Ladder Logic and Pid Control

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oakbarn
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Ladder Logic and Pid Control

Post by oakbarn »

I have a new Float Valve in my Electric HLT that also has an Electric Element.

If I set up in my Process where the HLT Element is controlled in PID Mode using the HLT Temp Probe AND

I set up in Ladder Logic the the HLT Element cannot be on except when the HLT Float Switch is closed,

Is the HLT Element still under PID Control when the HLT Float is Closed?
JonW
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Re: Ladder Logic and Pid Control

Post by JonW »

Off hand, I don't know if it would work with PID control or just go to the ON state. I suggest you try it.

You will probably need the output in the condition test as well. e.g. --[Din0]--[Out0]------------------------[Out0]
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Re: Ladder Logic and Pid Control

Post by oakbarn »

Yes. It worked with the Out in both a Contact and Coil position in addition to a DIN. Went to setpoint and Steady on the Setpoint. Turned off DIN and the Element was off.

I did test where the Out was just in the Coil and not also in the Contact slot and it just kept "on" without Temp control, so it does need to be there 2 times.

I am on my 3rd element in the sHLT because I have fried two. Fortunately they are the cheap $8 ones. But if they have power and are in Air, it is nano seconds before they are done.

I plan to add the Float Din and then I will not have to change elements mid brew :oops: .
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Bayoujeeper
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Re: Ladder Logic and Pid Control

Post by Bayoujeeper »

Glad it worked!
I have been playing around with ladder logic lately to learn it better. Out of curiosity, why does the output need to be in there twice? Maybe I don't understand ladder logic well enough, but why does it need to be in a contact position as well as a coil?
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Re: Ladder Logic and Pid Control

Post by JonW »

Bayoujeeper wrote:Glad it worked!
I have been playing around with ladder logic lately to learn it better. Out of curiosity, why does the output need to be in there twice? Maybe I don't understand ladder logic well enough, but why does it need to be in a contact position as well as a coil?
In the example above, if only Din4 (float switch) was in the contact position with Out1 (heating element) in the coil position, then Out1 would come on as soon as the float switch was activated and would turn off as soon as the float switch was deactivated. Since we don't want Out1 on all the time that the float switch is activated, we add the additional condition of testing if Out1 is also being requested to turn on.
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Re: Ladder Logic and Pid Control

Post by Bayoujeeper »

Got it! makes total sense now. Thanks Jon!
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