I am building a tendon-driven gripper for handling soft produce. It will comprise of three silicone fingers, each actuated via a tendon. The three tendons are all pulled by a single stepper motor and lead screw.
I would like to have force control, but it must use an indirect method of sensing the force at the fingertips. This is because I don't want to have to mount a hard fsr on the fingertips as that would defeat the purpose of using soft silicone fingers - the hard sensor is what would be in contact with the delicate target and could cause damage.
I would therefore like to measure the tensile force in the tendons, and work out the force at the fingertips based on that. I am unsure how to go about this. Tensile sensors all seem to be very large and very expensive.
My current idea is to attach a spring to the end of each tendon. With a magnet on one end of the spring, a hall sensor on the other could measure the extension of the spring and I could then find out the force using F=kx?
Is this a good idea or is there something easier that I could do? Would it be possible to measure the current in the stepper motor and get a force from that like you can with a dc motor?
Thanks 
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