No it's not ST, ST is just the type of language you use. most PLC's can be programmed in LAD STL FBD or ST and any combination. for example you can program one block in LAD and another in ST. Sequence is just a way of stepping through you could use bits like a drum sequencer but difficult to force forward or back. What most PLC's do is compile any of the blocks (LAD, ST, FBD) into the same or similar code for the interpreter to run. I still tend to use FBD as many site engineers can understand it better.
Sometimes ST is better as maths is easier to understand but you can do the same in LAD and if your very experienced it's just as easy and more visual in my opinion.
One other advantage of using a word sequencer if you have an HMI is that many HMI's have text lists that display a text based on the value of a register.
So...
If you have a Sequence word on some HMI's you drop a multi text field on the screen, populate the fields like:
10 Process Starting
20 Adding Water
30 Adding Milk
35 Mixing
and so on
This means that what ever value is in the sequence word displays the appropriate message. Often used in Scada systems where this information is held in a database I suppose you could call it a lookup table.
The sequence is very powerful an example:
You have recipes stored on an HMI, you load the recipe that contains maybe hundreds of words into the plc memory lets say registers 1 to 500
each recipe phase has 10 registers these contain info on any type of process
so
Reg 1 is type of sequence. i.e. 0 = no Addition 1 = Water 2 = powder 3 = manual addition
Reg 2 = the addition value
Reg 3 = heating type 0 = no heating 1 = heating
Reg 4 = Blender required Yes/No 0 or 1
Reg 5 = Mix time
Reg 6 = Heating time
and so on
when you set your sequence word to say 10 it compares the recipe reg 1 and if it = water you set the seq. word to 100
The program block for water is called for seq. values between 100 and 199
it runs the code in the block while Seq. is => 100 AND =< 199
The water block then increments the seq. word in steps to do what it needs to do i.e. Sets Seq. to 110 open the main & trickle valves.
When it detects the water addition is within 10% of set point then it steps on to 120, this closes the main valve but keeps the trickle valve open when the water reaches the recipe value it steps the sequence on to say 130 heating
You could use a main sequence and each addition block has a sub sequence if you wanted.
Anyway, at the point that the addition is complete, it forces the seq. word to say 1000, this makes the main program to run a sequence that increments the recipe block to the next stage and forces the seq. back to 100.
This repeats until end of recipe is detected in which case it moves onto last sequence i.e. discharge.
Here is part of a similar process, unfortunately it was converted to a later version of the IDE and has lost some comments.
Just looked at your pic & it does not quite make sense, you have moved batch to BK but then run Temp control on HLT ? and you state maintain HLT & MT but MT does not have heaters am I missing something.
