- Slides: 18
Timers Introduction to PLC DMT 353/3
Introduction • Timers are devices that count increments of time. • It is an instruction that waits a set amount of time before doing something.
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. 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. Time Delay Relay Solid-State Timer
On-Delay Timing Relay Operating coil NO Instantaneous contacts NC Nontimed contacts are controlled directly by the timer coil, as in a general-purpose control relay. When the coil is energized, the timed contacts are prevented from opening NO or closing until the time Time control delay period has elapsed. However, when the coil is contacts de-energized, the timed NC contacts return Time adjustment instantaneously to their normal state.
Timed Contact Symbols On-Delay Symbols Normally open, timed closed contact (NOTC) Normally closed, timed open contact (NCTO) 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 closing When relay is energized, there is a time delay in opening
Timed Contact Symbols Off Delay Symbols Normally open, timed open contacts (NOTO). Contact is normally open when relay coil is de-energized. When relay coil is energized, contact closes instantly. When relay coil is deenergized, there is a time delay before the contact opens. Normally closed, timed closed contacts (NCTC). Contact is normally closed when relay coil is de-energized. When relay coil is energized, contact opens instantly. When relay coil is deenergized, there is a time delay before the contact closes.
On-Delay Relay Timer Circuit (NOTC Contact) L 1 L 2 S 1 L 1 After 10 s, TD 1 closes, L 1 is switched on. 10 s Output S 1 open, TD de-energized, TD 1 open, L 1 is off. S 1 closes, TD energizes, timing period starts, TD 1 still open, L 1 is still off. 10 s Input Sequence of operation ON OFF Timing Diagram S 1 is opened, TD deenergizes, TD 1 opens instantly, L 1 is switched off.
On-Delay Relay Timer Circuit (NCTO Contact) L 1 L 2 S 1 Sequence of operation L 1 S 1 open, TD de-energized, TD 1 closed, L 1 is on. S 1 closes, TD energizes, timing period starts, TD 1 is still closed, L 1 is still on. 10 s After 10 s, TD 1 opens, L 1 is switched off. Input Output On Off Timing Diagram S 1 is opened, TD de-energizes, TD 1 closes instantly, L 1 is switched on.
Off-Delay Relay Timer Circuit (NOTO Contact) L 1 L 2 S 1 Sequence of operation S 1 open, TD de-energized, TD 1 open, L 1 is off. L 1 10 s Input S 1 closes, TD energizes, TD 1 closes instantly, L 1 is switched on. S 1 is opened, TD de-energizes, timing period starts, TD 1 is still closed, L 1 is still on. On Output Off Timing Diagram After 10 s, TD 1 opens, L 1 is switched off.
Off-Delay Relay Timer Circuit (NCTC Contact) L 1 L 2 Sequence of operation S 1 open, TD de-energized, TD 1 closed, L 1 is on. L 1 10 s Input Output On Off Timing Diagram S 1 closes, TD energizes, TD 1 opens instantly, L 1 is switched off. S 1 is opened, TD de-energizes, timing period starts, TD 1 is still open, L 1 is still off. After 10 s, TD 1 closes, L 1 is switched on.
On-Delay Timer • Timers used with PLCs can be compared to timing circuits used in hard-wired control line diagrams. • In the following example, a normally open (NO) switch (S 1) is used with a timer (TR 1). For this example the timer has been set for 5 seconds. When S 1 is closed, TR 1 begins timing. When 5 seconds have elapsed, TR 1 will close its associated normally open TR 1 contacts, illuminating pilot light PL 1. • When S 1 is open, deenergizing TR 1, the TR 1 contacts open, immediately extinguishing PL 1. • This type of timer is referred to as ON delay indicates that once a timer receives an enable signal, a predetermined amount of time (set by the timer) must pass before the timer’s contacts change state
Off-Delay Timer • This type of timer is the opposite of the ondelay timer. This timer simply “delays turning off”.
Operation of Timers • Timers are represented by boxes in ladder logic. • When a timer receives an enable, the timer starts to time. The timer compares its current time with the preset time. • The output of the timer is a logic 0 as long as the current time is less than the preset time. • When the current time is greater than the preset time the timer output is a logic 1. TIM XXXX # XXXX
Operation of Timers • When the instruction before the timer symbol are true the timer starts “ticking”. When the time elapses the timer will automatically close its contact. • Typically each manufacturer offers several different ticks. Most manufacturers offer 10 and 100 ms increments (ticks of the clock) * “ms” is a mili-second or 1/1000 th of a second
Example • In this ladder diagram, we wait for the input 0001 to turn on. When it does, timer TIM 000 starts ticking. It will ticks 100 times. • Each tick (increment) is 100 ms, so the timer will be 10 seconds (100 ticks x 100 ms = 10 s) • When 10 seconds have elapsed, the T 000 contact close and output 1000 turns on. • When input 0001 turns off, the timer TIM 000 will reset back to 0 causing its contact to turn off (open), thereby making output 1000 turn back off. 0001 TIM 000 #0100 TIM 000 1000
Timers Types • • • ON delay OFF delay ON-OFF cycle timer One-short timer
• When the Start Button is pressed at input 0001, the program starts pump 1 controlled by output 1001. • Pump 1 runs for 5 seconds, filling the tank with the first chemical, then shuts off. • The program then starts pump 2, controlled by output 1002. Pump 2 runs for 3 seconds filling the tank with the second chemical. After 3 seconds pump 2 shuts off. • The program starts the mixer motor, connected to output 1005 and mixes the two chemicals for 30 minutes. • The program then opens the drain valve controlled by output 1004, and starts pump 3 controlled by output 1003. • Pump 3 shuts off after 8 seconds and the process stops. • A manual Stop switch is also provided at input 0004. Designing a Mixer using PLC Pump 1 (1001) Pump 2 (1002) 0001 0004 Mixer Motor (1005) Pump 3 (1003) Drain valve (1004)