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hysteresis on analog input


dsula

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Hi,

 

I'd like to add some small amount of hysteresis to an analog 4/20mA input. How can that be done? I'd very much appreciate if anybody could point me in the right direction. I'm a seasoned programmer, but I'm brand new to PLC and Ladder programming.

 

Many thanks.

 

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Expand on what you mean by "hysteresis".  Is it a setpoint control with discrete outputs that you want to add deadband to?

 

Joe T.

I have some legacy jazz ladder code I inherited that must be modified. The code reads a temperature sensor and controls a heating element to reach the setpoint. The temperature sensor jitters around the setpoint causing excessive on/off switching of the heater. Because I'm new to ladder programming I figured it would be easiest to simply add some hysteresis (deadband) directly at the sensor reading instead of trying to fiddle with the rest of the code.

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Yes - the chattering output problem.  You need to disable your output until it falls a certain number of degrees below your setpoint.

 

If you post your code I can hack an example into it with comments, circles and arrows, and a paragraph on the back.

 

As far as your approach to programming the PLC, if you are schooled in other programming languages think of the PLC as a huge collection of If.. Then statements.  While there are a ton of function blocks for special features, the majority of programming is done with about 10 symbols that allow you to examine an manipulate boolean and signed integer variables.  The names are already set for you.

 

Joe T.

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Ok, thank you. I guess I have to go thru the code and actually understand what's going on. I was hoping there's a hysteresis option on the input similar to the filter one can enable. But if there isn't I'll just have to dive in and get it over with. *smile*.

 

Thank you

Daniel

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