Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

Introduction to PLC Timers and Timing Functions

10,069 views

Published on

Introduction to PLC Timers and Timing Functions for industrial control applications using The Learning Pit's LogixPro. From http://www.thelearningpit.com/

Published in: Education

Introduction to PLC Timers and Timing Functions

  1. 1. IEC2102 By Brian Khairullah
  2. 2. • Prior to the introduction of Programmable controllers in the late 1960s timing functions were implemented by the use of a variety of timers that included: • dashpot (pneumatic, hydraulic) • synchronous clock • motor driven • capacitor time limit relays • electronic (solid-state). • (Herman, 2014, p. 85 -88; Rockis & Mazur, 2014, p. 347-349
  3. 3. • ON Delay – When the timer coil is energized there is a delay before the contacts change position. The contacts remain in the changed position until the timer coil is de-energized at which time the contacts return to the normal position immediately.
  4. 4. • OFF Delay – When the timer coil is energized the contacts change position immediately and remain in the changed position. When the timer coil is de- energized there is a delay before the contacts return to the normal position.
  5. 5. • Interval (One-shot) – When the timer coil is energized the contacts change immediately and remain in the activated position for the duration of the preset time period after which they return to their normal position. • Retentive Timer: If power is removed from the timer coil during its timing phase it will retain the accumulated time and when power is restored to the coil it will begin timing at the point that it stopped. • Recycle – When the timer is energized the contacts change position at a rate determined by the preset time. • Symmetrical Recycle: The ON time and OFF time are equal. • Asymmetrical Recycle: The ON time and OFF time are not equal.
  6. 6. • An advantage of a PLC is that timing functions can be programmed eliminating the need for external timers. • All timing functions can be duplicated in a PLC.
  7. 7. • Timer: Identifies the timer and the location of the timer information. File type T4 followed by the timer number which can be a value from 0 to 99. e.g. T4:6 • Time Base: The duration of timebase interval that time is incremented in, in seconds. Can be 0.1 or 0.01 so a value of 100 would equal 10 seconds with a time base of 0.1 or 1 second with time base of 0.01. • Preset: Determines the amount of the time that must pass to activate the done (DN) bit. Expressed in units of the time base.
  8. 8. • Accumulator – Accum: While the timer is in timing mode (timing function activated) the accumulator increments in units of the time base. • Enable Bit - EN: True when the timer input is true. • Done Bit - DN: True when the accumulated time equals the preset time. • Timer Timing Bit - TT: True while the accumulator is incrementing.
  9. 9. • Counts timebase intervals when the instruction is true (Allen-Bradley, 2008, p. 2-9) • When the timer input is activated there is a delay, equal to the preset time, before the done bit (DN) changes from zero to one. i.e. DN is true when the accumulated time equals the preset time. • When the timer input is deactivated the done bit changes back to zero immediately.
  10. 10. • Counts timebase intervals when the instruction is false (Allen-Bradley, 208, p. 2-10). • When the timer input is activated the done bit (DN) changes from zero to one immediately. When the timer input is deactivated the accumulator begins incrementing and there is a delay, equal to the preset time, before the done bit changes back to zero. i.e. DN is true until the accumulated time equals the preset time.
  11. 11. • Counts timebase intervals when the instruction is true and retains the accumulated value when the instruction goes false or when power cycle occurs(Allen-Bradley, 208, p. 2-11). • RTO is an On Delay that retains its accumulated value if the input is deactivated while it is incrementing and then begins timing again at the same point once the input is reactivated. • The Reset instruction resets the accumulated value to zero.
  12. 12. • Other timing functions can be created using these timers in various combinations with each other and with other contacts: • Symmetrical Recycle: An XIO contact in series with the input of a timer controlled by the done bit of that timer will create a recycle timer by “self-resetting” the timer.
  13. 13. • Asymmetrical Recycle: A self-resetting recycle timer that controls an output with the addition of a comparative operator in series with the output. On time and Off time can be different values. • In this example for each 1 second cycle Lamp-3 will be off for 0.7 seconds and on for 0.3 seconds.
  14. 14. • Cascading Timers: The done bit of one timer activates the input of another timer. Multiple timers can be used in a “cascade.” • A self-resetting cascade can be created by the addition of an XIO controlled by the done bit of the last timer in the cascade placed in series before the first timer.
  15. 15. • Interval: Here is one way to create an interval timer.
  16. 16. • A variety of options • Reliable precision • Flexibility for any application
  17. 17. • Allen-Bradley. (2008). SLC 500 instruction set reference manual. Rockwell Automation. Retrieved from http://literature.rockwellautomation.com/idc/groups/literat ure/documents/rm/1747-rm001_-en-p.pdf • Herman, S. L. (2014). Industrial motor control 7th Ed. l. Clifton Park, NY: Cengage Learning. • Rockis, G. J. & Mazur, G. A. (2014). Electrical motor controls for integrated systems 5th Ed. Orland Park, IL: American Technical Publishers • Rockwell Automation. Retrieved from http://ab.rockwellautomation.com/
  18. 18. • Simpson, B. (2012). The learning pit dot com. Retrieved from http://www.thelearningpit.com/

×