Robot Hands and Grippers

Grippers, hands and the space between

End effectors range from two-finger parallel grippers and suction cups to three-finger adaptive grippers and five-finger humanoid hands. Parallel grippers cost less and are simpler to control, and vacuum gripping, from specialists such as Schmalz, handles boxes and flat items quickly. Multi-fingered hands can hold irregular objects, use tools and shift items within the grasp, but they add motors, sensors and possible points of failure. ISO 14539:2000 provides a shared vocabulary for grasp-type grippers and sets out how their characteristics should be presented, which helps when you compare datasheets from different makers. Some humanoids accept interchangeable end effectors, so ask whether the hand can be swapped for a gripper when the job suits it.

Why human-like hands are hard

A human hand packs many joints, dense touch receptors and tendons driven largely from the forearm into a small, tough package that lasts for decades. Copying that in metal and plastic means many small actuators, cables or linkages squeezed into a tight volume, each one a possible point of wear. In their Annual Reviews survey 'A Century of Robotic Hands', Piazza, Grioli, Catalano and Bicchi trace a hundred years of designs and identify two major trends: simplified actuation schemes, and soft materials and structures. Underactuated hands, in which one motor drives several joints that adapt to an object's shape, are one result. Ask makers how a hand's dexterity was measured and how often it needs servicing.

Touch sensing in the fingers

Vision alone struggles at the moment of contact, because the hand hides the object from the cameras. Tactile sensors fill that gap by reporting pressure, slip and shape where fingers meet a surface, and camera-based designs are a common research approach. MIT's GelSight Svelte, developed by Alan Zhao and Edward Adelson, places a camera at the base of a finger-shaped sensor and uses mirrors to view the flexing silicone skin along the whole finger, so the robot can feel power grasps that use the full finger as well as fingertip pinches. When evaluating a hand, ask whether tactile data is available to your own software and at what rate, not only whether sensors are fitted.

Choosing an end effector for the task

Start from the objects, not the hand. List what the robot must hold, with weights, sizes, surfaces and how gently each must be handled, then test candidates against that list. If every item is a box or a tote, a gripper or suction tool is often the simpler choice. If the work involves tools, cloth, cables or mixed shapes, a multi-fingered hand may earn its extra complexity. Check grip force limits for work near people, how the hand behaves when power is cut while it is holding something, and whether the maker publishes grasp test results. Run the same grasp trials on every candidate so the results are comparable.

Sources and further reading

• A Century of Robotic Hands — Annual Review of Control, Robotics, and Autonomous Systems.
  Peer-reviewed history of robot hand design, tracing the move to simpler actuation and soft materials and structures.
• ISO 14539:2000 Manipulating industrial robots — Object handling with grasp-type grippers — Vocabulary and presentation of characteristics — International Organization for Standardization (ISO).
  Shared terms for grasp-type grippers and a standard way to present their characteristics, useful when reading datasheets.
• Finger-shaped sensor enables more dexterous robots — MIT News.
  MIT's GelSight Svelte uses one camera and mirrors to give a robot finger touch sensing along its whole length.

Common questions

Can robot hands be replaced or repaired on site?

It depends on the design. Some hands detach at a standard wrist flange and can be swapped quickly, while others are built into the arm and need factory service. Ask about spare fingers and sensor pads, repair turnaround and whether keeping a spare hand in stock makes sense.

How much weight can a humanoid hand lift?

It varies widely between models and is often lower at the fingertips than at the wrist. Check the maker's stated payload for the hand and the arm separately, and test with your heaviest real object, since grip depends on friction and shape as well as motor strength.