PLC Online Training

scameron81 said:

Here's a lot of good information on general plc logic development and programming. Its geared towards the contrologix platform but the principles are all the same. I know AB has a bunch of sample programs that they have made available for people to use in getting started. I don't personally love the connected components software and the software is really bloated, but thats just me.

http://engineeronadisk.com/book_plcs/

What exactly do you mean by differential temperature controller? Are you trying to do a PID style control so that the vfd will vary its speed depending on the temperature?

Click to expand...

Unfortunately I am stuck using the Allen Bradley and there is no money for the RS Logic software.

We control the VFDs with a Newport differential temperature controller based on the differential temperature on a heating loop.

There are times that we run into loops that are used for heating and cooling based on the season.
On these loops the supply temp is higher than the return while heating but when cooling the supply temp is lower than the return.

What I am trying to design is a controller that I could use on these types of heating/cooling loops.

The Newport shows a negative differential when the season changes and I can't find a controller that will show a positive differential temp in heating or cooling mode.

I have tried to use a relay to swap the temp probes but I don't get the same temps once the probes are swapped. Either I am getting loss in the repays or something else.

If I use a PLC I should be able to do what I need along with integrating other controls.

I need some PLC programming basics, like analog inputs and outputs.

This is really more of a process control issue than a PLC programming issue. You need to understand some of the basics of process control to accomplish what you're trying to do. Basically what you need is a indirect acting PID during heating mode and a direct acting PID when in cooling mode.

In a classic control loop you have three components:

the PV = Process Variable the thing that you are trying to control
the SP = The set point which you want the PV to be at.
the MV = The manipulated variable. The output you change to make the PV match the SP.

In direct mode as the PV increases the so does the MV. This would be cooling mode, as the temperature increases you decrease the amount of heating water going to the process.

In indirect mode as the PV increase the MV decreases. This would be heating mode, as the temperature increase you decreases the amount of heating water going to the process.

This can be achieved a couple of different ways. You could use two PID's each configured for direct and inverse and then select which one feeds the VFD using a selector on the HMI, or you could use a single PID and invert the output based on the selected mode.

Something like this would be best programmed using function block which the connected components software supports.

For more information on process control and PID's in general check out www.controlguru.com


If you want more help trying to get this setup let me know.