Micrologix 1100 and Analog Input and Output

I am trying to put together a program for a control valve and scale combination. I will be using a ML1100 PLC, a 1762-IF4, and a 1762-OF4. I will have a Mettler Toledo IND560 scale head with a 4-20mA output module. Will send the 4-20mA signal to the 1762-IF4 input module to get my reading from the scale. I will have a HMI touchscreen so the operator can enter the amount of product to add to the tank. This number will go into the PLC and subtract from the scale value to give you the stop amount to get to. I will be sending a 4-20mA signal out to a control valve to control the flow of the liquid. Will be using PID to close the valve the closer you get to setpoint. I have never used Allen Bradley analog output modules before so I am not sure about the scaling on them. Does anyone have an example of something like this? Or an example of an analog output ladder diagram? Any help would be greatly appreciated.

Both the Analog Input and Output Card can be Scaled for PID in their configuration. That is the way I would go.

In the PID, I would do the Scaling for Engineering Units.

Stu....

Thanks, I am still learning the AB language. I have been working on DCS systems for te last 10 years and not much with PLC's. I know how to scale on a DCS but not sure on PLC's. More explanation would be appreciated.

rusty152 said:

Thanks, I am still learning the AB language. I have been working on DCS systems for te last 10 years and not much with PLC's. I know how to scale on a DCS but not sure on PLC's. More explanation would be appreciated.

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I am on my way out the door, but I am sure others will chime in to help you out. A good place to start is here;

http://literature.rockwellautomation.com/idc/groups/public/documents/webassets/browse_category.hcst


Stu....

Look at the "SCP" instruction. Scaled with Parameters. See pdf below.

Also see pdf below for the data format values for the cards you have. The values will depend on how you have configured the card.

If all you want to do is close a valve at setpoint then why a PID? Just use some compare instruction and close the valve when it gets to your setpoint value.

I could definitely do it with compare statements and just close the valve. Was trying to learn how to do the PID loops in the AB PLC's. I figured this would be an easy one to learn on.

Thanks Mickey

That module is configurable so make sure it is set up offline then download to the PLC for analog or voltage or your SCP will be off.

Mickey said:

Look at the "SCP" instruction. Scaled with Parameters. See pdf below.

Also see pdf below for the data format values for the cards you have. The values will depend on how you have configured the card.

If all you want to do is close a valve at setpoint then why a PID? Just use some compare instruction and close the valve when it gets to your setpoint value.

Click to expand...

IrishJerk said:

That module is configurable so make sure it is set up offline then download to the PLC for analog or voltage or your SCP will be off.

Click to expand...

Configurable by software and dip switches.

The PID algorithm is really intended for continuous process control, not the type of filling operation you are employing. I suggest instead you look at a ramp function type of control. When the fill level gets close to the final value start closing the valve to reduce the fill rate, and close the valve completely on reaching target. The easiest way to accomplish that would probably be to use several steps of position at several values of fill, but you could also do it as a continuous ramp.