All documents for our Subsystems Design are located here: Detailed Design Documents
Team Vision for Subsystem-Level Design Phase
During the subsystem level design phase we intended to:
- Consider all possibilities of concepts for each subsystem
- Select and explore further a concept for each subsystem
- Prove that the solutions we chose where feasible and the best possible solutions within the constraints and scope of the project
- Begin development of a Bill of Materials for each subsystem
- Define long lead items that may need to be selected before the conclusion of MSD I
During the Subsystem-Level Design Phase we completed:
- Refinement and adjustments to customer and engineering requirements following discussion with a wheelchair expert
- Feasibility analysis for our "Parallel-Links" concept on the lift subsystem
- Refinement of restraint possibilities
- Control System Layout (for one Linear Actuator)
- Calculation of lifting force required for current actuator placement
- Preliminary concept of footplate design
- Rework of documents of Systems-Level Design Phase to reflect current project goals and deliverables
Feasibility: Prototyping, Analysis, Simulation
Parallel Link Simulation
Actuator Lifting Force Analysis
The force required to lift the actuator was calculated using a static analysis at the worst case scenario (when the seat is all the way down), below is the free body diagram used for the analysis. The maximum actuator pull force is calculated to be 1531 Newtons. The current actuator has a max pull force of 4000 Newtons so that gives a factor of safety of 2.6, which is acceptable. Note that the lever arm in these calculations is 1 in longer than what is currently on the chair, so the chair will have to be raised 1 in, which will not greatly affect the design and/or performance of the chair.
To view the full hand calculations click the following link: Lift Analysis
Actuator pull vs. push analysis
- Utilize full 10000N push force of actuator
- Easier to overcome initial load of fully seated chair
- Actuator stroke is not long enough in current position
- Initial angle of push would be almost straight at the axis of rotation
- When standing, actuator would be fully extended, exposing the arm to damage
- According to calculations, current actuator pull force of 4000N is sufficient with current lever arm design.
- Lever arm can be lengthened, increasing mechanical advantage and factor of safety
- Actuator arm is fully retracted at standing position, reducing risk of damage
- Actuator fits under seat with no modifications to frame
- Actuator pull force is only slightly greater than current required force
Based on the analysis of our actuator, we have determined the pulling force of the actuator is sufficient to accomplish the task of raising the patient to a standing position. Although the push force is far greater than the pull force, there are more disadvantages to this configuration than benefits. Primarily, the current actuator stroke is not long enough to raise the chair to its full standing position. The actuator also cannot be moved to a different position due to the space constraints under the chair. When the chair is standing, the actuator would be in its fully extended position, exposing the arm. In this position, the arm could be bumped, possibly causing damage and failure of the actuator.
On the other hand, the current configuration of the actuator does provide enough force to lift the chair, but only just. To improve our factor of safety, we can relatively easily lengthen the lever arm. With this method we can also easily incorporate the parallel links system to raise the back of the chair.
Link BucklingEuler's Buckling Method was performed in order to determine the minimum thickness needed for bottom link of the parallel link system. The Force includes a factor of safety of 2. It was determined that the minimum thickness of the link needs to be at least 0.098 in. If we were to cut this link, we could use 12 (or lower) gauge steel.
The calculations are seen below.
Center of Mass Analysis
The center of mass is calculated using the coordinate system outlined in the image below. Note that the coordinate system is placed in the center of the front plane (looking face-on to the user) so that the z-axis center of mass is 0 due to an assumed equal weight distribution. The updated file for the COM calculations can be seen here, COM Calculations
Note that raising the seat 1 in (see Pull force calculations) will not greatly change the COM location.
Tipping Angle Calculation:
Using the calculated position of Center Of Mass while standing, we can calculate the tipping angle; Horizontal and vertical distance of COM with front and rear wheels forms 2 triangles. Calculating length of hypotenuses of these triangles allows us to find the angles between COM and the vertical lines going through wheel centers which are the tipping angles.
- The tipping angle from front: 22.6 degrees
- The tipping angle from the back: 49.0 degrees
- Tipping angle from sides (measured): 19.5 degrees
Upper body restraints analysisFinal decision is based on customer and PT recommendations for comfort and usefulness (ie. won’t offer enough torso support, too restrictive, not good for circulation etc.)
Option 1: Elastic Chest Band
- Buckles across chest
- offers small amount of flexibility for comfort and range of motion
- Single strap may not be enough to support torso
Option 2: Restraint bar (Current Design)
- Provides something to lean and hold on to
- Padding can increase comfort and safety
Option 3: Harness
- Greatest amount of support
- Highest level of safety
- Very intrusive, possibly uncomfortable
Option 4: Torso Pad
- Difficult to design
Drawings, Schematics, Flow Charts, etc.
Foot Plate CAD
To view the latest updated version of the Foot Plate Subsystems, click on the following link Floor Plate Subsystems
Power and Control Subsystems
To view the latest updated version of the Power and Control Subsystems, click on the following link Power and Control Subsystems
Bill of Materials (BOM)
Power and Control Subystem BOM
- A few different tasks where complete to minimize our risk based on the categories identified previously.
- For "Lack of field expertise", we met with Matt from Fonte Surgical, who is Megan's wheelchair fitter. Matt was able to provide us with a Permobil C300 Standing wheelchair to look at and was able to provide some tips for fitting the wheelchair to Megan.
- For "Actuator Fails", calculations are in process to analyze the force required for the actuator to pull at its worst case scenario.
- For "Injury to user,chair tips over" center of mass calculations where completed in both an uphill and downhill situation.
- For a link to the Risk Management document, click on the link below Risk Management.
Design Review MaterialsNo design review materials available at this time, action items from design review will be added following review.
Plans for next phase
- Discuss restraint options with Mary (physical therapist), Megan (patient), Matt (wheelchair fitter), and Debbie (mother)
- Select components for electrical/ control subsystem
- Complete BOM and drawings for lift system
- Complete Analysis on Linkage/Lift System Design
- Design foot-plate to customer requirements
- Establish expectations for customers of wheelchair
- Develop system to constrain knees tightly
- Estimate manufacturing or delivery time required for large/ long lead components