Rockwell software put out a VB demo that I used to learn OPC with RSLinx, but now I can't find it on their own website. It was published under tech note A7347.
You could have a local machine shop make you a special sprocket with larger arced teeth so that they will be equal distance from the prox while turning. You could have the inner collor the same dimension as the shaft and the outer dimension the same as the spocket; no great modifications...
Look up the tech note A7374 on http://www.software.rockwell.com/index.cfm
in the knowledgebase. It gives a visual basic example that is very good and has a link to the OPC Founation where you can download the OPC Specifications. Also look up the keyword OPC and you will get more information.
