Physical Human-Robot Interaction

• Also called: pHRI
• Category: Control › Human in the loop
• Tags: human-robot-interaction, safety, contact, force-control, assistive-robotics

Interaction in which a robot and a person exchange forces through direct or indirect physical contact, whether planned, as in guiding or assisting, or unplanned, as in a collision.

Definition

Physical human-robot interaction (pHRI) covers every situation in which a robot and a person exchange forces, either by touching directly or through a shared object. The contact can be intended, as when a worker guides a collaborative arm by hand, a rehabilitation robot moves a patient's limb, an exoskeleton assists walking, or a care robot brushes hair or helps someone dress. It can also be unintended, as when a moving arm collides with a person or traps a hand. A pHRI system has to handle both: it must do useful work through contact while keeping the forces and pressures on the body within safe and comfortable limits.

The field draws on several layers of engineering. Mechanical design reduces the harm a robot can do on contact, through low moving mass, padding, rounded edges, backdrivable drives and passive compliance such as series elastic actuators or soft fingers. Sensing tells the robot that contact is happening and how strong it is, using force-torque sensors, joint torque sensors, tactile skins or estimates from motor currents and momentum observers. Control decides how the robot responds: impedance and admittance controllers make the robot behave like a spring and damper rather than a stiff position servo, and collision detection and reaction strategies stop, retract or switch to a compliant mode when an unexpected contact is detected. Industrial robot safety standards such as ISO 10218, with their guidance for collaborative operation, describe modes such as hand guiding and power and force limiting that are built on these ideas.

pHRI is broader than any single technique on this site. A cobot is one type of robot designed for it, and impedance control is one control method commonly used for it; pHRI is the problem area that includes both, along with injury biomechanics, user comfort and trust. Contact with sensitive regions such as the head and face, as in hair care or feeding, raises the bar further, because acceptable forces are lower and the person's own motion is harder to predict.

Why it matters

More robots are leaving fenced cells to work next to and on people: in factories, rehabilitation clinics, homes and care settings. Any task that requires touching a person, or that risks touching one by accident, needs the robot's mechanics, sensing and control to be designed together for safe contact, and pHRI is the body of knowledge that does this.

Key applications

• Hand guiding and collaborative assembly with industrial arms
• Rehabilitation robots and powered exoskeletons
• Assistive care tasks such as feeding, dressing, bathing and hair care
• Collision detection and reaction on manipulators working near people
• Haptic teleoperation and shared control with physical feedback

In practice

A team builds a robot that combs and detangles hair. They mount soft, compliant fingers on a lightweight arm so that a missed estimate of the head's position produces a gentle press rather than a hard impact. A wrist force-torque sensor measures contact forces, and an admittance controller lets the arm yield along the head's surface while keeping the comb pressed with a small target force. Software limits cap speed and force near the face, and the arm stops and backs away if the measured force exceeds a threshold or rises faster than expected. Before testing with people, the team checks forces on a mannequin head fitted with a load cell, then runs user sessions that record both measured forces and participants' reported comfort.

Common misconceptions

• pHRI only means avoiding collisions — it also covers intended contact where the robot must apply useful forces to a person
• A cobot makes any task safe for physical interaction — safety depends on the full application, including the tool, payload, speed and which body regions can be touched
• Soft hardware alone solves pHRI safety — compliance reduces impact forces, but sensing, control limits and risk assessment are still needed
• Measured contact force is the only quantity that matters — pressure, contact duration, body region and whether the person can move away all affect injury risk and comfort

Who should know this

• Engineers designing collaborative and assistive manipulators
• Rehabilitation and exoskeleton developers
• Control engineers implementing force, impedance and collision handling
• HRI researchers running studies that involve touch

Related technologies

• Admittance Control
• Momentum-Based Collision Detection
• Series Elastic Actuators
• Tactile Skins
• Power and Force Limiting

Related terms

• Cobot
• Impedance Control
• Sensorless Force Estimation
• Protective Stop
• Pinch Point
• Perceived Safety

Related projects

• Drake: MIT robotics toolbox for model-based dynamics, optimization, and control system design
• Reachy Mini: Open-source desktop robot from Pollen and Hugging Face with an expressive 6-DoF head
• RAI: Vendor-agnostic agent framework for ROS 2 robots and physical AI
• NASA Ogma: NASA runtime monitor generator for safe robotics and flight systems
• Assistive Gym: Physics simulation framework for training assistive robots in caregiving tasks