Brooks Automation 3. Collaborative Robot Safety
Part Number: PF40-DI-00010 Rev. A Robot Testing and Safety Circuits
2. Impact force in free space (Transient contact). This is the dynamic case of the robot colliding
with person where the person is free to recoil from the collision. In some cases the speed and inertia
of the person should be added to the speed and inertia of the robot. The inertia of the robot will
include the payload, the robot structure, and the forward reflected inertia of the motor and gear train,
which can be quite significant. Impact force is considered to be a transient force of short duration.
3. Impact force against a surface (Trapping). This is the case where the person or appendage is
trapped between the robot and a hard surface with the robot moving at speed. While this can be rare
for many applications given proper workcell design, it should be considered. High speed impacts
which trap an operator against a surface may be avoided by teaching an “Approach” position which
is a greater distance from a fixture than any operator appendage that might enter the workcell, and
moving to this “Approach” position at high speed, then moving to the final position at a rigid surface
at a slow speed which will not create excessive force in the event of a trapped operator.
4. Pressure, or force per unit area. 130N of force applied to a large area, for example 50 mm X 50
mm is quite different from this same force applied to a small area, for example 1 mm X 1 mm.
ISO/TS 15066 does not differentiate between clamping/squeezing force (low speed) and impact
forces against a rigid surface (high speed) and refers to both cases as “quasi-static” even though
they are quite different, as the high speed impact will include dynamic forces from the moving mass,
while the low speed clamping forces will be mainly due to motor torques.
Robot Testing and Safety Circuits
While some robots have six or even seven axes and can move in many directions, generally testing
can be done in the horizontal plane and in the vertical direction. Since gravity adds to the force in the
downwards vertical direction, and since in the horizontal plane forces are symmetric in opposite
directions, testing in +X, +Y, and –Z (downwards) is generally sufficient to characterize robot forces.
PreciseFlex
TM
uses a test stand, to which a certified force gauge can be attached in either the
vertical or horizontal direction, for testing forces. A “compliance plate” assembly is attached to the
robot to simulate the compliance of the human hand of 75 N/mm.
Clamping/squeezing force is measured by moving the robot slowly into the force gauge until the
robot reaches its maximum force and generates an error. Based on Table A2 from ISO/TS 15066
(Appendix H: Table A2 from ISO/TS 15066: 2016, Biomechanical Limits), Brooks has selected the
maximum clamping force (quasi-static) to be 140 N for a collaborative robot.
Transient impact force in free space is measured by moving the robot at its maximum permitted
speed and payload with the compliance plate impacting the force sensor when the force sensor is
held by a person in free space. Based on Table A2 from ISO/TS 15066, Brooks has selected the
maximum impact force (transient force) in free space to be 280 N for the hand and forearm and 130
N for the skull for a collaborative robot.
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