GX developer: trying to understand relay logic...

[Mas01]

Not sure I understand the logic here...
This picture was taken when the rig was "idle".
Why would you energise the trip relay M3 when these contacts are NOT at their trip/high high values?
Thanks in advance.

 

[parky]

It does appear that the next rung the output is a trip state or healthy state, without knowing what they are trying to do with the logic, the N/C contacts etc. etc. it is only a guess.
1. assuming one of those inputs is true (as they are N/C) then the bit M3 will be off perhaps an interlock to stop some operation.
2. perhaps all need to be off so that M3 comes on to allow some type of operation.
As I said without finding out where else M3, M2, Y2F6 is used it is only a guess
If M3 is not used elsewhere then it dose appear to be some type of alarm condition (or healthy i.e. an enable).
Again it depends on what you call a trip value is it that the contacts should be on when healthy or off when healthy ?.
In many interlock systems for example all overloads have a N/O contact when healthy it is on, in this case you would use all in series as N/O so the bit M3 would be on when there is no overload.
Sometimes N/O auxiliary contacts on overloads may be used as interlocks in hard wired to disable the contactors, as most contactors have a N/O & N/C so rather than add an extra set of contacts they use the N/C that opens when healthy, (not a good idea as a broken wire means no overload) so it reports it as being ok even when tripped.

 

[Mas01]

Hope this 2nd picture helps.
M2 and M3 are only used in this bit of the code….

 

[parky]

The only problem is sometimes people do not make the symbols exactly what the function is, for example the emergency brake ON output could have a number of meanings.
Is the brake spring loaded so in effect on when not energised and when energised takes the brake off, or is it opposite ?.
It looks from the comments & symbols that the N/C contacts have to be off to allow the clutch/brake to energise allowing movement so in the event of any one alarm M2 drops out (dropping out Y2E5) to apply the brake & disable the clutch.

 

[Mas01]

For info Y2F6 is used for determining Plant Ready, which means the rig is in a good state to be run.
Y2F6 illuminates a physical bulb on the control console.

 

[Mas01]

Reason I was asking about the trip relay logic is because I've been asked to cause a trip when the water level reaches high high. It looks like I need to follow the same pattern and add an "if water level NOT high high" to the line of contacts which set M3, which seems counterintuitive (until I get my head around it). Cheers

 

[Mas01]

Many thanks again parky for taking the time to help me out. Much appreciated.

 

[parky]

It will depend on if the signal (high level) is on when in alarm or off.
Assuming as the logic driving M2 has to be true so M2 enables clutch/brake (clutch brake has to be energised to run) and not high level is false then yes a N/C contact.
However, if the High level signal is off when in alarm you would need to have a N/O. can be hard to get your head round. as it is the opposite compared to the others, the reason I talk about this is it is normal to have a true signal when not in alarm on things like high levels, so if the high level is reached, the input goes off, & if for some reason the cable got damaged it gives the same fault as high level stopping what ever feeds the tank.

 

[Mas01]

Thanks for this. I see what you mean about giving the contacts & relays meaningful names.
Your choice of "Enable_Clutch_1/2/3" makes the logic easy to understand.