You can build something: Fast/Cheap/Accurate [or usable]. I'd choose two of the three. This undertaking is going to wind up consuming a LOT of money OR time OR brainpower to do the right way. If we're truly talking about being competitive with a WISE or similar, you'll need at bare minimum some electronics knowledge and/or preferably plenty of mechanical knowledge. If your greatest worry at the moment is to figure out how to secure string to a tensioner, I'd recommend choosing something else for a science project...
I'm not sure what education level you're currently at, but at the very least this will require high school-ish physics. Tool(ing) cost for this project can be very expensive if you don't know/have anyone available that is handy in a shop setting. Materials won't be super cheap if you're building out of metals (Aluminum is probably where you want to start with this).
If it were me: I'd re-create a drop weight system and/or even build something with a variable pivot point like the current SW/LF drop weight machines (any angle true-CP tensioner). This will be by far the most bang for your buck in terms of research needed. There are design documents archived SOMEWHERE, and the designer at SW is accessible and willing to talk technical.
If you really want to pursue an 'electric' tensioner, you will need to understand (at minimum) electrical fundamentals (ohms law, basic safety/wiring, power [wiring relays and basic power distribution]). I would personally go with something like a load cell or stress/strain gauge coupled with a simple microcontroller (Arduino is probably the easiest to get started with). This is probably the quickest/dirtiest way to get started, as the load cell should be roughly linear within the range of operation you're interested in (from what I gather). You should use proven designs as far as tension securing, linear grippers OR rotating drum head. No need to reinvent the wheel here unless you are trying to achieve something specific.
Electric motor feedback should be pretty simple (and cheap) and can probably be done with extremely cheap/passive components, but a motor heavy duty enough to achieve this is probably not extremely cheap. I would imagine with enough google-fu, you could probably pull up patent documents with prior art and/or design notes to blatantly rip off.
IMHO, though, this is probably a pretty overwhelming project unless you're well into high school, and you'd probably need to be pretty mathematically/scientifically talented and/or self-motivated to do your own research to pull this off. I'm not trying to discourage you at all, just realize that this is probably going to be more work than you thought from the get-go
If you're still excited about this project, I'm excited for you