Grabbing oblong objects

[biojae]

I have a grabber mechanism that can pickup oblong objects.
How can I program it so that the grabber can sense when it has an object in its grasp, and change the setpoint of the closed loop controller to keep a gentle pressure on the object?

The only sensor that this mechanism has is a potentiometer that is used for position control.

The orientation of the object is not known by the robot beforehand, it has to be able to determine it only via the potentiometer.

I have tried using the first derivative of the potentiometer's values to determine if the grabber was moving or not, but that was very unstable.