Melsec FX2N Problem

Hello I am a college student and using Melsec for the first time and I am really stuck. I would be eternally grateful if someone could post a ladder diagram for the following. If I have the answer I can work backwards. I have wasted hours on his and haven't got very far at all.

Question:

A workshop has the sawdust created by two machines extracted through ducts by a three-phase motor operating an extractor fan. The control fo the fan is austomatic and is initiated by the machine ON/OFF pushbuttons.The system must operate to the following specification:

a) the fan must run for 20 seconds before power can be supplied to either of the machines

b) only one machien can be switched onat a time, a 10 second delay being required before the second machine can be started. The machines may be started in any order.

c) from the time that the last machine is turned off a 20 second delay is required before the exhaust fan may be stopped.

d) in the event of the fan motor overload protection operating, both the machines and the fan must stop immediately.

e) operating of the overload protection of a machine motor will only stop that machine as the workshop processes are independent of each other.

f) each machine is covered by a guard. The position of the guard is indicated by a changeover switch which changes position as soon as the guard starts to open but before any access can be obtained to the blades. The machien must not start until the guard is fully closed and must stop immediately if any attempt is made to open the guard.


Summary of inputs and outputs.

Inputs Contact type
Machine A
ON pushbutton NO
OFF pushbutton NC
overload NC
guardswitch terminal 1 NC
gaurdswitch terminal 0 NO

Machine B
ON pushbutton NO
OFF pushbutton NC
overload NC
guardswitch terminal 1 NC
guardswitch terminal 0 NO

Fan overload NC

Outputs

M/C A motor contact
M/C B motor contact
Fan motor contact



I know this is a lot to ask but help would be great.

While the program is a simple thing to write - the control description - or method of control is flawed.
I realise it is only a test of programming - but really the tutor who set it should have a better grasp of real world applications.

For starters - the fan should be seperately controlled by its own buttons and only when air flow is proved should either machine be able to run. Secondly the guard switches should go to a safety relay and an E stop if it was included

The way this is described is - you go to turn the wood machine on and nothing seems to happen. Somewhere outside maybe, a extract fan has started and a timer will start the machine 20 seconds later. meanwhile the operator is frantically pressing the start button wondering why the machine is not running. He might think it is broken and put his hands on the machine!!!

You might be lucky and someone might write it for you ( I hope not)
You will have to show us how far you have got - no matter how bad or wrong your code is. I would even have a word with the tutor and highlight the problems with this control method.

Agreed

Great Observation Goody

R

A solution to the machine not starting immediatly would be to sound a klaxon whilst waiting for the fan delay timer, this way the operator would be aware the machine was about to start

Hello Bilal_Baig;To get back to your original question:

I would be eternally grateful if someone could post a ladder diagram for the following

Click to expand...

I'm sure you would be, but that is not the way you will learn. I'm not sure I would hire someone who learned his trade that way.

Here are some suggestions to get you started; come back to us when you have started coding, some of us here are experts on Mitsubishi PLCs and can help you refine your program.


1) take all your nputs/ouputs as stated in your homework, give the real addresses as per your CPU.

2) note that you will need internal coils, timers/counters and registers to complete the logic.

3) start by stating in pseudo-code (plain language, English, or maybe Bangla if it is easier) the logic you have to use to implement this program.

For example:

a) the fan must run for 20 seconds before power can be supplied to either of the machines

Click to expand...

How to express this in logic terms?
--> if the pushbutton to start Motor A is pressed, a timer will run for 20 seconds; when the timer is done, then the M/C contact for motor A will be active;
--> if the pushbutton to start Motor B is pressed, a timer will run for 20 seconds; when the timer is done, then the M/C contact for motor B will be active;

One important point: outputs should be used only once in a program (do a search on "double-coiling" on this forum if you like). So we must modify the logic above to take into account conditions that will not allow the motor contacts to come on, or cause them to turn off:
--> if the pushbutton to start Motor A is pressed, a timer will run for 20 seconds; when the timer is done, AND there is no overlaod for Machine A, AND the stop pushbutton for Machine A has not been pushed, then the M/C contact for motor A will be active;

Once the logic has been worked out properly, change the pseudo-logic descriptiojn into ladder symbols:


AND --> --| |---| |--


OR -->

-----| |-----

|--| |---|

then --> ---( )

And place in the correct position the addresses selected earleir for your input/outputs.

Hope this helps,
Daniel Chartier