Using ML1100 w/ analog output and analog input module

[dakota152 0]

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.

[OkiePC 14]

I question the need for a PID. I think you simply need to find the error (desired minus actual) and scale that to your control valve. You may also want to set it up so that it completely shuts off early and allows time for the contents between the valve and the scale to settle before taking a final reading on the scale. In most cases, the scale will gain weight for a period of time after the flow shuts off. This time period is usually fairly constant. It might only be a fraction of a second, or several seconds depending on the physical arrangement. With the Micrologix, the inputs and outputs from the analog modules should appear as input and output words in the data table. There are often different scaling options available to determine the raw values that show up in hte data tables and how they relate to the analog signals. I don't have access to manuals for those cards at home, so I can't be more specific.

[Ken Moore 23]

The control valve is a waste of money in this application. You would be better served with a large fill valve and a trickle valve. Is this an existing system or one in the design phase?

[TimWilborne 108]

Also you only mention one input and one output point. If this is so you can save a little money with a 1762-IF2OF2.