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There are actually two common requirements for ESD-safe operation:

  1. Don't build up static charges
  2. Safely dissipate charge that may have built up.
Grounding the gripper fingers and using conductive or dissipative material for the fingertips can indeed prevent charge from building up. This will prevent charge that may be generated by the robot and gripper system (friction between cables and hoses, for example) or that is transferred from outside (like from an ungrounded human) from building up on the gripper and later being transferred to the workpiece.

The larger issue is what happens when the workpiece already has some charge on it. Suddenly discharging this to ground is equally damaging as transferring charge the other way.  Workpiece charge can be created by ungrounded humans, poor packaging, and even waving the part around in dry air. When a well-grounded gripper then touches the workpiece, the sparks fly. Many electronics manufacturers therefore require that there be a controlled resistive path to ground (generally around 1-10 MΩ) for anything that touches the workpiece.

If you can guarantee that only dissipative material (like dissipative synthetic finger tips) touch the workpiece, and that this dissipative material has a hard path to earth ground on the other side, that can fulfill the requirements.  If you must use metal finger tips, you may have to insulate them from the gripper, and connect the fingertips to an earth ground through a resistor.

Some manufacturers can get quite exacting about having absolutely no pieces of insulative material anywhere near their parts. Hence the requests for dissipative plastic caps on the robot.  That may seem like overkill, but some applications specify a maximum static voltage of 50V or even 10V.  Just scuffling your feet can generate thousands of volts.

Just as with risk assessments, ESD protection is a negotiation guided by standards, the application, and common sense.