I am not sure how one would control tension with an electric motor.
I saw an old prince machine which used a pneumatic cylinder. It seems like it would be much simpler design, and better amenable to controlling tension:
motor pumps the air --> the air reaches required pressure --> pulls the string. There are pressure relief valves. They will let excessive pressure off the cylinder. The ones I've seen were pre-set at manufacturing. I am not sure if adjustable pressure valves exist.
Alternatively, you can buy a tire pressure guage, overpump a bit, then open the valve until the pressue is just right -- similar to what you'll do when pumping a tire on a car. You can call overpumping a "pre-stretch" ( a sofisticated feature that only 3K+ machines provide).
Another alternative: get a reasonably tight spring. Calibrate its elongation under various tensions, say 50 lbs = + 1 in, 60 lbs = + 2 in.
Then attach string -> spring -> motor, and a ruler underneath. The elongation of the string will tell you the tension. If your motor is slow enough, so you can move in tiny steps.