Early PLCs only supported ladder logic and instruction list programming languages. Now, according to the International Electrotechnical Commission (IEC), five languages support PLC programming: Ladder Delete (LD), Instruction List Delete (IL), Function Block Diagram Delete (FBD), Sequential Function Chart Delete (SFC), Structured Text Delete (ST), etc. Today, we'll share some PLC control circuits and ladder diagrams, which are considered relatively basic and practical. Let's take a look!
PLC circuits and ladder diagrams for start, self-locking, and stop control
Start-up, self-locking, and stop control can be implemented using drive instructions (OUT) or set instructions (SET, RST).
1. Start-up, self-locking, and stop control are achieved using coil drive commands.
Circuit diagram and ladder diagram explanation:
When the start button SB1 is clicked, the start contact X000 in the PLC's internal ladder diagram program closes, the output coil Y000 is energized, the internal hard contact of the output terminal Y0 closes, the Y0 terminal and the COM terminal are internally connected, the contactor coil KM is energized, the KM main contact in the main circuit closes, and the motor is energized and starts.
When the stop button SB2 is clicked, the stop contact X001 in the PLC's internal ladder diagram program opens, the output coil Y000 loses power, the internal hard contact between the Y0 and COM terminals opens, the contactor coil KM loses power, the KM main contact in the main circuit opens, and the motor loses power and stops.
2. Start-up, self-locking, and stop control are achieved using set/reset commands.
Its PLC wiring diagram is similar to the one above.
Circuit diagram and ladder diagram explanation:
When the start button SB1 is clicked, the start contact X000 in the ladder diagram closes, the [SET Y000] instruction is executed, and the result of the instruction execution sets the output relay coil Y000 to 1. This is equivalent to the coil Y000 being energized, which connects the internal hard contacts between the Y0 and COM terminals, energizes the contactor coil KM, closes the KM main contacts in the main circuit, and energizes and starts the motor.
When the stop button SB2 is clicked, the stop contact X001 in the ladder diagram program closes, the [RST Y000] instruction is executed, and the result of the instruction execution resets the output coil Y000, which is equivalent to the coil Y000 being de-energized. The internal hard contact between the Y0 and COM terminals is opened, the contactor coil KM is de-energized, the KM main contact in the main circuit is opened, and the motor is de-energized and stops.
PLC circuit and ladder diagram for forward and reverse interlock control
The circuit diagram and ladder diagram are explained below:
1) Forward rotation interlock control
Clicking the forward rotation button SB1 → the forward rotation contact X000 in the ladder diagram program closes → coil Y000 is energized → the Y000 self-locking contact closes, the Y000 interlocking contact opens, and the internal hard contact between the Y0 terminal and the COM terminal closes → the Y000 self-locking contact closes, allowing coil Y000 to still be energized after the X000 contact opens; the Y000 interlocking contact opens, preventing coil Y001 from being energized even when the X001 contact is closed (caused by erroneous operation SB2), thus achieving interlocking control; the internal hard contact between the Y0 terminal and the COM terminal closes, energizing the contactor KM1 coil, closing the KM1 main contact in the main circuit, and energizing the motor to rotate forward.
2) Reverse Interlock Control
Click the reverse button SB2 → the reverse contact X001 in the ladder diagram program closes → coil Y001 is energized → Y001 self-locking contact closes, Y001 interlocking contact opens, the internal hard contact between Y1 terminal and COM terminal closes → Y001 self-locking contact closes, so that coil Y001 continues to be energized after X001 contact opens; Y001 interlocking contact opens, so that coil Y000 cannot be energized even when X000 contact is closed (caused by erroneous operation SB1), realizing interlocking control; the internal hard contact between Y1 terminal and COM terminal closes, contactor KM2 coil is energized, KM2 main contact in the main circuit closes, and the motor is energized and reverses.
3) Stop control
Clicking the stop button SB3 → both stop contacts X002 in the ladder diagram program open → coils Y000 and Y001 are de-energized → contactor coils KM1 and KM2 are de-energized → the main contacts KM1 and KM2 in the main circuit open, and the motor stops due to power loss.
PLC circuits and ladder diagrams for multi-location control
(1) Single person controlling multiple locations
Starting control at location A. When the start button SB1 is pressed at location A → the normally open contact of X000 closes → coil Y000 is energized → the normally open self-locking contact of Y000 closes, the internal hard contact of Y0 terminal closes → the normally open self-locking contact of Y000 closes to lock the power supply to the Y000 coil, the internal hard contact of Y0 terminal closes to energize the contactor coil KM → the main contact of KM in the main circuit closes, and the motor is energized and runs.
Stop control at location A. When the stop button SB2 is clicked at location A → the normally closed contact of X001 opens → coil Y000 is de-energized → the normally open self-locking contact of Y000 opens, the internal hard contact of the Y0 terminal opens → the contactor coil KM is de-energized → the KM main contact in the main circuit opens, and the motor stops due to power failure.
(2) Control by multiple people in multiple locations
Start-up control. Clicking buttons SB1, SB3, and SB5 simultaneously at locations A, B, and C energizes coil Y000, closes the normally open self-locking contact of Y000, closes the internal hard contact of the Y0 terminal, locks the power supply to the Y000 coil, energizes contactor coil KM, closes the KM main contact in the main circuit, and starts the motor.
Stop control. When one of the stop buttons SB2, SB4, or SB6 is pressed simultaneously at locations A, B, and C → coil Y000 is de-energized → the normally open self-locking contact of Y000 opens, and the internal hard contact of the Y0 terminal opens → the opening of the normally open self-locking contact of Y000 cuts off the power supply to the Y000 coil, and the opening of the internal hard contact of the Y0 terminal de-energizes the contactor coil KM → the KM main contact in the main circuit opens, and the motor stops due to power loss.
PLC circuit and ladder diagram for timed control
1. PLC circuit and ladder diagram for delayed start and timed operation control
It can achieve the following: the motor starts working 3 seconds after the start button is pressed, and automatically stops after working for 5 seconds.
The PLC circuit and ladder diagram are explained below:
2. PLC circuit and ladder diagram for multi-timer combination control
It can achieve the following: after clicking the start button, motor B will start immediately, motor A will start working after 30 seconds, motor B will stop after 70 seconds, and motor A will stop after 100 seconds.
The PLC circuit and ladder diagram are explained below:
PLC circuit and ladder diagram for extended timing control using a combination of timers and counters
The longest timing interval for Mitsubishi FX series PLCs is 3276.7s (approximately 54 minutes). The timing interval can be extended by using timers and counters.
The PLC circuit and ladder diagram are explained below:
The timer T0 in the figure has a timing unit of 0.1s (100ms). When used with counter C0, its timing time T = 30000 × 0.1 seconds × 30000 = 90000000 seconds = 25000 hours. If you need to reset the timing, you can open switch QS2, close the normally closed contact of [2]X000, and then execute the "RST C0" instruction. After that, counter C0 will be reset, and then close QS2. The 250000-hour timing will start again.
PLC circuit and ladder diagram for multi-output control
The PLC circuit and ladder diagram are explained below:
(1) Start-up control
(2) Stop control
PLC circuit and ladder diagram for overload alarm control
PLC circuit diagram and ladder diagram explanation:
(1) Start-up control
Click the start button SB1 → [1] X001 normally open contact closes → [SET Y001] command executed → Y001 coil is set, that is, Y001 coil is energized → Y1 terminal internal hard contact closes → contactor KM coil is energized → KM main contact closes → motor is energized and runs.
(2) Stop control
Click the stop button SB2 → [2] X002 normally open contact closes → [RST Y001] command is executed → Y001 coil is reset, that is, Y001 coil is de-energized → Y1 terminal internal hard contact opens → contactor KM coil is de-energized → KM main contact opens → motor is de-energized and stops.
(3) Overload protection and alarm control
PLC circuit and ladder diagram for blink control
Circuit diagram and ladder diagram explanation:
Close switch QS → X000 normally open contact closes → Timer T0 starts counting down for 3 seconds → After 3 seconds, timer T0 activates, T0 normally open contact closes → Timer T1 starts counting down for 3 seconds, simultaneously Y000 is energized, Y0 terminal internal hard contact closes, lamp HL lights up → After 3 seconds, timer T1 activates, T1 normally closed contact opens → Timer T0 resets, T0 normally open contact opens → Y000 coil is de-energized, simultaneously timer T1 resets → Y000 coil de-energized, lamp HL goes out; timer T1 resets, T1 closes, because switch QS is still closed, X000 normally open contact is also closed, and timer T0 starts counting down for 3 seconds again.
Then repeat the above process, with the HL lamp flashing at a frequency of 3 seconds on and 3 seconds off.
Related Exercises
PLC control of the fountain
The system requires two buttons to control the operation of three groups of nozzles (A, B, and C) (achieved by controlling the motors of the three groups of nozzles), as shown in Figure 4-32. The specific system control requirements are as follows:
When the start button is pressed, spray nozzles A spray for 5 seconds and then stop. Then, nozzles B and C spray simultaneously for 5 seconds. After that, nozzle B stops, and nozzle C continues spraying for 5 seconds before stopping. Next, nozzles A and B spray for 7 seconds, with nozzle C stopping for the first 2 seconds and spraying for the last 5 seconds. Then, all three nozzles (A, B, and C) stop simultaneously for 3 seconds, and this process repeats. Pressing the stop button stops all three nozzles simultaneously.
