My suggestion is that you take a look at Greg Needel's presentation on designing competitive manipulators, which can be found
here if you scroll down about half way to where it says Needel. The presentation was quite good when i sat in on it at nationals and has a few ideas that should be helpful on how to pull off what was posted by Joe and Bilfred above. Also i suggest that you look through the rest of it because it has some interesting solutions for some other things that i have a feeling that you will need for whatever you are building. If you have any other questions on specifics Greg is reachable through these forums and myself and many others here either heard the presentation or know its contents and can help you if need.
also
this might give you some ideas
-Simon