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Why so many relays?(Electro-pneumatics)

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On the basis of a school assignment I have drawn an electro-pneumatic with fluidsim. The circuit I have drawn contains much less relays than the solution of the assignment self. My question now is why in the solution of the assignment a total of 18 relays is used while my solution only needs 9 relays. For the rest, both circuits do the same action according to the sequence circuit. A + B + C + A-D + D- B-C-

What did i do wrong?

Thank you


Ian Rogers

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Saturdays are very quiet... Boolean algebra isn't for everyone... Our American friends will be online soon!!

I should assume this can be solved by logic just the same as gates... I should imagine you need to focus on what doesn't switch rather that what does


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I had a look earlier, but I did not understand what the thing was trying to do.
Also, there are some aspects of the pneumatic symbols which I did not understand. (In my day job, before I retired, I did work with some simple hydraulic systems).



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I assume this is some type of fluid-handling system (food processing/chemical manufacturing/water treatment).

It looks like the solution with more relays accounts for some safety as well as function. That is, some items must not turn on while others are on. For example the heater or agitator will not turn on until the door (hatch) relay indicates it is closed.


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The more complex circuit has both interlocking and latching functions.

Each relay circuit has a normally-closed contact in its feed to prevent it operating, or release it's latch, as required [Interlocking].

And, once one is energised via the proximity switch relay, it holds in (via one of its own contacts) and remains in rather than releasing as soon as the proximity switch opens [Latching].
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