PLC: Controladores Lógicos Programables
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This document discusses the use of timers in industrial control systems. It describes how timers are used to activate or deactivate devices after a preset time interval. Timers can be implemented using electromechanical relays or solid-state timers. The document discusses different types of timers including on-delay and off-delay timers. It provides examples of ladder logic programs that use timer instructions to implement on-delay and off-delay timing functions. Allen-Bradley programming is discussed as an example. Circuits are presented to illustrate applications of timers such as start-up warning signals and sequential control systems.
PLC: Controladores Lógicos Programables
- 1. Programmable Logic Controllers Third Edition Frank D. Petruzella McGraw-Hill
- 3. 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. Timers 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
- 4. On-Delay Timing Relay Operating coil Instantaneous contacts NO NC Nontimed contacts are controlled directly by the timer coil, as in a general-purpose control relay. Time control contacts NO NC Time adjustment When the coil is energized, the timed contacts are are prevented from opening or closing until the time delay period has elapsed. However, when the coil is de-energized, the timed contacts return instantaneously to their normal state.
- 5. Normally open, timed closed contact (NOTC) Contact is open when relay coil is de-energized When relay is energized, there is a time delay in closing Timed Contact Symbols On-Delay Symbols Normally closed, timed open contact (NCTO) Contact is closed when relay coil is de-energized When relay is energized, there is a time delay in opening
- 6. 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 de- energized, there is a time delay before the contact opens. Timed Contact Symbols Off Delay Symbols 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 de- energized, there is a time delay before the contact closes.
- 7. On-Delay Relay Timer Circuit (NOTC Contact) L1 L2 S1 L1 Sequence of operation 10 s S1 open, TD de-energized, TD1 open, L1 is off. After 10 s, TD1 closes, L1 is switched on. S1 closes, TD energizes, timing period starts, TD1 still open, L1 is still off. S1 is opened, TD de- energizes, TD1 opens instantly, L1 is switched off. 10 s OFF ON Input Output Timing Diagram
- 8. On-Delay Relay Timer Circuit (NCTO Contact) S1 open, TD de-energized, TD1 closed, L1 is on. Sequence of operation L1 L2 L1 S1 10 s After 10 s, TD1 opens, L1 is switched off. S1 closes, TD energizes, timing period starts, TD1 is still closed, L1 is still on. L1 L2 S1 is opened, TD de-energizes, TD1 closes instantly, L1 is switched on. 10 s On Off Input Output Timing Diagram
- 9. Off-Delay Relay Timer Circuit (NOTO Contact) L1 L2 S1 L1 Sequence of operation S1 open, TD de-energized, TD1 open, L1 is off. S1 closes, TD energizes, TD1 closes instantly, L1 is switched on. S1 is opened, TD de-energizes, timing period starts, TD1 is still closed, L1 is still on. 10 s After 10 s, TD1 opens, L1 is switched off. 10 s Input Output Off On Timing Diagram
- 10. Off-Delay Relay Timer Circuit (NCTC Contact) L1 L2 S1 L1 Sequence of operation S1 open, TD de-energized, TD1 closed, L1 is on. S1 closes, TD energizes, TD1 opens instantly, L1 is switched off. S1 is opened, TD de-energizes, timing period starts, TD1 is still open, L1 is still off. 10 s After 10 s, TD1 closes, L1 is switched on. 10 s Input Output On Off Timing Diagram
- 11. 1. Timers are used to activate or de-activate a device after a preset interval of time. (True/False) 2. With most timers the time delay period is fixed and can not be varied. (True/False) 3. ________ contacts are controlled directly by the timer coil, as in a general-purpose control relay. (a) timed (c) instantaneous (b) NO (d) NC
- 12. 4. When a relay timer coil is de-energized, the timed contacts return instantaneously to their normal state. (True/False) 5. Which of the following symbols represents a normally open timed closed contact?
- 13. 6. Which of the following symbols represents a normally closed timed open contact?
- 14. 7. The timed relay contact shown is designed to operate so that: a. when the relay coil is energized, there is a time delay in closing b. when the relay coil is energized, there is a time delay in opening c. when the relay coil is de-energized, there is a time delay in closing d. when the relay coil is de-energized, there is a time delay in opening
- 15. 8. The timing diagram shown is that of an a. on-delay timer circuit (NOTC contact) b. on-delay timer circuit (NCTO contact) c. off-delay timer circuit (NCTC contact) d. off-delay timer circuit (NOTO contact) 10 s Input Output Off On Timing Diagram
- 16. 9. In the circuit shown, the light will stay off a. as long as S1 is closed b. for 10 seconds after coil TD is energized c. for 10 seconds after coil TD is de-energized d. both a and c L1 L2 S1 L1 10 s
- 17. 10. In the circuit shown, the light will stay on a. as long as S1 is closed b. as long as S1 is open c. for 10 seconds after S1 is closed d. both b and c
- 18. 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: their settings can be altered easily the number of PLC timers used can be increased or decreased by programming changes without wiring changes timer accuracy and repeatability are extremely high
- 19. RSLogic Timer Commands Timer/Counter TON Command Name Description TON Timer On-Delay Counts time base intervals when the instruction is “true”
- 20. RSLogic Timer Commands Timer/Counter Command Name Description TOF TOF Timer Off-Delay Counts time base intervals when the instruction is “false”
- 21. RSLogic Timer Commands Command Name Description RTO Retentive Timer ON Counts time base intervals when the instruction is “true” and retains the accumulated value when the instruction goes "false" or when power cycle occurs Timer/Counter RTO RES RES Reset When this instruction is "true" it resets the count of the RTO counter
- 22. 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).
- 23. Coil-Formatted Timer Instruction The timer assigned an address XXX TON The type of timer is specified Preset value PR:YYY Time base 0.1 s Accumulated value AC:000 Contact determines rung continuity When the timer rung has logic continuity, the timer's accumulated value increases. When accumulated value equals the preset value, the output is energized and and the timed output contact associated with the output is closed. The timed contact can be used as many times as you wish throughout the program as a NO or NC contact.
- 24. Generic Block-Formatted Timer Instruction Timers are most often represented by boxes in a ladder logic. Preset time Time base Accumulated time Retentive timer block Control line controls the actual timing operation of the timer. Whenever this line is true the timer will time. Reset line resets the the timer's accumulated value to zero. Output line The timer continuously compares its accumulated time with its preset time. Its output is logic 0 as long as the accumulated time is less than the preset time. When the two become equal the output changes to logic 1.
- 25. On-Delay Timer Instruction The on-delay timer operates so that, when the rung containing the timer is true, the timer time-out period commences. Timer Input Rung condition Timed period False True On delay time duration Timed output bit False OFF True ON Timer Sequence The timed output becomes true sometimes after the timer rung becomes true; hence the timer is said to have an on delay.
- 26. Allen-Bradley On-Delay Timer Instruction Allen-Bradley PLC-5 and SLC-500 controller timer elements each take three data table words: the control word, preset word, and accumulated word. The control word uses three control bits: Enable (EN) bit, Timer-Timing (TT) bit, and Done-Bit (DN). TIMER TABLE T4:0 /EN /TT /DN 0 0 0
- 27. Allen-Bradley On-Delay Timer Instruction The Enable (EN) bit is true (has a status of 1) whenever the timer instruction is true. When the timer instruction is false, the enable bit is false (has a status of 0) Enable bit false TON TIMER ON DELAY Timer T4:0 EN EN T4:0 Enable bit true
- 28. Allen-Bradley On-Delay Timer Instruction The Timer-Timing (TT) bit is true whenever the accumulated value of the timer is changing, which means the timer is timing. TIMER ON DELAY TON Timer T4:0 Preset 50 Accumulated 10 EN TT T4:0 Timer-Timing bit true
- 29. Allen-Bradley On-Delay Timer Instruction The Done-Bit (DN) changes state whenever the accumulated value reaches the preset value. Its state depends on the type of timer being used. TIMER ON DELAY TON Timer T4:0 Preset 50 Accumulated 10 EN DN T4:0 DN Done-bit changes state 50
- 30. Allen-Bradley On-Delay Timer Instruction The preset value (PRE) word is the set point of the timer, that is, the value up to which the timer will time. The accumulated value (ACC) word is the value that increments as the timer is timing. The accumulated value will stop incrementing when its value reaches the preset value. TIMER TABLE T4:0 /EN /TT /DN .PRE .ACC 0 0 0 0 0
- 31. Allen-Bradley On-Delay Timer Instruction TON TIMER ON DELAY EN DN The information to be entered includes: Timer T4:0 Timer number which must come from the timer file. Time base 1.0 Time base which is expressed in seconds. Preset 15 Preset value which is the length of the time delay. Accumulated 0 Accumulated value which is normally entered as 0.
- 32. On-Delay Timer Program Ladder Logic Program L1 L2 Input A Input A T4:0 T4:0 T4:0 EN TT DN TON TIMER ON DELAY Timer T4:0 Time base 1.0 Preset 10 Accumulated 0 EN DN Output B Output B Output C Output C Output D Output D G R Y 10
- 33. On-Delay Timer Program Timing Diagram Input condition A On Off Timer-enable bit On Off Timer-timing bit On Off Timer-done bit On Off Timer accumulated value 0 4 s 4 s
- 34. On-Delay Timer Program Timers are 3-word elements Word 0 1 2 Word 0 is the control word EN TT DN Internal use Word 1 stores the preset value Preset value PRE Word 2 stores the accumulated value Accumulated value ACC
- 35. On-Delay Timer With Instantaneous Output Relay Ladder Schematic Diagram L1 L2 Stop Start 1TD M 1TD-1 (instantaneous contact) 1TD-2 (5 s) (timed contact) 1TD
- 36. On-Delay Timer With Instantaneous Output Programmed Circuit Motor Motor M Internal relay Internal relay Stop Stop Start Start Timer PR: 5 TB: 1 s Output line M L1 L2 Inputs Output Ladder logic program
- 37. Start-Up Warning Signal Circuit Relay Ladder Schematic Diagram L1 L2 Start-up PB1 Reset PB2 CR1 CR1-1 CR1-2 1TD-1 (10 s) CR1-3 1TD Horn
- 38. TON TIMER ON DELAY Timer T4:0 Time base 1.0 Preset 10 Accumulated 0 Inputs Output Ladder logic program Programmed Circuit Horn Horn Start-up Reset PB1 PB1 PB2 PB2 EN DN T4:0 EN DN T4:0 EN T4:0 Start-Up Warning Signal Circuit 10
- 39. Timed Closed Solenoid Value Program Input L1 Switch SW_1 SW_1 Ladder logic program Output L2 Valve Valve EN DN timer_1.dn TON Timer On Delay Timer timer_1 Preset 12000 Accumulated 0 12000
- 40. Automatic Sequential Control System Relay Ladder Schematic Diagram L1 L2 Stop PB1 Start PB2 Lube oil pump motor OL M1 M1-1 PS1 (lube oil pressure switch) Main drive motor M2 OL 1TD 1TD-1 (15 s) Feed motor OL M3
- 41. Automatic Sequential Control System Programmed Circuit Inputs Outputs Ladder logic program OL OL OL M1 M1 M2 M2 M3 M3 TON TIMER ON DELAY Timer T4:0 Time base 1.0 Preset 15 Accumulated 0 PB1 PB1 PB2 PB2 PS1 PS1 EN DN M1 T4:0 DN 15
- 42. Off-Delay Programmed Timer The off-delay timer (TOF) operation will keep the output energized for a period after the rung containing the timer has gone false. EN DN TOF TIMER OFF DELAY TIMER T4:3 Time base 1.0 Preset 15 Accumulated 0 I:1.0/0 I:1.0/0 O:2.0/1 O:2.0/1 T4:3/DN PL L1 L2 Input Output Ladder logic program S1 15
- 43. Off-Delay Programmed Timer Timing Diagram Input condition S1 True False Timed period Off delay timed duration Timed output O:2.0/1 True (logic 1) False (logic 0) Preset value = accumulated value
- 44. L1 Off-Delay Timer Used To Switch Motors Off Input L2 Output Ladder logic program 5000 10000 15000
- 45. Pneumatic Off-Delay Timer Relay Ladder Schematic Diagram L1 L2
- 46. Programmed Pneumatic Off-Delay Timer Equivalent Programmed Circuit L1 L2 Input Outputs Ladder logic program 5
- 47. Fluid Pumping Process When the pump start button is pressed, the pump starts. The button can then be released and the pump continues to operate. When the stop button is pushed, the pump stops. Operation Before starting, PS1 must be closed. PS2 and PS3 must be closed for 5 s after the pump starts. If either PS2 or PS3 opens, the pump will shut off and will not not be able to start again for another 14 s.
- 48. Fluid Pumping Process Program L1 L2 Inputs Output Ladder logic program 5 0
- 49. Retentive Timer A retentive timer accumulates time whenever the device receives power, and maintains the current time should power be removed from the device. Once the device accumulates time equal to its preset value, the contacts of the device change state. The retentive timer must be intentionally reset with a separate signal for the accumulated time to be reset. Electromechnical Retentive Timer Cam operated contact Motor-driven cam Once power is applied, the motor starts turning the cam. The positioning of the lobes determines the time it takes to activate the contacts. If power is removed from the motor, the shaft stops but does not reset.
- 50. Retentive On-Delay Timer Program The PLC-programmed RETENTIVE ON-DELAY timer (RTO) operates in the same way as the nonretentive on- delay timer (TON), with one major exception. This is a retentive timer reset (RTR) instruction. Unlike the TON, the RTO will hold its accumulated value when the timer rung goes false and will continue timing where it left off when the timer rung goes true again. This timer must be accompanied by a timer reset (RES) instruction to reset the accumulated value of the timer to zero. Same address
- 51. Retentive On-Delay Timer Program 3 9 0
- 52. Retentive On-Delay Timing Chart Enable bit is reset when input pushbutton PB1 is opened Accum value retained when rung goes false Accum = Preset When reset PB2 is closed, the T4:2/DN bit is reset to 0. Accumulated value is reset and held at zero until the reset pushbutton is opened.
- 53. Retentive On-delay Alarm Program The purpose of the RTO timer is to detect whenever a piping system has sustained a cumulative overpressure condition of 60 s. At that point, a horn is sounded automatically. You can silence the alarm by switching the key switch to the rest position. 60000 0 L1 L2 Ladder logic program
- 54. Bearing Lubrication Program Sequence Of Operation To start the machine, the operator turns SW on. Before the motor shaft starts to turn, the bearings are supplied with oil by the pump for 10 s. The bearings also receive oil when the machine is running. When the operator turns SW off to stop the machine, the oil pump continues to supply oil for 15 s. A retentive timer is used to track the total running time of the pump. When the total running time is 3 h, the motor is shut down and a pilot light is turned on to indicate that the filter and oil need to be changed. A reset button is provided to reset the process after the filter and oil have been changed.
- 56. 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. Three motors started automatically in sequence with a 20-s time delay between each motor startup. Relay Schematic Diagram
- 57. Equivalent Time-Delayed Motor-Starting Program 20000 20000
- 58. Annunciator Flasher Circuit Two timers can be interconnected to form an oscillator circuit. The oscillator logic is basically a timing circuit programmed to generate periodic output pulses of any duration. They can be used as part of an annunciator system to indicate an alarm condition. The oscillator circuit output is programmed in series with the alarm condition. If the alarm condition is true, the appropriate output indicating light will flash.
- 60. Cascading of Timers for Longer Time Delays 30000 12000
- 61. Control of Traffic Lights in One Direction A typical application for PLC timers is the control of traffic lights. Control of Traffic in One Direction Sequence of Operation Red 30 s Amber 5 s Green 25 s
- 62. Control of Traffic Lights in One Direction 30 25 5
- 63. 11. The timed contact of a PLC timer can only be used as a normally-open contact. (True/False) 12. The ______ bit operates the same as an instantaneous contact on a timer relay. a. enable b. done c. timer-timing d. timer number
- 64. 13. The preset time of a PLC timer represents the amount of time that has elapsed from the moment the timing coil became energized. (True/False) 14. If the preset time of a timer is 150 and the time base is 0.1 seconds, the time-delay period would be 1500 seconds. (True/False)
- 65. 15. In general, the three different types of PLC timers are: a. TON, TOF, and PRE b. TON, TOF, and RTO c. TON, ACC, and RTO d. TT, EN, and DN
- 66. 16. The amount of time for which a timer is programmed is called the a. preset b. set point c. Done Time (DN) d. accumulated time
- 67. 17. The timer reset instruction must be addressed to the same address as the ______ instruction. a. TON b. TOF c. RTO d. EN
- 68. 18. Which of the following statements is not true for a retentive on-delay timer? a. The timer accumulates time when it is energized. b.The timer requires a reset instruction to reset the accumulated value of the timer to zero. c. The timer does not reset to zero when it is de-energized. d. The reset input to the timer will not override the control input.
- 69. 19. The timer instruction is: a. an input instruction c. either a or b b. an output instruction d. both a or b 20. The interconnection of timers is commonly called : a. grouping b. programming c. sequencing d. cascading
- 70. 21. For the program shown, the pilot light will be: a. on at all times b. off at all times c. switched on 15 s after the switch has been actuated from the open to the closed position d. switched off 15 s after the switch has been actuated from the on to off position