefore as follows. Timing for sixty seconds,

switching its output on, then resetting itself after a very short time duration.

The actual on-time is dependent upon the scan time of the program pass. Each

occasion of the timer output switching on is recognised by the counter as a

count event. After five occurrences the counter output will switch on.

In the last rung the counter output is being used to drive physical output Y002.

The total arrangement, therefore, produces a delay in Y002 switching on of

five minutes after the closing of input X004 (provided that this input remains

closed for the whole period).

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________________________________________________________________________________________

COUNTER/COUNTER CASCADES ________________________________________________________________________________________

Having appreciated that some maximum value is stated for the operation of

timers then it should be apparent that the same reasoning applies to counters.

Maximum counts of 99 or 255 are common. Inevitably the problem must arise

which requires a count in excess of the maximum value of the counter.

Consider as an example a programmable controller used to control a coil

winding machine which is required to count 1920 turns of enamelled copper

wire onto a winding former. In such cases the total number of counts would

need to be examined to ascertain whether it is a number which is the product of

two other values. If it is, then the total count may be obtained by simply

cascading two counters i.e. having one counter counting the events as they are

happening and having a second counter counting the number of times the first

counter produces an output. The output from the first counter must be reset

before the next input event arrives so that the correct total number of events are

recorded. The total count will be deemed to have occurred when the second

counter produces an output.

In the example above the number specified (1920) fits very nicely into this

category. The two counter values may be chosen such that the product is 1920.

In this application the first counter value may also be required for the control

of an intermediate operation within the coil winding procedure (the application

of inter-layer insulation for example). The counter values may not, therefore,

be arbitrarily taken from a product table if such conditions apply. The table

below indicates some of the possible values.

Teesside University Open Learning (Engineering)

COUNTER 1 COUNTER 2 PRODUCT

VALUE VALUE

240 8 1920

192 10 1920

160 12 1920

120 16 1920

80 24 1920

Clearly not all numbers are the product of two others which means that a

slightly different solution sometimes needs to be found.

FIGURE 7 shows a ladder diagram having two counters cascaded to produce

the 1920 count required for the previous example. The chosen values are 120

for the first counter and 16 for the second.

Teesside University Open Learning (Engineering)

FIG. 7

Check through the ladder diagram to determine the operational sequence. The

Self-Assessment Questions on page 28 will examine your understanding.

001

MR000

000

CNT0

MR001

MR000

002MR000 CNT0

001 is push switch to start motor

000 is stop motor input

002 is the revolution sensor pick-up input

120 counts

MR000

OUT 020

CNT0

MR001

CNT1

MR000

OUT 021

CNT1

MR001

16 counts

Output to coil winder motor

Output to release the coil winder motor brake

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________________________________________________________________________________________

NOTES ________________________________________________________________________________________

....................................................................................................................................................

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1. With reference to the diagram of FIGURE 7 answer the following

questions.

(a) Which rung of the diagram is used to 'remember' the operation of the

start motor push switch?

(b) What causes counter CNT1 to recognise a count input and hence

decrement its counter value?

(d) When should the coil winder brake be on?

(e) Explain what you think happens when 1920 revolutions have been

detected.

2. Design a ladder diagram circuit which will fulfil the following

specification.

With the application of a momentary switch closure an output from the

PLC is to come on. After 3 seconds this output is to switch off and a

second output switch on for a further period of 6 seconds before it

switches off.

When the second output switches off the first output must switch back on

to begin the repeat of the 3 second on/6 second off operation.

When the second output switches off for the fourth time the sequence is

considered complete and both outputs remain switched off until the next

Teesside University Open Learning (Engineering)

momentary switch closure. FIGURE 8 shows the sequence in the form of

a timing diagram.

FIG. 8

Momentary input

First output

Second output

3 seconds

6 seconds

Off

Time axis

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS ________________________________________________________________________________________

1 (a) The pressing of the start switch is remembered by rung 1.

(b) A suitable logic state transition on the count input to CNT1 will

cause it to recognise a "count pulse". In the circuit of FIGURE 7 this

pulse is produced when counter CNT0 has an output.

switched on. This would normally happen as a consequence of input

contacts 001 being closed if input 000 and memory relay MR001 are

not on.

(d) The brake should be on whenever the motor is off. The brake should

be a spring applied/electrically released type to ensure that it is still

applied when the electrical supply is removed.

(e) After the second counter CNT1 has counted 16 inputs from the

output of CNT0, its output will switch on, bringing on memory relay

MR001.

MR001 resets memory relay MR000 which disables any further

counts, switches off the coil winder motor and applies the brake.

MR001 also resets counter CNT1.

Therefore, after 1920 turns the motor switches off, the brake is

applied and all counters are reset ready for the next coil winding

operation.

Teesside University Open Learning (Engineering)

2. There are many possible solutions to this sequence problem. The ladder

diagram of FIGURE 9 shows one version. Compare your solution with

FIGURE 9. If it is very different read FIGURE 9 to check that you can

follow it.

Points which you might note are:

(i) The momentary switch press is remembered by memory relay

MR030.

(ii) A normally closed stop switch has been included as input 000 in rung

(iii) The start input, 001, is also being used to reset the counter at the

beginning of the sequence. This ensures that if the sequence is

stopped part way through, it will complete a full sequence the next

time it is run.

Teesside University Open Learning (Engineering)

FIG. 9

001

MR030

000

MR030

CNT0

MR030

TIM00

001 is momentary switch input

000 is a stop input

MR030 TIM00

TIM01

OUT 020

CNT0

TIM01

001

4 counts

CNT0

MR030 OUT 020

OUT 021

TIM01 6 secs

TIM00 3 secs

Teesside University Open Learning (Engineering)

________________________________________________________________________________________

SUMMARY ________________________________________________________________________________________

This lesson started as a continuation of Lesson 1 by introducing applications of

timers. The main example used was the control requirements of a partially

completed pelican crossing (more on this later).

The explanation of the ladder diagram operation was a little longwinded but if

you persevered the understanding of the operation will prove beneficial.

Generalisations were given which can be usefully incorporated into later

exercises.

The theory progressed to using two timers working together to produce an

on/off freerunning output. This arrangement is itself quite useful and can be

found in many applications.

The theme of the lesson changed to the discussion of counters and counter

variations before finishing off with timer/counter and counter/counter

cascades. The majority of the topics covered in this lesson are valid for PLCs

in general and form the basis of many modern control system requirements.

Teesside University Open Learning (Engineering)

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