Capacitive Feedback??

[Bluenoser337]

Hard Question Coming Up...maybe...Have a slide valve position feedback device on a screw compressor. It is a STAL compressor, about 30 years old. The device is a capacitive 3-wire unit wired back to the ASEA-STAL controller. Want to tie this capacitive unit into a new PLC control but don't know what interface device to use...maybe the feedback device could be replaced...but with what??

[Andy]

Hi, Bluenoser, capacitive feed back is also used by Refrigeration Specialities on their vessel level controllers and also by Danfoss on their Adapt cool level controllers, perhaps a Danfoss controller could be made to give out a 4-20ma signal to the PLC. Halls compressors have a thing called a LVDT (liner voltage differential transfomer) on their slides to give feed back, I'am inclined to think that this uses a capactive rod to give the signal to the LVDT control board. Halls also make a thing called a Fridge Watch 2000, this acepts a signal from the LVDT again 4-20ma.
Danfoss have a web site. www.danfoss.com Halls also have a web site j&ehall.co.uk I think is right in both cases.
Regards. Andy.

[Andy]

Hi, Bluenoser, J&E Halls site is jehall.co.uk
Regards. Andy

[Andy]

Hi, Bluenoser
I took a quick look at the Danfoss site, the wiring on their capacitive probes is four wire, you mention that the feed back on the stall screw is three wire. Would that mean that the negative feed in has been commoned with the negative feed out and that moving the screw slide causes a change in the circuit voltage on the output (between positive output and negative). If you only require a feed back to make contacts at say minimum position for starting, intermediate for bringing in economiser/intercooler valves and max to start another screw, would an off the shelf voltage sensing relay not suffice.
You could always just fit micro switches for you position switches.
Let us know how you get on.
Regards. Andy.