2c. Design Constraints

 

All limiting constraints that the car seat would need to abide by needed to be determined so that they could be designed around properly.  For this project the three types of constraints would be limiting space, Federal Regulations, and Ben’s needs.

 

Limiting dimensions for the amount of space available in the car seats many applications where determined in a variety of ways.  First, seats in vehicles were looked at since this would be the primary application of the chair.  Front and back seats in a variety of different cars were measured for the following information:

 

 

The smallest measurements from this research were selected as the limiting dimensions for vehicle use.  As expected all of these dimensions came from small passenger vehicles.   Measurements of airplane seats were not taken directly.  For this task an article was found that detailed the limiting dimensions of a standard coach seat.  The article was found at the website: http://www.faa.gov/apa/publicat/crstips.htm

 

All of the limiting dimensions were combined and again the smallest dimensions found were used as limiting constraints for the prototyped car seat to fit inside of.  The results of this research showed that the car seat should be no more than 16 in. wide and be able to sit stably on a seat that is 19 in. front to back.  With the average car seat pushed all the way back there is approximately 1 ft. of distance between the front of the seat and the glove box.  There is approximately the same amount of distance between the front of an airplane seat and the seat in front of it.  The smallest amount of headroom the seat would have to operate around was found in a small car.  The measured distance from the seat bottom to the ceiling of the car was 3 ft. 

 

Next, Federal Motor Vehicle Safety Standards (FMVSS) were referenced.  It was decided that the prototype car seat should be designed to meet or exceed all applicable standards because it’s purpose is the same as that of a regular vehicle seat.  For this project, standards #207, #209, and #210 were applicable.

 

§         Standard #207 covers seating systems,

§         Standard #208 covers seat belt assemblies, and

§         Standard #210 covers seat belt anchorage methods.

 

From these standards it was determined that a 20-g impact load is the federal regulation for seating systems in passenger vehicles.  This means that all parts of a vehicles seats must be able to withstand a crash that cause the car to decelerate at twenty times the acceleration of gravity.  The EZ Journey car seat will be designed to withstand a 30g impact load giving it a factor of safety of 1.5 above the Federal Regulations.  The FMVSS also provided the range of weights and sizes of occupants which vehicle seats must be designed for.  This equates to a seat that can hold a 5th percentile female all the way up to a 95th percentile male.  A 5th percentile female weighs 103 lbs., has a hip breadth of 12.8 in., and has an erect sitting height of 31 in.  A 95th percentile male weighs 215 lbs., has a hip breadth of 16.5 in., and has an erect sitting height of 38 in.  Due to the limiting constraints of a small car seat, the EZ-Journey car seat is being designed for a person with an erect seating height of 33 in. and a hip breadth of no more than 15 in.  Our Design Review Document states that the final prototype will be able to recline a 200 lb occupant.  This was lowered to 180 lbs due to limitations on the motor systems available for this design.  Our client for the project tips the scales at a svelte 90 lbs.  By adding an additional potential 20 lbs of equipment to the seat makes the final weight of the occupant and equipment 200 lbs.  Applying the 30g impact load to this weight sets the impact load at 6000 lbs. 

 

The harness system and it’s anchoring to the EZ Journey car seat will also be designed to meet FMVSS standards.  To do this our harness system has to be able to withstand certain tension loads depending on what type of seat belt it is.  It also has to mount to a solid part of the seat with no smaller than a ½ in. bolt.  The harness will most likely be purchased retail and will therefore already meet the tension loads required.  To mount it to the EZ Journey a ½ in. bolt will be needed to meet Federal Regulations.  DDI decided to upgrade this to a 5/8 in. bolt for added safety.  Federal Regulations also state that seat belts are to be made no smaller than three inches.  This is important when determining how the EZ Journey will be secured to an actual car seat. 

 

2d.  Design Philosophy

 

For this project, we believe that our design skills can be utilized to better the lives of others.  With this in mind, our focuses for this project will be on safety, comfort, and simplicity as we reach a market that has been neglected in the past.  It is important that the established safety standards be exceeded in order to ensure that the proposed car seat and harness are secure and dependable.  A comfortable seat is vital to the design so that a long trip can be enjoyed to its fullest.  The project will also focus on simplicity to make an efficient product that is cost effective and easy to operate.  With these three criteria in mind, we feel that we can design, fabricate and deliver a superior car seat. 

 

 

3.  Design

3a.  Concept Generation, Selection, and Testing

 

We began the design phase for this project with a very general conceptual design.  The designs generated in this stage were quite general, and would be looked at more closely later in the process.  The conceptual design process was broken into three stages (division, brainstorming, and selection), which are discussed in the following paragraphs.

 

Division involved looking at the client wants and needs and formulating a very general list of probable subsystems likely to be present in any solution to the problem.  This list was not used to restrict the design to one form from the outset – rather, it was used to provide a useful framework with which to link all of the brainstormed concepts. It was also used to make the brainstorming process capable of producing subsystems that could be mixed and matched, to a certain extent.  If an idea was outside of the scope of the categories, or combined several of them, it was considered on its own, as a separate entity designed to fulfill the subsystems it was designed to fill the shoes of.

 

The following general subsystems were decided on:

 

§         Base Frame: The main frame that acts as the interface between the occupant seat system and the vehicle seat and safety belt system.

§         Seat Bottom: The bottom of the client seat, where the buttocks rest.

§         Seat Back: The back of the client seat, which supports the client’s back, and provides a mount for the headrest.

§         Seat Hinge: The hinge that controls the angle between the seat bottom and back, and provides a method of varying this angle.

§         Reclining Mechanism: The mechanism that reclines the client seat relative to the vehicle seat.

§         Rear Restraint Connection: The means by which the vehicle’s standard restraints (seatbelts, etc.) are made secure to the client seat.

§         Anti-Roll Mechanism: This subsystem prevents the car seat base from rolling forward and back during reclining.

§         Harness System: The client seat’s seatbelt/safety restraints.

§         Head Rest/Switch: This device must provide support to the client’s head and neck in the case of an accident, as well as providing head-motion capable switches for control of the reclining mechanism.

 

Brainstorming involved the generation of ideas by all team members.  Team members were strongly encouraged to brainstorm on the entire design over the winter break, and try to arrive at one or more ways to fulfill each want or need.  This resulted in an ample supply of possible choices, and a large pool of good ideas from which to choose.  Brainstorming was done individually and as a group, with the individual work being presented in design meetings often acting as the seed from which a new idea sprang.  Thusly, the two types of work which are usually at odds with one another, individual and collective, were brought together in design meetings and used to lengthen and improve the list of possible options available for the next stage.

 

The final stage in the conceptual design stage was selection.  In this stage, a design/decision matrix was used to select the preferred design for each subsystem.  Brainstormed ideas were submitted for each of the above subsystems (or a multi-subsystem design), and rated relative to other submissions in their category from best to worst in each case, for a number of criteria.  These criteria were (in no particular order):

 

§         Simplicity

§         Safety

§         Ease of use

§         Weight

§         Client Preference

 

Using this system, a design might rate quite high in several areas, but when the overriding concern of safety dictated another design, the safer design was unhesitatingly chosen.  Weighting of results of this type was at the discretion of the team, and was made use of frequently, as most designs were up to similar standards of safety.

 

The results were found to favor certain designs that combined all of the aforementioned characteristics: It was not only light, simple, and safe, but also easy to use and liked by Ben. The very few “ties” that occurred were decided upon based on the aforementioned weighting.  If one design had greater safety potential than he other, it was chosen.  If two designs had equal safety potential, the simpler of the two was chosen, and so on.   The results of this process are indicated on the Design Matrix by an “X”.   Please see the Design Matrix in the Appendix, page A-1, for complete details about this important phase and the results produced.  The selected designs then went through the Proof of Concepts/Modeling phase and the Analysis phase where they are explained in more detail.

3b.  Proof of Concept/Modeling

 

The next stage was the Proof of Concept/Modeling stage.  During this stage, the preferred subsystem designs were tested for workability, ease of manufacture, fulfillment of purpose, and overall viability.  Just as the name implies, the Proof of Concept stage gives proof that the design concepts generated in the previous stages function as they are intended, as well as providing additional opportunities for the refinement of the designs.

 

A great deal of time and money, as well as a number of headaches, was saved by finding and fixing bugs that pop up at this stage.  These were problems that could have been overlooked in an all-paper design, such as unforeseen interaction of components or unsuitability of a certain part or idea for its intended purpose.  Few of these occurred, but the few that did made the effort put into building a model worthwhile.

 

One of the most important parts that was tested during this phase was the Acme screw rack, the heart of the reclining mechanism.  When the reclining mechanism was acquired, it was unsure whether or not it would be capable of moving the client seat with its occupant.  This was one of the chief reasons for building the test rig.  By building this test rig, the suitability and viability of the rack mechanism was tested and verified, thereby eliminating a bone of contention, as well as a source of worry.  The test rig was made with certain portions to scale and other portions made larger.  This allowed for testing of various pieces as they would be used in the final prototype but also left open space to attach future ideas that needed to be tested in conjuncture with the reclining mechanism.  A 3D representation of the test rig can be seen in Fig. 3b-1. 

 

The general idea was “measure twice, cut once”.  The test rig was the yardstick used to ensure that each subsystem design measured up to the standard set for it.  It was also used as a yardstick of sorts in other ways.  By building a model approximating the final design, starting dimensions for the requisite analysis could be obtained, and manufacturing processes to be used later in the making of the finished product could be practiced and refined, and jigs could be made and reused.  This alone saved a great deal of time, as a very accurate estimate of the time required to assemble and otherwise create the finished product from scratch could be formulated. 

 
 

 
 


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