Hybrid Position/Force Control

• Also called: hybrid force/position control, hybrid force-position control, hybrid control
• Category: Control › Force & interaction control
• Tags: force-control, manipulation, contact-tasks, industrial-robotics, control

A manipulator control scheme that splits task space into directions controlled for position and orthogonal directions controlled for force, so a robot can move along a surface while pressing on it.

Definition

Hybrid position/force control is a method for controlling a robot in contact with its environment by dividing the task into directions where motion is controlled and directions where contact force is controlled. It was formalized by Raibert and Craig in 1981, building on Mason's analysis of natural and artificial constraints published the same year.

The key observation is that a contact constrains motion in some directions and force in others. When a robot slides a tool across a table, it cannot move into the table, so position control in that direction makes little sense, but it can choose how hard to press. Parallel to the table it is free to move but cannot build up a useful force against nothing. Hybrid control therefore defines a constraint frame at the contact and uses a selection matrix to assign each axis, translational or rotational, to either a position controller or a force controller.

The position loop tracks a trajectory along the free directions, and the force loop, typically using measurements from a wrist force/torque sensor, regulates the contact force along the constrained directions. Their outputs are combined into joint commands. The way the two subspaces are combined matters: the original formulation was later shown to be unstable for some revolute manipulators, which led to corrected formulations, including ones based on the operational space framework.

Hybrid control differs from impedance control, which does not separate directions but makes the robot behave like a spring-damper, trading position accuracy for compliance. Hybrid control gives explicit force regulation where it is needed, but it requires a fairly accurate model of the contact geometry; if the surface is not where, or not oriented as, the controller assumes, the selected directions are wrong.

Why it matters

Many industrial and service tasks, such as polishing, grinding, wiping and assembly, require a robot to control how hard it presses while moving along a surface. Hybrid position/force control gives a direct way to specify those tasks, and its ideas of constraint frames and selection matrices still shape how contact tasks are programmed.

Key applications

• Polishing, grinding and deburring
• Surface wiping and cleaning
• Peg-in-hole insertion and assembly
• Contour following and surface inspection
• Mobile and legged manipulators pushing or pressing on objects

In practice

A team setting up a robotic polishing cell defines a task frame aligned with the workpiece surface, assigns the surface-normal axis to force control with a target contact force, and assigns the tangential axes and the orientations to position control along the polishing path. The force loop reads a wrist-mounted force/torque sensor, usually filtered to reduce noise, and many industrial controllers offer this as a force-control mode in which the user picks which axes are force-controlled. Engineers tune the force gains conservatively, test on the real part with low target forces first, and watch for chatter or loss of contact at edges and corners, where the assumed surface normal changes.

Common misconceptions

• A robot can control position and force along the same direction at once — along any one axis only one of the two can be specified; the environment determines the other
• Hybrid control and impedance control are the same thing — hybrid control explicitly regulates force along chosen axes, while impedance control shapes a spring-damper relationship between force and motion
• Force control needs no model of the environment — hybrid control depends on knowing the contact geometry to choose the right axes

Who should know this

• Industrial robot programmers working on contact tasks
• Manipulation and control engineers
• Researchers working on mobile and legged manipulation
• Robotics students studying force control

Related technologies

• Force/Torque Sensor
• Impedance Control
• Admittance Control
• Operational Space Control
• Selection Matrix
• Compliance Control

Related terms

• Impedance Control
• Force/Torque Sensor
• Torque Control

Related projects

• LeRobot: HuggingFace ML library for real-world robotics in PyTorch
• Isaac Lab: NVIDIA robot learning framework: one task API across Newton, PhysX and Isaac Sim
• NVIDIA Isaac GR00T: NVIDIA open foundation model for humanoid robot reasoning and skills
• OpenVLA: 7B-parameter Vision-Language-Action model for robotic manipulation
• Drake: MIT robotics toolbox for model-based dynamics, optimization, and control system design
• OpenArm: 7DOF humanoid arm for physical AI research in contact-rich environments

Videos

• Modern Robotics, Chapter 11.6: Hybrid Motion-Force Control: Northwestern Robotics