Isolating PLC analog Inputs/Outputs

Gday,

I'm having trouble with the analog input of my PLC (S7-314IFM).

I am currently trying to run a DC VSD using voltage following, connected to the PLC's analog output with +-10V, and then a tachogenerator is used to measure the speed. The voltage from the tacho is stepped down from 105V to 10V (voltage divider) and connected to the analog input of the PLC.

The PLC outputs the correct voltage and the motor follows it exactly, the problem is the input value fluctuates dramatically. Example if the speed is set to 1750RPM it would chnage from 300 to 550 to 287 to 1300 to whatever.

The problem was eventually found to be caused by the connection of the analog output of the PLC as the VSD terminals were not isolated from the AC source and in the VSD user manual it states that a Isolated voltage source is needed and that it should not be connected to ground (which it is when connected to the PLC).

So by disconnecting the voltage following terminals of the VSD and connecting them to a DC power supply, the readings of the analog input of the PLC was steady and accurate.

So now I have the background situation explained, my question is... How do I go about isolating either the analog input or output of the PLC, so that I can use both to control and monitor the DC motor?

I hope I have given enough detail, any help would be greatly appreciated.

You can use PLC Modules with channel by channel isolation, or just use a signal isolator or two.
Example Signal Isolator

Thanks for your reply, unfortunately as the setup is for my final year uni project the availability of more modules is pretty much impossible and I have a feeling the PLC I am using is not expandable (not sure) .

The signal isolators seem like they could work in my situation dont suppose you know of any that would work with voltage instead of current and that dont cost an arm and a leg.

Otherwise are there any other approaches, methods I could try.