Heating and cooling

Kuhl-J

Member
Join Date
Feb 2019
Location
Minnesota
Posts
4
I'm looking for a way to determine if the temperature of a zone is rising or falling over a relatively long period of time, say 1 to 2 seconds, using RSLogix 5000 ladder logic. The zone in question uses resistance heat and chilled water for cooling. The idea is to reduce, or stop, chilled water cooling when the process variable above the set point but falling, as a result of the cooling, and increase cooling as the process variable moves above the set point.
 
You measure the temperature, and then compare old temps to new ones. This could be as simple as saving the temp every 2 s, or you could use various methods to smooth/filter the data.
 
You measure the temperature, and then compare old temps to new ones. This could be as simple as saving the temp every 2 s, or you could use various methods to smooth/filter the data.

Thank you for your reply.
I think maybe I should add the temperature is continuously being measured using a thermocouple into an analog input card.
My uncertainty is the code used in the various methods to save the temperature and to smooth/filter the date. Could you please elaborate by explaining how you would determine if the temperature is raising or falling over a period of time?
The frequency the process variable is compared is not necessarily that important.
 
Welcome to the forum.

what you are asking for depends on several factors.

how big is the zone?
how is the zone constructed?
how is it insulated?
how is the zone cooled?
what is the material you are treating?
What are you trying to measure? air, water?...
where is the temperature sensor?
is there any forced air cooling in the zone?
you may not be able to tell within 2 seconds, you may have to wait several minutes to 30 minutes or longer, depending on your system.

imo, you need to post the process in detail if possible so we can help.
include the heat / cool times if possible.
james
 
Last edited:
Welcome to the forum.

what you are asking for depends on several factors.

how big is the zone?
how is the zone constructed?
how is it insulated?
how is the zone cooled?
what is the material you are treating?
What are you trying to measure? air, water?...
where is the temperature sensor?
is there any forced air cooling in the zone?
you may not be able to tell within 2 seconds, you may have to wait several minutes to 30 minutes or longer, depending on your system.

imo, you need to post the process in detail if possible so we can help.
include the heat / cool times if possible.
james

The zone in question is one of ten barrel zones of an extruder constructed of steel. The barrel is cooled using chilled water. Each barrel zone has a thermocouple going to an analog input card. My question is, using PLC logic how can I determine if the temperature is rising or falling over a period of time.
 
That cold-heat regulation that you are studying is not an easy subject.
If you simply turn on the resistance when the temperature drops or you open the water when it rises then the regulation probably will be very bad due to the thermal inertias.

I would think about implementing a PID regulation with two sets of parameters, ones for cooling and others for heating.
 
That cold-heat regulation that you are studying is not an easy subject.
If you simply turn on the resistance when the temperature drops or you open the water when it rises then the regulation probably will be very bad due to the thermal inertias.

I would think about implementing a PID regulation with two sets of parameters, ones for cooling and others for heating.

I'm using a PID loop.
The issue is...Zone heats up and overshoots the Control Variable and the cooling starts. The cooling begins to lower the temperature but continues to cool in intervals until the process variable is lower than the control variable thus over cooling. I would like to reduce or eliminate cooling when the process variable begins to fall
 
I'm looking for a way to determine if the temperature of a zone is rising or falling over a relatively long period of time, say 1 to 2 seconds, using RSLogix 5000 ladder logic. The zone in question uses resistance heat and chilled water for cooling. The idea is to reduce, or stop, chilled water cooling when the process variable above the set point but falling, as a result of the cooling, and increase cooling as the process variable moves above the set point.


I'm going to let others recommend what you should do with your PID controllers and answer your original question.

You could detect whether a variable is rising or falling by taking x number of samples of the variable's value and storing their average value within a holding register. Then repeat the process and place the average value into a second holding register. Compare the two registers to determine whether your variable is waxing or waning.

If HR1 > HR2, then your variable is falling. If Hr1 < Hr2, your variable is rising. You could also implement a deadband in order to ignore very small differences between the two sampling periods. You could also find the difference between HR1 and HR2 and determine the rate of change amplitude.
 
Last edited:

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