Preliminary Detailed Design
Team Vision for Preliminary Detailed Design PhaseWhat We Hoped To Accomplish:
During this design phase the team set out to focus in on the technical details behind each design component. This included conducting research on the standards identified in the previous design phase, as well as understanding the range of limb sizes in the expected user population. The team also expected to draft design options with dimensions that support the research collected. From this point the team hoped to develop CAD models for each design component that would assemble to create a finished system.
The focus of the team was to develop a functional hydraulics system that addresses the shortcomings of the previous design. A stretch goal of the team was to complete CAD designs for all systems components as well as their corresponding bills of materials and manufacturing plans. We hoped to be able to deliver a project plan that would be carried out throughout the remainder of the projects life-cycle, as well as detailed descriptions of each of the systems components and how they interface with one another. The team also hoped to receive a donation for parts to use in the prototyping phase.
What We Accomplished:
During this phase the team quickly realized that a donation from home depot would not be possible, as the teams PM working there was seen as a conflict of interest, for this reason the team was told their request would be denied. The team was able to identify a source of extra supplies we could use for design via Marlon who had materials available from co-op.
Our team was able to successfully meet with two subject matter experts during this phase who helped us to identify important ergonomic considerations, and important hydraulic design considerations for our prototype. Through this our team was able to create designs that we feel will successfully fulfill both the system requirements and the customer requirements efficiently.
Our team began this phase by breaking down the prototype at the livability lab, where we were able to take measurements and disassemble the hydraulic cylinders. From this analysis we were able to move into the CAD design phase where the team used measurements, findings, and considerations from the subject matter expert meetings to create models of the system components.
As we worked to design these components the team developed a bill of materials for the system, as well as test plans for each of the critical components. At this point the team realized the final system would need to be tested by the user base after the design was completed.
The team attended human subjects testing and all project team members are in the process of completing training so that the team can submit the human test plan to the IRB for approval before MSD 2.
Below is a visual representation of the accomplishments of this phase, as well as some of the shortcomings we experienced. A detailed description of these successes and the reason for shortcomings is explained in the remainder of this section.
Meeting with Subject Matter Experts
In the beginning of this phase our team identified two subject matter experts that would be beneficial to meet with in order to ensure a successful design. These included an ergonomics, and a hydraulics expert.
Ergonomics SME Meeting
Our team met with Eshan Rashedi, an Ergonomics Professor in the Industrial and Systems Engineering department at RIT.
The major deliverables from this meeting was the receipt of the "The Measures of Man and Woman Revised Edition" book which details the measurements of both males and females from the 99th percentile to the 1st percentile.
The team used the following set of photos to determine appropriate ranges of design metrics when building our prototype.
Through this analysis it was determined that many of our designs should be designed to accommodate the largest person who would use them, as often chair widths and arm rests that are large enough to support the largest percentile of the population are large enough to also support the smallest percentile. The width adjustment was determined to be best set with a max of 26 inches or as close to that as possible as this is what a typical wheelchair is set at.
From the photos above we determined: The appropriate starting and lift height of the arm supports is 7 inches from the system base with adjust-ability every .5 inches up to 12 inches.
The arm supports should start at a width of 16 inches and extend to a width of 20 inches. This will accommodate the 99th percentile female.
The arm supports should be 20.8 inches long to support the 99th percentile male.
The back support should be a total of 26 inches to support the spine length of the 99th percentile male
Hydraulics SME Meeting
Our team met with Michael Schrlau, a Mechanical Engineering professor at RIT, with a PhD in Applied Mechanics. He has an extensive background in Fluid Dynamics and a lot of experience with pneumatics and hydraulics.
The major take-aways from this meeting were:
- Linear bearings for the guide rails
- Positioning the piston at the guide rails instead of in front of them
- Flow control valve will adjust speed of lift/tilt
- Position tilt pistons horizontally rather than vertically
Engineering Analysis and Design Feasibility - CAD Design Preparation
To further prepare for our CAD models our team completed feasibility analysis to determine the sizes, angles, and structures that would be appropriate for our design.
This included research of ADA requirements, human testing training, and materials research as well as a trip to the livability lab to deconstruct the current prototype and take measurement to consider when designing.
Livability Lab Measurements
While at the livability lab, Samantha, Kristina, and Christina took photos of and measured both a standard toilet with a tank, and a toilet without a tank.
Both sets of measurements are shown below
Toilet without Tank
Toilet with Tank
These measurements helped the team to ensure the design created would be compatible in both a public and private restroom.
While these measurements were being taken, Marlon and Dan worked to disassemble the prototype and complete diagnostics on the hydraulics system.
Key Findings in Hydraulics Analysis
The most critical parameters of the prototype's hydraulic system were inspected and measured. During inspection, the team noticed that the piston would not slide freely in the cylinder and was nearly impossible to actuate without hydraulic assistance. The fluid inside the cylinder seemed relatively clean and the piston was lubricated so there was immediately obvious cause of the piston binding. Once the piston was removed, dimensions were taken and compared to standards for PVC pipe and O-Ring design guidelines. The data are summarized in the tables below.
Figure 3.16: Prototype Piston O-Ring Calculations. Target values are based on the Parker O-Ring Design Guide
Based on these data, we were able to identify shortcomings of the prototype's design. The prototype used a schedule 40 PVC pipe with a nominal pipe size of 2-1/2", which has a nominal outer diameter of 2.875" and nominal wall thickness of 0.215" which gives an inner diameter of 2.445" which is certainly greater than the nominal piston outer diameter of 2.426. However, the ASTM standard calls out a +/-0.007" tolerance on the pipe outer diameter and +/-0.012 tolerance on the wall thickness which means that the inner diameter can actually fall anywhere between 2.426" and 2.464". This may result in a line on line or slight interference fit between the piston and cylinder, which can contribute to the binding issue.
Furthermore, the O-Ring calculations revealed that the main piston seal O-Ring is over sized for this application. Based on the Parker O Ring Design Guide, the target O Ring gland fill is 75% while the maximum compression is 16%. The O Ring used in the prototype could resulted in 90% gland fill with even the best case tolerances. In addition, the O Ring compression was calculated to be 37% which is also significantly higher than the target. The design guide points out that a higher compression leads and larger cross section diameter typically results in higher frictional forces.
Design Prototyping of Key System Components
With the prototyping analyses in mind, the team began design of the new project. The key dimensions of the hydraulic piston were determined to address the binding issue. The sizing is summarized in the tables below.
Figure 3.18: WAR4 Piston O-Ring Sizing Calculations. Target values are based on the Parker O-Ring Design Guide
As shown in the tables, the piston outer diameter has been downsized to accomodate the wide tolerances of the PVC cylinder inner diameter. Additionally, the O Ring cross section diameter has been downsized in order to improve the gland fill percentage and decrease frictional forces. A similar analysis will be completed for the piston rod seal as well. Another change is the implementation of a flow control valve to manage the lifting and tilting speed based on feedback from our hydraulics subject matter expert. Otherwise, the team's intention is to leave the hydraulic system design very similar to the WAR3 prototype since this system offered adequate hydraulic force and structural support.
As for the structural mechanics of the seat and frame structure, several changes have been made to allow for seat tilting and provide more rigid coupling between left and right pistons. The current geometry is shown in the images below.
Overall, the mechanics are roughly the same as the WAR3 prototype. Two hydraulic pistons toward the front of the toilet seat actuate the vertical lifting function while bearings and guides towards the rear constrain the seat from tilting. The tilting function is accomplished using a secondary tilting seat that is attached to the lift seat through a hinge at the front of the toilet seat and another set of pistons connected between the lift seat and tilt seat near the rear of the toilet seat.
For the WAR4 prototype, the guide rails will be 1"x2" box tube used in combination with standard ball bearings rather than plastic bushings to mitigate binding on the guide rails. Another change is the use of welded steel tube rather than rod-ends for the pistons for a more robust design that also constrains the seat from tilting left or right. Rigid coupling between left and right sides is achieved by 1"x1" box tube welded to the seat plates between left and right sides.
Another driving factor behind this design is to keep price low and allow for a straightforward manufacturing plan. The design of the WAR4 frame can be manufacturing with two lengths of box tube, one length of round tube and a sheet of steel. The sheet would be waterjet into the required seat profiles and gussets and primarily welded together.
Bill of Material (BOM)
About the Document:
The Bill of Materials (BOM) details all part components required for the intended design to come to fruition. This document confirms that all expenses and contingencies are afforded by the project financial allocation. The team was given an initial budget of $500 for the entirety of this project and through the completion of the BOM, our team realized an increase request would be likely be needed.
During this design phase the team was able to create an initial bill of materials for the parts we knew would be needed for this design. This document is a work in progress and will need to be updated before the end of the next phase.
To date it is understood that the materials we already know we need will cost ~$286.58 out of our $500 budget.
Given this information it is anticipated that the team will need to submit a budget increase request. This budget increase is a result of project scope changes. Initially, the goal of this project was to improve the current prototype, but with added customer requirements and an insufficient current system our team was forced to redesign the entire system.
Again, it should be noted that certain aspects of the prototype's design still have yet to be determined or discussed. Therefore, this initial BOM document is not considered complete and will most definitely require modifications/updates during the Detailed Design Phase. The information presented here is going to be used for giving the team an initial idea of what the final cost of the prototype could potentially be.
The Bill of Materials document is monitored and controlled by the teams purchasing agent Marlon Naveda.
Bill of Materials Document:
Shown above is the current BoM list. It contains a breakdown of the different systems that our final prototype will consist of. Each system can be broken down into individual sub-systems, which can then be broken down further into "sub-sub-systems." This will be used for determining what materials will be needed for which systems.
The different systems that make up our prototype are as follows:
- Support System (Mechanical)
- Seat System (Mechanical)
- Hydraulic System (Hydraulic)
- Arm Support System (Mechanical)
- Torso Support System (Mechanical)
- Back Support System (Mechanical)
The live document for the Bill of Materials list can be viewed here.
Figure 3.25: The non-human test plan document focuses on how we are going to test 5 different functions of the design.
In this phase we completed testing procedures designed to make sure that our device would conform to our engineering requirements. In order to determine that our device fulfills its purpose and is safe to use we developed six different plans to test the functions of the device. These test plans include: functionality, hygiene, piston leakage, safety, static load capacity, and visual appeal/human testing. Each test plan contains sections for detailed testing specifications (function tested, specification tested, unit of measure, and margin of error), equipment required for testing, a data collection strategy, procedure for testing, tables for data collection, and space to record any observations or conclusions we can draw from the test.
The functionality test will include procedures for testing the range of motion for the height as well as the tilt of the seat and the maneuverability of the arm supports. The ease of sanitation will be confirmed during the hygiene test. We designed the piston leak test to ensure that the hydraulic system does not have any leaks. The static load test will make sure that the device can hold the amount of weight specified in the engineering requirements. And lastly, the team members will complete a final safety check by attempting to use the prototype themselves.
The document containing our human test plans are separate from the rest of our test procedures as the human subjects test procedure will be submitted to RIT’s Institutional Review Board for approval before testing can take place. The team also decided to have every member complete the CITI human test subjects training in order to prepare for when we carry out the human testing for our device. At least one person with the training must be present at all times for the human testing, so by having the entire team complete the training we will not have to plan the testing around one person’s schedule.
All non-human test plans can be found here.
Design and Flowcharts
Below is an updated system flowchart that reflects changes that occurred during detailed design.
Since the previous Systems Design phase, the team was successfully able to add on to the list of potential risks that could be encountered at some point during MSD. The updated risk spreadsheet can be seen below. The owner of this document is Marlon Naveda and he is responsible for making sure that everything updated accordingly.
To view the live risk management spreadsheet, click here.
Plans for next phase
As a team we hope to have all of the part CAD files completed by the next design review. We hope to assemble these designs into a completed assembly to represent the prototype we plan to build. A stretch goal for the team is to design cardboard prototypes of the intended components. We also plan to understand additional budgetary needs the team will have so that we may request extra funds before moving into MSD 2.
Below is a hierarchy of items that we plan to accomplish in the next phase. Items at the top show the most risk, items at the bottom are likely to be accomplished without issue.
In order to accomplish these goals the team will adhere to the schedule below:
The Teams Individual Four Week Plans are as Follows: