*2.1. Mechanical Design*

Our module includes three different areas: sensors area, microcontroller area and charger area (Figure 1a). The sensors have been embedded inside the shaft, as close to the tip as possible at two different depths: 0.2 mm and 1 mm via a specially designed 3D printed plastic piece (Figure 1b). Thus, the tip and handgrip designs are not modified, and we preserve their original ergonomic properties. Placing electronics inside the shaft is actually much harder and makes access difficult, so they are externally attached. Their location has been split into two different areas to minimize changes to the center of gravity of the cane: the microcontroller (MCU) area (Figure 1c) and the charging area (Figure 1d). To connect the microcontroller (MCU) with the charger and with the sensor areas, only two holes are required in the shaft (H1 and H2 in Figure 1c,d). Hence, attaching the module only involves minor cane modifications.

The charging area has been designed to avoid connectors, so elderly people with visual impairment can use it in a straight, simple way. It relies on a *Qi* interface and it is divided into two different parts. The first part is a fixed piece that can be attached to a wall (**J** in Figure 1d). The second one is a flat piece attached to the shaft cane (**I** in Figure 1d). The *Qi* interface uses inductive charging over close distances. Hence, users do not need to plug any connector; they just need to support the cane on the wall piece and the battery starts charging. Each piece has been modelled using Autodesk-<sup>R</sup> Fusion 360 and it has been 3D printed with Ultimaker2 using PLA plastic (All 3D models are freely available online at https://github.com/joaquinballesteros/Smart-Cane).

**Figure 1.** Mechanical design overview (**a**) and its parts (**b**–**d**): (A) rubber tip; (B) force sensors locations; (C) microcontroller (MCU) box; (D) connectors between charger-sensors and MCU; (E) connector ring; (F) clamp for MCU box; (G) battery and charger box; (H1) hole to wires from charger and sensors areas to MCU box; (H2) hole to wires from MCU box to charger area; (I) charging receiver location; and (J) charging transmitter location.
