Pinch Point

• Also called: crush point, trapping point
• Category: Control › Human in the loop
• Tags: safety, risk-assessment, collaborative-robots, machine-safety, workcell-design

A place where part of a person's body can be caught between two parts of a robot, or between the robot and a fixed object, as the gap between them closes.

Definition

A pinch point is any location where a closing gap can trap part of a person's body, most often fingers or a hand. On a robot it can form between two of its own links, for example where the elbow folds tightly or where the wrist approaches the forearm, between gripper fingers, or between the robot and a fixed object such as a table, fixture, wall or conveyor. Machinery safety uses several related terms: crushing or clamping hazards where a body part is squeezed, shearing hazards where two edges slide past each other, and nip points where rotating parts draw material in. Pinch point is the everyday term covering the trapping cases.

Pinch points are especially important for robots that work near people, because a contact that would be harmless in open space can become dangerous when the body part has nowhere to go. Guidance on collaborative robots therefore distinguishes transient contact, where a person can recoil, from quasi-static or clamping contact, where the person is pressed between robot and object, and treats clamping more strictly. General machinery safety standards specify minimum gaps that avoid crushing particular body parts, so one design approach is to keep openings either large enough not to crush or too small for fingers to enter.

Pinch points depend on both mechanical design and motion. Designers reduce them with link shapes and covers that prevent tight folding, limited joint ranges and padding. Programmers reduce them by keeping trajectories away from fixed surfaces, avoiding poses where links fold onto each other near a person, slowing down where gaps close, and ensuring that a stopped robot can be moved or backdriven to free a trapped hand. Automated checks on robot programs can flag poses or paths in which link-to-link or link-to-environment distances fall below a threshold.

Why it matters

Pinch points are among the most common hazards identified in risk assessments of robot cells, and they remain a hazard on force-limited collaborative arms because clamping can produce high pressure even at low force. Spotting them in both the hardware and the program is a basic skill for anyone deploying robots near people.

Key applications

• Risk assessment of robot cells and collaborative workstations
• Mechanical design of links, covers and grippers
• Checking robot programs and trajectories for trapping hazards
• Placing fixtures and work surfaces around a collaborative arm
• Training operators on where to keep hands during robot motion

In practice

A technician uses a natural-language interface to generate a pick-and-place program for a collaborative arm on a workbench. Before running it, a checking tool steps through the planned trajectory in simulation and computes the distance between the forearm and the upper arm, and between the gripper and the bench surface. It finds a segment where the elbow folds sharply while the gripper passes a few centimeters above the bench next to where the operator's hands rest. The tool flags it as a pinch point; the program is revised to approach from above with a more open elbow and to slow down in the final approach, and the check is rerun before the program is approved.

Common misconceptions

• Force-limited collaborative robots cannot cause pinching injuries — when a hand is clamped against a surface, even moderate force concentrated on a small area can be harmful
• Pinch points only exist between the robot and its surroundings — robot links, wrist joints and gripper fingers can trap fingers on their own
• A pinch point is fixed by the hardware — the same robot can create or avoid pinch points depending on how it is programmed and where fixtures are placed

Who should know this

• Robot cell integrators and safety engineers
• Robot programmers and application engineers
• Mechanical designers of arms, grippers and covers
• Researchers building automated program-checking tools

Related technologies

• Risk Assessment
• Collision Checking
• Minimum Distance Computation
• Power and Force Limiting
• Safety Laser Scanners

Related terms

• Cobot
• Protective Stop
• Physical Human-Robot Interaction
• Workspace
• End-User Robot Programming
• Gripper

Related projects

• NASA Ogma: NASA runtime monitor generator for safe robotics and flight systems