Help with a PLC Program

Hello guys and gals. I am new here and new to PLCs.

I am attemting to learn about PLC. I am stuck on this program. It involves Latching ----(L)---- and Unlatching ----(U)----- and Bit Memory locate. My problem is that I have no clue how to use the latch and unlatch instructions and when to apply it. Can you help?

Here is what I need done:

We have a start SW!, stop SW2, bearing temp sensor TS_1, cabinet interlock INTR_1, and a motor overload OVL for the inputs.

For the outputs we have the pump motor M_1, motor overload indicator PL1, and motor bearing failure PL2.

• Closing SW1 start the pump motor if the following conditions are true: Stop switch is open, motor overload is closed, bearing temp sensor is closed, and the cabinet interlock is closed.
• If the motor overload contact opens, the motor should stop and turn on PL1. The motor cannot be re-started with the start switch, until first the motor overload fault is reset and the stop switch is pressed. (Stop will reset PL1)
• If the motor bearing temp sensor opens, the motor should stop and turn on PL2. The motor cannot be re-started with the start switch, until first the motor bearing temp. sensor is closed and the stop switch is pressed. (Stop will reset PL2)
• Safety interlock switch must be closed to start machine, however once motor is running interlock may be open and motor will continue to run.

Can anyone help me with something in RXLogic?



Thank you.

You did say learning PLC's, so I assume this is some type of lab assignment, therefore I'll give you a nudge, but not the answer.

I personally would not use the latch and unlatch instructions for this logic, but that's just my preference. I guess this assignment is to learn about latches and unlatches.

Anyway, think about all the conditions that must be true for the motor to start, use these conditions in series to latch the motor relay.
You didn't mention it, but I assumed your start and stop switches are momentary.

The faults will latch a fault bit and the stop button will unlatch them.

Use the fault bits to energize the fault indicators and stop the motor.

Use the conditions that stop the motor in parallel to unlatch the motor relay.

I'm not the best teacher in the world, and some times garble rather than clarify. Hope this helps.


Ken

I feel generous today

One step at a time. NOTE: Latch stays latch UNTIL you use conditions with an UNLATCH instruction....ie once you turn it ON with LATCH you have to use UNLATCH to turn it OFF.

Closing SW1 start the pump motor if the following conditions are true: Stop switch is open, motor overload is closed, bearing temp sensor is closed, and the cabinet interlock is closed.

Click to expand...

Technically, the STOP switch should be normally closed but if you are using a normally open switch then you invert the logic. I would not use Latch either but since that is what you asked for.


BUT you included the following

If the motor overload contact opens, the motor should stop and turn on PL1. The motor cannot be re-started with the start switch, until first the motor overload fault is reset and the stop switch is pressed. (Stop will reset PL1)
If the motor bearing temp sensor opens, the motor should stop and turn on PL2. The motor cannot be re-started with the start switch, until first the motor bearing temp. sensor is closed and the stop switch is pressed. (Stop will reset PL2)

Click to expand...

Now we have to include PL1 and PL2 to the start circuit.


DARN, I am old and got tired, think you can figure out how to UNLATCH PL1, PL2 and the motor?

It is the holiday season, see attached PDF. I threw this together really fast, so there may be mistakes, but it should get you headed in the right direction.

Did not think about PL being Pilot light/indicator...DOH me, good one.

I don't think I need the latch instructions. I was trying to do it without the latch instructions but at one point the motor and the PL will come on and I couldnt get around it.

Thank you for the nudge and since I dont have the software and hardware here with me I will cont. this poject after the holidays.

I understand the latch instruction to be basically like an ON and OFF button. It just takes me longer to understand things.

I will update y'all with the progress!

Thank you.

I understand the latch instruction to be basically like an ON and OFF button. It just takes me longer to understand things.

Click to expand...

It is more like an ON/OFF switch, like those that turn on lights in your house.

Its OK to use Latch/Unlatch but since they are retentive...ie stay on even with a power loss etc then you have to be careful how they are used.

Don't bother with the latch, also use a normally closed button for the stop as if you loose supply to the buttons it will stop, if you use a normally open & loose supply to the inputs you will not be able to stop it, but then again there should be a safety circuit somewhere i hope.
Start Stop Alarm Out
--| |---||------|/|--------------( )
Out |
--| |-+
this uses the out bit ored to latch the start button, if an alarm or stop button pressed it will drop out.
simple!

Fellows,

The instructions say to use a latch. Students (and employees) must learn to follow instructions. The latch is not to be used on the motor, however. A motor output should (almost) never be latched. It is the motor overload fault that is supposed to be latched, as in the following instruction:

The motor cannot be re-started with the start switch, until first the motor overload fault is reset and the stop switch is pressed. (Stop will reset PL1)

Click to expand...