Imposing Motion Variability for Ergonomic Human-Robot Collaboration

Figures & data

Figure 1. Two handover policies to govern the collaborative robot’s behavior. The arrows indicate the Object Transfer Point (OTP) from one handover to the next, as well as the intended motion of the human subject’s center of mass (CoM) around their base of support (BoS).

Figure 2. Workers packaging and transferring boxes of seafood in a seafood processing facility.

Figure 3. Experimental setup. Top: an overview of how the collaborative robot exchanges packages with a user while human posture is tracked by an external camera. Bottom: a top-down perspective of the experiment workspace.

Table 1. Demographic information of the robot-to-human handover study participants. Entries are means (SD).

Gender	Age Range	Height (m)	Mass (kg)
Male (n = 10)	21 – 34	1.73 (0.13)	75.6 (11.8)
Female (n = 6)	20 – 23	1.66 (0.05)	58.7 (5.0)
All Subjects (n = 16)	20 – 34	1.71 (0.11)	69.6 (12.8)

Figure 4. Box plots of the mean squared velocity and jerk values of participants’ Center of Mass (CoM) per handover.

Table 2. Aggregate statistics per handover of participants’ center of mass (CoM) velocities, torso rotation angles, and jerk in movement.

Experiment Mode	CoM Velocity (m/s)	Max. Torso Rotation Angle (°)	CoM Jerk (m/s^3)
Assistive	0.34	63.54	$2.1\times {10}^6$
Stimulating	0.90	68.73	$7.4\times {10}^5$

Figure 5. Box plot of the balance stability margins averaged across participant handovers. Larger values signify a more stable posture configuration.

Table 3. Mean and standard deviations per handover of Rapid Entire Body Assessment (REBA) scores for all samples collected across the three experimental conditions.

Experiment Mode	REBA Score	Coefficient of Variation	% >Risk Limit (7)	Balance Stability Margin (cm)
Training	5.06 ± 0.67	13.2	9.02 ± 6.48	8.33
Assistive	5.51 ± 0.65	11.9	13.86 ± 5.24	8.44
Stimulating	5.25 ± 0.67	12.8	8.87 ± 6.54	8.91

Notes: A higher REBA score denotes a higher degree of risk. Any score above 7 is categorized as “high risk”, therefore the percentage of task time spent in this danger zone is also reported. Coefficient of variation values in the REBA metric and balance stability margins are also provided. Higher margins indicate better postural support.

Table 4. Subjective questionnaire responses, with total NASA-TLX scores (Hart & Staveland, 1988), preference, alertness, and frustration choices indicated by experiment mode.

Experiment Mode	NASA-TLX Total	Preferred?	More Alert?	More Frustrating?
Assistive	19.8	7	3	6
Stimulating	20.9	6	9	6
No Difference	Not reported	3	4	4

Figure 6. Illustration of the difference between two consecutive handover exchanges using the Assistive mode (top row) and Stimulating mode (bottom row). Less constrained motion is depicted when using the Stimulating mode, where the center of mass displacement is clearly of greater magnitude across the two selected handovers.

Hart, S. G., & Staveland, L. E. (1988). Development of NASA-TLX (Task Load Index): Results of empirical and theoretical research. In Advances in psychology (Vol. 52, pp. 139–183). North-Holland.

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