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CHAPTER 5 FUNCTION OF OUTPUT CONTROLLER AND APPLICATION
Introduction • Timers are devices that count increments of time. • It is an instruction that waits a set amount of time before doing something.
TIMER 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
Programmed Timer Instructions PLC timers are output instructions that provide the same functions as timing relays and solid state timers. Some advantages of PLC timers: Ø their settings can be altered easily Ø the number of PLC timers used can be increased or decreased by programming changes without wiring changes Ø timer accuracy and repeatability are extremely high
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).
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
Timers Types • • • ON delay OFF delay ON-OFF cycle timer One-short timer
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
On-Delay Timer Example 1 The output will turn-ON 2. 5 seconds after the switch has been activated.
Off-Delay Timer • This type of timer is the opposite of the ondelay timer. This timer simply “delays turning off”.
Off-Delay Timer Address Instruction Data 00000 LD 000001 AND NOT 00002 OUT 01000 00003 LD 01000 00004 TIM 000 #0025 00005 END The output will turn-OFF 2. 5 seconds after the switch has been activated.
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.
Cascading Timers Address Instruction 00000 LD 00001 TIM Data 00001 000 #0200 00002 LD 00004 TIM 000 001 #0700 00005 LD 00005 OUT TIM 001 01000
Designing a Mixer using PLC Pump 1 (1001) Pump 2 (1002) 0001 0004 Mixer Motor (1005) Pump 3 (1003) Drain valve (1004)
• • Designing a Mixer using PLC When the Start Button is pressed at input 0000, 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.