Non-PID Heat Control

[Passman]

I am trying to build ladder logic in rsl5k to turn on a contactor with a digital output. I have a 1769-IT6 input card and a 1769-OA16 output card. What instruction would be necessary to achieve this, if it is possible?

 

[daba]

I assume your input card is for a thermostat switch.....

    Stat                           Contactor

|---|/|------------------------------( )---|

Or,

    Stat                           Contactor

|---| |------------------------------( )---|

depending on the thermostat being n/o or n/c

 

[Passman]

input card is for a thermocouple

 

[Mickey]

What kind of "Contactor" do you have? ( solid state or electromechanical).
Electromechanical contactors are not going to like being switch frequently.

How close do you want to maintain the temperature to setpoint.

 

[Passman]

What kind of "Contactor" do you have? ( solid state or electromechanical).
Electromechanical contactors are not going to like being switch frequently.

How close do you want to maintain the temperature to setpoint.

Electromechanical contactor. +/-3-5*F
I am currently using a controller in on/off mode instead of pid.

 

[daba]

apologies for my terse post

 

[Passman]

No problem, completely expected when asking a "dumb". I have never tried to program this sort of logic and any suggestions in the general direction will suffice. Thanks

 

[Mickey]

See picture below, It's not RsLogix5000 ( I don't have it) But you get the idea.

Your setpoints may vary. And hopefully you are not having problems switching your contactor often.

 

[Tom Jenkins]

I can't see Mickey's figure, so this may be redundant.

This is essentially a thermostat, that is, floating point control. You have a lower limit and an upper limit, where the upper limit is the lower plus your deadband or error tolerance. Scale the T/C input as real temperature, degrees F.

Use two comparison statements. If the actual temperature is less than the lower limit then turn on your contactor. Use a holding contact to keep it on until the actual temperature is greater than the upper limit. Then turn off the contactor by dropping out the holding contact.

You can add refinements to keep from chattering if hte measured temperature responds very quickly - time delays that don't turn on the contactor until the temp is low for a set period, and a timer to inhibit the contactor coming back on for a set period.