Preliminary Detailed Design
Table of Contents
All files related to the Preliminary Detailed Design Review are located in the Detailed Design Documents repository
Team Vision for Preliminary Detailed Design Phase
Team Goals for Phase
- Complete Lift System Design
- Complete Foot Plate Design
- Modify Lever Arm Location
- Complete Lever Arm and Actuator Pulling Design
- Complete Bill of Materials for all applicable designs
- Begin writing test plans for subsystems
Team Accomplishments For Phase
- Lift System Design Completed
- Foot Plate Design Completed
- Lever Arm Location Moved, Design needs to be finalized
- Bill of Materials for Electronics, Lift, and Foot-Plate Completed
- Hand Calculations and Finite Element Analysis for Lift System Brackets
- Preliminary Test Plans completed
InterviewsTo see the Question/Answer notes from the interview with Mary, Megan's PT, click on the following link Interview 11/17/15
Prototyping, Engineering Analysis, Simulation
Center of MassThe 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. Compared to the COM calculations on the Subsystem Design page, this COM analysis uses the new pivot point and changes the coordinate system so that the y-axis aligns in the center of the new pivot point. The updated file for the COM calculations can be seen here, COM Calculations Updated
Tipping AnalysisTipping analysis uses the same coordinate system defined in Center of Mass section. The calculations can be found in the "COM Calculations Updated" link above
Pivot Point Components Hand Calculations
A link to the hand calculations is provided here: Hand Calculations PDF
Lever Arm CalculationsChanging the pivot point of chair caused some changes to lever arm design in order to reach same 90 degrees displacement. The updated calculation for lever arm can be downloaded here, Lever Arm Calculations
Actuator Force CalculationsThe required actuator force was recalculated to account for the changes made in this phase. The calculations and free body diagrams can be seen in the images below. The force in each link is calculated to be 56 lb. The buckling analysis was redone following the same format as outlined on the Subsystem design page. The minimum thickness for the link is 0.105 in. This was calculated using a FOS of 2 on the compression force that acts on the link. The required actuator force that is needed to start the lifting motion is calculated to be 2180 N. If we use the same actuator, this gives a FOS of 1.83.
Drawings, Schematics, Flow Charts, Simulations
Lift SubsystemTo view a video of how the lift system will work, click on the following link Lift Video
Finite Element AnalysisAn FEA was conducted in Solidworks. The bracket analyzed was of a welded construction and will support the chair seat, back, and patient at the forward pivot point of the chair. The Finite Element Analysis was completed with a load double that of the maximum possible load on the bracket. The tubing was selected to be AISI 1020 Steel with a wall thickness of 0.125 inches. AISI 1020 Steel has a Tensile Strength of 60.9 ksi.
- Stress: The maximum stress in the analysis was 29.9 ksi, at location 1 in the image.
- Displacement: The maximum deflection is .0076 in, at location 2.
- Factor of Safety: With a yield strength of 60.9 ksi and double the anticipated load, the Factor of Safety is adequate.
To view the full report of the FEA, click on the following link FEA Report
Wiring SchematicThe wiring for the controls was reviewed and adjusted to provide better function and safety. A significant potential short circuit was removed in the process. For the up to date file, click this link: Wiring Schematic
Upper Limit SwitchThe upper limit switch is required to stop the lifting action of the chair at a specified point. The point will be set by physical therapists and must be easily adjustable.
Bill of Material (BOM)
Electronics BOMUsing the updated wiring schematic the components required for the electronic systems were selected from approved vendors. For the up to date file, click this link: Wiring Schematic
- The micro switches required have already been removed from the existing chair and will be reused for the proposed design. The switches were originally SUNS S19H, but due to damage have been converted to the SUNS S19E. Despite the damage to the casing, they are still in good working order.
- One rocker switch is already being used from the previous design but another one is needed. The existing model is a Nihon Kaiheiki LW3128-0100A. This switch has been verified to work so another one will be ordered.
- The selected circuit breaker is a TE Connectivity W58-XB1A4A-3.5.
- The heat shrink and wire connectors are generic components selected from the Lowe's inventory.
- Rebuilding the controllers requires new boxes. The REDDOT 17-cu in 1-Gang Metal Old Work Wall Electrical Box was selected as the new boxes for the primary and secondary controllers.
- Misc. is included to cover for minor unknown costs encountered while constructing the system. Extra hardware or wire may be required later.
Calf Pad BOM
The image below shows the BOM for the calf padding assembly.
Lift System BOM
The image below shows the BOM for the Lift System Assembly. Prices are based on material costs from McMaster Carr. Manufacturing cost was not applied with the possibility of completing manufacturing in-house.
Upper Limit Switch BOM
The preliminary BOM for the upper limit switch is shown below.
- The Push Button Nut is a Morton Machine Works KN-725 and has a 1/4-20 internal thread.
- The threaded rod was selected from the Lowe's general inventory
- Estimated Material / Hardware is included as an approximation for the material and bolts that will be required to manufacture and assemble the system. This will become refined as the system design is completed.
The image below shows the BOM for the Floorplate. Prices are based on material costs from McMaster Carr.
Overall Assembly View
Test PlansTo see the current version of the test plan, click on the following link Test Plan 11/17/15
Plans for next phaseTo view the latest updated version of the schedule, click on the following link Schedule
Key Points for Next Phase
- Send expectations letter to customer
- Design tray and create BOM
- Decide on hip pad and its attachment
- Decide on chest restraint and its attachment
- Decide on pommel and its attachment
- Finalize part dimensions and create drawings
- Finalize complete BOM with suppliers, sizes, and costs
- Contact PTs to see if the chair back needs to recline