Analog problem

I have just had a curve ball thrown at me.

I have a ML 1100 setup that already had 3 I/O cards attached to it. Since I am nearly completed with the system design, I now find that I need to add 3 analog sensors. the analog signals are to be relayed through the PLC and outputted to the analog inputs of several VFDs. I am pretty sure that the purpose of running the signals through the plc like this (it is how the customer would like it done) is simply to interupt the signals on demand, i.e. I don't want the vfd to run at the moment, so make the signal 0. Since the combo analog card will only give me 2 in and 2 out, that will not work, and I already have 3 cards so I can not add the 4 in and the 4 out cards. I was thinking that I could get "creative" and wire the analog signals through relays and then use the PLC to switch the relays. However I am a bit skeptical as to how that will work, I would suspect that the relays would not give me a good signal on the back side.

Does anyone have any ideas? Or has anyone ever tried running an analog signal through a relay? Did it work?

Thanks,
Greg

never tried that but i would be willing to bet it would work on a realy type output card!

or how about just run the ana inputs right to the drive, and i would be willing to make another bet the VFD's could take a digital input to set them in run mode!?!?


maybe you should ask the client what the idea is all about!

Yea, I am trying to get some more info out of them. I am hoping they doing come back and tell me the signal needs to be scaled based on some funky math or something.

Definitely see about just running the signals straight to the drive. Many drives you can scale the analogs to compensate for any scaling issues. You really don't want to send a 0 speed reference for the purpose of stopping a drive.