Here's a narrative of a 'gripper hand' done about 30 years ago - it worked - and could pick up a raw egg or 1kg lead weight.
Two jaws closed by a motor (may now be able to use servos?) on a vertical shaft (for lifting)
Inside of the jaws clad in rubber (to improve grip)
One (each?) jaw contained a small wheel, with rubber around the diameter, at 90 degrees to the gripper and protruding slightly.
A touch sensor - this was built into the wheel above.
So the gripper would continue to close until the touch sensor indicated contact.
The gripper would then start to move up the shaft (ie lift the object)
Since the grip would initially be too light then the object would slip, causing the small wheel to rotate.
Use a feedback (PID?) system to tighten the jaws until the slip is zero. (ie the egg gets less grip, and the 1kg weight gets more)
The applied grip is 'just enough' to hold the object without slip.
The encoder for the wheel was done as follows:
1. The wheel protruding from the clamps had a number of slits. Its movement was generated by the slip of the object.
2. The wheel had 'another wheel' behind it that was fixed (couldn't rotate) and also had slits.
3. The slits of wheel 1 across wheel 2 caused an interference pattern that could be monitored to detect movement and this was the feedback signal. ie the more slip then the more it caused the jaws to tighten until the slip became zero.