14632_Ch9-PLC Timer Functions

8/19/2019 14632_Ch9-PLC Timer Functions

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Ch-9: PLC TIMER FUNCTION

ELE528 Programmable Logic

Controllers & Industrial Applications


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Fundamental types of Timers


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Examples of Timer Function Industrial Applications

• Non-Retentive Timers:

• On-Delay: Output B comes on at a specific set time after output A

is turned on. When A is turned off, B also goes off.

• Off-Delay: Both A and B have been turned on at the same time.

Both are in operation. When A is turned off, B remains on for a

specific set time period before going off.

• Retentive Timers: A retentive timer will sum all of the on or off

time for a timer, even if the timer never finished.

• A non-retentive timer will start timing the delay from zero each

time. Typical applications for retentive timers include tracking the

time before maintenance is needed.

• A non retentive timer can be used for a start button to give a

short delay before a conveyor begins moving.


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Examples of Timer Function Industrial Applications

• Limited On time: A and B go on at the same time. B goes off afterspecific set time period, but A remains on.

• Repeat cycling: An output pulses on and quickly off at a constant presettime interval.

• One-shot operation: Output B goes on for a specified time after outputA is turned on. Output B will run for its specified time interval even if A isturned off during the B timing interval.

• Alternate On and Off of two outputs: For example, two alternatelyflashing signal lights. Timings could be same or set to different intervals.

• Multiple On Delay: Two different events start at different time intervalsafter an initial starting time reference point.

• Multiple Off-Delay: Two different functions remain on for two differenttime intervals after a process is turned off.

• Interval time within a cycle: We may require that an output come on 7.5seconds after system start up, remain on for 4.5 seconds, and then goesoff and stay off. The interval would then be repeated only after thesystem is shut off and ten turned back on.


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Timer On-Delay

• On-Delay: Output B comes on at a specificset time after output A is turned on. When A

is turned off, B also goes off.


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Timer Off-Delay

• Off-Delay: Both A and B have been turned on at the sametime. Both are in operation. When A is turned off, Bremains on for a specific set time period before going off.


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Retentive Timer

• A retentive timer will sum all of the on or off time for a timer,

even if the timer never finished.• Typical applications for retentive timers include tracking the time

before maintenance is needed


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Ex.1: Metal Grinding Process

• For a grinding operation on a metal part, the coolant

flow on the part must be on for an interval before

the grinding process starts.

• When the process circuit is turned on, the coolant

motor (CM) is turned ON. Eight seconds later thegrinding process (GM) starts.

• Both Relay logic and PLC programs for this process

are next.

• Modify it for grinding off after 15 seconds and

coolant continues


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Metal Grinder Process• The sequence for this example is as follows;

•

1. When switch 1 (I:1/0) is closed. Coolant (CM) goes ON.• 2. Eight seconds later Grinder Motor (GM) goes ON.

• 3. When the grinding is completed, opening switch 1 turns CM andGM off


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Ex: Coolant-Grinder off delay


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Ex2: Time Delay-Off

• A motor and its lubrication pump motor are

both running.

• Lubrication for main motor bearings is

required during motor coast-down.

• After the main motor is shut-off, the

lubricating pump remains on for a time

corresponding to coast-down time (e.g. 10sec)


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Ladder Diagram for Motor-Lubricant


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Ex.3 Wood Sawing Operation

• The sawing operation takes 4.6seconds. After

the sawing operation takes place, a blower is

used to blow away the sawdust to keep

running until the operation is turned off.


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Ex.4: Pulsed Timer

• In a paper mill, as finished paper comes out of apaper machine to the finished roll, a roll number is to

be printed on one edge at intervals.

• It is determined that the number should be printed

every 12 seconds.

• The PLC circuit will produce the required pulses to

actuate the high speed print head.

• The timer then after one scan (1Sec) immediatelyshuts itself off and gets reset

• Add an emergency switch to stop operations


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Ex.4: Pulsed Timer


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More Exercises

• Ex5: An alarm is to sound for 17 seconds after a

machine is started to alert personnel in the area that

the machine has been started. The alarm goes on

when the machine is started. The alarm stays on for17 seconds, even if the input is turned off during the

17 second timing period.

• Ex6: Design a warning light system on a machine to

indicate it is running. The warning light shouldconsist of two lights flashing alternately.


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Ex5: An alarm is to sound for 17 seconds after a machine

is started to alert personnel in the area that the machine

has been started. The alarm goes on when the machine

is started. The alarm stays on for 17 seconds, even if theinput is turned off during the 17 second timing period

W i Li ht E 6 D i i li ht t hi t i di t it i


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Warning Lights Ex6: Design a warning light system on a machine to indicate it is

running. The warning light should consist of two lights flashing alternately


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Industrial Process Timing Application

• Multiple timing programming as well as contact/coil logic

• It consists of a single, operational, heat-treating machine.• The station carries out a surface-hardening process on a steel

ring. Hardening is accomplished by heating the steel ring to ahigh temperature, then immediately quenching it (cooling itvery rapidly). The metallurgical result is relatively hard surfaceon the steel ring.

• The heating is done by a noncontact induction heatingprocess. A high current in the circular coil around the outsideof the part induces high circulating currents in the part. The

part therefore heats up very rapidly.• The coil has cooling water circulated through its outer half to

keep the heating ring from overheating or even melting. Thequench is then done by spraying cold water on the part.


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Heat/Quench Station Layout


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Processing Sequence• 1. The master pushbutton is depressed, turning the system

on• 2. The part is placed on the mandrel.

• 3. Both left and right start-up buttons are depressed

• 4. At this point, or at any other time, pushing any stop

button stops all processing action.

• 5. The part is raised from bottom to top by pneumatic air

action. A solenoid valve supplies this air to a pneumatic

elevating cylinder. The lower-limit switch must be actuated

before the part will rise. Lower- limit switch actuation

indicates that there is a part on the mandrel and that the

mandrel is down. Note that the limit switch opens as the

part leaves the bottom position.


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Processing Sequence • 6. The mandrel makes contact with a limit switch at the

top of the travel.• 7. Heat comes on for 10 seconds and goes off.

• 8. Quench comes on after 8 seconds and goes off.

• 9. The part returns down by gravity and spring action.

The upper-limit switch becomes inactivated when the

mandrel starts down.

• 10. The part and the mandrel reach the bottom. The

down-limit switch is again actuated.• 11. The system should reset.

• 12. The part is removed.


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Some optional features not included in this program are:

• If you assume the heat generator and both water pumps are

on, interlocks could be added to insure they are runningthroughout the process.

• The same ring part could be processed two or more times.We could require the ring to be removed after step 12 beforeresetting takes place.

• Is proper temperature reached? A thermocouple sensorcould be incorporated to monitor temperature.

• Manual controls for setup could be added. These are Up,Heat and Quench.

• Safety features could be added, such as a safety shield thatlowers during heat. Where does the process restart afterinterrupted power is restored?

• Other features as required


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PLC Register and Inputs/Outputs

INPUTS OUTPUTSI:1/1 Master Stop CR19-O:2/2 Solenoid Valve-Up

I:1/2 Master Start CR20-O:2/3 Heater On

I:1/3 Left Stop-Up CR21-O:2/3 Quench Spray Water

Solenoid

I:1/4 Left Start-Down Options

I:1/5 Right Stop-Up CR17-O:2/0 System On Pilot Light

I:1/6 Right Start-Down CR18-O:2/1 Machine On/Up Pilot

Light

I:1/7 Limit Switch Down

I: 1/8 Limit Switch Up


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Connection Diagram for PLC Module,

Inputs and Outputs

14632_Ch9-PLC Timer Functions

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Transcript of 14632_Ch9-PLC Timer Functions