Hardware & control
Force closure
Force closure is a property of a set of contacts whose admissible forces can collectively generate a wrench opposing any external disturbance wrench under the chosen contact model. It describes directional force and moment capability without, by itself, accounting for finite actuator strength, object damage or uncertainty in the contacts.
Also known as: Force-closure grasp, Force closure grasp
Updated
Contacts must resist moments as well as forces
A wrench combines force and moment. Each contact contributes an admissible wrench set determined by its location, normal and friction model. Force closure requires the combined set to span the entire wrench space with nonnegative contact-force combinations. Modern Robotics gives this condition and its computational test.
Checking whether a grasp can support gravity in one orientation is a narrower question. That tests a particular load, not whether the contact arrangement can oppose disturbances in every wrench direction.
The friction cone constrains each contact's force. Contact locations then determine the moments those forces can generate about the object. Consequently, adding squeeze force cannot compensate for every unfavourable contact arrangement.
The contact model changes the answer
Two ideal point contacts on a spatial object cannot resist a moment about the line joining them. Soft fingertips can instead form contact patches that support a frictional moment about the contact normal. Modern Robotics explains why two soft-finger contacts can permit force closure where two point contacts cannot.
Force closure is also distinct from form closure, which concerns geometric immobilisation by contacts. Frictionless force closure coincides with first-order form closure in the source's formulation; frictional force closure can rely on tangential contact forces instead of geometry alone.
A grasp label is not a payload rating
The theoretical condition does not ensure that a hand's motors can produce the required contact forces. Modern Robotics explicitly warns that finger-joint strength can prevent a force-closure grasp from resisting a particular wrench.
For humanoid grasp data, record contact positions, normals, friction assumptions, point-versus-soft-finger modelling and the test used. Keep an analytical force-closure label separate from an observed successful lift or disturbance-rejection trial.
Finite force limits and uncertainty require additional evaluation. A binary closure label should not be used as evidence that a grasp is safe for a fragile object or reliable after the fingertips shift.
Sources
Related terms
Hardware & control
Friction cone
A friction cone is the set of contact-force vectors allowed by a Coulomb friction model: the normal force is nonnegative and the tangential force magnitude cannot exceed the friction coefficient times that normal force. It represents a force constraint at a contact, not a physical cone or a guarantee that the robot will not slip.
Hardware & control
Dexterous manipulation
Dexterous manipulation is the controlled, skillful reconfiguration of an object through coordinated motion and contact, often using multiple fingers. It can involve changing an object's pose within a hand, regrasping, sliding, rolling, finger gaiting, or making precise contact with the environment. The term describes capability, not a fixed minimum number of fingers or joints.
Hardware & control
Gripper
A gripper is a robot end effector designed to seize and hold an object. It may use fingers, jaws, suction, magnetism, adhesion, or another grasping mechanism. A gripper can open and close with one command or expose several independently controlled joints, but it is not synonymous with every end effector or with a complete robot hand.
Hardware & control
Tactile sensing
Tactile sensing is the detection and measurement of physical contact properties at a robot's surface or contact interface. Depending on the sensor, it can report pressure or force distribution, contact location, shear, vibration, slip, texture, temperature, or deformation. Tactile data complements vision by measuring interactions that may be hidden at the point of contact.
Hardware & control
Force–torque sensing
Force–torque sensing is the measurement of forces and rotational moments transmitted through a robot interface, commonly with a six-axis sensor reporting three force components and three torque components. The combined quantity is a wrench expressed at a specified origin and in a specified coordinate frame. It measures the resultant load at the sensor, not a spatial pressure map across every contact.