AD’s 260 PID Control Output to turn Contactors

[Anibal Hernandez]

AD’s 260 PID Control Output to turn Contactors





Does anyone here have a suggestion on how to use the Control Output value of a PID to turn a discrete Contactor ON-OFF?



This is a 3 Compressors application in which the goal is to maintain a healthy 80 PSI pressure in the plant. One contactor per compressor.



I once saw a logic that used this Control Output value as the preset value of a Timer; I just cannot remember how it was done.



Thanks for your input.



Andy

 

[Eric Nelson]

I once saw a logic that used this Control Output value as the preset value of a Timer; I just cannot remember how it was done.

I'm not sure about what your trying to accomplish, but you can change a timer's preset value by using a V-memory location rather than a constant (K) value. Then it's simply a matter of moving the value to this V-memory location.

For example:

|                   +-----------+

|----] [------------| TMR  T10  |

|                   | V2000     |

|                   +-----------+

....

|                   +-----------+

|----] [--------+---| LD Vnnnn  | Value you want to use as preset

                |   +-----------+

                |   +-----------+

                +---| OUT V2000 |

                    +-----------+

-Eric

 

[elevmike]

Im getting that Andy want to start a compressor based on a low pressure reading mabey???

Likey using a compairative contact to compair the pressure you need with the pressure you've got. Start with page 5-27 of the D205 manual and read on from there...As a mater of fact look at all the compare instructions.

 

[Lancie1]

Andy,

I agree with Mike. You only need to use the AD 260 Comparative Contacts, Less Than Equal To, Not Equal, or Greater Than or Equal To. Use these to compare your Pressure with some known arbitrary constant value switching point. For example, "If Pressure >79 but < 82, run the Lead Compressor. If Pressure > 75 but < 80, run Lead and 1 Back-up. If Pressure < 76, run all compressors."

You CAN use the Control Output value of a PID block, if has Air pressure as the Process Variable input, but all your really need is a Pressure Input and the above Comparative contacts.

It takes a little experience to know how to set up 3 compressors so that they will share the load and wear evenly. It depends on the size of the compressors relative to how much air is needed. There are many ways to do it, but the most common is to alternate the lead, or main compressor, on a daily basis, with say #1 being the "lead" today, #2 and #3 helping out as the air demand increases and the pressure drops. Then tomorrow #2 becomes the lead, with #1 and #3 helping out as needed, and the third day, #3 becomes the lead, and so on.

Lancie

 

[Tom Jenkins]

The DL-205 User Manual has a section showing ow to use P-I-D control with discrete outputs - they call it Time-Proportioning Control.

I'm not sure that P-I-D control will give you what you want, though. Lancie's technique looks like a better bet, based on the info you provided.

 

[Anibal Hernandez]

Guys:

Thanks for all your answers. I found the logic I was referring to. See picture below.

It appears that %Q00023 is driving the relay or contactor that turns ON-OFF the Heaters for this Furnace. My guess is that the time that the relay is ON or OFF is based on:

PID CV times 100 divided by 32,000 times (whatever is in %R00196) divided by 100.

Can anyone explain this logic or perhaps tell us what kind of value does the variable %R00196 is expecting from the HMI.

Thanks all.



Andy