Safety First: Collaborative Robotics

is where it is supposedly safe for a person to work with and in close proximity to a robot arm. However this is illusionary.
Risks should still be assessed.

Newtons First Law

states an object will not change its motion unless a force acts on it. In the robot case the force is the reaction force from whatever the robot hits, be it solid or human.
For a collaborative robot as soon as an impact is detected the robot stops but it can not stop dead. The kinetic energy must be absorbed by the object being impacted even when that inertia is being arrested by the system controlling the robot.
When an ST robot hits a stationary object the motors come out of synchronism and the robot stops with an error on the screen. All further motion is canceled. However it does not stop dead; no robot can.

Example impact forces

Energy recovery

With energy recovery option selected the robot will immediately back off from a collision, partly meeting the PFL (power and force limiting) requirement of ISO 10218.

ST Robotics Workspace Sentry: collaborative robotics safety system

The safest robot system is one where the robot stops before an impact

ST Robotics have developed an IR ToF (Time of Flight) sensor called Sentry that sends 3 beams of IR pulses across the workspace in front of the robot. When the beams are interrupted the robot stops at maximum deceleration. Sensors can be strategically positioned around the shared workspace. When the user breaks any of the beams the robot stops. This is considerably safer than a robot that simply stops on contact.
Video: ST Workspace Sentry in action Data sheet: download
Sentry is an additional safety feature and is not essential for working with an R12.

However, a risk assessment should always be carried out.

How we measure impact of a collision between an R12 robot and a human arm.

We make the assumption that unlike in this video the human arm in question is not able to be bumped out of the way but may be hypothetically jammed against a solid surface, for example between robot and bench.
1. We set up a force plate and programmed a robot position in contact with the plate.
2. We coated the plate with a layer of foam of approximate density and compliance as human muscle in a human arm.
(note that there is not much of it between the skin and bone)
3. We then sent the robot to the programmed position which it could not reach due to the layer of foam. The force recorded was 6.2Kg
4. We then used a volunteer wearing a jacket with his forearm resting on the force place and sent the robot to a position below his arm. The robot stalled with a force of approx 6Kg.