Programmable Logic Controllers. Second Edition

Transcription

1 Programmable Logic Controllers James A. Rehg Second Edition Glenn J. Sartori

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3 FIGURE 16: Off-delay timer diagrams. locks the machine doors at the start of the process and lights a doors locked indicator. The operator turns off the machine when the part is finished and the TOF timer keeps the doors locked for an additional 5 seconds for the motor to come to a stop. Figure 16(b) displays the timing diagram for this operation. When the NO selector switch is true, the machine start input instruction is true, the T4:0/EN and T4:0/DN bits are true, the accumulator value is reset to zero, and rung 0 becomes active. As a result, rung 1 is true because T4:0/DN is true and the machine door is locked and the process light is illuminated. When the machine is turned off, the switch contacts return to the NO state, rung 0 returns to the false state, and the timer accumulator begins incrementing toward the preset value while T4:0/DN remains true. When the preset value is reached ( seconds), the timer output (T4:0/DN) becomes false and the door is unlocked and the process light is extinguished. Note that this timer example is the standard configuration in Figure 13(b), but it has a maintain contact switch. Compare the ladder logic and the timing diagrams for the TOF timer in Figures 13(b) and 16 to see how the standard ladder logic was adapted to this application. 168

4 EXAMPLE 2 Draw a ladder diagram for a pumping system where the pump requires a 5-second delay before pumping; when the pump is shut off, it requires a 15-second delay before it can be restarted. Start and stop switches are NO momentary contact push buttons. Solution Refer to Figure 17(a), the ladder solution, and 17(b), the timing diagram, as you read the following description. The pump control is implemented with T4:0, an on-delay timer, and T4:1, an off-delay timer. The activation of the momentary start switch makes rung 0 true, which initiates the on-delay timer (T4:0/EN and T4:0/TT are true). Since the start switch is FIGURE 17: Pumping system ladder diagram for Example

5 a momentary contact type, the I:1/10 instruction must be sealed with the T4:0 timer timing bit (T4:0/TT) to keep the rung active while the timer is incrementing the accumulator for the on-delay time of 5 seconds. Upon completion of the on-delay time of 5 seconds, the T4:0 timer done bit (T4:0/DN) is active, which makes the XIC instruction in rung 2 true. The XIO instruction in rung 2 is also true (continuity) because the T4:1 timer done bit (T4:0/DN) addressing the XIO instruction is false or 0. As a result, the pump output, O:2/5, is true, so the pump starts 5 seconds after the start switch is pressed. The sealing instruction (O:2/5 around T4:0/DN in rung 2) is necessary because the T4:0/DN bit starts the pump after 5 seconds but is a 1 or true for only one scan. The sealing instruction in rung 2 keeps the pump on after the delayed start. T4:0/DN is true for only one scan because the timer resets immediately after the preset time is reached. This occurs because T4:0/TT bit is used to seal the XIC start instruction in rung 0, which is a momentary start push button. At 5 seconds the timer timing bit becomes false, which makes the T4:0 timer rung false and the timer resets. As a result, one scan after the done bit is true it returns to the false or 0 state. Review the operation of the standard ladder logic in Figure 12(c), which is used for this pump delay timer. Now when the stop PB2 is pressed, the I:1/11 instruction in rung 1 is true, and the T4:1/DN bit of the TOF timer in rung 1 is true. When the stop switch is released, the T4:1 TOF timer starts timing and keeps the T4:1/DN bit true for 15 seconds. Thus the initiation of the stop push button makes the XIO instruction in rung 2 false because the T4:1/DN output is true. This action stops the pump because output O:2/5 false. Rung 2 is held in this false condition by the XIO instruction for the duration of the T4:1 time, so the start push button cannot restart the pump for 15 seconds. Upon completion of the off-delay time of 15 seconds, the T4:1/DN output becomes false, the XIO instructions returns to true state, and the pump can be restarted. Note that this example uses standard timer ladder logic from Figures 12(c) and 13(b). 8 ALLEN-BRADLEY RETENTIVE TIMERS Review the operation of the retentive timer in Table 1(c) and Figure 9(c). The retentive timer (RTO) operates the same as a TON timer, except the accumulator (ACC) is not reset when the timer enable returns to the false state. The accumulator will continue to increment from the previous value whenever the EN bit goes from false to true. When the ACC equals the PRE value the timer timing bit goes false and the done bit becomes true. The done bit remains in that state until a reset (RES) instruction for the timer is executed. The reset instruction is covered in the next section. Compare and study Table 1(c) and Figure 9(c) until you understand the logical operation of an RTO timer instruction. The RTO instruction operates the same for all three Allen-Bradley processors. The RTO ladder logic symbol for the PLC 5, SLC 500, and ControlLogix systems is the same as their TON symbol. After the reset instruction is introduced in the next section, an example is used to illustrate how the RTO and RES instructions operate. 8-1 Reset Instruction for RTO Timer and Other Allen-Bradley Instructions Since the retentive timer does not automatically reset itself, a reset instruction is used to return the timer accumulator to zero and turn off the done bit. The reset (RES) instruction must have the same program address as the timer you want to reset. The reset instruction can reset the timer at any time during its operation and is independent of the input conditions. The reset instruction is also used for the TON and TOF timers and with other Allen-Bradley instructions. The operation of the reset instruction is the same for all three types of Allen- Bradley PLC processors. Example 3 illustrates the operation of an RTO and RES instruction in an automation system. Heater sequential control application. In large furnaces the electric heaters are often turned on or off in a sequence to control the heating and cooling of the product. In this application three heaters come on at the same time and remain on as long as the momentary start switch is held. When the switch is released the heaters turn off in sequence at 30-second 170

6 EXAMPLE 3 The pumping system in Figure 17 from Example 2 has an additional requirement to shut the pump down and illuminate a red pilot lamp after four hours of operation. The illuminated pilot lamp indicates that it s time to check the pump since it moves very abrasive material. A NO momentary push button reset switch is used to reset the system when the maintenance is completed. Draw the new ladder diagram for the pumping system with these additional requirements. Solution Refer to Figure 18 as you read the following description. The operation of rungs 0, 1, and 2 are similar to the ladder logic in Example 2; the last three rungs are new. Every time the pump is running, output O:2/5 (rung 2) is active. This makes the XIC instruction (O:2/5) and the retentive timer (T4:2) in rung 3 active, which increments the accumulated time in T4:2. Note that the retentive timer preset value is 14,400 seconds, which is 4 hours. When the accumulated number equals 14,400 seconds, the retentive timer done bit (T4:2/DN) is true, which causes a true FIGURE 18: Pumping system ladder diagram for Example

7 condition on output O:2/6 in rung 4, and the maintenance light to illuminate. A second T4:2/DN bit, added in rung 2, is assigned to the examine if open instruction (- -) called maintenance timer. This XIO instruction prevents the pump from being restarted until after the retentive timer is reset and the done bit is a 0. Finally, when the reset push button switch is pressed the I:1/6 instruction in rung 5 is true, which makes RES true. This resets the accumulator in the retentive timer to zero and turns off the T4:2/DN. Note that the address on the reset instruction is the address of the RTO instruction to reset. intervals. Figure 19(a) depicts a ladder diagram using the ControlLogix syntax where the done bits from three off-delay timers (TOF) are used to turn on three heaters and then sequentially turn them off at the 30-second interval rate. (Note that the arrowed lines in the ladder logic FIGURE 19: Control of heaters with off-delay timers. 172