A Statistical Model to Determine Biomechanical Limits for Physically Safe Interactions With Collaborative Robots

Abstract

Collaborative robots (cobots) provide a wide range of opportunities to improve the ergonomics and efficiency of manual work stations. ISO/TS 15066 defines power and force limiting (PFL) as one of four safeguarding modes for these robots. PFL specifies biomechanical limits for hazardous impacts and pinching contacts that a cobot must not exceed to protect humans from serious injuries. Most of the limits in ISO/TS 15066 are preliminary, since they are based on unverified data from a literature survey. This article presents a human-subject study that provides new and experimentally verified limits for biomechanically safe interactions between humans and cobots. The new limits are specifically tailored to impact and pinching transferred through blunt and semi-sharp surfaces as they can occur in the event of human error or technical failures. Altogether 112 subjects participated in the study and were subjected to tests with emulated impact and pinching loads at 28 different body locations. During the experiments, the contact force was gradually increased until the load evoked a slightly painful feeling on the subject’s body location under test. The results confirm that the pain thresholds of males and females are different in specific body regions. Therefore, when defining biomechanical limits, the gender difference must be taken into account. A regression model was utilized to incorporate the gender effect as a covariate into a conventional statistical distribution model that can be used to calculate individual limits, precisely fitted to a specific percentile of a mixed group of male and female workers which interacting with cobots.

Experimental Comparison of Biofidel Measuring Devices Used for the Validation of Collaborative Robotics Applications

“…If the collision area is semi-sharp, a second safety mechanism is important. The specific contact pressure needs to be observed to avoid local trauma to the human skin [ 1 , 2 , 3 , 4 , 26 , 27 ]. However, the influences of the pressure measuring equipment are not part of this investigation.…”

“…On the other hand, measuring devices are developed to test if a robot meets these limits. Both activities have been driven, for more than 10 years, by the IFA working together with different partners from industry, research, and OSH organisations [ 1 , 2 , 3 , 4 ].…”

“…Behrens distinguishes between quasi-static and dynamic (transient) contact , between constrained and unconstrained situations, and between blunt and semi-sharp surfaces . He investigates up to 28 different body parts as part of a volunteer study [ 1 , 2 ]. In terms of pain onset, the hand area is the most hardened to endure a collision.…”

Safety first in rehabilitation robots! Investigating how safety-related physical human-robot interaction ca be assessed

“…There is very little knowledge on magnitudes and durations of forces that are acceptable for rehabilitation robot use. Previous research focused on pain pressure thresholds for incidental contacts [225], timestress relationships for sliding contacts [116], and discomfort and pain thresholds in pneumatic circumferential pressure [182], [183]. The repetitive force application through a (semi-)rigid interface over long durations which is characteristic for rehabilitation robot use had not yet been investigated in terms of comfort thresholds.…”

“…Due to the use of (semi-)rigid orthoses in exoskeletons, the interaction is different from that in exosuit use. 12 Discomfort detection threshold for ramp inflation of pneumatic cuff [183] 32 Discomfort detection threshold for staircase inflation of pneumatic cuff [183] 38 Averaged thigh pressure during walking with REX exoskeleton [40] 36 Pain detection threshold for ramp inflation of pneumatic cuff [183] 61 Pain detection threshold for staircase inflation of pneumatic cuff [183] 133 Sit-to-stand peak pressure in REX exoskeleton [40] 480 Pain pressure threshold thigh [180] 700-2500 Pain/safety threshold quasi-static impact thigh muscle [146], [225] Some research has been performed on safe limit values for continuous sliding contacts. The relationship of shear stress and rubbing time beyond which blisters are generated has been described for durations of up to about 30 minutes [102], [184] and was adopted in a standardization document for physical assistant robots [147].…”