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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