# Designing a User-Centered Inspection Device's Handle for the Aircraft Manufacturing Industry
## 2 Materials and Methods  
### Description of the Device and Handles Tested  
The traditional inspection tasks were previously assessed regarding the ergonomic workplace analysis (EWA [25]), the postural assessment was carried out by Rapid Upper Limb Assessment (RULA) [26] using IMU for the joint angles measurement, and the workers' perceptions collection about cognitive workload was achieved by NASA Task Load Index (NASA-TLX) [27]. This previous assessment [20] allowed us to identify the main occupational risk factors and the requirements of the inspection task. The adoption of awkward postures, frequently aggravated by the handling of different tools needed to perform the task, is pointed out as the main problem that requires an ergonomic intervention. In the traditional tasks, the workers visually inspect the surfaces of the airplane's metallic structural parts and (1) take photos of the defects found, usually using a smartphone (where the images are stored), as well as (2) take measurements of said defect using a magnifying lens and a ruler. Later, the inspectors use the images and the measurements to create a report about the inspected parts. To digitalize this process, our multidisciplinary team, composed of researchers of Ergonomics, Design, Electronics Engineering, and Computer Vision, conceptualized a novel device that consists of a portable artificial vision system able to  
1. Capture and store images of the defects;
2. Classify the type and measure the dimensions of the defects;
3. Automatically generate digital reports with the inspection results. A prototype of this device, including all electronic components, was developed and supported by a modular physical structure that allows for handle changes. The prototype device is built upon the Raspberry Pi platform and it is comprised of the following modules: (1) a monochromatic 3.17 MPix 2K industrial camera for image acquisition; (2) a 7-inch touchscreen display for intuitive menu interfacing; (3) three infrared distance sensors for image perspective correction and camera-to-defect distance verification; (4) an accelerometer for implementing hand-stabilization algorithms during acquisition; and (5) a push button to trigger image acquisition. In our previous study [20], we defined the main anthropometric dimensions relevant to the design of the handle device, such as diameter and length metrics. Moreover, a set of ergonomic requirements were defined, foreseeing the suitability of the handle device for its end-users and inspection tasks, namely:  
1. Handle design potentiating the alignment of the device's center of mass with the center of the hand holding it;
2. The handle diameter shall not exceed 40 mm, allowing a power grip for the majority of the workers;
3. Curved handle shape to avoid deviations of the hand-wrist system during the tasks' performance;
4. Handle with a physical protrusion on the extremity to avoid slippage;
5. The button position to activate focus and image capture must be easily reached by the index finger;
6. Handle length between 125 and 145 mm to avoid compression of hand's soft tissues. In addition, following an ergonomic participatory approach, we involved five inspectors and managers of an aircraft manufacturing industry, as described in [20]. Along with the ergonomic assessment, three handle prototypes were created (hereinafter denoted as \"handles 1, 2 and 3\") and tested with a sample of participants, adopting a user-centered design approach, as adopted by [28]. The prototype \"handle 1\" was designed to put the user's hand and wrist in the most natural position possible when using the device. The shape was inspired by the handle design of an FN P90, a submachine gun produced by the Belgian company FN Herstal in 1990 for NATO soldiers. However, \"handle 2\" followed a more traditional approach as it is based on the shape most adopted for handheld devices. Its shape derives from an inclined cylinder that is warped to better adapt to the hand shape [9; 29]. Grooves for the fingers were also added to avoid slipping during use.