Solar-Powered Wheelchair Design: Proposal & Methods

Introduction

In Africa, many elderly people face myriad physical challenges including impaired mobility, muscular and nerve degeneration, and reduced balance and motor function. As a result, the wheelchair is considered a vital tool for enhancing quality of life, mobility, and dignity for elderly users (Chien, 2014). Current wheelchairs fall into one of three categories: electric-powered, manual, or power-assisted (Gurrama et al., 2012). Traditional manual wheelchairs produce a higher respiratory exchange ratio and greater oxygen consumption, which can be beneficial to the user's health when used effectively (Yang et al., 2007). Nevertheless, their gross mechanical efficiency is only about 2–13.8 percent — measured as the ratio of metabolic to external power — depending on propulsion technology, injury level, exercise intensity, and wheelchair interface adjustments (Wang & Chiang, 2012). The low mechanical efficiency and increased physical strain placed on users can lead to fatigue or strain-related injuries in some instances (Pasion, 2019). Designing and developing electric-powered wheelchairs has therefore been a matter of great importance and need over the past decade.

Electric-powered wheelchairs offer numerous advantages over manual ones. For instance, they carry minimal risk of strain-related injuries and require significantly less user effort (Chien, 2014). Consequently, they have become more prevalent in recent decades, prompting users to shift from manually propelled chairs to motor-propelled electric-powered wheelchairs (Pasion, 2019). Nevertheless, current electrically powered wheelchairs still exhibit several drawbacks (de Groot et al., 2013). They are heavy, expensive, physically large, and require lengthy charging times (Bhatnagar et al., 2022). In addition, due to their electrical and mechanical components, most electric-powered wheelchairs are difficult to maintain (Chien, 2014). They cannot be easily disassembled and folded, making them cumbersome to transport and store. These limitations notwithstanding, they hold a significant market share compared to traditional manual wheelchairs.

In response, wheelchairs combining electric and human power — known as push-rim-activated power-assist wheelchairs — have been developed (Wang & Chiang, 2012). The human-powered component uses arm action on push rims, while the electric component uses motor torque from a battery (Gurrama et al., 2012). A typical push-rim-activated power-assist wheelchair detects the torque applied to the push rim and produces corresponding assist torques via its motors (De Groot et al., 2008). These wheelchairs can also help users maintain physical condition by reducing the risk of upper-limb injuries and pain (Bhatnagar et al., 2022). However, due to their strong dependence on user interaction, push-rim-activated power-assist wheelchairs can still present challenges for some individual users.

The motors in electric wheelchairs rely on batteries, meaning the chairs cannot travel long distances without frequent recharging (Wang & Chiang, 2012). Many research studies have investigated ways to overcome the limitations of electric wheelchairs by developing chairs fitted with solar power to ensure a constant power supply (De Groot et al., 2008). However, only a limited number of solar power-assisted electric wheelchairs exist to date. In most existing designs, the solar panel is rigidly fixed at the back of the chair using a metal frame that cannot be easily disassembled (Chien, 2014). Furthermore, such wheelchairs operate in electric mode only, forfeiting the benefit of the chair as an exercise device. Messenger and Melanson developed a proposal to merge electric-powered wheelchairs with solar panels (de Groot et al., 2013), but both designs retained most of the classic electric-powered wheelchair problems, including heavyweight construction, an inability to fold, and substantial physical size (Wang & Chiang, 2012). Subsequently, Curran et al. introduced modifications to the mainframe to reduce weight (Bhatnagar et al., 2022); however, the wheelchair's power module and frame could still not fold, and the chair could not be propelled manually.

To address these challenges, this study proposes the design and development of a solar-powered electric wheelchair that allows the user to select either electric or manual propulsion via a mode switch (Chien, 2014). A mechanism is also included to allow easy removal of the solar panels and batteries, enabling quick disassembly and folding for convenient transportation and storage.

Quality Function Deployment

The Quality Function Deployment (QFD) perspective is used to design the proposed wheelchair. QFD involves translating customer needs into engineering characteristics or design requirements, which are then transformed into product requirements and process plans (Yang et al., 2007). The initial step is to identify the users, their needs, and how to meet those needs. For this study, users are elderly persons from Africa, specifically Kenya (Chien, 2014). After identifying customer needs, a statistical approach is applied to questionnaire data to assign a degree of importance to each customer requirement through a weighting factor ranging from 1 to 5, where 5 represents very important and 1 represents not essential (De Groot et al., 2008). The relationship between design and customer requirements is displayed in a house-of-quality matrix.

The design requirements will reflect the user's preferences and needs for the solar-powered electric wheelchair as informed by the QFD process. They will also specify various design criteria to be addressed in order to achieve market success and customer satisfaction (Wang & Chiang, 2012). A cross-functional team — comprising industrial designers, clinicians, therapists, and wheelchair engineers — will identify the design requirements (Gurrama et al., 2012). The design team will assign strength weightings according to the relationship between designer and customer requirements (Chien, 2014). Relative and absolute weighting values will then be assigned to each designer requirement (Liu et al., 2010). The total weighting value for each design requirement will be computed using the following equation:

AI_j = Σ W_i R_ij, 1 ≤ m

where W_i is the weighting value allocated to CR_i, i = 1, …, m; R_ij is the weighting value illustrating the strength of the relationship between DR_j and CR_i; and AI_j is the conclusive weighting rating of DR_j, j = 1, …, n (Gurrama et al., 2012).

The proposed wheelchair's solar power system performance will be evaluated based on statistical results obtained for a 5-kW solar power system, assuming daily sunshine exposure of 3.44 hours in Tainan.

The prototype wheelchair's maximum travel range will be calculated from energy consumed over an experimental track, supported by depletion measurements of a fully charged battery of known capacity. The prototype's performance will be assessed both with and without a solar panel module connected to the power supply system (Gurrama et al., 2012). To ensure the reliability of test results, the test will be conducted by a non-disabled subject. The test procedure will be carried out between 9 a.m. and 1 p.m., with a sunlight intensity of approximately 860 W/m².

The wheelchair's static stability will be tested in accordance with the International Organization for Standardization (ISO) 7176-1 standard. The wheelchair will be secured on a platform using restraining straps positioned so as not to hinder tipping movement (De Groot et al., 2008). A dummy weighing 120 kg will be placed in the wheelchair, and the platform angle will be adjusted slowly until the tip-over angle is established (Chien, 2014). Occupied wheelchair stability is characterized by the lateral tip-over angle, the rearward tip-over angle, and the forward tip-over angle.

Conceptual Design and Prototype

A schematic of the electric/manual wheelchair prototype was developed from recent studies (Gurrama et al., 2012). The wheelchair is built on a commercially available manually propelled chair frame designed to retract and assemble conveniently. The capability for electric propulsion is provided by a battery as the primary power source, along with two electric motors (Bhatnagar et al., 2022). A solar panel auxiliary power supply extends the wheelchair's travel range (Pasion, 2019). As illustrated in the conceptual design figure, the solar panel is fixed so that it acts as a roof over the user's head, serving not only as a secondary power source but also protecting the user from rain and sun (De Groot et al., 2008). A manually operated clutch mechanism enables the user to switch between electric and manual propulsion modes at will (Chien, 2014). The solar panel assembly is also built on a modular design with quick-release mechanisms to allow the chair to be collapsed and folded for transportation and storage.

Figure 1: Conceptual design of solar power-assisted electric/manual wheelchair (Chien, 2014).

The major components are as follows:

(1) Solar panel (single crystal, 60 W); (2) foldable frame for the solar panel; (3) steering joystick and controller; (4) manual/electric mode switch; (5) quick release for solar panel frame; (6) wheelchair handle; (7) batteries; (8) motor (50 W × 2); (9) planetary gear.

Figure 2 shows the proposed wheelchair's electric circuit (Wang & Chiang, 2012). The battery and the solar panel are connected with a diode to prevent power reversal from the battery back to the solar cells (Chien, 2014). During electric propulsion mode, the control unit receives power from either the solar panel or the battery, then sends commands to drive the two wheel motors according to the velocity and direction inputs provided by the operator through a hand-activated joystick.

Figure 2: Electric circuit prototype of solar power-assisted electric/manual wheelchair. M = motor (Chien, 2014).

The prototype wheelchair weighs 16.4 kg and is built on a commercially available manual wheelchair frame (Yang et al., 2007). The auxiliary compartments — including the battery set, planetary gears, and solar panels — together weigh approximately 20.5 kg, bringing the total weight of the wheelchair to approximately 37 kg (Chien, 2014). The proposed wheelchair uses a set of planetary gears as a switching mechanism between electric and manual driving modes, as well as a deceleration mechanism, as shown in the mode-switch diagram.

Table 1: Comprehensive specifications of the solar power-assisted electric/manual wheelchair (Wang & Chiang, 2012).

Contour Dimension: 1050 × 610 × 870 mm; Minimum Turning Radius: 650 mm; Additional Extra Weight: 21.6 kg; Solar Panels: 8 kg; Driving Hardware: 13.6 kg; Manual Wheelchair Weight: 17.4 kg; Net Weight: 39 kg; Maximum Load: 130 kg; Front Wheel Specifications (×2): 8 in.; Rear Wheel Specifications (×2): 24 in.; Motor Voltage: 24 V (DC); Motor Power (×2): 100 W; Battery (×2): Lead-acid battery; Charger Input: 12V/12Ah, AC 110V/AC 220V. (AC = alternating current, DC = direct current.)