Blog #2

The Wheelchair Lifting Aid Progress

Over the work period April 1 - April 15, 2023, Team 8 is to finish the 3rd milestone (Sketches and 3D models created for the connecting parts and the selection of materials to be used). Team 8 has started a 3D CAD of the wheelchair frame shown in Fig. 1, which is to be used to perform the force reaction and moment analysis for the lifting procedure through COMSOL. In addition to this analysis, the 3D CAD model is to detail how the seat mechanism is to be accommodated within the modified structural frame of the wheelchair. Following, the components which connect the linear actuator to the wheelchair frame are to be sketched. Team 8 is to buy the linear actuator to use as reference and to follow the exact measurements closely during the development of the CAD model, since all these figures and designs are to be made following the dimensional constraints given by the wheelchair. Fig. 1 below, shows the base structure of the modified wheelchair frame. Lastly, Team 8 is also to research and choose the material to be used for the connecting parts between the linear actuator and wheelchair frame. 

   

Figure 1: Side and back-side views of the wheelchair. 

As it can be observed from Figure 1 above, the modification of the structural wheelchair frame consists of including a horizontal platform (highlighted in red) behind the cross bars that will connect the left and right side of the wheelchair frame. The linear actuator will be placed on this platform and its purpose is to avoid introducing stress concentrators on the wheelchair frame and instead evenly distribute the weight being pushed by the actuator. Currently, the planned platform dimensions are 17x6 [inches] with 1/8 in thickness, and is to be made of a steel alloy. The Team plans to utilize "U" brackets, highlighted in blue, to mount the platform onto the cylindrical frame of the wheelchair. Since the wheelchair frame is made of aluminum, the material for these U brackets must be composed of aluminum to ensure proper welding can be performed. Focusing further up on the wheelchair frame by the seat, a supporting frame has been included for the wheelchair seat to rest upon. This supporting frame is to give the wheelchair seat a starting position and ensure it does not "cave in" beyond the designed initial position once it is being lowered to its starting point. 

Challenges or obstacles > follow up with solution implemented.

The introduction of the linear actuator as a lifting mechanism and only connection mean between the seat and the wheelchair frame, adds a factor of lack of stability during the extension of the actuator when the load is being lifted. This is due to the existing connecting part between the linear actuator and the wheelchair seat, which can be seen in Figure 2. The Team believes this connecting part does not provide enough surface contact area between the seat and actuator, therefore introducing the risk of instability. The team is to re-design the connecting point, so it has a greater surface attachment area between the linear actuator and wheelchair seat to counter the equilibrium loses. A rough sketch of such part can be seen in Figure 3, which will act as an intermediate part and be connected to the existing part in Figure 2.  Furthermore, another obstacle the team must deal with pertains to the folding mechanism of the wheelchair. Since the crossbar serves as the folding mechanism of the wheelchair, it is connected to the main wheelchair frame and thus provides high structural support. The folding mechanism can no longer be used since the linear actuator is to be placed behind the crossbars, but the team cannot sacrifice the structural stability the crossbar provides. Therefore, the team decided it is best to fix the crossbar into place and restrict any movement. This will be accomplished by removing the vertical sliders which allows the wheelchair seat to fold upwards. Lastly, the team has been facing a non-technical issue which regarded to conducting a survey on current caregivers to obtain data on different caregiver weight and statures for the calculations of different lumbar forces experienced. The team has been unable to schedule an appointment with nursery homes, but will continue searching for volunteering caregivers. If, for any case a meeting cannot be arranged, the team will conduct further preliminary experiments with volunteering candidates on the UH campus. 

Figure 2: Connecting part for linear actuator. 

Figure 3: Intermediate Connecting Part.

For the working period from April 16 to April 22, Team 8 is to start working on milestone 4^th. Team 8 plans to have their first meeting with the UH machinist, where the finished 3D model of the connecting part (actuator to wheelchair frame) is to be presented and evaluated by the machinist. Team 8 expects to strengthen the quality of the part by modifying the 3D model with the machinist’s suggestions.

Figure 4: Free Body Diagram. 

Continuing with the milestone 4^th, team 8 plans to perform a rigid body analysis to guaranty the stability of the wheelchair. In addition, once the location of every component of the system has been determined, by using COMSOL, the team will try to identify points of high stress and modify the design as needed to avoid plastic deformation.

To this day, the team has anticipated that there are several things that could compromise the stability of the chair. For example, the reacting forces experienced by the U brackets and the joint point of the wheel to the frame, identified as Wrfy, Brfy and Brfx on the figure 4, could exceed the elastic strength of these or the frame, deforming them plastically. Also, the force exerted horizontally originated by the chair seat identified as Fx in figure 4, could overcome the friction force exerted by the back wheel, causing the chair to slide, compromising the integrity of the patient.