I would like to have a claw or similar with an adjustable force, that does not try to crush whatever it should pick up. Does something like this exist already?
Of several approaches the following one looks most promising to me:
The claws are not immediately moved by an servo, as is, instead a servo stretches a spring connected to the claws. The applied force is measured by a strain gauge/sensor, or simply by measuring the motor current. With an self-locking gear mechanism no current is drawn while something is picked up, only while picking or releasing an object. In the "open" position the servo will not only release the spring, but also should open the claw.