P16005: Motorized Standing Wheelchair 2
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Subsystem Design

Table of Contents

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:

  1. Consider all possibilities of concepts for each subsystem
  2. Select and explore further a concept for each subsystem
  3. Prove that the solutions we chose where feasible and the best possible solutions within the constraints and scope of the project
  4. Begin development of a Bill of Materials for each subsystem
  5. Define long lead items that may need to be selected before the conclusion of MSD I

During the Subsystem-Level Design Phase we completed:

  1. Refinement and adjustments to customer and engineering requirements following discussion with a wheelchair expert
  2. Feasibility analysis for our "Parallel-Links" concept on the lift subsystem
  3. Refinement of restraint possibilities
  4. Control System Layout (for one Linear Actuator)
  5. Calculation of lifting force required for current actuator placement
  6. Preliminary concept of footplate design
  7. Rework of documents of Systems-Level Design Phase to reflect current project goals and deliverables

Feasibility: Prototyping, Analysis, Simulation

Parallel Link 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.

FBD Lift System

FBD Lift System

To view the full hand calculations click the following link: Lift Analysis

Actuator pull vs. push analysis

Push:

Pro's

Con's

Pull:

Pro's

Con's

Decision:

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.

Euler'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.

Buckling Analysis

Buckling Analysis

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

COM Schematic

COM Schematic

COM Results

COM Results

Note that raising the seat 1 in (see Pull force calculations) will not greatly change the COM location.

Tipping Angle Calculation:

Falling Angle Calculation

Falling 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.

Upper body restraints analysis

Final 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

Option 2: Restraint bar (Current Design)

Option 3: Harness

Option 4: Torso Pad

Upper body restraint options

Upper body restraint options

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

Foot Plate CAD

Foot Plate CAD

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

Wiring Diagram

Wiring Diagram

Bill of Materials (BOM)

Power and Control Subystem BOM

Wiring BOM

Wiring BOM

Risk Assessment

Risk Management

Risk Management

Design Review Materials

No design review materials available at this time, action items from design review will be added following review.

Plans for next phase


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