I am fairly familiar with RSlogix 500, though not an expert with PID loops. I am revising a program for one of our vessels that uses a steam HX and a chilled water HX to control product temperature (product is heated indirectly via circulation of water through vessel jacket).
The current control scheme uses 2 PID blocks, one for cooling and one for heating. My concern is each has a trigger on the rung, i.e, when B3:11/7 is energized, the heating PID is enabled. When B3:11/9 is energized, the cooling PID is enabled. The heating and cooling bits are energized depending on whether heating or cooling is required, but never both at the same time.
I have read that there should never be a condition on a PID rung, as data is updated only when enabled.
Here are my 2 questions:
1.)With the condition on the rung, when the rung goes false to true, wouldn't the relevant PID data update almost instantaneously (within a few scans)? How big is the danger of operating this way? Please note that I have not been able to observe operation of the system, as it has been out of service due to unrelated issues.
2.)If I want to remove the conditions from the rungs, is it as simple as using the existing bits to switch the PID block status from Auto to Manual. For example, if my control block is N9:0, can I just have a rung that says XIO B3:11/7 OTE N9:0/1 for the heating PID? When in manual, does the output automatically go to zero, or do I have to move that value there?
Thanks very much for any help.
The current control scheme uses 2 PID blocks, one for cooling and one for heating. My concern is each has a trigger on the rung, i.e, when B3:11/7 is energized, the heating PID is enabled. When B3:11/9 is energized, the cooling PID is enabled. The heating and cooling bits are energized depending on whether heating or cooling is required, but never both at the same time.
I have read that there should never be a condition on a PID rung, as data is updated only when enabled.
Here are my 2 questions:
1.)With the condition on the rung, when the rung goes false to true, wouldn't the relevant PID data update almost instantaneously (within a few scans)? How big is the danger of operating this way? Please note that I have not been able to observe operation of the system, as it has been out of service due to unrelated issues.
2.)If I want to remove the conditions from the rungs, is it as simple as using the existing bits to switch the PID block status from Auto to Manual. For example, if my control block is N9:0, can I just have a rung that says XIO B3:11/7 OTE N9:0/1 for the heating PID? When in manual, does the output automatically go to zero, or do I have to move that value there?
Thanks very much for any help.