Blog # 3

  Over the work period of April 22-29, Team 8 has finished the 3D cad assembly of the final design of the wheelchair system, its stress analysis for the lifting procedure, and the static analysis of the reaction forces during the lifting procedure. The 3D CAD assembly was conducted through SOLIDWORKS, and includes the key components in action such as the linear actuator and its corresponding connecting parts. Figure 1 below is a short video which demonstrates the linear actuator in action and how the team plans to angularly translate the wheelchair seat. 

Figure 1: Wheelchair Lifting Aid in action

    The Team's final design concept has been carried out as previously discussed, however small modifications have been made. For instance, for the intermediate part which will connect the linear actuator to the wheelchair seat, its dimensions and design have been slightly altered. Figure 2 below illustrates the new intermediate part which has dimensions 6in width x 7 in length, with a center protruding height of 1.1in and is to be constructed of aluminum. The team believes this intermediate part to be a key feature to the overall design, as it will be increasing the surface contact area between wheelchair seat and linear actuator, resulting in a greater region for the center of gravity of the patient to stay within and thus providing more stability during the lifting process. 

Figure 2: New Intermediate Part

    Within our final design, the team has officially concluded that the stroke length of 10 inches from the linear actuator has allowed the seat to achieve an angle of 68 degrees from the horizontal. Figure 3 below illustrates with the usage of trigonometric functions and the built in dimensions from SOLIDWORKS, by sin^(-1)[6.3/6.8] = 68 degrees the wheelchair seat's angle was able to be calculated when the linear actuator was fully extended. One of the team's primary goals was for this angle to be at least 65 degrees, and although this goal has been achieved the team will further work to continue to increase this angle. The larger the angle from the horizontal of the wheelchair seat, the smaller the angle of inclination the caregiver will have to undertake when lifting the patient off of the elevated wheelchair seat. The angle can be increased by experimentally positioning the linear actuator at different positions within the newly created bottom platform.

       Figure 3: Seat Angle created from the horizontal 

   

         

Figure 4: Stroke Length of the Linear Actuator 

    Initially the team planned to conduct the stress analysis of the lifting procedure through COMSOL, however due to licensing issues that platform has become unavailable, and the team opted to instead conduct the analysis through SOLIDWORKS. Although the linear actuator has a maximum rated load of 330lb, the wheelchair itself only has a maximum load capacity of 250lb, therefore the distributed load used on the seat for the simulation was 250lb. Figure #5 illustrates how the distributed load was placed on the wheelchair seat. The stress analysis conducted has allowed the team to highlight the highest areas of stress experienced on the wheelchair system during the lifting procedure. Figures 5-7 below illustrates that the highest areas were at the connecting point between the linear actuator and wheelchair seat, the linear actuator and the platform, as well as between the U-brackets and the bottom platform. High stresses were also observed at the hinge connection between the wheelchair seat and seat platform; thus, the team will be selecting heavy duty hinges to sustain the induced loads.  To minimize these induced stresses, the team is to fix the cross members to have a better stress distribution throughout the whole frame, as well as continue experimenting with the geometry, dimensions, and material of the connecting parts. In addition team 8 is to consult with the UH machinist in order to select the best fillet radius, thickness, and machining process for our desired connecting parts to prepare for the execution phase.  

Following, a stress analysis was made to the wheelchair system through Simulation Express in SolidWorks, in which a 250lb distributed force was applied to the wheelchair seat as shown in figure 5. This was to simulate the maximum load the actuator would be able to hold. As we can see in the figures 6&7 below, the hinges would experiment some of the load applied, so the team must make sure heavy-duty hinges are installed, the supporting intermediate connecting part would also undergo stress front and back as observed in Fig 6 and 7.

Figure 5: Distributed force applied to wheelchair seat

Figure 6: stress applied to hinges and intermediate part (Front)

Figure 7: stress applied to hinges and intermediate part (Back)

The parts and areas that would hold the major stress are the four U brackets and the actuator platform as observed in figures 8-10 below.

Figure 8: stress applied to actuator platform and U brackets (Bottom)

Figure 9: stress applied to actuator platform and U brackets (Top)

Figure 10: displacement done to actuator platform 

    As for the static analysis, team 8 has anticipated that the possibility of the wheelchair to slip while using the lifting aid system represents a big challenge. Because of the rotational motion of the seat, the force applied to the seat (250 lbf) would originate two new components: one horizontal and another vertical (x and y axis). The horizontal component of the force (BF) would develop a back push that would be counteracted only by the friction force (FF) of the back wheels against the floor once these have been locked. Considering the possibility to slip, the worst case scenario would be using the lifting aid system while the wheelchair is on a wet surface or any tile material. In any of these scenarios mentioned before, team 8 has estimated that the coefficient of static friction would have a value of 0.6. Figure 11 below shows the forces acting on the wheelchair while the lifting aid system is being used.

Figure 11: Static reaction force analysis on wheelchair system 

    Because of the friction force, counteracting the back push depends on the reacting force normal to the floor against the back wheels and friction coefficient, the interaction between the angular position of the seat and the low coefficient of static friction can add up to make the back push of the wheelchair to exceed the friction force. Figure 12 below, shows the results of the force analysis of the wheelchair and how, if the chair is being used over a slippery surface, the back push would exceed the friction force once the seat reaches an angular position of about 31° from the horizontal line. Even though the stability analysis only represents a theoretical scenario, team 8 is considering testing the lifting aid system while the wheelchair is placed on different slippery surfaces and installing a new braking system to counteract the back push if needed.

Figure 12: Force Vs Seat Angle

    To continue preparing for the execution phase, the team plans to utilize the summer period to test the fitment and motional clearance of the connecting parts within the wheelchair system as the linear actuator extends. In order to save money on materials, the parts will be 3D printed. The team will 3D print the mounting U-brackets, the bottom platform, and the intermediate mounting part which connects to the wheelchair seat. Although the stresses will not be able to be analyzed, the team will primarily focus on experiment with the positioning of these connecting parts. Refraining from placing a load on the seat will allow the team to study the motion of the wheelchair seat whenever the 3D parts are connected with the linear actuator and wheelchair frame.