Development of an Ergonomic Design Framework for Determination of Smartphone Hard Key Locations

Abstract

The location of smartphone hard key need to be ergonomically designed to improve grip stability and operational efficiency for better usability. Smartphone hard key includes power key to screen on/off for efficient energy management and volume key to volume up/down for intuitive volume control. Hard key could cause various usability problems such as grip loss, discomfort, and unintended operation if the location is improperly designed. In addition, the locations of commercial smartphones are different from manufacturer to manufacturer and from device to device even designed by the same manufacturer, which implies ergonomic design guide for hard key location is not available. While many studies for ergonomic user interface design have been conducted on the graphical user interface (GUI) design on a touchscreen and design dimensions of a device, research on the ergonomic design location of the hard key is a few. Hard keys need to be carefully located as they are impossible to be relocated once manufactured unlike GUIs on a touchscreen, also, changing the location during design is difficult as well because the locations of the other parts are all related to each other. Therefore, development of an ergonomic design method for determining the location of hard keys on smartphones with various sizes is necessary. The objectives of the present study are (1) development of a design methodology for hard key location which determines recommended design location based on the analysis of preferred control range in preferred grip postures by users with various hand sizes, (2) application of the methodology on the design of hard key locations for various smartphone sizes, and (3) validation of the methodology by evaluation of operational satisfaction for the hard key locations. The developed design methodology first analyzes the characteristics of target device, user, task, and use context, then preferred grip postures by users are analyzed. Next, preferred hard key control range in the preferred grip postures are investigated to derive preference distribution for control area. Finally recommended design location is determined by considering the size of hard key and preference for the control area. Preferred grip postures of 45 participants were analyzed for operating power key and volume key on 9 smartphones with 3.0” to 7.0” screens. Out of 9 identified grip postures, 3 fingers at the left side-1 finger at the right side-1 finger at the back (L3-R1-K1), 4 fingers at the left side-1 finger at the right side (L4-R1), and 3 fingers at the left side-1 finger at the right side-1 finger at the top (L3-R1-T1) were the major grip postures with more than 95% of preference in total. Effect of device size on the preference for each preferred grip posture was significant that the preference of L3-R1-K1 increased from 32.2% to 84.4% when screen size increased from 3.0” to 7.0” (p < 0.01). Recommended design locations for power key and volume key of 9 smartphones with 3.0” to 7.0” screens were investigated from preferred control ranges of 52 participants. The recommended design locations for power key and volume key were derived by accumulating the preference for each control location by participants with various hand sizes. The recommended hard key design locations moved from 69 mm to 116 mm above the bottom for power key and moved from 61 mm to 104 mm above the bottom for volume key. Effectiveness of the design methodology of smartphone hard key location was validated by the evaluation of operational satisfactions from 70 participants for the three hard key locations (recommended location, 10 mm above/below) on 4 smartphones with 5.0” to 6.5” screens. Mean operational satisfactions for the recommended locations on the 4 smartphones were 1.2 point higher than the others with averages of 4.2 to 4.9 points (p < 0.01). The developed design methodology for smartphone hard key location would be usefully applied to the design for user interface of various portable products in addition to smartphones.