Mitsubishi Q series: Another SET/RST logic question...

[Mas01]

Bottom line: the logic prefers a low level (pump cavitation) than a high level (overflow?). Do I have that right?

TL;DR

So it protects against the logic turning the pump off when FltSw018 is 0, indicating that the level is below 18cm, if any of the other higher level switches indicate the level is above that higher switches level.

That works if the 18cm switch's signal is in error and the rest are correct. However, it also prevents the logic from turning the pump off if the 18cm switch's signal is correct and 0 but one of the other level switch's signals is in error and stuck at 1.

(P.S. I know what braces are, and I like to wear them with my belt)

I thought you called braces "suspenders" on your side of the pond.

Not sure what you mean by the logic"prefers" a low level. By its name, the sump pump is at the bottom of the tank. It's purpose is to provide circulating cooling water

 

[parky]

@DR: like to be doubly sure they don't fall down eh...
DR is thinking like me should one of the higher level sense positions was faulty i.e. was on when not covered then the pump will not shut off.
As I posted just before DR I would do some illegal logic to set an alarm i.e. hh level was on but low level was off this is illegal either the high level is reporting a high level when it should be off as the low level is off so is an error, that goes for the others as well.

 

[Mas01]

The problem is without a seal in bit like I posted, the low level pulse bit will reset the pump, however, because the original start button will not be on there is no way to restart it without pushing the button again, if you follow what I did it will work so what it actually does is sets a bit (seal in if you like) this stays active true until the stop button is pressed, the two timers then give a slug time on the low level probe so when the level drops below the low level after 10 seconds the pump will stop because the interlock will energise, on reaching low level or above for 10 seconds the pump will re-start because the original start/stop latch will still be active.

You've got a way with words. Cracking explanation.

 

[Mas01]

@DR: like to be doubly sure they don't fall down eh...
DR is thinking like me should one of the higher level sense positions was faulty i.e. was on when not covered then the pump will not shut off.
As I posted just before DR I would do some illegal logic to set an alarm i.e. hh level was on but low level was off this is illegal either the high level is reporting a high level when it should be off as the low level is off so is an error, that goes for the others as well.

I should point out that as well as the 4 float switches, there's also a water level sensor at the bottom of the tank which is configured in the PLC to measure tank water depth. I suppose I could use this (analogue input) and not rely on the float switches. Sinces there's a "mechanical" element to the float switches, maybe the depth sensor is a better option.(?)

 

[drbitboy]

I thought you called braces "suspenders" on your side of the pond.

I lived on your side for a couple years many moons ago, so I am little less separated by a common language than others.

Not sure what you mean by the logic"prefers" a low level. By its name, the sump pump is at the bottom of the tank. It's purpose is to provide circulating cooling water

by "prefers" I meant what the logic allows to happen if any switch provides a false (inverted) signal e.g. 1 from switch A when level is below switch A or 0 from switch A when level is above switch A.

The way 547 is "wired" with the ANDed series of NCs, if any switch fails to 1 (high) when level is actually low, the pump will keep running i.e. keep driving the level towards low i.e. the logic "prefers" low levels, and avoids high levels, even if any high level indications are false.

 

[parky]

Yes but analogue sensors go wrong as well, for example it could give you a full level when the tank was empty so perhaps use both one to check the other, does not need to be accurate but for example on the low level, if the analogue signal was say 5% (no idea what scaling you have) then if the low level was off you would expect the analogue level to be below 5%, so a window i.e. if the analogue signal was above say 8% but low level was not covered then there is a problem with either the analogue sensor or the float. You could generate probably quite a few alarms but perhaps a generic one i.e. level sensors fault.
Quite a few combinations Ored together but at least it would give both operators & engineers some indication what the problem was.
Example:
IF HHL ON AND (HL OFF OR ML OFF OR LL OFF) Set Alarm
Do the illegal combinations then OR them together to set the alarm & stop the pump, display the alarm on the HMI.

 

[drbitboy]

@DR: like to be doubly sure they don't fall down eh...
DR is thinking like me should one of the higher level sense positions was faulty i.e. was on when not covered then the pump will not shut off.
As I posted just before DR I would do some illegal logic to set an alarm i.e. hh level was on but low level was off this is illegal either the high level is reporting a high level when it should be off as the low level is off so is an error, that goes for the others as well.

This is best: anomalous behaviour means alert the operator to monitor and figure out the problem.

E.g. you could also do a vote: if any three are 0, then shut off pump; but that means nominal operation (with all four working) would cycle around the second switch.

I guess my point is that if you don't trust the belt, then it doesn't matter if you bring in braces when you have so much money in your pockets that even together they cannot keep your trousers up (I am not convinced that analogy works).

 

[drbitboy]

I think the problem is that the level is being controlled around the lowest switch. If it was one of the middle two switches then the outer switches could be used as backup and it would require a double contingency (i.e. two switches on the same side) to fail.

 

[Mas01]

So although M413 is clicked based on the choice of a pump, and it multiplexes a button press event to control "something" about that pump, it does not feed into that "something" itself; i.e. the value of M413 never by itself determines whether the pump itself is running.

Correct again. Once you click on M413 It goes TRUE , the operator then has 10 seconds in which to select a pump to start.
After 10s idle time, M413 goes FALSE.

 

[Mas01]

I guess my point is that if you don't trust the belt, then it doesn't matter if you bring in braces when you have so much money in your pockets that even together they cannot keep your trousers up (I am not convinced that analogy works).

LOL I'm not convinced either, but it's funny.

 

[Mas01]

I understand that M413 controls which pump turns on or off when the start or stop pushbuttons are pressed.

But what would control which pump should turn on when the level goes above the 18cm switch?

Sorry, I missed this question...only the sump pump is switched off based on water level <18cm. The other pumps are e.g. oil pumps, vacuum pumps, fans etc

 

[drbitboy]

Sorry, I missed this question...only the sump pump is switched off based on water level <18cm. The other pumps are e.g. oil pumps, vacuum pumps, fans etc

Is there more than one sump pump? E.g. when you choose M413 and press start, are you choosing one of several sump pumps? Or is M413 the sump, M414 is e.g. the oil pump, M412 is the vacuum pump, etc.?

 

[Mas01]

Is there more than one sump pump? E.g. when you choose M413 and press start, are you choosing one of several sump pumps? Or is M413 the sump, M414 is e.g. the oil pump, M412 is the vacuum pump, etc.?

No, there's only one sump pump.
Correct again. M413 is specific to the sump pump.
Yes, you've got it - M412 is vac pump, etc.

 

[drbitboy]

Going back to the original post:

We've got a sump pump which is switched on/off at the HMI by clicking ... physical pushbutton on a console,

We've also got another step ... where the pump will automatically turn off if the water level is below 18cm

HOWEVER, as the code stands, once the pump turns off due to the low water level, it does not turn back on once the water level starts rising again (and this is a requirement)

There are two cases here:

• Reading requirement as contingent on past operator action
  • I.e. if the operator has activated the sump stop button ([sump select HMI], then [Motor Stop PB] within 10s), then the requirement is relaxed
  • Maybe I have not looked closely enough, but I think @parky has coded this, where the start and stop operator PB events put the system in and out of [Auto Run] mode, and the level switch controls the pump when [Auto Mode] is 1
    • I am pretty sure @parky's Rung 21 is a clear indication that that code implements this approach: the pump cannot rung unless [Auto Run] = 1.
  • So [Auto Run] = 1 means run the pump during any period, plus 10s, that the level is at or above 18cm.
  • The [Motor Start PB] and [Motor Stop PB] events put the system in and out of [Auto Run] mode, and have nothing directly to do with running the motor.
  • In this requirement-contingent case, we are trusting an operator, who has activated the Stop PB, to re-engage [Auto Run] mode before the sump overflows.
• Reading requirement as absolute
  • In this case, the sump will always run during any period, plus 10s, that the level is at or above 18cm, no matter what the operator does.
  • In addition, if the [Motor Start PB] is pressed (during the select sump 10 duration), then the motor will also run and/or continue to run, regardless of the level, until the [Motor Stop PB] is pressed (during ...).
    • This logic for the Start/Stop PBs effects a [Sump Override] mode i.e. it overrides the level logic from controlling the pump by keeping the pump running.
  • In this requirement-absolute case, we are trusting an operator, who has activated the Start PB, to disengage [Sump Override] mode before the sump goes dry and causes damage to the pump.

I suppose @Mas01 could be looking for a more nuanced approach, but it there are so few inputs (start pb, stop pb, level18cm) that I don't think much else is possible.

So my "drbitboy, DMD" queries to @Mas01 are: which is it, if either; how much money do you have in your pockets; do you have a source for industrial strength braces?

Having heard both, "belt and suspenders" is more melodic to my ears, especially as my dad said it so often, but still, ya' gotta like the alliteration of "belt and braces."

 

[drbitboy]

On the left is the requirement-absolute, [Sump Override] mode, case.

On the right is the requirement-contingent, [Auto Run] mode, case. This should be pretty close to equivalent to @parky's most recent PDF, although I think there is a very minor typo at his rung address 4: M2 should be in a [Normally Open] instruction, and in series (ANDed) with the [Normally Open M4] instruction; as it is, the system will leave [Auto Run] mode any time the [M413 SumpSelect]-active period has expired; i.e. it should be DeMorgan'ed relative to the stop logic below for the AUTO_RUN/SUMP_OVERRIDE rung below.

Not too different really.

I did it without SET/RST because I'll never be able to improve on anything @parky does.

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