Determining ergonomic forms for rollable display devices
Authors: Lee Songil , Choi Donghee , Choi Hyeeun , Hwang Kitae , Park Seonghyeok , Kim Minjoong , Yi Jihhyeon , Kim Si Jung , Kyung* Gyouhyung 
Published: September 2018

Following commercialization of curved displays, foldable and rollable displays are under development. The rollable display should be unrolled first using a pulling motion to access the screen. The corresponding pulling force acting on the lateral grip (bezel) areas of the device should be higher than the spring force typically used for retracting the screen. The objective of the current study was to examine the effects of hand length and device thickness on the required lateral grip area sizes of the rollable display device and the grip comfort for the screen unrolling motion, and to ultimately determine the ergonomic bezel width and device thickness associated with high grip comfort.

Thirty young individuals with the mean (SD) age of 22.1 (2.2) years participated in this study. All participants were recruited from a university population, right-handed, and healthy without any musculoskeletal diseases on their upper limbs. This study was a 3 (Hand length) × 3 (Device thickness) mixed factorial design. Hand length (HandS/M/L; between-subjects factor) consisted of HandS (short hand length; ≤162.5 mm, 10th percentile), HandM (medium hand length; 174.6–177.3 mm, 45th–55th percentile), and HandL (large hand length; ≥189.4 mm, 90th percentile). Device thickness (DeviceThin/Medium/Thick; within-subjects factor) consisted of DeviceThin (2 mm thick), DeviceMedium (6 mm thick), and DeviceThick (10 mm thick). Each of three rollable display device prototypes was comprised of Acrylonitrile Butadiene Styrene plastic panels, a roll of paper screen (to show a default screen), a roller, and a spring (to roll the screen). The thickness of the right side of the device was manipulated, whereas that of the left side was fixed at 10 mm to house the three parts described above (a rollable screen, a roller, and a spring). When fully unrolled, the sizes of each prototype and the screen were 140H × 300W × 2.5R (mm) and 130H × 260W (mm), respectively. The prototype was equally split into two sides, with each grip part (bezel) 20 mm wide. A 1 mm-interval grid image (130H × 20W (mm)) was attached to each bezel to measure the bezel area involved in gripping. The initial pulling force for unrolling the screen was 2.5N. A desk (150 × 60 × 73 cm) and a height-adjustable chair were used. First, participants unrolled and rolled the prototypes freely for five min to familiarize themselves with how to use the prototypes. A randomly assigned prototype was evaluated three times as follows. Each seated participant repeated unrolling and rolling motions with the assigned prototype until they found the most comfortable grip. While the screen was fully unrolled using the most comfortable grip, each grip area was photographed from four different directions. Then, each individual rated the grip comfort of each hand on a 100mm Visual Analogue Scale (0: Very uncomfortable, 100: Very comfortable). A paper-and-pencil method was used for comfort ratings. The entire procedure to evaluate the three prototypes required about 30 min per participant.

Regardless of hand length, the width of the grip area from the device side edge was up to 20 mm. The mean (SD) height of the grip areas for HandS/M/L was 108.8 (3.1), 116.8 (2.5), 124.2 (2.3), respectively. Regardless of hand length, the lower end of the grip area reached the bottom of the device, while the upper end moved more upward with hand lengths. The thinner the device was, the smaller the difference in the grip areas was across the three hand-length groups. In addition, grip comfort increased with device thickness. When gripping a thinner object, the grip posture becomes more deviated from a relaxed hand posture to make more flexions of the thumb and fingers.

The simple linear regression model for the left-hand grip comfort on the right-hand grip comfort was constructed (R2 = 0.68 and p-values <0.001):

Y(left-hand grip comfort)=23.1+0.74×X(right-hand grip comfort)

This regression model indicates that the two grip comfort ratings were positively correlated. In addition, the right-hand grip comfort ratings were lower than the left-hand grip comfort ratings. The mean (SD) comfort ratings for the left and right hands were 75.1 (19.2) and 78.6 (17.3), respectively, with p-value for a paired t-test < 0.001. Thus, device thickness appears to be an important design dimension that influences the grip comfort associated with screen unrolling.

There are some limitations in the current study. First, the initial pulling force required for screen unrolling was fixed at 2.5N. The screen unrolling motion involves external rotation of the shoulders. To the authors’ knowledge, no study has investigated an ergonomic force range for this motion. Second, some measurement errors may have been involved in manually identifying the grip area based on the photographs. Using touch sensors would provide more accurate and faster measurements. Finally, it is necessary to analyze the grip areas more in detail.

The current study investigated the effects of hand length and device thickness on the grip area and the grip comfort of each hand for rollable display devices. The findings suggested that regardless of hand length, the side bezel of a rollable display device should be at least 20 mm wide and the device should be sufficiently thick (preferably 10 mm thick) to ensure high grip comfort. These findings will be useful when designing ergonomic rollable display devices with high grip comfort.